Contamination and ESD control in high-technology manufacturing

CONTAMINATION AND ESD CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

ROGER W. WELKER R. NAGARAJAN CARL E. NEWBERG

A JOHN WILEY & SONS, INC., PUBLICATION

CONTAMINATION AND ESD CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

CONTAMINATION AND ESD CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

ROGER W. WELKER R. NAGARAJAN CARL E. NEWBERG

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data: Welker, R. W. Contamination and ESD control in high-technology manufacturing / by Roger W. Welker, R. Nagarajan, Carl E. Newberg. p. cm. ISBN-13: 978-0-471-41452-0 ISBN-10: 0-471-41452-2 1. Electronic apparatus and appliances—Protection. 2. Electric discharges. 3. Electrostatics. 4. Contamination (Technology) 5. Cleanrooms. I. Nagarajan, R. (Ramamurthy) II. Newberg, Carl E. III. Title. TK7870.W36 2006 670.42—dc22 2005058119 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

CONTENTS PREFACE 1

2

FUNDAMENTALS OF CONTAMINATION CONTROL

xv 1

1.1 Introduction 1.1.1 Contamination Sources 1.1.2 Contamination Adhesion Forces 1.1.3 Contamination Control Methods 1.2 Glossary of Contamination Control Terms 1.3 Specifying Contamination in Air and on Surfaces 1.4 Sources of Contamination 1.5 Contamination Control Requirements 1.5.1 Airborne Particle Requirements 1.5.2 Chemical Vapor Contamination Control Limits 1.5.3 Ionic Contamination Control Limits 1.5.4 Magnetic Contamination Control Limits 1.5.5 Surface Contamination Rates and Air Ionization 1.5.6 Contact Transfer and In Situ Contamination 1.5.7 Airflow Requirements 1.5.8 Pressure Requirements and Enclosure Exhausts 1.5.9 Maintenance Requirements 1.5.10 Other Requirements 1.5.11 Summary of Requirements 1.6 Pertinent Standards References and Notes Additional Reading

1 1 3 9 10 13 16 18 18 33 35 37 37 38 39 39 40 43 43 43 46 46

FUNDAMENTALS OF ESD CONTROL

48

2.1 Introduction and Historical Perspective 2.2 Glossary of Electrostatic Charge Control Terms

48 52 v

vi

CONTENTS

2.3 Sources of Electrostatic Charge 2.3.1 Static Electricity 2.3.2 Effects of Electrostatic Charge and Discharge 2.3.3 Failure Modes in High-Technology ESD-Sensitivity Devices 2.4 Requirements of ESD Control 2.4.1 Determining ESD Damage Sensitivity 2.4.2 Electrically Explosive Device ESD Modeling 2.5 Building the ESD-Safe Workplace 2.5.1 Surface Resistivity of Materials 2.5.2 Grounding 2.5.3 Identification of and Access to an ESD-Safe Work Area 2.5.4 ESD-Protective Floor Coverings 2.5.5 Work Surfaces and Table Mats 2.5.6 Wrist Strap Ground Points 2.5.7 Air Ionization Systems 2.5.8 Relative Humidity 2.5.9 Chairs and Stools 2.5.10 Trash Cans 2.5.11 Cathode-Ray Tube Displays 2.5.12 Field Potential Limits 2.5.13 Tools and Fixtures 2.5.14 Conveyors 2.6 ESD Controls for People 2.6.1 Wrist Strap and Coiled Cord 2.6.2 Training and Certification Program 2.6.3 Cleanroom Gowns and ESD Lab Coats 2.6.4 Footwear 2.6.5 Gloves, Liners, and Finger Cots 2.7 Consumables and Accessories 2.7.1 Packaging 2.7.2 Desiccants 2.7.3 Tote Boxes, Bins, and Other Shipping Containers 2.7.4 Notebooks and Sheet Protectors 2.7.5 Swabs and Wipers 2.7.6 Paper 2.7.7 Tape 2.8 Personnel Equipment and Procedures for Its Use 2.8.1 Wrist Straps and Wrist Strap Monitors 2.8.2 Sit–Stand Protocol 2.9 Transportation of ESD-Sensitive Products 2.10 Inspections and Record Keeping 2.10.1 Daily Visual Inspection 2.10.2 Periodic Instrumental Inspection 2.10.3 Testing Protocols 2.11 ESD Control Program 2.12 ESD and Contamination Control 2.13 Useful Reference Standards References and Notes

56 57 65 67 68 69 74 75 75 77 78 78 81 83 83 88 90 90 91 93 94 94 95 95 95 97 98 100 100 100 102 102 104 104 104 104 105 105 106 106 106 106 107 109 112 115 116 117

CONTENTS

3

4

vii

SAMPLING AND ANALYSIS METHODS

119

3.1 Introduction 3.2 Classification of Analysis Methods 3.2.1 Functional Laboratory Tests 3.2.2 Nonfunctional Tests: Objective Laboratory Tests 3.3 Sampling of Contaminants in Air, in Liquids, and on Surfaces 3.3.1 Contaminants in Air 3.3.2 Contaminants in Liquids 3.3.3 Surface-Borne Contaminants 3.4 Organic Contamination Analysis Methods 3.4.1 Water Break Test 3.4.2 Contact Angle Measurement 3.4.3 Optically Stimulated Electron Emission Technique 3.4.4 Nonvolatile Residue Test 3.4.5 Organic Sampling Techniques 3.4.6 Central Atmospheric Monitoring System 3.4.7 Electron Spectroscopy for Chemical Analysis 3.4.8 Gas Chromatography/Mass Spectroscopy 3.4.9 Secondary Ion Mass Spectroscopy 3.5 Ionic and Inorganic Contamination Analysis Methods 3.6 Electrostatic Discharge Methods 3.6.1 Tribocharge Testing 3.6.2 Bulk and Surface Resistance Measurements 3.6.3 Air Ionizer Testing 3.6.4 Typical ESD Field Instruments 3.7 Numerical Simulation 3.8 Algebraic Predictive Modeling 3.9 Statistical Analysis Methods 3.9.1 Basic Statistical Analysis Tools 3.9.2 Gage Capability Analysis of Cleanliness Measurement Methods Additional Reading References and Notes

119 119 121 124 133 133 134 135 136 136 136 137 137 137 138 139 139 139 139 141 141 142 144 145 146 147 150 150 151 156 156

FACILITIES DESIGN: CONTAMINATION- AND ESD-SAFE WORK AREAS

158

4.1 Introduction 4.2 Basics of Cleanroom Design 4.2.1 What Can Be Called a Cleanroom 4.2.2 What It Takes to Make a Cleanroom Work 4.2.3 How Filters Work 4.3 Cleanrooms 4.3.1 Non-Unidirectional-Flow (Conventional or Mixed-Flow) Cleanrooms 4.3.2 Air Ionization for Non-Unidirectional-Flow Cleanrooms 4.3.3 Unidirectional Flow: 100% Filter Coverage 4.3.4 Air Ionization in Unidirectional-Flow Cleanrooms 4.3.5 Adding a Perforated Raised Floor

158 159 159 161 162 165 166 168 169 174 174

viii

5

CONTENTS

4.3.6 Balancing a Room Using a Perforated Raised Floor 4.3.7 Airflow Balancing After Tool Installation 4.3.8 Solid vs. Perforated Work Surfaces 4.3.9 Parts Storage Locations 4.3.10 Horizontal Unidirectional-Airflow Cleanrooms 4.4 Cleanroom Construction and Operating Costs 4.5 Modern Energy-Saving Approaches 4.5.1 Unidirectional-Flow Clean Benches 4.5.2 Isolators and Minienvironments 4.5.3 Point-of-Use Clean Air Cleanrooms 4.5.4 Tunnelizing an Existing Ballroom Cleanroom 4.5.5 Minienvironments 4.6 Other Design Considerations 4.6.1 Doors and Air Showers 4.6.2 Pass-Throughs 4.6.3 Equipment Pass-Throughs 4.6.4 Service Areas References and Notes

175 176 181 181 182 183 184 184 186 187 188 190 191 191 192 193 193 193

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

195

5.1 Introduction 5.2 Historical Perspective 5.3 Gross and Precision Cleanliness Protocols 5.3.1 Approaches to Specifying Cleanliness Levels 5.4 Design for Manufacturability and Cleanability 5.4.1 Design-for-Manufacturability Guidelines 5.4.2 Design-for-Cleanability Guidelines 5.4.3 Cleanability Indexes for Indirect Cleanliness Measurements 5.4.4 Design-for-Cleanability Planning Considerations 5.4.5 Design-for-Cleanability Management Considerations 5.5 Process Design Guidelines 5.5.1 Use of Water-Soluble Cutting Fluids 5.5.2 Minimizing Work in Progress by Implementing Continuous-Flow Manufacturing 5.5.3 Rinsing After Machining 5.5.4 Parts Handling After Final Cleaning 5.5.5 Soldering and Flux Removal 5.5.6 Clean–Then Assemble vs. Assemble–Then Clean 5.6 Cleaning Processes 5.6.1 Particles in Liquid Baths 5.6.2 Boundary Layers 5.6.3 Ultrasonic Cleaning 5.6.4 Spray Cleaning 5.6.5 Spin-Rinse Dryer Cleaning 5.6.6 Vapor Degreasing 5.6.7 Chemical Cleaning

195 196 197 199 202 202 203 203 206 216 216 217 218 218 218 219 219 220 221 221 221 225 228 230 230

CONTENTS

6

ix

5.6.8 Solvent Cleaning 5.6.9 Mechanical Agitation Cleaning 5.6.10 Manual Cleaning 5.6.11 Specialty Cleaning 5.7 Drying Processes 5.7.1 Spin-Rinse Drying 5.7.2 Forced-Air Drying 5.7.3 Vacuum Drying 5.7.4 Adsorption Drying 5.7.5 Chemical Drying 5.8 Cost of Cleaning 5.9 Vendor Process Contamination Checklist 5.10 Case Studies: Cleaning Equipment and Cleaning Process Design 5.11 Details on the Clean–Then Assemble and Assemble–Then Clean Procedures 5.11.1 Cleaning Strategies 5.11.2 Case Studies: CTA and ATC 5.11.3 Case Study Results and Discussion 5.12 Particle Size Distributions 5.12.1 MIL-STD-1246 5.12.2 Analytical Methods 5.12.3 Extraction Methods Tested 5.12.4 Results 5.13 Tool Part Cleanliness References and Notes

230 231 231 232 234 234 234 235 235 235 236 236 246

TOOLING DESIGN AND CERTIFICATION

276

6.1 Introduction 6.1.1 Tooling Design Process 6.1.2 Applications and Limitations of Tooling Design 6.2 Contamination and ESD Control Requirements 6.3 Maintenance Requirements 6.3.1 (Basics of a) Wipe-Down Procedure 6.3.2 Maintenance Wipe-Down 6.3.3 Engineering Changes 6.3.4 Summary of Requirements 6.4 General Design Alternatives 6.4.1 Eliminating Contamination Generators 6.4.2 Relocating Contamination Generators 6.4.3 Enclosing and Evacuating Contamination Generators 6.5 Materials 6.5.1 Guidelines for Materials 6.5.2 Guidelines for Wear 6.5.3 Guidelines for Plastics 6.6 Surface Treatments 6.6.1 Paints 6.6.2 Anodizing and Related Treatments

276 277 278 279 280 280 281 282 282 283 283 284 285 293 293 297 301 308 309 310

252 253 255 262 264 264 265 266 266 272 273

x

7

CONTENTS

6.6.3 Electroplating, Electropolishing, and Other Treatments 6.6.4 Cautions About Coatings 6.6.5 Synergistic Coatings 6.6.6 Relative Wear Properties of Coatings 6.6.7 Surface Texture and Porosity 6.7 Selection and Evaluation of Components 6.7.1 Pneumatic Devices 6.7.2 Linear Motion Guides 6.7.3 Electric Motors 6.7.4 Process Piping and Point-of-Use Filtration 6.7.5 In Situ Monitoring Equipment 6.7.6 Hand Tools 6.8 Tool and Workstation Layout 6.8.1 Flow Control Enclosures, Minienvironments, and the Standard Machine Interface 6.8.2 Putting the Cleanroom Tool Together 6.9 Cleanroom Certification of Automated Tooling 6.9.1 Statistical Requirements for Sampling 6.9.2 Analytical Equipment and Methods References and Notes Additional Reading

311 311 311 312 312 313 314 314 314 315 316 317 318 318 322 325 327 331 334 334

CONTINUOUS MONITORING

336

7.1 Introduction 7.1.1 Approaches to Monitoring 7.1.2 Traditional Airborne Particle Measurements 7.1.3 Critical and Busy Sampling 7.1.4 Modified Data Collection Protocol 7.1.5 Ongoing Use of Critical and Busy Sampling 7.1.6 Case Studies: Traditional vs. Critical and Busy Sampling 7.1.7 Trend, Cyclic, and Burst Patterns of Particle Generation 7.1.8 Case Studies: Other Applications of Continuous Monitoring 7.1.9 Summary and Conclusions 7.2 Continuous Contamination Monitoring 7.2.1 Electronically Multiplexed Monitoring 7.2.2 Pneumatically Multiplexed Particle Monitoring 7.3 Continuous Monitoring of Manufacturing 7.3.1 Air Quality 7.3.2 Process Fluid Purity 7.3.3 The Value of 100% Sampling 7.3.4 Cleanliness of Surfaces and Electrostatic Charge 7.4 Evaluation of In Situ Monitoring in an Aqueous Cleaning Application 7.4.1 Description of Experiment 7.4.2 Experimental Results 7.4.3 Management Using ISPM 7.4.4 Conclusions 7.5 Antennas for Electrostatic Charge Monitoring References and Notes

336 337 338 339 339 340 341 346 348 350 350 350 351 352 352 355 356 358 359 360 362 370 371 372 372

CONTENTS

8

9

xi

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

374

8.1 Introduction 8.2 Cleanroom and ESD Gloves 8.3 Functional vs. Nonfunctional Testing 8.3.1 Functional Materials Qualification Tests 8.3.2 Nonfunctional Testing: Objective Laboratory Measurements 8.3.3 ESD Considerations in Glove Selection 8.4 Glove Use Strategies 8.5 Initial Qualification vs. the Need for Ongoing Lot Certification 8.6 Glove Washing 8.6.1 Early Observations with Natural Rubber Latex Gloves 8.6.2 Gloves Washability 8.6.3 Nitrile Glove Performance 8.6.4 Glove Washing Conclusions 8.7 ESD Performance of Gloves 8.7.1 Materials Selection for ESD Properties 8.7.2 Specifying the ESD Performance of Cleanroom Gloves and Glove Liners 8.7.3 Testing Considerations 8.7.4 Factors That Affect the ESD Performance of Gloves 8.8 Glove Laundering 8.8.1 Cost–Benefit Problem 8.8.2 Polyurethane Glove Laboratory Properties 8.8.3 ESD Performance 8.8.4 Chemical Contamination 8.8.5 Wear Characteristics 8.8.6 Laundering Tests 8.8.7 Impact of Laundering and Reuse on Glove Cost 8.8.8 Conclusions 8.9 Wipers and Swabs 8.9.1 Selecting the Correct Wiper or Swab 8.10 Reusable and Disposable Packaging Materials 8.10.1 ESD Consideration in Packaging 8.10.2 Carbon-Filled Polymers 8.10.3 Metal Loading 8.10.4 Topical and Incorporated Organic Agents 8.10.5 Copolymer Blends 8.11 Facial Coverings References and Notes

374 375 376 376 377 379 381 381 383 383 384 387 388 388 389

CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

410

9.1 Introduction 9.2 People as a Source of Contamination 9.2.1 Skin and Hair 9.2.2 Fingerprints 9.2.3 Bacteria and Fungi 9.2.4 Spittle Droplets

410 410 411 413 414 414

389 391 392 396 397 397 398 399 399 401 401 402 402 403 405 405 405 406 406 407 407 408

xii

CONTENTS

9.2.5 Street Clothing 9.2.6 Other Forms of Contamination 9.3 Typical Gowning Protocols 9.3.1 Inner Suit 9.3.2 Hair Cover (Bouffant) 9.3.3 Woven Gloves 9.3.4 Barrier Gloves 9.3.5 Facial Cover 9.3.6 Hood and Powered Headgear 9.3.7 Frock, Coverall, and Two-Piece Suit 9.3.8 Shoe Covers, Booties, and Special Shoes 9.3.9 Suggested Frequency of Change 9.4 Procedures for Entering a Cleanroom 9.4.1 Pre-Change Room Procedure 9.4.2 Wipe-Down 9.4.3 Hairnet and Face Mask 9.4.4 Shoe Cleaners 9.4.5 Handwashing 9.4.6 Changing into Cleanroom Garments 9.4.7 Powered Headgear 9.4.8 Footwear 9.4.9 Shoe Cleaners and Tacky Mats 9.5 Behavior in a Cleanroom 9.5.1 Working in a Cleanroom 9.5.2 HEPA Filters 9.5.3 Raised Floors 9.5.4 Glove Awareness 9.6 Procedures for Exiting a Cleanroom 9.6.1 Knee-High Booties 9.6.2 Frock or Jumpsuit 9.6.3 Head Covering 9.6.4 Hairnets, Gloves, and Disposable Shoe Covers 9.7 Relationship between Attire and Class Achieved 9.8 Procedures for Entering an ESD-Safe Work Area 9.8.1 Behavior in an ESD-Safe Work Area 9.8.2 ESD-Safe Work Area in a Cleanroom 9.9 Garments and Laundry Services 9.9.1 Garment Options 9.9.2 Measurements of Garment Cleanliness 9.9.3 Selection of Fabrics 9.9.4 Design and Construction of Garments 9.9.5 Selection of a Cleanroom Laundry Service References and Notes 10

415 416 417 418 419 419 420 420 421 422 424 426 426 427 427 428 429 430 431 433 433 436 437 438 439 439 439 439 440 440 440 441 441 443 444 445 446 446 446 448 448 449 449

LAYOUT OF CHANGE ROOMS

451

10.1 Principles of Efficient Change Room Design 10.2 Case Studies: Change Rooms

451 454

CONTENTS

11

xiii

10.3 Entering the Cleanroom 10.3.1 Planning a Trip into the Cleanroom 10.3.2 Pregowning Actions 10.3.3 Dressing in Cleanroom Garments 10.3.4 Finishing Dressing 10.4 Exiting the Cleanroom 10.5 Other Considerations References and Notes

467 468 469 469 469 470 472 474

PROCEDURES AND DOCUMENTATION

475

11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8

475 476 478 479 480 481 483 488 489 489 491

Hierarchy of Documents and Audits Operator Self-Check Noninstrument Audits Instrument Audits Independent Audits Managing Use of the Audit Scorecard Typical Survey Case Study: Broken Magnet Procedure 11.8.1 Definition of a Broken Magnet 11.8.2 Recommendations for the Broken Magnet Procedure Reference INDEX

493

PREFACE

Contamination and electrostatic discharge (ESD) are now becoming recognized as factors affecting yield and reliability in an ever-increasing number of industries. Whereas contamination traditionally was recognized as affecting the semiconductor, disk drive, aerospace, pharmaceutical, and medical device industries, today such industries as automobile and food production are also discovering the benefits of contamination control. ESD control has experienced a similar growth in applications. An engineer or scientist cannot obtain a degree in contamination or ESD control from a college or University. It is possible to obtain certification as an ESD control engineer or technician through an independent certification agency. However, there is not a similar certification program for contamination control. Indeed, engineers and scientists with diverse degrees in mechanical engineering, chemistry, physics, microbiology, industrial engineering, electrical engineering, and many other fields are found in the ranks of those who consider themselves contamination or ESD specialists. Despite the large number of degreed professionals working in contamination control and ESD control, these fields remain misunderstood and underappreciated. The misunderstandings often arise because of the interdisciplinary nature of contamination and ESD control. Because so many different academic disciplines are needed to provide a comprehensive understanding, the problems and solutions often appear confusingly complex. At the same time, the vast majority of contamination or ESD problems are solved using very simple analysis, making them appear to be child’s play. In addition, there is a long-standing perception that what is good for contamination control is bad for ESD control, and vice versa. This book is an attempt to rectify these two problems. ●

●

We begin with two general chapters on the fundamentals of contamination and ESD control. This is followed by a chapter on analysis methods useful for solving contamination and ESD problems. xv

xvi ●

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●

● ●

●

PREFACE

We next begin to build the contamination and ESD control environment. We start with a description of cleanrooms, their components, and how processes are arranged within them. Construction of an ESD-protected environment is described similarly. The next subject covered is cleaning processes and the equipment used to support them. This subject is dealt with from the perspective of both the supplier and the user. Tooling is discussed in some detail. This involves material selection and evaluation problems common to all industries affected by contamination and ESD. We turn next to a discussion of consumable materials and supplies. We talk about contamination originating from people and how that is contained. This involves discussions of behavior and discipline. Finally, we discuss management of the cleanroom and ESD-protected workplace environments. Companies dealing with contamination and ESD range in size from those having a single sensitive facility to multinational corporations having cleanrooms and ESD-protected workplaces on virtually every continent.

The emphasis is to provide a working knowledge of contamination control and ESD control. This is done by introduction of control standards and examples of how they are employed. In this regard, the book is considered to be a “how-to” guide. It is filled with many case studies to illustrate and illuminate the lessons of contamination and ESD control engineering.

CHAPTER 1

FUNDAMENTALS OF CONTAMINATION CONTROL

1.1

INTRODUCTION

Contamination control is a process. It is the process of limiting contamination to below some tolerable amount. It does not mean absolute elimination of contamination. Indeed, due to the limitation of measurement capability, it is impossible to verify zero contamination. The best we can do is to reduce the amount of contamination to below the lower detection limit of the measurement technique in use at the time. Contamination can be defined in several different ways. One popular definition is that contamination is any form of matter or energy that has a detrimental effect on products or processes. This is a functional definition, since it assumes nothing about the nature of the contaminant, but rather, is concerned with what the contaminant does. Contamination can be matter, such as particles, films, ions, or gases. Contamination in the form of excess static electric charge can cause product damage due to the process of electrostatic discharge. Contamination can also be electromagnetic radiation, such as the wrong wavelength of light. Some contamination engineers even include temperature or vibration outside specified limits as forms of contamination. Contamination can be in plasma, gaseous, liquid, or solid form and can appear within other solids, liquids, and gases. 1.1.1

Contamination Sources

It is often convenient to think of contamination in two broad categories: functional contamination and nuisance contamination. Functional contamination is contamination that has a detrimental effect on products or processes; nuisance contamination does not have a Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

1

FUNDAMENTALS OF CONTAMINATION CONTROL

ce

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45 40 35 30 25 20 15 10 5 0 g

Percent

2

Source Category

FIGURE 1.1 Hypothetical contamination source distribution represented as a bar chart. This representation is particularly useful because it highlights problem areas.

directly detrimental effect on products or processes. Clearly, this distinction is useful, as it provides focus when investigating and eliminating problems caused by contamination. Our primary focus should be on functional contamination; however, nuisance contamination can become a problem if it is so extensive that it interferes with the orderly process of identifying and eliminating the source of a functional contaminant. An argument can be made that the indirect benefit of performing good nuisance contamination control is that it facilitates our primary objective, control of functional contamination. Contamination can originate from many different sources, take many forms, and appear in many places. Among the sources are the cleanroom, tooling, chemicals, processes, parts, consumable supplies, and people. There is a widely held belief within the contamination control community that people are the most significant source of contamination. This generalization must be approached with caution. For example, the relative contribution of people vs. tooling is process dependent. A factory where people do all material handling and manufacturing operations will have a different proportion of contamination from people than that of a fully automated process that has relatively few people present. A second example can be cited for the use of isolation enclosures and standard machine interfaces (SMIFs) around products and processes. Early in the promotion of SMIFs there was a widely communicated belief that utilizing SMIF technology would lessen the demands on and need for cleanrooms, would turn the cleanroom into a shirtsleeve environment, and would eliminate the need for continuous contamination monitoring. This has not been realized in the vast majority of cases because the isolation enclosure environment must be entered during maintenance and the isolation enclosure can actually amplify the contribution of tool-generated contamination, reemphasizing the value of continuous monitoring. Several different ways of visualizing the relative importance of various source categories to overall contamination are useful. Pareto diagrams (Figure 1.1) and pie charts (Figure 1.2) are particularly useful graphic representations, because they allow one to quickly visualize where problems originate and where corrective action must focus. Clearly, tooling and people are the most important sources for this hypothetical environment, accounting for more than half of all contaminants. How might a chart like this be generated? One approach would be to examine all the failure analysis reports for field returns and select all failures where contamination was found and suspected to be a contributing factor

INTRODUCTION

Chemicals 3%

Piece Parts 16% Consumables 13% People 26%

3

Maintenance 2%

Other 8%

Tooling 37%

Facilities 3%

FIGURE 1.2 Hypothetical contamination source distribution represented by a pie chart. This representation is useful in showing the relative importance of sources.

in the failure. Analysis of the materials of the contaminants could then be sorted into the appropriate source category. For example, clothing fibers or skin flakes are clearly associated with people contamination one failure producing this conclusion would be attributed to people. Analysis might also identify materials that could be associated with more than one category. For example, suppose that aluminum particles made of a particular alloy are identified. If some of the piece parts and some parts of the tooling are made of this particular alloy of aluminum, it would not be possible to associate that failure unambiguously with either source category. In this case, because both categories could be the source, half of the failure would be attributed to each category. Another approach might also be useful, especially where there are relatively few field failures. In this approach the yield loss is analyzed. Tests in which contamination can be a contributing factor to yield loss are then analyzed in the same manner as for field failures. It can be reasonably argued that analyzing yield loss is a more productive approach to solving contamination problems than is analyzing field failures (being proactive rather than reactive). This is especially true where the tests are designed to be good predictors of field failures. 1.1.2

Contamination Adhesion Forces

The forces that affect adhesion of contamination also vary widely. Van der Waals Forces which are created anytime that two bodies approach one another, are universal. As the bodies approach one another, the force of attraction increases. This is theorized to be due to charge displacement: Like charges repel. Thus, electrons on one surface that are tightly held will repel electrons on a nearby surface that are less tightly held. This electron repulsion creates an oppositely charged surface, drawing the two surfaces together. At some point the materials approach each other so closely that they begin to repel each other, by either electron or proton repulsion. Figure 1.3 illustrates the attractive and repulsive forces of van der Waals attraction. Van der Waals attractive forces are considered to be the weakest of the forces that bind contamination to surfaces. However, the force of van der Waals attraction should not be

4

FUNDAMENTALS OF CONTAMINATION CONTROL

Repulsive

Force 0

Attractive

Interatomic distance

FIGURE 1.3 Van der Waals forces of attraction and repulsion.

underestimated. It is theoretically directly proportional to the surface area in contact between different surfaces. If the materials in contact are rigid and relatively nondeformable, as the surfaces approach one another, the attractive force between them reaches a maximum and stabilizes relatively quickly. Conversely, if one or more of the materials are deformable, they will change shape to accommodate the approach of the contacting surface. This will increase the area in contact and increase the adhesive force between them. (It is for this reason that elastomeric polymers can often be very difficult to clean.) For a spherical particle contacting a flat surface, the adhesive force due to van der Waals interaction is given by

Fadh 

Ad 12x 2

where A is the Hamaker constant, which depends on the materials of which the particle and surface are made, with a typical order of magnitude of 1019 or 1018 J; d is the diameter of the spherical particle; and x is the separation distance between the particle and the surface. The separation between the particle and the surface is never zero. Electrostatic Attraction A second force binding particles to nonconducting surfaces is electrostatic attraction. This force is given by FE 

K E q2 x2

where KE is a constant (9.0  109 N  m2/C2 in SI units), q is the charge on the particle in coulombs, and x is the particle diameter. For particles larger than 100 nm or so, the equilibrium charge, q, is roughly proportional to the square of the particle diameter, so the electrostatic force ends up being directly proportional to the particle diameter (like the van der Waals forces). Charged particles become attracted to an oppositely charged surface by simple coulombic attraction. However, both surfaces do not need to be charged. A common example occurs when particles are charged and are attracted to neutral surfaces. Excess electrical charge plays a significant role in contamination. It can cause an energetic discharge [electrostatic discharge (ESD)] that causes damage. ESD is the usual concern

INTRODUCTION

5

-q

-q +Q

(a)

+Q

(b)

FIGURE 1.4 Electrostatic attraction of a charged particle to an oppositely charged surface. (a) The particle is not yet affected by the charge on the surface; it has only a vertical component to its motion. (b) The particle’s vertical motion has brought it into the electrostatic field surrounding the charged surface; the ESA of the particle imparts horizontal motion to the particle vis-à-vis the surface.

expressed regarding charge. However, charge also introduces a contamination concern. That is, charge on surfaces or particles can increase contamination of surfaces by a process called electrostatic attraction (ESA). The presence of excess charge on a surface creates an electrostatic field that will accelerate deposition of oppositely charged particles and thus accelerates contamination. The number of particles deposited on a surface is proportional to (1) particle charge and concentration, (2) the electrostatic charge per area on the surface, and (3) the duration of exposure. Experiments have shown that charged particles are attracted to oppositely charged surfaces and to neutral surfaces but that charged surfaces have little effect on attraction of neutral airborne particles. Of course, since most particles become charged when they are generated, the latter case probably occurs seldom under normal circumstances in cleanrooms. Figure 1.4 shows the situation schematically, where a particle of charge q is attracted to a surface having a charge Q (polarity opposite that of the particle) per area A. Figure 1.5 illustrates the situation where the surface is neutral but the particle is charged. The charge on the particle repels like charges on the surface. The surface thus acquires a net charge opposite in polarity to that on the particle locally around the vicinity of the particle. Opposite charges attract. Thus, the charge on the particle causes the particle to suck itself to the surface, even when the surface started out neutral. For the case shown in Figure 1.4, where the particle has charge q and the surface has charge Q of opposite polarity, the total number deposited on a surface is roughly N  cqEBAt

6

FUNDAMENTALS OF CONTAMINATION CONTROL

(a)

(b)

(c)

FIGURE 1.5 Electrostatic attraction of a charged to a neutral insulating surface by induced surface charging. (a) A particle with charge q approaches a neutral surface. (b) The charge on the particle locally induces a negatively charged region on the surface, producing an opposite polarity “image charge” on the surface. (c) The charged particle is attracted to the surface by its image charge.

where N  number of particles deposited c  airborne particle concentration q  charge in coulombs per particle E  electrical field strength on the surface B  mechanical mobility of the particle A  area of the surface over which the charge Q is uniformly distributed t  time Figure 1.6 illustrates the effect of charge on contamination deposition on horizontal surfaces in vertical unidirectional-flow cleanrooms. Data are plotted from 0.01 to 10 m (above approximately 5 m, the effect of electrostatic charge on deposition rate is negligible). The term nb describes the charge state of particles in terms of the Boltzmann equilibrium distribution. When nb  0, the particles are uncharged. This condition is expected to occur only rarely, as most mechanisms generating particles produce charge on the particles. Particles with nb  1 correspond to the charge distribution that results when particles are exposed to a cloud of bipolar air ions, generally considered the minimum charge state of aerosol particles. The curve for particle deposition with nb  10 probably is a more realistic charge state for aerosols in the cleanroom. The term E describes the charge state of surfaces in a cleanroom. E  100 V/cm (250 V/in.) lies between the charge states expected for rooms with air ionization, which typically will be controlled to less than 100 and the actual charge state of surfaces in cleanrooms with no air ionization. Also shown in Figure 1.6 is the deposition velocity curve for nb  10 and E  1000 V/cm (2500 V/in.) which probably represents deposition rates that would be expected in many cleanrooms without air ionization.

INTRODUCTION

7

100

Deposition Velocity (cm/s)

10 nb = 10 E = 1000 V/cm

1

nb = 10 E = 100 V/cm

0.1

0.01

nb = 1 E = 100 V/cm

0.001

0.0001

0.01

nb = 0 E = 100 V/cm

0.1

1.0

10

Diameter (μm)

FIGURE 1.6 Effect of particle charge and surface charge on surface contamination rates in cleanrooms. The lower curve, with nb  0, E  100 V/cm, probably is not observed in practice since processes that produce particles usually charge them. The second curve, where nb  1 and E  100 V/cm, probably represents the lower limit of deposition rates to be seen in cleanrooms. The upper curve, nb  10, E  100 V/cm and nb  10, E  1000 V/cm, probably represents the range that includes most cleanrooms without air ionization.

Much work has been done concerning the deposition of particles on charged silicon wafers in a cleanroom. The predictions of Liu and Ahn [1] and of Cooper et al. [2] were that deposition velocities (without electrostatic effects) would be near 0.001 to 0.01 cm/s. Pui et al. [3] confirmed this in the laboratory using monodisperse fluorescent particles having minimal charge (thus minimal electrostatic effects). Wu et al. [4] found that deposition velocities were about an order of magnitude higher for ungrounded wafers in a cleanroom than for grounded wafers. Cooper et al. used a minimal charge distribution (Boltzmann equilibrium distribution) and a Federal Standard 209 (FED-STD-209) class 100 (ISO 14644 class 5) cleanroom particle size distribution and predicted that an electric field as low as E  100 V/cm would produce an order-of-magnitude greater deposition with gravity plus diffusion than these two mechanisms combined produced without the electrostatic field. Further details are available in the book edited by Donovan [5]. Control The basic formula for the number of particles (of size d) attracted to the surface: N  cqEBAt suggests various alternatives for control [6]: 1. The concentration of airborne particles, c, should be kept to a minimum using standard contamination control approaches.

8

FUNDAMENTALS OF CONTAMINATION CONTROL

2. The particle charge q should be keep to a minimum by using air ionizers. 3. The electrostatic field E  kQ/A should be minimized by preventing charging of surfaces, by draining charge from the surface by grounding, by using a bipolar (positive and negative) air ionizer to neutralize charge on the surface, or by wet wiping with a grounded static-dissipative cloth. The beneficial effect of the use of bipolar air ionization in manufacturing cleanrooms has been demonstrated clearly [7]. 4. The duration of exposure, t, should be minimized. Charged surfaces increase the deposition of charged particles. Deposition rates per surface area are proportional to the electric field strength, particle charge, and particle concentration. Current standard contamination control procedures can minimize concentration. Air ionization and other static control techniques are able to reduce particle charge and surface charge, reducing particle contamination. Capillary Attraction The third force binding contamination to surfaces is capillary attraction, in which a film forms between two bodies, concentrated as a meniscus between their contacting surfaces. This capillary film increases the surface area of contact between the two objects, increasing van der Waals forces attracting the objects. As the film dries, the adhesion force increases rapidly. The force of capillary attraction is, to a first approximation, proportional to the surface tension of the liquid forming the capillary bridge between the particle and the surface. It should be kept in mind that adsorption by the liquid of materials on the particle, on the surface, or in the air can alter the surface tension of the liquid, so that the pure liquid surface tension might not be a good absolute predictor of the capillary force. The effect of the formation of a capillary bridge between two objects and the resulting increase in attractive forces between them can have a profound influence on the ability to clean surfaces. The attractive forces are increased in direct proportion to the increase in surface area in contact, thus increasing the amount of force required to separate them. Capillary drying is the principal reason why “just-in-time” cleaning is very critical. Figure 1.7 illustrates the increase in surface area in contact due to formation of a capillary bridge between a particle and a surface. The chemical history of the material forming the capillary bridge can have a profound effect on cleaning efficiency. For example, material forming the capillary bridge may be soluble in polar liquids such as water. Nonpolar solvents may not be able to dissolve the material forming the capillary bridge and thus may be found to be ineffective at cleaning a part whose prior history includes exposure to polar materials. Chemical history can also include exposure to elevated relative humidity. Contaminants may have solid chemicals adsorbed on their surface that do not wet the interface between the contaminant and the surface. If these solid chemicals are exposed to elevated relative humidity, they may absorb moisture and dissolve in the moisture, a process referred to as deliquescing. This concentrated solution may then wet the interface between the contaminant and the surface, forming a capillary bridge. If the relative humidity later drops and the moisture evaporates, solid may precipitate out at the interface, forming a strong, solid bond. Chemical Reactions A fourth force binding contamination to surfaces is the result of chemical reactions. Chemical reactions can result in adhesion between surfaces that are so strong that ordinary cleaning processes are rendered entirely ineffective. After chemical

INTRODUCTION

9

Soluble material on surface

(a)

Soluble material dissolves – meniscus forms

(b)

Soluble material concentrates at interface, forms capillary bridge

(c)

FIGURE 1.7 Effect of capillary bridge formation in the adhesion of contamination: (a) there is three-point contact; (b) the material is exposed to a solvent-rich atmosphere; (c) the solvent evaporates.

reactions have occurred, often the only way to clean is to find an alternative chemical reaction to reverse the chemical bond. This can be very difficult. 1.1.3

Contamination Control Methods

The effects of contamination are as varied as their sources and forms. Contamination can result directly in product failure or yield loss. Occasionally, the damage caused by contamination can be corrected by rework. Rework increases the cost of products. Contamination can also result in reliability loss. Many tests are performed to detect the presence and effects of contamination on products. If contamination was not a problem, these tests could be eliminated or reduced to a sampling frequency, further reducing production cost. Thus, the overall objective of contamination control is to optimize cost through maximization of yield and reliability. Contamination is controlled using many different techniques: ●

Development of contamination control plans. Here the activity focuses on identifying the known or suspect contamination tolerances of the product or processes. From these, contamination budgets can be developed and mitigation strategies planned. This planning involves management early in the contamination control plan so that resources can be planned for and utilized effectively. This process is often referred to as the systems approach to contamination control.

10

FUNDAMENTALS OF CONTAMINATION CONTROL ●

●

●

●

●

● ●

1.2

Construction and operation of clean facilities. The establishment of a contaminationcontrolled workplace is the first step in the implementation of an overall contamination control program. The plans may call for various architectural approaches, including the use of unidirectional-flow clean benches, minienvironments, glove boxes, and other nested architectures to achieve the desired level of cleanliness while minimizing capital and operating cost (see Chapter 4). Selection and approval of materials. These may be materials of construction for the workplace, materials for construction of tooling and fixtures, materials for consumable supplies and packaging, process chemicals, and so on. See Chapter 3 for guidance on material selection and ongoing control. Development of cleaning processes and the design of equipment to support the cleaning processes (see Chapter 5). Control of contamination from tools and fixtures. This aspect of control is especially, important in the modern highly automated workplace (see Chapter 6). Control of people, the most significant source of contamination in virtually all contamination-sensitive industries (see Chapters 9 and 10). Control of consumable supplies (e.g., gloves). See Chapter 8 for a general discussion. Continuous monitoring to verify compliance. This step can be indispensable in identifying problems and establishing proper control. (see Chapter 7). GLOSSARY OF CONTAMINATION CONTROL TERMS

Many of the terms used in this book are common to descriptions of cleanrooms, tooling, piece parts, consumable supplies, and so on. A general knowledge of them is useful for any engineer or designer working in an industry where contamination is a potential issue. Where appropriate, common abbreviations and acronyms are included. In general, these acronyms are not used in the text but are provided for reference value. The terms listed below apply specifically to contamination control. Terms particular to electrostatic charge and discharge control are defined in Section 2.2. Additional terms are defined where appropriate, such as in Chapter 3 on analysis methods. ●

●

●

● ● ● ● ● ●

Absorption: penetration of one substance into the interior of another: a sponge absorbs liquids. Chemical absorbents include activated charcoal and silica gel. Adsorption: a condition in which one substance is attracted to and held on the surface of another. Adsorption is responsible for chemical contamination of nonporous materials, such as machined metal parts. Aerosol: a quasistable gaseous suspension of liquid or solid particles about 100 m in diameter or smaller. Airborne molecular contamination (AMC): vapor-phase contamination in air. Anemometer: an instrument for measuring air velocity. Anion: an atom or molecule with a net negative charge. Busy periods: During normal production operation. Cation: an atom or molecule with a net positive charge. Chimney effect: vertical movement of air due to buoyancy created by temperature differences.

GLOSSARY OF CONTAMINATION CONTROL TERMS ●

●

● ●

●

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●

● ● ● ●

●

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11

Class: in the traditional U.S. federal standards, the airborne particle concentration limit, such as class 100 and class 10,000, as monitored using airborne optical particle counters. Class in FED-STD-209 is equal to the maximum allowable concentration of 0.5-m diameter and larger particles per cubic foot of air. Cleanroom classification were made metric in FED-STD-209 revE. In the metric version, the class is an M followed by a number, where the number is the approximate power-of-10 number of particles 0.5 m and larger per cubic meter of air. In ISO 14644 class is n, where n is the power of 10 number of particles equal to or larger than 0.1 m per cubic meter of air. Cleanroom: an enclosed area employing control over particulate matter and other forms of contamination in air, with airflow, relative humidity and temperature, and pressure control as needed. Clean benches, downflow units, minienvironments, and so on, are considered to be cleanrooms in this context. Colloid: a stable suspension of particles in a fluid. Contamination: an unwanted foreign substance or energy, including particles, organic and inorganic vapors, electromagnetic radiation, vibration, and electrostatic charge. (See also functional contamination and nuisance contamination.) Contamination control: the process of limiting contamination to within specified amounts. Critical and busy sampling: sampling that satisfies the criteria of critical location and busy periods. Critical location: as close to the product or process as possible without interfering physically with the movement of tools, people, or product. Critical operation: an operation in which contamination of the product or process results in yield loss or field failures in excess of desired amount. For example, in a disk drive manufacturing operation, critical operations are those in which customer heads or disks, or items that come in contact with heads or disks, are exposed. Critical surface: a surface that requires precision cleanliness. Deionized (DI): water or other liquids that have been purified to remove ionized material. Densitometer: a commercial instrument for measuring the opacity of photographic film. EDX (energy dispersive x-ray analysis): a technique for elemental analysis used during scanning electron microscopy. Factory environment: portions of a facility outside a contamination-controlled or staticsafe workplace. FED-STD-209: the federal standard cleanroom Work Station Requirements, Controlled Environment, which describes and defines cleanrooms and is the basis for specifications for cleanrooms [8]. Fiber: a particle with a length/width ratio in excess of 10 : 1. Some measurement standards define a fiber as a particle with a length/width ratio greater than 3 : 1. FID (flame ionization detector): used in combination with gas chromatography to quantify volatile contaminants. FTIR (Fourier transform infrared spectroscopy): a technique for the identification of molecular contamination. It is most often used to analyze organic contaminants. Functional contamination: contamination that has a detrimental effect on product or processes.

12

FUNDAMENTALS OF CONTAMINATION CONTROL ●

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●

GC/MS (gas chromatography/mass spectroscopy): a gas chromatograph allows the separation of complex mixtures of volatile molecules; mass spectroscopy is a detection technique used to identify the separated molecules. HEPA (high-efficiency particulate air filters): filters that have at least 99.97% capture efficiency at the maximum penetrating particle size, usually rated at 0.3 m. Hydrophilic: liking water. Hydrophilic substances tend to be wetted by water and occasionally also absorb water. The tendency of a material to absorb or repel water is an important consideration in the selection and efficacy of cleaning processes. Hydrophobic: hating water. Substances that are hydrophobic tend not to be wetted by water. The tendency of a material to absorb or repel water is an important consideration in the selection and efficacy of cleaning processes. ISO 14644: a standard being developed by the International Standards Organization to unify cleanroom standards on a worldwide basis. Level: the amount of contamination specified for a surface. Level is the size in micrometers above which less than 1 particle per square foot of surface is expected to be found. Defined in Military Standard 1246. Liquid-borne particle counter (LPC): an automated electronic device that separates, sizes, and counts individual particles suspended in liquids. Maximum penetrating particle size: in filtration, the particle size above and below which all particles of other sizes are collected with higher filtration efficiency. Micron: a unit of length equal to one millionth of a meter; more properly called a micrometer and abbreviated m. MIL-STD-1246: the U.S. military standard defining contamination per unit surface area. Molecular contamination: contamination in gaseous or molecular form. Nonparticulate matter: matter that does not have a definable length or width dimension, such as a film or vapor. Nonvolatile residue (NVR): soluble or suspended matter remaining after controlled evaporation of a filtered volatile solvent. Filtration is normally performed through a 0.45- or 0.8-m filter prior to evaporation to distinguish between filterable particles and nonfilterable liquid or soluble contamination. Some laboratories do not filter prior to evaporation, so particle matter ends up being included in the NVR total. Nuisance contamination: contamination that does not have a functional effect on a product or process but which interferes with the discovery of functional contamination or interferes with the orderly management of a cleanroom. Optical particle counter (OPC): automatic electronic devices that size and count individual particles. The abbreviation OPC is limited to airborne particle counters. Outgassing: the process of production of matter in gaseous form. Particulate matter: matter with definable width and thickness. Scanning electron microscope (SEM): an instrument used to physically characterize the shape and size of an object. It produces an image using a beam of electrons and is capable of resolving structures smaller than 1 m. Semiclean zone: an area with restrictions on contamination from behavior and/or materials and/or attire but without an airborne particle classification. Surface contamination rate (SCR): the rate at which surfaces accumulate contamination. Contamination on surfaces is described by contamination levels.

SPECIFYING CONTAMINATION IN AIR AND ON SURFACES ●

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1.3

13

Tooling: any mechanism or device used in processing, handling, and assembly, including robots, hard automation, materials-handling systems, and processing equipment. Turbidimeter: an instrument for measuring the quantity of contamination suspended in a liquid by extinction and scattering. Turbulence: in cleanrooms, any flow that is not unidirectional. Turbulent flow is generally rotational and includes flow directions and magnitudes not in compliance with specified limits. Not to be confused with the fluid dynamic definition of turbulent. Today, cleanrooms that are not described as having unidirectional airflow are referred to as non-unidirectional- or mixed-flow cleanrooms. ULPA (ultralow penetration filter): usually, a filter with better than 99.997% particle removal efficiency at the most penetrating particle size, usually rated at 0.12 m. Unidirectional: in cleanrooms, airflow characterized by straight streamlines that flow parallel to one another. Previously, the word laminar was used to describe this type of flow. Because the cleanroom use of the term laminar did not fit the fluid mechanical definition of laminar flow, the term unidirectional airflow is now preferred. Viable contamination: contamination by bacteria, spores, or viruses. Wipe-down: any procedure for manual cleaning of objects within or to be placed within a cleanroom. A complete wipe-down procedure must include descriptions of cleaning supplies, chemicals, procedures, and acceptance criteria. Witness plate: a bare, clean, unpatterned silicon wafer, disk, microscope slide, or part used as a surrogate for production parts, assemblies, and other critical surfaces in a cleanroom, for the purpose of measuring surface contamination rates. SPECIFYING CONTAMINATION IN AIR AND ON SURFACES

Contamination in air is specified in units of contamination per unit volume. For example, airborne particle contamination is usually expressed in particles per cubic foot or cubic meter, airborne molecular contamination in parts per billion or micrograms per cubic meter, and airborne bacteria in colony-forming units per cubic foot or cubic meter of air. In general, the methodologies are intended to demonstrate compliance with requirements of ISO 14644 or FED-STD-209. Contamination on surfaces is described in units of contamination per unit area. Contamination on surfaces is expressed in two different ways. Aerial density, in particles or mass per unit surface area, is analogous to particles per unit volume used to describe airborne particle concentrations. An alternative way of describing surface contamination is defined in MIL-STD-1246, which defines a model particle size distribution and a cleanliness term, level, the particle size at which one particle is expected per square foot of surface area. MIL-STD-1246 also defines nonvolatile residue in mass per unit area, outgassing in percent weight loss, volatile condensable matter, and condensed volatile condensable matter. The standard also defines percent obscuration, a term used almost exclusively in the aerospace industry [9]. It is important to differentiate terminology to clarify the difference between volumetric contamination in the air in the cleanroom vs. aerial contamination on surfaces. The two terms used to classify airborne and surface-borne contamination are class and level, respectively. Class refers to the concentration of airborne particulate matter (or other airborne

14

FUNDAMENTALS OF CONTAMINATION CONTROL

9

8

Number of Particles, log(N)

7

6

5

4 1000 750 500

3 300

2

200 100 50

1

25 10

5 0 1 1 0

2

3 4 5 6 7 Particle Size, (log x)2 (μm)

8

9

FIGURE 1.8 MIL-STD-1246 size distribution model.

contaminants) per unit volume, whereas level refers to the concentration of contaminants on surfaces per unit area. The terms are not interchangeable and there is no simple analytical relationship between them, although several attempts have been made to establish such a correlation [10]. MIL-STD-1246 is a method of specifying surface contamination levels. The particulate portion of the specification is based on a particle size distribution where the log of the concentration of the particles per unit surface area is plotted vs. the log of the square of the particle size (Figure 1.8). The model particle size distribution assumes that the maximum particle concentration occurs at 1 m. MIL-STD-1246 also specifies cleanliness for nonvolatile residues. A typical MIL-STD1246 specification could be MIL-STD-1246 level X, Y, Z, where X Y Z PAC PC

numerical particle cleanliness level, corresponding to the particle size occurring at 1 particle per unit area, as shown in Table 1.1 nonvolatile cleanliness level in g/cm2, as shown in Table 1.2 alternative or additional cleanliness levels, consisting of one or more abbreviations from the following list and the maximum limits described: percent area covered particle count specified independent of Table 1.1

15

SPECIFYING CONTAMINATION IN AIR AND ON SURFACES

TABLE 1.1 Particle Cleanliness Levels Level

Particle Size (m)

Count per 1 ft2

Count per 0.1 m2

Count per Liter

1

1

1.0

1.08

10

5

1 2 5

2.8 2.3 1.0

3.02 2.48 1.08

28 23 10

10

1 2 5 10

8.4 7.0 3.0 1.0

9.07 7.56 3.24 1.08

84 70 30 10

25

2 5 15 25

53 23 3.4 1.0

57 24.8 3.67 1.08

530 230 34 10

50

5 15 25 50

166 25 7.3 1.0

179 27.0 7.88 1.08

1,660 250 73 10

100

5 15 25 50 100

1,785 265 78 11 1

1,930 286 84.2 11.9 1.08

17,850 2,650 780 110 10

200

15 25 50 100 200

4,189 1,240 170 16 1.0

4,520 1,340 184 17.3 1.08

4,190 12,400 1,700 160 10

300

25 50 100 250 300

7,455 1,021 95 2.3 1.0

8,050 1,100 103 2.48 1.08

74,550 10,210 950 23 10

500

50 100 250 500

11,817 1,100 26 1.0

12,800 1,190 28.1 1.08

118,170 11,000 260 10

750

50 100 250 500 750

95,807 8,919 214 8.1 1.0

105,000 9,630 231 8.75 1.08

958,070 819,190 2,140 81 10

100 250 500 750 1,000

42,658 1,022 39 4.8 1.0

46,100 1,100 42.1 5.18 1.08

426,580 10,220 390 51 10

1000

16

FUNDAMENTALS OF CONTAMINATION CONTROL

TABLE 1.2 Nonvolatile Residue Cleanliness Levels Limit, NVR

CVCM VCM NTU TML 1.4

Level

g/cm

A/100 A/50 A/20 A/10 A/5 A/2 A B C D E F G H J

0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 3.0 4.0 5.0 7.0 10.0 15.0 25.0

2

mg/L 0.1 0.2 0.5 1.0 2.0 5.0 10 20 30 40 50 70 100 150 250

volatile condensable material collected in accordance with ASTM E595 volatile condensable material determined by methods other than ASTM E595 nephelometric turbidity units total mass loss in accordance with ASTM E595

SOURCES OF CONTAMINATION

One classification scheme often found useful for addressing contamination problems is to classify them generically according to source categories. Here we present a useful scheme for classifying sources of contamination. Each source can have a chemical composition or fingerprint that points to it uniquely or to one or two of the many other source categories. Facility The facility of a cleanroom, sometimes referred to as the cleanroom ambient environment, is often an important source category. The facility is often detected as a source during at-rest certification and during surveys using airborne optical particle counters (OPCs). When this occurs, an OPC is used to track the source of contamination to its origin by identifying the highest concentration point along the cleanroom ceiling, walls, and floors. Causes of failures in a cleanroom facility include underpressurization of the room with respect to the factory, failures of seals in the ceiling filter grid, holes in filters, and fan failures. Failures also can be associated with walls, floors, doors and other architectural features if these degrade or become damaged in use. When these types of failures occur, the composition of the contamination usually matches the composition of materials in the factory or outside air and often includes terrestrial dust, factory emissions, automobile exhaust particles, and so on. The cleanroom ambient environment can also be a contributing factor in the failure to adequately control contaminants generated within a cleanroom, due to inadequate or misdirected airflows. In this case, the problem usually must be clarified using flow visualization to locate the improper airflows. The composition of the contamination can be anything

SOURCES OF CONTAMINATION

17

within the cleanroom, so often, composition analysis is less helpful than flow visualization for solving this type of problem. Finally, materials of construction of the facility can contribute. Several examples are useful to illustrate these types of problems: ●

●

●

●

The cleaning chemicals used in facilities maintenance may leave behind residues that become airborne. Here the composition matches that of the cleaning chemicals. The cleaning chemicals attack surfaces within the cleanroom, causing them gradually to break down. Paint discoloration, chalking, and flaking are common examples. Here the composition of the contaminant often better matches the materials of the walls and floors than it does the chemicals causing the deteriorization. Materials within the cleanroom, such as those used for walls and doors, become physically damaged. Calcium sulfate is a common facility contaminant in cleanrooms built with gypsum walls. Gel-type ceiling grid systems and other materials in a cleanroom can outgas, contributing to airborne molecular contamination. This form of contamination is often discovered when an older facility, where the material was qualified without regard to airborne molecular contamination, is used to manufacture a new product that is sensitive to airborne molecular contamination.

Tooling Tooling within a cleanroom can be a second important source category. Tooling makes use of electromechanical and pneumatic (or hydraulic) devices that can be inherent sources of contamination. In addition, tooling is subject to wear and can gradually change in contamination performance. Improper maintenance is often a source of gradual or sudden deterioration in contamination performance. Contamination from tooling generally will have the composition of the materials and surface treatments of the tooling components, especially those subject to wear. This can be helpful in distinguishing the source of contamination, as the sources are often unique to the tool. Lubricants on tooling are also a significant source. Piece Parts Piece parts used in assembly operations in industries, such as disk drives, aerospace, flat-panel display, and medical devices, are often found to be a source of contamination. In essence, the piece parts of a product are contaminating the product itself. These contaminants often are the result of inadequate supplier or in-house cleaning. They can also be generated during the assembly or manufacturing process in a cleanroom, which is often referred to as in situ contamination. The composition of the contaminant will be identical to that of the piece parts, or if the results of generation by wear against a tool, may contain both piece part and tooling materials. Piece parts can also contribute molecular contamination. Where molecular contamination is a known concern, piece parts will be scrutinized for outgassing during the qualification process. However, changes in raw materials, inadvertent contamination during shipping and handling, and other factors can alter the outgassing performance of materials otherwise qualified. Consumable Supplies Consumable supplies include such items as wipers, swabs, gloves, packaging materials, labels, tape, and paper products. As with piece parts, the contamination characteristics of consumables are subject to scrutiny in the qualification process. In the majority of cases where consumable supplies contribute to contamination, it is the result of natural variations in their manufacturing process (lot-to-lot variability) or the

18

FUNDAMENTALS OF CONTAMINATION CONTROL

result of misuse and abuse. The composition of the contaminant will be identical to that of the consumable, or if a result of generation by wear against a tool, may contain both consumable and tooling materials. Chemicals Chemicals and their distribution systems are now understood to be a potentially significant source of contamination, often either from the chemical as formulated or as a result of contact with the distribution system. As with other source categories, the relative contribution of chemicals to overall contamination is a function of the process. Processes that make intense use of chemicals (including water) can have a relatively high proportion of their contaminants from chemicals (e.g., manufacturing of semiconductors flat panel displays, and thin-film disks). Conversely, processes that use relatively few chemicals in small quantities may see only a trivial contribution from chemicals (e.g., assembly of disk drives). People People are a notorious source of contamination. The amount of contamination contributed by people is often highly variable, due to differences among individual attire, discipline, and activity. The types of contamination contributed by people are also highly variable. These include skin and hair, which can often be identified through a combination of their physical appearance and chemical analysis. People also contribute fibers from their street clothes. Again, these are often identified through their physical appearance and chemistry. People also are carriers of terrestrial dust, food particles, particles of cosmetics, and other contaminants. People can also be a significant source of viable contamination: an important consideration in aerospace, pharmaceutical and medical device industries.

1.5

CONTAMINATION CONTROL REQUIREMENTS

The single most important step in the design of a contamination control program is the definition of what contamination requirements apply. The selection of what to control and what limit to impose are not arbitrary choices. They are dictated by the needs of the product and the process. The contamination engineer nearly always depends on the product design and the production engineering functions to define these requirements. Requirements may also be dictated by the manufacturer of the process equipment. In this section we describe some of the more important factors that must be specified. Some standards describe various control limits, such as ISO 14644-1 and FED-STD-209 for airborne particle contamination. However, these specifications do not specify to the user the appropriate control limit for a given situation. There are also standards that specify methodologies for determining performance in contamination control situations, such as ASTM 595 for NVR. Again, the standard analytical methods do not tell the user what the appropriate control limit should be. Determining the appropriate control limit remains the user’s responsibility. In our discussion of standards and methods, ways of determining appropriate control limits are offered. Where experience allows, appropriate starting points for control limits are suggested. 1.5.1

Airborne Particle Requirements

Airborne particle contamination limits in cleanrooms have traditionally been described by U.S. Federal Standard 209, which defines the term class. In FED-STD-209, the class of the

CONTAMINATION CONTROL REQUIREMENTS

19

cleanroom is equal to the limit of the number of particles per cubic foot of air equal to or larger than 0.5 m in diameter. The E revision of FED-STD-209 included metric units and became the basis for ISO 14644-1. More recently, ISO 14644 has undergone further changes in nomenclature and has added the term class. The ISO definition has lost some of the recognizable characteristics of the old FED-STD-209 term, where the number corresponded to the number of particles 0.5 m and larger per cubic foot of air. The current ISO 14644 definition of class 8 corresponds to FED-STD-209 class 100,000; ISO 14644 class 5 corresponds to FED-STD-209 class 100; and so on. In general, airborne particle concentrations must not exceed the requirement of the product or process. The cleanliness requirement may be specified as class 10 (ISO class 4), class 100 (ISO class 5), and so on. To understand the airborne particle contamination requirement, it is important to understand the language and meaning of airborne particle cleanliness. In addition, it is important to understand how particle counts are measured and how the particle count limit is gradually approached. Thus, three questions will be answered: ●

● ●

What is the meaning of cleanroom class limits in terms of both FED-STD-209 class and ISO 14644 (metric) units? How is cleanroom performance measured? What is the process of staged certification?

ISO 14644-1 and FED-STD-209E: Classification of Air Cleanliness In the original version of FED-STD-209, the class of a cleanroom equals the number of particles 0.5 m in diameter and larger per cubic foot of air, as measured using airborne optical particle counters. Thus, a class 100 cleanroom was expected to contain no more than 100 particles 0.5 m and larger per cubic foot. Fewer particles are expected at larger sizes, and more particles are expected at smaller sizes. This original definition was based on observation of airborne particle size concentrations measured using airborne optical particle counters, which at the time the original standard was written, could resolve particles no smaller than 0.5 m. Particles 5 m in diameter and larger were also measured. Straight lines were drawn between the 0.5- and 5.0-m concentrations, resulting in particle size distributions. The E revision of FED-STD-209 and ISO 14644-1 defined additional particle sizes. Concentrations of particles are specified at 0.1-, 0.2-, 0.3-, 0.5-, 1.0- and 5.0-m sizes, depending on the class of cleanroom. This modernization included particles smaller than 0.5 m, in recognition that the modern class 10, class 1, and cleaner cleanrooms required excessive sample times to obtain statistically meaningful particle counts at 0.5 and 5.0 m. Particles smaller than 0.1 m in diameter were defined as the ultrafine fraction. Provision for cleanroom classification of ultrafine particles was made using U designators. Cleanliness class designations and quantity have changed from FED-STD-209E (see Tables 1.3 to 1.5). Along with the obvious change to a metric measure of sampled air volume, ISO 14644-1 adds three additional cleanroom classes: 1, 2, and 9. Note that in the ISO 14644 classification, the class is equal to the power-of-10 number of particles equal to or larger than 0.1 m in diameter per cubic meter of air. Estimating the Class of Cleanroom Needed Predicting the class of cleanroom needed for any given situation was largely a matter of guesswork in the past, but unnecessarily so. The relationship between the surface contamination rate and the cleanroom class was determined using computation fluid dynamic models and verified experimentally in

20

FUNDAMENTALS OF CONTAMINATION CONTROL

TABLE 1.3 ISO 14644 Airborne Particulate Cleanliness Classes Number of Particles per Cubic Meter Size: 0.1 m

0.2 m

0.3 m

0.5 m

1 m

5 m

10 100 1,000 10,000 100,000 1,000,000

2 24 237 2,370 23,700 237,000

10 102 1,020 10,200 102,000

4 35 352 3,520 35,200 352,000 3,520,000 35,200,000

8 83 832 8,320 83,200 832,000 8,320,000

29 293 2,930 29,300 293,000

ISO Class 1 2 3 4 5 6 7 8 9

TABLE 1.4 Comparing FED-STD-209E to ISO 14644-1 ISO 14644-1

FED-STD-209E

1 2 3 4 5 6 7 8

1 (M1.5) 10 (M2.5) 100 (M3.5) 1,000 (M4.5) 10,000 (M5.5) 100,000 (M6.5)

TABLE 1.5 Class Limits in Particles of Size Equal to or Larger Than the Size Limit per Cubic Foot of Aira Class Standard

Measured Particle Size

FED-STD-209E

ISO 14644-1

0.1 m

1 (M1.5) 10 (M2.5) 100 (M2.5) 1,000 (M6.5) 10,000 (M7.5) 100,000 (M8.5)

3 4 5 6 7 8

35 350

0.2 m 7.5 75 750

0.3 m

0.5 m

5.0 m

3 30 300

1 10 100 1,000 10,000 100,000

7 70 700

a

Class limit particle concentrations are defined for class purposes only and do not necessarily reflect the size distribution of airborne particles to be found in any particular situation.

the late 1980s for unidirectional-flow cleanrooms. To estimate the class of cleanroom required, the following information is needed: ● ●

Critical particle size, the size below which no defects are produced. Exposure time of the surface to the cleanroom environment.

CONTAMINATION CONTROL REQUIREMENTS ●

●

●

21

Average charge level on the surfaces. This can be estimated from historical data, where air ionization is not employed, to be in the range 1000 to 2000 V. Conversely, if ionization is used, the average surface charge level will be less than the float potential to which the ionizers are adjusted and maintained. As shown in Chapter 2, the average charge level on surfaces in cleanroom equipped with air ionization systems will depend on the allowable float potential specification used for control of the ionization system. Percent of the exposed area of the surface that is vulnerable to contamination. For example, in a photolithography step, no more than 5% of the surface may be likely to produce a defect if a particle lands on that area. Yield desired at any given step in the process.

There is a simple algebraic method for determining the class of a cleanroom needed to support any process step. This simple model is illustrated using airborne particles in the cleanroom as an example. The process can easily be extended to airborne molecular contamination. The overarching factor for determining the class of a cleanroom is the yield desired at the process step. For this example it is assumed that a particle that is larger than the critical particle size landing on a critical area of a product will cause a defect. Thus, the surface contamination rate will be the most important factor driving the class of cleanroom to be used for a given process. The surface contamination rate in any class of cleanroom is highly influenced by the charge level on particles and surfaces in the room. The charge level on particles and surfaces will vary depending on whether or not air ionizers are used. Table 1.6 illustrates this phenomenon. Table 1.6 assumes that the cleanroom conforms exactly to the size distribution specification of ISO 14644 class 5 or FED-STD-209 class 100. Since this is a design problem intended to select the class of cleanroom, the actual size distribution in the cleanroom affecting the process is immaterial. Moreover, in the size range 0.1 to 1.0 m, the standards have been shown to be reasonably good predictors of size distributions in operational cleanrooms. The table applies specifically to FED-STD-209 class 100 (ISO 14644 class 5) unidirectionalflow cleanrooms. It can be extrapolated to other class unidirectional-flow cleanrooms by

TABLE 1.6 Cumulative Surface Contamination Rates, SCR5, for FED-STD-209 Class 100 (ISO 14644 Class 5) Vertical Unidirectional-Flow Environments with and Without Air Ionization Surface Contamination Rate (particles/cm2  h)

Particle Size (m) 0.1 0.2 0.3 0.5 1 3 Source: Ref. 7.

With Air Ionization (100-V Float Potential)

Without Air Ionization (1000- to 2000-V/in. Surface Charges Measured)

10.1 1.03 0.31 0.11 0.049 0.028

88–176 8.5–19 2.4–4.9 0.6–1.2 0.12–0.25 0.03–0.06

22

FUNDAMENTALS OF CONTAMINATION CONTROL

adjusting for the order of magnitude of the class. For example, a FED-STD-209 class 10 (ISO 14644 class 4) unidirectional-flow cleanroom will have an order-of-magnitude lower surface contamination rate than that shown in Table 1.6. For this discussion we refer to this order-of-magnitude adjustment factor as M. For mixed-flow cleanrooms, the surface contamination rate can be adjusted based on the residence time of airborne particles in the cleanroom. The adjustment factor, R, shown in Table 1.7 is based on the fact that particles (and airborne molecular contamination) remain suspended within the cleanroom for a longer time, simply on the basis of an ideal stirred tank model. The first step is to determine the critical particle size, that size above which a defect will be produced. Assume for this example that the critical particle size is 0.1 m. The second step is to determine the percent of the area of the part that is vulnerable to contamination in the process, the percent vulnerable active area, V. For many photolithography processes this is only about 5% of the area of the part. Next, the assumption is made that the landing site of the contamination is random and uniform. Based on this 5% vulnerable area, only 5% of the particles equal to or larger than the critical particle size will land in an area that leads to a defect. This term may be thought of as the probability of failures per particle, % fail./particle. (In many processes, 100% of the product surface is vulnerable to producing a defect.) For this exercise, assume that 5% of the surface area is vulnerable. Therefore, in this example V  0.05% fail./particle. The third step is to predict the surface contamination rate for the class of cleanroom. This can be done by multiplying the baseline surface contamination rate in Table 1.6, SCR5, by the class adjustment factor, M, and by the residence time factor, R, given in Table 1.7. For this example, let us determine if a FED-STD-209 class 10,000 cleanroom with air ionization is suitable for a given process step. The baseline surface contamination rate, SCR5, must be multiplied by M  100, times R  10. The surface contamination rate in class 10,000, SCR7, is predicted to be approximately 1000 SCR5. Thus, SCR7  1000  10 ⯝ 10,000 particles/cm2  h. The last step is to determine the amount of time, T, in hours that the product will be exposed to the cleanroom environment. In this example, assume that T  0.5 h. Now let us suppose that the surface area, A, of the part undergoing the process is 10.0 cm2. (This might be a 10-cm2 LCD display chip on a 6-in.-diameter wafer.) Then, to a first approximation, we can predict that the failure rate is the product of the surface contamination

TABLE 1.7 Relationship Among Airborne Particle Count Class, Volume Air Exchange Rate, and Average Residence Time, Assuming That the Cleanroom Behaves as an Ideal Stirred Reactor

FED-STD-209 Class (ISO 14644 Class) 100,000 ( M8)

Typical Volume Exchange Rate V/h

20

10,000 (M7)

20–60

1,000 (M6)

60–200

100 ( M5)

200–600

V/min

–13 1 –3 –1 1–3 –13 3 –13 –10

Average Residence Time (min)

Multiplication Factor vs. Unidirectional Flow, R

3

10

3–1

10

1– –13

3

1 –13 – — 10

1

CONTAMINATION CONTROL REQUIREMENTS

23

of the critical particle size times the area per part times the time of exposure times the percent vulnerable area of the part. Thus, for our class 10,000 cleanroom, % fail./particle  SCR 7 (particles/cm2 ⋅ h)  T (h)  A (cm2 /particle)  V (% fail./particle)  10,000 particles/cm 2 ⋅ h  0.5 h  10.0 cm 2 /particle  0.05% fail./particle

or 2500 failure sites per particle. If there is no redundancy on the part (typical for an LCD display chip), all will fail and the class 10,000 cleanroom is not acceptable. If we move the process to a FED-STD-209 class 100 unidirectional-flow cleanroom, the calculation changes: % fail./particle  SCR 5 (particles/cm 2 ⋅ h)  T (h)  A (cm2 /particle)  V (% fail./particle)  10 particles/cm 2 ⋅ h  0.5 h  10.0 cm 2 /particle  0.05% fail./particle

or 2.5 failure sites per LCD display chip. If we move the process to a FED-STD-209 class 10 unidirectional-flow cleanroom, the calculation changes again: % fail./particle  SCR 4 (particles/cm2 ⋅ h)  T (h)  A (cm2 /particle)  V (% fail./particle)  1.0 particles/cm 2 ⋅ h  0.5 h  10.0 cm2 /particle  0.05% fail./particle

or 0.25 failure site per LCD display chip. At this point we would predict that 25% of the chips will fail in a FED-STD-209 class 10 cleanroom. Suppose that we want the yield at this process step to be 95%. To achieve this, the failure rate must be reduced to 5%. One way to accomplish this would be to reduce the exposure time from 30 minutes to 6 minutes. How Is Cleanroom Performance Measured? This document determines the type and frequency of testing required to conform to standard ISO 14644-1. Several mandatory and optional tests are specified with recommended test intervals. Table 1.8 lists the tests considered critical for demonstration of cleanroom class requirements. The particle count is the most essential test, as the results determine conformance with the airborne particle class requirement of the cleanroom. The air pressure difference and airflow are important to keep in control, primarily as an end to achieving the airborne particle count requirement. This is not to say that they are unimportant on their own. They are critical to maintaining the overall cleanliness of the cleanroom. However, they do not measure the conformance of the cleanroom with particle count limits. They are a means to achieving this end. The frequency of each of these tests: particle count, air pressure difference, and airflow, should be adjusted to reflect the risk of failure for any given facility. Testing the airborne particle count once every six months for an ISO 14644 class 5 facility (a FED-STD-209 class 100 facility) is probably inadequate for biosafe cleanrooms. Indeed, the testing frequency should be based on the allowable airborne cleanroom concentration contribution to surface contamination. Similarly, air pressure differences and airflow should be tested more frequently than suggested in Table 1.8. At the very least, each time a facility maintenance operation is performed that may affect room pressurization and airflow, the pressure and airflow should be verified before returning the room to full use.

24

FUNDAMENTALS OF CONTAMINATION CONTROL

TABLE 1.8 Strategic Testing: Schedule of Tests to Demonstrate Continuing Compliance Test Parameter Particle count Air pressure difference Airflow

TABLE 1.9

ISO 14644-1 Class

Maximum Interval (months)

ISO 14644 Procedure

5 (209 class 100) 5 All All

6 12 12 12

Annex A Annex B5 Annex B4

Strategic Testing: Schedule of Additional Optional Tests from ISO 14644-3

Test Parameter Installed filter leakage Containment leakage Recovery Airflow visualization

Class

Maximum Time Interval (months)

ISO 14644-3 Test Procedure

All All All All

24 24 24 24

Annex B6 Annex B4 Annex B13 Annex B7

One major consideration for the three parameters (particle count, pressurization, and airflow) is the availability and affordability of continuous monitoring systems. Today, these systems are so affordable that they compete favorably with the cost of annual or semiannual room certification, because the automatic data logging and reporting eliminates the time and effort of preparing reports to document manual surveys. One important feature that makes a cleanroom work is that it is kept at a positive pressure with respect to its surrounding environment. This can be measured using relatively simple gages, such as inclined tube manometers or diaphragm pressure gages. In many cases, these are installed in the rooms permanently. Pressure differences can be maintained within an overall cleanroom as well. For example, if a support enclosure containing electronic, pneumatic, or other equipment is maintained at a negative pressure with respect to the surrounding cleanroom, the requirements for contamination control can generally be waived for its contents. In addition, change rooms and large equipment pass-through enclosures are often maintained at a pressure intermediate between that of the general cleanroom and that of the surrounding environment. Small pass-through boxes are not generally treated like large equipment pass-through devices and are not generally subject to pressure difference monitoring. Additional optional tests are listed in Table 1.9 and discussed below. Installed Filter Leakage Test The installed filter leakage test is generally considered mandatory for biosafe cleanrooms but optional for other high-technology room types. In this test an aerosol may be injected into the recirculation path ductwork or plenum, and particles escaping through pinholes or leaks in seals are sought using an airborne optical particle counter in the cleanroom. In an alternative technique, where HEPA filters are used to clean makeup air brought into the cleanroom for room pressurization, the makeup air filters are temporarily bypassed. This technique uses the concentration of particles in the ambient air as the filter leakage challenge. It has the advantage that a particle generator is not required. It has several potential disadvantages, among which are ambiguity about the magnitude of the challenge and the possibility of contaminating the intervening ductwork.

CONTAMINATION CONTROL REQUIREMENTS

25

Airflow Visualization Airflow visualization tests are used to demonstrate airflow characteristics in a cleanroom. Techniques recommended are by dangling thread or tracer injection. The ISO recommendation is for the flow visualization to be repeated every 24 months. Experience has shown a better approach. The as-built room should be flow visualized. During tool installation, airflow should be visualized with each major installation. Installation of tools often involves making penetrations through walls and floors. If these penetrations are not sealed adequately, they become leakage paths that spoil airflow. These undesirable flows can be found most efficiently using an injected tracer. Of the injected tracer methods, the safest is to fog the room using a cleanroom safe source of fog. Many foggers are sources of contamination. Methods of generating fogs are described in Chapter 3. Recovery Time Test These are tests to evaluate the capability of the airflow in the cleanroom to recover from a particle-generating event within the cleanroom. They are applied principally to non-unidirectional-flow cleanrooms. Theoretically, the average residence time in any cleanroom can be modeled as an ideal stirred tank reactor, where the average residence time, T, is equal to the volume of the cleanroom, V, divided by the volume airflow exchange rate, Q, as a first-order approximation: T

V Q

V/Q can be thought of as the volume air exchange rate of the airflow in the room, in units of volume per unit time. It is convenient to roughly estimate the class of cleanroom using a given volume air exchange rate. The average residence time is the inverse of the volume air exchange rate, as shown in Table 1.10. These predicted average contamination residence times will be affected by the specific layout of filters, return vents, and flow obstructions within the room: hence the need to perform recovery time measurements. These estimates do provide a reasonable starting point for estimating expected recovery times based on some simple room design parameters. Recovery time tests are best performed at critical product locations. Significant deviation from predicted recovery times is often a sign of local airflow problems. For this reason airflow visualization is often a more valuable than recovery time tests.

TABLE 1.10 Relationship Among Airborne Particle Count Class, Volume Air Exchange Rate, and Average Residence Time, Assuming That the Cleanroom Behaves as an Ideal Stirred Reactor FED-STD-209 Classa (ISO 14644 Class)

Typical Volume Exchange Rate V/h

100,000 (8)

20

10,000 (7)

20–60

1,000 (6)

60–200

100 ( 5)

200–600

a

Class limits are shown graphically in Figure 1.9.

V/min

–13 1 –3 –1 1–3 –13 1 3 –3 –10

Average Residence Time (min) 3 3–1 1– –13 1 1 –3 – — 10

26

FUNDAMENTALS OF CONTAMINATION CONTROL

100,000

s1 as Cl 00

10,000

0 ,0 00 0, 0

00 0 s1 as Cl

100

00 ,0 s1 as Cl

s1 as Cl

Particles per cubic foot

0 s1 as Cl

1,000

Cl s1

as

10

1

0.1 0.01

0.1

1

10

Particle Size (µm)

FIGURE 1.9 FED-STD-209 class limits in particles per cubic foot of size the particle size shown. Note: Class limit particle concentrations are defined for class purposes only and do not necessarily reflect the size distribution of airborne particles to be found in any particular situation.

Other Tests Ultrafine Particle Count (for Characterization) Airborne testing for particles with size distribution thresholds below 0.1 m for characterization of a cleanroom is done using a condensation nucleus counter (CNC). Conventional CNCs include particles larger than 0.1 m in their cumulative counts. The lower size limit for most CNCs is in the range 0.02 to 0.05 m. The ultrafine particle count test is included where the critical particle size is less than 0.1 m in diameter. Conventional optical particle counters generally do not detect particles below 0.1 m in diameter. Macro Particle Count (for Characterization) These are tests for particles with size distribution thresholds above 5 m for characterization of the cleanroom. They generally make use of settling plates, such as a bare silicon wafer for the semiconductor industry. Any object can take the place of the silicon wafer. For example, in the aerospace industry, glass plates are used as a surrogate to determine the percent obscuration of solar panels for satellites. In the disk drive industry, where a liquid-borne particle count is used to control particle cleanliness of piece parts, it can also be used to characterize the accumulation of contaminants on surrogate parts used as settling plates.

CONTAMINATION CONTROL REQUIREMENTS

27

Log of Concentration

Particle distribution predicted by extrapolation of standards

Measured size distributions

0.1 1.0 Log of Particle Size (μm)

FIGURE 1.10 Comparison of actual particle size distributions with the particle size distribution predicted in ISO 14644 or FED-STD-209.

The macro particle count test is included where large particles are a concern. Conventional particle counters are not efficient samplers for particles larger than 5 to 10 m in diameter, generally due to line losses in the sampling apparatus or internal losses within the particle counter. Thus, it is necessary to utilize indirect sampling using settling plates or surrogate parts for macro particle characterizations. The model on which the airborne particle concentration limit is based is a log-log distribution. We need to look at how actual size distributions measured in cleanrooms compare to the model. In addition, we are increasingly concerned with particles smaller than 0.1 m and whether we can extend to below 0.1 m by simple extrapolation of the model. Research has shown that in the size range 0.1 to 1 m, the log-log distribution is a reasonable fit, especially for cleanrooms sampled under stage 1 operating conditions. However, the model size distribution tends to underestimate particle concentrations larger than 1 m, especially for stage 2 and 3 operation of cleanrooms. The situation for particles smaller than 0.1 m is the opposite. Far fewer particles smaller than 0.1 m are measured than would be predicted by simple extrapolation of the concentration limit model. This is illustrated in Figure 1.10 [11]. Actual airborne particle size distributions in cleanrooms can be partially understood by looking at a general model for aerosols and the underlying mechanisms controlling particle generation and behavior. The general model is a trimodal size distribution, shown in Figure 1.11. The smallest particle mode, typically consisting of particles below 0.1 m in diameter, is referred to as the ultrafine fraction in contamination literature and as the condensation mode in aerosol literature. It is also sometimes referred to as the Aitken nuclei mode. These particles are generally produced by homogeneous nucleation, sometimes called gas-to-particle conversion, combustion, or evaporation processes. Their atmospheric lifetimes are very short, due to their high diffusion and electrical mobilities. Particles tend to agglomerate and

28

FUNDAMENTALS OF CONTAMINATION CONTROL

Condensation or Aitken Nuclei Mode

0.01

FIGURE 1.11

Supermicron Mode

Accumulation Mode

0.1

1

10

100

Generalized atmospheric trimodal size distribution.

grow in size, so they are removed from this mode very rapidly. The agglomeration rate is proportional to the square of the capturing particle diameter, so matter from the condensation nuclei mode preferentially tends to combine with the larger particles in the accumulation mode. Processes in cleanrooms that tend to produce particles in the condensation mode size range include evaporation and condensation of volatile matter from heated surfaces and gas-to-particle conversion of reactive gases such as acids and bases. Another mechanism is chemical decomposition of gases in the corona surrounding electrically charged objects, especially the emitter needles of corona discharge air ionizers. Sputtering also produces particles in this size range; sputtering of emitter needles on corona discharge air ionizers is an important source for cleanrooms. Open flames are an extremely important source of particles in this size range. Generally, open flames are contained within exhausted enclosures and are rarely an important source of condensation nuclei in cleanrooms. The accumulation mode particles are those in the size range from about 0.1 to 1 or 2 m. Removal forces of diffusion, electrostatic attraction, impaction, and gravitational sedimentation are all at a minimum for particles in this size range, so particles in this range have the longest residence time in the atmosphere. Particles in this size range tend to dominate particle counts in the atmosphere, including the air in cleanrooms. Particles in the accumulation mode size range are produced by a number of mechanisms. One of these is the agglomeration of matter from the condensation nuclei mode. Some wear processes will produce large numbers of submicrometer particles. Evaporation of sprayed liquids is also an important mechanism. The largest particles, typically those larger than 1 or 2 m in diameter are usually referred to as the supermicron mode. These particles usually are produced by wear mechanisms. They are removed by the processes of impaction and gravitational sedimentation. Supermicron aerosols are almost always generated within the cleanroom itself. Despite the fact that they quickly settle onto surfaces, they can easily become resuspended. In general, the number of particles in the supermicron fraction of air samples is relatively small compared to the number of particles in the accumulation mode. Their contribution to contamination problems must not be underestimated based on their relatively small abundance. For example, the contamination effect of the accumulation mode and supermicron mode in the aerospace industry is dominant because of obscuration of optics, solar collectors, and the increase in adsorption of heat by the surfaces of spacecraft [12]. Here the contamination

CONTAMINATION CONTROL REQUIREMENTS

29

TABLE 1.11 Properties of Airborne Particles in the Size Range of Interest for Cleanrooms Electrical Particle Size (m)

Diffusion Mobility (cm2/V  s)

Settling Coefficient (cm2/s)

Surface Area Velocity (cm/s)

Volume Factor (m2)

Factor (m3)

0.001 0.005 0.01 0.05 0.1 0.5 1 5 10 50 100

2 8  102 2  102 9  104 2.6  104 2.5  105 1.1  105 2  106 9  106 1.8  107 9  108

5  102 2  103 5  104 2.5  105 7  106 6  107 2.8  107 5  108 2.5  108 4.8  109 2  109

7  107 3.5  106 7  106 3.8  105 8.8  103 1  103 3.5  103 7.5  102 0.3 7.5 25

1  106 2.5  105 1  104 2.5  103 1  103 2.5  101 1 2.5  101 1  102 2.5  102 1  104

1  109 1.25  107 1  106 1.25  104 1  106 1.25  101 1 1.25  102 1  103 1.25  105 1  106

TABLE 1.12 Assumptions Used to Calculate the Aerosol Properties in Table 1.11 Property

Symbol

Unit

Air viscosity Air density Particle density Gravitational acceleration Boltzmann’s constant

a p g K

183  106 poise 1.205  103 g/cm3 1 g/cm3 981 cm/s2 1.38  1016 erg/K

Elementary unit of charge

E

Mean free path



4.8  1010 electrostatic unit (esu) 1.6  1019 C 0.653  105 cm

Conversions 1 poise  1 g/cm  s

1 erg  1 dyn  cm (1 dyne  1 g  cm/s2)

contribution is proportional to the projected area of the particles (the square of the particle diameter). In almost all industries, the mass of the particles is more important than the number of particles (e.g., as in poisoning of liquid chemical baths). Mass is proportional to the cube of the particle diameter, making the supermicron fraction an important source once again. Aerosol Properties Several of the most important properties of particles are illustrated in Tables 1.11 and 1.12. Electrical mobility, diffusion coefficient, and settling velocity are important properties describing particle behavior. The area and volume factors are important when considering the effects of contamination that are proportional to area, such as obscuration of optics, or to mass (volume), such as chemical reactions. One priority not shown in Table 1.11 is thermal mobility, also called therophoresis. This is a property of particles in which they are attracted to cold surfaces and repelled by hot surfaces. Thermophoretic velocity is independent of particle size [13]. One obvious application of this property of aerosols is to protect surfaces from particle contamination by keeping them warm with respect to the ambient environment. In addition, a cold surface placed at critical locations in the cleanroom can act as both a particle getter and an AMC getter. The assumption on which the tables are based is that the particles are in air at standard temperature and pressure, 20°C and 1 atm. It is also assumed that the particles are unit density spheres. Values of constants

30

FUNDAMENTALS OF CONTAMINATION CONTROL

used in the calculation of particle dynamic properties are given in Table 1.12. The following equations were used in calculating the particle properties: 1. Electrical mobility, Z (cm2/V  s): Z

neC 3 Dp

where n  1 (assuming singly charged particles), Dp is the particle diameter, and C is the Cunningham slip correction factor. The dimensionless Cunningham slip correction factor is C  1  1.246

Dp ⎞ ⎛ 2 2  0.42 exp ⎜ 0.87 2 ⎟⎠ Dp Dp ⎝

To obtain the electrical mobility units, cm2/V  s, it is necessary to use the conversion 1 statvolt  300 V. 2. Diffusion coefficient, D (cm2/s): D

kTC 3 Dp

where T is the absolute temperature in kelvin. 3. Settling velocity, V (cm/s): V

g p Dp2C 18

Staged Certification Clearly, a technique is needed to allow for an orderly approach to the class limit of a cleanroom. The technique used is called staged certification. The three most commonly encountered stages are (1) after the cleanroom is built, (2) after it is populated with functioning tooling, and (3) when it is placed in full-production operation. There is also a fourth defined stage of cleanroom certification, the energy conservation stage, seldom seen in high-volume manufacturing cleanrooms but seen occasionally in development cleanrooms, where control of cost can be important. The four stages are defined as follows: ● ● ● ●

Stage 1: Stage 2: Stage 3: Stage 4:

after construction; often called the cleanroom “as built” after equipment installation; often called the cleanroom “at rest” during full operation during energy conservation operation

Stage 1 fits the following description: ● ●

Cleanroom construction is complete. The blowers for room HEPA or ULPA filters are operating.

CONTAMINATION CONTROL REQUIREMENTS ● ●

31

Wipe-down is finished. All construction equipment and personnel are gone.

Stage 1 certification criteria are typically 20 to 25% of the room’s specified class. The amount of contamination that can be contributed in the remaining stages of certification is dependent on the amount of contamination in the cleanroom at rest. In a well-designed cleanroom or unidirectional-flow clean bench, the particle count during stage 1 certification seldom exceeds 1 to 5% of the specified class. Stage 2 is the condition after tooling has been installed and made functional. This is the appropriate stage for certification of contamination from tooling and workstations. Stage 2 fits the following description: ● ●

●

● ●

The cleanroom is operating. All tooling, workstations, and material handling and peripheral equipment have been installed. All tooling is fully functional and operating as intended for normal manufacturing operations. Tooling and workstation wipe-down has been completed. Manufacturing personnel are absent.

It is important to make certain that the tooling is operating. That is, all of the utilities (e.g., power, compressed air, contamination evacuation systems) must be functional. In addition, the tools must be running (i.e., ovens turned on, cure lamps operating, robots moving, etc.) to assure that all potential sources of contamination from the tooling are present as they will be when the cleanroom is later certified in the fully operational stage. Stage 2 certification criteria are usually no more than 50% of those of the cleanroom class specified. This allows for the remaining 50% of the class’s airborne particle count to be allocated to the people. There is an exception: If safety or flow control barriers effectively protect portions of a tool from contamination from people, the protected portions of the tool may be certified at 100% of the class limit. This can be illustrated using a hypothetical tunnel oven. In Figure 1.12 the interior of the tunnel oven is at a positive pressure with respect to the cleanroom. The portion of the oven between the input and the output of product in the oven is shrouded so that no one can come in contact with the hot interior surfaces of the oven

Oven Heat Shield Positively Pressurized with Respect to Cleanroom, Stage 3 Applies Input, Stage 2 Applies

Output, Stage 2 Applies

Conveyor Section

FIGURE 1.12

Tunnel oven, illustrating the use of stage 2 and 3 certification criteria.

32

FUNDAMENTALS OF CONTAMINATION CONTROL

(safety enclosure). The enclosure effectively isolates the interior of the oven from contamination generated in the cleanroom, because it is positively pressurized with respect to the cleanroom. The interior of the oven can thus be certified at 100% of the class limit for the process. The input and output portions of the oven are exposed to the cleanroom and thus are exposed to contaminants generated in the cleanroom. Since they are exposed to contamination from people working in nearby areas of the cleanroom, they must be certified at 50% of the class limit. Stage 2 certification sampling is preferably done at critical and busy product locations in and around the tool. These are areas where the probability of product contamination is at the maximum. The selection of sample points is discussed in detail in Chapter 7. Stage 3 limits are applied to a fully operational cleanroom. Stage 3 fits the following description: ● ●

●

● ● ● ●

The cleanroom is operating. All tooling, workstations, and material handling and peripheral equipment have been installed. All tooling is fully functional and operating as intended for normal manufacturing operations. Workstation wipe-down has been completed. All piece parts, containers, and indirect materials are present. All manufacturing personnel are present. Normal work activity is taking place.

Stage 3 certification criteria are 100% of those of the cleanroom class. It is recommended that during monitoring of ongoing operations, a warning alarm level be established at 70 to 85% of the room class, since it is inevitable that contamination levels will increase to some extent once regular operation begins. Recall that stage 3 sampling criteria may be applied to the certification of tools that have flow enclosures or physical safety barriers, or where another provision is made that effectively eliminates contributions from personnel and stops production when open to personnel. Thus, for tools that will be physically isolated from operating personnel, stage 3 control limits may be applied during the tool certification phase. Once again, a safety barrier consisting of clear plastic panels that isolate a tool from contamination by an operator may allow the tool to be certified at 100% of the class limit. A light curtain that provides a safety barrier but permits airflow into a tool does not satisfy this requirement. Stage 4 is not often encountered in high-volume manufacturing cleanrooms, as there are seldom periods in which these rooms are not in use. When a cleanroom is to be unoccupied for an extended period of time, stage 4 may be applied to save cleanroom operating costs. Stage 4 fits the following description: ●

●

●

The cleanroom is operating at reduced fan speed, usually with the lights off and the doors locked. All tooling, workstations, and material handling and peripheral equipment have been installed. All tooling is operating in idle mode. That is, all utilities that can be shut off without endangering the rapid return of the tools to operating status are shut off. This is especially

CONTAMINATION CONTROL REQUIREMENTS

33

TABLE 1.13 Airborne Particle Concentration Limits for Staged Certification of Cleanrooms (particles/ft3 0.5 m)

●

●

FED-STD-209 (ISO 14644) Class

Stage 1

Stage 2

Stage 3

10 (4) 100 (5) 1,000 (6) 10,000 (7)

2–2.5 20–25 200–250 2,000–2,500

5 50 500 5,000

10 100 1,000 10,000

important for portions of the tooling, such as ovens, that tend to produce a strong chimney affect. The thermal plumes from heat sources tend to keep contamination suspended in the air. All product in the room is enclosed within protective packaging or stored in protected locations. No personnel are present.

In stage 4 operation, the fans in the cleanroom are slowed down to conserve energy. The cost of running the fans is proportional to the cube of the linear velocity. Thus, a cleanroom whose linear velocity can be cut in half operates at one-eighth the cost of a room running at full speed. The criterion for stage 4 certification is usually 100% of the cleanroom class. Table 1.13 shows how staged certification is applied to FED-STD-209 class 10 through class 10,000 (ISO 14644 Class 4–7) cleanrooms. The important column for tooling is that under the stage 2 heading. 1.5.2

Chemical Vapor Contamination Control Limits

Airborne contamination can also be in the form of organic and inorganic chemical vapors, which are not detectable by airborne optical particle counters. Contamination in vapor form is often referred to as molecular contamination. When it is in the air of the cleanroom, it is referred to as airborne molecular contamination (AMC). Airborne molecular contaminants can originate in the air brought into the cleanroom by the air-handling system or can originate from materials within the cleanroom. Inorganic AMC originating in the air is generally in the form of acidic or basic vapors. The most common of these are oxides of nitrogen (NOx), oxides of sulfur (SOx), and process chemicals such HCl. Airborne basic vapors include ammonia and organic amines. Materials in the cleanroom can be a source of AMC by a process called outgassing, the slow evaporation of volatile substances from within another material. The amount of outgassing that occurs is proportional to the vapor pressure and concentration of the volatile contaminants on the surface of the material. In addition, the diffusivity of the molecular contamination from within the bulk of the material is a factor affecting its concentration in the vapor phase over time. Another important consideration is hydrolysis and other chemical reactions. Reaction products from hydrolysis can have much higher vapor pressure than the reactants, so exposure of materials to moisture, chemicals and elevated temperature can result in the formation of AMC in materials where they did not exist previously.

34

FUNDAMENTALS OF CONTAMINATION CONTROL

Deep ultraviolet photoresist is particularly sensitive to organic and inorganic amines. In some cases, airborne amine vapor concentrations as low as 1 to 2 parts per billion (ppb) may have adverse affects. By way of comparison, typical outdoor airborne concentrations of organic and inorganic amines range from 10 to 50 ppb; ammonia in human breath can exceed 100 ppb for nonsmokers and 1000 ppb for smokers. This duality of origins, from both air and surfaces, makes it necessary to use a combination of approaches for specification and control. AMC originating in air generally is specified in units of contamination per unit volume of air. AMC in the air is usually controlled using filters containing activated charcoal or other absorbents. Conversely, outgassing from materials is usually measured as a percent by unit weight. The molecular contamination content of a material is usually controlled by materials acceptance tests. Not all vapors are considered harmful in all situations. For example, in the disk drive industry, the volatile plasticizer dioctyl phthalate (DOP) is considered very dangerous and its use is not permitted. Thus, one seldom sees vinyl curtains, poly (vinyl chloride) (PVC) gloves, or other plasticized PVC products in use in cleanrooms where disk drives are manufactured. Conversely, in much of the semiconductor industry, DOP can usually be tolerated and is not considered to be a dangerous material, and thus plasticized PVC products are in common use. Inorganic chemical vapors can also be present. In addition to harming product, corrosive chemical vapors may severely shorten the useful life of tools and equipment in their presence. The inorganic materials typically are the common acids, such as hydrochloric or sulfuric acid, or the basic gas, ammonia. These may not initially exist in acidic or basic form within materials, but may be formed as the result of hydrolysis or other chemical reactions that may take place. For this reason, materials are tested for their outgassing properties under conditions that promote and even accelerate hydrolysis or other chemical reactions. Acidic and basic molecular contaminants are especially of concern in semiconductor and other industries where thin-film devices are manufactured, due to the extreme corrosion susceptibility of these structures. Two types of tests are used for the qualification of materials that might outgas: functional tests and objective laboratory tests. These are described in more detail in Chapter 3. A typical outgassing requirement could be as follows: ●

●

There shall be less than a 1% weight loss from the material when heated to 100°C for 24 h at 0.1 torr. The effluent from the material must contain no killer chemicals when analyzed by the method specified. The methods usually are Fourier transform infrared (FTIR) spectroscopy or gas chromatography/mass spectroscopy (GC/MS).

For approving materials, there is some latitude in the interpretation of these requirements. Among the considerations are (1) proximity of exposure, (2) degree of exposure, and (3) identity of the vapor. Proximity of exposure refers to the distance between the product and the material. Materials that are located relatively far away in a unidirectional-flow environment may be allowed a greater percent weight loss because the probability that the outgassing material will be adsorbed by the critical components is small. In this context, proximity is a relative term. An object located 60 cm directly above the product in a vertical unidirectional-flow environment is relatively closer to the product than is an object located 10 cm below or downwind of the product.

CONTAMINATION CONTROL REQUIREMENTS

35

TABLE 1.14 Examples of Known and Suspect Killer Chemicals Material Class

Status and Industries Affected

Typical Source

Low-molecular-weight siloxanes

Known killer chemicals in the disk drive and plasma display industries

Elastomeric silicone tubing, silicone rubber seals, roomtemperature-vulcanizing (RTV) caulking compounds, mold release agents

Organotin compounds

Known killer chemicals in the disk drive industry

Industrial humidifying biocides, gypsum wallboard, latex paint, adhesives (especially acrylates)

Organic amines

Suspect killer chemicals for disk drives; known killer chemicals for deep ultraviolet photolithography

Topical antistatic sprays and coatings, blue and pink polyethylene antistatic trays and bags, mold release agents, detergents

Plasticizers

Suspect drive killers; known killer chemicals in optical coatings

DOP or diethylhexyl phthalate (DEHP) in plasticized PVC

Acid and basic gases

Known to all industries as killer chemicals

Metal pickling solutions, electropolishing solutions, chemical deburring solutions, plating baths, combustion gases

Degree of exposure refers to the time that an object is in proximity to a product. Extremely short exposure times pose less of a risk of product contamination than do long exposure times. Again, a short-duration exposure may be allowed a higher percent weight loss than a material held in contact or close proximity for an extended period of time. The identity of the vapor is also important. Some materials have particularly adverse effects; others are relatively benign. For most applications, water vapor is excluded from the total. An exception occurs in vacuum processing equipment. Water vapor evaporating from a material may increase the time required to pump the vacuum chamber down to its target pressure. This would have an adverse affect on productivity for the vacuum tool, and water vapor would then be included in the weight loss total. For certain industries, there are materials referred to as known killer chemicals, chemicals known to cause defects and that interfere with either yield or reliability. Generally, they are banned from use. Any material containing or producing the killer chemicals usually cannot be qualified for use in a cleanroom. There is also a list of materials that are suspected to be killer chemicals but for which the proof is not definitive, referred to as suspect killer chemicals. These chemicals are avoided but not forbidden. A few examples are given in Table 1.14. 1.5.3

Ionic Contamination Control Limits

Materials that dissolve in liquids and form ions can be ionic contaminants. Ionic contamination can result in corrosion of piece parts, can interfere with the solution chemistry of process baths, and can promote corrosion of tool surfaces. Ionic contamination may originate in air or may be found within and on the surface of materials. Quite often these materials

36

FUNDAMENTALS OF CONTAMINATION CONTROL

absorb moisture from the atmosphere and hydrolyze to form acidic or basic gases. They can then appear as vapor-phase contaminants. In addition, some materials absorb moisture from the atmosphere and dissolve themselves in this moisture. Materials that exhibit this phenomenon are referred to as deliquescent. If these materials also release ions into the moisture, they can greatly accelerate corrosion. The liquid pool often will wet the surface, causing it to ooze out of crevices. This can result in the “bloom” of solid material on the surface after the moisture evaporates, resulting in both ionic and particle contamination, as these evaporation deposits are often easily disturbed, forming aerosols. Possible sources of ionic contamination include: ● ● ● ●

Residual surface treatment chemicals Residues in coatings and polymers Piping and components that contact process liquids Hydrolysis reaction products

Some nonaqueous liquids, much as alcohols, can be surprisingly strong sources of ionic contamination. For example, isopropyl alcohol can easily become contaminated with chloride ions if it is dehydrated (made to be anhydrous or free of water) using calcium chloride, a good drying agent. Several forms of surface treatment are used to improve the chemical resistance or wear resistance of materials. They include: ● ● ● ●

Electropolishing Anodizing Electroless and electroplating Paints and other coatings

Unless properly finished, each surface treatment or coating involves the use of chemicals that can be a source of ionic contamination. The finishing process can involve neutralizing and/or rinsing. The interiors of hollow frame sections are difficult to rinse and often contain ionic contamination unless special procedures are followed to ensure that they do not. Thus, when ordering electropolished or plated metal work surfaces, it is important to anticipate the possible contamination of these parts by chemical residues. It is possible to specify the limit to the amount of ionic contamination on plated or painted surfaces. This requires a test method. Fortunately, simple methods exist. One test for ionic contamination can be run by rinsing the parts in deionized (DI) water. The conductivity of the rinse water is then measured using a conductivity cell. Typically, the resistivity of DI water can be as high as 18.2 M  cm (0.05 S/cm). However, after contact with air, the conductivity of DI water usually drops to between 10 and 1 M  cm. After exposure to ionizable contamination, this will drop even further. The change in conductivity of the DI water exposed to air vs. DI water used to rinse the part can then be used as a measure of the ionic cleanliness of the surface. Less than 0.1 g/cm2 equivalent sodium chloride (NaCl) is considered acceptable in most applications. Here, the conductivity of all ions combined is reported as the conductivity of one easily calibrated ionizable salt (0.1 g/cm2 is approximately equal to 0.1 mg/ft2 and 0.1 mg/0.1 m2).

CONTAMINATION CONTROL REQUIREMENTS

1.5.4

37

Magnetic Contamination Control Limits

Magnetic contamination is especially dangerous to disk drives, magnetic recording tapes, removable magnetic media, and plasma displays. Magnetic contamination needs to be dealt with as a special form of particle contamination. Magnetic contaminants originate almost exclusively within and on the surface of materials, but may be transported as airborne contaminants. Two classes of magnetic contamination can be thought of: soft or low-energy magnetic materials and hard or high-energy magnetic materials. Soft (or low-energy) magnetic materials are those materials in which magnetism may be induced (e.g., iron, steel, certain forms of stainless steel) but which are not generally permanent magnets. They are easily demagnetized. These types of magnetic contaminants generally are a problem for low-coercivity magnetic recording (e.g., floppy disks, magnetic recording tape). Hard (or high-energy) magnetic materials are generally the materials from which permanent magnets are made. Some common high-energy magnetic materials include barium ferrites, samarium cobalt, neodymium ferrites, and so on. Plasma display panels and magnetic recording media may experience performance degradation due to the presence of high-energy magnetic materials. As far as tooling design is concerned, avoid high-energy magnetic materials. If highenergy magnets are unavoidable, as in linear variable displacement transducers, coated rather than bare magnets are used. If uncoated magnets cannot be avoided, they should be encapsulated, preferably within evacuated enclosures. In any case, magnets or components containing magnets should be qualified and inspected periodically to verify that they are magnetically clean. Tools or items that contain magnetic material must be tested for loose magnetic debris (see Section 11.8 for possible test methods) on a regular basis to ensure that contamination control coatings or other measures are not defeated. Degaussing must be part of regular tool maintenance. In severe cases, tools that contact surfaces of parts (e.g., tweezers) must be made of nonmagnetic materials.

1.5.5

Surface Contamination Rates and Air Ionization

The most direct measure of the performance of a cleanroom or tool is to directly measure the level of contamination on their surfaces. This may be done by measuring the product directly, by extracting the product, or by measuring the contamination indirectly, by the use of witness parts (surrogate for real parts). Measurement of the surface contamination level on parts is an especially attractive way of demonstrating tooling conformance to contamination requirements. The part may be inspected directly (e.g., silicon wafers, magnetic recording media), or indirect methods may be used. Where part cleanliness is specified, this method can be used to measure the combined accumulation of contamination by airborne transport, plus contact transfer. In fact, if the part is actually from an assembly operation or is run through the fabrication process (e.g., sputtering chamber, chemical deposition) in situ–generated contamination can also be measured. An example of in situ–generated contamination in a precision assembly factory is debris generated when a fastener is inserted or removed. Table 1.15 shows the relation between the rate at which surfaces become contaminated in a FED-STD-209 class 100 (ISO 14644 class 5) environment as a function of particle size and air ionization. The cumulative surface contamination rate (SCR), in particles/cm2  h, is based on particle collection by a 3-in.-diameter bare silicon wafer held horizontally in a 100 ft/min vertical unidirectional-flow environment. FED-STD-209 class 100 particle distribution is assumed. SCR values for other classes may be estimated by dividing by 100

38

FUNDAMENTALS OF CONTAMINATION CONTROL

TABLE 1.15 Cumulative Surface Contamination Rates (particles/cm2  h) for Vertical Unidirectional-Flow Environments with and Without Air Ionization Particle Size (m) 0.1 0.2 0.3 0.5 1 3

With Air Ionization

Without Air Ionization

10.1 1.03 0.31 0.11 0.049 0.028

88–176 8.5–19 2.4–4.9 0.6–1.2 0.12–0.25 0.03–0.06

Source: Ref. 7.

and multiplying by the class desired. The SCR values in the table have been verified experimentally in real manufacturing facilities. In an ionized air cleanroom, the surfaces in the room were permitted to swing between 100 V. In a room without air ionization, surfaces were measured to have charges averaging 1000 to 2000 V. In cleanrooms where surfaces are maintained at 10 V, the surface contamination rate would be expected to be one-tenth that of the ionized air cleanroom in Table 1.15, where surfaces were maintained at 100 V. The data in Table 1.15 may be used in several ways. First, they may be used to estimate the accumulation of contamination on parts to determine if the class required is appropriate for protection of the parts. The surface contamination rate is multiplied by the part exposure time to determine the aerial density of contamination accumulation. Second, the data can be used to determine if sources other than airborne contamination (contact transfer or in situ contamination) are dominant. The expected accumulation from the class is calculated. Parts are then measured to determine if significantly more additional contamination than expected has been accumulated. If much more is accumulated than would be expected on the basis of the airborne particle concentration, effort can focus on identifying and reducing contract transfer and in situ generation. Third, Table 1.15 may be useful in determining if air ionization should be incorporated in the design of a cleanroom or workstations.

1.5.6

Contact Transfer and In Situ Contamination

Contamination of surfaces can occur without generation of airborne particles, due to contact transfer. This is important for several reasons. First, most cleanrooms are monitored using airborne OPCs, witness plates, and other techniques that do not come in direct contact with product surfaces. Second, the tooling generally does come in direct contact with product. Tools become contaminated due to wear, damage, or improper maintenance. This allows them to become an undetectable source of surface contamination of product by contact transfer (unless sticky-tape sampling for all critical contact locations is implemented systematically). Since no airborne transport is involved, the dominant means of monitoring cleanrooms are not able to detect this important mechanism. Contamination due to contact transfer can exceed the amount of contamination from all other mechanisms combined. Wear and parts damage is unavoidable, even with optimum material selection. As a consequence, wipe-down is necessary to keep contamination from these sources within control. This topic is dealt with in detail in Section 1.5.9.

CONTAMINATION CONTROL REQUIREMENTS

39

TABLE 1.16 Airflow for Stage 2 in Cleanrooms Type

Class

Unidirectional

100 and lower

Mixed

1000 10,000 and higher

1.5.7

Certification Criteria About 200 to 600 air changes/h Vertical flow: 0.45  0.1 m/s (90  20 ft/min) Horizontal flow: 0.8 m/s. (150 ft/min), unless specifically provided for and directed toward the operator About 60 to 150 volume air changes/h About 20 to 60 volume air changes/h

Airflow Requirements

Product exposed to tooling at the workstation or other locations depends on clean airflow to stay clean. An important rule is that tooling must not unnecessarily obstruct the flow of clean air to the product. There may inevitably be design situations where some obstruction of the airflow occurs. The idea is to minimize such occurrences. Airflow in cleanrooms is measured by two methods, depending on the type of cleanroom in which the tooling is located. In unidirectional-flow cleanrooms, airflow in measured as linear velocity. In mixed-flow cleanrooms, airflow is measured in units of volume air changes per hour. If the tool is located in a unidirectional-flow cleanroom, either horizontal or vertical, the airflow will be measured in linear feet per minute or meters per second. Flow disturbances have the most noticeable affect on contamination performance of tools in unidirectional-flow rooms. Mixed-flow cleanrooms are generally measured in terms of the volume air exchange rate in the room, since the flow is generally not unidirectional. However, in modern cleanroom designs, local areas of unidirectional flow are being created specifically for certain processes. It is thus important to review both the class of cleanroom and the local environment to provide guidance for a reliable design guide. You must consult with the cleanroom owner or designer for details. Table 1.16 provides some guidance about how cleanrooms are generally specified and the types of numbers you can expect. Keep in mind that in some modern mixed-flow cleanrooms, local areas of unidirectional flow will have been created by design specifically for a particular tool or process. Note also the following requirements that are imposed on tools: ● ●

● ●

●

Tool cross-sectional area upwind of the product should be kept to a minimum. Tool installation must not have unnecessary air leakage to adjacent non-cleanroom areas. This requirement is in recognition that bulkhead mounting of tools is considered a desirable installation option. Flow direction should be from product toward operators or tooling whenever possible. Exhaust from fans on enclosures must be directed away from products (e.g., flow from fans should be ducted to raised floor or return ducts). Hot objects can reverse airflows (chimney effects) and must be accounted for in design.

1.5.8

Pressure Requirements and Enclosure Exhausts

One of the most versatile design strategies for contamination control is to remove the potential contaminating parts from the cleanroom. This can be done in many ways, one of

40

FUNDAMENTALS OF CONTAMINATION CONTROL

which is to enclose the part within an evacuated enclosure. The advantage of this approach is that the components in an enclosure kept at a negative pressure with respect to the cleanroom are generally exempt from cleanroom requirements. Components within evacuated enclosures can be exempt from cleanroom requirements if the following conditions exist: ● ● ●

The enclosure is evacuated to approximately 12 Pa (1.3 mmHg or 0.05 in. H2O). Exhaust is directed to a safe location. Certification tests demonstrate that the enclosure is effective.

There are many safe exhaust locations, but several are potentially not safe. Air exhausted below raised floors and into return plena is generally considered to be safe. However, if the air exhausted from a tool contains potentially harmful vapors (airborne molecular contamination), exhausting into the recirculation air path may not be acceptable. The standard HEPA and ULPA filters in cleanroom air recirculation systems are not capable of removing airborne molecular contamination from the air stream as they filter out particles. In cases where the tool exhaust may contain chemical contaminants, consult with the facility engineers for the cleanroom. Determine if they have the necessary equipment and capacity to remove your form of chemical contamination. For example, chemical scrubbers may be in the recirculation air path or there may be an exhaust system, essentially a big vacuum cleaner that has sufficient capacity to support emissions from the tool. If neither of these exist, it may be necessary to build the required chemical scrubbing capacity into your tool. 1.5.9

Maintenance Requirements

One of the most common deficiencies in contamination control is lack of proper documentation of maintenance procedures. In addition, the need for maintenance must be considered during the conceptual design phase. There are four concerns: ● ● ● ●

Cleaning by production operators (part of manufacturing process) Cleaning by maintenance personnel (part of the maintenance process) Maintenance access to enclosed components (a design consideration) Cleaning by engineers (part of the engineering process)

The engineer who designs or directs the design of a tool by a contractor is responsible for describing wipe-down procedures. Engineering is also responsible for performing wipe-down procedures after the tool has been subjected to engineering changes. Wipedown chemicals may impose a restriction on the selection of materials for a tool or workstation. For example, if a tool or workstation is to be cleaned with isopropyl alcohol and the alcohol causes paint to chalk or an ESD mat to become ineffective, the paint or ESD mat material may not acceptable for use in the tool or workstation. Similarly, the selection of wipers may effectively limit the surface finish of a tool or one of its parts. If the finish on the surface shreds or tears the required wiper, the surface finish is too rough for use in the application. It may then be necessary to change the surface to a smoother finish to eliminate this problem. Conversely, the finish on tools and work surfaces may place a restriction on the selection of wipers. In addition, tools are often made compatible with cleanroom requirements by the addition of evacuated enclosures. These normally are applied over some mechanical or pneumatic

CONTAMINATION CONTROL REQUIREMENTS

41

device that produces an unacceptable amount of wear-generated contamination or which requires period adjustment. Thus, maintenance access by operators, engineers, or maintenance personnel will be required. This issue is so important that special emphasis is required. After all else is done, ongoing maintenance is the most critical factor. As maintenance is often an overlooked issue, we focus special emphasis on it. Features of a Wipe-Down Procedure Wipe-down instructions that are general in nature are inadequate. General instructions, such as “wipe with a cleanroom wiper wetted with isopropyl alcohol” are inadequate. Wipe-down procedures must specifically call out critical areas that are to be wiped, how they are to be wiped, what materials are to be used to wipe, and how to know when wipe-down has been completed successfully. The wipedown procedures should not be in a general cleanroom procedures document, but in each workstation or operation procedure document. Critical areas are areas that come in contact with or close proximity to product surfaces, the operator’s hands, or cleanroom garments. Close proximity is a relative term. An object 60 cm upwind of product in a vertical unidirectional-flowroom is relatively closer than an object 10 cm downwind of the product. Similarly, the undersurface of a workstation where an operator sits is critical, as it is in close proximity to the operator’s cleanroom garments and potentially the operator’s gloves, if the gloves are placed on the operator’s lap. Here are some guidelines on what to specify in a wipe-down procedure: ●

●

● ● ●

●

● ● ●

●

Only cleanroom-approved wipers, swabs, and wipe-down chemicals may be used. In order of preference: DI water; DI water–detergent; isopropyl alcohol (IPA); IPA–DI water; other solvents. Consult with your local contamination engineering people to determine what materials are considered to be acceptable. The wiper should be folded to form a pad. A random wad of material should not be used as a wiper. Wipe-down begins at the top and proceeds downward. Wipe-down begins at the back and proceeds forward. Wipers must be thoroughly wet (but not dripping wet), or shredding, linting, and tearing can occur. The wetting agent provides lubrication to prevent wiper shredding, can provide some added chemical extraction ability that aids in cleanup of organic contaminants, and provides surface tension, which aids in particle removal. Inspect the wiper often. When the wiper becomes visibly soiled, fold it to expose a fresh surface, rewet, and wipe the offending surface again. Continue until no soil remains. Change the wiper often so that a clean wiper surface is always available. Through- and blind holes, crevices, and slots may need to be cleaned with swabs. Pay special attention to areas of workstations and tools that come in contact with product (wear and contact transfer). Vacuum, then wipe, then vacuum again for best effectiveness.

There are several precautions that should be observed when using a vacuum wand to clean. Experiments conducted in the late 1980s showed that touching the surface of a silicon wafer with either a plastic or a metal tip of the vacuum wand left behind more particles than it removed. These experiments also showed that vacuuming was relatively ineffective

42

FUNDAMENTALS OF CONTAMINATION CONTROL

at removing particle smaller than about 30 micrometers: about the smallest particle one can normally see. The wear debris generated by touching the surface consisted primarily of particles smaller than 30 micrometers and were thus invisible [14]. The surface is considered to be clean when no visible residue is left on the surface and when no residue can be removed from the surface when it is wiped. This is a form of “white glove” inspection. There has always been an element of controversy about this acceptance criterion. People often are concerned that the visible cleanliness criterion may be inadequate because we know that many particles are smaller than the eye can see. But the only way we can see smaller particles is through the use of some form of magnification, and this limits our field of view, so some areas may be overlooked. In addition, it requires the use of an instrument, which makes the inspection inefficient. The requirement that a cleanroom always be visibly clean is easy to check and easy to enforce. In addition, wiping is effective at removing contaminants too small to be seen with the unaided eye. Visible contamination is an indication that an area has not been wiped. Maintenance Wipe-Down Tools and workstations require scheduled and unscheduled maintenance. Maintenance activities often require access to parts of a tool that are not ordinarily exposed to the cleanroom. Maintenance may use materials that are not easily removed by the operator during standard wipe-down. That is, maintenance may require the introduction of lubricants and other materials that require special chemicals, wipers, and cleaning procedures for their removal. These need to be evaluated and specified for a comprehensive tooling installation. In addition, maintenance will have access to areas of the tools not normally accessible by or of concern to the production operator. Hence, special cleaning procedures must be performed by maintenance. Parts of tools that require seals or evacuated enclosures probably are covered because they are a source of contamination. Several design considerations are important. Parts behind covers often need adjustment or replacement. The designer must consider ease of maintenance access. For example, adhesive sealants should not be used where access is needed because they are difficult to remove and difficult to replace. Enclosures should have a minimum number of easy-to-use fasteners. One of the popular solutions to containment of contamination on components that move has been to supply bellows, usually made of polyurethane or polyethylene. These may not be the ideal solution. Bellows wear and require replacement and need mounting adapters and exhaust tubes. This adds maintenance requirements and introduces mechanical complexity. A better arrangement may be to provide close-fitting and evacuated but noncontact enclosures around moving members. Another solution that has been employed is a moving belt carried by the moving components, which travels in a noncontact guide system that acts as a labyrinth to contain contamination. Engineering Changes Engineering changes to a tool inevitably occur during its lifetime. Engineering has many responsibilities when changes occur. Among these are the following: ●

●

●

Engineering must revise operator wipe-down instructions as needed by engineering changes to the tool. Engineering must revise maintenance wipe-down instructions as needed by engineering changes to the tool. Engineering changes must maintain particle count and other contamination control requirements.

PERTINENT STANDARDS

1.5.10

43

Other Requirements

Other requirements may include issues such as electromagnetic radiation, vibration, and electrostatic charge. For example, many chemicals are sensitive to ultraviolet radiation (light). Ordinary fluorescent lamps produce sufficient ultraviolet light that they are unacceptable for use in photolithography processes for semiconductor, flat-panel displays, magnetic recording head manufacture, and so on. Where lighting is required in tools, it is important to determine if there are unacceptable wavelengths. Similarly, for some types of tools, vibration is a critical issue. A good example is photolithography exposure tools, often referred to as steppers. Manufacturers of steppers and other extremely vibration-sensitive tools understand and specify the vibration requirements of their tools very carefully. These requirements may place restrictions on the design of nearby tools, so it is wise to ask about any vibration requirements. In addition, if vibration is specified for a tool, it is important to know what the intended installation facility is capable of. Electrostatic charge is by far the most commonly encountered “other” requirement. As a consequence, it is described in some detail in Chapter 2. 1.5.11 ●

● ● ● ● ●

● ● ● ● ●

1.6

Summary of Requirements

The airborne particle concentrations at product locations must be certified to staged cleanroom acceptance requirements. Airflow must comply with directions and velocity. Materials must not outgas harmful vapors. Ionic contamination must be controlled. Surface wear and parts damage must be minimized by design. Materials and surface finishes must be compatible with cleaning fluids and wiper materials. Obstructions to airflow must be minimized. Air should be delivered to the product first. Wipe-down procedures must be fully documented. Fans for enclosures must exhaust to safe places. Electrostatic charge requirements must be understood and designed for. PERTINENT STANDARDS

Many of the methods used to characterize cleanrooms and direct and indirect material have been standardized. One important source of these standards is the Institute of Environmental Sciences and Technology. Some of the documents available include: ●

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ISO 14644-1, Cleanrooms and Associated Controlled Environments, Part 1: Classes of Air Cleanliness (defines air cleanliness classes in the size range 0.1 to 5.0 m; also allows for definition of U descriptors, the concentration of airborne particles smaller than 0.1 m, and M descriptors, the concentration of airborne particles larger than 5.0 m) ISO 14644-2, Cleanrooms and Associated Controlled Environments. Part 2: Specifications for Testing and Monitoring to Prove Compliance with ISO 14644-1 (defines the requirement for periodic verification)

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ISO 14644-3, Cleanrooms and Associated Controlled Environments. Part 3: Metrology and Test Methods (defines test methods for characterizing cleanrooms and clean zones) ISO 14644-4, Cleanrooms and Associated Clean Zones, Part 4: Design, Construction and Start Up (defines responsibilities of customers and suppliers as well as those of regulatory agencies, consultants, and service organizations) ISO 14644-5, Cleanrooms and Associated Clean Zones, Part 5: Cleanroom Operations (defines basic principles for operating a cleanroom. Included are sections on clean-room clothing, furnishings and tooling, portable equipment and consumables, and housekeeping) ISO 14644-6, Cleanrooms and Associated Clean Zones, Part 6: Terms, Definitions and Units (defines terms used in related ISO documents pertaining to cleanrooms) ISO 14644-7, Cleanrooms and Associated Clean Zones, Part 7: Separative Enclosures (Clean Air Hoods, Glove Boxes, Isolators and Minienvironments (defines requirements for these environments where they differ from cleanrooms) ISO 14644-8, Cleanrooms and Associated Clean Zones, Part 8: Classification of Airborne Molecular Contamination (defines standards for testing and classification of airborne molecular concentrations between 100 and 1012 g/m3) ISO 14698-1, Cleanrooms and Associated Controlled Environments, Biocontamination Control, Part 1: General Principles and Methods (defines principles and methods for control of biocontamination) ISO 14698-2, Cleanrooms and Associated Controlled Environments, Biocontamination Control, Part 2: Evaluation and Interpretation of Biocontamination Data (defines principles and methods for sampling biocontamination in cleanrooms) ISO 14698-3, Cleanrooms and Associated Controlled Environments, Biocontamination Control, Part 3: Measurement of the Efficiency of Processes of Cleaning and/or Disinfection of Inert Surfaces Bearing Biocontamination Wet Soiling or Biofilms (not released as a standard; issued as an information document) IEST-G-CC1001, Counting Airborne Particles for Classification and Monitoring of Cleanrooms and Clean Zones IEST-G-CC1002, Determination of the Concentration of Airborne Ultrafine Particles IEST-G-CC1003, Measurement of Airborne Macroparticles IEST-G-CC1004, Sequential Sampling Plan for Use in Classification of the Particulate Cleanliness of Air in Cleanrooms and Clean Zones FED-STD-209E, Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones MIL-STD-1246C, Product Cleanliness Levels and Contamination Control Program IEST-RP-CC001.3, HEPA and ULPA Filters IEST-RP-CC002.2, Unidirectional Flow Clean-Air Devices IEST-RP-CC006.2, Testing Cleanrooms IEST-RP-CC007.1, Testing ULPA Filters IEST-RD-CC009.2, Compendium of Standards, Practices, Methods and Similar Documents Relating to Contamination Control IEST-RD-CC011.2, A Glossary of Terms and Definitions Relating to Contamination Control

PERTINENT STANDARDS ● ● ● ●

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IEST-RP-CC021.1, Testing HEPA and ULPA Filter Media IEST-RP-CC034.1, HEPA and ULPA Filter Leak Tests IEST-RP-CC003.2, Garments Required in Cleanrooms and Controlled Environments IEST-RP-CC004.2, Evaluating Wiping Materials Used in Cleanrooms and Other Controlled Environments IEST-RP-CC005.2, Cleanroom Gloves and Fingercots IEST-STD-CC1246D, Product Cleanliness Levels and Contamination Control Program

Another important source of documents is the American Society for Testing and Materials (ASTM). Some useful ASTM documents include: ●

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ASTM E595, Standard Test Method for Total Mass Loss and Collected, Volatile, Condensable Materials from Out-gassing in a Vacuum Environment ASTM E1216, Standard Practice for Sampling for Surface Particulate Contamination by Tape Lift ASTM E1234, Standard Practice for Handling, Transporting, and Installing Nonvolatile Residue (NVR) Sample Plates Used in Environmentally Controlled Areas for Spacecraft ASTM E1235, Standard Test Method for Gravimetric Determination of Non-volatile Residue (NVR) in Environmentally Controlled Areas for Spacecraft ASTM F25, Standard Test Method for Sizing and Counting Airborne Particulate Contamination in Cleanrooms and Other Dust-Controlled Areas Designed for Electronic and Similar Applications ASTM F50, Standard Practice for Continuous Sizing and Counting of Airborne Particles in Dust-Controlled Areas and Cleanrooms Using Instruments Capable of Detecting Single Submicrometer and Larger Particles ASTM F51, Standard Test Method for Sizing and Counting Particulate Contaminants in and on Cleanroom Garments ASTM F302, Standard Practice for Field Sampling of Aerospace Fluids in Containers ASTM F303, Standard Practice for Field Sampling of Aerospace Fluids from Components ASTM F306, Standard Practice for Sampling Particulates from Man-Accessible Storage Vessels for Aerospace Fluids by Vacuum Entrainment Technique (General Method) ASTM F307, Standard Practice for Sampling Specialized Gas for Gas Analysis ASTM F311, Practice for Processing Aerospace Liquid Samples for Particulate Contamination Analysis Using Membrane Filters ASTM F312, Methods for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters ASTM F318, Standard Practice for Sampling Airborne Particulate Contamination in Cleanrooms for Handling Aerospace Fluids ASTM F327, Standard Practice for Sampling Gas Blow Down Systems and Components for Particulate Contamination by Automatic Particulate Monitor Method ASTM F331, Standard Test Method for Non-volatile Residue of Halogenated Solvent Extract from Aerospace Components (Using Rotary Flash Evaporator)

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ASTM F1094, Standard Test Methods for Microbiological Monitoring of Water Used for Processing Electronic Devices by Direct Pressure Tap Sampling Valve and by the Pre-sterilized Plastic Bag Method

REFERENCES AND NOTES 1. B. Y. H. Liu and K. H. Ahn, Particle deposition on semiconductor wafers, Aerosol Science and Technology, 6:215–224, 1987. 2. D. W. Cooper, R. J. Miller, J. J. Wu, and M. H. Peters, Deposition of submicron aerosol particles during integrated circuit manufacturing: theory, Particulate Science Technology, 8(3–4):209–224, 1990. 3. D. Y. H. Pui, Y. Ye, and B. Y. H. Liu, Experimental study of particle deposition on semiconductor wafers, Aerosol Science and Technology, 12:795–804, 1990. 4. J. J. Wu, R. J. Miller, D. W. Cooper, J. F. Flynn, D. J. Delson, and R. J. Teagle, Deposition of submicron aerosol particles during integrated circuit manufacturing: experiments, Journal of Environmental Sciences, 32(1):27, 28, 43–45, 1989. 5. R. P. Donovan, Ed., Particle Control for Semiconductor Manufacturing, Marcel Dekker, New York, 1990, pp. 312–320. 6. D. W. Cooper, R. P. Donovan, and A. Steinman, Controlling electrostatic attraction of particles in production equipment, Semiconductor International, 22(8):149–156, 1999. 7. R. W. Welker, Equivalence between surface contamination rates and class 100 conditions, Proceedings of the 34th Annual Technical Meeting of the Institute of Environmental Sciences, King of Prussia, PA, May 3–5, 1988, pp. 449–454. 8. The U.S. General Services Administration (GSA) released a Notice of Cancellation for FEDSTD-209E, Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones, on Nov. 29, 2001. The document may still be in effect for older contracts. 9. MIL-STD-1246 has been supplemented by a consensus industry standard, IEST-STDCC1246D. 10. Two notable early examples are O. Hamberg, Particulate fallout predictions for cleanrooms, Journal of Environmental Sciences, May–June 1982; and S. E. Keilson, Work area characterization to control product cleanliness, Journal of Environmental Sciences, Mar.–Apr. 1986. 11. This was often believed to be the case, but was first reported based on systematic experimentation in 1987. D. S. Ensor, R. P. Donovan, and B. R. Locke, Particle size distributions in cleanrooms, Journal of the Institute of Environmental Sciences, July 1987. 12. See, for example, S. M. Peters, Particle fallout in a class 100,000 high-bay aerospace cleanroom, Journal of the Institute of Environmental Sciences, 1995, pp. 15–17. 13. S. K. Friedlander, Smoke, Dust and Haze, Fundamentals of Aerosol Behavior, John Wiley & Sons, New York, 1977, pp. 42–44. 14. R. W. Welker, Previously unpublished laboratory data.

ADDITIONAL READING Bae, G. N., C. S. Lee, and S. O. Park, Measurement of particle deposition velocity toward a horizontal semiconductor wafer by using a wafer surface scanner, Aerosol Science and Technology, 21:72–82, 1994. Fosnight, W. J., V. P. Gross, K. D. Murray, and R. D. Wang, Deposition of 0.1 to 1.0 micron particles, including electrostatic effects, onto silicon monitor wafers (experimental), Microcontamination Conference Proceedings, San Jose, CA, Sept. 21–23, 1993.

ADDITIONAL READING

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Greig, E., I. Amador, and S. Billat, Controlling reticle defects with conductive air, presented at the SEMI Ultraclean Manufacturing Symposium, Austin, TX, Oct. 1994. Murakami, T., Togari, H. et al., Electrostatic problems in TFT-LCD production and solutions using air ionization, EOS/ESD, Conference Proceedings, Orlando, FL, Sept. 10–12, 1996. Riley, D. J., and R. G. Carbonell, Effects of charge reversal on particle deposition onto silicon wafers, Proceedings of the 38th Annual Technical Meeting of the Institute of Environmental Sciences, Nashville, TN, May 3–8, 1992, pp. 450–459. SEMI Draft Document E78-0998, Electrostatic Compatibility: Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment. SEMI E43, Recommended Practice for Measuring Static Charge on Objects and Surfaces. A. Steinman, Electrostatic discharge: MR heads beware! Data Storage, July–Aug. 1996.

CHAPTER 2

FUNDAMENTALS OF ESD CONTROL

2.1

INTRODUCTION AND HISTORICAL PERSPECTIVE

In this discussion we use two different terms: electrostatic charge and electrostatic discharge. Electrostatic charge (ESC) is the result of an imbalance between the number of negatively charged electrons and positively charged protons in the atoms comprising the surface of a material. If the material has fewer electrons than protons on its surface, the surface is positively charged. If the surface has more electrons than protons on its surface, the surface is negatively charged. Electrostatic discharge (ESD) occurs when the charge imbalances cause electrons to flow, either between two different objects or on different areas in or on a single object. If the charge flows in an uncontrolled manner through an ESD-sensitive device or structure in a device, damage can result. Electrostatic charge is a special form of contamination. It consists of excessive or depleted amounts of electrons resident on the surface of insulative materials or on the surface and within ungrounded static-dissipative or conductive material. Excess electrostatic charge, or the electrical field that it supports, can also charge other objects in a static-safe work area. These objects can then become damaged when they discharge. The similarities between electrostatic discharge control and contamination control are surprising. That is why they are dealt with together in this book. The similarities include the pervasive nature of contamination and electrostatics and the fact that the quantities of contamination and electrostatic charge that cause problems are too small to perceive under most circumstances. The training, engineering, and management approaches that must be used to gain effective control of electrostatic charge are surprisingly similar to those used to control contamination. Electrostatic charge is generally controlled by two methods: by controlling the level of charge on objects and by controlling the rate at which they discharge. Of equal importance is the design of special circuitry to protect ESD-sensitive devices from accidental exposure Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

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49

to electrostatic discharge. The design of ESD protective circuits is beyond the scope of this book. Fortunately, several good references are available for those interested in the design of ESD protective circuits [1,2]. Electrostatic charge and discharge have been identified as one of the most significant sources of reliability loss, yield loss, and productivity loss in high-technology industries. High-technology industries affected by electrostatic discharge include semiconductors, disk drives, flat-panel displays, and aerospace, among others. Electrostatic discharge damage of integrated-circuit (IC) assemblies became a major issue in the 1970s as manufacturing of electrostatic-sensitive devices became more common. The designers of semiconductor devices responded quickly to the ESD concern by developing on-chip protection against ESD, which worked quite well in ensuring that the ESD-sensitive devices could be handled and packaged without damaging them. The continued shrinking of critical dimensions on semiconductors creates an ongoing challenge to designers and manufacturers. In fact, today even chrome-on-glass photomasks that are used in the manufacture of semiconductor devices are sensitive to damage by electrostatic discharge [3]. As semiconductors have become smaller and faster, the relative cost of the area of an IC chip has become more expensive. In some cases, manufacturers and designers of ICs have begun to remove the ESD protection circuitry from the chips to free this valuable “real estate” for more productive use. In any case, ESD protective circuitry, if available, is provided only on the input lines for the devices. As a result, when these types of devices are tested for their ESD sensitivity, the damage is found to occur in the output lines rather than the input lines. Prior to the late 1980s, disk drives were not considered to be as sensitive to ESD as semiconductors. In the late 1980s, magnetoresistive (MR) heads were introduced into disk drives, making them even more sensitive to damage from ESD than their semiconductor contemporaries. Initially, disk drive manufacturers adopted a state-of-the-art semiconductor-style ESD control approach, reflecting the folk wisdom that semiconductors were far more sensitive to damage by ESD than were disk drive components. The MR heads were found to be more sensitive than the ICs being manufactured at the time. As a consequence, the control measures initially applied were found to result in unacceptably high yield and reliability loss. These were soon discovered to be inadequate, and ever-tighter controls were gradually adopted [4]. Flat-panel displays have evolved to larger screens with smaller pixels. Electrostatic charge on the panels increases surface contamination [5]. Even the loss of a single pixel due to contamination or electrostatic discharge can result in a visual defect, seriously decreasing the value of the display. The recent trend in the construction of spacecraft in the aerospace industry is to increase the use of commercial-off-the-shelf (COTS) electronic components. As a result, the ESD sensitivity of spacecraft closely follows that of semiconductor devices. In addition, the evolution of sensors for scientific instruments to improve their sensitivity has led to their being manufactured using thin-film techniques, so modern sensors for spacecraft instruments are becoming extremely sensitive to ESD. ESD damage was the original effect that attracted the attention of design and process engineers. Today, engineering focus is increasingly on the driving cause of ESD: the electrostatic charge that develops on materials. Static charge can cause several problems that can affect the manufacturing process adversely. The most obvious problem is the damage caused during an electrostatic discharge event. A second effect of ESD events is the electromagnetic interference (EMI) that occurs when an ESD event occurs. EMI can lock up microprocessors, which can result in unreasonable downtime of equipment in manufacturing

50

FUNDAMENTALS OF ESD CONTROL

processes [6]. Another critical problem in clean environments is the attraction of contamination to an item that has a high static charge on its surface. The good news in all of this is that if one controls the static charge on items in the process and provides safe discharge paths for items that may have become inadvertently charged, one can prevent both ESD and the deleterious effects of electrostatic charge– induced contamination problems (electrostatic attraction). One final historical note is of interest. In the past, the solutions for contamination control and ESD control were viewed to be mutually exclusive. What was good for contamination control was bad for ESD control, and vice versa. Today, this is no longer true, due to the development of materials that satisfy the requirements of contamination control and ESD control simultaneously. These advanced materials must still be qualified for their suitability for the contamination and ESD control requirements, but this process is far less restricted than in the past. Control of electrostatic charge generation and electrostatic discharge focuses on several areas: the facility, people, packaging, and inspections and record keeping. Everyday Experiences with ESC and ESD Electrostatic charge and electrostatic discharge are part of our daily lives. Most people seldom notice electrostatic charge and discharge, and even if they do, they seldom understand why things happen the way they do. A review of some of these events in an ESD training class is important in order to convey to people an understanding of how common electrostatic charge and electrostatic discharge really are. One of the most familiar of these is clothing that has electrostatic charge, resulting in an effect often referred to as static cling. Static cling occurs when dissimilar materials are worn in contact with one another in an environment favorable for charge generation, such as a low-relative-humidity climate. The cling can result in an unsightly and occasionally slightly embarrassing appearance. There are commercial sprays on the market sold specifically to eliminate this problem. A related example is the buildup of static charge when clothes are dried in an ordinary home laundry. Nearly everyone has experienced this. When the clothes are initially removed from the dryer, cotton socks will stick to polyester-blend shirts. The cotton and polyester charge to opposite polarities and so are attracted to one another. When the socks are peeled off the shirts, a crackling sound is heard. This audible report is evidence of electrostatic discharge. When this is done in a well-lit laundry room, the sound is heard but no sparks are seen. Peel the socks off the shirts in a darkened room and light from the sparks will be visible because your eyes have adapted to the dark. The same phenomenon occurs when a T-shirt is removed at nighttime in a darkened room. The fact that small electrostatic discharges are invisible under normal room illumination is an important point. ESD-protected workplaces are usually well lit. Small electrostatic discharges are invisible given the brightness of this background illumination. Plastic food wrap is another good example of electrostatic charge in action in our daily lives. When plastic wrap is peeled off the roll, it becomes charged electrostatically. Often, this results in the plastic food wrap clinging to itself. When charged plastic wrap is brought near an uncharged food container, the charge on the wrap induces the opposite charge on the surface of the food container. The opposite charges attract and the plastic wrap sticks tightly to the food container. After an extended period of storage, usually in the refrigerator, the electric static charge is dissipated. That is why the same piece of plastic wrap seldom sticks as well the second time it is used as it did the first time. When we arrange our hair with a plastic comb or brush, electrostatic charge is generated and electrostatic discharges can be heard. The comb or brush becomes charged to the opposite polarity of the hair. All of the hair fibers tend to take on the same charge, so they repel

INTRODUCTION AND HISTORICAL PERSPECTIVE

51

one another, giving the hair extra body or, if excessive, resulting in a “bad hair day.” Excessive electrostatic charge can result in uncontrollable hair. This is one of the reasons that we use cream rinses and conditioners after shampooing our hair. A conditioner, which is left on the hair after rinsing, contains chemicals that dissipate static electric charge and help reduce the unmanageable charge on the hair. Fabric softeners in our laundry operate in much the same way, reducing static charge and providing lubricity. Sometimes the effect of electrostatic discharge results in catastrophic effects. People occasionally set themselves or their cars on fire during automotive refueling. This effect is so well known that gasoline truck drivers always ground their truck to the underground storage tanks before they begin filling them. The next time you’re at the airport, watch carefully how the ground crews refuel aircraft. Before they begin pumping fuel, they connect ground wires between an aircraft and the fuel truck. This is done to neutralize the difference in electrostatic charge between the fuel truck and the aircraft so that no electrostatic discharge will ignite fuel vapors during the refueling operation. Electrostatic discharge is also believed to be a major suspect ignition source in grain dust explosions in grain elevators. This also occurs in flour mills, explosive plants, and is possible any time that a combustible material is conveyed pneumatically. Electrostatic charge interferes with the handling of sheet and film materials in many industries. Good examples include handling of bedclothes and handling of fabrics when garments are sewn. The lifetime of film and the audience enjoyment of their display are affected adversely by dust that contaminates and scratches film during display. Static electric charge is used beneficially in many industrial processes. Photocopying is enabled by the use of papers that charge. Electrostatic precipitators clean contaminants in the exhaust of many industrial processes. Small electrostatic precipitators are now appearing on the market for residential use in controlling household airborne dust. Electrostatically augmented air filters have been known since the 1930s. Electrostatic augmentation is used to improve the efficiency of dust collection by bag filters used to control airborne dust without increasing their flow resistance. More recently, manufacturers have learned how to charge filter materials permanently to take advantage of this electrostatic effect without having to provide a powered precipitator. Lightning One of the most spectacular and dangerous forms of electrostatic discharge is lightning (Figure 2.1). Benjamin Franklin proved that lightning is static discharge. One of the earliest inventions resulting from an understanding that lightning is electrostatic discharge was the lightning rod, which is used today in its original form hundreds of years after its invention. Before Franklin started his scientific experimentation, it was thought that electricity consisted of two opposing forces. Franklin showed that electricity consisted of a “common element” which he named “electric fire.” Further, electricity was “fluid,” like a liquid. It passed from one body to another—however, it was never destroyed. Franklin’s work, including his now famous kite experiment, led to a much refined theory of electricity. One problem was that the language of electricity had not yet been clearly defined: Scientists working at the time often had to invent new words or word combinations to describe their concepts: hence, Franklin’s use of the term “electric fire,” which we now know as electricity. Some of the electrical terms that Franklin coined during his experiments include battery, charge, condensor, conductor, plus, minus, positively, and negatively. We still use these terms today in their original context. Among Franklin’s attributes was a keen desire to make practical use of his discoveries. Among his enduring inventions are bifocals, the Franklin wood stove, and the lightning

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FIGURE 2.1 Lightning.

rod, which is used today in essentially the same form as Franklin originated. Today’s world includes items that did not exist in Franklin’s time which need additional protection against the effects of lightning: home appliances and electronic devices such as home computers. To protect them from power surges induced by lightning and other sources, we use surge protectors and other noise-isolation devices. Scientists cannot agree on the mechanism by which static electricity is generated in clouds to result in lightning. However, some observations are agreed upon. During fair weather, a potential difference of 200,000 to 500,000 V exists between Earth’s surface and the ionosphere, with a fair weather current to Earth of about 2  1012 A/m2. It is widely believed that this potential difference is due to the lightning activity of thunderstorms. Present measurements indicate that an average of almost 1 A of current flows into the stratosphere during the active phase of a typical thunderstorm. Although present theory suggests that thunderstorms are responsible for the ionospheric potential and atmospheric current for fair weather, the details are not fully understood. The bottoms of thunderclouds are negatively charged and the tops are positively charged. The negative charge on the bottoms of thunderclouds induces positive charge on the Earth below. When the difference in charge between the bottom of a thundercloud and the Earth exceeds 25,000 V/cm, the air begins to conduct, resulting in a lightning bolt. Research continued at a steady pace until the late 1960s, when lightning research became particularly active. This increased interest was motivated by the danger of lightning to both aerospace vehicles and solid-state electronics used in computers and other devices. Improved measurement capabilities have been made possible by advancing technology.

2.2 ●

GLOSSARY OF ELECTROSTATIC CHARGE CONTROL TERMS Air ionizer: a source of ionized air. Air naturally contains ions, but they are not sufficiently abundant in most cases to neutralize static charges rapidly enough to protect static-sensitive devices. Further, air ions are removed completely by HEPA and ULPA

GLOSSARY OF ELECTROSTATIC CHARGE CONTROL TERMS

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filters in cleanrooms. For this reason, ESD problems in cleanrooms require air ionization to be used. Antistat, topical: a chemical compound applied to a surface or impregnated within a material to render an insulative material static dissipative. Plastics made static dissipative through impregnation of topical antistatic agents can become depleted by water and alcohol cleaning, rendering them ineffective. Most topical antistatic agents have significant vapor pressure. For this reason, topical antistatic agents should not be used in cleanrooms. Antistatic: traditionally, a material that inhibits or resists triboelectric charging. The term has fallen into disfavor, as it implies that no static charge will be generated, which is not true. Charged-device model: a model in which the ESD-sensitive device is charged and then discharged. The fine structures of a MR head are so sensitive to damage that heads must not become tribocharged at any time in their processing. In addition, disks to which heads are merged must not become charged, or head damage will result during merge. Cold healing: a phenomenon whereby device characteristics, changed by ESD stress, return to normal at room temperature. Conductive material: a material with volume resistivity less than 105   cm and surface resistivity less than 106 /sq. Conductivity: the ability of a material to conduct electricity. Coulomb: a unit of electrical charge, equivalent to 6.24  1018 electrons: Q  CV

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where Q is the charge in coulombs, C is the capacitance in farads, and V is the voltage in volts. Decay time: the time required for voltage to reduce to a specified percentage of its initial value. One of the two principal criteria for evaluating acceptability of air ionizers and materials used in workstations provided with air ionizers. Dielectric: a nonconductor that can sustain an electric field. Dielectric breakdown voltage: the voltage at which an electrically conductive path is created through a dielectric. Dielectric strength: the rated voltage (or electric field) above which the creation of an electrically conductive path through a dielectric is possible. Electrostatic discharge (ESD): the transfer of electrostatic charge between bodies at different electrostatic potentials. Electrostatic discharge sensitive (ESDS): the property of a component or assembly that it can be damaged by electrostatic discharge. Generally reported as voltage using one of the three test models: human body, machine, or charged device. Electrostatic field: the lines of force surrounding an electrically charged object. Electrostatic overstress (EOS): the exposure of an electronic component or assembly to current or voltage greater than its maximum rating. EOS may or may not result in catastrophic failure. Electrostatic potential: the voltage difference between a point and an agreed-upon reference.

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Electrostatic shield: a barrier or enclosure that limits the penetration of an electrostatic field. ESD ground: the plug-in point, busbar, conductive braid, bare wire, or metal strip designated as a connection point to eliminate electrostatic charge on connected objects. ESD-protected work area: a work environment with materials and equipment to limit electrostatic voltage, also called a static-safe workplace, and which provides controlled means for neutralizing electrostatic charges on ESDS items. ESD protective: a property of materials capable of one or more of the following: limiting the generation of static electricity, dissipating electrostatic charges, or providing shielding from ESD or electrostatic fields. ESD protective workstation: a work position with materials and equipment to limit electrostatic voltages and provided a controlled means for neutralizing electrostatic charge on ESDS items. ESD protective work surface: a work surface that is intended to discharge electrostatic charges from materials placed on the surface or from the surface itself. ESD susceptibility (sensitivity) (ESDS): a measure of the susceptibility (sensitivity) of an item to ESD damage. Susceptibility or sensitivity is defined as that level of ESD that produces changes in characteristics such that the item fails to meet its specified parameters. Faraday cage: an enclosure that provides an electrostatic shield (may or may not affect electromagnetic waves). Field-induced model: a charged-device model event where the charging method is by electrostatic induction. Floating-device model: a model of an isolated device subjected to an electric field producing a voltage across the device. Float potential: the second measure of performance of an air ionizer. The highest positive and negative potential measured using an uncharged, ungrounded charged plate monitor or ionizer verifier. Ground: (1) a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and Earth, or to some conducting body that serves in place of Earth; (2) the position or portion of an electrical circuit at zero potential with respect to Earth; (3) a conducting body, such as Earth or the hull of a steel ship, used as a return path for electric currents and as an arbitrary zero reference point. Ground straps: an item intended to provide a conductive path to ground. Groundable point: a designated connection, location, or assembly used on an ESD protective material or device that is intended to accommodate electrical connection from the device to an appropriate electrical ground. Human body model: a model representing the ESD from a human being. Input protection: external or internal structures, devices, or networks connected at the terminals of an item to prevent damage due to ESD. Insulative material: a material that has a surface resistivity greater than 1  1012 /sq and volume resistivity greater than 1  1011   cm. Ionization: the process by which a neutral atom or molecule acquires a positive or negative charge. Joule: a unit of energy. One joule (J) is equal to 1 V multiplied by 1 coulomb. One joule is equal to 0.2391 cal. One calorie is defined as the amount of energy required to

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raise the temperature of 1 g of water 1°C at 20°C. The quantity of energy in a spark necessary to ignite the optimum mixture of methane (the principal gas in natural gas) in air is about 0.25 mJ. The quantity to ignite an optimum mixture of hydrogen in air is about 0.017 mJ (about 17 J). [NFPA 77, Recommended Practice on Static Electricity, 1993.] By comparison, the quantity of energy to damage most electronic devices is in the range 2 to 1000 nJ: H  12 CV 2

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where H is the energy in watt  seconds or joules, C is the capacitance in farads, and V is the voltage in volts. Junction damage: a power-dependent mechanism resulting in a change in semiconductor current–voltage (I–V) characteristics. Latent failure: a malfunction, attributable to earlier exposure to ESD, that occurs following a period of normal operation. Oxide punch-through: dielectric breakdown of an oxide layer, as in a semiconductor device. Sensitive electronic device symbols: the symbols placed on hardware assemblies and documentation for identification of ESDS items. Shunting bar: a device that shortens together the terminals of an ESDS item, forming an equipotential surface. Spark: an electrical discharge of very short duration, normally between two conductors separated by a gas (such as air). Static decay test: a procedure that specifies contact-charging a material and measuring the decay time to a specific voltage. Decay to 10% of the initial voltage is frequently used. Static dissipative: a material having a surface resistivity greater then 1  106 /sq or 1  105   cm volume resistivity but less than 1  1012 /sq surface resistivity or 1  1011   volume resistivity. Static electricity: electrical charge at rest. The electrical charge is due to the transfer of electrons within a body (polarization) or from one body to another. Static eliminator, electrical: electrical static eliminators generally consist of one or more electrodes and a high-voltage power supply. Ion generation from electrical static eliminators occurs in the airspace surrounding high-voltage electrodes. (See also air ionizer and ionization.) Static eliminator, induction: a passive device having an electric field of sufficient intensity to supply ions for static elimination. Static eliminator, nuclear: static eliminators which create ions by the irradiation of air molecules. Most models use an -particle-emitting isotope to create ion pairs to neutralize static charges. (See also air ionizer and ionization.) Static-safe workplace: a workplace that has been designed to protect ESD-sensitive devices from damage by electrostatic charge. Also called an ESD-protected work area. Step stress testing: a test consisting of increasing stress levels applied sequentially to a sample for periods of equal duration. Surface resistivity: the ratio of dc voltage drop per unit length to the current per unit width that passes across the surface of a system. In this case the surface consists of a

56

FUNDAMENTALS OF ESD CONTROL

square unit of area. In effect, the surface resistivity is the resistance between two opposite sides of a square and is independent of the size of the square or its dimensional units. Surface resistivity is expressed in ohms per square. When using a concentric ring fixture, resistivity is expressed in ohms per square. When using a concentric ring fixture, resistivity is calculated by using the following expression: surface resistivity, s 

2 R ln( D2 /D1 )

where D2 is the inside diameter of the outer electrode, D1 is the outside diameter of the inner electrode, and R is the measured resistance in ohms. [Note: This term is in review by EOS/ESD Standards Subcommittee 11.0 because the unit ohms per square (/sq) is so confusing.] ●

●

●

●

●

2.3

Triboelectric charging: the generation of electrostatic charges when two pieces of material in intimate contact are separated. Substantial generation of static electricity can be caused by contact and separation of two materials or by rubbing two substances together. (See also triboelectric series.) Triboelectric series: a list of substances arranged so that one can become positively charged when separated from one farther down the list, or negatively charged when separated from one farther up the list. The main utility of the series is to indicate likely resulting charge polarities after triboelectric generation. However, this series is derived from specially prepared and cleaned materials tested in very controlled conditions. In everyday circumstances, materials reasonably close to one another in the series can produce charge polarities opposite to that expected. This series is only a guide. Unprotected ESDS device: an ESD-sensitive device that is not protected by static shielding enclosures (bag, box, or cabinet) and/or having exposed electrical connectors. Voltage suppression: a phenomenon where by increasing the capacitance of the object rather than decreasing the charge on the object reduces the voltage from a charged object. Volume resistivity: the ratio of the dc voltage per unit thickness to the current per unit area passing through a material. Volume resistivity is generally reported in   cm. SOURCES OF ELECTROSTATIC CHARGE

People are usually considered to be the most common source of electrostatic charge causing ESD damage. The human body is a capacitor and thus can store charge. Measured body capacitance ranges from about 100 to greater than 500 pF, depending on a person’s size and attire [7]. In addition, human skin is usually a good conductor, with resistance typically in the range 1000 to 3000 . In a small percentage of the population, skin resistance can be so high that grounding through a wrist strap (typically, monitored to be less than 10 M) is found to fail when tested. Skin resistance is affected by relative humidity and the use of skin moisturizers. This combination of relatively large body capacitance and low skin resistance makes people an important source of potential electrostatic charge. Another factor contributing to the importance of people as a source of ESD is the relatively unpredictable nature of human behavior. For this reason robots and other forms of automation are often preferred in high volume manufacturing of devices with extreme ESD sensitivity.

SOURCES OF ELECTROSTATIC CHARGE

2.3.1

57

Static Electricity

Static electricity is electricity at rest: It is not moving. It consists of an imbalance of electrons resident on the surface of materials. Materials that lose electrons take on a positive charge, and materials that have acquired extra electrons take on a negative charge. Excess electrostatic charge, or the electrical field that it supports, can also charge other nearby objects, by a process called induction charging (more below). Objects that have been induction charged can then be damaged when they discharge or become contaminated. Objects become electrostatically charged by three primary mechanisms: conduction charging, induction charging, and tribocharging. Objects also can become charged by ionizing radiation, plasma and photoionization. These are seldom encounted so this discussion will be limited to contact, induction and tribocharging. Conduction Charging Electrostatic charge can be transferred to conductive or staticdissipative materials by contact with other conductive or static-dissipative items that are charged. Figure 2.2 illustrates a charged metal tool contacting an uncharged item. The charge on the metal tool, upon contact, is shared with the item that was initially at a different potential. After contact, both objects are charged to the same polarity. It is important to recognize that conduction charging is not an important charging mechanism for materials that are insulators. Induction Charging During induction charging the charged object and the ESDsensitive (ESDS) item are never in contact. The charge on the charged object supports an external electrical field. This electrical field can induce electrons to flow in the conductors of an ESDS object placed in the field. Opposite charges attract and similar charges repel. If an ESDS item is moved into the external electrical field, current will flow in one direction, and then when it is moved out of the electrical field, current will flow in the opposite direction. If the ESDS item is moved quickly into a strong electrical field and out of the strong electrical field, a high surge of electrical current can occur. The conductors on the ESDS part can become polarized while it is in the electrical field. If the ESDS part is

+

+

+

+

+

+

+

+

+

(a)

+ + +

+

+

+

+

+

+

+

+

+

+ + +

(b)

FIGURE 2.2 Conduction charging: (a) starting condition (charged tweezers and neutral chip); (b) charged tweezers charge chip during contact.

58

FUNDAMENTALS OF ESD CONTROL

grounded or touched by an object that is a source of or sink for electrons, the surge of current can also damage the ESDS device. Touching the polarized ESDS device while in the external electrical field can result in neutralizing the polarization. When we remove the ESDS device from the external field, we find that it has become charged. An ESDS item in this state is known as a field-induced charged device. The charged device can also be damaged when it comes in contact with a grounded surface such as a tool or other conductor. It is also possible for damage to occur if the ESDS item contacts with an unground conductor, if the conductor has sufficient capacitance. One example of induction charging of an ESDS device is illustrated in Figure 2.3. Suppose that we have a charged cathode-ray tube (e.g., a television monitor, computer display, oscilloscope) and an uncharged ESDS item. As the ESDS item is brought within the charge field, electrons are induced to flow through the conductors on the ESDS item. The rate at which electrons are repelled or attracted (and thus the induced current flow) is proportional to the variation in field strength over the charged object and proportional to the speed at which the uncharged object is moved through this varying field. The same factors affect current flow as the object is removed from the external field. Moving rapidly through strongly varying external electrical fields thus provides at least two opportunities for fieldinduced current flows to cause damage to ESDS devices. Other opportunities for ESD damage to occur in the presence of electrostatic field potentials are also illustrated in Figure 2.3. If a conductor on the ESDS item is momentarily contacted, the charge on the ESDS object may neutralize so rapidly that ESD damage can occur. An ESDS device can become polarized. Here we show an ESDS device in the electrostatic field from the glass front surface of a positively charged cathode-ray tube. This polarizes the ESDS device, so the conductors near the CRT become negatively charged (opposites attract), and the portions of the conductors away from the CRT become positively charged (to conserve the total number of electrons).

+ + + (a)

+

(b) _

_ _

_

+ + +

_

+ + +

_

_

_

_

_

(c)

_

__

_

_

(d )

FIGURE 2.3 Example of induction charging commonly found in static-protected workplaces: (a) a neutral chip; (b) the chip becomes polarized when put into the electrostatic field; (c) adding ground or other source or sink of electrons neutralizes the charge; (d) the field is removed. The negatively charged chip could now suffer ESD damage.

SOURCES OF ELECTROSTATIC CHARGE

59

If one of the conductors on the ESDS device is touched by an object that is a source of or sink for electrons while in the static electric field, charge will transfer to the device. In Figure 2.3 we show electrons flowing into the ESDS device to neutralize the positively charged portions of the conductors. If the ESDS device is subsequently removed from the external electrostatic field, the charge will be stored on the device. If the charged device is subsequently grounded, it is possible for the discharge to be so rapid that a damaging ESD event could occur. Triboelectric Charging The prefix tribo in triboelectric charging is derived from ancient Greek. Ancient peoples understood that if you rubbed dissimilar materials together, such as amber against fur, something changed. The amber would attract small items (e.g., hair, wood chips). The earliest attribution of this observation is to Thales of Melitus around 585 B.C. [8]. Today we understand that when two dissimilar materials become charged, one acquires a positive charge and the other acquires a negative charge. Tribocharging occurs any time that dissimilar materials are placed in contact with one another and then separated. Rubbing tends to increase the charge generation caused by contact between dissimilar materials. Triboelectric charging is the source of many electrostatic charging problems. When joined and separated or rubbed across each other, all dissimilar materials will generate charge through tribocharging. There are many familiar examples of generating electrostatic charge by tribocharging. Clothes tumbling together in a dryer can charge. When the clothes are removed from the dryer, they are often stuck together. When separated, they often crackle, the audible report of electrostatic discharge. Combing or brushing hair, walking across carpeting, and a trick that is commonly done at children’s birthday parties, where a rubber balloon is rubbed on hair or clothing and then stuck to a wall, are all familiar examples of triboelectric charge generation. The tendency of materials to develop positive or negative charge due to tribocharging has been arranged in the triboelectric series. More than 800 different material combinations are included in the most complete triboelectric series. However, most of these materials are unlikely to be used in an ESD-controlled workplace. Some common materials that are often found in an ESD-controlled workplace are: Positive End of the Series Human hands Plexiglas Glass, quartz Nylon Human hair Wool Lead Silk Aluminum Paper Cotton Steel

More Positive Charge

60

FUNDAMENTALS OF ESD CONTROL

Wood Hard rubber Nickel, copper Brass, silver Gold, platinum Polystyrene foam Acrylic Rayon Polyester Polyurethane Polyethylene, polypropylene PVC Teflon Negative End of the Series

More Negative Charge

The triboelectric series gives some idea of how materials will probably charge after being placed in contact with and separated from other materials in the series. When two of the materials in the series are joined and separated, the series helps to determine the polarity and the magnitude of the charge resulting on each surface. For instance, if brass and polyurethane are joined and separated, the brass should emerge with a positive charge and the polyurethane should emerge with a negative charge. The relative magnitude of the charge would probably be greater than if steel and polyurethane were joined and separated. We are careful here to say “probably” because triboelectric charge generation is affected by many variables. Some of the variables that have been identified include the area in contact, the presence or absence of surface contaminants, the speed of separation, and the relative humidity. For this reason, if you compare several different published series, the exact order of the items in the series will not always be the same. However, the general overall position in the series is quite consistent among the various series. Basis for Tribocharging The precise explanation for the order of materials in the triboelectric series has not been determined. One proposed explanation is that the relative tenacity with which electrons are attached to different materials varies. When dissimilar materials are brought into contact and then separated, the material with a weaker hold on its electrons loses some electrons, resulting in that material becoming positively charged. The material with a stronger hold on its electrons acquires the electrons lost from the other surface, acquiring a negative charge. One explanation for this phenomenon is based on the work function of materials, which is most easily measured using the photoelectric effect. In the photoelectric effect, a metal surface is illuminated by light. If the wavelength of light has sufficient energy, low-energy electrons are emitted from the material. The production of an electron is a function of the wavelength of illumination. The energy at that wavelength of light is used to describe the work function in units of electron volts. The higher the work function, the more tightly electrons are bound, and vice versa. A competing theory is that the tendency for materials to tribocharge can be associated with the electronegativity of the elements that comprise the surface of the material. The two are closely related and many attempts have been made to establish a correlation between them [9]. However, values of workfunctions for non-metals

SOURCES OF ELECTROSTATIC CHARGE

61

do not exist. Among the elements that comprise materials that are important for the ESDsafe workplace for which there a no work functions are nitrogen, oxygen, fluorine, sulfur and chlorine. Electronegativity and Work Functions Electronegativity is a parameter originally introduced by Linus Pauling which describes, on a relative basis, the tendency of an atom in a molecule to attract bonding electrons. Although electronegativity is not a precisely defined molecular property, the electronegativity difference between two atoms provides a useful explanation of the bonding behavior between different atoms in a molecule. Electronegativity can be used to explain the dipole moment of interatomic bonds (i.e., the degree of polarization of the interatomic bond) and also the nature of the bond as being ionic or covalent. Table 2.1 gives the electronegativity, X, on the Pauling scale, for the most common oxidation state of each element. Other scales are also described in Refs. 10 to 12 Table 2.1 also gives the work functions of the metallic elements [13–15]. Most of the work functions are measured using photoelectric stimulation. Where a dash appears in the table, no value exists. In the case of electronegativities, only the inert gases have no electronegativity. The inert gases do not generally form stable compounds. For the photoelectric work functions, only metallic elements release photoelectrons. Not all metallic elements have been characterized for their photoelectric work functions. Table 2.2 shows the materials that are likely to be found in the ESD-safe workplace from the triboelectric series shown earlier. The materials are listed along with the elements which are expected to be significant constituents of their surfaces. The relative surface abundance of elements for many of the materials can not be easily predicted so elements electronegativity and work functions are not shown for those materials. The polarity of charge generated by tribocharging should be predictable on the basis of the electronegativity or the work function of the elements on the surface of the materials. When dissimilar materials are placed in contact and then separated, the materials whose surface consists of elements with higher electronegativity should become negatively charged and the surface with lower electronegativity should become positively charge. Or, when dissimilar materials are placed in contact and then separated, the materials whose surface consists of elements with higher work functions should become negatively charged, and the surface with a lower work function should become positively charge. Comparison of dissimilar materials suggests that the tribocharging of materials is better explained by their electronegativities than by their work functions. Notice that the comparison is limited to pairs of materials whose atomic surface composition can be predicted with reasonable accuracy. For this reason, natural organic materials and synthetic polymers, which consist of molecules with a broad mixture of C, H, N, O, and S atoms, are not included. The atomic composition of the surfaces of these types of materials is likely to be unpredictable. Limitation of Tribocharge Testing for Materials Qualification Many users try to qualify materials using tribocharge tests. Care should be taken in making qualification decisions based on the results of tribocharge tests. In one standard test [16], Teflon or quartz cylinders are rolled down an inclined plane that has been covered with a material under evaluation. The charge generated on the cylinders is measured using a Faraday cup and electrometer. Although this procedure can be demonstrated to be relatively repeatable, it does not test all combinations of materials that could be in a static-protected workplace that can result in tribocharging. Table 2.3 illustrates the effect of materials on charge generation. The average charge generation was measured in nanocoulombs at 53% RH.

62

Atomic Symbol

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti

Atomic Number, Z

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

2.20 — 0.98 1.57 2.04 2.55 3.04 3.44 3.98 — 0.93 1.31 1.61 1.90 2.19 2.58 3.16 — 0.82 1.00 1.36 1.54

Pauling Electronegativity, X — — 2.93 4.98 4.45 5.0 — — — — 2.36 3.66 4.15 4.79 — — — — 2.29 2.87 3.5 4.33

Work Function,  (eV)

TABLE 2.1 Electronegativities and Work Functions of the Elements

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69

Atomic Number, Z Cd In Sn Sb Te l Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm

Atomic Symbol 1.69 1.78 1.96 2.05 2.1 2.66 2.60 0.79 0.89 1.10 1.12 1.13 1.14 — 1.17 — 1.20 — 1.22 1.23 1.24 1.25

Pauling Electronegativity, X

4.08 4.09 4.42 4.55 4.95 — — 1.95 2.52 3.5 2.9 — 3.2 — 2.7 2.5 2.90 3.0 — — — —

Work Function,  (eV)

63

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag

1.63 1.66 1.55 1.83 1.88 1.91 1.90 1.65 1.81 2.01 2.18 2.55 2.96 — 0.82 0.95 1.22 1.33 1.6 2.16 2.10 2.2 2.28 2.20 1.93

4.3 4.5 4.1 4.74 5.0 5.21 4.76 4.26 4.32 5.0 3.75 5.9 — — 2.26 2.59 3.1 4.05 4.35 4.57 — 4.71 4.98 5.41 4.63

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94

Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Th Pa U Np Pu

— 1.0 1.3 1.5 1.7 1.9 2.2 2.2 2.2 2.4 1.9 1.8 1.8 1.9 2.0 2.2 — 0.7 0.9 1.1 1.3 1.5 1.7 1.3 1.3

— 3.3 3.9 4.3 4.55 4.72 5.93 5.6 5.64 5.37 — 3.84 4.25 4.34 — — — — — — 3.4 — 3.63 — —

64

FUNDAMENTALS OF ESD CONTROL

TABLE 2.2 Triboelectric Series, Their Respective Electronegativities, and Their Work Functions Position of Material in Triboelectric Series Human hands Plexiglas Glass, quartz Human hair Nylon Wool Lead Silk Aluminum Paper Cotton Steel Wood Hard rubber Nickel, copper Brass, silver Gold, platinum Sulfur Rayon Polyester Polyurethane PVC Teflon

Atomic Number, Z

Atomic Symbol

Pauling Electronegativity, X

Work Function  (eV)

11 19

Na K

0.93 0.82

2.36 2.29

14

Si

1.90

4.79

82

Pb

1.8

4.25

13

Al

1.61

4.15

26

Fe

1.83

4.74

28 29 47 79 78 16

Ni Cu Ag Au Pt S

1.91 1.90 1.93 2.4 2.2 2.58

5.21 4.76 4.63 5.37 5.64 —

17 9

Cl F

3.16 3.98

— —

TABLE 2.3 Tribocharging of Various Material Combinations at 53% RHa (nC) Inclined Plane Material (Resistivity, /sq)

Cylinder Material Brass Steel Teflon Acrylic Polycarbonate Glass-filled phenolic Glass-filled epoxy Glass a

Polyethylene (1012 /sq) 1.30 1.21 0.15 1.06 0.78 0.97 0.31 2.92

Static-Dissipative Polyethylene Polyolefin Blend (1011 /sq) (1012 /sq) 0.24 0.13 0.52 0.72 0.27 0.05 0.39

0.01

0.47 0.12 1.50 0.67 0.28 0.17 0.56 0.64

Conductive Carbon-Filled Polyolefin Blend (103 /sq) 0.01

0.01 0.83 1.30 0.54 0.03 0.19 0.03

One convenient apparatus for making such measurements is made by Electro-Tech Systems: the Model 705 triboelectric charge generation test system (Electro-Tech Systems, Inc., 3101 Mt. Carmel Avenue, Glenside, PA 19038).

SOURCES OF ELECTROSTATIC CHARGE

65

Case Study: Tribocharge Testing In the early 1990s, one large disk drive manufacturer began manufacturing a new model of an older head disk assembly (HDA) and switched from inductive heads to magnetoresistive (MR) heads. The switch to MR heads held the promise of increasing the storage capacity of the HDA o ver the old inductive head model. This came at the expense of introducing a read/write head with serious sensitivity to ESD, whereas the old inductive read/write head it was replacing had no sensitivity to ESD. This manufacturer had previous experience with MR heads in another one of its factories. It was known that it was desirable to replace the standard insulative natural rubber latex cleanroom gloves with the new (at the time) thin static dissipative nitrile cleanroom gloves to reduce yield loss from electrostatic discharge. So nitrile gloves were implemented for the new HDA to try to avoid ESD problems during manual assembly of development production. During the building of development HDAs, ESD failures in the new MR heads occurred, although at a relatively low percentage rate. One engineer in the development lab made the observation that if a nitrile rubber glove of the type being worn in the assembly area was rubbed over a polyimide portion of the assembly to which the head wires were bonded and touched the head wire bond pad, the heads would always be damaged by ESD. He also noted that rubbing the same way with the old natural rubber latex glove did not result in ESD damage to the MR heads. These were observations of the tendency of different materials, natural latex rubber versus nitrile rubber to tribocharge when in contact with Kapton. He immediately ordered that the nitrile gloves be removed and replaced by the natural rubber latex gloves. To his surprise, the result was that, now nearly 100% of the MR heads were damaged by ESD. His mistake was in considering the tribocharging between the glove and the polyimide portion of the assembly to be the only important factor in glove selection for this application. He forgot that the gloves would touch and be tribocharged by many other materials in the workplace. Nitrile gloves, being static dissipative, would not tend to retain this charge for a long time. Conversely, the natural rubber latex gloves, being insulative, would retain charge for a very long time. The solution to this problem was more complex. Operators were instructed not to touch the critical areas of the parts. The bond pads were conformally coated after wire bonding to insulate their surfaces. Nitrile gloves were returned to production. This case study illustrates the importance of considering tribocharging as only a single part of an overall ESD control program. Considering tribocharging properties alone can lead to selecting ESD control approaches that are inadequate, or as illustrated by this case study, counterproductive. This should not be taken to mean that all tribocharge tests are meaningless. Take the example of an automated pick-and-place machine. In an automated process, the material for the contact pads and the objects to be picked and placed are well known and do not vary. In this case the relative tribocharge generated by different candidate pad materials might become a valid criterion for choosing the correct type of material from which to make the contact pads. 2.3.2

Effects of Electrostatic Charge and Discharge

The effects of ESD are numerous. Among them are: ● ●

Disruption of microprocessors Corruption of data

66

FUNDAMENTALS OF ESD CONTROL ● ●

Increased rates of surface contamination Failures in sensitive devices

Of these effects, disruption of the performance of microprocessors is one of the most annoying and in some cases costly. When an electrostatic discharge gets into a microprocessor, the microprocessor could interpret the resulting noise on the signal lines as improper commands. This could alter the function of the microprocessor in such a way that the microprocessor stops functioning. When this happens on a stand-alone desktop or laptop computer, the result can be improper termination of a task, leading to loss of data since the last update command was processed by the user. The problem can be especially annoying if more than one application is open simultaneously. If one of these program interrupts occurs during a lengthy download, download may have to be redone completely, usually resulting in added, costly connect time. Disruption of microprocessors also occurs during manufacturing operations. In this case the sophisticated computer controlling an automated work cell simply stops running, causing the process to stop running. This can be especially expensive if the process being run fails so badly that a very expensive batch of valuable product becomes ruined. The cost of a disrupted microprocessor should not be underestimated. Granted that little or no permanent damage is usually done to the computer itself in this type of “interruption” failure. However, the loss of work or valuable product must be factored into estimating the need to protect against such a work stoppage. The problem becomes even more perilous where the loss of a microprocessor can result in serious personal injury or loss of life. To avoid these types of outages, many risk analyses result in the deployment of redundant systems, where more than one microprocessor will be running simultaneously to prevent the loss of function if one or more system halts operation. These types of electrostatic charges get into microprocessors via several mechanisms. The most common of these are line voltage transients. Transients that are too large in magnitude or beyond the frequency limits of the power supply circuit supplying the microprocessor can get in and disrupt the action of the microprocessor. These line voltage transients are, in turn, caused most frequently by electrical storms or power surges created when heavy-power-drain equipment is switched on and off on the same power circuit. This is one of the primary functions of surge protectors, which are now commonly found in most home and business offices. Another way that microprocessors become disrupted is by electromagnetic interference. Electromagnetic Interference and ESD Although electromagnetic radiation is not a charging mechanism per se, it is an important phenomenon because it induces current flow in conductors and causes various electrostatic effects. When a spark occurs, it creates electromagnetic waves. This production of electromagnetic waves is familiar to most people. The static we hear on our AM radios, especially during electrical storms, is a common example. One of the earliest forms of electrostatic discharge detectors was an AM radio tuned between broadcast frequencies with the volume turned up. Today, arrays of antennas and computers equipped with triangulation software can be used as ESD locators. Electromagnetic radiation that is detected by an AM radio has been given the name of radio-frequency or electromagnetic interference. Electromagnetic interference can induce current flows in conductors on electronic circuits. These current flows can be superimposed on analog and digital electronic signals in the circuit and can result in the shutdown of microprocessors and other abnormal performance. This well-understood phenomenon

SOURCES OF ELECTROSTATIC CHARGE

67

forms the basis of EMI shielding requirements on modern electronic circuits. The ESD sensitivity of some state-of-the-art electronic devices has increased so much that electromagnetic interference has been shown to be a cause of ESD damage [11–15]. Electrostatic Charge and Increased Surface Contamination Rates Electrostatic charge on surfaces also has an effect on the accumulation of contamination by surfaces. This phenomenon, referred to as electrostatic attraction, is discussed extensively in Chapter 1.This is an especially important mechanism contributing to failures in virtually all technologies that are adversely affected by particle contamination. Indeed, for this reason, in many industries where electrostatic discharge does not represent a significant failure mechanism, much effort is still spent on elimination of static electric charge. Good examples are the manufacture of magnetic recording media, the manufacture and reproduction of films, the manufacture of compact disks and DVDs, and the production of precision optical components such as lenses, mirrors, and contact lenses. The charge level on surfaces in a cleanroom have been measured and reported by several authors. Surfaces with charges up to 35,000 V are observed in cleanrooms that do not have ionization systems.

2.3.3

Failure Modes in High-Technology ESD-Sensitive Devices

Three general ESD damage failure modes can be defined: 1. Catastrophic failures. When a catastrophic failure occurs, the device does not function at all. This is bad, because increased cost and time will be needed to locate, replace, and retest the failed components. But the fact that the failure could be detected during testing is good. 2. Parametric performance failures. A parametric performance failure occurs when the device has been slightly damaged so that it still performs but not to specification. For example, the device may not oscillate at the correct frequency, may exhibit intermittent performance, or may be unstable. The device still works when tested, but some performance parameters may be out of the acceptable tolerance limits. Parametric performance failures are often found during stress tests: tests performed at the extremes of temperature, humidity, pressure, vibration, and so on, under which the device is expected to operate. Again, this is bad, because increased cost and time will be needed to locate, replace, and retest the failed components. But the fact that the failure could be detected during testing is good. 3. Latent failures. A latent failure occurs when a device has been damaged so slightly that it does not fail but continues to perform within its parametric tolerance limits. The device passes all the tests, even under stress, and will be used. These latent defects manifest themselves as reliability losses days, weeks, or months later [16]. The types of failures may be thought of in terms of their relative severities and relative frequencies. Catastrophic failures are the most severe, parametric performance failures are moderately severe, and latent failures are the least severe. Conversely, catastrophic failures occur the least frequently, parametric performance failures occur more frequently, and latent defects occur most frequently. This makes sense when one considers the frequency of ESD events in terms of the magnitude of their voltage. High-voltage ESD events, those

68

FUNDAMENTALS OF ESD CONTROL

that induce catastrophic failures, occur the least often. Intermediate-voltage ESD events occur more often and produce parametric performance failures. Low-voltage ESD events occur the most frequently and produce the latent defects. That this is so is illustrated by reliability testing. Batches of parts that show a high frequency of parametric performance failures show a high proportion of reliability loss. Batches of parts with a low frequency of parametric performance failure have proportionately lower reliability loss. In both cases, for each parametric performance failure, there are typically 4 to 10 times as many parts that will fail to meet their reliability goal.

2.4

REQUIREMENTS OF ESD CONTROL

First it is necessary to define what types of devices could be electrostatic discharge sensitive (ESDS). From ESD-sensitivity data it is possible to define acceptance limits for charges and discharge times on surfaces, people, packaging materials, procedures, and processes in setting up the ESD control program. In addition, these data are used to demonstrate compliance of the ESD protection during surveys and audits. The types of devices exhibiting ESD sensitivity include the following: ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

BJTs (bipolar junction transistors) CCDs (charge-coupled devices) CMOS (complementary metal-oxide semiconductor) devices GaAs (gallium arsenide) devices Hybrid microcircuits Integrated circuits JFETs (junction field-effect transistors) Laser diodes Magnetoresistive heads MCMs (multichip modules) MEMSs (microelectromechanical systems) Microwave devices MLC (multilayer ceramic) MMIC (monolithic microwave integrated circuit) MOS (metal-oxide semiconductor) Operational amplifiers Oscillators Photomasks Piezoelectric crystals Resistor networks/chips Silicon rectifiers Small signal diodes Surface acoustic wave devices Thin-film displays

REQUIREMENTS OF ESD CONTROL

69

Passive electronic components such as resisters and capacitors traditionally were not considered to be ESD sensitive. However, many discrete passive devices today are thinfilm devices manufactured using the same types of photolithography processes used to make logic and memory integrated circuits and thus are ESD damage sensitive. After ESDsensitive devices have been assembled into printed circuit boards or other subassemblies, there is a widespread misconception that they are less susceptible to damage by ESD. This is erroneous. The printed wiring board can store much more charge than the discrete component itself, and the conductive traces on the board can act as antennas to collect charge and direct it to the ESDS devices. State-of-the-art printed wiring boards can themselves be sensitive to ESD damage when they consist of multiple layers of metal separated by thin layers of dielectric insulation. Subassemblies can eventually be combined into systems. Even when these systems are contained within metal chassis, ESD damage can still occur if the input and output connectors are not properly protected. There is another important consideration. The ESD sensitivity of a device and the probability that a damaging ESD event can be delivered to the device can vary depending on its state of manufacture. Two familiar examples illustrate this point. Semiconductor chips on a wafer are relatively immune to damage because it is difficult to deliver a discharge to them. Conversely, in back-end processes, such as packaging and testing, the presence of the lead wires makes them highly vulnerable to ESD [17]. Magnetoresistive heads are relatively robust prior to lapping, which thins the ESD-sensitive elements of the heads. After lapping MR heads acquire their ESD sensitivity, but it is still relatively difficult to deliver an ESD event. Conversely, after head wires are attached, it becomes relatively easy to deliver a damaging ESD event. Once the items that are susceptible to damage by electrostatic discharge have been identified, it is necessary to determine the electrostatic discharge sensitivity of the devices.

2.4.1

Determining ESD Damage Sensitivity

The first step in the engineering of an ESD-controlled workplace is to determine the ESD sensitivity of the most sensitive parts to be handled in the ESD-safe workplace. Once the ESD sensitivities of the devices have been defined, it is then possible to build a suitable ESD-safe workplace and establish procedures for its operation. Three general approaches to determining ESD sensitivity are available: Testing the ESD-sensitive components using the four accepted ESD testing sensitivity models; inquiring from the device manufacturer what the ESD sensitivity is; and finally, estimating the sensitivity based on analogy with similar structures for which the ESD sensitivity is known. The most direct method for determining ESD sensitivity is to test the devices using the four ESD simulation models. There are four commonly accepted modes of damage due to electrostatic discharge, represented by four models for conducting these tests: ● ● ● ●

Human body model (HBM) [18] Machine model (MM) [19] Charged-device model (CDM) [20] Transmission-line pulse (TLP) modeling [21]

The ESD sensitivity of the devices should be reported using at least one and preferably all three of the HBM, MM, and DCM test models. The various classes of HBM, MM, and

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FUNDAMENTALS OF ESD CONTROL

TABLE 2.4 Current and Past ESD Sensitivity Classes Current ESD Model

Class

Past

Voltage Range (V)

Class

Voltage Range (V)

0 1A 1B 1C 2A 2B 3

250 250 to 500 500 to 1000 1000 to 2000 2000 to 4000 4000 to 8000 8000

MM

M1 M2 M3 M4 M5

25 to 100 100 to 200 200 to 400 400 to 800 800

CDM

C1 C2 C3 C4 C5 C6 C7

125 125 to 250 250 to 500 500 to 1000 1000 to 1500 1500 to 2000 2000

C1 C2 C3 C4 C5 C6 C7

0 to 124 125 to 249 250 to 499 500 to 999 1000 to 1499 1500 to 2999 3000

HBM

1

0 to 1999

2

2000 to 3999

3

4000 to 15,999

M1 M2 M3 M4 M5

0 to 100 101 to 200 201 to 400 401 to 800 800

CDM models are shown in Table 2.4. The classifications based on old versions of the ESD Association test methods are included for comparison. The TLP model is becoming increasingly popular. However, there currently is no agreed-upon standard for TLP testing, but rather several divers commercial and “in-house designed” systems are in use. Device classifications based on TLP modeling are thus not standardized. The most significant change is to definition of class 0 HBM parts. The precautions required to protect devices with ESD sensitivity below 250 V are considerably more elaborate and expensive to implement than the precautions acceptable to protect a device with ESD sensitivity of 2000 V. Mathematics of Electrostatic Discharge Very few simple mathematical relationships are required for a basic understanding of electrostatic charge and electrostatic discharge control. One of the simplest is Ohm’s law, which relates current, voltage, and resistance: E  IR

(1)

where E is the electromotive force in volts, I the current in amperes, and R the resistance in ohms. One way to think of Ohm’s law is that if a potential difference (electromotive force) of 1 V is applied across a resistance of 1 , a current of 1 A will result. Power in simple electrical devices can be calculated using the following equation: W  EI

(2)

REQUIREMENTS OF ESD CONTROL

71

where W is the power in watts. A 100-W light bulb on a 110-V ac line draws a current of 1.1 A. Substituting E from equation (1) into equation (2) results in W  I2R

(3)

A 100-W light bulb drawing a current of 1.1 A has a resistance of 82 . It is often informative to know how much energy is expended in a given ESD situation. The energy is equal to the integral of the power times the time: H  WT

(4)

where H is the energy in watt  seconds and T is the time in seconds. The charge stored in a capacitor is equal to the capacitance times the voltage: Q  CV

(5)

where Q is the charge in coulombs and C is the capacitance in farads. The energy stored in a capacitor is given by the expression U  12 CV 2

(6)

where U is the energy in watt  seconds, C the capacitance in farads, and V the voltage in volts. Note that 1 W  s  1 J. Mathematical Prefixes Picafarads, nanojoules, megohms, . . . . What language is this? It is the language of scientific and engineering notation. Most of the basic units of measure— ohms, farads, coulombs, joules, and so on—were defined long ago. The basic definitions of these units were are based on simple numerical relationships, often a lot of them. For example, a potential of 1 V across a 1- resistance will allow 1 A of current to flow. One farad is equal to 1 Coulomb/V, the capacitance of a capacitor that requires a charge of 1 Coulomb to establish a potential difference of 1 V between its plates. In today’s modern electronics, we must deal with very large and very small numbers. Ordinarily, this would mean we would need to use a lot of zeros to represent the numbers. If you are a millionaire, you measure your wealth in millions of dollars: $1,000,000, for example. You have the megabucks! Mega is a prefix meaning million. The prefixes in front of basic units are a shorthand way to represent zeros when we speak. We also use scientific and engineering notation when writing very large and very small numbers. For example 1,000,000 equals 1  106, a 1 followed by six zeros. (Generally, the power of 10 is divisible by 3.) Thus, 10,000,000  equals 10  106  or 10 megohms (M). (Yes, for every convenient rule there are inconvenient exceptions. When we talk about millions of things, we generally use the prefix mega—hence we say “megabucks” or “megawatts.” Ohms is an exception: We drop the “a” from the mega prefix and say “megohms.”) Table 2.5 is a convenient summary of the prefixes and what they mean. Notice that when written out longhand, groups of three zeros are separated by commas in the United States but are separated by spaces in many European countries. In very small numbers, groups of three zeros are always separated by spaces. You can see the value of the shorthand: 4.7 pF would be 0.000 000 000 004 7 F if written out; 4.7 pF could also be written 4.7  1012 F in engineering notation. Also notice that one of the prefix symbols is a Greek letter: lowercase mu ().

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FUNDAMENTALS OF ESD CONTROL

TABLE 2.5 Mathematical Prefixes Scientific or Engineering Notation 18

10 1015 1012 109 106 103 102 101 100 101 102 103 106 109 1012 1015 1018

Written Form

Prefix

Symbol

1,000,000,000,000,000,000 1,000,000,000,000,000 1,000,000,000,000 1,000,000,000 1,000,000 1,000 100 10 1 0.1 0.01 0.001 0.000 001 0.000 000 001 0.000 000 000 001 0.000 000 000 000 001 0.000 000 000 000 000 001

exa peta tera giga mega kilo hecto deca

E P T G M k h da

deci centi milli micro nano pico femto atto

d c m  n p f a

Human Body Model The human body model is the oldest and the most widely accepted. The simplified circuit for the human body model simulator is shown in Figure 2.4. The human body model test is intended to model discharge through the human hand. In the human body model, the capacitance of the human body is represented by a 100-pF capacitor. The actual capacitance of the human body varies from less than 100 pF to more than 500 pF. This must be kept in mind when interpreting the results from human body model simulation of ESD sensitivity. The charge on the capacitor is delivered through a 1500- resistor. As with capacitance of the body, skin resistance at the tip of the finger varies from person to person. Not shown in the simplified schematic is the inductance of the discharge line through the resistor and switch. Inductance is kept as low as possible by design and generally is less than 0.1 microhenry (H). The human body model circuit is connected to the device under test and to a highvoltage power supply, as shown in Figure 2.4. The procedure is to charge the capacitor using the power supply and then discharge the capacitor through the 1500- resistor through the device under test. The capacitor is initially charged to a low voltage to test the ESD sensitivity of the device. The charge on the capacitor is gradually increased until an adverse affect is seen on the device. Usually a sample of three devices are tested at each voltage and are replaced with each step increment of voltage. This is done because testing at each voltage may wound the device under test, increasing its susceptibility to fail at the next voltage level. Machine Model The machine model is also widely accepted. A simplified circuit for the machine model simulator is shown in Figure 2.5. The machine model test is intended to model the discharge through the conductive surface of tooling, transport equipment, carts, and so on. The schematic for the machine model is very similar to that for the human body model. In the machine model, the capacitance of the portion of the machine making contact with the ESD-sensitive device is modeled as a 200-pF capacitor. The actual capacitance of

REQUIREMENTS OF ESD CONTROL

Charge body

High-voltage power supply

73

Discharge body

C = 100 pF (body capacitance) R = 1500 (body resistance)

FIGURE 2.4 Simplified schematic for the human body model ESD damage simulator.

Charge machine

High-voltage power supply

C = 200 pF (machine capacitance)

Discharge machine

Nil-resistance

FIGURE 2.5 Simplified schematic for the machine model ESD damage simulator.

the portion of the tool contacting the ESD-sensitive device may be far different than the capacitance used in the accepted model. The charge on the capacitor is delivered through a short piece of wire having nearly zero resistance. That is, the surface that comes in contact with the ESD-sensitive device in the machine model is assumed to be conductive. Again, the inductance of the circuit is kept below 0.1 H. The machine model simulator circuit is connected to the device under test and to a highvoltage power supply, as shown in Figure 2.5. The procedure is to charge the capacitor using the power supply and then discharge the capacitor through the short length of nilresistance wire through the device under test. The same procedure is used with the machine model simulator that is used with the human body model simulator. The charge on the capacitor is gradually increased until an adverse effect is observed on the device under test. Charged-Device Model In the human body model and the machine model the conductors on the device being tested are physically connected to the simulation tester. This is intended to simulate direct contact by a person or machine to the conductors on the device. The charged-device model is intended to measure the sensitivity of an ESD-sensitive device to exposure to external electrical fields without charge being delivered through direct physical contact. In charged-device simulation, the object is placed in an external electrostatic field. The connectors on the device are contacted by a ground wire. The external field strength is gradually increased until an adverse effect is observed on the device under test. In general, the machine model ESD sensitivity will be a lower voltage than the human body model sensitivity, because the machine model employs a larger capacitor (storing

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FUNDAMENTALS OF ESD CONTROL

more charge) and discharges through a much smaller resistance (allowing higher current). In turn, the human body model ESD sensitivity will generally be a lower voltage than the charged-device model ESD sensitivity, because the ESD-sensitive device usually has a much lower capacitance than that of the human body and can store less energy than the capacitor in the human body model simulator. However, the structures of modern electronic devices are becoming so small that charged device failures are becoming the dominant failure mode in some industries. These ESD-sensitivity tests are destructive. In addition, relatively large numbers of parts must be tested to estimate the ESD sensitivity of the parts accurately. On reason for this is the phenomenon of conditioning. Parts tested at voltages below their true ESD sensitivity may be slightly damaged in an undetectable fashion. This conditioning predisposes the parts to fail at voltages below their inherent ESD sensitivity. Thus, each time a new, higher voltage is applied, a new set of parts is tested. Additional parts are then tested to verify the ESD sensitivity. As can be seen from this description a large number of parts are sacrificed in the tests. Another point is important to consider. The observation of conditioning implies that repeatedly subjecting a part in rapid succession could cause damage below the inherent ESD sensitivity of the part. In some industries, the number of available ESD-sensitive devices is limited. Sacrificing parts in simulation model tests may not be an option since the simulation model tests are destructive. If insufficient parts are available to perform sacrificial tests, other alternatives to defining the human body, machine, and charged-device model should be used. Manufacturers of commercial off-the-shelf (COTS) electronic devices usually know the ESD sensitivity of the devices they manufacture. They need to know the ESD sensitivity of their products so that they can prepare their manufacturing work area to prevent damage from occurring. Thus, where a customer is using COTS devices, the ESD sensitivity may already be known to the manufacturer. There are some devices for which the ESD sensitivity is unknown and for which so few devices are available that destructive tests are not a feasible option. In this case it is often necessary to estimate the sensitivity by comparing the structures and materials on the unknown device to similar structures on devices with known ESD sensitivity. For example, aluminum oxide insulation layers are typically damaged by 0.1 V per 1 angstrom (Å) of film thickness (1 V/nm) using the HBM challenge. 2.4.2

Electrically Explosive Device ESD Modeling

The ESD sensitivity of electrically initiated explosive devices, generally referred to as electroexplosive devices (EEDs), is a valid concern in many industries. Such devices are common in aerospace applications. With the widespread use of airbags, they are common in modern automobiles. These are tested using the machine, human body, and charged-device models, but using different values for resistance and capacitance than are normally used with semiconductor and thin-film device tests. Table 2.6 compares the resistances and capacitances used in ESD-sensitivity testing of EEDs vs. those used in ordinary ESDS electronics. The results of tests for EEDs and electronic ESDS devices are surprising different. Generally, EEDs are tested at 25,000 V in the EED HBM test. This corresponds to an energy discharge of roughly 0.16 W  s (16 mJ). By comparison, many modern electronic devices can easily be damaged by 100 V on the electronic ESDS HMB test. This corresponds to roughly 500 nJ. The charged-device model test of an EED is usually the most severe, whereas the charged-device test for an electronic ESDS is usually the least severe. This difference is attributable to the fact that EEDs usually have high internal capacitance

BUILDING THE ESD-SAFE WORKPLACE

75

TABLE 2.6 Comparison of Resistances and Capacitances for ESD Sensitivity of EEDs vs. Electronic ESDS Devicesa Electroexplosive Devices Model

Capacitance (pF)

Human body HBM variation Machine Charged device

b

500 330d 500b n.a.

Resistance () b

5000 150d 0b 0

Electronic ESDS Devices Capacitance (pF) c

100 n.a. 200e n.a.

Resistance () 1500c n.a. 0e 0f

a

n.a., Not applicable. MIL-STD-322B-1984, MIL-STD-1512-1972 and MIL-STD-1576-1984. c ANSI/EOS/ESD STM5.1. d IEC 801-2-1991. This model is intended to represent a person discharging through a screwdriver or other metal object. e ANSI/EOS/ESD STM5.2. f ANSI/EOS/ESD STM5.3. b

and can store a great deal of energy, whereas electronic ESDS devices have small internal capacitances compared to the capacitances within the HBM and MM apparatus.

2.5

BUILDING THE ESD-SAFE WORKPLACE

The work areas where items are handled need to be designated as ESD-safe work areas. ESD-sensitive devices must only be handled in ESD-safe work areas. Some of the requirements for the ESD-safe work area are listed below. 2.5.1

Surface Resistivity of Materials

After the human body, machine, and charged-device model ESD sensitivities of a device have been defined, it is possible to engineer an ESD-safe workplace and the procedures to be followed therein. In engineering an ESD-safe workplace, one of the most important properties for the selection of materials is the surface resistivity of the materials to be used. Since static charge resides on the surfaces of materials, the surface resistivity of materials becomes critical in controlling the charge on those surfaces and in designing a static control program. For static control purposes, materials are classified into three categories: conductive, static dissipative, and insulative. Conductive and static-dissipative materials allow current to flow at relatively fast rates. Insulative materials pass current so slowly that they are considered to be nonconductive. Materials are defined by their surface resistivity and bulk resistance. The term antistatic is misused in the description of materials for use in the static-safe workplace. In the past, a material labeled antistatic was assumed to be incapable of becoming charged. This misconception regarding terminology gradually led to the term being abandoned as too badly defined and misleading. Today, other assumptions are made about the terminology regarding materials’ discharge properties. For example, some people assume that a static-dissipative material would be incapable of generating a charge during contact and separation. Any material can become charged, regardless of its surface resistivity. Ungrounded conductive surfaces or ungrounded static-dissipative surfaces can become charged as easily as insulative materials. The difference between charging of static-dissipative or conductive materials and insulative materials is

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FUNDAMENTALS OF ESD CONTROL

TABLE 2.7 Resistance Ranges and Materials Classification Materials Class Conductive Static dissipative Insulative

Surface Resistivity Range (/sq)

Bulk Resistance (/cm)

Less than 106 Greater than 106 to less than 1012 Greater than 1012

Less than 105 Greater than 105 to less than 1011 Greater than 1011

Discharge Rate Extremely fast Moderately fast Extremely slow

that charge does not remain localized on static-dissipative or conductive materials as it does on insulative materials. Thus, the same amount of charge (number of coulombs) applied to the surface of a static-dissipative or conductive surface results in the entire surface having a low and uniform voltage (as measured using a field potential meter) distributed uniformly over the object’s surface, whereas an insulator is found to have localized hot spots. Surface resistivity is an important characteristic of materials for electrostatic discharge control, because many ESD applications require that charge be dissipated through the surface of materials. The lower the surface resistivity of a material, the faster it will discharge. Table 2.7 defines surface resistivity and bulk resistance for the three classes of materials: conductive, static dissipative, and insulative. Rigorously speaking, the rate at which an object is discharged is a function of both the resistivity and the electrical permitivity of the material from which the object is made [22]. This is expressed mathematically as   0et/ where   charge density, in Coulomb/m2 at any time 0  charge density at time t  0 t  time, in seconds ε  electrical permitivity or dielectric constant of the material   resistivity of the material The dielectric constant of most materials is in the range 2 to 10. By comparison, the volume or surface resistivity of materials varies over more than 16 orders of magnitude, so for all practical purposes one need consider the resistivity of materials only when characterizing them from the perspective of their discharge rate. Each of these classes of materials has utility in the ESD control program. Table 2.8 lists some conductive, static-dissipative, and insulative materials commonly found in ESDprotected workplaces. Conductive materials are not favored for surfaces that come in direct contact with ESDsensitive devices because they do not provide a safe path for controlled discharge of devices that may have become inadvertently charged. However, conductive surfaces are easy to neutralize by grounding, a desirable property for controlling charge in an ESDcontrolled workplace. In addition, conductive materials can be used to provide static shielding packaging. More about each of these subjects later. Controlling Static Charge on Conductive and Static-Dissipative Materials The basic premise behind controlling static charge on conductive materials is grounding. If the conductive surface is connected electrically to ground, charge will be eliminated.

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77

TABLE 2.8 Common Materials Found in High-Technology Manufacturing Environments Insulative Materials Plastic wafer carriers Tapes (wafer tapes, etc.) Painted surfaces

Static-Dissipative Materials

Conductive Materials

Modified plastics Specially designed work surfaces Silicon wafers

Stainless steel tables Tooling People

Occasionally, it is not possible to ground a conductive surface: Occasionally, this situation is encountered for portions of test equipment. Where grounding cannot neutralize charge on a conductor, air ionization must be employed. Controlling Static Charge on Insulative Materials Items that are insulative must be dealt with differently. Because the charge will not move on these materials, grounding them cannot neutralize static electric charge. The most effective method in preventing charged insulative materials from creating damage in a manufacturing environment is to remove them from the process completely. In many cases, insulators are used in critical applications, and a conductive or static-dissipative material would work as well or better in the application. Plastics manufacturers are continuously developing new materials for the static control industry. Unfortunately, removing insulative materials from the process is not always feasible. Often, even the products we are trying to protect are made of insulative materials—it is nearly impossible to make wires without insulated coatings. In these cases, the only solution to controlling static on these materials is to install ionization. This topic, as well as details on grounding, are covered later in the chapter. Insulative surfaces are not favored for use in an ESD-controlled workplace because they retain charge for such a long time that there is an increased probability that a field-induced ESD damage could occur. However, many of the items that must be used in an ESD-controlled workplace are made from insulative materials, so insulative materials must be used. Static-dissipative materials are considered to be most suitable for an ESD-controlled workplace. Static-dissipative materials have high enough resistance that they will discharge charged devices slowly. In addition, static dissipative surfaces can be grounded, so static charges can be relatively rapidly neutralized, minimizing the risk that an ESD event will occur from them. 2.5.2

Grounding

Grounding is the foundation of any static control program. In addition to being the conduit through which the entire ESD program is based, poor grounding principles can result in safety and electromagnetic interference problems. It is advisable for personnel designing a facility to consult not only with architects and mechanical/civil engineers but also with a good ESD and RF engineer to ensure that adequate grounding is supplied. A poorly designed grounding system can become a conduit for noise to enter the facility and disrupt sensitive equipment. Good design incorporates the following items: ● ●

Grounded static dissipative or conductive flooring and appropriate footwear Grounded work surfaces

78

FUNDAMENTALS OF ESD CONTROL ● ● ● ●

Grounded wrist strap connection (with monitoring as an option) Properly installed and maintained air ionization system Relative humidity control Signs and markings identifying the area as a static-safe work area, prohibiting the entry of personnel who are not certified to work in an ESD-safe work area

2.5.3

Identification of and Access to an ESD-Safe Work Area

An ESD-protected work area should be clearly identified by prominently placed signs and markings. This can be accomplished by signs on entrances, by floor markings, and by workstation signs. A partition, rope, or chain may also be used if properly labeled and can be especially useful to designate temporary work locations or field operations. Access to ESDprotected work areas should be controlled and limited to certified personnel. All noncertified personnel accessing an ESD-protected work area should be escorted continuously by certified personnel. When unoccupied, ESD-protected work areas should be locked. Access to workstations where unprotected ESD-sensitive items are located should be limited by a protective perimeter. The protective perimeter is set at some distance from the location of the ESD-sensitive parts to prevent access by personnel and to define the limits on charge that will be tolerated on personnel and objects close to the ESD-sensitive items. In practice, the choice of a protective perimeter is sometimes set arbitrarily. For example, in the aerospace industry, many follow the NASA practice of a 1-m perimeter inside which charge is limited to less than 200 V. Conversely. ANSI-ESD Standard S20.20 recommends that charge be limited to less than 2000 V within 12 in. Prohibited Materials and Activities An ESD-protected work area must be maintained in a clean and orderly fashion. Prohibited materials include personnel items such as purses and briefcases. Materials that have not been qualified for use in an ESD-protected work area should be purged if found. In general, all equipment not essential for conducting work at an ESD-protected workstation should be removed, even if it is otherwise qualified as acceptable for use in an ESD-protected work area. Smoking, eating, and drinking are prohibited.

2.5.4

ESD-Protective Floor Coverings

In building an ESD-protected workplace, it is logical to start with the floor. Flooring is available in both static-dissipative and conductive forms. To simplify this discussion, staticdissipative and conductive floors will together be termed grounded flooring. The decision regarding the use of a grounded floor in a facility is an important one. The installed cost of a grounded floor is typically $5 to $7 per square foot, making the floor one of the most expensive parts of the construction of an ESD-protected work area. The decision to use a grounded floor must be made based on both product sensitivity to ESD damage and the processes to be supported in an ESD-protected work area. For example, if the use of a wrist strap becomes restrictive or dangerous, personnel must be grounded through their feet, and grounded flooring is mandatory. Keep in mind that the effectiveness of grounding through footwear and the floor is compromised if a person is not standing. Thus, in workplaces where there is a combination of sitting and standing workstations, a sit–stand protocol must be adopted. In the sit–stand protocol, it is mandatory for the operator to be plugged in using a wrist strap when sitting, and use of a wrist strap is optional when standing.

BUILDING THE ESD-SAFE WORKPLACE

79

If the operations in the process are mostly standing operations, a grounded floor provides the most convenient grounding method for the workforce. A grounded floor also provides a convenient method for grounding rolling equipment such as carts and tables, and furniture such as chairs and stools. Grounded flooring and footwear grounders are mandatory if wearing a wrist strap might be hazardous to personnel and processes. For example, in a machine shop there are many types of rotating tools. A wrist strap cord that gets tangled in a drill bit could damage the work being performed or could act like a whip, resulting in personal injury. In a solder reflow operation a wrist strap cord could get tangled in the chain conveyor. The wrist strap cord will unsnap from the wrist strap, so direct physical injury to the wearer will not occur, but the cord could cause damage to the reflow equipment. A grounded floor and footwear simplify the process of moving sensitive products from one location to another. Wrist straps limit how far a person can move away from his or her workstation while being grounded. If a person needs to carry an ESD-sensitive item from one workstation to another in a facility without conductive flooring, he or she would have to package the device in a shielding container before removing the ESD-sensitive device from the workstation. If the person is grounded via the grounded floor, the operator can move freely with sensitive products anywhere within the facility. Types of Floors Grounded flooring is available in many forms and materials. Everything from raised floor access panels to carpeting is available for flooring. Numerous materials are also available for flooring. The additives that make a floor groundable are almost always carbon based. The most common base materials are vinyl, rubber, and epoxy, each of which has strengths and weaknesses, all of which will be pointed out by the various suppliers. As mentioned earlier, carbon in one form or another (i.e., fibers, powders, etc.) is typically used to make a floor groundable. The conductivity of a floor is defined by the type and amount of conductive material present in the matrix. A careful study by the purchaser is necessary to ensure that the right material is chosen. The resistance and triboelectric voltage generation characteristics are also widely variable. The electrical properties of floor materials should be evaluated carefully (see Chapter 3), as should the chemical properties for environments that could be damaged by chemicals released by the floor materials. The chemical compatibility of the floor should also be checked against the chemicals used in the workplace, to make sure that they will survive exposure to occasional spills, cleaning chemicals, and other intended or unintended exposures. In an existing facility that has an insulative floor, the floor can be made temporarily static dissipative by coating it with a static-dissipative floor sealant. Topical floor treatments such as waxes should typically not to be used in cleanrooms, as they sometimes do not adhere well to the floor and may contain chemical contaminants incompatible with a cleanroom environment. If one chooses to use a dissipative coating such as a floor wax, a rigorous maintenance schedule must be maintained. These coatings are truly temporary, and the user must understand the life of the material, especially in high-traffic areas. The floor coating will occasionally need to be stripped and reapplied. The chemicals used to strip waxes and other coatings need to be evaluated to make certain that they are compatible with the products and processes in the workplace. Floor Installation Installation of the floor is critical to the final electrical performance of the floor. Again, this is not the place to cut corners. A poorly installed floor will end up performing poorly, and its cosmetic appearance may well be the subject of many “who did this?” meetings! Vinyl and rubber flooring utilize adhesives that are conductive and must be

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FUNDAMENTALS OF ESD CONTROL

applied very carefully. Ensure that whoever is installing the floor has been trained by the floor manufacturer so that the floor manufacturer will honor the warranty. Epoxy floors are typically poured in place, and again, the manner in which this is done is critical to the final properties. A critical part of the installation is the grounding grid that is used to connect the floor to the grounding system. Typically, the grounding grid consists of copper tape laid in conductive epoxy, which is then connected to the ac grounding system. The floor manufacturer commonly defines the distance between the copper grids. Make sure that whoever installs the floor follows the manufacturer’s recommendations closely. Many reputable flooring companies will send a representative to ensure that the floor is installed properly. This can sometimes be negotiated during purchase of the floors. Temporary floors such as mats or carpet are often installed using double-sided tape formulated specially for use in this application. One side of the tape has a more permanent adhesive that is applied to the temporary floor. The less permanent adhesive is then used to attach the temporary floor to the room. Then, if the temporary floor is removed, minimal tape and adhesive residue will be left behind in the room, simplifying cleanup. The floor is typically grounded to the facility ground using copper tape. Maintenance of Floors Dirt on grounded floors can increase the resistivity of the floor so that it fails to remain static dissipative. Floors should be kept clean through regular wet mopping, the mopping interval being based on the results of regular periodic testing. Cleaning floors using sticky roller mops has often been found to be inadequate. Temporary coatings must also be maintained properly. Resistance Guidelines for Floors The resistance of a floor is a critical parameter, as it defines how well the floor will perform in grounding personnel and items in contact with it. Flooring can be purchased with resistance ranging from 102 to 1012  (or greater). Some companies have even installed metal floors in hypercritical and specialized applications (e.g., munitions). The low end of the conductive range is typically limited by safety regulations. The ESD coordinator typically dictates the high end of the resistance range. The best guidance on floors for critical areas is to install the lowest resistance floor that is feasible. The National Electrical Code (NFFA 70) recommends a minimum of 105 /sq surface resistivity. One needs to consider many other things that affect the final resistance. The choice of footwear, dirt and debris on the floor and footwear, and even how much operators perspire in their shoes all affect the final resistance. Floors with a starting resistivity greater then 109 /sq surface resistivity will become insulative through accumulation of contamination much more rapidly than will a floor with starting resistivity less than 109 /sq. Floors with a resistivity greater than 109 /sq require more frequent cleaning and inspection than do lowerresistivity floors. More critical applications have specified floor resistivity below 106 /sq. Case Study: Floors In audit of a large aerospace ESD-protected work area that was also a class 10,000 cleanroom, it was found that a static-dissipative floor measured greater than 1012 /sq. The spot that had been tested was cleaned with isopropyl alcohol and a cleanroom wiper and retested. It was now found to measure 109 /sq. The floor-cleaning procedure in this room consisted of daily mopping using a sticky roller mop. A more conventional cleaning procedure would have included wet mopping. Sticky roller cleaning is effective for large particle removal but not for small particle removal and was thus ineffective at preventing the floor from failing because of inadequate cleanliness.

BUILDING THE ESD-SAFE WORKPLACE

2.5.5

81

Work Surfaces and Table Mats

An ESD-safe workstation is a workstation where unprotected ESDS items can be handled safely. A workstation may be a single workstation in a floor area (which may or may not be an ESD-protected work area itself), or it may be an entire set of workstations that are linked together by a grounded floor or a grounded automated handling system. It could also be an entire room in which many different parts are handled, assembled, and packaged. The first critical consideration is that the workstation be designed to limit and control the static electricity that is generated during the handling of components and that safe means of dissipating charged objects be provided. A number of things go into the proper design of an ESD-protected workstation. The second critical consideration is that once the workstation is designed and installed, static control tools must be used by everyone and used properly. This is where training comes in. The final is that static-controlled workstations must be checked on a regular basis to ensure that they continue to work properly after they are installed. Recall that all conductive and static-dissipative materials are grounded to keep them at the same potential. To accomplish this on a workstation, a common point ground is frequently used. A common point ground is a single location on a workstation to which all of the conductive and static-dissipative items on the workstation are connected. One connection is then made from the common point ground to the facility grounding point, preferably the ac reference ground. Figure 2.6 is a typical workstation that is grounded properly. If operator safety is a concern, the ac power should be provided through a ground-fault-circuit interrupter (GFCI).

Main service panel circuit breaker

120-V outlet

Neutral Hot Ground

White Black Green Main bonding jumper

Static-dissipative mat

Neutral Hot Ground Earth grounding electrode

R

ESD ground

Wrist strap jack

Grounded floor mat (optional)

FIGURE 2.6 Typical workstation grounding scheme.

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FIGURE 2.7 Daisy chain grounding.

For ESD-protected work surfaces in cleanrooms, the work surface should not be perforated stainless steel. The work surfaces should be connected directly to the ac reference ground; they should not be connected through a 1-M resistor. Where safety is a concern, the outlet to which the workstation is connected should be the ac reference ground of a GFCI. However, keep in mind that a grounded conductive surface is not always considered to be safe for contact with an unprotected ESD-sensitive device. Conductive surfaces allow too rapid current flow to discharge charged objects safely. For this reason, grounded static-dissipative materials almost always cover conductive work surfaces for ESD applications. Several types of static-dissipative table surfaces are available, including laminated work surfaces and vinyl or rubber table mats. Where needed for shock absorption, vinyl ESD table mats may be a good alternative. These must be grounded, preferably through a workstation monitor. Unfortunately, vinyl table mats tend to perform poorly at temperature extremes. Vinyl mats can be burned or melted by soldering operations, and low-temperature operations can embrittle vinyl mats. For high- and low-temperature extremes, it is often preferable to use hard rubber mats. ESD-safe workstations should be equipped with a grounding bar or other common ground path. This should be visible to the user of the workstation from the front of the workstation. All individually grounded items should be routed to the common ground bar. A single ground wire from the workstation to the ac reference ground should terminate the circuit. Individual workstations should not be interconnected, daisy chain fashion, to a single ground point (Figure 2.7). One of the risks is that if one of the connections between workstations is broken, several workstations may become ungrounded. Avoiding daisy chain grounding eliminates that risk. An argument can be made for specifying a minimum resistance for a work surface. This makes sense in operations where direct contact between the work surface and the ESDsensitive device can occur. Examples of these types of work areas include printed circuit board assembly, packaged IC assembly (in particular, ball-grid-array, pin-grid-array, and similar packaging). In these processes, the devices are routinely set down on the work surface, and if they are charged (which is almost always the case) at the moment of contact with a conductive work surface, a charged-device model ESD event can occur. If that work surface were made of a static-dissipative material rather than a conductive material, the discharge would move from the device to ground in a more controlled fashion and thus be less likely to damage the device. Fallacy of the Workstation Ground Resistor It has often been assumed that placing a 1-M resistor between the conductive table, rack, or shelves and the facility ground will limit the current flow and provide electrostatic discharge protection to ESD-sensitive devices. The use of a 1-M resistor between the work surface and facility ground is unwise for two reasons. A large conductive surface can be thought of as a large vessel for electron flow. The surface will absorb a large amount of charge, so grounding through a current-limiting

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resistor may not limit the initial current flow. In addition, the resistor will limit potentially hazardous currents between the work surface and the facility ground, defeating the protection intended by the circuit breaker. The use of a 1-M resistor between work surfaces and the facility ground is probably a misinterpretation of the purpose of the 1-M resistor in the wrist strap or footwear, which is to protect personnel from electrical shock. The resistance is not provided to protect ESDsensitive items from excessively rapid discharge; that is provided by other means. If a building’s electrical ground system is designed properly, there should be no need to install an ESD ground system separate from the building’s electrical ground. Resistance between the workstation ground and the facility ground is generally very small. These lowresistance grounds are often referred to as hard grounds. Typically, a surface is considered to be hard grounded if its resistance to the facility ground is less than 10 . Earth grounds are commonly used as ESD grounds in place of the facility ground, but this can be accomplished only if the resistance between the earth ground and the facility ground is small, typically less than 10 . This ensures that the potential difference between equipment plugged into ac power and items grounded for ESD protection will be minimal. 2.5.6

Wrist Strap Ground Points

The wrist strap ground point should be connected directly to the facility ground. Although at first this may seem to present a safety hazard, it does not. The cord in the wrist strap contains a 1-M current-limiting resistor. This resistor prevents the wearer from becoming electrically shocked, should he or she accidentally plug into a ground point that has power on it. The wrist strap ground point may be a passive plug-in point or may be part of a continuous monitoring system. Studies have shown that a significant percentage of the workforce is actually grounded through wrist straps only 20 to 30% of the time [24]. Several types of wrist strap monitoring systems are available. Some of these monitor resistance between the wearer and ground; others measure charge on the wearer’s body. The wrist strap monitoring system may be part of a larger monitoring system also designed to monitor workstation grounding, air ionizer performance, temperature and relative humidity, and other parameters. For example, monitoring systems are available to monitor virtually every contaminant imaginable. This subject is sufficiently complex that an entire chapter is devoted to it: Chapter 8. 2.5.7

Air Ionization Systems

Ambient air contains both positive and negative ions. Most of these are produced as a by-product of radioactive decay or bombardment by interstellar radiation such as gamma rays. Equal numbers of positive and negative ions are produced by these processes. The lifetime of these natural ions are relatively short, as they are rapidly attracted to their oppositepotential brethren. As a consequence, their concentration in ambient air is relatively low, and ambient air is a relatively good insulator (with a resistivity of approximately 1015 /m). Typical ambient atmospheres produce about 10 ion pairs per second per cubic centimeter. The small number of ions in the ambient atmosphere can be eliminated by neutralizing charge on surfaces. However, these ions can be also be removed by filters, especially the HEPA and ULPA filters used to clean the air in cleanrooms. Figure 2.8 shows an interesting illustration of this phenomenon. A 20-pF metal plate charged to 5000 V in an ambient atmosphere discharges to less than 500 V in about 30 minutes. The same charged plate

FUNDAMENTALS OF ESD CONTROL

Residual voltage

84

5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 34 Time (hours)

FIGURE 2.8 Discharge of an isolated charged plate in a unidirectional-flow cleanroom.

charged to 5000 V in a unidirectional-flow cleanroom discharges to less than 500 V in over 31 hours. The slow discharge in ambient atmospheres is inadequate to provide protection in the average workplace where there are ESD-sensitive devices, even given the presence of chargeneutralizing ambient air ions. This problem becomes acute when an ESD-protected workplace is simultaneously a cleanroom. For this reason, air ionizers form an important (in some situations, indispensable) portion of the ESD control program in a cleanroom. Ionizers create positive and negative ions in the air. Three types of air ionizers are in common use: radioactive air ionizers, corona discharge air ionizers, and photonic air ionizers. A radioactive air ionizer usually contains polonium 210 or krypton 85. (Americium 241 is used to ionize air in the familiar residential ionization smoke detector.) In corona discharge air ionizers, a high electrical field around a sharp needle creates air ions. Photonic air ionizers use short-wavelength ultraviolet light to ionize air molecules. Each of these types of air ionizers has advantages and disadvantages. The ions produced by the three types of air ionization systems are attracted to oppositely charged surfaces, neutralizing charges on these surfaces. Unfortunately, the air ions are also attracted to each other. The positive and negative ions eventually neutralize each other, limiting their effective range unless some means is provided to disperse them. In some designs of corona discharge air ionizers, separate emitters produce positive and negative ions. If one polarity emitter is mounted too close to a grounded conductor in the workplace, ions of that polarity can be lost to ground. If one polarity of ion is preferentially shunted to ground, an excess of the opposite polarity ion can exist, and the air ionizers may induce voltage imbalance in the workplace. For this reason, an ESD expert must be consulted during the layout or rearrangement of a workstation that is equipped with corona discharge air ionization. This problem is not a concern with radioactive or photonic air ionizers because air ions of both polarities are produced in the same volume of space. However, there are other concerns with radioactive and photonic air ionizers that must be considered. Radioactive Air Ionizers Radioactive air ionizers generally use an isotope of radioactive polonium, polonium 210 (210Po) to create the air ions. Radioactive air ionizers are inherently self-balancing, as they produce the same number of positive and negative ions. This eliminates the maintenance associated with corona discharge ionizers, where the ionizer must periodically be inspected, maintained, and balanced. Often, there is a safety concern associated with the use of a radioactive material in processing. Fortunately, with the radioactive sources used in air ionizers, the source is sealed,

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so that it is classified as a generally licensed radioactive device. This means that the radioactive material has been packed in a way that it would not be expected to release radioactive material even after a serious accident such as a plane crash. Many states still require periodic surveys of radioactive air ionizers for radiation leakage, partially offsetting their maintenance advantage for calibration. Incidents have occurred in which improperly manufactured radioactive ionizers leaked encapsulated polonium particles. The most widely publicized such incident, which occurred in the 1980s resulted in the recall of all of the company’s polonium air ionizers and ultimately resulted in the company’s withdrawal from the business of making radioactive air ionizers [25]. Air ions have a fairly limited range unless some means of moving air is provided. The positive and negative air ions recombine with one another and neutralize the ionization unless airflow is provided to spread them apart. This introduces a slight need to complicate a system in which radioactive air ionizers are used. Airflow can be provided by filtered compressed air or by fans to disperse the ions. In the absence of air movers, the effective diameter of ionization is about 6 in. Radioactive air ionizers are found to be particularly effective where the size and shape of a tool will obstruct airflow. In this case, even a fan-powered or compressed air radioactive ionizer may be limited in its effectiveness. Radioactive air ionizers without fans or compressed air can be extremely small, making it possible to fit them into places where other types of ionizers would not fit: for example, the pick-and-place device on tools. The tools can then carry the ionization to where it is needed to work, usually a very restricted area. Another advantage of radioactive air ionizers is that they are safe to use in explosive environments. However, they have a relatively short half-life, requiring periodic replacement. 210 Po has a half-life of approximately 134 days. Thus, after a year the ionizer produces only about 15% of the ions it produced when newly manufactured. Two years from the date of manufacture, the ionizer produces barely 5% of its original ion output. Fortunately, manufacturers of radioactive air ionizers are aware of this and emboss the manufacturing date on the ionizer label. The gradual decrease in efficiency may not be critical for all processes. The only way to be certain is to establish discharge-time control limits based on product sensitivity and the chance that hazardous charge levels will be generated in the workplace. Once these control limits are known, the performance of the radioactive air ionizer can be controlled to the end of its usable life. Once a radioactive air ionizer is exhausted, it must be replaced. In the replacement process, the old ionizer must not be discarded. It should be returned to the manufacturer, who knows how to dispose safely of the radioactive waste. Radioactive air ionizers are labeled as to the date of manufacture. The date is usually embossed indelibly on the label so that it cannot be erased accidentally. Photonic Air Ionizers Photonic air ionizers use short-wavelength ultraviolet light (typically, around 0.15 m wavelength) to ionize air. Photonic ionizers are inherently selfbalancing. However, ultraviolet radiation is known to cause skin cancer, especially at the high power levels needed to provide high-speed charge elimination. Ultraviolet air ionizers do not require airflow and thus are used in low-pressure reactors. Shielding must be provided to prevent exposure of personnel to hazardous ionizing radiation. Thus, photonic air ionizers must always be used enclosed within lighttight enclosures. Their use is thus limited to processes that can accommodate these types of enclosures. Corona Discharge Ionizers Corona discharge air ionizers are readily available. They require routine maintenance, which must be considered as part of their cost of ownership

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and is a factor affecting the selection of a particular manufacturer and model for a given application. They can go out of balance, adversely affecting the work area they are intended to protect. Even a self-balancing corona discharge air ionizer can eventually go out of balance. To create a corona, high voltages must be produced. The efficiency of production of air ions is a function of the electrical field gradient. The maximum field gradient is produced when a sharp needle is used as the emitter. Both positive and negative ions are produced in the corona. When the emitter is positive, negative ions are attracted to the emitter point and positive ions are repelled. When the emitter is negative, the positive ions are attracted to the emitter and negative ions are repelled. Thus, positive and negative emitters appear to produce only positive and negative ions, respectively. Just as the high field potential from a cathode-ray tube can damage an ESD-sensitive component, so can the high field potential from a corona discharge air ionizer. Thus, products should be kept well away from corona discharge air ionizers. In general, the effectiveness of the shielding for the emitters on a corona discharge air ionizer should be checked using a field potential meter before the ionizer is put into service. In addition, because the ionizer interacts with its environment, it should not be moved unless its performance is verified. This is an especially acute problem with hand held corona discharge ionized air blowoff guns. Guns with improperly shielded emitters can cause ESD damage. Performance of a corona discharge air ionizer can vary for many reasons, including fouling of the needles by foreign material to create fuzz balls, needle wear, component aging, and so on. As a consequence, routine periodic maintenance is required to keep the ionizers performing correctly. In addition, wear particles from emitter needles or contamination shed from fuzz balls contribute to contamination. The rate of erosion of emitters varies according to the material from which the emitters are made. Emitters made from single-crystal silicon and germanium have the lowest erosion rates and are generally materials of choice for class 1 or cleaner cleanrooms. Thoriated tungsten emitters have moderate erosion rates. Stainless steel emitters have the highest erosion rates. Emitter point erosion is observed with positively charged emitters. Several different types of corona discharge ionizers exist. In ac ionizers, the sinusoidal waveform of ac current is transformed to high voltage to produce both positive and negative ions. These ionizers are usually provided with fans to disperse the ions produced. Pulsed dc air ionizers are frequently used in cleanrooms with significant airflow from HEPA filters. Because they work with the HEPA airflow, they do not stir up contamination by introduction of turbulence. They can neutralize charges from 1000 V to less than 100 V in tens of seconds if mounted near the ceiling, typically 1 to 2 m above the work surface. Many pulsed dc ionizers can be programmed so that there is a period of no ion production, lasting from a few tenths of a second to several seconds, between the production of positive or negative ions. This allows like charges to diffuse away from one another and limits recombination of positive and negative charges. When mounted in front of HEPA filters in clean benches 6 to 24 in. from work locations on horizontal- or vertical-flow clean bench surfaces, discharge times of less than 10 seconds can be achieved. Steady-state ac or dc air ionizers generally require fans or filtered compressed air to disperse ions before they combine and neutralize themselves. They generally are used in locations where airflow is inadequate to disperse ions effectively. They can neutralize 1000 V to less than 100 V in less than 10 seconds. Airflow from the fans can cause turbulence in laminar flow cleanrooms, increasing the dispersion of contamination. Corona discharge ionizers are also used in compressed air lines for blow-off guns. Compressed air or nitrogen is generally used.

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Emitter points are available in a large number of materials. Stainless steel emitters are generally used where contamination control is not a major consideration. They are relatively inexpensive but have the highest wear rate and highest rate of contamination generation. Some ionizer manufacturers use narrow-diameter wires in place of precision-ground needles to compensate for the decrease in ionization efficiency that occurs when emitter needles become blunted by wear. Thoriated tungsten has a much lower sputtering rate than stainless steel and is thus preferred for contamination control applications. In general, stainless steel emitters are acceptable for class 100,000 and class 10,000 cleanrooms. Thoriated tungsten emitters are preferred for class 1000 and class 100 cleanrooms. Cleanrooms for class 10 and cleaner applications generally require the use of single-crystal silicon or germanium emitters. The processes that cause corona discharge air ionizers to go out of balance are complex. Positive emitters become eroded due to sputtering by air ions. The efficiency of production of air ions is a function of the radius of curvature of the emitter tip. As the tip becomes blunted, the production of positive ions decreases. The negative emitters become fouled with dust balls, but the dust balls do not originate as particles; a process called gas-to-particle conversion produces them. Since the efficiency of production of negative ions is also a function of the radius of curvature of the negative emitter tip, as the tip becomes fouled, production of negative ions goes down. Because the erosion of positive emitters and fouling of negative emitters does not occur at the same rate, the ionizers can go out of balance. In the 1980s, research at IBM indicated that erosion and fouling of emitter tips is catalyzed by water vapor in the air. Enclosing the emitter tips in a jacket of clean dry air or dry nitrogen eliminates both the erosion and fouling [26]. Today, clean air ionizers using sheath air systems are commercially available. The preferred systems for use in unidirectional flow cleanrooms, with air velocities in the range 0.45  0.05 m/s, are bipolar or rapidly pulsing dc air ionizers without built-in fans. The airflow in unidirectional flow workstations and cleanrooms is sufficient in most cases to achieve the desired ionizer performance while avoiding the turbulence associated with fanpowered air ionizers. Generally, these airflow conditions will exist in class 100 or better cleanrooms and unidirectional flow units. (Note: Where conditions in a unidirectional flow workstation prevent a bipolar or rapidly pulsing dc ionizer from achieving the required performance, a fan-powered air ionizer should be used.) For areas not equipped with unidirectional air flow, such as ordinary factory work areas or cleanrooms of the mixed airflow type, only fan-powered air ionizers will probably be capable of achieving the desired ionizer performance. Generally, cleanrooms of class 1000 or poorer will require the use of fan-powered air ionizers. Ionized air guns are also available. These use either radioactive sources or corona discharge sources to ionize the air. They can discharge surfaces very rapidly, typically discharging from 1000 V to less than 10 V in less than 2 seconds. Ionized air blow-off guns often are used to blow off particle contamination or to dry surfaces after cleaning. They are also very useful where charge must be eliminated quickly from large objects. Air Ionizer Performance Ionizer performance is specified by discharge times and float potentials. ●

Discharge times. For critical ESD-safe work areas, typically those in which the device has human body model ESD sensitivity of less than 50 V, the discharge time should be less than 20 seconds from 1000 V to less than 20 V. For highly sensitive but not

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●

critical ESD-safe work areas, where the human body model ESD sensitivity is greater than 50 V but less than 200 V, the discharge time should be less than 20 seconds from 1000 V to less than 50 V. For conventional ESD-safe work areas, such as printed wiring board assembly and testing, where human body model ESD sensitivity is greater than 200 V, the discharge time can be less than 45 seconds from 1000 V to less than 100 V. Float Potentials. For critical ESD-safe work areas, the float potential should be less than 20 V. For highly sensitive but not critical ESD-safe work areas, the discharge time should be less than 50 V. For conventional ESD-safe work areas, the float potential should be less than 100 V.

Ionizer performance should be certified prior to initial release of a workstation to manufacturing or development use, and subsequently at specified intervals. In addition, the performance of ionizers should be verified on a regular basis and any time that a change is made to the layout of equipment on the workstation or if the workstation is relocated. As mentioned before, ambient air will gradually discharge a charged plate: typically from 5000 V to less than 500 V in about 30 minutes or from about 1000 V to less than 100 V in about 45 minutes. By comparison, in a unidirectional flow cleanroom, the discharge time from 1000 V to less than 100 V is about 45 hours. A corona discharge ionization system in an ambient environment can achieve the discharge in 50 to 500 seconds; in a unidirectional flow cleanroom in typically 10 to 15 seconds. The selection of ionizers for a given application must consider many different factors, among them the inherent stability of the ionizer. Unstable ionizers frequently drift outside their desired float potential limit. The rate at which this occurs determines the frequency of calibration and adjustment of the ionizer. Thus, long-term stability tests are an essential part of ionizer evaluations [27]. The cost associated with calibration and adjustment of an ionizer must be factored in with the cost of acquisition and installation. It is also important to inspect corona discharge ionizers thoroughly for the presence of unshielded electrical fields. These can be present anywhere around the ionizer. 2.5.8

Relative Humidity

It has long been known and documented that elevated relative humidity plays an important role in reducing the amount of static charge that is generated on items in the environment (Table 2.9). Most industry standards call for a minimum relative humidity in the range 25 to 40%. Some standards provide for operation below the minimum relative humidity if air ionization is used. For example, in the aerospace industry it is not uncommon for the lower relative humidity limit to be set at 30%, but work can continue if the relative humidity in the workplace falls below 30% as long as fan-powered bipolar air ionizers are used. A question that is asked frequently is why elevated relative humidity reduces the level of charge. One often hears that high relative humidity makes air more conductive. A second frequently offered explanation is that humid air helps to dissipate electrostatic charges by keeping surfaces moist, therefore increasing surface conductivity. Neither of these explanations is satisfactory. Several studies have looked at ion electrical mobility vs. relative humidity. The electrical mobility of ions in the air is what controls the conductivity of the air. Contrary to popular wisdom, if you increase the relative humidity of air, it becomes less conductive. Thus, people who explain the reduction in tribocharging that occurs at elevated relative humidity as attributable to an increase in conductivity of air at high relative humidity are incorrect.

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TABLE 2.9 Tribocharging and Relative Humidity Charge Generation at: Activity Walking across vinyl floor Walking across synthetic carpet Arising from foam cushion Picking up polyethylene bag Sliding styrene box on carpet Removing Mylar tape from PC board Shrinkable film on PC board Triggering vacuum solder remover Aerosol circuit freeze spray

20% RH 12 kV 35 kV 18 kV 20 kV 18 kV 12 kV 16 kV 8 kV 15 kV

80% RH 250 V 1.5 kV 1.5 kV 600 V 1.5 kV 1.5 kV 3 kV 1 kV 5 kV

Source: Ref. 28.

You are probably still curious about how raising relative humidity is observed to decrease the amount of tribocharging that occurs. This is often explained on the basis that the increased moisture content of the surface allows charges to spread out over the surface more uniformly, reducing the apparent voltages. Again, this is not a satisfactory explanation of the phenomenon observed. If spreading the charge over a larger area were the only explanation, the total charge would still be constant and the beneficial effects of elevated relative humidity would not be realized in any practical way. Walking across a carpeted room in a humid environment would generate the same amount of charge as walking across a carpeted room at low relative humidity. But we know we don’t “have as severe an ESD problem” at high relative humidity, whereas we do at low relative humidity. Something else must be going on. To understand, we need to look at what is controlling charge generation in the tribocharging process. If you plot the electronegativity of the elements comprising the surface of the materials vs. their relative positions in the triboelectric series, they line up quite nicely. The electronegativity of the atom is a measure of its tendency to gather or donate electrons. Surfaces that are rich in elements with high electronegativity tend to become negatively charged after separation from materials whose surface consists of atoms with low electronegativity. Suppose that we have two dissimilar materials that have begun to adsorb moisture. (Adsorption is occurring on the surface; absorption occurs within the bulk of the material.) When these two dissimilar materials are brought in contact with one another, some areas will actually be water molecules in contact with water molecules. When the water molecules are separated, no charge will be generated. Atomic force microscopic studies of surfaces with adsorbed water films support this hypothesis. Very dry surfaces show no liquid islands of adsorbed water molecules. As humidity is increased, ever larger percentages of the surface exhibit islands of water molecules. At high relative humidity, the surface becomes more or less continuously covered by a film of water molecules, in some places several atomic layers thick. So now we have a plausible explanation for our everyday observations about ESD and relative humidity. As the humidity of the air goes up, the atomic composition of the two contacting surfaces become more alike and the amount of tribocharge that is generated goes down. The upper relative humidity control limit is dictated by at least three factors. One is the corrosion susceptibility of the devices in the process. A second consideration is

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the absorption of moisture by materials that could result in dimensional changes, changes in hardness, or other physical changes. Finally, trace contaminants on the surface of parts may absorb moisture, forming conductive solutions, leading to current leakage problems. Typical upper control limits for relative humidity range from 50 to 70%. Some standards provide continued operation for a limited period if the upper relative humidity limit is exceeded. For example, one company specifies an upper control limit of 50% for one of their locations. They are in a region where the weather can change rapidly, so they allow for continued operation of the facility for an additional 30 minutes if the relative humidity does exceed 50% as long as the humidity does not exceed 55%. This allows the building dehumidification systems some time to catch up with a sudden change in ambient relative humidity. 2.5.9

Chairs and Stools

The coverings on chairs and stools should be static dissipative. Fabric coverings should not be used in cleanrooms; polyurethane covering are preferred. Grounding of chairs should be through the use of conductive castors or furniture slides to the floor, not through the use of drag chains. Drag chains are prone to breakage, so the reliability of the ground through the drag chain is questionable. This is especially true for ESD-protected work areas in cleanrooms, where holes in perforated floor tiles or floor exhaust grates can break the drag chain. Chairs with pneumatic cylinder height adjusters must be checked when received. If a nonconductive grease is used during the assembly of the pneumatic cylinder, the chair will not ground through the cylinder. This condition cannot be detected by visual inspection; the chair must be tested using a high-resistance ohmmeter and a conventional ohmmeter to verify grounding. Case Study: ESD Chairs A manufacturer was preparing its work areas to deal with the increased sensitivity of MR heads. The area was outfitted with a grounded floor and chairs with conductive casters. These had been in place for several years. When the chairs were tested with a high-resistivity ohmmeter, the seat pan and seat back cover materials were found to be static dissipative. The ohmmeter indicated that all of the connected components in the legs and height-adjust cylinder were electrically connected. However, the conductive casters did not ground to the conductive test plate. The problem was found after inspection of the contact surface on the plastic wheels of the casters. These were coated with wear debris from the floor tile (Figure 2.9). The floor tile was a hard vinyl (nonconductive) material with an embedded matrix of carbon-rich material, giving the tiles a spider web–like appearance. The wheels on the chairs were wearing nonconductive vinyl particles from the floor, which subsequently became embedded in the contact surface of the conductive rollers. Scraping the embedded material off the wheels corrected the grounding problem. 2.5.10

Trash Cans

Trash cans should all be made of static-dissipative or conductive material. Trash can liners may be insulative or static dissipative. If insulative liners are used, trash may be removed from the can only during nonproduction periods or by relocating the trash can outside the static-safe work area prior to removing the liner. Several high-technology companies have adopted a novel approach to providing a trash receptacle at individual workstations in high-volume manufacturing operations. They fasten

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FIGURE 2.9 Contaminated caster.

a static shielding bag to the edge of the workstations using ESD-safe pressure-sensitive adhesive tape to form a temporary waste bag. At the end of each shift, the operators take the waste bags out with them as they exit the ESD-protected work area. 2.5.11

Cathode-Ray Tube Displays

One of the most notorious sources of charge in an ESD-protected workplace (and in most homes) is the cathode-ray tube (CRT), best known to most consumers as the picture tube in a television set. To produce an image in a CRT, an electron gun inside the picture tube shoots a stream of electrons at a phosphor-coated metal screen. The phosphor-coated screen is just behind the front of the CRT. The stream of electrons from the gun illuminates tiny dots of phosphorescent material on the screen, producing the image. (This process is called cathode luminescence, hence the term cathode-ray tube.) This also charges the screen to a negative potential. The outside of the glass of the picture tube becomes charged to the opposite potential by induction. This is illustrated in Figure 2.10. This is one of the most familiar examples of electrostatics in everyday life, although we don’t usually think of it as a manifestation of electrostatics. The face of a television set is notorious for being the dirtiest surface in a room. This is because the charged surface of the television picture screen attracts and holds more dust than does any other surface in the house. CRTs are widely used in high-technology applications such as computer displays and microscopes. Often overlooked are oscilloscope displays. When testing and providing protection for CRTs in an ESD-protected workplace, it is a common mistake to ignore oscilloscopes displays. Another common mistake is to ignore the effect of the electron gun and the phosphorcoated screen, both of which can induce charge on the outside of the cabinet in which they are contained. This problem is illustrated in Figure 2.11. A solution is shown in Figure 2.12. Modern displays often consist of liquid-crystal display (LCD) panels. These have no electron gun and thus do not become charged by the mechanism described here. They can

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Phosphor-Coated Metal Screen Becomes Negatively Charged Stream of Electrons Electron Gun

-

Heavy Glass Envelope of Picture Tube

FIGURE 2.10

+ + + + + + + + + + + +

Front of CRT (Picture Tube) Becomes Positively Charged

Working principle of a cathode-ray tube.

Field from Glass Field from Phosphor Coated Screen Field from Gun

FIGURE 2.11

Possible fields around a display containing a CRT. Provide Protection as Needed

Screen Cover

FIGURE 2.12

Ventilated housing

One way to fix a display that cannot be moved out of an ESD-protected work area.

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become charged when they are wiped because their surfaces are normally good insulators. Another factor is that many manufacturers of television sets have come to recognize the annoyance factor of dust accumulation on their sets and have begun manufacturing screens that have conductive grounded coatings so that no charge appears. It is not possible to determine if a particular display is safe or not by looking at it. It is necessary to test with suitable inspection instruments under normal conditions of use to determine if the particular display is a threat and what corrective actions should be taken. Oscilloscopes are of particular concern with regard to high field strength on the front of a display. However, oscilloscopes usually have metal cases, so fields generated by the electron gun are shielded by the metal housing of the instrument and thus are seldom detected around the case of an oscilloscope. Unfortunately, very few displays and monitors have metal cabinets. For displays that cannot be removed to a safe distance from ESD-sensitive devices, a static-shielding “doghouse” can be fabricated from perforated metal. The static shielding screen protector can then be installed as a window in the doghouse to fully enclose the display or monitor. 2.5.12

Field Potential Limits

The limit to charge that can be tolerated in an ESD-safe work place varies, depending on which standard one refers to: For example, ESD Association standard S20.20 limits field potential to 2000 volts for objects within 12 inches (approximately 30 cm) of an ESD sensitive item. By comparison, JPL standard D1348 [28] limits field potential to 200 volts within 1 meter (100 cm) of an ESD sensitive item. The static electric field on charged surfaces is measured using a field potential meter or static locator. Field potential limits are measured using a type of non-contact voltmeter referred to as a static locator or field potential meter. The resolution limit specified for the electrostatic field meter described in several standards is 100 volts. A preliminary gage capability analysis (measurement system analysis) of several brands and model this type of meter estimated that the smallest field which could be measured at a gage capability of 20% was in the range of 600 to 900 volts [29]. This is adequate gage capability for measuring to the requirements of S20.20, but is inadequate for measuring to JPL standard D1348. The field potential limit specifies how much charge can be permitted on a surface and the distance between the ESD sensitive item and the charged surface. One way of thinking of this specification of charge over a distance is as a field potential gradient. The field potential gradient assumes the ESD protected work station and/or the ESD sensitive device is grounded and at zero volts. The field potential gradient permitted at 12 inch separation using S20.20 equals approximately 67 volts/cm. (2000 volts/30 cm.). The field potential gradient permitted at 1 meter separation using D-1348 equals 2 volts/cm. (200 volts/100 cm.). Thus, the field potential gradient permitted under S20.20 is more than 30 times greater than permitted under D-1348. To put this difference into perspective, the net effect of the control limits must be compared. Consider the field potential when a charged object that just meets the control limit is permitted within the control perimeter. This is calculated by dividing the upper control limit for surface potential, in volts, by the separation between the charged object and the ESD sensitive device, in cm, and is plotted in Figure 2.13. As can be seen in Figure 2.13, ESD sensitive parts are exposed to an order of magnitude greater field potential gradient under S20.20 than under D-1348 at all separation distances. A second net effect is equally important. This is related to the separation specification of the requirement. D-1348 control distance is set at one meter, because this is about the

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Field potential gradient (V/cm)

1000

2 5 10

100

2

20 30

5 10 20

10

30 100

1 Separation between charged object and ESDS device (cm) S20.20

FIGURE 2.13

D-1348

The field potential gradient possible under ESDA S20.20 versus JPL D-1348.

limit of normal reach for a person seated at an ESD protected work station. Objects outside the 1 meter limit are not subject to the 200 volt surface potential limit. But it is highly unlikely that ESD sensitive parts will be exposed to excessively charged objects because these are outside of the normal reach envelope of an operator. Conversely the S20.20 control distance of one foot is well with a person’s normal reach envelope. It is much more likely that ESD sensitive devices will be exposed to surfaces with excessive charge under the control requirement of S20.20 than under D-1348. 2.5.13

Tools and Fixtures

Portions of tooling in direct contact with ESD-sensitive devices must be hard grounded through a resistance not to exceed 10 ; some facilities require less than 1 . In general, do not ground through current-limiting resisters. Ground paths containing current-limiting resisters can allow potentially lethal voltages to exist on tooling by preventing circuit breakers from tripping. Where safety is a concern, the tool should be grounded to the ac reference ground of a GFCI. Where shock absorption is needed, pieces of approved workstation mat may be affixed to product contact points. Portions of tools that articulate through bearings pose an especially difficult problem. When the bearings are stationary, the tool segments on either side of the bearing are grounded through the bearing. Conversely, when the tool moves, metal-to-metal contact within the bearing can be lost, due to grease buildup in the bearing. If this occurs, it is usually necessary to provide a parallel ground path across the bearing. Damaged coiled cords that do not contain a current limiting resistor are particularly useful ground wires for this application because they accommodate motion easily (see Section 2.6.1). 2.5.14

Conveyors

Conveyors should be grounded via hard ground not to exceed 10 . In rare cases, conveyor rollers may be made of static-dissipative material connected to the grounded surfaces of the conveyor: Conductive rollers are more common in conveyors for ESO applications.

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2.6

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A favorite saying among contamination control experts is that the perfect cleanroom is one with no equipment or people! Similarly, a nearly perfect ESD-safe work area is one where no people are moving around and thus generating static electricity. Personnel are the single largest generator of static electricity in a manufacturing environment. Recognition of this fact is why the first and most common model of ESD damage to electronic devices used is the human body model (HBM), which simulates discharge from a charged person to a sensitive device. Because people are large charge generators and most manufacturing involves people to at least some degree, controlling static charge on personnel is one of the most important aspects of a static control program. 2.6.1

Wrist Strap and Coiled Cord

The most commonly encountered grounding system for people working in an ESDprotected work area is the wrist strap and coiled cord. Many different styles of wrist straps are available, the most common being cloth wrist straps, although these have been associated with electronic failures in the aerospace industry [29]. The failures were traced to conductive fibers shed from the cloth band of wrist straps. In some aerospace applications, expandable metal wrist straps are favored for this reason, but these can be very uncomfortable for persons with hairy wrists. Another option is the use of molded plastic wrist straps, which eliminate the pinching problem of expandable metal straps. For safety reasons the coiled cord contains a 1  0.2 M current-limiting resistor. The cord and its self-contained resister can become damaged and if the resister becomes shorted, a safety hazard will be created. Conversely, if the wires or resistor in a coiled cord are broken, an open-circuit condition can result in failure to discharge the wearer. For this reason, it is necessary to test wrist straps and coiled cords on a regular basis using a wrist strap tester. Continuous wrist strap monitors provide additional security. 2.6.2

Training and Certification Program

The most important step in controlling people-related static problems is training. Most industry specifications require training for all personnel who enter static-controlled work areas or who work around static-sensitive devices. A partial list of work areas whose personnel should be trained follows: Management Receiving Traffic Manufacturing Shipping Facilities Purchasing

Engineering Inspection Development Industrial engineering Maintenance Contractors Quality assurance

Some of these areas are overlooked in many programs. The training of various groups of people should include subject matter appropriate for their responsibilities: applicable specifications and standards, copies of the training presentation, appropriate enrichment materials, demonstrations, and so on. Certification of

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training should be verified through demonstration of proficiency by examination. Evidence of certification should be kept in the work area, either by personal badges or by a posted list of qualified personnel. Increasingly, this information can be accessed online, even in relatively remote manufacturing locations. There are many levels of training, from very basic to highly advanced. The most basic training is awareness training, which should cover the fundamentals of electrostatic charge generation, mechanisms by which devices become damaged by electrostatic discharge, and basic principles of electrostatic discharge control. A reasonable argument can be made that this training should be mandatory for all employees, including personnel who rarely, if ever, enter an ESD-safe work area. This makes everyone aware of the minimum requirements and expectations for working in an ESD-safe work area. The benefits of this universal awareness approach include the following: ●

●

●

Managers understand what is expected of their employees, so managers can respond to good or bad performance appropriately. The manager is then better able to evaluate reports of behavior that does not conform to requirements. Inspectors understand what is expected of personnel working in an ESD-protected work area. This applies to both internal inspectors and outside auditors. Operators can confidently enforce the discipline required for the workplace. This is especially important when operators have to confront engineers and scientists who are only occasionally in an ESD-safe work area and who might be perceived by operating personnel as their superiors.

More advanced training is required of many personnel. Auditors will need to understand the ESD control requirements in greater detail to enable their inspections. For example, an operator might need to know only that a ground wire must be connected to the point to which it was connected previously. The auditor would need to know the acceptance criteria, such as the resistance that is required to accept the ground. Auditors should also receive training on how to conduct an audit, how to operate ESD audit instruments, and how to perform simple repair and corrective actions for an ESD-protective workplace. Design engineering personnel should also receive training in ESD sensitivity tests for ESDS items and circuit design approaches to protect ESDS items from damage. Process engineers should understand the need to evaluate alternatives when selecting alternative processes. The best basic awareness training usually involves one to three hours of lecture and demonstrations, followed by a short period of “hands-on” training, including application of all the principles necessary to carry out the various tasks. For example, it should include how to test wrist straps, how to put on and test footwear, and how to inspect ground connections. This training should be done prior to actually working in a static-controlled work area. Many programs include a written test with a minimum passing score necessary to become certified. For inspectors, the hands-on training should include an inspection exercise. This provides candidate inspectors with an opportunity to use the inspection equipment, fill out applicable inspection documents, and test their knowledge of appropriate corrective actions which can be implemented immediately. For engineers, additional training should focus on various damage mechanisms and circuit designs to provide protection for ESD-sensitive devices. Training should be repeated for all personnel at least once a year. This is necessary because the ESD protection requirements for devices can change, because the choice of

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control technology has changed, and because peoples’ recollection of ESD control requirements can deteriorate over time. Some practioners perform more frequent training, especially where device sensitivities are changing rapidly. Training documents and record keeping are very important. Records of the training for each member of the team should be kept with the member’s personnel records. Periodic audits should be performed of personnel performance in static-safe work areas to ensure that the training program is doing its job. Violations of static-safe protocol should be recorded and reported to management so that problems can be addressed. If global problems are found (many of the same type of errors in one or more areas), a reassessment of the training program should be done to correct those problems. Possibly more frequent training is necessary for some personnel. ESD Training Documents Training can be delivered a number of ways. Prerecorded video training packages, widely used, offer several advantages. One is uniformity: The information delivered is the same every time. A second advantage is the reduced time required for a professional trainer to deliver training. Training can be on an as-needed basis as new employees are hired or when retraining is required. Many demonstrations used for ESD awareness training are difficult to see in a large classroom but can be captured effectively by cleaver camera work. There are disadvantages as well. First, the video presentation can get out of date rather quickly. Second, the presentation is not interactive, so questions by the audience must be deferred until a live instructor is present. Finally, the expertise of the instructor proctoring the video instruction may not be very high. Any video instruction should be accompanied by student handouts. Live instruction is generally preferred over video instruction, since it is usually more up to date and an opportunity exists for questions and answers. Demonstrations can be live or shown as video. Both video taped and live instruction should be accompanied by student handouts. The handout should consist of copies of the slides used in the presentation, a copy of the specification the students are expected to conform to, and supplemental information such as a general ESD guidebook. Qualification criteria should also exist for instructors, especially for live instruction. Instructors’ with extensive ESD Control experience will be better able to answer student questions. 2.6.3

Cleanroom Gowns and ESD Lab Coats

The requirements for garments vary widely. There are many concerns in garment selection that are totally unrelated to static control: contamination control, safety, protection of operator’s clothing, and uniform appearance. On the other hand, if the correct garment is worn, it can also provide a significant benefit in static control. The most important aspects of a garment’s performance from an ESD perspective is its ability to dissipate static electric charge and to shield static electric fields on the wearer from [30]. Smocks, also called frocks, and jumpsuits, also called “bunny suits,” are types of garments used in the cleanroom and static control industries. Smocks and frocks are typically jacketlike garments that cover only the body above the knee. For machining operations, shortsleeved frocks are often preferred for safety reasons. Cleanroom jumpsuits are typically designed to cover the entire body. These are available in both two- and one-piece designs. Gowns and lab coats for static-safe work areas must be worn when within these areas. They must completely enclose the wearer’s street clothes. Sleeves must be rolled down completely to cover street clothes, and buttons or zippers must be fastened to cover the clothes. These

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must be constructed of fabric containing a grid of conductive fibers to form a Faraday cage barrier, which suppresses the charge on the street clothing. The garment to be worn, when tested to EOS/ESD Standard DS2-1995, should show a sleeve-to-sleeve decay from 1000 V to less than 10 V in less than 1 second, and panel-topanel decay should be less than 10 V in less than 1 second. In addition, panel-to-panel resistance should be less than 109 . The laundry service should provide evidence of the ESD performance of the garment throughout its usable life, through periodic testing. Garment material for cleanroom applications is typically made of single-filament polyester. Polyester is an insulative material that tends to generate and accumulate significant electrical charge. One would think that garments made for static control would employ a different material, but single-filament polyester is the best material for cleanroom applications. For this reason, garments made for cleanroom applications still employ polyester as the base material. Garment manufacturers have found that a woven grid pattern of conductive threads in the polyester fabric can control the charge on street clothing by forming a shield. If the grid pattern is designed properly, the garment can provide a Faraday cage to suppress the electric fields generated by any clothing that might be worn under the garment. A common problem with ESD control garments is loss of good electrical connections between the various panels of the garment. This is true because the connection between the panels are most vulnerable to damage at the seams between panels. Some manufacturers of cleanroom garments have found a way to overcome this problem: by sewing conductive ribbons into the seams between panels. However, rigorous testing is still required to ensure that the garment performs well after many uses and launderings. Many uses and launderings can result in failure of a previously acceptable garment [31]. Note about Spun-bonded Polyolefin Garments Garments made of spun-bonded polyolefin are a special concern. One of the most familiar brand names for this material is Tyvek. Many disposable cleanroom garments are made from spun-bonded polyolefin. Polyolefins are made from the ordinarily insulative materials polyethylene and polypropylene. Interestingly, cleanroom garments made from spun-bonded polyolefins are usually found to be static dissipative. This is true because the fabrics must be treated with chemicals to minimize static charge buildup to facilitate their handling during garment fabrication. The chemicals used to make the fabric easy to handle during fabrication are usually water soluble. Thus, washing the spunbonded polyolefin garment may eliminate the static-dissipative properties. 2.6.4

Footwear

For a grounded floor to work properly, the people working on the floor must be connected to it electrically. Groundable footwear must be worn to provide this connection. Fortunately, many types of footwear are available to connect a person to a grounded floor. This provides many options for selection of footwear, although there are subtleties for footwear that must be observed to avoid problems. Options available for groundable footwear include heal straps, toe straps, disposable shoe covers, shoes, and booties, sometimes referred to as overshoes. Knee-high booties are a common solution for footwear grounding in cleanrooms. Some installations require the use of dedicated ESD shoes in their cleanrooms. Often, visitors wear shoe covers with conductive ribbons sewn into the soles. Cleanroom Booties Footwear for the cleanroom should consist of static-dissipative booties with sewn-in conductive soles. Where personnel wear street shoes inside booties,

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means should be provided to connect the feet to the conductive sole of the bootie, such as through the use of a sewn-in ground strap. Case Study: Groundable Footwear—An Incomplete System The importance of complete footwear systems cannot be overemphasized. Two ESD control programs in hightechnology companies have fallen prey to failure in their footwear system due to a lack of either proper qualification tests or ongoing surveillance during audits. In both cases the user did not know that personnel wearing static-dissipative knee-high shoe covers in a cleanroom are not grounded through their street shoes. In both cases, a means of connecting cleanroom ESD booties to the person had to be adopted. In one case, the booties were modified by sewing a conductive ribbon into the sole. In the other case, the gowning procedure was modified to include wearing a disposable shoe cover with a conductive ribbon between the shoe and the sock, under an ESD knee-high cleanroom bootie. Both of these problems could have been avoided if a footwear test protocol had been included as part of an overall ESD control program. Visitors’ Shoe Covers Visitors’ shoe covers usually consist of disposable material with a conductive ribbon sewn into the sole, used to connect the sole of the shoe cover to the wearer. The conductive ribbon should normally be worn between the socks and the street shoes. Many people are especially conductive through their socks. This can provide a potentially dangerous conductive pathway to the floor through the conductive ribbon since disposable shoe covers do not have current-limiting resisters. Conversely, some people have especially high resistance through their socks and will fail the footwear test due to excessive resistance. There is a temptation to wear the conductive ribbon in direct contact with the skin, tucking it in between the skin and the sock. If the facility is equipped with conductive floors as is often the case in aerospace applications, an electrical shock hazard can be created. Again, this problem can be avoided if a footwear tester is a part of a comprehensive ESD control program, since most footwear testers alarm if resistance is too low. Static Dissipative Shoes Alternatively, static-dissipative shoes can be provided. These should be considered as factory shoes and should not be worn out of the building. One advantage of using a dedicated factory shoe is that all persons working in an ESD-safe work area can have shoes of the same color and style on their feet, making it instantly obvious if someone is not wearing the appropriate footwear. A second advantage is that ESD shoes can be purchased with steel toes and metatarsal protection, an important occupational safety consideration in many industries. Testing Footwear Prior to entering a static-safe work area, personnel must use a footwear tester to verify that their footwear is functional. Each foot must be tested individually by standing on one foot while holding a conductive element in a bare hand. The total system resistance should be less than 109 . A log should be conveniently posted near the tester, in which the test results will be recorded. Regardless of the footwear grounding system chosen, it is important to test footwear to verify its performance. Footwear such as dedicated shoes, heel grounders, or toe grounders can become contaminated, which can insulate the footwear grounder. Contamination can accumulate rapidly, so it is necessary to test footwear frequently. At the very least, footwear should be tested at the beginning of the shift each day the footwear is used. In some applications, footwear is retested upon every entry to an ESD-protected work area.

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The ground strap attached to footwear may not work well for all wearers. Dry skin can interfere with grounding through the conductive ribbon (dry skin can be treated using skin moisturizers). The ribbon on a footwear grounder may not be long enough to work with hightop shoes such as cowboy boots. In the latter case, grounding garters are available to attach the ribbon from the footwear to the person’s calf. Finally, it has been found that some kneehigh booties do not ground consistently, depending on the type of street shoes the person wears. This can be corrected by sewing a conductive ribbon to the inside sole of the bootie and wearing the ribbon between the street shoe and sock. 2.6.5

Gloves, Liners, and Finger Cots

Gloves, liners, and finger cots may consist of any material that is capable of meeting the following criteria. The person conducting the test must not be grounded during the test. This can be accomplished by standing on an insulated surface that is elevated at least 12 in. above the floor. The person should also wear the garments that will be required in the area where the gloves will be used. For example, if the gloves are to be used in a non-cleanroom application but the operators would wear ESD frocks, the test should be conducted while the frock is worn. While wearing a glove or finger cot and the applicable wrist strap, the surface of a 20-pF plate on a charged plate monitor should be touched firmly. The operator is then charged to 1000 V. The operator then discharges himself or herself using the approved wrist strap, and the time to discharge to 10 V is measured. (Different industries and different companies within a given industry will require discharge to other voltage levels.) The glove or finger cot becomes an approved part of the grounding system if the discharge time meets the acceptance criteria. (Note: Gloves, liners, and finger cots for use in cleanrooms will also be required to meet cleanliness criteria.)

2.7 2.7.1

CONSUMABLES AND ACCESSORIES Packaging

Packaging for ESD application comes in a variety of forms and materials. This includes film stock and preformed bags made from either static-dissipative or conductive materials, plastic film bubble wrap, and others. Totes, bins, and shipping containers are generally made of rigid polymers or carbon-coated paper or cardboard. Packaging foams are also in wide use. These materials have a wide variety of properties. The principal feature that ESD packaging provides that sets it apart from ordinary packaging is protection from electrostatic charge and electrical fields. Thus, ESD packaging can be characterized for four different properties: surface resistivity, discharge time, charge retention, and shielding properties. For packaging that is to be used outside an ESD-protected work area, these properties must be retained at the low-relativity-humidity conditions that may exist in shipment or storage. The ANSI/EOS/ESD standard test methods for surface resistance and electrostatic discharge shielding are S11.11-1993, Surface Resistance Measurement of Static Dissipative Planar Material—Bags and S11.31, Shielding Bags respectively. Electrical Industry Association (EIA) Standard 541, Appendix E [32] is the only test method that measures the transmission of electrical energy through film material resulting from an electrical discharge to the outside of the bag, referred to generally as EMI shielding. EIA 541, Appendix E, defines materials as static shielding if their surface resistivity is less than 104 /sq. However,

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it is doubtful that a surface resistivity this high would provide adequate shielding. A more conservative approach is to require that resistivity be less than 102 /sq. One final test is to place the most sensitive ESD-sensitive parts to be handled inside a candidate bag and cause an electrostatic discharge on the outside of the bag using a machine model or human body model tester. Although this type of test is the most direct method available, it is destructive and expensive to perform. An alternative test is to measure the charge decay property of the material [33]. Plastic film sheet stock, preformed bags, and bubble wraps are available in a number of materials. Polyethylene can be impregnated with chemicals to render it static dissipative and can be dyed pink to indicate its suitability for ESD applications. Acceptable materials generally measure in the static-dissipative range and are generally considered to be adequate protection for use in static-safe work areas where electrical fields are limited in field strength but are generally not considered adequate protection for transport out of a static-safe work area. Pink polyethylene is humidity sensitive and may not perform well at 12% RH. In addition, the additives tend to have high vapor pressure and may not be suitable for use in many contamination applications. In addition, the chemicals used to make pink polyethylene static dissipative are usually soluble in water, alcohol and detergent-water solutions. Depletion of the chemicals render pink polyethylene unsuitable for use. The general trend today is to utilize laminated films containing a buried conductive layer of aluminum or nickel rather than pink polyethylene. Referred to as static shielding bags they contain a buried metal layer sandwiched between layers of polymers (Figure 2.14). The films come in moisture barrier forms, usually with a thick aluminum layer, and in static shielding forms, which do not provide an effective moisture barrier. In addition, the films are available with either conductive or static-dissipative surfaces. Thus, it is important to characterize the type of bag required; all bags are not the same. Static shielding bags can be sealed using a variety of methods, including tape sealing and heat sealing. Some static shielding bags are provided with a zipper closure, making them highly reusable. In general, closing bags by using staples is not advised. Staples puncture a bag, which can result in tearing. In addition, staples occasionally shed metal fragments; which could cause shorts in electronic devices. Everyone who purchases static shielding bags is told that the bags can be reused until they are excessively crumpled. Excessively crumpled bags develop microscopic cracks in the plastic film which allow oxygen to oxidize the underlying metal layer. Metal oxides are insulators, not conductors. These patches of oxide act like pseudoholes and allow static

1. Static-dissipative, heat-sealable polyethylene layer (inner layer) 2. Polyester layer (adds strength)

1 2

3. Metal layer (Ni, Al) 4. Static-dissipative abrasion layer (outer layer)

FIGURE 2.14

3 4

Cross section through a static shielding bag.

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FIGURE 2.15

Visualizing pseudoholes in a badly oxidized static shielding bag.

fields into a bag, potentially damaging the ESD-sensitive parts inside. Holes are easily visualized as nonreflective semitransparent patches in the metallized film by placing a hand inside the bag, as shown in Figure 2.15. 2.7.2

Desiccants

Desiccants are an important adjunct to packaging to protect high-technology products from the harmful effects of moisture. One important consideration in the use of desiccants is the fact that they use highly porous materials as absorbents to gain the advantage of high surface area per unit weight. As a consequence, packages used to contain desiccant must be selected carefully when they are to be used in applications where contamination is a concern. Various materials are used as desiccants, including molecular sieves, clays, and silica gels. Of these, silica gels are the most widely used in the pharmaceutical, disk drive, and electronics industries. The name silica gel is misleading as the material is actually a hard, brittle solid rather than a soft, rubbery gelatin as the name implies. Although chemically inert, silica gel particles are troublesome, in that they are very abrasive. They can cause rapid wear-out of bearings and are a known cause of scratches and ESD damage in disk drives. 2.7.3

Tote Boxes, Bins, and Other Shipping Containers

Tote boxes, bins, and other shipping containers have also been made from impregnated polyethylene, although these are usually dyed blue. They can suffer from the same outgassing properties as those of film materials. In addition, where boxes are reused, if they are cleaned using aqueous cleaning processes, water dissolves the additives from the surface, raising the surface resistivity. They also loose their static dissipative properly at low relative humidity. For these reasons, treated polyethylene has fallen out of favor as a material for use in ESD packaging.

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103

Rigid polymers can be made static dissipative or conductive via a variety of methods. Some polymers are inherently static dissipative and need no additives to impart ESD properties. Other polymers that are normally insulative can be compounded with conductive polymers (copolymerization) or can incorporate carbon black, carbon fibers, or metal fillers. These generally do not exhibit outgassing problems. Care must be taken in the use of polymers incorporating carbon or metal filler particles to avoid sloughing. Conductive materials sloughed from these packages can create shorting problems in electronic components and can contribute to general contamination in a cleanroom. Metal fillers are generally used to make conductive containers that provide not only ESD protection but also electromagnetic interference (EMI) and radio-frequency interference (RFI) protection. Rigid polymers are often used in vacuum forming of packaging materials. Care must be taken in the selection of sheet stock to ensure that the molded package retains its ESD properties in all locations. Where the sheet stock is thinned, the conductive fillers may become so spread out that locally, ESD properties are lost. [34] Totes, bins, and shipping containers for use inside a static-safe work area can be static dissipative. Totes, bins, and shipping containers for product to be taken out of or stored outside a static-safe work area should be static shielding. When not in a static-safe work area, these containers must be covered with the appropriate conductive lid or overwrap. Case Study: Packaging Polyethylene has frequently been made into static-dissipative packaging through the incorporation of organic amines, which function by absorbing moisture from the atmosphere. At low relative humidity there is insufficient moisture to have the material retain its static-dissipative property. For this reason, organic amines were quickly recognized to be unsuitable in shipping applications where humidity cannot be controlled. However, this failure at low relative humidity was not seen as a problem in manufacturing applications where the lower relative humidity was controlled. Many companies began using amine-modified plastics in their manufacturing operations in the early 1980s, when the dominant cleaning solvents were chlorofluorocarbons (CFCs). In the late 1980s, CFCs became recognized as detrimental to the environment because of their tendency to deplete ozone in the upper atmosphere. Many high-technology companies responded by replacing their CFC cleaning processes with cleaning processes using detergent–water solutions as the cleaning solvent. The organic amines in the packaging materials, which had previously been insoluble in CFCs, were now removed completely by the detergent–water washing. The packaging materials gradually lost their static-dissipative properties, creating ESD problems. This problem appeared gradually, however. The surface of the packaging became depleted of the organic amines, but additional chemicals gradually returned to the surface by diffusion from within the bulk of the material. Thus, the ESD problem appeared and disappeared, until the materials lost all of their ESD properties after numerous washings. The industries where this occurred were mostly in semiconductor and disk drive manufacturing. Many aerospace companies delayed replacement of CFCs because of various exemptions. Thus, amine-filled polyethylene packaging materials continued to be used in many aerospace companies long after their use had been abandoned by semiconductor and disk drive companies. A second problem with organic amines used to impart static-dissipative properties to packaging materials is that they have significant vapor pressure. This results in contamination problems associated with outgassing. As recently as 2004, contamination of valuable aerospace materials by outgassed contamination from amine-impregnated polymers was still occurring.

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2.7.4

Notebooks and Sheet Protectors

All documents in a static-safe work area should be printed on static-dissipative materials or protected within static-dissipative sheet protectors. All binders should be static dissipative. Ordinary binders can be modified by taping static shielding film stock over their surfaces. 2.7.5

Swabs and Wipers

Swabs and wipers used to clean ESD-sensitive components should be conductive or static dissipative. Swabs to be used on products must be capable of discharging a 20-pF plate from 1000 V to less than 10 V in less than 5 seconds. Personnel performing the test must wear approved static-dissipative gloves and/or finger cots and be grounded using an approved wrist strap through an acceptable workstation ground or workstation monitor. Swabs not used in direct contact with ESD-sensitive product can be exempt from discharge-time requirements. In general, wipers should not be allowed to come in contact with ESD-sensitive components. Wipers used for housekeeping or other processes that do not contact ESD-sensitive product need not be static dissipative or conductive. However, where wipers must be used to provide a cushioning, absorbent, or visually contrasting surface and thus must be in contact with ESD-sensitive components, conductive or static-dissipative wipers should be used. 2.7.6

Paper

Cleanroom paper should not be used in a static-safe workstation unless it is static dissipative. Static-dissipative spun-bonded polyolefin should be used in static-safe work areas in class 100 or cleaner cleanrooms. Paper that is not static dissipative must be kept in static-dissipative sheet protectors or bags. Impregnated paper must be tested for contamination compatibility. 2.7.7

Tape

Tapes consist of a plastic, paper, or metal film to which is laminated pressure-sensitive adhesive on one or both sides. Tapes appear in a variety of forms and are used in a wide range of applications including temporary sealing, permanent sealing (metal or plastic foils), as selfadhesive temporary paper labels, and for fixturing and mounting (double-sided tapes used in sawing and dicing operations and tapes used for masking operations in coating processes). Unless specially formulated for ESD properties, tapes can generate thousand of volts when removed from dispensers or from product surfaces where they have been used for temporary applications [35]. Tape usually can be considered to be acceptable for use in an ESD-protected work area when a 5-in. strip applied to a 20-pF plate immediately after removal from the roll charges the plate to less than 5 V. In addition, when a 5-in. strip of tape is removed from a charged plate, it is considered static safe when less than 5 V of charge is generated. All tape application and tape removal operations should be conducted while in the airflow from a balanced, fan-powered air ionizer. Pealing tape off the roll or off product surfaces can generate high charge levels. One common practice is simultaneous removal of tape and protective dust covers from connectors. A better procedure is to remove the tape first and then remove the dust covers.

PERSONNEL EQUIPMENT AND PROCEDURES FOR ITS USE

2.8

105

PERSONNEL EQUIPMENT AND PROCEDURES FOR ITS USE

After completion of engineering design, analysis and certification of items in a static-safe workplace, the last line of defense becomes personnel. Personnel can be effective in the control of ESD through thorough knowledge of their personal protective equipment and its use, and by following ESD procedures faithfully. 2.8.1

Wrist Straps and Wrist Strap Monitors

All personnel in a static-safe work area should wear wrist straps, which must contain a 1-M current-limiting resister. But there is an exception. If wearing a wrist strap could create a hazardous condition, wrist straps must not be worn. Two examples can be cited of work situations where one probably should not be wearing a wrist strap. In a machine shop there usually are many rotating tools. If the wrist strap cord gets entangled in a piece of rotating tooling, it probably will not draw the wearer’s wrist into the tool because the cords are designed to unsnap from the wrist strap at relatively low detachment force, but the detached cord, still entangled in the rotating machinery, could cause injuries as it spins around. Consider a second case: that of a solder reflow process. Many of these processes use tools that are equipped with chain conveyors, in which a wrist strap cord could become entangled. Again, there is little risk that a person’s arm will get drawn into the hot interior of the solder reflow oven because the cord will detach from the wrist strap, but the wrist strap could melt inside the oven, causing machine damage and downtime. If grounding though a wrist strap is unsafe, personnel must be grounded through their footwear. A grounded static-dissipative or conductive floor must be provided, and the process must be made a stand-only operation. Wrist straps must be tested on a regular basis. At the very least, wrist straps should be tested once a day each day they are used. This is because the resistor in the strap can become damaged. If a damaged resistor in a wrist strap cord results in a short, the current-limiting safety protection will be lost. The wrist strap test should include the skin of the wearer in the test. This is because people can suffer from dry skin, and a combination of the resistance of the resistor in the wrist strap cord plus the wearer’s skin resistance could exceed the acceptance limit for the wrist strap tester. If a person suffers from dry skin, a moisturizing cream can be used on the wrist. Tests of wrist straps that do not test the entire grounding system should not be used. For example, testing the resistance of a wrist strap cord or cord plus wrist strap using a volt ohmmeter does not include the wearer’s skin in the test. Having a designated person test all the wrist straps in a given work area using a wrist strap tester again does not include the resistance of each wearer in the test. In many operations, a wrist strap must be tested more frequently. Often, the best approach is to monitor the wrist strap using a wrist strap continuous monitor: some of these can log failures. The wrist strap monitor may monitor the capacitance of the wearer, the resistance of the wearer to ground, or the charge on the body of the person wearing the wrist strap. Some guidelines should be followed for plugging into a wrist strap monitor. A capacitancebased wrist strap monitor is useful in environments where the human body model ESD sensitivity is greater than 200 V. A resistance-based wrist strap monitor should be used at all workstations where the HBM ESD sensitivity is less than 200 V but greater than 50 V. A body charge monitor should be used for any ESD-sensitive operation where the HBM sensitivity is less than 50 V. Any monitor should trigger an alarm if either too low or too high a resistance is measured through the wrist strap.

106

FUNDAMENTALS OF ESD CONTROL

2.8.2

Sit–Stand Protocol

●

●

●

2.9

Seated operations If an operation can be performed in a seated position, the operator must be grounded using a wrist strap. In a work area that relies on both wrist straps and footwear for personnel grounding, a sit–stand protocol must be adopted. This is because the protection provided by the footwear can be lost completely when a person sits down. Thus, the usual procedure is that the person plug into his or her wrist strap ground point before sitting down, and stand up before the wrist strap is unplugged. This protocol ensures grounding continuity. Standing operations Workstations designed so that it is impossible for an operator to be seated are exempt from the requirement that a wrist strap be used. A good strategy to ensure compliance with this requirement is to configure the workstation so it is at standing height and to not permit chairs or stools at the work station. Sitting or standing optional Workstations where it is possible to either sit or stand must follow the rules of seated operations above when an operator sits. If the operator performs the tasks while standing, he or she is exempt from plugging in the wrist strap.

TRANSPORTATION OF ESD-SENSITIVE PRODUCTS

Within an ESD-protected work area, products need not be packaged in static shielding materials, but static-dissipative trays or containers should be used. If packaging is required for other reasons, it must consist of either static-dissipative or conductive material. Insulative packaging materials should not be permitted. When products are taken out of an ESD-safe work area, they must be protected by enclosure within conductive containers. These may consist of metallized plastic static shielding bags, metal boxes, carbon-impregnated cardboard boxes, and so on, depending on the application. Transportation of static-sensitive devices out of a static-safe work area inside static-dissipative containers is prohibited without the addition of static shielding packaging. The additional protection by placing the static-dissipative container inside a conductive container is required. Several static-dissipative containers may be combined inside a single conductive container. Conductive containers may only be opened in a static-safe work area. These transportation requirements have added complexity where the ESD-protected work area is within a cleanroom, but the added complexity is only slight. First, the packaging materials need to be qualified from both a contamination control and an ESD control perspective. Up to the early 1980s, this would have been viewed as one way in which the requirements of ESD control and those of contamination control were mutually exclusive. Fortunately, this is no longer the case. Packaging materials that satisfy both sets of requirements simultaneously are readily available.

2.10 2.10.1

INSPECTIONS AND RECORD KEEPING Daily Visual Inspection

Operators should inspect their workstations visually, prior to the start of work, every time they return to their workstations at the beginning of a shift and after returning from a break. Operators should look for and verify the presence and proper connection of all ESD equipment

INSPECTIONS AND RECORD KEEPING

107

provided for the workstation as described in the operation process instruction. This equipment check will consist of at least the following: ●

●

● ●

Verification that the ionization system is present and operating, as usually indicated by pilot lamps; where portable ionization equipment is used (e.g., bench top fan-powered ionizers), verification that they are present, operating, and aimed at the proper locations(s). Verification that the ground wires are still connected to the pieces of equipment, are connected to the grounding bus, and that the grounding bus is connected to the ac reference ground (ground wires on moving fixtures and equipment should be tested by gently tugging on the wires to verify connection). Verification that the workstation monitor is present and operational. Verification that chairs, carts and other portable equipment at the workstation are static safe. That is, unfamiliar chairs, carts and other portable equipment should be moved outside of the protected perimeter of the ESD-safe work station and called to the attention of ESD auditors or management.

Any visible deviations must be reported to supervisory personnel and the workstation must not be used until after the deviations are corrected. Where the deviation cannot be corrected immediately, as by removing from the work area carts that are not static safe, signage must be posted indicating that the station is no longer ESD safe. The site ESD coordinator or his representative must be notified. Other additional engineering personnel will also be notified if their assistance in correcting the deviation(s) is required. After deviations have been corrected and verified by the site ESD coordinator or his or her representative, the workstation may be placed back into use. 2.10.2

Periodic Instrumental Inspection

The site ESD coordinator should schedule periodic instrumental inspections of static-safe work areas and workstations according to the following requirements. In addition, the site ESD coordinator should maintain and make available the instruments required for periodic inspections. The results of all periodic instrumental inspections should be recorded. A minimum of the last seven consecutive instrumental inspections should be kept for each workstation or area. A variety of instruments are available for these periodic inspections. Surface Resistivity Meter A surface resistivity meter conforming to the requirements of EOS/ESD Standard S4.1 should be used. This consists of a self-contained megaohm meter, at least one 2.27-kg weight, and appropriate interconnect electrodes, apparatus capable of measuring at open-circuit voltages of 10 and 100 V, 10%. For extremely sensitive ESD-sensitive-component ESD-protected work areas, tests should be conducted at 10 V. For ESD-protected work areas that do not handle the most sensitive ESD-sensitive components, tests may be conducted at 100 V. Electrostatic Field Meter A chopper-stabilized noncontact electrostatic field meter should be used. These typically have 10 or 1 V resolution. Electrostatic Locator An alternative to an electrostatic field meter is the lower-cost electrostatic locator. These generally have 100 V resolution. Electrostatic locators with 100 V resolution will not be a capable gage for measuring surface charge below approximately 500 V (see Section 3.9.2).

108

FUNDAMENTALS OF ESD CONTROL

Charged Plate Monitor A charged plate monitor conforming to the requirements of the EOS/ESD Association Standard S-3.1 should be used. This consists of a 20-pF plate and a charging system capable of charging the plate to a minimum of 1000 V and timing the discharge to at least 100 V. For discharges to lower voltage, the timing should continue manually, observing the voltage display on the monitor. Charged plate monitors capable of timing discharge to less than 100 V are available and are especially useful for monitoring ionizer performance where the ESD sensitivity of the product requires float potentials of less than 100 V. Charged Plate Verifier Charged plate monitors are relatively bulky and heavy. For this reason they are inconvenient to use to survey discharge performance in large areas that may contain many ionizers. An alternative instrument to use for surveys is the ionization verifier: a battery-powered field potential meter outfitted with a capacitor plate. An auxiliary batterypowered charger is used to charge the plate to measure discharge times. The performance of the verifier should nearly match that of charged plate monitors conforming to the requirements of ESD Association Standard S-3.1. Figure 2.16 compares a charged plate monitor conforming to ESD Standard S-3.1 with a charged plate verifier using a 15  2 pF capacitor plate. Volt-Ohmmeter A portable volt-ohmmeter should be used to verify tool, fixture, and workstation hard grounding. The voltmeter is also useful in identifying tools where stray voltage exists, such as on ac-powered drivers, soldering irons, wire strippers, and wire bonders. Temperature–Humidity Testers Portable battery-powered temperature-humidity testers are readily available. These are important inspection tools, especially in locations where low relative humidity is a common occurrence. Unfortunately, they can drift in accuracy and are generally not designed to be calibrated. Annual calibrations probably are not adequate. One good practice is to maintain a test vessel at a constant relative humidity using humectant solutions. The portable humidity testers can then be routinely verified for accuracy before they are used in an inspection program. In general, the critical control point is the lower % relative humidity limit. A saturated CaCl2 solution will maintain 30% rh at 60

Time (sec)

50 40 CPM CPV

30 20 10

FIGURE 2.16 monitor.

20 ve ps rh ig ea C le d an Fa n Be nc R oo h m G rid @ R 50 oo ft m @ /min 10 ft / m in O

ne ,

In -li

Be nc

ht op

Fa n

0

Comparison of an Ion Systems 775 ionizer verifier vs. an ESDA S3.1 charged plate

INSPECTIONS AND RECORD KEEPING

109

75  °F; 30% rh is one of the most common lower %rh control points for ESD control. %rh should be tested at product locations in the ESD-safe work place. Outlet Testers Ac outlets should be tested during periodic audits using portable outlet testers. The selection of tester type to be used depends on the grounding requirements for the facility. If GFCI outlets are used, an ordinary outlet tester is unsuitable for this type of inspection. Experience has shown that improperly wired outlets in facilities are more common than most people imagine. This is especially true with outlet strips or the wiring in clean benches. Outlets should be tested for correct polarity and grounding at least annually, any time new equipment is purchased (including wiring strips, surge protectors, etc.), and any time that electrical repairs or modifications are done to the facility. It is also important to test GFCI outlets. This requires a different tester than is used in an ordinary ac outlet. Wrist Strap Tester and Footwear Tester Verifiers Many companies have mature ESD programs that include wearing wrist straps and/or footwear. In these companies the need to test the resistance of the wrist strap and footwear regularly is recognized as mandatory from a maintenance standpoint. However, a surprisingly large number of companies do not test their footwear testers or wrist strap testers but rely on annual calibration as the sole maintenance consideration. Given the safety concern involved in deliberately grounding people using a personnel grounding system, more frequent verification of testers seems prudent. 2.10.3

Testing Protocols

Continuous Workstation Monitoring Continuous workstation monitoring for ESDsafe work areas can take many forms. One of the simplest is to monitor the grounding of the workstations. Wrist strap monitors are in widespread use. Many monitors will monitor both workstation grounding and wrist straps. Finally, there are workstation charge monitors that monitor workstation grounding, charge on people through their wrist strap, and charge on the workstation though a variety of antennas [36]. Floors and Floor Mats New Material New material under evaluation or following installation should be cleaned following the manufacturer’s recommendations. Probes for the surface resistivity meter should be cleaned following the operating instructions for the instrument. The surface resistivity of the material is measured using either a single concentric ring probe or a pair of 2.27-kg (5-lb) probes. Acceptable material will fall in the static-dissipative range. For newly installed material, a minimum of five tests per 500 m2 (5400 ft2) should be made. Tests are conducted in expected high-traffic areas, at standing operations where high sensitivity to ESD damage is expected, and in areas where soils are expected to accumulate rapidly. Existing Material Existing floor materials should not be cleaned prior to testing. This is intended to demonstrate if the periodic maintenance interval for the flooring maintains the resistivity within target values. Probes for the surface resistivity meter are cleaned according to the operating instructions for the instrument. The surface resistivity of the material is measured using either a single concentric ring probe or a pair of 2.27-kg (5-lb) probes. Acceptable material will fall in the static-dissipative range. Failing material should be retested following cleaning. If the cleaned material passes, tests should be repeated in another failing area. This area must be cleaned using the standard

110

FUNDAMENTALS OF ESD CONTROL

cleaning method and then retested to verify the effectiveness of the cleaning. If failure occurs due to excessive cleaning interval, the cleaning interval must be shortened. If the failure occurs due to ineffectiveness of the cleaning method, the floor manufacturer should be contacted for improved cleaning methods. Test locations are those identified during initial testing: in expected high-traffic areas, at standing operations where high sensitivity to ESD damage is expected, and in areas where soil is expected to accumulate rapidly—unless these have changed due to rearrangement of processes. Work benches and Table Mats Work benches should be tested once a month. Tests should verify the resistivity of mats, if present, using a surface resistivity meter. In addition, the resistance to ground of the table and mat should be verified. Tooling and Fixtures Initial Tribocharging of Materials Tooling and fixtures should be tested initially for the presence of materials that tribocharge. In order of decreasing importance, materials of interest are: materials that come in contact with the product, materials that are close to the product (e.g., within 25 cm), and materials that are far from the product. The objective is to identify and eliminate any material that can become tribocharged under normal conditions of use, which are not discharged to acceptable voltage limits in less than the time limit by the air ionization system provided for the workstation. Normal conditions of use means that the person producing the tribocharge must wear the gloves and wrist strap specified and be plugged into a wrist strap ground point monitor while conducting the test. Alternately, for standing operations footwear grounders and a grounded floor should be used. In addition, the attempt to tribocharge must imitate the worst case that would normally be expected during processing. Thus, the person must handle knobs, packaging materials, documentation, and so on, in the normal fashion. Also, if materials are to be rubbed vigorously, as in cleaning, the cleaning wipers and fluids specified must be used. After the attempt to tribocharge is completed, a field potential meter is used to characterize charge retention on the part. If the material fails to discharge in the time specified for the workstation, it must be relocated, modified, or changed to a material that does not fail the test. Equally important, if the material fails to discharge completely, it may be unsuitable for its intended application. Failure of a material to discharge completely is often referred to as charge retention. Ongoing Tribocharge Testing Once a month, a survey of materials on the workstation should be repeated to verify their acceptability for use. This survey should look especially for the introduction of new materials to the work area. In general, materials that have been introduced to the work area without undergoing prior qualification through initial ESD control testing should be confiscated. Grounding Grounding of tools and fixtures must be verified using a volt-ohmmeter. Parts that move often fail the 10- maximum resistance criterion when they are moved unless ground wires in parallel to ground paths through bearings are provided. The auxiliary ground wires are often subject to stress and can sometimes break. For this reason, the integrity of the ground must be verified for all tools and fixtures on a monthly basis.

INSPECTIONS AND RECORD KEEPING

111

AC-Powered Tools Ac-powered tools include soldering irons, wire strippers, wire-wrap tools, screw/torque drivers, and others. All soldering irons and powered wire strippers should be tested according to the requirements of MIL-STD-2000A. (Note: MIL-STD2000A has been abandoned. The ESD Association is working on a replacement standard.) In MIL-STD-2000A, potential on the solder tip should not exceed 2 mV at operating temperature. In addition, resistance to ground at the solder tip should not exceed 10  at operating temperature. Unfortunately, this standard is probably impractical. Contact between dissimilar metals produces voltage due to the thermocouple effect. Contact between a gold-coated connector and a solder iron tip produces about 2 mV, regardless of voltage leakage through the solder iron. A more reasonable standard proposes limiting the voltage to less than 20 mVac, based on the maximum power that ESD-sensitive circuits can tolerate (2  104 W). This would also allow a maximum current of 10 mA if the ac power ground is faulty. Use of a GFIC that limits maximum current drain to 5 mA to power ac-powered tools would prevent overcurrents from occurring. In addition, because GFICs are highly reliable, their continuous monitoring operation could eliminate the need for periodic inspection of ac-powered tools [37]. Resistance to ground on the rotating bit of powered tools, such as drivers and wirewrap tools should not exceed 10 , either stationary or rotating. In addition, potential on the tip should not exceed 2 mV either stationary or rotating. This can be difficult to achieve with lathes, drill presses and other tools in a machine shop. Spring loaded wipers are sometimes useful to achieve grounding. Test Equipment Hot plugging of test equipment is a known cause of damage to ESDsensitive components. In addition, power surges during power-up of test equipment can damage ESD-sensitive components. Test equipment must be tested using a high-speed oscilloscope to identify voltage spikes that can occur during connection or power-on of test equipment. Ionizers Air ionizers should be tested at least once a month. Ionizer emitters should be inspected for wear and accumulation of contamination and cleaned or replaced as necessary. A charged plate monitor should be used at least once a month to verify that discharge times and float potentials are within specification. The charged plate is positioned as close to ESDdamage-sensitive product locations as practical during testing. If monthly inspection reveals the presence of air ionizers that are out of balance or fail discharge-time requirements, the ionizer must be cleaned, rebalanced or replaced. In addition, the sampling frequency for locations of out of balance or slow discharge must be increased to verify stability of performance. Charge plate verifiers can be used in place of monitors in many cases. Discharge Times For critical ESD-safe work areas, typically those in which the human body model ESD sensitivity of an ESD-sensitive device is less than 50 V, the discharge time should be less than 20 seconds from 1000 V to less than 20 V. For highly sensitive ESD-safe work areas, typically those where human body model ESD sensitivity of the ESD-sensitive device is less than 200 V the discharge time should be less than 20 seconds from 1000 V to less than 50 V. For ordinary ESD-safe work areas, where the human body model ESD sensitivity of the ESD-sensitive device is greater than 200 V, the discharge time should be less than 45 seconds from 1000 V to less than 100 V. The latter is a very conventional discharge performance that has been used for over a decade in the semiconductor industry.

112

FUNDAMENTALS OF ESD CONTROL

Float Potentials For critical ESD-safe work areas, the float potential should be less than 20 V. For highly sensitive ESD-safe work areas, the discharge time should be less than 50 V. For ordinary ESD-safe work areas, the float potential should be less than 100 V. Note that the float potential for each of the defined ESD-protected work areas is the same as the target voltage for discharge times. Static Event Detectors Static event detectors have been in existence for a long time. One of the simplest consists of an AM radio tuned to a frequency where no local broadcast signal is found. ESD in that vicinity can be heard as static on the radio: This phenomenon is a familiar experience to AM radio listeners during electrical storms. One drawback is that the simple radio receiver cannot locate the source of the ESD event. More recently, sensors have been developed to detect ESD events using magnetoresistive materials. These sensors can be wired into circuits and components or into tools. They can be inspected visually to reveal the evidence of an ESD event using a polarized-light microscope and can be reset magnetically for reuse [38,39]. 2.11

ESD CONTROL PROGRAM

An ESD control program should be established to satisfy the requirements of applicable supplier or customer standards, such as a government agency, an industrial supplier or customer, or an internal supplier or customer. In general, it is recommended that a person be designated to act as the ESD control coordinator. The coordinator would be responsible for implementation of the ESD control program. The program should include the following: ●

●

●

●

● ● ● ●

●

●

●

Ensure conformance of ESD-safe areas to applicable requirements. This should be done prior to their use and periodically during their use. Use appropriate protective clothing, equipment, and procedures throughout the program where ESD-sensitive items are unprotected. Establish and operate an appropriate training program to ensure that all personnel entering an ESD-protected work area are aware of and follow applicable control measures. This should include certification of proficiency and maintenance of appropriate records. Perform and maintain adequate records of audits and inspections, including documentation of corrective actions where required. This should apply to suppliers and customers as well as to internal operations. Ensure adequate packaging and labeling of ESDS items. Ensure proper identification of ESDS items when not packaged. Include ESD requirements on purchasing documents. Describe applicable procedures for handling of ESDS items at receiving, during inspection, assembly, fabrication, test, failure analysis, and shipping. This can extend to procedures at customers’ locations, field operations, installation, maintenance, and returns. Establish and maintain approvals for all materials and equipment used in the ESDprotected work area. This can be in the form of materials qualification and lot certification acceptance testing where applicable. Ensure adequate training and performance of support staff, operators, technicians, and management personnel for the ESDS items. This should include special training for ESD auditors. Maintain and have available all required ESD test equipment.

ESD CONTROL PROGRAM

113

The ESD control coordinator should stay informed of the available and emerging technology in the field, if necessary through attendance at domestic and overseas technical meetings. He or she should keep management informed of the state of ESD awareness on the site through regular summary reports of audits and inspections. If appropriate, the coordinator should organize an ESD control committee, and should provide periodic meetings and seminars to reinforce training, to provide enrichment, and to provide updates when new developments occur. Certification Audits and Verifications Surveys and inspections should be performed on a regular basis. Two types of surveys and inspections should be provided in a comprehensive ESD control program: formal audits and verifications. A formal audit results in a formal survey report certifying the acceptance of a facility or process to an accepted level of requirements and usually results in a formal document, usually using a standard survey form of approval or acceptance. Audits are usually performed on a scheduled basis, such as quarterly or annually. A verification is more a confirmation that an area remains suitable for use or that a new project that will be introduced into a previously certified area will be successfully protected from ESD. Verifications usually make note of an existing formal audit and should provide a positive statement that the area is verified to be suitable for use by the new project. For small projects using only a fraction of a large facility, the verification can be limited only to that portion of the facility the small project will use. Policies regarding documentation of verification surveys range from formal documentation using the standard survey form with acceptance to a memo to file kept as part of project files. Case Study: Audits and Verifications at a Multiple-Use Test Facility Many large companies maintain centralized test facilities for cryogenic, vibration, EMI, vacuum, and other highly specialized test operations. These general-use facilities are built and certified for use by many projects on an as-needed basis. The facility may be certified to some general-use condition through a formal audit and certification. When a new project is planned to move into the facility, a verification procedure should be carried out. In this process the facility is inspected to verify that all of the ESD controls required for the new project are in place in the existing facility. A verification notice can then be placed in the project records, ensuring ESD safety for the new project but avoiding unnecessary costs associated with a full formal audit, especially if only a small part of the existing facility is to be used for the new project. If additional control measures are found to be needed for the new project, a safe path can be carved through the existing facility, incurring additional expense only where needed. This type of ESD control audit and verification approach provides added protection where needed but avoids unnecessary expense. A suggested schedule for various surveys and inspections is included in Table 2.10. These surveys and inspections should assess and document the ongoing adequacy of the ESD control program. ● ● ● ● ●

Plan implementation Design, construction, and maintenance of ESD-protected work areas Protective procedures used in the program Training and certification of all personnel Performance of all personnel

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FUNDAMENTALS OF ESD CONTROL

TABLE 2.10 Suggested Frequency of Audits and Inspections Item Wrist straps and coiled cords Footwear Garments Wrist strap during operations Workstation grounding Floor mats

Frequency

Go/no-go gauge

Daily Each entry Continuously monitored

800 k to 108  Worn properly Plugged in when not standing

Go/no-go gauge Visual Visual & audible

Log sheeta waived if monitored continuously Log sheet None None

Each entry

Secure connection

Visual

Nonea

Each entry

Damage; secure connection Resistivity and resistance to ground Clean and orderly

Visual

Nonea

Surface resistivity meter

Log sheeta,b

Visual

None

Surface resistivity meter

Log sheeta,b

Surface resistivity meter Visual

Log sheeta,b

Daily

Monthly

Chairs, carts, Daily other mobile equipment Monthly

Wrist strap Quarterly tester Footwear tester Quarterly Ionizer Semiannual

Resistivity and resistance to ground Resistivity and resistance to ground ESD-approved equipment

None

Resistivity and Surface Log sheeta,b resistance to resistivity meter ground Resistance verification Calibration resistor Log sheeta,b Resistance verification Calibration resistor Discharge time Charge plate and balance monitor or ionizer verifier To requirements Auditor with complete kit Expiration date Training records Typical: VOM

0.020 mVac

10 

ESD system

Annually

Training Ac-powered hand tools

Annually Daily

Relative humidity

Daily

To requirements

Temperature– RH meter

Footwear

Each entry

To requirements

Footwear tester

a

Records

1  0.2 M

Monthly

Floors

Test Methods

Daily

Monthly

Workstation mats

Acceptance Limits

b

Log sheeta,b Log sheeta,b

Certification report Certification report Log sheeta waived if monitored continuously Log sheeta waived if monitored continuously Log sheeta

Record exceptions and corrective actions. Adjust cleaning methods and inspection frequency as needed.

ESD AND CONTAMINATION CONTROL ● ● ●

115

Maintenance of training records Packaging and storage Corrective actions, including identification of inventory at risk

Processes The processes by which static-sensitive components are manufactured are where some of the most critical but most overlooked problems and solutions to static control are found. This is where the “rubber meets the road” for a successful manufacturing process. The ESD and/or contamination control engineer must work with process engineers, equipment engineers, manufacturing personnel, and management to ensure that the static controls that are put in place not only assist in controlling static but also do not affect the process deleteriously.

2.12

ESD AND CONTAMINATION CONTROL

The requirements of ESD and contamination control should be treated separately, except to the extent that one influences the other. In this regard the following statements are true: 1. Control of charges on surfaces in cleanrooms derive a benefit for contamination control. In this regard, selecting materials with a low tendency to tribocharge and that can be grounded because they are static dissipative or conductive is desired. However, alone these two solutions are insufficient in the vast majority of cases and the use of air ionization provides further benefit. Thus, the use of air ionization can provide a benefit for contamination control, regardless of the ESD sensitivity of products or processes within the cleanroom. 2. Control of electrostatic discharge in cleanrooms can provide a benefit by minimizing ESD and EMI induced microprocessor upset, regardless of the ESD sensitivity of products or processes within the cleanroom. 3. Airflow in mixed flow cleanrooms is not optimized for performance of air ionizers not equipped with fans or compressed air sources. Fan powered or compressed gas ionizers are preferred in mixed flow cleanrooms for this reason. However, fan powered or compressed gas ionizers can increase redistribution of contamination and can have a detrimental effect. The requirements of both ESD (discharge time and float potential) must be balanced against contamination (airborne particle count, surface contamination rates, and so forth. 4. Ionizers placed near ceiling or bench mounted HEPA filters can perform well in unidirectional flow applications, particularly for controlling surface contamination. Conversely airflow in unidirectional flow cleanrooms and clean benches can cause isolation effects or flow stratification that prevents ionizers from achieving discharge time performance. Proper ionizer deployment must take into consideration cleanroom airflow effects. 5. Ceiling mounted room type ionizers may not provide rapid discharge times. As the ESD sensitivity of devices tend toward lower voltages, only local fan or gas powered ionizers can achieve acceptable ESD performance. In these cases, balancing the requirements of ESD failures versus contamination failures becomes precarious.

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FUNDAMENTALS OF ESD CONTROL

6. All ESD control materials and equipment must be qualified for contamination performance where they are used in contamination sensitive applications. Similarly, all contamination control materials and equipment must be qualified for ESD performance where they are used in ESD applications. 7. Continuous monitoring systems are available for both contamination and ESD control applications. Their use in work stations that simultaneously require both forms of control can serve to minimize upsets in processes.

2.13

USEFUL REFERENCE STANDARDS

Many procedures for neutralizing charge can be the subject of process capability studies: a good example is neutralization of the charge generated when pressure sensitive. Adhesives are pealed. This neutralization is typically accomplished by holding the charged item in the airflow from an ionizer for several seconds before the item is applied to a surface. A process capability study could consist of pealing samples, holding them in the ionized airflow for various times and then measuring the residual charge. On a time is found that appears to neutralize the charge this time is used to neutralize many samples. The average and standard deviation of residual charge can then be used to calculate how long the charged item must be neutralized to achieve a 3, 6 or more sigma process. ●

● ●

DOD HNDBK-263, Handbook for Protection of Electrical/Electronic Parts, Assemblies and Equipment MIL-B-81705, Barrier Materials, Flexible, Electrostatic Protective, Heat Sealable MIL-STD-1686, Program Requirements to Establish and Implement an ESD Control Program

ANSI/EOS/ESD Association standards, as follows:

● ● ● ● ● ● ● ● ● ● ● ●

● ● ●

STD S1, Personnel Grounding Wrist Strap STD DS2-1995, Garments STD S3.1-1991, Ionization STD S4.1-1990, Work Surfaces: Resistive Characterization STD S5.1-1993, Human Body Model STD S5.2-1994, Machine Model STD DS5.3-1993, Charged Device Model STD S6.1-1991, Grounding: Recommended Practice STD S7.1-1994, Floor Materials: Resistive Characterization of Materials STD S8.1-1993, Symbols: ESD Awareness STD DS9.1-1994, Footwear STD S11.11-1993, Surface Resistance Measurement of Static Dissipative Planar Material: Bags STD S11.31-1994, Shielding Bags STD S20.20-1999, ESD Control Program STD S541-2003, Packaging Materials for ESD Sensitive Items

REFERENCES AND NOTES

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REFERENCES AND NOTES 1. A. Z. H. Wang, On-Chip ESD Protection for Integrated Circuits: An IC Design Perspective, Kluwer Academic, Boston, 2002. 2. A. A. Amerasekera, C. Duvvury, W. Anderson, and H. Gieser, ESD in Silicon Integrated Circuits, Wiley, New York, 2002. 3. J. Wiley and A. Steinman, Investigating a new generation of ESD-induced reticule defects, MiCRO, Apr. 1999, pp. 35–40. 4. A. Steinman, Electrostatic discharge: MR heads beware, Data Storage, July–Aug. 1996, pp. 69–72. 5. T. Murakami, H. Togari, and A. Steinman, “Electrostatic Problems in TFT-LCD Production and Solutions Using Ionization”, Proceeding of EOS/ESD symposium, pp. 364–371 Orlando, FA Oct. 1996. 6. L. B. Levit, L. G. Henry, J. A. Montoya, F. A. Marcelli, and R. P. Lucero, Investigating FOUPs as a source of ESD-induced electromagnetic interference, MICRO, 2002, pp. 41–48. 7. N. Jonassen, Human body capacitance, EOS/ESD Conference Proceedings, Las Vegas, NV, Oct. 6–8, 1998, pp. 111–117. 8. Thales is credited with discovering that amber rubbed with wool or fur attracts light bodies. None of Thales’s manuscripts is known to have survived to modern times. Everything we know about him comes from the writings of others, particularly Aristotle. 9. H. B. Michaelson, Relation Between an Atomic Electronegativity Scale and the Work Function. IBM J. Res. Develop, V. 22, No. 1 Jan, 1978, pp. 72–80. 10. L. Pauling, The Nature of the Chemical Bond, 3rd ed., Cornell University Press, Ithaca, NY, 1960. 11. J. Hölzl and F. K. Schulte, Work functions in metals, in Solid Surface Physics, G. Höhler, Ed., Springer-Verlag, Berlin, 1979. 12. J. C. Riviere, Work function: measurement and results, in Solid State Surface Science, Vol. 1, M. Green, Ed., Marcel Dekker, New York, 1969. 13. One of the most frequently used procedures for evaluating planar material is the inclined plane test method described in ESD ADV 11.21. The test measures the charge developed on Teflon and quartz cylinders when rolled down a test sample mounted to a plane inclined at 15°. 14. R. C. Allen, IC susceptibility from ESD-induced EMI, Evaluation Engineering, May 1998, pp. 116–121. 15. A. Wallash and D. C. Smith, Damage to magnetic recording heads due to electromagnetic interference, IEEE EMC Symposium Proceedings, 1998 pp. 834–836. Denver, August 26, 1998. 16. A. Wallash and D. C. Smith, Electromagnetic interference (EMI) damage to giant magnetoresistive (GMR) recording heads, EOS/ESD Conference Proceedings, Las Vegas, NY, Oct. 6–8, 1998, pp. 368–374. 17. O. J. McAteer, R. E. Twist, and R. C. Walker, Identification of latent ESD failures, EOS-2, Sept. 9–11, 1981, pp. 54–57. 18. J. Markey, D. Tan, and V. Kraz, Controlling ESD damage of ICs at various steps of back-end process, EOS/ESD Conference Proceedings, Portland, OR, Sept. 11–13, 2001, pp. 120–124. 19. ANSI/ESD STM5.1-2001, Electrostatic Discharge Sensitivity Testing: Human Body Model (HBM) Component Level. 20. ANSI/ESD STM5.2-1999, Electrostatic Discharge Sensitivity Testing: Machine Model (MM) Component Level. 21. ANSI/ESD STM5.3.1-1999, Charged Device Model (CDM): Component Level. 22. ANSI/ESD SP5.5.1-2004, Electrostatic Discharge Sensitivity Testing Transmission Line Pulse (TLP) Component Level.

118 23. 24. 25. 26. 27.

28. 29. 30. 31. 32.

33. 34. 35.

36. 37. 38. 39. 40. 41.

FUNDAMENTALS OF ESD CONTROL

N. Jonassen, How fast does a charge decay? Compliance Engineering, 17: 2 (2000). T. O’Connell, the case for continuous monitoring, EOS/ESD Technology, Oct. 1992, pp. 16–17. Microcontamination, Staff Industry News section of Magazine May 1988, pp. 10–11. K. D. Murray, V. P. Gross, and P. C. D. Hobbs, Clean corona ionization, ESD Journal Dec 1991/Jan. 1992. C. E. Newberg, Analysis of the electrical field effects of ac and dc ionization systems for MR head manufacturing, EOS/ESD Conference Proceedings, Orlando, FL 1999, Sept. 28–30 pp. 319–328. JPL Standard D1348, Standard for Electrostatic Discharge Control, March, 2003 R. W. Welker previously unpublished laboratory data. R. Moss, Exploding the humidity half-truth and other dangerous myths, EOS/ESD Technology, Apr. 1987, p. 10. See http://llis.nasa.gov/llis/plls/index.html, PLSS data base entry 0301, Electrostatic discharge (ESD) wrist strap contamination of Magellan flight hardware. M. J. D. Dyer, The antistatic performance of cleanroom clothing: Do tests on the fabric relate to performance of the garment within the cleanroom? EOS/ESD Conference Proceedings, Sept. 1997 Santa Clara 23–25 pp. 276–286. W. Boone, Evaluation of cleanroom/ESD garment fabrics: test methods and results, EOS/ESD Conference Proceedings, Las Vegas, NV, Oct. 6–8, 1998, pp. 10–17. Currently replaced by ANSI/STD S541-2003, Packaging Materials for ESD Sensitive Items. One test method for charge decay time is described in J. Passi, S. Nurmi, R. Vuorinen, S. Strengell, and P. Maijala, Performance of ESD protective materials at low relative humidity, Journal of Electrostatics, V51–52 May 2001, pp. 429–434. R. L. Benson and S. V. Patel, Exploring ESD thermoformable packaging materials, Evaluation Engineering, Nov. 1998 pp. S-4 to S-11. R. J. Pierce and J. Shah, Potential ESD hazards when using adhesive tapes, Evaluation Engineering, 1996, pp. S-30 to S-31. S. Heyman, C. Newberg, N. Verbiest, and L. Branst, Voltage detection systems help battle ESD, Evaluation Engineering, Nov. 1997, pp. S-6 to S12. G. Baumgartner and J. S. Smith, EOS analysis of soldering iron tip voltage, EOS/ESD Conference Proceedings, Las Vegas, NV, Oct. 6–8, 1998, pp. 224–232. N. Jackson, W. Tan and D. Boehm, Magneto optical static event detector, EOS/ESD Conference Proceedings, Las Vegas, NV, Oct. 6–8, 1998, pp. 233–237. N. Jackson, Nelson, D. Boehm, and B. Odum, Advances in magneto optical static event detector technology, EOS/ESD Conference Proceedings, Anaheim, CA, 2002, pp. 187–193.

CHAPTER 3

SAMPLING AND ANALYSIS METHODS

3.1

INTRODUCTION

Sampling and analysis are among the most important aspects of ESD and contamination control. Reflecting this, there are many books on sampling and analysis of contamination. These are useful references in their own right. Unfortunately, these books all too often focus all of their attention on laboratory methods. Often, these laboratory methods involve the use of complex instruments that are unsuitable for use outside a sophisticated laboratory setting. An understanding of these varied methods is important because there will often be a need to identify contaminants as an aid to identifying and eliminating their sources. However, there has often been an emphasis on the use of these types of analysis in materials science laboratories to the detriment of consideration of methods better suited for analysis and control of contamination more proximate to their source: in the cleanroom or the ESD-safe workplace. Here a different approach is taken to sampling and analysis. Here the emphasis is more on the application of analysis, especially as applied to control of manufacturing locations, both in-house and at suppliers. 3.2

CLASSIFICATION OF ANALYSIS METHODS

There are several ways of classifying contamination analysis methods: 1. Classify the methods into categories reflecting the matrix in which the contamination is found (Figure 3.1a). 2. Classify the methods with regard to the phase of contamination (Figure 3.1b). 3. Classify the methods with regard to the category of contamination (Figure 3.1c). Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

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Solid Air Liquid Surface Vacuum Solid

(a)

Liquid Gas or vapor Plasma or ionized

(b)

Particles Films Gases and vapors Ionic Organic Viable Electromagnetic radiation Electrostatic charge Other

(c)

FIGURE 3.1 Methods of categorizing contamination sampling and analysis methods: (a) by the matrix in which the contaminant is found; (b) by the phase of the contaminant; (c) by the type or category of the contaminant.

Each of these categorization methods has advantages and disadvantages. The primary advantage is that each categorization provides a structure in which to organize a discussion comparing and contrasting the various methods. The principal disadvantage is that there is considerable overlap within each categorization scheme. For example, some forms of viable contamination must be considered as particles that may be sampled in liquids, in air, or on surfaces. Gases and vapors may be organic or inorganic. One way to categorize contamination sampling and analysis methods is to divide them into functional tests and objective laboratory tests. A second way is to describe them as tests that are suitable for field measurements versus tests that must be performed in a laboratory. These two ways of categorizing contamination sampling and analysis methods are defined as follows: ●

●

Functional laboratory tests are those that do not necessarily describe an amount of contamination but describe the effect that contamination inherently produces. Objective laboratory tests tend to describe a quantity of contamination without regard to the effect of that quantity of contamination.

This distinction is important. Functional tests are often used to qualify materials and processes to determine their suitability for use in manufacturing applications. For this reason, functional tests are critical in the process of selecting materials and processes. Objective laboratory tests usually are not concerned with functional effects but with determining the concentration of a contaminant that can be detected with some level of accuracy and precision.

CLASSIFICATION OF ANALYSIS METHODS ●

●

121

Many laboratory tests involve use of expensive instruments, consume considerable time, and may require the involvement of highly skilled personnel. Field tests should be low cost, rapid, and easy to conduct and interpret.

This distinction is important. A laboratory test that must be performed in a laboratory generally uses relatively expensive equipment that is difficult or impossible to move. These types of tests are often time consuming and require the attention of highly trained personnel. Laboratory tests are often prohibitively expensive and time consuming and would be considered generally unsuitable for use in field tests. This limits their utility in many practical applications. Conversely, they may be measuring the quantity of a contaminant that is of functional importance. In addition, laboratory tests often provide chemical identification. Field tests that are simple, rapid, low-cost and easy to interpret are better suited for use in manufacturing for process capability studies and longterm process control. The disadvantage of field tests is that they generally do not provide chemical identification information and must be supplemented with laboratory methods where out of control conditions are discovered. This discussion focuses on a discussion of sampling and analysis methods from this perspective: Identify a suitable field test that is easy, fast, and low in cost that can be used to quantify the amount of a functional contaminant in a setting outside the laboratory: in the field at the closest point of contamination generation as possible.

3.2.1

Functional Laboratory Tests

In functional tests, the product is exposed to conditions that either realistically represent the actual use conditions or to conditions expected to accelerate the expected failure modes using realistic acceleration factors. Functional tests can be especially useful for consumers because they are both simple and measure directly the effect of the material being tested on the product to which it is being exposed. Examples of functional tests include corrosion tests, particle shedding tests, and outgassing tests. Several considerations are important for functional tests. One of the most often encountered is the question: Are the accelerated test conditions realistic? Some accelerated stress tests can be so unrealistic that they do not reflect the true risk of damage to product from the material under test. For example, let us suppose that we conduct an outgassing test at 150°C. If the material being tested would never experience this high a temperature and would chemically degrade at that temperature, the acceleration conditions would be excessive and will produce misleading results. One of the oldest forms of functional tests relevant to contamination control is the accelerated corrosion test, in which a material is placed in a chamber and subjected to higherthan-ambient temperature and relative humidity. The mechanism of acceleration and the resulting acceleration factor are the subject of much debate. In the construction and automotive industries, accelerated corrosion test conditions are designed to shorten time to failure for some mechanical or cosmetic characteristic of the material under consideration. These test conditions often include salt spray and alternating periods of surface wetness and dryness. In these tests, obvious material degradation is the criterion for evaluation. Conversely, the vast majority of consumer products do not see service conditions in which exposure to salt or alternating periods of wet and dry are realistic. For these types of materials, test conditions generally are milder. Conversely, the failure modes in high-technology

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products occur at a microscopic level. The acceptance criteria for corrosion resistance for high-technology materials are correspondingly more restrictive. It can be argued that an accelerated stress test, no matter how seemingly arbitrary, that separates good materials from bad is acceptable as long as every candidate does not fail and a selection can be made based on test results. Conversely, if the only candidate that does not fail is many times more expensive than the candidates that failed, the debate regarding the validity of the acceleration conditions will continue. Corrosion Test Acceleration Factors There are 15 ASTM standards relating different variations of creating and controlling fog and humidity in cabinets for corrosion testing of a broad spectrum of products. Elevated humidity tests are most commonly used to evaluate the corrosion resistance of materials or the effects of residual contaminants. Cyclic humidity tests are conducted to simulate exposure to the high humidity and heat typical of tropical environments. These tests usually include periods of exposure to sprayed salt solutions for automotive or construction materials. The most frequently used standard tests for corrosion resistance are cyclic and static accelerated corrosion tests such as ASTM G85 and B117. However, high-technology products rarely see service conditions in which exposure to weather is the primary factor under consideration. In most high-technology applications, the cleanliness of the item under consideration dominates its acceptance for corrosion resistance. Thus, most high-technology accelerated corrosion tests do not introduce chemical accelerants such as acidic gasses or salt spray. Rather, these tests simply use elevated temperature and relative humidity to accelerate corrosion response of the materials under consideration. The relative acceleration factor is an important consideration. Fortunately, there is a large database in the semiconductor industry for electrochemically driven corrosion failure mechanisms. The model used in these estimates of equivalent field corrosion times was developed by Weick [1]. This model compares corrosion at the test temperature (in kelvin) and percent relative humidity (% RH) vs. corrosion at ambient temperature and humidity in the field. The specific equation is acceleration test/field  e6444(1/Ttest 1/ Tfield )e0.0828(%RHtest %RHfield ) Two examples are useful: ●

●

A 96-hour soak test at 60°C/80% RH in a chamber would be equivalent to 25,570 hours at 25°C/40% RH in an ambient environment. A 24-hour soak test at 80°C/80% RH in a chamber would be equivalent to 700 hours at 25°C/80% RH in an ambient environment.

Functional Testing Problems Functional tests are problematic on a second level, aside from the suitability of test conditions and the production of unrealistic failures. Many of the suppliers for products do not have the facilities or expertise to conduct such tests, and even if they did possess the appropriate resources, it may be impossible for them to determine what constitutes a failed outcome. This is due partly to the fact that most suppliers do not have access to the specific materials that are critical for their susceptibility to corrosion; only their customers have these materials. An excellent example exists in the disk drive industry. The critical surfaces for corrosion susceptibility in the disk drive industry are the disks and

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123

the magnetic recording heads. The items are not generally available to suppliers of items such as packaging materials. The best that suppliers can hope for is that the customer will provide materials to test. If the customer’s products are in development, the materials may be in limited supply or so sensitive that releasing them to a supplier may not be a good business practice. Thus, in many circumstances, suppliers have severely limited access to the products for which they are trying to qualify. Contact and Near-Contact Stain Tests Many materials used in cleanrooms sometimes come in contact with products or are in close proximity to, but not incontact with, products. Two types of tests are applied to evaluate the functional suitability of materials for cleanroom applications for these two cases: contact stain and near-contact stain. Others may be specified, depending on the users’ functional requirements. In a contact stain test, apparatus suitable to hold the test material and product is prepared so that apparatus contribution to the test is negligible. Several strips of the material under test are held against the product. The apparatus is then sealed in a thin polyethylene plastic bag to prevent interaction with gases from adjacent bags containing test specimens. Occasionally, samples are also protected with aluminum foil or, better yet, copper strips to prevent crosstalk from other bags. The sacrificial metals normally are placed outside the bags containing the items under test, to prevent them from masking adverse results. The bags are then placed in a temperature–relative humidity (TRH) chamber for conditioning. Many different companies use this test. Typical conditions are 60 to 80°C and 70 to 85% RH for a period of 4 to 7 days. (Some companies test for as long as 21 days, but this is unusual.) At the end of the test the TRH chamber is returned to ambient temperature and humidity under noncondensing conditions. The product is removed from the chamber and inspected for signs of stains, discoloration, or corrosion. This may be done by unaided eye inspection or by inspection using magnification. One particular form of this test is used in the qualification of materials for the disk drive industry [2]. A near-contact stain test is virtually identical to a contact stain test. The primary difference is that the material under evaluation is held in close proximity to but not incontact with the product. Care is taken to ensure that the material under test cannot drip or sag onto the product. The material under test is usually beneath the product. Spacing between the material under test and the product is typically 250 to 1270 m (0.01 to 0.05 in.). There is one other consideration regarding contact and near-contact stain tests. The material under test may come in contact with water, isopropyl alcohol, or other chemicals that extract damaging substances. If this is the case, extracts obtained by soaking the materials in appropriate solvents are used as the challenge material in the functional tests, often in the form of dried residues. Functional ESD Tests Several different types of functional ESD tests are available. These are based on four models: the human body model (HBM), the charged-device mode (CDM), the machine model (MM) and the transmission line pulse (TLP) model. Each is intended to estimate the sensitivity of the device to exposure to various voltages or fields. The human body model is intended to imitate the electrostatic discharge of a charged person through the tip of a bare finger. The charged-device model is intended to imitate the discharge that occurs when an electrostatic-sensitive device becomes inadvertently charged in an external electrical field and is then discharged. The machine model is intended to estimate the voltage that must be present on a conductive surface that could cause a damaging electrostatic discharge. The transmission line pulse model is intended to estimate the

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effects of power surges. Functional ESD tests are very useful but are never used as field inspections. 3.2.2

Nonfunctional Tests: Objective Laboratory Tests

Materials qualified under functional tests are generally considered acceptable for use in a specific application. Many of these tests require the use of product materials that are unavailable to a supplier. The supplier cannot duplicate the qualification test to verify that their product will continue to conform to requirements. For this reason it is imperative that materials be characterized using objective laboratory tests to provide an objective means for a supplier to ensure the consistency of the product on an ongoing basis. The results of these tests are then used to specify to the supplier the properties desired. The tests will quantify such parameters as extractable particles, anions, cations, organic contaminants, nonvolatile residue, and viable contamination. Electrostatic charge can also be specified to the supplier. These tests are generally used for qualification of materials at the supplier. Many of the instruments and facilities for these objective laboratory tests are strictly laboratory instruments, some of which are portable. In many cases, field inspection instruments are used in place of laboratory test instruments. Most large companies either possess a state-of-the-art materials analysis lab or have access to one. The types of analysis equipment and methods available at these laboratories can be vast, complicated, and expensive. The expertise required to use the instruments and interpret their output seldom is available to small producers. Therefore, in developing control limits on contamination or ESD performance from materials for use by the material supplier, it is important to use instruments and methods that are “supplier friendly”: inexpensive, easy to use, and producing understandable results. Next we summarize some of the most common techniques available in objective laboratory analyses for characterization of contamination. Light Microscopy Light microscopy is one of the simplest and lowest-cost methods for routine analysis of contaminants. Low-power light microscopy (Figure 3.2), usually at a magnification below about 50, is where many contamination analyses begin. The value of low-power microscopy as a starting point in any contamination control program has long been recognized. A sample is examined at low magnification (typically about 7), and the magnification is increased gradually until a general understanding of the composition is obtained. During this examination, the sample or illumination is usually manipulated to aid in understanding the nature of the specimen: ● ● ● ●

Is it reflective? Is it uniform? What shape is it? Do surface features exist?

An experienced person using a low-power microscope can identify objects as small as about 50 m with reasonable accuracy based on the appearance of the object alone. A lowpower light microscope can often be used to determine the type of further analysis required to produce definitive chemical identification. An example of how low-power microscopy can enable an overall contamination analysis scheme is shown in Figure 3.3. Low-power light microscopes are also used in the field, in both contamination and ESD control.

CLASSIFICATION OF ANALYSIS METHODS

125

FIGURE 3.2 Typical low-power light microscope. These instruments typically feature stereo vision, magnifications of up to 40 (80 with a 20 objective), and long working distances. A long working distance of 5 to 8 cm allows samples to be manipulated by hand while maintaining focus.

Analytical Light Microscopy Higher-power light microscopes can typically have magnifications up to around 1000. At this magnification, objects as small as 1 or 2 m can be seen. In addition to simple shape, size, and color information, high-power light microscopes offer special illumination and observation conditions that make it possible to determine additional information about a specimen. This additional information can be helpful in identifying the contaminants being observed. In a bright-field microscope, the entire field of view is illuminated. Contrast between the specimen and the sample holder (e.g., filter, microscope slide) is low. Observation is enhanced by using dark-field illumination because the relatively dark background of the sample allows scattering from the specimen to be observed under higher contrast. If the light used to illuminate the specimen is polarized, the tendency of the specimen to depolarize the light can be used as an aid to identification. Finally, illuminating the specimen at ultraviolet wavelengths provides a dark-field image that is enhanced by the emission of light from particles that fluoresce. The color of the fluorescence can be an aid to contamination identification. Figure 3.4 is an example of a dark-field polarized light microscope suitable for analytical microscopy. Light microscopes with magnification greater than about 100 are generally considered to be high-power as opposed to low-power microscopes (Figure 3.2). These microscopes were among the first instruments used for sizing and counting contamination, simply by comparing the size of the particles to an eyepiece reticule used as a measurement scale. Because of their veneration, the light microscope has often been cited as the referee method for determining the truth of a particle count where there is a disagreement between two or more other techniques that are not based directly on microscopy. However, even when a specimen is available for repeated counting using a light microscope, particle sizing has been shown to have its limitations. In high-technology applications, the repeatability

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Liquid to: Ion chromatography, FTIR, GC/MS, TOFSIMS, ICP

Filter to low power light microscopy

Inorganic fraction

SEM/EDX AFM/MFM

Organic fraction

FTIR, Raman

Contamination Inventory FIGURE 3.3 How a low-power microscope fits into an overall contamination analysis scheme. Results of the inspection of the filter using this microscope determine the relative portion of the sample to be used for subsequent analyses.

and reproducibility of particle counting using a light microscope must be understood in order to comprehend its overall role in a contamination control program. Analytical light microscopes are occasionally used as field instruments in contamination control, although less often than are low-power binocular microscopes. This is true because they are more expensive, complicated, and require a greater degree of skill in their use than do low-power microscopes. Analytical light microscopes are seldom used in the field in ESD applications. Particle Counting with a Light Microscope Counting particles using a light microscope is almost certainly the oldest particle-counting technique. Counting particles inherently implies measuring particle size and often results in a particle size distribution. To size particles accurately using the microscope it is necessary to provide a comparison scale of some sort. The most common way of doing this is to outfit the eyepiece of the microscope with an eyepiece reticle (sometimes called a graticule). The venerability of this method is

CLASSIFICATION OF ANALYSIS METHODS

127

FIGURE 3.4 Dark-field polarized light microscope suitable for analytical light microscopy.

illustrated by the fact that one of the definitive references for the design and use of reticles for particle counting is now over 50 years old [3]. Two examples of commonly used eyepiece reticles are illustrated here. Figure 3.5 is a combination horizontal and vertical scale. Figure 3.6 is an illustration of a Patterson globe and circle reticle. Each of these is calibrated by measuring a stage micrometer: a precisely ruled size-reference standard. Once the eyepiece reticle has been calibrated, it is then used to measure the size of the particles on a specimen. There are several ways to measure the diameter of particles using the microscope. Microscopic Particle Sizing Techniques For perfectly spherical particles, those most often encountered in calibration of particle-counting equipment but rarely observed in realworld samples, there is little or no ambiguity in measurement of the size of the particles. When counting odd-shaped particles, which represent the vast majority of particles in realworld samples, the method of sizing could produce a number of different results. To achieve some degree of standardization, there are some definitions for describing particle sizes (see Figure 3.7): ●

●

Minimum linear diameter: shortest distance between two parallel lines tangent to the perpendicular projection of the particle in the measuring plane Maximum linear diameter: longest distance between two parallel lines tangent to the perpendicular projection of the particle in the measuring plane

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SAMPLING AND ANALYSIS METHODS

0 10 20 30 40 0

10

20

30

40

60

70

80

90

100

60 70 80 90 100

FIGURE 3.5 Combination horizontal/vertical-scale eyepiece reticle. 25

20

15

12.5

10

8

6

4

2

1

25

20

15

12.5

10

8

6

4

2

1

FIGURE 3.6 Patterson globe and circle eyepiece reticle. ●

●

Feret’s diameter: perpendicular projection onto a fixed direction of the distance between two parallel lines, parallel with the fixed direction, tangent to the extremities of the perpendicular projection of the particle in the measuring plane Martin’s diameter: length of the line parallel to a fixed direction that divides into equal areas the perpendicular projection of the particle in the measuring plane

CLASSIFICATION OF ANALYSIS METHODS

129

Feret's diameter

Minimum linear diameter

um ter im me x a a M r di a e lin

Martin's diameter

FIGURE 3.7 Definitions for sizing particle diameters using a light microscope.

One of the most convenient of these to use is Feret’s diameter. In this application, the sample is traversed under the objective and each particle is positioned next to the vertical/horizontal-scale eyepiece reticle. The sample is traversed in a systematic way to avoid recounting fields of view containing particles that were counted previously. This counting technique (and indeed, all of the counting techniques for irregularly shaped particles) makes the assumption that orientation of the particles on the surface being examined is random. This will tend to cancel out orientation to produce a meaningful average particle size. This assumption tends to be violated when the concentration of particles is very low, for which it has been shown that the repeatability and reproducibility of microscope counting of irregularly shaped particles can limit its value as a referee particle-counting technique [4]. Infrared Spectroscopy In infrared spectroscopy, the tendency of materials to absorb light at infrared wavelengths is used to identify their chemical composition. In the modern contamination control lab, the state-of-the-art manifestation of infrared spectroscopy is the Fourier transform infrared (FTIR) spectrometer. In this instrument, the entire infrared spectrum is sampled hundreds of time, typically one spectrum every ½ second, and the resulting spectra are combined to produce a spectrum having an improved signal-to-noise ratio. This work is typically done using an infrared microscope (Figure 3.8), allowing routine analysis of particles as small as 30 m in diameter. The primary application in contamination control is identification of organic materials. Inorganic materials also have identifiable IR spectra but are seldom analyzed by FTIR spectroscopy in contamination control applications. The diameter of an organic particle that can be identified by ordinary FTIR microscopy is often a limiting factor in the use of this technique. Analysis of particles smaller than 30 m in diameter is difficult and time consuming, and few analysts are successful in producing definitive results. Portable FTIR spectrometers are used in remote-sensing applications to detect air pollution or for homeland security applications. They have not yet found their way into the cleanroom or ESD environment. Conversely, nondispersive infrared spectrometers can be used for sensing of gas-phase contamination in continuous monitoring applications. Raman Spectroscopy Raman spectroscopy is a complementary technique to FTIR microscopy. In Raman spectroscopy, a sample is illuminated at a single wavelength and its

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FIGURE 3.8 Typical FTIR microscope. Instruments like this can be used routinely to identify organic particles down to about 30 m in diameter. With great care, a long analysis can produce infrared spectra suitable for identification of particles as small as about 15 m in diameter. However, this is very difficult and time consuming.

emission in the infrared is measured. The amount of light emitted is vanishingly small. The emission is easier to detect than is the light absorbed in FTIR microscopy, because the detection of Raman emission is a dark-field technique, whereas absorption is a bright-field technique. Combining this with the use of lasers to illuminate a sample using light sources as small as 1 m in diameter, Raman spectroscopy extends the range of organic particle analysis down to about 1 m in diameter. Figure 3.9 shows a typical Raman microscope. Other Adjunct Infrared Spectroscopy Methods Infrared spectroscopy can be used with a wide variety of other sampling techniques. Some of these allow for the analysis of gases and thin films. One of the most popular of these for contamination analysis is diffuse reflectance FTIR (DRIFT) spectroscopy. Diffuse reflectance occurs when light impinges on the surface of a material and is partially reflected and transmitted. Light that passes into the material may be absorbed or reflected out again. The light that is detected is a combination of the reflected and emitted components. Scanning Electron Microscopy In scanning electron microscopy (SEM), a focused beam of electrons is scanned over the surface of an object in a controlled fashion. Electrons scattered or emitted from the surface of the object are collected by a detector. The intensity of the location emitting the electrons is known from the position of the incident beam on the surface. The surface can then be imaged on a cathode-ray tube (CRT) by synchronizing the position of the electron beam in the CRT with that of the electron beam on the sample. The SEM is capable of producing images with greater depth of focus than is achievable using ordinary light microscopy. However, there is a great deal more happening, due to interaction of the incident electron beam with the sample, than in simple electron scattering, as illustrated by Figure 3.10.

CLASSIFICATION OF ANALYSIS METHODS

FIGURE 3.9

131

Typical Raman microscope.

X-rays Depth composition

Incident beam Primary scattered electrons

Auger emitted electrons

Primary and secondary scattered electrons

(Topography)

(surface composition)

Secondary scattered electrons

Sample

Sample holder

FIGURE 3.10 In a scanning electron microscope the incident beam of electrons is scattered back toward the detector. The primary scattered electrons reveal a contrast in brightness that indicates highatomic-number materials (brighter) vs. low-atomic-number materials (darker). Secondary scattered electrons do not provide atomic number contrast. Auger electrons are emitted by atoms very near the surface, typically within 2 nm, and their energies are characteristic of the element from which they originate. The energies of x-rays are characteristic of the element from which they are emitted. X-rays are produced from the volume within a sample, generally to a depth of about 1 to 5 m.

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FIGURE 3.11

Typical atomic force microscope.

Primary scattered electrons provide atomic number contrast. Atoms within a sample with high atomic numbers scatter incident electrons more strongly than do atoms with a low atomic number. Secondary scattered electrons lose much of this atomic number contrast and are the most frequently seen SEM images. The incident beam of electrons can disrupt the electrons surrounding the atoms of the samples, giving them additional energy. When these electrons relax, an electron may be emitted, called an Auger electron, whose energy is characteristic of the element from which it is emitted. When the excited electrons relax, they may also emit x-rays, whose energy is proportional to the atomic number of the element emitting the electron. (There is another energy emission, called cathode luminescence, not shown in this figure. Cathode luminescence is light emission. The wavelength of the light emitted is also used to identify some atoms.) The scanning electron microscope provides both topography information (i.e., shape, size, texture) and elemental composition information. There are applications for which an SEM will be deployed in a contamination control factory. Field-emission SEMs are commonly used within process facilities to allow rapid-turn around inspection within the process itself. Robotic SEMs have been used in the disk drive industry for magnetic contamination inspection of incoming magnets or as part of a broken magnet procedure (see Chapter 11). Atomic Force Microscopy The atomic force microscope (AFM; Figure 3.11) is not like other analysis methods called microscopic: Microscopes generally produce an image using light or electrons. It is better to think of an AFM as a stylus gage rather than as an optical or electron optical imaging system. In an AFM a tiny stylus is moved up and down over a surface (Figure 3.12). The surface is moved in a precise x–y pattern under the moving stylus. The interaction of the stylus with the surface is measured as the deflection of a cantilever beam. The measurement is made by observing the deflection of a laser beam reflected off the back of the cantilever. This combination of motions, back and forth for the sample and up and down for the stylus, produces a three-dimensional image of the surface. An AFM is capable of nanometer resolution. Various probes are available to perform magnetic force microscopy and some chemical analysis. Surface-enhanced Raman spectroscopy

SAMPLING OF CONTAMINANTS IN AIR, IN LIQUIDS, AND ON SURFACES

133

Incident Laser Beam

Reflected Laser Beam Reflected Laser Beam Deflection

AFM Probe Motion, Z

Cantilever

Cantilever Deflection

Probe Tip

Surface Under Analysis

Motion of the Surface Under Analysis, X and Y

FIGURE 3.12

Principle behind the atomic force microscope.

is claimed to be able to produce spectra from single molecules. Atomic force microscopes find application in some semiconductor factories as process inspection tools.

3.3 SAMPLING OF CONTAMINANTS IN AIR, IN LIQUIDS, AND ON SURFACES Any discussion of analysis methods must consider how the sample is obtained before a discussion of the subsequent analysis can proceed. Contamination can be present in air, in liquids, and on surfaces. Next we discuss the sampling techniques used for these three media. 3.3.1

Contaminants in Air

Contamination in the air may be in the form of particles consisting of solids or liquids or in the form of airborne molecular contamination, either as ionic or organic substances in the gaseous or vapor phase. The most common method for sampling airborne particles is filtration. Depending on subsequent analysis, a variety of filters may be chosen. For example,

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if the subsequent analysis will be primarily chemical in nature, the filter medium can have a fibrous surface. Conversely, if the analysis is to be by microscopy, the preferred filter medium is a membrane filter. Airborne particles and molecular contamination can be sampled by a variety of methods. The choice of sampling technique must take into consideration the state of the contaminant and the location from which it is sampled. Airborne particles may be either liquid or solid. Both can be sampled using filters. However, subsequent analysis can vary, depending on the state of matter of the contaminant. One useful method for deciding on the analysis method to be used following collection of the sample is to examine the filter surface under low-power magnification using a binocular microscope. If the materials on the filter surface appear to be metallic or ceramic (e.g., metal oxides), scanning electron microscopy is the preferred analysis method. Conversely, if the materials appear to be fibrous or based on polymers, Fourier transform infrared (FTIR) spectroscopy or Raman microprobe may be a better alternative. Finally, if the materials on the surface of the filter appear to be liquid in nature, FTIR, Raman microprobe, or gas chromatography/mass spectrometry (GC/MS) may be the best choice. All airborne filter samples should be examined using an optical microscope. Optical inspection of the filter allows one to determine a crude estimate of the nature of the contaminants. This is important information for guiding the selection of subsequent, often expensive additional analysis. To a trained eye, many materials in the sample, such as clothing or paper fibers, will be identified unambiguously and further analysis will not be required. Inspection of the filter will also be able to provide an estimate of the metallic/ceramic fraction, for which electron microscopy is ideally suited, vs. the organic fraction, for which FTIR, Raman microprobe, and GC/MS are ideally suited. Airborne contamination in the form of solids or liquids should preferentially be sampled using membrane filters. These types of filters present a flat plane with precisely bored holes as opposed to a fibrous surface. A flat surface is ideal for optical inspection and analysis and provides a surface from which individual particles may be removed for additional analysis. Particles on fibrous membrane filters can be difficult to manipulate for follow-on analysis. Airborne contamination can also be in the form of vapors. Vapor-phase contamination can be loosely divided into two forms: organic vapors and inorganic vapors. Organic vapors include such classes of compounds as solvents, which generally have high vapor pressure and evaporate relatively rapidly. Organic vapors can also include some low-volatility compounds, such as the common plasticizer dioctyl phthalate (DOP), silicone oils, and organotin compounds. These must generally be sampled using absorbents such as activated charcoal, Tenax, or a molecular sieve. The contaminant collected is then removed from the absorbent for subsequent analysis by solvent extraction or thermal desorption. 3.3.2

Contaminants in Liquids

Contamination in liquids can be sampled using similar techniques. Filtration is effective where the contaminant is not fully dissolved in the liquid. Again, membrane filters are particularly useful, in that they allow rapid location and identification of contaminants. Membrane filters do have one distinct disadvantage, particularly noticeable in sampling liquids: They do not allow high-volume flow rates. Contamination in liquids can also be sampled by evaporating away the liquid, leaving the contaminants behind. This evaporation technique is effective if the volatility of the contaminant is less than that of the liquid matrix in which the contaminant is sought. In cases where the contaminants are more volatile than the liquid matrix, headspace analysis can be performed, sampling from a headspace analysis vessel.

SAMPLING OF CONTAMINANTS IN AIR, IN LIQUIDS, AND ON SURFACES

3.3.3

135

Surface-Borne Contaminants

One of the earliest tests to be applied to materials was the extractable particle test, developed originally for characterization of piece parts for precision assembly. It was soon discovered that 40-kHz ultrasonic extraction was unsuitable as an extraction technique for some materials (in particular, soft metals and natural polymers), as these were extremely sensitive to damage by ultrasonic waves. Ultrasonic extraction has been replaced by the orbital shaker to remove particles for materials exhibiting extreme sensitivity to ultrasonic erosion or by low-pressure spray extraction. Where either oscillation or spray extraction have been used, considerable air can be entrained into the liquid as tiny bubbles. Typically, turbidimeters or liquid-borne particle counters (LPC1) will detect and count air bubbles as if they are particles. A procedure then had to be developed to degas the resulting suspension. Two procedures for degassing are available. One uses ultrasonic degassing. The beaker containing the suspension is immersed in an ultrasonic tank. The power to the tank is pulsed on and off rapidly. This procedure is repeated 10 to 20 times, until the suspension no longer effervesces. An alternative procedure allows the suspension to stand, undisturbed, for 20 minutes. The 20-minute stand results typically in a five- to 10-fold reduction in particle count vs. ultrasonic degassing for samples extracted from typical parts. Conversely, for extracts containing particles that are similar in density as water, little difference in particle counts are seen between the two degassing methods. An example of the latter is particles extracted from unused cleanroom gloves [5]. Following degassing the suspension is measured using a turbidimeter or counted using a liquid-borne optical particle counter. The current practice is to count using a 0.5-mresolution particle counter. Turbidimeters respond to all particles sizes, although their response is not linear. The apparatus used to perform turbidimetry or liquid-borne particle counting is relatively inexpensive and easy to use, so its use is considered a field analysis method. After liquid-borne particle counts or turbidimetry have determined particle concentration, the remaining liquid can be filtered or evaporated for subsequent chemical analysis. Witness plates have a venerable history in the semiconductor and other industries. Automatic wafer scanners are commonly found in semiconductor factories where bare silicon wafers are used as witness plates. Similarly, bare disks may be used as witness plates and analyzed using an automated disk scanner in the magnetic recording industry.

Case Study: SEM/EDX Corrosion Analysis One of the most useful applications of SEM/EDX (energy dispersive X-ray analysis) is in the investigation of corrosion problems. When a part fails due to corrosion, SEM/EDX is often used to analyze the corrosion site. The objective is to identify elements normally associated with corrosion, especially chlorine and sulfur. One disk drive manufacturer had a serious field failure problem with its disk drives. When the drives were disassembled for failure analysis, the problem appeared to be corrosion of the surface of the magnetic recoding disks. SEM/EDX analysis did not reveal the presence of detectable quantities of either chlorine or sulfur. A field investigation at the customer location revealed the source. The disk drives were being contaminated with oxides of nitrogen, NOx. Excessive quantities of NOx were detected in the air in locations in the building where the disk drives were failing. (Disk drives in the same building that were located in a computer “white room” that had a separate air system were not failing. Excessive NOx was not detected in this computer room.) The high concentrations of NOx in the general air

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in this building could be accounted for easily. The building was located less than ¼ mile from a turbine engine test stand. Workers in the building confirmed that they could smell the engine exhaust inside the building when engines were being tested. So why didn’t SEM/EDX analysis discover this? There are two basic reasons. One, nitrogen atoms are not very efficient x-ray emitters. The x-ray signals from the nitrogen atoms causing the corrosion were present but very weak. Second, nitrogen is considered ubiquitous and a trace x-ray emission from nitrogen is observed in many SEM samples. The analysts, used to seeing a small signal for nitrogen, tended to dismiss it as unimportant.

3.4

ORGANIC CONTAMINATION ANALYSIS METHODS

A wide variety of methods are used for analysis of organic residues. Several of these are suitable for online measurements. 3.4.1

Water Break Test

In the water break test, water is drained off the surface. If the water forms a continuous sheet, the part is considered to be free of hydrophobic films that could interfere with subsequent plating and coating processes. If the water does not form a continuous sheet, the part is considered to be contaminated. Several limitations of the water break test must be considered: ●

● ● ●

The test is subjective; each person will have slightly different criteria for what constitutes an acceptable surface. The test cannot be applied effectively to hydrophobic surfaces. Extremely small quantities of contaminants might not be detected. Residual surfactants from inadequate rinsing will serve as positive interference and enhance formation of the water break film.

The water break test is suitable for use online as a process control technique. It is easy to perform, and the results are easy to interpret. It finds its greatest use in the plating and painting industries, where large surfaces must quickly be inspected for cleanliness prior to coating. The water break test is generally considered a field-use technique that must be applied with caution. 3.4.2

Contact Angle Measurement

Contact angle measurement is relatively simple, requires only inexpensive equipment, and can be performed as an online inspection. It can be thought of as a companion to the water break test, since it also is intended to reveal the presence of hydrophobic contaminants on hydrophilic surfaces. A droplet of water is placed on a surface. If the hydrophilic surface is clean, the water droplet wets the surface and spreads out, forming a low contact angle. On the other hand, if the surface is contaminated with hydrophobic contaminants, the droplet will bead up, forming a high contact angle. The contact angle can be measured using a goniometer [6].

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A chief drawback of the contact angle measurement test is that it is highly localized. Areas on the parts that are not being tested may be contaminated locally. Contact angle measurements can be used as a process control technique in production environments. It is thus considered to be a field technique with better quantitative results than those of a water break test, but can look at only a limited region of a specimen. 3.4.3

Optically Stimulated Electron Emission Technique

In optically stimulated electron emission (OSEE), short-wavelength ultraviolet light is used to illuminate a surface. This stimulates the emission of electrons from the surface (the photoelectric effect). These low-energy electrons are detected as a return current. Clean surfaces will produce the highest return current. Surfaces contaminated by ionic or organic films will produce lower currents. One drawback of the OSEE technique is that the emission is difficult to relate quantitatively to the amount of contamination on a surface. For this reason, its best application is where the chemical nature of the contaminant is known. Alternatively, if the result of OSEE inspection can be correlated to process outcome, either by yield or by some other measure of goodness, it can be utilized in a process control application. It is thus generally considered to be a field-deployable technique. The OSEE technique uses moderately expensive equipment. The equipment is portable, compact, produces results in a relatively short time, and its results are easy to interpret, so it can be considered for use as an online inspection technique for process control [7]. 3.4.4

Nonvolatile Residue Test

In the nonvolatile residue (NVR) test, the surface is flushed with water or solvent. The flush liquid is evaporated and the nonvolatile residue is weighed and reported as weight per unit area. This is usually a laboratory measurement and is quite time consuming, so it is unsuitable as an online measurement. In a conventional NVR test, the material is washed with a suitable solvent, often isopropyl alcohol, and the solvent is allowed to evaporate in a preweighed weighing dish. The resulting added mass is reported in milligrams per square foot of surface area. Drawbacks of the NVR test are that it is time consuming and procedurally difficult, occasionally resulting in gross errors. It can be shown that there is a direct linear correlation between the cumulative particle count greater than 0.5 m per unit area and the NVR results for gloves, as shown in Figure 3.13. The strong correlation between results of NVR and LPC tests indicates that the NVR test may be redundant, at least for cleanroom gloves. Where laboratory determination of NVR is found to correlate with yield problems, the correlation between it and other field techniques, such as turbidimetry or liquid-borne particle counts should be investigated. In this way, liquid-borne particle count or turbidimetry could be used as a field-deployed substitute for NVR analysis. 3.4.5

Organic Sampling Techniques

Organic materials can be extracted from certain types of materials by various organic solvents. Isopropyl alcohol, which is commonly used in cleanrooms, might be a good starting point for extracting organic residues. In other cases it might be desirable to extract with

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0.2

mg/ft2

0.15

0.1

0.05

0 0

2000

4000

6000

Particle/cm2

FIGURE 3.13

8000

10000

12000

≥ 0.5 μm

Correlation between liquid-borne particle count and nonvolatile residue for gloves.

more aggressive solvents such as acetone, methylene chloride, or hexane, to enhance recovery of hydrocarbons, soluble oligomers, plasticizers, siloxanes, or other molecules considered undesirable. After recovery of the soluble material, the samples can be concentrated by evaporation, as in the NVR procedure. However, rather than weighing the concentrate, some of it is analyzed by Fourier transform infrared spectroscopy, or in the case of extremely complex mixtures, gas chromatography with mass spectrometric detection. Many organic compounds are so detrimental to products or processes that the acceptance criterion is “none detected”. There are, however, instruments that measure nonvolatile residue in near real time. In one, a small drop of liquid is evaporated and the residual NVR is weighed. This is a relatively small instrument with good response time (minutes), making it suitable for online applications. In a second instrument, the liquid is nebulized, dried, and the subsequent hyperfine aerosol is measured using a condensation nucleus counter [8]. Unfortunately, this instrument is relatively expensive. Its best application is in monitoring the purity of process fluids in-line, allowing near-real-time control over process fluid purity [9].

3.4.6

Central Atmospheric Monitoring System

In 1975, the Naval Research Laboratory (NRL) developed the Central Atmosphere Monitor System (CAMS) for use in U.S. submarines. The prolonged underwater operation of nuclear submarines makes monitoring of gases in their atmosphere a critical concern. For the first time, the CAMS allowed submarine crews to monitor the air aboard their boats reliably. In this application, CAMS is a combination carbon dioxide detector and fixed-collector mass spectrometer that monitors hydrogen, water, nitrogen, carbon monoxide, oxygen, carbon dioxide, and refrigerant gases. NASA also uses a variant of this system for manned space vehicles. In the early 1980s, IBM adapted this system for monitoring wet chemical fabrication facilities. It remains a feasible approach to continuous monitoring, as multiple gas species can be sampled from several locations within a facility. This subject is covered in more detail in Chapter 7.

IONIC AND INORGANIC CONTAMINATION ANALYSIS METHODS

3.4.7

139

Electron Spectroscopy for Chemical Analysis

X-ray photoelectron spectroscopy (XPS) or (electron spectroscopy for chemical analysis (ESCA)) was developed in the 1950s. A beam of x-rays is used to irradiate a surface. The electrons ejected from the surface are analyzed for their energy and relative abundance. This information allows a determination of the chemical composition of a surface. The technique is particularly valuable for contamination analysis when the contaminant is absorbed on the surface of a sample, which is often the case. This is true because the depth of ESCA analysis typically is the upper 2 to 10 nm of the surface. This is a high-vacuum technique and can only be performed in a laboratory. It is unsuited for use as a process monitor. It is highly suitable as a materials qualification technique. Its best application is in failure analysis. 3.4.8

Gas Chromatography/Mass Spectroscopy

GC/MS is complementary to FTIR. In GC/MS, complex mixtures are injected into a gas chromatograph. Absorbent columns in the GC separate the individual components, where they are analyzed by mass spectrometry. The mass spectrometer breaks the molecules into small fractions, which are then analyzed to determine their atomic weight. Computer programs then assist in reconstructing the identity of the original molecule. GC/MS is often used in combination with FTIR and other organic contamination analysis techniques. GC/MS is definitely a laboratory technique. The equipment is large and expensive, sample preparation is critical, and a well-trained professional is needed to interpret the results. 3.4.9

Secondary Ion Mass Spectroscopy

Secondary ion mass spectroscopy (SIMS), including time-of-flight SIMS, is used to characterize the composition of contamination on surfaces. An energized beam of primary ions, typically Ar, Cs, or N2, is beamed at the surface. These primary ions eject and ionize atoms from the surface being analyzed. The ions ejected are analyzed to determine their atomic composition. No chemical binding information is determined. SIMS can be helpful where the contamination is present as a very thin film. SIMS is another laboratory technique that is best applied to material characterization and failure analysis. 3.5

IONIC AND INORGANIC CONTAMINATION ANALYSIS METHODS

Dissolved ions can be a significant source of contamination. The most important technique for identification of ionic contamination is ion chromatography. In ion chromatography a sample is injected into a packed column, which retards the flow of ions vs. pure ion-free water. The time for ions to emerge is calibrated using known standards. In addition, the relative response of the detector is calibrated, so the quantity of ions in solution can be estimated. Both negatively charged ions (anions) and positively charged ions (cations) can be measured. Following extraction, anions are usually analyzed using an ion chromatograph, and atomic absorption spectroscopy (AAS) is generally used for cations. Anions of interest generally are chloride, nitrate, and sulfate, although some end users specify phosphate as well. Cations of interest include aluminum, copper, iron, magnesium, silicon, sodium, and zinc. Figure 3.14 illustrates a typical ion chromatograph. Figure 3.15 shows a sample being aspirated into a typical AAS.

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FIGURE 3.14

FIGURE 3.15

Typical ion chromatograph.

Sample being aspirated into an AAS for cation analysis.

An alternative method for control of ionic contamination is represented by the ionograph™. The alpha metals, Inc ionograph™, the industry standard ionic contamination tester, is specified in most commonly used cleaning standards. The instrument is used to measure the amount of ionic contamination on a variety of products, notably printed circuit boards after soldering. The instrument recirculates a mixture of isopropyl alcohol and water

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141

through an extraction chamber, then a conductivity cell, and finally, a deionizer cartridge. In this instrument the change in conductivity of the recirculating fluid is reported as equivalent NaCl concentration. The identity of the ions causing an increase in conductivity of the recirculated fluid after the part is immersed in the extraction chamber is not determined. This limits the instrument’s usefulness for failure analysis. However, the instrument is very easy to use, making it a very good field-deployable process control tool. In an experimental adaptation of the ionograph, the isopropyl alcohol solution was replaced by 100% DI water. This increased the lower election limit by more than a Factor of 10. At this sensitivity it was necessary to provide a nitrogen purged load/unload chamber and sample elevator, because the baseline conductivity responded to atmospheric Carbondioxide and the operator’s breath.

3.6

ELECTROSTATIC DISCHARGE METHODS

Selection of material for ESD applications can be critical. The ESD performance of a cleanroom material can be specified using a number of different parameters. Among these are bulk and surface resistivity, discharge time, residual charge retention, generation of electrostatic fields, and tendency to tribocharge. Bulk and surface resistivity are classical methods for specifying conductive properties of materials; these are often important in selection or qualification of materials for use in a static-safe workplace. Discharge time is an important parameter, since it is the arrival at a safe voltage level that often determines the material’s suitability for use in a given application. Residual charge is especially important in laminated or composite structures, where the continuous phase material in contact with the external environment can be highly insulative compared with the bulk of the laminate or composite structure. Generation of electrostatic fields is important anytime high voltage electronic devices are used, such as items containing cathode ray tubes and corona discharge air ionizers. Tribocharging beyond acceptance limits is determined after handling under normal and intended use conditions or using standard test methods. Of these, the tendency to tribocharge, that is, to acquire and/or impart a charge when rubbed against or separated from a dissimilar material, is by far the most controversial. The repeatability and appropriateness of tribocharge testing is so in question that “no one test currently available can predict general tribocharging properties for a specific material” [10]. Since there is no agreed-upon standard for tribocharge testing of materials, attempting to specify materials solely from the standpoint of tribocharge properties is presently a difficult prospect at best. 3.6.1

Tribocharge Testing

There remains a need to include tribocharging characteristics of materials. Although there is no currently accepted standard test, instruments are commercially available for performing tribocharge analysis. Careful use of this apparatus greatly improves the repeatability of tribocharged tests. In this apparatus, Teflon or quartz cylinders are rolled down a 15° inclined plane covered with a material to be tested and dropped into a Faraday cup. The charge generated by this action is measured with a nanocoulomb meter [11]. (Alternatively, cylinders can be formed from other materials, coatings can be applied to metals, cylinders can be covered with other materials, and so on, to test other combinations.) An example is shown in Figure 3.16. Many times, materials qualification tests must be done at 50% and 12% RH.

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FIGURE 3.16 Inclined plane apparatus for testing a material’s tribocharged properties, shown in a TRH chamber with the glove box door open for clarity.

As such, the inclined plane in the figure is shown inside a TRH chamber. The glove box door is open and not seen in this image, to give a clearer view of the apparatus within. 3.6.2

Bulk and Surface Resistance Measurements

Most materials require testing from the viewpoint of their bulk and surface resistivity, discharge time, and residual charge retention. Bulk and surface resistivity tests are reliable, as they are based on currently accepted test methods. Discharge time tests are useful, in that they are based on accepted test standards and reflect the expected performance of materials in their intended application. Residual charge retention tests are based largely on the experience with packaging materials. Bulk or surface resistivity can be measured using a number of different standards. Standards considered particularly appropriate are those of the American National Standards Institute/Electrostatic Discharge Association (ANSI/ESD) [12,13], the American Society for Testing and Materials (ASTM) [14], and the International Electrotechnical Commission (IEC) [15]. It is interesting that a direct correlation can be established between bulk or surface resistivity and discharge time. Conversely, there is no correlation between bulk or surface resistivity and tribocharging. Discharge-time measurement is described in FED-STD-101C [16]. A residual charge method is described in IEC 61340-2-1-2002 [17]. Discharge-time performance has become an industry norm in the specification of many materials for use in the manufacture of hard disk drives. Discharge times are measured for a person holding a hand on a 20-pF charged plate. The plate and operator are charged to some starting voltage and the time to discharge to a target voltage is measured. The most generous disk drive discharge requirement is from 1000 V to less than 100 V in less than 5 seconds. The most demanding requirement is for discharge from 1000 V to less than 10 V in less than 500 ms.

ELECTROSTATIC DISCHARGE METHODS

FIGURE 3.17 weights.

143

Testing the surface resistance of a static-dissipative benchtop using a pair of 5-lb

FIGURE 3.18 Testing the volume resistance of a candidate static-dissipative packaging film. This method uses the guarded ring electrode.

Figure 3.17 shows an example of a Monroe Electronics 272A surface resistivity/resistance meter being used to measure point-to-point resistance of a static-dissipative benchtop. This test is shown using a pair of 5-lb weights, following the procedure described in ANSI/ESD S4.1 [18]. Figure 3.18 shows the same instrument set to measure the bulk (volume) resistance

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FIGURE 3.19 Testing the surface resistivity of a candidate static-dissipative packaging film. This test also uses the guarded ring probe, but the sample is supported on the insulated surface of the specimen support plate.

of a sample of candidate packaging film. This arrangement uses the guarded ring electrode called for in ANSI/ESD STM11.12-2000 [13]. Figure 3.19 shows the same instrument set to measure the surface resistivity of a sample of candidate packaging film using the probe arrangement called for in ASSI/ESD STM11.11-1993 [12]. This instrument is rugged and compact. It also comes with a convenient carrying case with shoulder strap, so it can legitimately be described as portable. It weighs 7.6 kg (16.7 lb) fully loaded, so it can hardly be called light. Considering the large number of accessories that could get lost in the field, it is better to consider it a laboratory tool and a good one at that. Conversely, there are much smaller surface resistivity meters that are useful in the field. There will be more on this subject when ESD field test kits are discussed. Many other brands of surface resistivity meter are on the market. This one was chosen to illustrate its use in three of the most common probe configurations used to test ESD materials. 3.6.3

Air Ionizer Testing

Air ionizers are subject to qualification tests, periodic calibrations, and periodic verification. The most common instrument used to calibrate an air ionizer is the charged plate monitor (CPM), an example of which is shown in Figure 3.20. In use, the plate of the CPM is positioned in the airflow containing the ions. The plate is charged to a starting voltage and discharged to a given end voltage. The discharge is timed. Typical starting voltages are 5000 and 1000 V. The most common end voltage is 10% of the starting voltage. This and most state-of-the-art CPMs will time to any manually selected end voltage. The plate is also left uncharged, and the voltage induced by the air ionizer is measured. The maximum offset voltage during a time period, typically 1 minute, is called the float potential and is a measure of the balance between the number of positive and negative ions generated by the ionizer.

ELECTROSTATIC DISCHARGE METHODS

FIGURE 3.20

145

Typical charged plate monitor.

This and similar CPMs are portable, so they could be used in the field. This and similar CPMs also typically weigh between 7 and 9 kg (about 15 to 20 lb), including their carrying case, tripod, and so on. Adding the weight of the CPM to the weight of the resistivity/surface resistivity meter and one begins to realize that these are probably not suitable instruments for conducting field studies and audits. 3.6.4

Typical ESD Field Instruments

In addition to the ability to measure resistivity and surface resistivity and to verify calibration of air ionizers in the field, several other instruments should be included in the test kit used to perform field surveys and certifications. These include: ●

●

●

●

A means of testing ac outlets for wiring integrity. Where the facility is equipped with GFIC protectors, the ac outlet tester should be capable of testing to determine if they trip when current leakage exceeds 5 mA. A volt-ohmmeter (VOM) to verify hard grounding of ESD ground points and ac-powered tools. The VOM will also be used to test for voltage leakage from the tip of ac-powered tools. A means of verifying relative humidity in ESD-protected work areas. Most ESDprotected work areas will be equipped with temperature–humidity sensors, although many of these will be found to be unacceptable. Often, labs simply use a dial-type TRH meter that does not actuate an alarm and/or record the TRH. In other cases, the TRH meter is not located on the workstations where work is to be done. A small handheld TRH probe can allow one to verify rapidly that the TRH is correct at the workstation. The accuracy of handheld TRH meters can be verified more frequently than can most TRH meters used in laboratories [18]. A means for testing wrist straps and footwear.

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FIGURE 3.21 Typical ESD auditor’s test kit. Starting from the upper right hand corner this kit contains an a.c. outlet tester, a TRH sensor, a cutout for storing cables and accessories, a VOM, a field potential meter, a high-resistivity ohmmeter, and a combination wrist strap/footwear tester. In the cover (not shown in this image) is the footplate for testing footwear, chair and cart grounding, the operating manuals, and an auxiliary ground cable.

Figure 3.21 illustrates a typical ESD test kit used to verify the suitability of ESDprotected work areas. The kit illustrated in Figure 3.21 weighs only 3.5 kg (7.7 lb). A second portable test kit useful for verifying ionizer calibration in the field is shown in Figure 3.22. This type of ionizer verifier must be hand timed. It has been shown that the verifier agrees with a conventional CPM within 3% under normal-use conditions. This verifier kit weighs just 0.5 kg (1.1 lb), including two spare 9-V batteries, a metal wrist band and 6-foot coiled cord, and the carrying case. 3.7

NUMERICAL SIMULATION

Starting from a modest beginning in 1987 at the IES annual technical meeting in San Jose, California, the application of computation fluid dynamics (CFD) to the study of contamination has become a widely accepted and applied technique. CFD techniques were well developed but were largely limited to government or university institutions that possessed the computing power (supercomputers) and academic resources to apply them. By 1987, the simulation computer programs were compact enough and the computing resources available to use these software models no longer required the use of supercomputers [20]. Today, numerical simulations can be performed on a personal computer. The models are capable of predicting airflow performance and the behavior of contaminants within the airflow. Whereas previously the models could only be applied affordably to sophisticated aerodynamic or hydrodynamic problems affecting the aerospace industry, in 1987 they could be applied to something as seemingly mundane as the airflow around a unidirectional flow clean bench [21]. Many different models were in use in the late 1980s, raising concern about

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FIGURE 3.22 Portable test kit for verifying ionizer calibration in the field. The items from left to right are the charger for charging the 15-pF capacitor plate, a hole for holding the capacitor plate during transport, and a field potential meter, modified to be an ionizer verifier by slipping a 15-pF capacitor plate over the top as shown. Also visible is the ground cord, which is snapped to the back of the field potential meter and can be used as a banana plug, or as shown, with a slip-on alligator clip.

the uniformity of prediction among the models. Six different simulation models were subsequently tested using a standard two-dimensional section of the cleanroom. These showed excellent agreement among the six models for airflow but some variations among the six for particle concentrations [22]. High-technology manufacturers of cleanrooms, flow benches, and process equipment interface enclosures began to recognize the value of the contamination simulation modeling tools and began to apply them to their design problems. Examples include modeling within diffusion furnaces [23], spin coaters [24], minienvironments [25], and wafer transport systems [26]. 3.8

ALGEBRAIC PREDICTIVE MODELING

Algebraic predictive modeling takes a slightly more primitive approach than numerical modeling. In analytical predictive modeling, simple algebraic expressions are used to predict the relationship between input variables and outcomes. These simple analytical models can then be used to predict the class of cleanroom needed for any particular process step or can assist in selection of the type of monitoring equipment that might be justifiable in any given situation. Case Study: Estimating the Class of Cleanroom Needed There is a simple algebraic method for determining the class of a cleanroom needed to support any given process step. This simple model will be illustrated using airborne particles in the cleanroom as an example. The process can easily be extended to airborne molecular contamination by analogy. The overarching factor for determining the class of the cleanroom is the yield desired at the

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process step. For this particle example it is assumed that a particle that is larger than the critical particle size landing on a critical area of a product will cause a defect. Thus, the surface contamination rate will be the most important factor driving the class of cleanroom to be used for any given process. The surface contamination rate in any class of cleanroom is highly influenced by the charge level on particles and surfaces in the room. The charge level on particles and surfaces will vary depending on whether or not air ionizers are used. Table 3.1 illustrates this phenomenon. Table 3.1 assumes that the cleanroom conforms exactly to the size distribution specification of ISO 14644 or FED-STD-209. Since this is a design problem, the actual size distribution in the cleanroom affecting the process is immaterial. Moreover, the size range 0.1 to 1.0 m in the standards has been shown to be a reasonably good predictor of size distributions in operational cleanrooms [28]. The table applies specifically to FED-STD-209 class 100 (ISO 14644 class 5) unidirectional-flow cleanrooms. It can be extrapolated to other classes of unidirectional-flow cleanrooms by adjusting for the order of magnitude of the class. For example, a FED-STD-209 class 10 (ISO 14644 class 4) unidirectional-flow room will have an order-of-magnitude lower surface contamination rate than that shown in the table. For this discussion we refer to this order-of-magnitude adjustment factor as M. For mixed-flow cleanrooms, the surface contamination rate can be adjusted based on the residence time of airborne particles in the cleanroom. The adjustment factor, R, is shown in Table 3.2. This adjustment factor is based on the fact that particles (and airborne molecular contamination) remain suspended within the cleanroom longer, simply on the basis of an ideal stirred tank model. The first step is to determine the critical particle size, that size above which a defect will be produced. Assume for this example that the critical particle size is 0.1 m. The second step is to determine the percent of the area of the part that is vulnerable to contamination in the process. This is called the percent vulnerable active area, V. For many photolithography processes this is only about 5% of the area of the part. Next, the assumption is made that the landing site of the contamination is random and uniform. Based on this 5% vulnerable area, only 5% of the particles equal to or larger than the critical particle size will land in an area that leads to a defect. This term may be thought of as the probability of failures per particle, % fail./particle. (In many processes, 100% of the product surface is vulnerable to producing a defect.) For this exercise, assume that 5% of the surface area is vulnerable. Therefore, in this example, V  0.05% fail./particle. TABLE 3.1 Cumulative Surface Contamination Rates, SCR5, for FED-STD-209 Class 100 (ISO 14644 Class 5) Vertical Unidirectional-Flow Environments Surface Contamination Rate (particles/cm2  h) Particle Size (m) 0.1 0.2 0.3 0.5 1 3 Source: Ref. 26.

With Air Ionization

Without Air Ionization

10.1 1.03 0.31 0.11 0.049 0.028

88–176 8.5–19 2.4–4.9 0.6–1.2 0.12–0.25 0.03–0.06

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149

The third step is to predict the surface contamination rate for the class of cleanroom. This can be done by multiplying the baseline surface contamination rate in Table 3.1, SCR, by the class adjustment factor, M, and the residence time factor, R. For this example, let us determine if a FED-STD-209 class 10,000 cleanroom with air ionization is suitable for a given process step. The baseline surface contamination rate, SCR5, must be multiplied by M  100 times R  10. The surface contamination rate in the class 10,000, SCR, is predicted to be approximately 1000 SCR5. Thus, SCR  1000  10.1 (10,000 particles/cm2  h). The last step is to determine the amount of time, t, in hours that the product will be exposed to the cleanroom environment. In this example, assume that t  0.5 h. Now let us suppose that the surface area, A, of the part undergoing the process is 10.0 cm2. (This might be a 10cm2 LCD display chip on a 6-in.-diameter wafer.) Then to a first approximation we can predict that the failure rate is the product of the surface contamination of the critical particle size, multiplied by the area per part, multiplied by the time of exposure, multiplied by the percent vulnerable area of the part. Thus, for our class 10,000 cleanroom, % fail./particle  SCR (particles/cm 2  h)  t (h)  A (cm 2 /particle)  V (% fail./particle)  10,000 particles/cm 2  h  0.5 h  10.0 cm 2 /particle  0.05% fail./particle or 2500 failure sites per part. If there is no redundancy on the part (typical for an LCD display chip), all will fail and the class 10,000 cleanroom is not acceptable. If we move the process to a FED-STD-209 class 100 unidirectional-flow cleanroom, the calculation changes: % fail./particle  SCR (particles/cm2  h)  t ( h)  A (cm2 /particle)  V (% fail./particle)  10 particles/cm 2  h  0.5 h  10.0 cm2 /particle  0.05% fail./particle or 2.5 failure sites per LCD display chip.

TABLE 3.2 Relationship Among Airborne Particle Count Class, Volume Air Exchange Rate, and Average Residence Time, Assuming That the Cleanroom Behaves as an Ideal Stirred Reactor

V/hr

V/min

Average Residence Time (min)

20 20–60 60–200 200–600

–13 to 1 1 to 3–13 3 –13 to 10

3 3 to 1 1 to –13 1 –3 to 10 –1

Typical Volume Exchange Rate FED-STD-209 Class (ISO 14644 Class) 100,000 (M8) 10,000 (M7) 1000 (M6) 100 (M5)

–13

Multiplication Factor vs. Unidirectional Flow, R 10 10 3 1

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If we move the process to a FED-STD-209 class 10 unidirectional-flow cleanroom, the calculation changes again: % fail./particle  SCR (particles/cm 2  h)  t (h)  A (cm 2 /particle)  V (% fail./particle)  1.0 particle/cm 2  h  0.5 h  10.0 cm 2 /particle  0.05% fail./particle

or 0.25 failure site per LCD display chip. At this point we would predict that 25% of the chips will fail in a FED-STD-209 class 10 cleanroom. Suppose that we want the yield at this process step to be 95%. To achieve this, the failure rate must be reduced to 5%. One way to accomplish this would be to reduce the exposure time from 30 minutes to 6 minutes. 3.9

STATISTICAL ANALYSIS METHODS

Modern management of contamination and ESD-controlled work areas is increasingly dependent on data collected in the work area. Unfortunately, with this proliferation of data collected, statistical analysis tools are needed to draw correct conclusions and establish correct policies. Since the actions taken based on decisions can cost millions of dollars, a firm understanding of the statistical significance of data is critical. For example, if the precision and accuracy of the collected data are far away from the control limits, detailed statistical analysis of the data may not be necessary. Conversely, where the measured parameter is close to the measurement limit of the technique being employed or close to the control limit for the process, statistical analysis becomes critical. Techniques that are employed in these analysis processes include calibration, determination of detection limits, gage capability analysis, estimation of confidence intervals, regression analysis, analysis of variance, and many others. Control techniques that subsequently become useful include regression analysis and process control charts. 3.9.1

Basic Statistical Analysis Tools

One of the easiest statistical analysis tools to apply is Student’s t-test, which is often applied where there are fewer than 30 data points. The results of a t-test will show if a comparison between a data group and a standard or between two data groups is a meaningful difference, and the probability that such a difference could occur purely by random chance. Calibration is used to establish a correlation between measurements and some accepted standard. Lower detection limits can be estimated for a tool based on repeatability studies performed during calibration. However, a more proper determination of a lower detection limit must include how the measurement instrument is used in the test. This often includes sample preparations that could influence the outcome and are often subject to reproducibility contributions by personnel. To deal with this problem of instrument repeatability and procedure reproducibility, a better way to estimate lower detection limits is to determine the gage capability of the entire analysis method. Design of experiments and analysis of variance are very powerful tools for process study and control. However, experiments designed without a firm understanding of underlying physics, chemistry, or mechanics can result in data that are more or less useless. Most textbooks on statistics present thorough discussions of the basic analysis tools, such as analysis of

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variance and regression analysis. One subject not found commonly in standard textbooks on statistics is described here: gage capability analysis. It is extremely important to know the gage capability of measurement methods, whether for qualification testing or acceptance testing. 3.9.2

Gage Capability Analysis of Cleanliness Measurement Methods

Cleanliness measurement methods can be broadly divided into direct and indirect methods. Direct cleanliness measurements are those that do not alter the concentration of contamination on the part being measured. Contamination measures that do not alter contamination concentration on surfaces include: ● ●

●

●

Optically stimulated electron emission (OSEE) of films Wafer scanner measurement of silicon wafers used in particles per wafer per pass process studies Scanner measurements of magnetic recording disks or thin-film heads in magnetic recording Reflectivity or transmission measurements of optics

The conventional gage capability analysis method can be used for direct cleanliness measurement methods because the direct cleanliness measurement does not alter the contamination on the surface. Gage capability analysis is a fundamental statistical tool for characterizing measurement methods. Gage capability is the analysis of the variability associated with the repeatability of an instrument and the reproducibility of more than person using that instrument to make a measurement by some proscribed method. For that reason, gage capability is often referred to as gage R&R and is usually expressed as a percent of the tolerance of a measurement consumed by the repeatability and reproducibility of the measurement. We often thus see gage capability expressed as % R&R, the symbol we will use for the remainder of this discussion. Gage capability analysis was originally developed for mechanical measurements, such as the use of a micrometer to measure the diameter of a shaft. The general process is as follows. An operator makes several measurements of interest on a standard using a tool. Another operator then repeats those measurements using the same standard, procedure, and tool. These measurements are typically repeated two or three times. The percent repeatability of the tool and the operators’ ability to reproduce each other’s measurements are combined and the total estimated error is compared to the tolerance of the measurement. To be considered capable for mechanical measurements it is expected that the gage consumes no more than 30% of the tolerance for a given measurement; less than 10% is considered ideal. Thus, suppose that a shaft is 0.5 cm in diameter with an acceptable tolerance limit of 0.005 cm. Ideally, the sum of the repeatability and reproducibility should not exceed 10% of the tolerance of the measurement. Thus, the sum of the repeatability and reproducibility of the measurement method in this case should not exceed 0.0005 cm. It should be apparent from this description that it is important that the measurement method not change the magnitude of the measurement being made. For most mechanical measurements it probably can reasonably be assumed that measurements do not change the object being measured. The measurement method is considered to be nondestructive. For most ESD measurements and some contamination measurements this assumption is equally true. Because it is a noncontact voltmeter, a field potential meter has little or no effect on the charge on a surface as long as reasonable care is taken in handling of the

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surface. Surface contamination measurements using a noncontact inspection tool, such as a wafer surface scanner, or a spectroscopic technique would also not be expected to alter surface concentrations of contaminants, again as long as reasonable handling precautions are observed. Unfortunately, a significant number of the measurement methods used to characterize electrostatic discharge sensitivity of materials or cleanliness of materials cannot be considered to be nondestructive. For example, ESD-sensitivity tests using the human body model, charged-device model, or machine model simulators are by definition destructive. Similarly, a significant number of cleanliness measurements depend on removal of the contaminant of interest from the surface. Clearly, a repeat measurement using the same operator or another operator could not be expected to get the same result. Several useful references exist for classical gage capability measurements, including measuring the gage capability of wafer surface scanners. We do not discuss the classical method here. Instead, we focus on an adaptation of the gage capability method for measurements that alter the material being tested. We focus on cleanliness measurement methods that extract contamination from the surface, rendering it impossible for a second person to extract the surface and obtain the same result. These types of cleanliness measurement methods are often referred to as indirect cleanliness measurement methods, a term used in the remainder of this discussion. Four assumptions central to classical gage capability analysis are violated in the case of indirect cleanliness measurement methods. The first assumption, that the measurement does not change the object being measured, has already been mentioned. To get around this problem, samples are obtained from split lots of parts from a parent population. There will be inherent variability from part to part. It is not practical to distinguish between variability contributed by persons sampled within a lot from variability contributed by the measurement method. Because of this limitation, it is considered reasonable to accept indirect cleanliness measurement methods to be acceptable gages when the % R&R is less than 30% rather than 10%. The second assumption is that the measurement standards used for classical gage capability are stable. In contamination measurements, some types of standards, such as photographic film used in densitometry or particle suspensions used in calibration of turbidimeters or optical particle counters, are sufficiently stable that they can be used reliably in gage capability measurements. However, contaminates removed from surfaces are seldom stable. This difference can be used to advantage. In the first part of the gage capability analysis, stable calibration standards can be used to characterize the instrument. In a subsequent step, real-world samples can be used to characterize the contribution of sample nonuniformity on the measurement method. Several examples will illustrate this point. The third assumption is that the gage tolerance is largely independent of the magnitude of the measurement. A micrometer would be expected to measure with the same tolerance over its entire range. Tolerance for most mechanical measurements can be expressed as nt where n is the magnitude of the dimension and t is the tolerance. Conversely, contamination measurements are usually expressed as 0  t (the upper limit) 0  0 (the lower limit)

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153

TABLE 3.3 Multiple-Tape Sample of a Silver–Carbon Ground Button Trial 1 2 3 4 5 6 7

Optical Density

% R&R

0.75 0.5 0.32 0.21 0.16 0.04 0.02

21 19 21 23 28 93 170

Indeed, to express the contamination specification as n  t would imply that if the total contamination fell below the n  t limit, that contamination should be added to bring the part into control, a truly ludicrous notion. What we see, then, is that the specified cleanliness limit for a part becomes its tolerance. Thus, we would not expect the gage capability of an indirect cleanliness measurement to be independent of the magnitude of the measurement. One way around this dependence on the magnitude is to measure several lots of samples at various cleanliness levels and plot the gage capability versus the magnitude of the measurement. The fourth assumption is that the gage is evaluated using two-sided statistics. Clearly, cleanliness is a one-sided test. Therefore, instead of using calculation factors for two-sided statistics, calculation factors for single-sided tests are used. This modification of the gage capability analysis makes a small but not insignificant difference, as shown in Table 3.3. Gage Capability of Densitometry The densitometer is calibrated using a stable, uniform piece of photographic film. If this piece of film is used to measure the gage capability of the densitometer, one concludes that the densitometer is a capable gage down to 0.01 optical density units (ODU). Since some measurement leeway is desired, the lower specification limit for controlling contamination is set at three times the gage capability limit, 0.03 ODU. Unfortunately, this does not adequately represent the method by which a densitometer is used. In practice, a piece of transparent tape is applied to a spot and the spot is lifted. The operator must then search for and measure the optically densest location on the spot. Thus, use of the uniform optical density calibration film misses an important part of the densitometer procedure: the ability of one or more operators to locate the optically densest part of the spot repeatably and reproducibly. In addition, use of the calibration film does not demonstrate that the process of sampling a spot alters the amount of contamination on the surface, making it impossible for a spot to be sampled consecutively by two different operators and obtain the same result. To illustrate these points, a simple experiment was performed. A silver–graphite ground button was touched seven times using a piece of adhesive tape, producing seven progressively lighter spots. Each touch altered the amount of contamination on the surface, so that the optical density of the spot was gradually reduced from an average of 0.75 ODU to 0.02 ODU, as shown in Table 3.3. This demonstrates that multiple extractive sampling using adhesive tape alters the amount of contamination on the spot. A second feature of these data is also interesting. The percent repeatability and reproducibility remained relatively constant in the range 0.75 to 0.02 ODU. This illustrates that rather than being constant, the % R&R is not independent of the magnitude of the

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measurement. This is typical of indirect cleanliness measurement methods. Finally, the % R&R rises rapidly starting at about 0.16 ODU. From these data we estimate that the % R&R of densitometry is about 0.15 ODU (the point at which the % R&R is 30% of the magnitude of the measurement). This is very important. Let us suppose that the gage capability was believed to be 0.01 ODU, based on the stable, uniform film measurements. One might be tempted, then, to specify a lower contamination limit of 0.03 ODU, assuming that the % R&R should consume no more than about 30% of the tolerance of the measurement. We might then specify that no part shows a densitometer value greater than 0.03 ODU. However, the experiment with a real-world sample shows that a more correct estimate of the gage capability of the densitometry method is closer to 0.15 ODU. Specifying at the lower level would result in unjustifiable rejections. The Value of Densitometry Densitometry has a value for contamination inspection in its ability to discover the removability of a contaminant. If the spot cannot be removed by pealing it off using tape, the spot would normally be considered to be a nonfunctional or cosmetic defect. Where to set the control limit becomes the subject of further gage capability analysis or negotiation between the supplier and the consumer. The tape that has been lifted is attached to a microscope slide. The particles and films trapped between the tape and the microscope slide can be examined microscopically, providing clues to their identity and origin. Finally, the material sampled can subsequently either be removed from the tape for other analysis or the sample can remain undisturbed, providing a durable record. Gage Capability of Turbidimetry The turbidimeter is commonly used in the measurement of drinking water quality. It was adapted to the measurement of cleanliness of parts in high-technology manufacture in the early 1980s. Turbidimetry cannot measure the amount of contamination on a part directly; some method must be used to remove the contamination from the part and place it in suspension for measurement. Thus, turbidimetry is an indirect cleanliness measurement method, just like densitometry. The three dominant methods for particle removal are ultrasonic extraction, spray extraction, and undulation (swishing about in a standing pool of liquid). The repeatability of a turbidimeter can be measured using relatively stable suspensions of polystyrene latex microspheres. This alone is insufficient for characterizing the turbidity test when used as an indirect cleanliness measurement method. The dominant extraction method for turbidity testing is ultrasonic extraction, so we limit our discussion to the ultrasonic extraction method. Three calibration standards of monodisperse polystyrene latex microspheres were used: 0.1 nephalometric turbidity unit (NTU), 3.9 NTU, and 26.1 NTU. These standards represent the range of use of the turbidimeter for measuring parts cleanliness. In addition, four sets of parts were selected for ultrasonic extraction, yielding cleanliness values from 1.5 to 32 NTU for comparison. The results are shown in Table 3.4 from these data, several observations are important. First, the repeatability of the turbidimeter for measurement of stable standards is fairly constant over the measurement range. Conversely, when the turbidity from real parts was measured following ultrasonic extraction, the gage capability falls below 30% at a turbidity value of about 1.5%. % R&R increases as the turbidity decreases when the extraction method is included. From these data we would draw the conclusion that the % R&R is not dominated by the influence of the turbidimeter but is dominated by the influence of the extraction method.

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155

Gage Capability of Enhanced Turbidity Enhanced turbidity was developed as a method for measuring extractable particles from plastic parts. Plastic parts were in general much cleaner than metal parts, so a method for measuring lower levels of particle contamination was needed. In the enhanced turbidity method the part was extracted in a water solution containing a trace of detergent. The aqueous suspension was then filtered and the filter was dissolved using acetone, releasing the particles back into suspension in a much smaller volume of acetone than the original water. Enhanced turbidity values were reported as enhanced turbidity units (ETU), to distinguish them from nephalometric turbidity units (NTU), normally used to describe conventional turbidity measurement. The ratio of water to acetone is known, so the relative concentration vs. the original water suspension and thus the original turbidity could be calculated. The gage capability of enhanced turbidity was studied at two cleanliness levels, as summarized in Table 3.5. Both of these enhanced turbidity levels correspond to original calculated turbidity values below 1.5 NTU, the gage capability limit of turbidity. It is therefore not surprising that both show % R&R values greater than 30%, since the dominant source of variability, ultrasonic extraction, was used for the enhanced turbidity method. A study was done examining the values of the blanks measured along with the samples. The blanks ranged from 0.65 to 5.6 ETU, indicating that the variability of the contribution of the filter had a significant detrimental effect on the outcome of the enhanced turbidity test. Gage Capability of Liquid-borne Particle Counting Liquid-borne particle counters differ from turbidimeters in many ways. Among the most important is that the liquidborne particle counter measures the light scattered or obscured by individual particles, whereas the turbidimeter measures the scattering from a large number of particles simultaneously. The liquid-borne particle counter thus offered the possibility of measuring contamination extractable from very clean plastic parts, too clean to be measured using turbidimetry. In the gage capability study reported in Table 3.6, a set of parts was extracted repeatedly so that the particle count gradually decreased. The % R&R could then be compared to the particle count to determine when % R&R fell below the gage capability limit. TABLE 3.4 Ruggedization Analysis of Turbidimetry and Ultrasonic Extraction plus Turbidimetry Using Gage Capability Analysis Turbidimeter Only Using Standards

Ultrasonic Extraction plus Turbidimetry

NTU

% R&R

NTU

% R&R

0.10

2.5

3.9

4.1

26

4.5

1.5 3.9 8.2 32

30.4 25 8.6 10

TABLE 3.5 Gage Capability Analysis of Enhanced Turbidity Measurement of Plastic Parts Enhanced Turbidity (ETU) 4.7 9.0

% R&R

Sample Concentration Ratio

Calculated Turbidity (NTU)

123 86

13 10

0.37 0.90

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TABLE 3.6 Gage Capability Analysis of Liquid-Borne Particle Counts Particle/ml  0.5 m 104 86 57 44 26 23 15 13 6.3 4.3 1.7 0.66

% R&R 9.5 13.1 11.4 16.5 20.3 15.3 29.2 24.9 25.5 41.2 52.9 101

ADDITIONAL READING ASTM G31, Standard Practice for Laboratory Immersion Corrosion Testing of Metals. Benninghoven, A., F. G. Rüdenauer, and H. W. Werner, Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends, Wiley, New York, 1987. Guthrie, J., B. Battat, and C. Grethlein, Accelerated corrosion testing, AMPTIAC Quarterly, 6(3), Fall 2002. Kellner, R., J.-M., M. Otto, M. Valcarcel, and M. Widmer, Analytical Chemistry: A Modern Approach to Analytical Science, Wiley, New York, 2004. Roberge, P. R., Handbook of Corrosion Engineering, McGraw-Hill, New York, 1999. Which accelerated test is best? Problem Solving Forum, JPCL, Aug. 2000, pp. 17–28 (comments by various experts). Journal of Protective Coatings and Linings.

REFERENCES AND NOTES 1. W. W. Weick, Acceleration factors for IC leakage current in a steam environment, IEEE Transactions on Reliability, 29(2):109–115, 1980. 2. IDEMA Standard M6-98, Environmental Testing for Corrosion Resistance and for Component Compatibility. 3. R. J. Hamilton et al. Factors in the design of a microscope eyepiece graticule for routine dust counts, British Journal of Applied Physics, 5:S101–S104, 1954. 4. R. Coplen, R. Weaver, and R. W. Welker, Correlation of ASTM F312 microscope counting with liquidborne optical particle counting, Proceedings of the 34th Annual Technical Meeting of the Institute of Environmental Sciences, King of Prussia, PA, May 3–5, 1988, pp. 390–394. 5. Previously unpublished laboratory observations. 6. B. Wettermann, Contact angles measure component cleanliness, Precision Cleaning, Oct. 1997, pp. 21–24. 7. M. Chawla, Measuring surface cleanliness, Precision Cleaning, June 1997, pp. 11–15. 8. D. R. Blackford, K. J. Belling, and G. Sem, A new method for measuring nonvolatile residue for ultrapure solvents, Journal of Environmental Sciences, 30(4): 33–47, 1987.

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9. P. D. Kinney, D. Y. H. Pui, B. Y. H. Liu, T. A. Kerrick, and D. B. Blackford, Evaluation of a nonvolatile residue monitor for measurement of residue after evaporation of IPA and acetone, Journal of the Institute of Environmental Sciences, Mar.–Apr. 1995, pp. 27–35. 10. D. W. Cooper and R. Linke, ESD: another kind of lethal contaminant, Data Storage, Feb. 1997, pp. 45–49. 11. This apparatus is available from Electro-tech Systems, Inc. of Glenside, PA. It conforms with ESD Association Advisory ADV11.2-1995, Appendix C. 12. ANSI/ESD STM11.11-1993, EOS/ESD Association Standard for the Protection of Electrostatic Discharge Susceptible Items: Surface Resistance Measurement of Static Dissipative Planar Materials. 13. ANSI/ESD STM11.12-2000, EOS/ESD Association Standard for the Protection of Electrostatic Discharge Susceptible Items: Volume Resistance Measurement of Static Dissipative Planar Materials. 14. ASTM Standard D257, Standard Method for DC Resistance of Conductance of Insulating Materials, 1993 Annual Book of ASTM Standards, Vol. 10.01, pp. 103–119. 15. IEC-93-60093, Methods of Test for Volume Resistivity and Surface Resistivity of Solid Electrical Insulating Materials. 16. FED-STD-101C, Method 4046. 17. IEC 61340-2-1-2002, International Standard: Electrostatics, Part 2-1, Measurement methods: ability of materials and products to dissipate static electric charge. 18. ANSI/ESD-S4.1-1997, Work Surfaces: Resistance Measurements. 19. The lower % RH is of primary concern. Where the lower % RH control limit is 30%, as it is in many ESD-protected work areas, the accuracy of the handheld TRH meter can be verified by placing it in the atmosphere above a saturated solution of calcium chloride in water at about 75°F. 20. A. Busnaina, Modeling of clean rooms on the IBM personal computer, Proceedings of the 33rd Annual Technical Meeting of the Institute of Environmental Sciences, San Jose, CA, May 5–7, 1987, pp. 292–297. 21. I. Shanmugavelu, T. H. Kuehn, and Y. H. Liu, Numerical simulation of flow fields in cleanrooms, Proceedings of the Institute of Environmental Sciences, San Jose, CA (May 57, 1987), pp. 298–303. 22. T. H. Kuehn, D. Y. H. Pui, and J. P. Gratzek, Results of the IES cleanroom flow modeling exercise, Journal of Environmental Sciences, 35(2):37–48, 1992. 23. R. Alchalabi, F. Tapp, N. Verma and F. Meng, In-situ gas analysis with contamination modeling for diffusion furnaces, Proceedings of the 44th Annual Technical Meeting of the Institute of Environmental Sciences, Phoenix, AZ, Apr. 26–May 1, 1998, pp. 531–539. 24. R. J. Bunkofske, Optimizing airflow, elimination backside contamination in a photoresist spincoater, MiCRO, Oct. 1995, pp. 35–44. 25. A. G. Tannous, Optimization of a minienvironment design using computational fluid dynamics, Journal of the Institute of Environmental Sciences, Jan.–Feb. 1997, pp. 29–34. 26. H. Schneider, P. Fabian, R. Sczepan, S. Hollermann, and A. Honold, Air flow modeling and testing of 300 mm minienvironment/load port systems, Proceedings of the 44th Annual Technical Meeting of the Institute of Environmental Sciences, Phoenix, AZ, Apr. 26–May 1, 1998, pp. 411–418. 27. R. W. Welker, Equivalence between surface contamination rates and class 100 conditions, Proceedings of the 34th Annual Technical Meeting of the Institute of Environmental Sciences, King of Prussia, PA, May 3–5, 1988, pp. 449–454. 28. D. S. Ensor, R. P. Donovan, and B. R. Locke, Particle size distributions in clean rooms, Journal of the Institute of Environmental Sciences, Nov.–Dec. 1987, pp. 44–49.

CHAPTER 4

FACILITIES DESIGN: CONTAMINATIONAND ESD-SAFE WORK AREAS

4.1

INTRODUCTION

In this chapter we consider some basic facilities design concepts in a very limited way. Our aim is to provide the reader with an understanding of how a decision in the layout of rooms has an effect on airflow behavior in a cleanroom. Proper airflow is fundamental to contamination and electrostatic charge control in the design and layout of facilities. There is a strong interaction between the location and layout of tools within cleanrooms and their affect on the airflow in the room. Random placement and orientation of workstations within cleanrooms without regard to their effect on airflow is a common practice. Proper placement and orientation of workstations in cleanrooms, considering the effect of room airflow and the workstations influence on it, allow the benefit of the cleanroom to be maximized. Consideration of the airflow in cleanroom also provides guidance for optimum selection and placement of air ionizer. There are two essential principles for cleanroom design. One is to provide an adequate volume of clean air to achieve the level of airborne cleanliness desired. The second is to deliver that airflow in an optimal fashion to maximize its effectiveness at achieving cleanliness. The airflow in a cleanroom can have a dominant affect on the airborne cleanliness class of the room. The design and layout of the tooling within the cleanroom has a profound effect on both airflow around the tools and the location and strength of contamination sources. The effect of tooling on airflow is dealt with in this chapter. More details on tooling design are provided in Chapter 6. The performance of the airflow system in a room has a profound influence on the selection of air ionizers for a given room, clean or otherwise. Specific recommendations regarding ionizer selection and placement to deal with various cleanroom designs are described. Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

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159

Width of the T-bar in the ceiling

FIGURE 4.1 The modern teardrop light fixture takes up only the ceiling space of the T-bar ceiling support system.

The earliest cleanrooms were of conventional or mixed-flow design, referred to today as non-unidirectional-flow cleanrooms. In these rooms, a relatively small percentage of the ceiling would contain HEPA filters. The resulting airflow within the room is rather unpredictable. These room designs relied on dilution as a major factor in elimination and control of airborne contamination. The terms non-unidirectional flow and mixed flow both refer to the limited deployment of HEPA filters and the resulting effect on airflow in the cleanroom. As the need grew for cleaner environments than could be achieved using mixed-flow cleanrooms, unidirectional-flow cleanrooms came into use. In these rooms, the ceiling coverage with HEPA filters approaches 100%. One of the more modern developments to help achieve essentially complete ceiling coverage was the development of the teardrop light fixture to replace the conventional 1 ft  4 ft or 2 ft  4 ft light fixture, as illustrated in Figure 4.1. An alternative to the unidirectional-flow cleanroom is to place the process inside clean flow benches, minienvironments, and other types of isolation. All of these alternatives are discussed with respect to their inherent airflow characteristics. 4.2 4.2.1

BASICS OF CLEANROOM DESIGN What Can Be Called a Cleanroom

There are many different structures that we refer to as cleanrooms. As defined previously, a cleanroom is an enclosed area employing control over particulate matter and other forms of contamination in air, with airflow, relative humidity and temperature, and pressure control as needed. This definition of a cleanroom is a good one in that it makes no assumption about the size and shape of the cleanroom. Indeed, it might be better to refer to a cleanroom as a contamination-controlled space, since the term room implies a place in which you could stand up and walk around. Cleanrooms range in size from extremely large to very small. Among the largest-volume cleanrooms in existence are truly enormous structures used in the aerospace industry for assembly, test, and launch preparation of satellites and launch vehicles. These types of cleanrooms are often referred to as high bays. An example of an ISO 14644 class 7 (FEDSTD-209 class 10,000) high-bay cleanroom is shown in Figure 4.2. Large open-space cleanrooms with a standard ceiling height of 10 to 25 ft are often used in the manufacture of CDs, DVDs, disk drives, and some semiconductors. These are sometimes referred to as ballroom cleanrooms. Going toward smaller sizes, many cleanrooms are arranged as long, narrow rooms, often with many side branches. These are often referred to as tunnels and are very commonly used in semiconductor manufacture. The side branches are separated by service cores, allowing for bulkhead installation of process tools. Continuing

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FIGURE 4.2 Typical high-bay cleanroom prior to acceptance testing. Fan-Filter Units with Built-in Lights

Legs on Casters

Legs on Casters

Transparent Curtains

FIGURE 4.3 Typical portable cleanroom.

to shrink, there are modular cleanrooms, which can be as small as a closet. A good example is the body box, a closet-size cleanroom used to measure total particle emission from a person to evaluate the effectiveness of cleanroom garments. Modular cleanrooms come in many shapes and sizes. Some of them can be mounted on casters and equipped with removable side curtains. When used this way, they are often referred to as portable cleanrooms. An example is shown in Figure 4.3. Minienvironments are modular cleanrooms that may include control over contaminants other than particles. For example, minienvironments may provide very tight temperature, relative humidity, and airborne molecular contamination control for photolithography processes. Minienvironments are the logical extension of the use of flow control panels in cleanrooms to isolate products and processes from the general room environment. However, rather than just controlling airflow, they are usually designed to isolate products and processes completely and usually enhance other controls. For example, minienvironments for deep

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FIGURE 4.4 (a) Typical horizontal flow bench with an ionizer grid in front of HEPA filters; (b) typical vertical setup, with the flow bench mounted over a conventional laboratory workbench and cabinet. (Photo courtesy of Terra Universal.)

ultraviolet photolithography usually include airborne molecular contamination filters if the photoresist housed within them is sensitive to inorganic and organic amine vapors. Clean benches may be thought of as the original manifestation of the isolation/minienvironment approach to design and construction of cleanrooms. Clean benches come in two basic forms: horizontal and vertical unidirectional flow. Figure 4.4a shows an example of a horizontal unidirectional-flow clean bench: Figure 4.4b is a typical verticle flow bench. Finally, we end up at the ultimate in isolation technology, the glove box (Figure 4.5). In a glove box, the product within the box is protected from the contamination in the ambient environment, and the ambient environment is protected from the contamination inside the box. When used in the medical/pharmaceutical industry, these are referred to as isolators or biological safety cabinets. They also have application in the aerospace industry for handling samples returned from space. A wide variety of biological safety cabinets are in use today. The selection of the type of cabinet to use is a function of the type of activity to be performed and the associated risk of contamination exposure to personnel, the product, or the environment. This subject is dealt with in detail in Refs. 1 and 2. 4.2.2

What It Takes to Make a Cleanroom Work

Several elements must be combined to make a cleanroom. The room must have appropriate filtration. The type (HEPA, ULPA, AMC), number, and placement of filters are determined by the contamination requirements and class of the cleanroom. Materials of construction must be selected that are compatible with the product and process. This includes requirements such as freedom from sloughing, little or no outgassing, and proper surface finishes. The materials must be impervious to chemicals to be used in the process or in housekeeping. Finally, consideration in the design and layout of the room must take into consideration the airflows that will occur during full operation. In this chapter, filtration and the

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FIGURE 4.5 Typical glove box. Three of the glove ports are outfitted with arm-length gloves; the fourth is sealed with a transparent cover. A double-door pass-through is shown on the right side of the cabinet. HEPA-filtered air is introduced to the top of the cabinet via the blower at the upper right. Exhaust is filtered before being returned to the ambient lab environment. (Courtesy of Terra Universal.)

consideration for room layout and their effects on airflow are discussed. Analysis method that are suitable for materials selection is discussed in Chapters 3 and 6. 4.2.3

How Filters Work

Filters may be grouped into two general categories: particle filters and airborne molecular contamination filters. Particle Filters HEPA and ULPA filters collect particles by five different mechanisms: sieving, impaction, impingement, electrostatic attraction, and diffusion. These five mechanisms combine to produce the overall filter efficiency curve, as shown in Figure 4.6. Most HEPA and ULPA filters consist of a mat of glass fibers bound together with a polymeric binder. This medium is then pleated and potted into a filter housing. Modern filter housings are nearly all metal. In older facilities, many of the filter housings will be wood products. Pleating materials include metal and filter media. The potting compounds are adhesives. Generally, the fibers are a few micrometers in diameter, with gaps between the fibers of 10 to 100 m. No straight-line path is available through the filter because the filter paper is very thick. HEPA and ULPA filters are sealed to the ceiling support using elastomer or foam gaskets or a gelatin like substance held in a U shaped trough. Sieving Particles that are too large to fit between fibers are trapped on the surface of the filter media by the process of sieving. In many cases, the particles that accumulate on the surface of the filter consist of a large percentage of clothing and paper lint fibers. As these accumulate on the surface of the filter, they add depth to the filter. Since they do not penetrate the body of the filter medium, they do not contribute significantly to pressure drop increase in the filter. Instead, when a large percentage of the contamination trapped by sieving consists

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Overall Collection Efficiency 100

Capture Efficiency (%)

Small Particle Collection Efficiency

Large Particle Collection Efficiency Mechanisms:

Mechanisms: Diffusion, Electrostatic Attraction

Impaction Impingement

Sieving

Maximum Penetrating Particle Size

99.9 0.01

0.1

1.0

10

100

Size (μm)

FIGURE 4.6 Typical filter efficiency curve for a HEPA filter. The decrease in efficiency of large particle collection mechanisms as the particles get smaller combine with the decrease in efficiency of small particle collection as the particles get larger to produce an overall filter efficiency curve that must have a minimum collection efficiency at some size, referred to as the maximum penetrating particle size. HEPA and ULPA filters are tested and rated at their maximum penetrating particle size.

of fibers, this acts as additional filter medium. This can increase the efficiency of the filter without significantly increasing the pressure drop. As particle size decreases, two other mechanisms become dominant: ●

●

Impaction: In impaction, particles have too much inertia to follow the air movement around the fiber. The particle crosses streamlines and strikes the surface of the fiber. When the particle touches the surface of the fiber, it is captured by van der Waals forces. This process is illustrated in Figure 4.7. Particle a has too much inertia and crosses the streamlines, contacting and being collected by the surface of the fiber. Impingement: In impingement, particles follow the streamlines. If the streamline approaches within one particle radius of the fiber, the particle touches the fiber and is collected by van der Waals forces. This is illustrated as particle b in Figure 4.7. Particles swept into the turbulent flow field behind the fiber are also collected, mostly by impingement. This is illustrated by particle c, trapped in the rotational flow field at the trailing side of the fiber.

Impaction and impingement are 100% effective at collecting very large particles. As particle size decreases, these two mechanisms decrease to less than 100% efficiency. Very small particles are collected by the processes of diffusion and electrostatic attraction. Diffusion In diffusion, the particles are so small that they are not bombarded uniformly on all sides by collisions with air molecules. The collisions that are not canceled out by collisions in the opposite direction impart motion to the particles superimposed on the bulk motion of the volume of air carrying the particle. This results in random motion of the particle around the primary streamline defining the particle’s average path. This random walk

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b c

a

FIGURE 4.7 Impaction and impingement collection of particles by a filter fiber. Particle a is too large to follow the streamlines as they change direction to pass around the fiber and is collected by inertial impaction. Particle b is small enough to follow the change in direction of the air, but the streamline carrying the particle approaches within one particle radius. Particle b is captured by impingement. Particle c made its way around the fiber but was captured by the rotational flow field in the fiber’s wake. Particle c is collected by impingement on the fiber surface. Filter Fiber a

b

FIGURE 4.8 Particle collection by diffusion. The airflow streamlines are solid. Particle a is larger than particle b. It has relatively less diffusion, so its meandering path about the airflow streamline does not intercept the filter fiber, the path of particle a is represented by the line with short dashes. Conversely, particle b, which is smaller than particle a, has larger diffusion. Its path, shown by the line with long dashes, intercepts the fiber. Particle b is collected by van der Waals forces when it contacts the fiber surface.

is also called Brownian motion. The smaller the particle, the greater the tendency to take the off-centerline walk or diffuse and be captured by van der Waals attraction when the particle touches the surface of a fiber. Figure 4.8 illustrates this diffusion phenomenon. Electrostatic Attraction Nearly all naturally generated particles are charged either positively or negatively and nearly all have some imbalance of electrical charge [3]. The charged particles become attracted to charges on the surface of filter fibers. The charges on the fibers can occur because fibers are excellent collectors of ions from the air. In fact, because of the very small diameter of air ions, they are collected very efficiently by HEPA and ULPA filters. Charges on the surface of filter fibers attract oppositely charged particles. Electrostatic attraction is very strong for small particles but decreases with increasing particle size. Particle filtration by electrostatic attraction and diffusion is essentially 100% effective for very small particles, those less than about 0.01 m in diameter, but decrease as particle size increases. Impaction and impingement are essentially 100% effective for collection of large particles, those above about 10 m in diameter, but decrease as particle size decreases. As a consequence, these four mechanisms must combine to produce an overall filtration efficiency curve that has an observed minimum somewhere between 0.1 and 1.0 m in diameter. This minimum in particle collection efficiency occurs at the maximum penetrating particle size. This is the size at which HEPA and ULPA filters are rated.

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Airborne Molecular Contamination Filters HEPA and ULPA filters are very effective at removing particles from air but have virtually no capacity for retaining gaseous or vapor-phase contaminants. These materials, often referred to as airborne molecular contamination (AMC), are adsorbed momentarily but not retained permanently by HEPA and ULPA filters. Airborne molecular contamination filters remove chemicals from the air by physical absorption (sometimes the term physisorption is used) and chemical reactions. Physisorption is the process of locking up molecules mechanically in other solid material, the most common of which is activated charcoal. Activated charcoal is a very effective absorbent for organic molecules but is not very effective for inorganic acidic or basic gases. To absorb inorganic acidic and basic acids such as HCl and sulfuric acid or basic compounds such as ammonia, a different type of absorbent is needed. This different absorbent reacts chemically with the acids or with the bases and binds them to the AMC filter. The most common chemicals used for this purpose is a mixture of sodium carbonate and sodium bicarbonate, referred to as a buffer. A buffer is a mixture of chemicals that tends to try to maintain a constant pH. The treatment of the AMC filter with this mixture of carbonate compounds tends to keep the filter at a constant pH while absorbing acidic or basic gases. Life of Filters Often novice users of cleanrooms ask about the usable life of filters. There is no single answer to this question because filters are not replaced after a time of operation but for other reasons. HEPA and ULPA filters may need to be replaced if they are physically damaged or if they have gotten wet. HEPA and ULPA filters that are not damaged and continue to show acceptable particle counts are usually replaced when the pressure drop across the filter is double the initial value. Thus, particle filter are generally not replaced because of their particle filtration performance, but are replaced because they restrict the airflow more that the fans supplying them can accommodate. AMC filters are generally constructed of several layers of absorbent separated by air gaps. Removable absorbent pouches can be placed in the air gap preceding the last layer of absorbent. These are periodically analyzed to determine if AMC is penetrating past the next to last layer of absorbent, indicating that its capacity is being exceeded. When AMC is detectable on the absorbent pouch, it is time to replace the AMC filter.

4.3

CLEANROOMS

Once a project has been defined, the requirements for the performance of the cleanroom are quite easy to define. From these cleanroom performance requirements, the structure, layout, and selection of materials can then be defined based on the performance requirements. One of the more important considerations in the design and layout of tooling and workstations is the way that airflow affects contamination or ion movement in the room. To appreciate the importance of airflow in workstation design and placement within a room, the way that airflows in a cleanroom should be understood. The effects or airflow must be considered for all phases of design and construction of the cleanroom, installation of tooling, and operation of the cleanroom. Factors that must be considered include: ● ● ●

The material-handling system Parts storage locations Workstation location and orientation within the room

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Product location on process fixtures on workstations Operator movements

Airflow has an influence on many cleanroom performance factors. First, airflow can dilute contaminant generated in the room. Airflow can aid in control of the temperature and relative humidity in the room. Airflow is essential to the comfort and safety of operating personnel. Maintaining proper pressure differentials is essential to control direction of airflow for contamination isolation with the cleanroom and its contained enclosures, between the cleanroom and the external “factory” environment, and for extracting and exhausting waste and hazardous materials. Proper airflow is essential in most cases to ensure adequate performance of air ionization systems. Excessive airflow can also lead to as much operator discomfort as inadequate airflow, particularly dry skin, irritated eyes in the cleanroom and cold hands in the ESD-safe workplace. Finally, some processes are extremely sensitive to airflow disturbances and require special contamination control approaches. A good example is in lubrication of rigid disk recording media, where lubricant is applied during a slow pull from a quiet liquid interface. In the following discussion of cleanroom designs, the initial focus is on mixed- or turbulent-flow cleanrooms. Next, the discussion focuses on unidirectional-flow cleanrooms. Airflow in clean benches is described next. This brings us finally to a discussion of isolation enclosure, glove boxes, and biological safety cabinets and minienvironments. The latter are specialized environments (forms of cleanrooms) that have always had niche applications but are finding increasingly important applications now and will be used increasingly in the future. 4.3.1

Non-Unidirectional-Flow (Conventional or Mixed-Flow) Cleanrooms

The current terminology for describing the two types of airflow in cleanrooms is unidirectional and non-unidirectional. These two terms replace the older terms laminar and turbulent. Sometimes mixed flow is used in place of turbulent flow. The new terminology is used in recognition of the fact that airflow in cleanrooms can be neither laminar or turbulent in fluid mechanical terms, where the two words have definite meanings. Reynolds Numbers and Cleanroom Airflow Fluid mechanics defines airflows in terms of a dimensionless unit, the Reynolds number. The Reynolds number between parallel plates is defined as ReL: Re L 

Lub 

where L is the length, in feet; ub is the bulk velocity, in feet per second;  is the density, in lb/ft3; and  is the viscosity, in lb/ft-hr. Under representative cleanroom conditions, 70°F (21°C) and 1 atm pressure, at a bulk velocity of 90 ft/min (1.5 ft/sec), this becomes ReL  2.58L Given a ceiling height of 10 ft in a vertical unidirectional-flow cleanroom, we get a Reynolds number of 25.8. Clearly, this is nowhere near a Reynolds number of 2200, which is the onset

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of turbulent flow. A Reynolds number below 2100 is normally associated with fluid mechanical laminar flow. Our Reynolds number of 25.6 is less than 2100, but does this mean that our laminar flow cleanroom is fluid-mechanical laminar flow? It is not. Creeping flow is normally associated with a Reynolds number around 1, so clearly the flow in the cleanroom could be more accurately described in fluid-mechanical terms as creeping flow than laminar flow. In mixed-flow cleanrooms, airflow is characterized by streamlines that do not remain parallel to one another, but rather, intermingle or mix on a macroscopic scale. Contamination moves about in an unpredictable manner. Standing recirculation zones tend to allow the contamination to remain present for a long time. The amount of surface contamination accumulated by product is proportional to the time the contamination is present. Figure 4.9 illustrates a conventional cleanroom that has a limited deployment of filters in the ceiling and that is built on a grade-level floor. In this figure, the return ducts for the room are at the perimeter walls, with return diffusers mounted near the floor. If the ceiling-mounted filters are operated at a nominal face velocity of 90 ft/min, the room as illustrated would have about 120 air exchanges per hour and could reasonably be expected to achieve class 1000 performance. In this illustration, six HEPA filters are mounted near the center of the room in two rows. They are separated by rows of solid ceiling tiles. In addition, each of the two rows of HEPA filters are separated from the outside walls by rows of solid ceiling tiles. The airflow from this arrangement of HEPA filters in the ceiling and the returns along the walls is particularly instructive. The high-velocity airflow from the HEPA filters, labeled a in Figure 4.9, forces its way down into the room vertically but quickly loses momentum. The vertical airflow from the HEPA filters changes direction so that the air is removed from the room by the suction provided by the return air vents located near the floor on the walls. Note that this illustrates an important principle about airflow. Airflow is analogous to positioning a string. If you want to control where the air goes, you must pull it out of a room as if it were a piece of string. You cannot position a string where you want it by pushing. Two Rows of HEPA Filters

Solid Ceiling Tiles

a b

Solid Ceiling Tiles

Solid Ceiling Tiles

a c

b

d

Return Near Floor

Grade-Level Floor

Return Near Floor

FIGURE 4.9 Typical mixed-flow cleanroom design shown in cross section, with simplified airflow patterns.

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The airflow from the HEPA filters induces airflow from nearby areas. First, note the airflow between the left bank of HEPA filter and the left wall, under the solid ceiling tiles, labeled b in the figure. The air in this location is moving due to the fact that there is friction between the air moving out of the HEPA filter and the stagnant air under the adjacent solid ceiling tiles. The air flowing from the HEPA filter induces a rotational flow in the room airspace under the adjacent solid ceiling tiles. This rotational flow tends to increase the time that contamination remains in the room, because it tends to recirculate rather than to exit the cleanroom immediately. If contamination is generated in this standing recirculation zone, it will remain in the room for an extended period of time. The mirror image of this flow pattern occurs under the ceiling tiles on the opposite wall. Note the airflow pattern in the center of the room under the solid ceiling tiles between the two rows of HEPA filters. This airflow occurs for the same reason as before: Friction between the moving air from the HEPA filter and the initially stagnant air under the solid ceiling tiles causes the initially stagnant air to move. However, the motions are more complex than the flow between the HEPA filters and the walls. Friction between airflow from the HEPA filter and the adjacent blank ceiling areas induces a rotational airflow. In this case, air flows down on the perimeter and must flow up in the center, labeled c in Figure 4.9. Finally, note the airflow pattern near the floor in the area between the HEPA filters. The air flows from the two rows of HEPA filters diverge to exit the room through the return grills on the walls near the floor. This diverging airflow creates a standing recirculation zone over the floor in the center of the room, labeled d in Figure 4.9. Note that the direction of the airflow in the center of this recirculation zone is upward from the floor in the center. This standing recirculation zone is particularly troublesome because it provides a pathway for contamination on the floor to become resuspended in the air in the room. There is a high probability that the upward flow from recirculation zone d will mingle with the upward flow in recirculation zone c. Standing recirculation zones in conventional airflow cleanrooms can carry particles from the floor up to the ceiling in the cleanroom, where other standing recirculation zones can keep them suspended in the air for extended periods of time. These zones of standing recirculation are the primary reason why non-unidirectionalflow cleanrooms are often referred to as mixed-flow cleanrooms. The long residence time for contamination in a non-unidirectional-flow cleanroom is one of the reasons why this type of cleanroom is usually unable to operate cleaner than FED-STD-209 class 1000 (ISO 14644 class 6) unless extraordinary precautions are taken to eliminate sources of contamination within the room. 4.3.2

Air Ionization for Non-Unidirectional-Flow Cleanrooms

Room-type air ionizers installed in the ceiling near HEPA filters can be effective over a very large area of the room. The same turbulence that can keep contamination airborne for an extended period of time will spread ions produced by the ionizers airborne over an extended area. The turbulence tends to spread the air ions around so that a few ionizers mounted near the ceiling filters can ionize a large area of the room. However, discharge times will tend to be long and can vary from place to place in the room, depending on the proximity to the ionizer. This type of ceiling-mounted room air ionizer is often supplemented by ionizers installed for specific tasks. For example, it is not unusual to use fan-powered air ionizers in mixed-flow cleanrooms where the airflow is locally inadequate to achieve the desired discharge times or float potentials using ceiling mounted ionizers alone.

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169

Unidirectional Flow: 100% Filter Coverage

Vertical Unidirectional-Flow Cleanroom One method to eliminate the standing recirculation zones near the ceiling is to cover the ceiling with filters completely and operate them all at the same velocity. With no differential velocity among ceiling HEPA filters, no standing recirculation zones will be generated. The airflow pattern expected in a cleanroom with 100% ceiling filter coverage but without a perforated raised floor is depicted in Figure 4.10. The elimination of standing recirculation zones near the ceiling is dependent on the balance of airflow among all the ceiling-mounted filters in the room. Indeed, this is one of the most perplexing problems faced by test and balance firms that maintain and certify cleanrooms. Even when a ceiling is essentially 100% covered by filters, adjustments to eliminate turbulence and horizontal airflows can require skill and time. To some degree, modern fanfilter units have simplified this balancing act. With a fan-filter unit, the discharge velocity of each filter can be adjusted independent of all the others. By contrast, when a central fan is used, supply branches and individual filters would be equipped with dampers to provide flow control. Adjusting the damper on an individual filter would change the pressurization within the supply duct, so each filter adjustment would affect other filter discharge rates. Covering the ceiling with HEPA filters so there are no areas of the ceiling under which standing recirculation zones can form does eliminate standing recirculation zones high above the floor in the room. However, complete ceiling coverage does not eliminate one of the most important standing recirculation zones: that located near the floor in the center of the room between the returns, labeled d in Figure 4.10. This standing recirculation zone in the center of the room tends to loft contaminants toward the ceiling. However, unlike the conventional cleanroom depicted in Figure 4.9, there are no standing recirculation zones extending down from the ceiling above. Thus, in the unidirectional-flow cleanroom shown in Figure 4.10, complete coverage of the ceiling by HEPA filters causes contamination to remain near the floor. Ceiling Covered by HEPA Filters

d

Return Near Floor

FIGURE 4.10

Grade-Level Floor

Return Near Floor

Ceiling filter coverage of 100% in a conventional cleanroom without a raised floor.

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Work Surface Height and Room Certification The standing recirculation zones near the floor in a cleanroom with grade-level floors (i.e., not on perforated raised floors) has led to an approach to cleanroom certification and use rules that have continued to be applied to rooms with perforated raised floors. Rooms are certified for particle count at the level of the work surface. If you measuring the particle count directly below and close to the face of the HEPA filter in a vertical unidirectional-flow cleanroom, there are virtually no particle counts unless the filter is damaged or the seals are leaking. Counting at this height in a cleanroom measures the contribution of the HEPA-filtered airflow to the total room particle count. This is not necessarily a bad thing to do, as long as it is recognized that this represents only a portion of the sources of contamination that can contribute contamination during a stage 1 certification. It is only a measure of ceiling and filter integrity. A complete stage 1 certification should also be measured at the level of the work surface, so that samples can include any room contribution intervening between the ceiling and the work surfaces. Why, you might ask, even do a particle count so close to the ceiling? The answer is: To do a filter integrity test. This is how you sample for an installed filter leakage test, ISO 14644-3, Annex B6. This eliminates all other possible contribution within the room and ensures that any counts seen are attributable to the filters and their installation. The suggested interval for the installed filter leakage test is 24 months. There is also a second reason for sampling so close to the ceiling, which is probably more useful and will certainly be done more frequently than an installed filter leakage test. The second reason is to measure, locally, the installed filter contribution to particle counts detected at the level of the work surface during production operations. Thus, when investigating a particle count problem detected at the level of the work surface, the first step is to verify the integrity of the installed filters before proceeding with the investigation. Rooms are certified at the height of the work surface because that is where product is supposed to be located. Samples taken closer to the floor often are much higher than samples taken at the work surface because of the standing recirculation zones near the floor. This imposes a restriction on where product can be stored in the cleanroom. Storing product below the level of the work surface, even if contained within packaging materials, is one of the most common violations of cleanroom protocol observed during cleanroom audits. 100% Ceiling Filter Coverage If the top of standing recirculation zone d shown in Figure 4.10 rises above the level of the work surfaces located near the center of the room, contamination will be carried over the work surfaces from the floor, contaminating products and processes. Airflow studies have shown that the height of the recirculation zone is a function of the distance between the two return walls. These visualization studies have shown that periodically, the top of the room-centered standing recirculation zone will rise above the level of a typical workbench (28 to 29 in. above the floor) when the distance between the return walls is about 24 ft. Turbulence generated by people and other objects moving about in the room amplifies this effect. Narrow cleanrooms with complete ceiling coverage but without a perforated raised floor are sometimes referred to as unidirectional-flow tunnels. They are often used in room designs where only one or two rows of process tools will be installed. Quite often the tunnel will consist of a main aisle with several branches called process aisles, as shown in Figure 4.11, as a floor plan and as a three-dimensional drawing, shown in Figure 4.12. This type of cleanroom is very commonly used in semiconductor, photomask, magnetic recording head, and rigid disk manufacturing. There are two approaches to installing tooling

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Process Aisles

Main Aisle

Return/Service Cores

FIGURE 4.11

FIGURE 4.12

Typical tunnel floor plan.

Typical tunnel floor plan shown in three dimensions.

in tunnels. Tools can be lined up in the center of the room, as shown in Figure 4.13. One advantage of locating the workstations down the centerline of the room is that this takes advantage of the inherent airflow within the room. Air will flow down from the ceiling and diverge to exit the room. Contamination from personnel in the room will be directed away

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Utility Drop Airflow Patterns

Floor-Located Returns

Grade-Level Floor

Workstations

Floor-Located Returns

FIGURE 4.13 Tools aligned down the centerline of a process branch in a typical tunnel-style cleanroom on a grade-level floor. Note the prevailing airflow pattern in the room. (The end wall is removed for clarity.)

from the workstations and product processing locations, across the personnel space, and out of the room. There are drawbacks to this type of tool installation, however. One disadvantage is the need to deliver utilities to the tools. As illustrated here, when a grade-level floor is used, this is often provided by utility drops through the ceiling. Liquid Waste lines exiting the room through these utility drops must be pumped. All maintenance must be done within the cleanroom. For some tools, the distance between support hardware and the “business” part of the tool is limited, so precious floor space ends up being occupied by portions of the tools that do not necessarily need to be inside the cleanroom. An alternative strategy for installation of workstations is to locate them along the return walls to take advantage of bulkhead mounting of the tools. This provides several advantages: ●

● ●

Portions of the tools not requiring a cleanroom environment may be located behind the return wall, outside the cleanroom, but still close enough to facilitate interconnection with the portion of the tool remaining inside the cleanroom, the “business” end of the tool. In some extreme cases, the only portion of the tool actually within the cleanroom is the load/unload chamber door. There is easy routing of power, communications, fluid supplies, and waste removal. Much of tool maintenance can be done from within the service core behind the bulkhead.

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a

Floor-Located Returns

Grade-Level Floor

Floor-Located Returns

FIGURE 4.14 Three-dimensional view of workstations bulkhead mounted on the return wall in a process aisle for a tunnel-style vertical unidirectional-flow cleanroom on a grade-level floor. Airflow patterns around workstation a are shown in Figure 4.15.

An example of bulkhead mounting of workstation in a tunnel-style vertical unidirectional-flow cleanroom on a grade-level floor is illustrated in Figure 4.14. Airflow from the ceiling to the floor-located return openings, unimpeded by an intervening workstation, is illustrated by bold dashed lines. Air exiting the room past a bulkhead-mounted workstation is shown in the light dashed lines. A detail of the airflow past workstation a is illustrated in Figure 4.15. Unfortunately, locating the tools along the return walls places them in a location where contaminants generated in the center of the tunnel will be carried by the airflow toward the tools. To minimize the contamination of tools installed on the return walls, care must be taken to control airflows over the tools. To prevent contamination from streaming over the critical contamination-sensitive areas of the process tools from the center of the tunnel and affecting the process adversely, it is important to detail the installation of the tooling. This detail includes sealing all openings between the tool and bulkhead above the level of the work surface or load-unload station on the tool. Many tools do not have horizontal surfaces, which act to direct airflow toward the center of the tunnel before the air exits the room below the critical product locations at the floor-mounted return. In cases where horizontal surfaces may need to be added to control airflow, flow visualization can be helpful in designing these flow control barriers.

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Unsealed Opening in Bulkhead

Undesirable Airflow Patterns

Desired Airflow Pattern

a

Cleanroom Portion of Bulkhead-Mounted Tool Floor-Mounted Return

FIGURE 4.15 Airflow leakage through improper sealing of a workstation to the return wall bulkhead.

4.3.4

Air Ionization in Unidirectional-Flow Cleanrooms

In a non-unidirectional-flow cleanroom, the mixing within the room tends to spread air ions about. Conversely, in a unidirectional-flow cleanroom, the airflow tends to keep the air ions stratified. As a consequence, ceiling-mounted ionizers seldom are found to provide satisfactory discharge-time performance for devices with human body model ESD sensitivity below 200 V, where discharge times much less than 45 seconds are required. In unidirectional-flow cleanroom, it is best to take advantage of the stratified airflow by placing bar-type ionizers directly in the air path that will deliver the ions to the critical location on the workstation. The relatively high velocity in unidirectional-flow cleanrooms [typically, 70 to 110 ft/min (0.35 to 0.45 m/s)] allows the use of bar-type ionizers in most cases, avoiding the extra expense, noise, and mixing introduced by fan-powered air ionizers. 4.3.5

Adding a Perforated Raised Floor

Obviously, there is a need to build cleanrooms where the distance between the return walls is greater than about 24 ft. There is a design adaptation that overcomes this problem successfully in most cases. Perforated raised floor tiles are installed across the entire room, as illustrated in Figure 4.16. The perforations in the floor tiles allow air to exit from the room everywhere instead of just from out of the room at the return walls. This provides a

CLEANROOMS

175

Ceiling Covered by HEPA Filters

Raised Floor

Return Under Raised Floor

FIGURE 4.16

Grade-Level Floor

Return Under Raised Floor

Cleanroom with 100% ceiling HEPA filters and a raised floor.

tremendous airflow control advantage. However, it comes at a price. The typical perforated raised floor increases the price of the cleanroom by $25 to $35 per square foot. The improved airflow control afforded by a raised floor has its limitations. The pressure differential between the cleanroom and the subfloor area is a function of the distance between the location of the floor tile in the room and the return wall. The closer to the return wall, the greater the pressure difference between the cleanroom and the subfloor area and hence the greater the air velocity exiting the room near the return wall. Conversely, the pressure differential in the center of the room, away from the return walls, is minimal, so the airflow exiting the cleanroom in the center of the room is much less than that near the return wall for wide rooms. This can result in horizontal flows in the cleanroom that are unacceptably high. 4.3.6

Balancing a Room Using a Perforated Raised Floor

Again, an easy solution exists. Cleanroom floor tiles that are equipped with flow control dampers are available to compensate for the variable pressure differences across the raised floor in cleanrooms. Figure 4.17 shows one approach to balancing airflow in a cleanroom where the distance between returns is 48 ft. Half of the room is shown: the other half of the room is a mirror image. This approach uses a mixture of solid floor tiles, perforated floor tiles with adjustable flow control dampers, and perforated floor tiles without flow control dampers. The solid floor tiles are installed in two rows immediately next to the return wall. This is the area where the pressure differential is a maximum, so the air preferentially wants to exhaust from the room next to the return wall. Perforated floor tiles with flow control dampers installed in these two rows would have their flow control dampers completely closed, negating the purpose of the perforations. The next eight rows of floor tiles have flow control dampers. The flow control dampers would be nearly closed in the row adjacent to the solid tiles and the degree of openness of the dampers would gradually increase as the center of the room is approached. In the center of the

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FIGURE 4.17 Balancing airflow in a cleanroom with a raised floor. The gray scale is an indication of the degree of flow restriction; the darker the color, the more restricted the airflow. Each square represents a 2 ft  2 ft floor tile. (Only half of the room is shown: the other half of the room is a mirror image of this.)

room several rows of perforated floor tiles without flow control dampers would be installed. Since the pressure differential across the floor in the center of the room is at the minimum, adding additional flow restrictions such as employing dampers is counterproductive. By using a mix of floor tiles, the cost of the floor can be kept to a minimum. The cost of floor tiles varies according to the following: solid perforated, no dampers perforated, dampers The problem of lack of uniformity of airflow above a raised floor is most noticeable in rooms with relatively shallow subfloor areas. Many modern cleanrooms, especially large rooms built for semiconductor fabrication, have very deep subfloor areas, to accommodate the massive utilities needed to support the process tools. Deep subfloor areas tend to produce uniform subfloor pressure drops, optimizing airflow uniformity through the raised floor above. 4.3.7

Airflow Balancing After Tool Installation

The airflow in a cleanroom should be balanced using the floor after the floor has been installed initially. In addition, it may be necessary to rebalance the floor after installation of tooling. Quite often it is necessary to cut the floor tiles to accommodate the tool support pedestals. Utilities and communications wiring for the tooling are usually routed under the raised floor, again necessitating floor penetrations. These cuts to the floor will alter airflow in the immediate vicinity of the tooling. In general, it is prudent to have extra solid floor tiles to be used where the floor must be cut to accomodate tools and utilities. Perforated floor tiles, especially those with dampers, are expensive and troublesome to cut. As tools are being installed, floor tiles often get moved away from their initial installation location, further increasing the need to rebalance the room after installation of tooling, where air flows have been balanced by selection of the type of tile or by adjustment of flow control dampers.

CLEANROOMS

a

177

b

FIGURE 4.18 Airflow patterns around workstations installed in a vertical unidirectional-flow cleanroom. Workstation a has a standing recirculation zone at the corner formed by the vertical wall and the horizontal work surface. Workstation b, in the center of the room, has a pyramid-shaped standing recirculation zone centered over the table.

When workstations are added to a vertical unidirectional-flow cleanroom with a balanced raised floor, the vertical airflow of the cleanroom becomes altered. Figure 4.18 illustrates two workstations installed in the vertical unidirectional-flow cleanroom discussed earlier. One workstation is installed against the return wall. The second is installed away from the return wall, closer to the center of the room. First, focus on the workstation installed next to the wall. It forms a horizontal obstruction to the vertical unidirectional flow of clean air. The airflow must change direction from vertical to horizontal away from the wall when the vertical flow encounters the horizontal obstruction. The wall is an obstruction to the horizontal airflow. We can call the wall a flow control barrier and use it to control the horizontal flow in a desirable fashion. Assume that the product is located on the center of the workstation. The horizontal airflow now moves over the product location toward the edge of the table at the center of the room. Personnel walking past the edge of the table can be a significant source of contamination. The horizontal airflow then could protect the product on the table from the contamination generated by the personnel by pushing the contamination back toward the personnel. To maximize this protection, one logically desires that the horizontal airflow from the product to the personnel be at a maximum. Decreasing the horizontal airflow toward the source of contamination increases the chance that the contamination will get to the product. Note the airflow in the corner between the horizontal surface of the workstation and the cleanroom wall. It is an area of standing recirculation. Contamination generated in this area will remain in the air above the workstation for a long time. The standing recirculation zone occupies about one-third of the width of the workstation in a 90-ft/min (0.4-m/s) vertical flow cleanroom. One strategy to deal with this standing recirculation zone is to mark the surface of the workstation to indicate a zone in which product or other critical surfaces should not be placed. An example is shown in Figure 4.19. Flow visualization would permit positioning a strip of warning tape on the work surface. Workers would be instructed not to place anything behind the tape. The tape could be marked with arrows, warning logos, or simply cover the entire area of the work surface that is not to be used.

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FIGURE 4.19 One method of marking a work surface to indicate a zone that is not to be used because of a standing recirculation zone.

FIGURE 4.20 Reduction of a standing recirculation zone by allowing a gap between the flow control panel and the horizontal surface.

A second strategy is to reduce the volume of the standing recirculation zone by allowing some air from this corner to spill over the back edge of the table. Moving the workstation slightly away from the wall results in the airflow shown in Figure 4.20. This reduces the size of the standing recirculation zone at the rear edge of the work surface. The location of the standing recirculation zone and its size are determined by how far away from the wall the workstation is located.

CLEANROOMS

FIGURE 4.21

179

Standing recirculation on a horizontal obstruction in the center of a room.

Next, we focus our attention on the airflow over the workstation, in the center of the room, located away from the return walls. In cross section, shown in Figure 4.21, the airflow over the horizontal obstruction of the workstation looks exactly like the airflow in the center near the floor of a room with no raised floor (Figure 4.9). Suppose that an operator stands at one side of a workstation. The operator’s body acts like a small wall, forcing the airflow to spill off the opposite, unobstructed side of the tool, as shown in Figure 4.22. Again, suppose that the product is located in the middle of the table. Now the airflow induced by the obstruction moves across the table from the person to the product. This is clearly undesirable. This can be corrected by placing a flow control barrier on the side of the workstation opposite the person’s location, as shown in Figure 4.23. Now the vertical unidirectional flow is forced to move horizontally, but it moves from the product location toward the operator. If the horizontal surface of the workstation and flow control barrier formed a solid corner through which no airflow could spill, a large standing recirculation zone would be formed in the corner. One note is of importance in managing airflow. It is important to keep the horizontal air velocity moving from the product toward the person as high as possible. For example, if it is not necessary for the workstation to be accessed from the sides, flow control panels should be added to the sides to force all of the horizontal flow toward the operator. Any changes in the design of the workstation and its flow control barriers that reduce the velocity of this airflow will degrade the protection provided. Therefore, it is important that airflow design be carried out with care. At this point, adjustment of the floor can be helpful. The floor tiles immediately under the work surface can have their damper closed or can be replaced by solid floor tiles. The floor tiles under the operator’s position can be opened up to assist in pulling horizontal flow toward the operator.

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FIGURE 4.22 Airflow due to interaction between an operator and the workstation. Note the airflow spilling off the back of the workstation.

FIGURE 4.23 Airflow due to adding a flow control barrier to correct the airflow interaction between an operator and a workstation. The airflow cannot spill off the back of the workstation completely and is now forced to flow from the product toward the operator.

CLEANROOMS

Rack open at back for loading/unloading

181

Upper Shelf – solid surface with opening at back for airflow. Incoming parts. Middle Shelf – solid surface. Incoming parts.

Bottom Shelf – wire rack. Scrap, rework and empty baskets. (Below work surface height.)

FIGURE 4.24 cleanroom.

4.3.8

Conceptual design for multiple-level parts storage in a vertical unidirectional-flow

Solid vs. Perforated Work Surfaces

In the early 1980s, the thinking was that tooling design should not interfere with the vertical unidirectional flow in the cleanroom. As a consequence, many cleanrooms were outfitted with tooling and workstations that featured perforated horizontal surfaces, open wire shelving for storage locations, and so on, to help preserve vertical flow. Unfortunately, this has the adverse effect of reducing the velocity of the desirable horizontal flow toward the operator. Experiments and fluid dynamic models have shown that solid surfaces, in combination with an understanding of horizontal flow and strategically placed flow control barriers, result in much lower product contamination than do perforated work surfaces. In addition, after the perforated surfaces are populated with tools, fixtures, trays, parts, and so on, most of the perforations are obstructed. From an airflow viewpoint, they are almost solid work surfaces anyway. 4.3.9

Parts Storage Locations

There is a second detrimental effect that is created by the use of perforated or open wire shelves in multilevel carts and storage locations. With multiple shelves stacked vertically, contamination generated by moving parts on upper shelves can fall on parts stored on lower shelves. Here are a few recommendations for design of storage locations within and around tooling: ● ●

Single-level storage in containers tilted toward the operator is best. Tilting allows retrieval of a part without reaching over parts.

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Return Wall

HEPA Filter Wall

Return Grills with Control Dampers

Grade-Level Floor

FIGURE 4.25 Typical horizontal unidirectional-flow cleanroom cutaway showing the filter wall, return wall, and airflow pattern in between. ●

●

Features must be provided to prevent the tray from sliding. This is considered desirable even on a horizontal surface, because you want to discourage personnel from sliding the tray on and off the shelf. Airflow disruptions are minimized.

For multiple-level storage, solid shelves are superior to perforated or open wire shelves. Figure 4.24 is a conceptual design for a parts storage rack for a vertical unidirectional-flow cleanroom. 4.3.10

Horizontal Unidirectional-Airflow Cleanrooms

Horizontal unidirectional-flow cleanrooms are not as common as vertical unidirectionalflow cleanrooms, but still exist and under certain circumstances, are a useful design. Their primary application has been found when adapting a cleanroom to an existing structure when ceiling height is so restrictive that there is insufficient space to locate filters, plena, ductwork, fans, and so on, above the cleanroom ceiling. Figure 4.25 illustrates the airflow patterns in a newly built horizontal-flow cleanroom. Air exits the face of the HEPA filter wall as a uniform front and moves across the room without macroscopic mixing. The return illustrated here used return grills equipped with dampers to ensure that air is pulled out of the room as a uniform front. (If dampers were not present, the air would preferentially exit the room near the ceiling because the pressure drop across the return wall is greatest at the ceiling.)

CLEANROOM CONSTRUCTION AND OPERATING COSTS

183

Horizontal-flow cleanrooms are common in the aerospace industry, where very high ceiling heights are often required. An important feature of horizontal-flow-cleanroom performance is that contamination generated near the filter wall adds to contamination downwind. Thus, as one gradually moves away from the filter wall, the class of the cleanroom can change. It is important, therefore, to place the operations requiring the best cleanliness close to the HEPA filters and that operations that can tolerate more contamination be placed farther away from the filter wall. Often, this utilization policy is impossible to implement. One of the most common applications of this type of retrofit of a horizontal-flow cleanroom into an existing structure is in the medical industry. The benefits of performing surgery in operating rooms that are also cleanrooms has been realized, especially for surgeries such as hip and knee joint replacement, where infection control is especially important.

4.4

CLEANROOM CONSTRUCTION AND OPERATING COSTS

It is not uncommon to hear of a cleanroom that cost $1000 to $2000 per square foot to build. This large number is a little misleading, since the cost of installing utilities such as ultraclean gas piping, waste drains, acid and organic chemical exhausts, and other utilities is often included in the construction cost of the facility. This is usually included because these utilities must be planned for and installed as part of the cleanroom design and construction. The cost associated with the process supply, exhaust piping, and other utilities associated with the process should be considered separate from the basic cleanroom. For example, a state-of-the-art semiconductor process will require far more elaborate and extensive supply and exhaust utilities than will a simple electromechanical assembly process. The cost elements that should be included are the common cleanroom architectural elements: ● ● ● ● ● ● ● ● ● ● ●

HEPA/ULPA filters Airborne molecular contamination filters (optional) The ceiling structure Recirculation fans if fan-filter units are not used Smoke and fire detectors and sprinklers above those required by the factory Special cleanroom flooring or a raised floor and floor tiles Walls, windows, doors, and pass-throughs Lights Monitoring and measurement instrument for cleanroom operation Air-conditioning equipment, including chillers, humidifiers, and dehumidifiers Utilities to support the room and its maintenance

Studies have estimated the average cost of building pharmaceutical, biopharmaceutical, and semiconductor cleanrooms ranging from FED-STD-209 class 1 to class 100,000 (ISO 14644 class 3 to class 8). This study separated 12 ducted cleanrooms from eight rooms built with fan-filter units. In the ducted cleanroom, the average facility cost was about $360 per square foot. In the cleanrooms built with fan-filter units the average cost was about $200 per square foot. In each case, the cost of supply piping and exhaust systems was about half of the cost of the facility. Subtracting the cost of these utilities, we end up with a realistic

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estimate of the cost of building the cleanrooms. The ducted cleanrooms cost $177 per square foot, whereas the rooms built with fan-filter units averaged $75 per square foot, exclusive of the cost of supply and exhaust utilities. Included in these costs are the recirculating fans or fan-filter units, the air-conditioning units (cooling, reheat, humidification), lights and controls, and the makeup-air handlers needed to pressurize the rooms with respect to their ambient factory environments. The operating cost for the eight ducted cleanrooms was estimated to be $26 per square foot per year. The operating cost for the 12 cleanrooms built with fan-filter units was estimated to be about $18 per square foot per year. These are the costs associated with operating the recirculating fans or fan-filter units, the air-conditioning units (cooling, reheat, and humidification, lights, and control systems), and the makeup-air handlers needed to pressurize the rooms with respect to their ambient factory environments. Thus, the operating cost of the cleanroom will meet or exceed the construction cost of the cleanroom within three to seven years. Given that the average life of a cleanroom is 15 to 20 years, it is obvious that operating cost is as important a consideration in cleanroom design as is construction cost. Among the biggest consumers of energy in the cleanroom are the fans moving the air through the filters. The amount of energy required to move air through the filters is directly proportional to the cube of the linear velocity. For this reason, many rooms are operated at or near the lower velocity rating of the filter. However, there are alternatives to adjusting fan speed to minimize energy consumption. In these alternative approaches, the area of the filters is minimized. We shall explore a few of these approaches. 4.5 4.5.1

MODERN ENERGY-SAVING APPROACHES Unidirectional-Flow Clean Benches

One approach to minimizing the deployment of filters is to confine the product or process under class 100 (ISO class 5) or class 10 (ISO class 4) unidirectional-flow clean benches that are installed in a class 10,000 (ISO class 7) or class 100,000 (ISO class 8) cleanroom. This approach makes sense if the total energy consumed by the unidirectional-flow bench and cleanroom combination is less than that consumed by providing a total cleanroom approach. Typical unidirectional-flow bench layouts often include only one or two benches in a factory environment. In a factory environment the airflow entering the room is unfiltered. Here it is important to consider the direction of airflow from the factory heating, ventilating and air conditioning (HVAC) ceiling diffuser. It may be necessary to plan the position of unidirectional-flow benches to avoid intrusion of unfiltered airflow from the factory HVAC air diffusers. Alternatively, it may be necessary to install flow control barriers to deflect the unfiltered airflow and prevent it from entering the front of the clean bench. Another approach is to install a fan filter unit in each factor HVAC diffuser. Some unidirectional-flow clean benches do not perform as well as others. This problem is more often associated with design of the air return system than with improper filter selection, fan selection, and so on. There are two primary types of unidirectional-flow clean benches: horizontal flow and vertical flow. Figure 4.26 illustrates a typical horizontal unidirectional flow clean bench and the airflow it supports. A horizontal unidirectional-flow clean bench typically has a filter mounted vertically the length of the work surface. The overhead lights, the side panels, and the work surface confine airflow so that it moves across the work surface with unidirectional streamlines. The return for the clean bench is

MODERN ENERGY-SAVING APPROACHES

185

HEPA Filter

Fan

Table

Return Filter

FIGURE 4.26

Horizontal unidirectional-flow clean bench and airflow patterns.

located below the work surface, so the air must return to the fans in the most desired fashion, vertically downward. Contrast the airflow in the horizontal unidirectional-flow bench illustrated in Figure 4.26 with the improperly designed vertical unidirectional-flow clean bench shown in Figure 4.27. In a vertical unidirectional-flow clean bench the filters are mounted horizontally. Airflow from these filters is intended to flow downward and across the work surface. There are two problems with this airflow arrangement. First, there is a standing recirculation zone at the back of the work surface next to the vertical wall, just as there was over the wall-mounted work surface shown in Figure 4.18. This problem can be partially solved by moving the work surface away from the back wall of the hood, allowing the air to spill off the work surface as before. A second, more serious problem is created by the location of the air return. The air return on most vertical unidirectional-flow clean benches is located overhead, either on the front or on the top of the cabinet over the operator’s head. This location prevents the air from reaching most of the work surface since air near the front short circuits to the return, minimizing its effectiveness at controlling contamination. One solution to this problem is to install a vertical sash to force the air down toward the work surface. Installing a sash is not without its problems. A sash can interfere with access to the interior of a clean bench. Looking through the sash may be unacceptable for precision assembly and inspection activities. Addition of a sash to the front of a vertical-flow bench does not correct the second problem: the less than ideal airflow outside the clean bench. For the air to return to the fan, it travels up in the room. This creates a large standing recirculation zone in the front of the hood that maintains contamination in suspension in front of the hood. In addition, the upcast airflow is fighting gravity. If the clean bench is installed in a factory or cleanroom where the prevailing room flow is downward, the return flow from the hood is also fighting the room airflow. This contamination can get swept inside the hood

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Return Filter

Fan

HEPA Filter

Recirculation Zone

Recirculation Zone

Table

FIGURE 4.27

Vertical unidirectional-flow clean bench and airflow patterns.

whenever a person reaches into the hood, and can also get swept into the hood by the turbulence created when a person walks by the front of the hood. Figure 4.28 illustrates one possible fix for the flow problems illustrated in Figure 4.27. In this fix, the return air openings at the top of the air-handling module have been covered by a new duct. A new return air opening has been created on the back of the unit. To ensure that the air entering this new return opening comes from under the work surface, a duct has been installed so that it opens to the space below the work surface. 4.5.2

Isolators and Minienvironments

Isolators and minienvironments are one of the later developments in contamination control facilities. Although developed primarily for reasons other than energy savings, they provide energy cost advantages. Some would argue that these should more properly be reclassified as process tools for tax advantages. (Facilities are usually taxed as real estate, using a longer depreciation period than that for capital equipment.) Indeed, many have argued that all cleanrooms should be treated as capital equipment. Regardless, isolators and minienvironments should be included as part of this discussion regarding their airflow characteristics. Isolators are used to isolate a product or process from the general ambient environment and also to isolate the ambient environment from the content of the isolator. They are used

MODERN ENERGY-SAVING APPROACHES

187

Return filter Sheet metal duct to control return path Fan

HEPA Filter

Table

FIGURE 4.28 An added sheet metal duct forces the air from the vertical flow hood to return under the table, correcting the airflow problem.

in a number of industries, including pharmaceutical, chemical, and food preparation, and in the military. In the medical industry, isolators are also used to treat burn patients and other medical conditions. The famous movie The Boy in the Plastic Bubble [4] is a good example of isolation as used in the medical industry. Figure 4.29 illustrates a typical class III biological safety cabinet, which is a good model for the principles behind isolators. In ordinary cleanroom applications differential air pressure is used to exclude contamination from areas needing protection. These areas are kept at a positive pressure with respect to their surroundings and are permitted to leak into their surroundings. With class III biological and medical/pharmaceutical isolators, differential pressure may exist, but no leakage is permitted. In general, the airflow in a biological/medical/pharmaceutical isolator need not be unidirectional. The isolator is equipped with at least one double-door passthrough, so materials can be passed into and out of the cabinet. These pass-throughs are usually equipped with filtered purge air systems. 4.5.3

Point-of-Use Clean Air Cleanrooms

There is another approach to the design of facilities to minimize energy consumption by filters: the point-of-use clean air cleanroom, also referred to workstation isolation. In this

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Factory air in

Exhaust air out

Exhaust HEPA filter

Inlet HEPA filter Viewing window

Glove ports Double-door pass-through enclosures

FIGURE 4.29 Class III biological safety cabinet. This illustrates the important principles of airflow for isolators.

approach to cleanroom design, filters are concentrated at critical product and process locations. Airflow from the filters is directed to the critical product and process location through the use of flow control barriers, which create an area around the critical product or process locations that are slightly positively pressurized with respect to the surrounding environment. More important, flow control barriers prevent contamination from the cleanroom from entering the critical product or process location. In point-of-use clean air cleanroom design, the ceiling is designed to accept filters, but filters are installed only over critical process locations. Airflow from the filters is directed toward the critical product or process location by the use of flow control barriers. Access to isolated workstations can be via a variety of options. The least restrictive access is afforded when the isolation barrier is minimized, like a sneeze shield on a salad bar. This is most often used where manual access is required frequently. An alternative isolation uses a sliding or swing door, which must be opened and closed to provide access. A slightly more cumbersome alternative is a double-door pass-through, which affords better contamination isolation at a slightly higher cost. The SMIF approach is by far the most expensive but provides the best contamination isolation. The performance of these alternatives versus FED-STD-209 classes 10, 1000, and 10,000 (ISO classes 3, 6, and 7) ambient cleanroom conditions outside the isolation enclosure is illustrated by the data in Table 4.1. 4.5.4

Tunnelizing an Existing Ballroom Cleanroom

Next we describe an upgrade that was given to an existing class 10,000 (ISO class 7) cleanroom to create a virtual tunnel down the center of the room. This was accomplished by relocating existing HEPA filters over the process aisle. Solid floor tiles are installed in the

MODERN ENERGY-SAVING APPROACHES

189

TABLE 4.1 Average Particles per Wafer per Pass  0.3 m for Various Access Methods into Isolation vs. Ambient Class Class

SMIF

Pass-Through

Swing Door

Sneeze Shield

10 1000 10,000

0.5 0.0 1.3

0.5 0.0 1.5

0.5 7.0 40

3.6 7.5 52

Source: Ref. 5.

FIGURE 4.30 First step in tunnelizing a ballroom cleanroom. Filters are consolidated over the intended main process aisle.

center of the room under the location of conveyors and work stations. The process conveyors and tooling were installed. Finally, flow control panels were installed in and around the tooling to create a virtual tunnel. Figure 4.30 illustrates the changes that were made to the ceiling. Filters from throughout the room were concentrated in the center of the room to provide approximately 80% ceiling filter coverage. In the second step of tunnelizing a ballroom, the tooling is arranged under the HEPA filters in the main process aisle. Flow control barriers are erected around the tools. The flow control barriers are left open where the operators must access the workstation and conveyor. FED-STD-209 class 10,000. The flow control dampers on the tiles beneath the conveyor and workstations were closed to create a horizontal flow from the product and toward the operators. A conceptual design for a portion of this step in tunnelization is shown in Figure 4.31. A second innovation was in the design of the flow control barriers. The flow control barriers stopped 18 in. below the ceiling. This was done to prevent creating new fire control spaces, which would have required installation of new sprinkler heads. The original facility estimate to convert the 26,000 ft2 of a class 7 cleanroom into a class 5 cleanroom was approximately $9 million. Converting the entire area in class 5 space would require installation of a new blower center, running additional ductwork, and installing a new power distribution center. By tunnelizing the room, class 10 performance was achieved in the workstations for only $900,000, including the cost of the static dissipative polycarbonate flow control panels and their frame.

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FIGURE 4.31 Second step in tunnelizing a ballroom cleanroom. The tools and workstations are arranged under the clustered HEPA filters.

4.5.5

Minienvironments

Safety enclosures often can act as flow control enclosures. Enclosures that do not provide HEPA filters, air ionizers, or blowers must consider the following: ● ●

Cleanliness of the interior is dependent on cleanroom air supply, air ionization, and so on. Openings must be provided in the enclosure ceiling, walls, and floor for air from the cleanroom to pass through.

Minienvironments are enclosures with HEPA filters, blowers, air ionization, and so on. They may be fully sealed and accessed using a load–unload feature. Considerations for the design of a minienvironment include the following: ●

●

High-velocity air jets induce flow within enclosures. This can result in the creation of standing recirculation zones, as illustrated in Figure 4.32. High-velocity air produces localized negative static pressures, which can cause air to leak into the enclosure by the Venturi effect.

Minienvironments may be manually loaded and unloaded, or they may be fully automated. One common approach to a fully automated minienvironment is to enclose the product within a sealed box, often referred to as a SMIF (standard mechanical interface) pod. An SMIF is a robot mounted on all of the minienvironments to allow one type of SMIF pod to be used throughout the process. The SMIF carries the notion of workstation isolation one step further. In a factory designed using SMIF isolation, not only are the workstations isolated by tool enclosures but the products are isolated within sealed product

OTHER DESIGN CONSIDERATIONS

191

Induced airflows

Induced airflows

Primary clean airflow

FIGURE 4.32 Induced airflow formed by a high-velocity free air jet. Standing recirculation flow and turbulence induced by the high-velocity jet redistributes contamination and increases contamination residence time.

enclosures called pods, which protect products in the cleanroom. When the pods are loaded on the process tool, they are unlocked, allowing the tool to have access to the product within the pod [6]. The interior of the pod can be purged with dry nitrogen or another inert gas.

4.6 4.6.1

OTHER DESIGN CONSIDERATIONS Doors and Air Showers

Doors need to be considered carefully. In small facilities with relatively little traffic in and out, swinging doors can be an acceptable alternative. However, in larger facilities with more in-and-out traffic, a swinging door might not be suitable. The surface of the door can quickly become damaged, especially if some of the traffic in and out includes material being moved on carts. In addition, swinging doors are often propped open to get them out of the way of traffic flow. This can result in airflow control problems if the door separates areas at different pressures. In larger facilities with a lot of traffic in and out, automatic sliding doors are often a wise alternative to swinging doors. They may seem to be expensive initially, but can actually be less expensive than swing doors, which require frequent replacement due to wear and tear. In addition, the automatic sliding door usually eliminates the problem of doors being propped open, since they get out of the way automatically. The entrances to cleanrooms from the change room are often equipped with air showers. Air showers are also used within cleanrooms where the adjacent cleanroom areas are of different cleanliness classes or are operated at different pressures. Air showers usually consist of interlocked entrance and exit doors and an interior equipped with high-velocity filtered air jets. Often, the interior is also equipped with air ionizers. The air shower is intended to remove loose contamination from the exterior of cleanroom garments of personnel entering the cleanroom from the change room. Occasionally, air showers are long tunnels, often not equipped with entrance or exit doors.

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The effectiveness of air showers at removing contamination from the surface of cleanroom garments has been the subject of much controversy. Several studies have concluded that air showers are ineffective at particle removal. Few have concluded otherwise. Perhaps the greatest benefit to be derived from the air shower is the psychological impact it creates. When entering a cleanroom through an air shower there is no doubt where you are going. The author has conducted two separate studies to examine the effectiveness of air showers. In the first experiment operators walked through the air shower with the fans and ionizers turned off and then walked past a series of witness plates (bare silicon wafers) in the cleanroom. The operators then walked back out of the cleanroom and returned using the functioning air shower (fans and ionizers turned on). Five operators walked past 10 plates for each trial. These were then analyzed using a Hamamatsu C1515 wafer scanning system with 0.5-m resolution. There was statistically no difference in the number of particles collected on the two sets of plates [7]. Later, a serious fiber contamination problem arose in a disk manufacturing facility. The change room for this facility had no air shower, so an alternative method of decontaminating the cleanroom garments after changing was needed. Maintenance personnel in this cleanroom wore dark blue cleanroom coveralls. These could be inspected using a black light. This inspection revealed that the exterior of the coveralls was contaminated by loose fibers. (Black-light inspection of the white cleanroom coveralls worn by everyone else was not feasible; the polyester of the white garments fluoresced so intensely that loose fibers could not be seen.) In place of an air shower, self-adhesive sticky rollers of the type used to remove lint from street clothes were implemented. Black-light inspection of the blue coveralls verified the effectiveness of sticky rollers at removing loose contamination. At another facility this experiment was repeated where an air shower was available. In this study it was possible to show that use of an air shower had no effect on the amount of fluorescent contamination found on the sticky roller used to remove contamination from a garment. Conversely, use of the sticky roller was shown to be effective. 4.6.2

Pass-Throughs

Pass-through enclosures are often used in conjunction with air showers. Often, air showers are so small that there is no room to carry even simple items into the cleanroom. Passthrough enclosures usually are double-door boxes which allow parts to be passed into and out of the cleanroom. Often, the doors are interlocked so that it is not possible to have two doors open at the same time. Pass-through enclosures can be especially useful when combined with parts packaging– repackaging stations. Figure 4.33 illustrates a plan view of a pass-through working with a packaging–repackaging station. When parts are placed in the pass-through, they are usually double bagged. In a double-bagged package, the outer surface of the outer bag is dirty, having been exposed to the environment outside the cleanroom. To get the clean parts into the cleanroom, an orderly process for removing the packaging must be established. When a passthrough is used with a repackaging station inside the cleanroom, an orderly process can be established. The repackaging process proceeds as follows. The double-bagged parts are removed from the pass-through from the cleanroom side of the pass-through by the dirty-side operator. The outside of the package is contaminated, so it should not be moved around the cleanroom indiscriminately. It should be placed on the repackaging station and the outer bag opened so that the inner bag can be removed by the clean-side operator without touching

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Inside the cleanroom

Operator for clean side of unpacking station

Clean side of unpacking station

Operator for dirty side of unpacking station

Dirty side of unpacking station

Waste basket

Doubledoor passthrough Cleanroom wall Outside the cleanroom

FIGURE 4.33

Plan view for an unpackaging station working with a double-door pass-through.

the outside surface of the outer bag. The outer surface of the inner bag has been kept clean, so the material may now be moved to the clean side of the repackaging station and then on to the rest of the cleanroom. When used as a packaging station the entire work surface can be used. It must be thoroughly decontaminated before product is brought to the workstation. 4.6.3

Equipment Pass-Throughs

Large pieces of equipment often require elaborate cleanroom entry procedures. This is best accomplished in an area dedicated for the purpose of bringing equipment into the cleanroom. Large pieces of equipment are usually packaged in material that has been exposed to the environment outside the cleanroom. This outer packaging should be removed before the equipment is moved into the cleanroom. This is best done in the equipment pass-through. Several examples of equipment pass-throughs are illustrated in Chapter 10. 4.6.4

Service Areas

Many cleanrooms contain equipment that requires intense maintenance. One of the most effective ways of providing such an equipment maintenance area is to place it adjacent to the change room; an example is shown in Chapter 10.

REFERENCES AND NOTES 1. Primary Containment for Biohazards: Selection, Installation and Use of Biological Safety Cabinets, 2nd ed., U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, and National Institutes of Health, Washington, DC, Sept. 2000.

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2. HHS Publication (CDC) 93-8395, Biosafety in Microbiological and Biomedical Laboratories, U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, and National Institutes of Health, Washington, DC, 1993. 3. B. Y. H. Liu, D. Y. H. Pui, W. O. Kinstley, and W. G. Fisher, Aerosol charging and neutralization and electrostatic discharge in clean rooms, Journal of the Institute of Environmental Sciences, Mar.–Apr. 1987, pp. 42–46. 4. The movie is about a boy born with a complete lack of a functioning immune system. He lived his childhood inside a plastic tent, completely isolated from sources of infection. 5. K. Mitchell and D. Briner, A comparison of manual and automated access to microenvironments, Journal of the Institute of Environmental Sciences, July–Aug. 1992, pp. 55–60. 6. S. Gunawardena, R. Hoven, U. Kaempf, M. Parikh, B. Tulis, and J. Vieter, The challenge to control contamination: a novel technique for the IC process, Journal of Environmental Sciences, 27(3):23–32, 1984. 7. R. W. Welker, previously unpublished observations.

CHAPTER 5

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

5.1

INTRODUCTION

It has often been estimated that the single most important source of contamination in a cleanroom is the piece parts themselves, along with packaging material in which they arrive. This makes sense in a way. Since we are trying to protect product from contamination, any contamination arriving in the cleanroom on the product or its packaging material is most proximate to the material we are trying to protect. By comparison, the workstations and garments of workers in the cleanroom are not likely to come in contact with the product. In addition, the probability that contaminants from the cleanroom will find their way to the product is similarly remote. Who should be concerned with parts cleanliness is an important question. Parts cleanliness is an issue for designers, manufacturing engineers, cleaning process engineers, procurement engineering, quality assurance personnel, and their management. Indeed, because of the dominant importance of contamination on piece parts and subsequent attempts to remove this contamination and preserve the cleanliness of the parts after cleaning, virtually every discipline will become involved. Chemists, chemical engineers, mechanical engineers, facilities engineers, and others will contribute to the multidisciplinary solutions required. The types of products adversely affected by contamination vary widely by industry and by volume of production. High-volume products such as semiconductors, disk drives, flatpanel displays, and CD-ROM or DVD are affected. Low-volume manufacturers such as equipment manufacturers or manufacturers in the aerospace industry are equally concerned, although they may choose different (e.g., batch-oriented) approaches to deal with contamination issues. In this chapter we describe some of the techniques available for controlling contamination on parts and how these are applied in various situations. Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

195

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5.2

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

HISTORICAL PERSPECTIVE

Early in the manufacture of high-technology products, little attention was paid to the surface cleanliness of parts. Even to this day, some high-technology manufacturers use relatively primitive methods to characterize and control contamination on piece parts. A few examples will suffice. In the 1970s and 1980s, many manufacturers assumed that their in-house cleaners provided adequate protection to prevent contamination incoming on piece parts from affecting their processes. The view was the result of a lack of quantitative measurement of parts cleanliness. Cleaning processes can be viewed as being relatively fixed in efficiency. The cleaning process removes a fraction of the contamination on the parts going into it. Figure 5.1 illustrates a simplified cleaning process. This should not be taken to suggest that the cleaning efficiency of all cleaning processes does not vary. Quite to the contrary, the fraction of contamination removed is a function of the contamination level on the parts going into the cleaning process. If parts put into the process are very dirty, a very high percentage of the incoming contamination will be removed. Conversely, if relatively clean parts are put into the process, the percent contamination removal will be less. This is illustrated by a simplified cleaning efficiency curve, shown in Figure 5.2. Parts with high incoming contamination levels are relatively dirty. Parts with large amounts of contamination can be cleaned to apparently very high cleaning efficiency. This may be misleading. Much of the contamination on heavily contaminated parts may be relatively easy to remove. Parts with low incoming contamination levels can be relatively clean. The contamination on these surfaces may be extremely difficult to remove, resulting in apparently low cleaning efficiencies. An interesting extrapolation here is that very clean parts can actually get dirtier going through a cleaning process (i.e., demonstrate negative cleaning efficiency). This can be

Parts In

Parts out

Fixed Efficiency Cleaning Process

Cleaning Efficiency Curve

50

70

90

12 5

17 5

30 0

50 0

70 0

90 0

100 80 60 40 20 0 20 00 12 50

Percent Removal

FIGURE 5.1 Simplified fixed-efficiency cleaning process.

Incoming Cleanliness Level

FIGURE 5.2 Cleaning efficiency curve showing the relationship between incoming cleanliness level and percent removal.

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observed frequently in practice when clean plastic parts of a hard disk drive assembly are washed right after a basket of dirty screws has been washed in the same cleaner. Historically, the cleanliness level of parts was not measured. As a consequence, parts were deemed to be clean based on visual inspection. This is often described as gross cleanliness measurement. These visual inspections were most often performed under ambient illumination, without magnification. The gage capability of these visual inspections was impossible to measure. However, visual inspection remains a useful safety net. The eye is indeed a sensitive device, just like the nose, which can smell parts-per-trillion levels of substances. Parts that have obvious visual contamination should be rejected, regardless of any quantitative cleanliness measurements used. The rejected contamination may be limited to such a small area of the part that the part does not fail a more general, quantitative measurement. In fact, at least one major computer manufacturer allows parts to be rejected on the basis of visual criteria, even though the parts are simultaneously measured rigorously using objective cleanliness measurement techniques. If a stain or other evidence of contamination is seen on a part, the cleanliness of the part is questioned. The suspicious area of the part is then subjected to a tape lift test. Stains that cannot be lifted using tape are ruled to be cosmetic defects and the parts can be used as is. Stains that result in a visible transfer of material to the tape are reason for rejection of the part: The part will not get used in manufacture and will probably be subjected to detailed analysis to identify the material of the stain and how it was probably produced. Small pits on the surface of a part can sometimes be mistaken for particle contamination. Pits seldom transfer features to tape, and the parts are often used. There is another reason for relying on the visual inspection of parts to back up the quantitative measurement of cleanliness. Personnel who handle parts on a regular basis often are very sensitive to the slightest deviation in their appearance from the norm. This can be leveraged to great advantage, since instead of having only a small number of cleanliness inspectors looking at the parts, in essence everyone handling the parts becomes a cleanliness inspector. Of course, there can be drawbacks to using this approach. Aside from a relatively minor loss in productivity that can result from so many people peering at the parts, there is a real risk of expending valuable engineering resources responding to false alarms. In most cases, the causes of these false alarms are identified and eliminated very quickly, minimizing chasing false alarms. The addition of magnification for parts visual inspection should be approached carefully. Increasing magnification to verify contamination can be very effective. Unfortunately, increasing magnification results in a decrease in the field of view, so increased magnification can result in contamination being missed. In the extreme, scanning electron microscopy at magnifications in excess of 10,000 may be used on a sample basis to inspect nano-dimensions, such as the edges of the air-bearing surface of the slider portion of a read/write head. In such cases, criteria must be statistically based rather than being absolute pass/fail specifications. Cleanliness control based solely on visual inspection can be effective for very smooth and nonporous materials. Indeed, hard disks, semiconductor substrates, flat-panel display media, and several other surfaces are suitable for direct visual inspection. Conversely, the vast majority of surfaces are unsuitable for direct visual inspection.

5.3

GROSS AND PRECISION CLEANLINESS PROTOCOLS

Visual inspection for cleanliness of parts that are not smooth, flat, and nonporous is often referred to as a gross cleanliness inspection protocol. In general, these visual inspections

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result in a high degree of cleanliness variability because these irregular surfaces are not well suited to direct visual inspection. Parts not well suited to direct visual inspection can be tested using indirect cleanliness measurement methods. These methods were discussed in detail in Chapter 3. These indirect cleanliness inspections produce quantitative results. They employ standardized instruments using standardized procedures. The availability of precision cleanliness data allows formal gage capability analysis to be done and thus renders these methods of use in establishing statistical process controls for product cleanliness. Historically, one of the most commonly used specifications for surface contamination levels has been MIL-STD-1246. This standard has now been converted to a consensus industry standard [1]. This standard consists of a particle size distribution that should be customized for most applications. The need for customization reflects the fact that few measured particle size distributions agree exactly with the standard particle size distribution. However, the model size distribution in the standard is a plot of the log of the particle concentration vs. the log squared of the particle size and results in linear particle size distributions. Most measured particle size distributions are also linear when plotted using this model. In summary, gross cleanliness versus precision cleanliness protocols may be compared as follows: Gross Cleanliness ● ● ● ● ● ● ●

Uses visual inspection (visual acuity not specified) Uses ambient light (lighting criteria not specified) Does not use magnification Is not a gage-capable inspection method Results in a high variability of cleanliness Often results in the use of component “suspect” contamination acceptability Can create a lack of respect for contamination criteria

Precision Cleanliness ● ● ● ● ● ●

Uses objective cleanliness measurement methods Has quantitative results Uses standardized instruments of known precision and accuracy Demonstrates gage capability Is suitable for statistical process control Results in known and controllable variability of parts cleanliness

The evidence suggests that a precision cleanliness control protocol has overwhelming evidence in its favor. However, this is not to say that there is no value to retaining a visual inspection element, even after adopting a precision cleanliness protocol. Many institutions retain a visual inspection protocol so that parts that appear to be irregular can be called into question. This “safety net” allows operators, engineers, and other personnel who discover parts with apparent visual defects to question their usability. Parts identified as visually suspicious are then subjected to objective cleanliness measurements to establish their usability.

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5.3.1

199

Approaches to Specifying Cleanliness Levels

Cleanliness can be specified based on any of the following approaches: ● ● ●

● ●

Material-dependent generic values Benchmarks using comparable competitive parts The cleanability characteristics of a part, as determined by multiple extraction measurements or other tests on prototype parts Receiving inspection data for lot certification Vendor process capability (source inspection)

Generic Value Approach The ability to utilize the generic value approach depends on the availability of an existing database for parts made of similar materials. These data can come from external reference values, supplier databases, or internal experience in measuring similar parts. For example, one may contemplate manufacturing using a part cast of A300 aluminum alloy, half of which is bare machined aluminum and the other half of which is coated with electrophoretic paint. If the average cleanliness level for bare machined A300 aluminum is known for several other parts, the cleanliness level per unit area can be calculated. Similarly, if the average cleanliness level for electrophoretic paint is known per unit area, this knowledge can be used to predict the cleanliness level for the new part solely from the part dimensions on the drawings. Clearly, the broader the database available, the more accurate a prediction can be made for the cleanliness expected of newly designed parts. The limitation of this approach is that it is based on old data. Changes in new processes may render the old database questionable. For example, all of the old data may be based on parts that were machined using oil-based cutting fluids, cleaned by solvent dip only. All of the new machine shops propose to cut using water-soluble cutting fluids and plan to spray clean the parts with hot water. The old data might not accurately predict the cleanliness of the new parts under this circumstance. Benchmark Approach A tried-and-true method of establishing acceptance criteria is to obtain data based on benchmarking the competition. This method can be used to establish contamination acceptance criteria as well. The primary limitations of this technique are as follows: ●

●

The competition may not control the same parameters to the level required to maintain the reliability level desired. For example, if a competitor’s product is not as sensitive to ionic contamination as your own, they may be able to tolerate a higher level of contamination than you can. Competitive components will have additional handling after receiving. They may be dirtier, due to accumulation of processing debris, or they may be cleaner than as received from their supplier if substantial in-house cleaning has been done.

Multiple-Extraction Approach In the multiple-extraction approach, several prototype parts that are representative of the expected final parts design and process are repeatedly extracted and the extracts are analyzed. The extraction method used for this test is the same method as that to be used for receiving inspection and source inspection/process control

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

Cleanliness Level

200

Specification limit: Ts = K × Asymptote

Asymptotic Cleanliness Limit

1

2

3

4

5

6

7

8

9

Number of Extraction Steps

Cleanliness Level

FIGURE 5.3 In a typical multiple ultrasonic extraction, the cleanliness-level measurement levels at an asymptote. Cleanliness specifications based on cleanliness measurements using multiple ultrasonic extraction are a multiple of K times the asymptotic cleanliness limit. K typically is in the range of two to four times the asymptotic limit.

Specification limit: Ts = K × Gage Capability Limit

Gage Capability Limit

1

2

3 4 5 6 7 Number of Extraction Steps

8

9

FIGURE 5.4 In a typical multiple spray extraction, the cleanliness-level measurement levels at an asymptote. Cleanliness specifications based on cleanliness measurements using multiple spray extraction are usually two to three times the spray extraction limit.

at the supplier. Many extraction methods result in a contamination vs. number of analyses curve that approaches an asymptote. For other extraction methods, a linear decrease in contamination concentration is seen that gradually reaches the gage capability limit for the measurement method. The control limit can then be based on a reasonable multiple of the asymptotic cleanliness limit for the part or the gage capability limit for the measurement method. This is illustrated in Figures 5.3 and 5.4. Inspection Data Approach Many materials require lot certification as part of receiving inspection. This is necessary when a supplier is incapable of performing the necessary measurements. The frequency of lot receiving inspection can be determined based on typical quality assurance principles. One advantage of receiving inspection is that it eliminates

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201

the need to correlate among a large number of measurement labs located in a number of different locations. There are several important limitations to this approach: ● ●

●

Feedback to the supplier regarding problems with lots involves delay. Lack of supplier measurement capability inhibits the ability of the supplier to implement statistical process control based on part cleanliness data. (Suppliers can implement process controls to keep a process stable, but these are not necessarily the optimum conditions for cleanliness levels.) Receiving inspection in a just-in-time manufacturing environment can create a conflict between the need to maintain cleanliness levels and the need to keep production running.

Source Inspection Approach Of the various techniques for establishing cleanliness limits, use of data based on source inspection is the best approach. To implement source inspection, the supplier must have capable gages for measuring cleanliness. These can be used to study the factors that influence cleanliness. Because the measurement capability is based at the supplier, feedback is immediate. Nonconforming lots can be sequestered at the source. Rework is enabled. The primary drawback for a source inspection program is the need to correlate cleanliness measurements among multiple sites: This is considered to be a minor inconvenience compared to the other advantages of a source inspection program. The limitations of this technique are:

●

A small number of parts are used to set the specification limit. This can result in a poor estimate of the true cleanliness capability of the part. Variation in the estimate of the variability of the cleanliness of the part can have a larger effect than the mean where a six-sigma specification approach is used. Some parts are not well suited to multiple ultrasonic extraction. Examples of multiple ultrasonic extraction behavior that makes it difficult to spec are illustrated in Figure 5.5.

Cleanliness Level

●

b a

1

Asymptotic Cleanliness Limit

2

3

4

5

6

7

8

9

Number of Extraction Steps c

FIGURE 5.5 Different types of asymptotes.

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

Curve a is a well-behaved multiple-extraction curve. Assignment of a cleanliness specification based on curve a is reliable. Parts exhibiting this type of multiple ultrasonic extraction curve include stainless steel, T6 hardness aluminum, titanium, and electroless nickel plate. Curve b is a type of part that exhibits ultrasonic erosion sensitivity. Setting a cleanliness limit based on multiple extractions assumes that all parts will be cleaned only to their minimum cleanliness level. Parts that must be cleaned because of rework may degrade in cleanliness. Conversely, setting a more generous cleanliness limit means that all parts will be dirtier. Materials exhibiting this type of multiple ultrasonic extraction behavior include cast aluminum, polycarbonate, and other, relatively soft materials. Curve c is observed for parts that are so sensitive to ultrasonic erosion that they immediately degrade in cleanliness. This is an indication that ultrasonic cleaning and ultrasonic extraction for cleanliness measurement are unsuitable for this part type. Materials exhibiting this type of behavior include carbon fiber–filled rigid polymers and high pressure water–cleaned bare aluminum castings.

5.4

DESIGN FOR MANUFACTURABILITY AND CLEANABILITY

In the 1970s and 1980s, design for manufacturability was a very hot topic in design. This was due in part to the increased interest in automated assembly processes. In the 1990s an interest developed in developing guidelines for design for cleanability. Interestingly, many of the guidelines of design for manufacturability also improve cleanability. The motives behind design for cleanability are multiple. Many industries in the 1990s were actively eliminating cleaning processes that used chlorofluorocarbons (CFCs) and other solvents with ozone-depleting or global-warming potentials. Since cleaning processes were to be changed anyway, many companies seized the opportunity to improve overall product yield and reliability by exploring some of the cutting-edge cleaning technology in contemporaneous development. In many cases these changes in product design for manufacturability and cleanability could bring about productivity, cost, and quality improvements at vendors. 5.4.1

Design-for-Manufacturability Guidelines

Design-for-manufacturability guidelines are as follows: ●

●

●

Design for single-axis assembly, where all of the parts are stacked up from one side of the part. Minimal part orientation changes are built into the design to allow relatively simple automation to complete the assembly process. Reduce the number of parts and part numbers. The fewer the parts to assemble, the easier is the task of automation. In mechanical assemblies, using only two or three fasteners rather than more simplifies automation, since fewer bowl feeders and part grippers are required. Use standard parts. This quickly became a consideration where several products could be manufactured on a given production line. By maximizing the commonality of parts used with several products, the conversion of a production line among several products minimizes switchover time. In addition, many manufacturers maximized the commercial content of their products, as opposed to using custom-designed components. This was an especially important design for manufacturability of printed wiring board assemblies.

DESIGN FOR MANUFACTURABILITY AND CLEANABILITY ●

●

● ●

203

Eliminate threaded fasteners, springs, and molded seals. Where possible, self-aligning features, snap-together fasteners, and film-type self-adhesive seals are favored. Design for robustness. For example, if the design provides for delayed assembly of very damage-sensitive components at the latest possible steps in the operation, the amount of damage during assembly can be minimized. Eliminate adjustments. Again, self-aligning features are highly desired. Design for ease of orientation. This often was a critical design consideration for cleaning and drying of products, where the objective was to minimize cleaning and drying time. Ease of orientation facilitates placement of piece parts on assembly tooling.

Often, various design for manufacturability guidelines are measured on productivity parameters, such as the number of completed products per hour, the average cost of production per part, and the profitability per product. Since design often influences the cost and complexity of the manufacturing process and tooling, these must be considered in the initial investment and the ongoing cost of production [2]. 5.4.2

Design-for-Cleanability Guidelines

We begin by listing the basic design-for-cleanability guidelines; each will be discussed in some detail. It will be seen that many of the design-for-cleanability rules are compatible with the design-for-manufacturability rules. ● ● ● ● ● ● ● ● ● ● ● ● ●

Use easily cleanable materials. Use surface treatments to enhance cleanability. Maximize compatibility of parts with water cleaning. Eliminate blind holes and reduce through-holes. Use materials and adhesives that do not outgas or emit corrosive vapors. Seal products effectively from external contamination. Design for ease of drying. Design for ease of packaging. Involve the supplier early in the design. Involve purchasing early in the design. Provide the part designer with feedback when cleaning problems are identified. Implement a “sign-off” by a cleaning engineer on the prints. Provide close collaboration between design, development, quality control, and contamination control functions.

5.4.3

Cleanability Indexes for Indirect Cleanliness Measurements

To understand cleanability, it is important to see how cleanability curves are developed. In indirect cleanliness measurements, the parts are subjected repeatedly to the same contamination extraction procedure using a standard extraction method. For example, the part may be extracted ultrasonically for 1 minute in each step. Conversely, the part could be spray extracted using a standardized spray pressure, flow rate, and extraction pattern at each step. A simple multiple ultrasonic extraction curve is illustrated in Figure 5.6. One way to normalize these

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

Normalized Cleanliness

204

1 0.8

Part Data

0.6 Asymptotic Cleanliness

0.4 0.2 0 1

6 Extraction Number

FIGURE 5.6 Well-behaved multiple ultrasonic extraction curve.

1

Cleanliness Ratio

0.9 0.8

Elastomers

Soft aluminum alloys

0.7 0.6

Rigid plastics

0.5

Stainless steel screws

0.4 0.3 0.2 Hard aluminum or steel alloys 0.1 Initial 1 2 3 4 Number of Extractions

5

FIGURE 5.7 Idealized cleanability curves for various materials.

multiple extraction curves is to divide the individual cleanliness values by the initial cleanliness value. An ultrasonic multiple extraction defines the cleanliness and cleanability of a part as measured using ultrasonic extraction to remove residual contamination. In many cases an asymptote is reached that represents both the limit of cleanliness that can be achieved and the limit of our ability to measure the cleanliness of the surface. Where ultrasonic extraction is employed, this often is the erosion limit for the part. Cleaning Parameters Figure 5.7 illustrates the multiple ultrasonic extraction behavior of several different materials and part types. The cleanliness ratio is the ratio of the contamination concentration of the nth extraction, Ci, to the initial cleanliness, C0. In general, the harder the material, the steeper the initial cleanup curve and the smaller the asymptotic cleanliness. Thus, hard aluminum alloys and steel alloys tend to clean up faster and arrive at a lower asymptotic cleanliness than most other materials. Not surprisingly, elastomers, which have high surface deformability and thus higher particle adhesion, clean up slowly. Elastomers also have a relatively low tendency to erode under ultrasonic extraction and thus very slowly approach a relatively low asymptote. Stainless steel screws also have a very shallow slope to their cleanup curve, this time because of their complex geometry and because screws are usually cleaned in bulk.

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These two portions of the cleanability curves may be described by two different fundamental cleaning factors, the surface cleanability, SC, and maximum cleanability potential, MCP. The surface cleanability is a measure of the relative ease of cleaning materials during early stages of cleaning: SC 

C0  C1 C0  Ca

where C0 is the initial cleanliness, C1 the cleanliness after the first extraction, and Ca the asymptotic cleanliness. The maximum cleaning potential, MCP, is more representative of the final cleanliness value: MCP  1 

Ca C0

Both SC and MCP may be expressed as a percentage. Figure 5.8 shows the relationship between percent SC and percent MCP vs. surface energy for a wide range of material types when measured using ultrasonic extraction. Repeated ultrasonic extractions produce cleanability curves, which can be used to estimate how clean a surface can be made and to determine the suitability of ultrasonic cleaning for a particular surface. To understand many of the figures used to illustrate design for cleanability guidelines, one needs to understand how the cleanability is defined. Two important indices are the maximum cleaning potential (MCP) and the surface cleanability (SC), defined as asymptotic T or LPC initial T or LPC initial T or LPC  first-stage T or LPC surface cleanability (SC)  initial T or LPC  asymptotic T or LPC

Aluminium Alloys

90

30

40

% SC

Electroless Nickel

80

20

80

Stainless Alloys

85

Rubbers

% MCP

90

Rigid Plastics

95

Epoxy Coated Metal

maximum cleaning potential (MCP)  1 

70

60

50 60 70 80 90 100 110 120 Surface Energy (erg/cm2)

FIGURE 5.8 Relationship among % MVP and % SC for a variety of surface materials when extracted ultrasonically.

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

Cleaning Parameter Value

206

100

90

MCP %

80 SC % 70 -5

-4

-3

-2

-1

0 )2

log (A132

1

2

3

4

5

/H

FIGURE 5.9 Cleaning parameters vs. adhesive force between materials. The percent surface cleanability is the proportion of the total removable contamination removed in the first extraction. This represents the ease of cleaning a part. The maximum cleaning potential is the ratio of the total amount removed to the initial cleanliness.

where T is the turbidity (primarily for metallic parts) in nephalometric turbidity units (NTU) and LPC  cumulative LPC counts (primarily for polymeric parts) in particles per unit volume. The force of adhesion between materials may be modeled by van der Waals force alone. With the exception of electrostatic attraction, nearly all adhesion may be explained on the basis of van der Waals attraction, including capillary adhesion. The force of adhesion can be characterized by the Hamaker constant, A132, which describes the forces prevailing between the particle, the surface to which it is attached, and the medium in which the particle and the surface interact. In addition, it is necessary to account for the inelastic deformation of one or more of these three components. Deformation of the surface accounts for the difficulty of cleaning elastomers vs. rigid polymers, which undergo comparatively less deformation. In addition, gradual deformation of materials accounts for the time dependence of adhesion forces [3] with particle adhesion to surfaces and also the aging effect of pressure-sensitive adhesive bond strength. Thus, the force of adhesion may be expressed as

Fadhesion ∝

(A132 )2 H

Various materials have been studied for both their MCP and SC properties. A correlation has been found between these two parameters, as shown in Figure 5.9. 5.4.4

Design-for-Cleanability Planning Considerations

Easily Cleanable Materials Particle-to-surface adhesion characteristics should be considered. Soft surfaces such as rigid polymers or elastomers can be very difficult to clean. Harder surfaces such as stainless steel or hardened aluminum alloys can be much easier to remove particles from. Soft alloys such as cast aluminum are generally intermediate in their ease of cleanability.

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Surface Treatments Where a material does not have inherently favorable cleanability characteristics, it may be necessary to improve cleanability through the use of surface treatments. Surface treatments include mechanical deburring and polishing to improve the smoothness of surfaces to reduce sites where contamination can hide. However, mechanical deburring and polishing operations seldom change the composition of the base material, so subsequent chemical treatments are often required. These chemical treatments can improve the cleanliness or cleanability of a surface and can also affect corrosion susceptibility, wear resistance, and the electrical and magnetic properties of the surface. Chemical treatments include chemical conversions, sealing with paint or conformal coatings, plating, electropolishing, synergistic coatings, sputter etch and sputter coating, chemical vapor–deposited coatings, and surface passivation (which also improves corrosion resistance). Mechanical deburring and polishing are accomplished using a wide range of methods. Dry blasting with abrasive media can use glass beads, steel shot, nut shells, polymer beads, and water-soluble materials. Impaction with carbon dioxide pellets may also be considered a dry deburring method. In slurry deburring, water or other liquids replace gas. Unfiltered high-pressure water spray also can be an effective deburring and polishing process. Tumbling of parts in metal or ceramic beads is especially popular. In most cases, mechanical deburring and polishing processes do not alter the chemical composition of the surface being treated. However, there are notable exceptions. Steel shot can become fragmented and be embedded in softer alloys. Subsequently, severe galvanic corrosion problems can occur. The fragmentation of the steel shot can result in magnetization of the shot fragments, leading to surfaces that are contaminated with embedded magnetic contamination. Hard, brittle media such as glass or ceramics can become fragmented and be embedded in softer surfaces. For this reason, deburring is often performed using polymeric or nut shell–based media to avoid subsequent galvanic corrosion problems. Other reasons exist for blast cleaning a surface. The surface may have become fouled with contamination that requires removal prior to further treatment. Rust and marine contaminants such as barnacles and oxidation are good examples of materials that need to be removed. Sanding is an option for flat surfaces. Unfortunately, few surfaces are flat enough for sanding to be a suitable surface treatment process. In this case, air blasting using abrasive or abrasive slurry cleaning is often chosen. A wide range of abrasives ranging in Moh hardness from 9 (natural and synthetic aluminum oxides) down to approximately 3 (sodium bicarbonate) [4] are commercially available. Other then unintentional alteration of the chemistry of surfaces mentioned above, mechanical deburring and polishing rarely result in changes to the chemical composition of the surface. For this reason, subsequent chemical treatments may be necessary to impart desired surface properties such as corrosion resistance and surface adhesion. Many sources of standards are available for specifying surface treatments. These include ASTM, Japanese, European, and military and industrial standards, including standards from materials suppliers and surface treatment applicators. In many cases, these standards must be customized to achieve optimum performance of the surface treatment for the intended application. Anodization for aluminum is available in a number of forms. Anodization from chromic acid solutions usually results in clear to dark gray coatings, depending on the alloy being anodized. Thickness from 0.5 to nearly 10 m can be achieved. However, chromic acid treatment is not recommended for aluminum alloys containing greater than 5% copper or 7% silicon or total alloying elements greater than 7%. In general, anodization from chromic acid shows poor wear resistance and should not be considered for use as a final surface treatment. The surface resistivity of the coating ranges from static dissipative for

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thin coats to insulative for thick coatings. Anodization for aluminum may also be accomplished in sulfuric acid baths. These coatings tend to be thicker than chromate bath coatings and can be as thick as 25 m. They are available in a wide range of colors. Anodization from sulfuric acid bathes are hard, wear resistant, and corrosion resistant; hence, they are suitable as final surface finishes. The coating generally is static dissipative when less than 10 m thick. Thicker coatings tend to be insulative. The coating consists of approximately 50% penetration and 50% buildup. Chemical conversion coatings such as Alodyne or Iridite are used on aluminum or magnesium to impart corrosion resistance and to prime for subsequent painting. In general, the coatings are of low resistance and suitable for electrical contacts. Chrome electroplating is applied to most metals as a decorative surface treatment or as a hard, wear-resistant treatment. Decorative chrome is generally plated over a base coat of nickel or copper. It ranges in thickness from 0.25 to about 1 m, exclusive of the base coat thickness. It is conductive. Hard chrome is generally thicker and is applied directly over the base metal. Final dimensions with hard chrome plate are usually achieved by grinding and polishing. Copper may be electroplated or electroless plated over most metals. It is used primarily as an undercoat for other metallic surface treatments, so its lack of corrosion resistance is seldom a contamination concern. Copper may also be used as a decorative finish, in which case it is coated with transparent polymers to impart corrosion protection. Some plating shops assume when copper has been specified as a final surface that the customer has forgotten to request the polymer topcoat and apply it without being requested to do so. It is important to state clearly that bare copper, with no subsequent protective coating, is required. Case Study: Surface Treatment Some disk drive manufacturers use self-adhesive copper foil in their disk drives as an acid gas getter. One disk drive manufacturer transferred one of its manufacturing lines to a relatively inexperienced overseas subsidiary. The overseas manufacturer tried to source as many of the components with local suppliers as possible. Their local supplier for the adhesive-backed copper foil getter assumed that the manufacturer had forgotten to call for a polymer overcoat on the copper strips. (As implausible as this may sound, it really happened.) The overseas manufacturer did not inspect the parts and did not subject them to near-contact stain tests, which they should have in any case, because the copper strips are coated with pressure-sensitive adhesive, a material known to promote corrosion in disk drives. Needless to say, the disk drives began failing in unexpectedly large numbers. A great deal of unnecessary failure analysis was done before someone noticed that the copper strips were not protecting the drives. Magnesium is a particularly corrosion-susceptible material and must always be used with surface treatments and coatings. The most common surface treatment is dichromate (Dow No. 7). This is the most corrosion protective of all chemical coatings for magnesium and provides a good base for painting. An alternative is to anodize (Dow No. 17). This provides a good paint base and excellent corrosion protection. The coating thickness is approximately 40% base penetration and 60% buildup, so allowances must be made for final dimensioning. Magnesium is generally used as castings that are not pore free. It is difficult to plate the interior of pores, so residual corrosion and contamination problems can occur. Electroless nickel may be applied over virtually any surface. It provides 100% uniform plating on all accessible internal and external surfaces. Good corrosion and wear resistance make it a nearly ideal coating for contamination-controlled applications. It is also conductive.

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However, when applied over softer base materials such as aluminum, cracks can form where parts are joined and fastened. These tiny cracks become a pathway through which galvanic corrosion can occur if exposed to moisture and can also be a source of particle shedding. One solution to minimize crack formation where parts are joined is to increase the loadbearing surface area so that the compressive strength of the underlying base metal is not exceeded. Electroplated nickel may be applied to almost any metal. Corrosion resistance is proportional to coating thickness. Thickness is typically from 5 to about 50 m. The coating is conductive and slightly magnetic. Passivation is a process used to improve corrosion resistance and surface cleanliness of steels, and sometimes stainless steels. Passivation can be accomplished in nitric acid solutions or citric acid solutions. Care must be taken in rinsing, especially in citrate-processed passivation. Passivation works by dissolving away oxidation-sensitive materials, including the base metal iron. The resulting surface is enriched in nickel oxide and/or chrome oxide, imparting high corrosion and wear resistance. Passivation over 400 series stainless steel reduces surface hardness and thus may have a detrimental wear effect. Gold plating or, commonly, gold over palladium is used for copper electrical contacts. It imparts excellent corrosion resistance. Plating is usually performed in a drum rather than on the stamping trees so that the surface is 100% covered. Corrosion resistance is proportional to coating thickness. Painting or conformal coatings are applied as decorative finishes and corrosion-resistant coatings. The range of materials available includes alkyd, acrylic, epoxy, and polyurethane enamels and silicone base coatings. Conformal coatings are applied over corrosion-sensitive electronic assemblies such as printed circuit boards. Their primary drawback is difficulty in rework if individual electronic components must be replaced. Masking is required to ensure that electrical contact surfaces do not get coated, with significant impact to cost. For this reason, conformal coating is seldom seen in consumer electronic applications and is found most commonly in military and aerospace. In chemical conversion and plating processes, material is added to the surface being treated. By contrast, in electropolishing, material is removed. This is accomplished by a combination of chemistry and electric current. Electropolishing tends to work fastest on peaks and slowest on valleys. Thus, material removal is fastest on the peaks, sharp points, and burrs, resulting in smooth surface finishes. Many foreign materials, such as polishing debris and machining debris, are released as a result of the electropolishing process, leaving more uniform surface composition. The result is a smooth, burr-free, and relatively corrosionresistant surface. Material removal can be made as little as a fraction of a micrometer to as much as desired by careful control of solution chemistry and electrical current. Electropolishing is effective on a variety of metals, including aluminum, beryllium, brass, molybdenum, nickel, and steel and stainless steel. “Bulk” or “barrel” electropolishing may be used for small parts (such as threaded fasteners) where positioning of parts using precision fixtures of individual parts is not economical or not feasible. In such cases, chemical polishing may be used as an alternative. As with many other processes, special precautions may be needed to apply electropolishing safely for contamination control applications. Ordinary industrial standards seldom include the need for measurement of ionic residues after electropolishing and cleaning of the parts. Electropolishing residues left within parts due to inadequate rinsing can result in serious contamination problems. Many of the residues remaining on stainless steel parts after electropolishing form a friable, crystalline contamination deposit. In addition, some of the contaminants produced in the electropolishing of stainless steel are deliquescent. Exposure of the residues to varying

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ambient relative humidity creates puddles of dissolved deposits that can wet the surface of the part and migrate to other locations. Thus, exposing a part that has been rinsed inadequately will eventually cause electropolishing residues to migrate to the surface of the part from within to locations where the friable contaminants can be disturbed and spread in the cleanroom. Hence, there exists a need for customization of the specification to meet the needs of precision clean parts. Similar concerns would apply to chemical polishing as well. Synergistic Coatings Synergistic coatings utilize more than one type of material in providing a surface treatment. The materials are chosen and combined in ways that take advantage of complementary properties to produce a surface that cannot be achieved with any single surface treatment. Synergistic coatings were originally developed for the aerospace industry and found some of their earliest applications in spacecraft. This is true because spacecraft operate in environments where conventional lubricants are unsuitable: high vacuum, singlet oxygen, with extremes of radiation and temperature exposures. Synergistic coatings have been developed for all of the metal alloys commonly found in piece parts and tooling parts, including steel, aluminum, copper, and magnesium. Many of these surface treatments impart wear resistance and chemical inertness to metals that cannot be achieved by any comparable method. Sputter Etching and Sputter Coating Sputter etching and sputter coating have also emerged as important methods to modify surface properties of materials. Sputter etching can be useful as an in situ surface preparation technique for subsequent chemical surface treatments, often in the same chamber or in a different chamber within a cluster tool. Sputter coating can be used to apply coatings of virtually any type of material, including metals and ceramics. Chemical Vapor–Deposited Coatings One interesting chemical vapor coating process is ion vapor deposition. In this process, metal or nonmetal vapors are generated at high vacuum and electrically charged to produce an ionized vapor. The parts to be coated are charged to the opposite polarity and the ionized vapor is attracted to the oppositely charged surface. This process is dependent on the cleanliness of the surface to be coated. Surfaces are generally chemically cleaned and then loaded into the processing chamber, where they are plasma cleaned prior to coating. Coating is uniform (nondirectional). The coating is conductive and not outgassing. Even the evolution of surface treatment specifications has potential pitfalls. One large disk drive manufacturer had a specification for electrophoretic painting (e-painting) cast aluminum parts. The need to eliminate organotin compounds from the paint formulas led to the revision of the e-paint specifications. In this revision the rejection and rework provisions for pinholes and scratches in the older version of the specification were inadvertently dropped. The e-paint supplier now had no alternative but to strip e-paint off the parts using hot sulfuric acid because touching up scratches and pinholes was no longer permissible. This led to enormous sulfate contamination problems. Compatibility with Water Cleaning Concerns about ozone depletion have put halogenated hydrocarbon cleaning chemicals in disfavor. Further, concerns about global warming have created concerns about the use of organic solvents in general. Fortunately, studies have shown that the vast majority of materials can be cleaned effectively using water-based

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(aqueous) cleaning processes. In some cases, materials may need to be modified to make them compatible with aqueous cleaning. For example, a magnesium part that had previously been left uncoated when washed with organic solvents may need a surface treatment to prevent it from reacting with water. Chromate and paint are two good options for magnesium parts. Many advances have occurred in the development of water-soluble solder flux. Three options are currently available: no clean flux, water-soluble flux, and solvent cleaning using acceptable alternatives to CFCs. The development of low- and medium-residue fluxes has enabled the option of not cleaning where reliability is not a concern. This simplifies the processing of printed circuit boards by the elimination of process steps. Where high reliability is required, no-clean fluxes may not be acceptable. In this case, cleaning using water with water-soluble fluxes is an option. Water-based cleaning often is performed using semiaqueous chemistries. A good example is mixtures of water and terpenes or limonenes. Finally, environmentally friendly solvents have been developed, including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and hydrofluoroethers (HFEs). The drawback to these solvent alternatives is the high cost of the materials [5]. Elimination of Blind Holes/Through-Holes There are several considerations in the design of parts that have holes. There are two types of holes: blind holes and through-holes. Blind holes have the advantage that they trap assembly debris when filled with a fastener. However, this advantage can be lost if the part requires removal of the fastener for rework. Blind holes also have the advantage that they do not allow contamination to migrate into the assembly. Cleaning of blind holes can be difficult. Depending on the orientation of the hole, they may trap air, preventing cleaning fluid from reaching the contamination in the blind hole. Conversely, a blind hole may fill with the cleaning fluid but then not drain, so that a portion of the contamination dislodged during the cleaning process may remain in the hole during the drying process, reducing cleaning effectiveness. These two problems are illustrated in Figure 5.10. Through-holes are easier to clean and dry. However, through-holes do not trap assembly debris generated during assembly. In addition, through-holes can be a leakage path. Two common alternatives have been tried, using a thread locking agent, and sealing the opening

Particle removed by diffusion and acoustic streaming

Cavitation

No air pocket, but particle does not drain out of blind hole

Air pocket: no cavitation

FIGURE 5.10

Problem with cleaning blind holes in immersion cleaners.

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of the through-hole using tape seals. Both alternatives work. However, both can introduce additional potential sources of chemical contamination, both require the addition of an assembly step, and both can introduce adhesive removal problems for rework. Clearly, with these competing advantages and disadvantages, compromise is needed between parts cleaning and assembly needs. For example, a threaded screw hole can be more difficult to clean than an unthreaded hole. An unthreaded hole may require the use of press-fit, snap-in, or riveted assembly processes. Screws can be difficult to clean, so their elimination can simplify the overall cleaning process. Conversely, holes may require the use of slip agents to facilitate assembly. Low-molecular-weight alcohols have been effective slip agents in several assembly processes and offer the advantage that they evaporate quickly and thus minimize contamination problems. Other alternatives that have been used successfully include lubricating the hole or fastener with a trace of the detergent used in the cleaning process or using an inert lubricant, such as a perfluorinated polyether. No Outgassing/No Corrosive Vapors As mentioned above, the need to seal assemblies can introduce thread sealants and pressure-sensitive adhesive-coated tapes. These must be selected carefully to avoid introducing chemical contamination into the finished product. In the 1970s and early 1980s the primary contamination concern was particles. Then starting in the 1980s and continuing to today, outgassing has become a dominant consideration. The outgassed chemicals can be detrimental to the product and process. Outgassed chemicals can be in the form of acidic or basic vapors. These can promote corrosion reactions. Organic vapors can also cause problems. Adsorbed films of organic vapors can interfere with adhesion of surface coatings or can prevent chemical bonding. Organic vapors can polymerize. For example, adsorbed organic vapors on spacecraft mechanical components can later vaporize to condense on the cold surface of optical windows. There, the intense ultraviolet radiation of outer space can cause them to polymerize, clouding the optics. Effective Sealing from External Contamination The sealing of the product must be considered during the design process. The difficulty of this design problem is definitely product specific. Many electronic devices are hermetically sealed or conformally coated, effectively protecting them from environmental contamination. Other products, such as disk drives, cannot be hermetically sealed, and indeed, must be allowed to breathe. To allow a disk drive to breathe, filters are installed over the breather holes that contain particle and chemical filters. The chemical filters typically contain both activated charcoal to absorb organic vapors and buffers (such as mixtures of sodium carbonate and sodium bicarbonate) to absorb inorganic vapors. Some products, such as disk drives, do not function well if the air inside the drive gets too dry. These products often are designed to contain humectants, which keep the relative humidity inside the disk drive from becoming too dry at the elevated temperature at which the disk drive operates. Ease of Drying When halogenated solvents were the dominant cleaning media, drying was not a problem, due to their low boiling point and heat of vaporization. Replacement of halogenated solvents with water has definitely complicated the drying process since water boils at a higher temperature and requires more energy for evaporation. Failure to eliminate water in drying of semiconductors prior to packaging is a serious cause of reliability loss [6].

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Several guidelines for the designer are useful to aid in drying of parts. Among these are the following: ● ● ●

●

The design should eliminate or minimize the use of blind holes. The design should prevent accumulation of pools of liquid on surfaces. The part should be designed for optimization of orientation in the cleaning/drying basket. The designer should work closely with process engineers.

Spin-rinse drying should always be considered for rotationally symmetric parts. Even parts that are not exactly circular can often be cleaned using the spin-rinse drying technique, because the rotor for spinning the part can be designed to compensate for imbalances. Spin-rinse drying is especially attractive because heat is generally not needed to dry the parts. The centrifugal force generated by high-speed rotation mechanically removes the cleaning fluids from the parts. In some cases, filtered dry nitrogen or air is introduced into the chamber to assist in the drying. Introduction of filtered dry nitrogen or air into any type of drying chamber can introduce electrostatic charge problems, since the filters are very efficient at removing ions, and the parts may have become charged in the cleaning and rinsing process. Several methods have been developed to compensate for this and to reduce static electricity on the parts being dried. One technique in vogue in the 1980s was to add some carbon dioxide to nitrogen to increase conductivity of the gas. The use of carbon dioxide for this has the drawback that it can increase the chemical reactions that occur in the drying chamber. Chemical inertness of the drying fluid is also a consideration in the choice between nitrogen and air: The presence of oxygen may not be desirable for some materials. A second technique is to ionize the air directly, through the use of either radioactive or corona discharge air ionizers. The selection of air ionizers was discussed in detail in Chapter 2. Ease of Packaging Packaging is used in many ways. Considerations in the selection of the packaging must include the part that is to be packaged as well as the process that the part will go through after it is received by the customer. A few examples: ●

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Direct-line-feed parts will not be cleaned by the customer. Responsibility for achieving the final part cleanliness rests with the supplier. The packaging must preserve this final cleanliness. Parts to be cleaned by the customer may require packaging to protect them from the elements and from damage during shipping and handling. In-process packaging to handle production introduces new concerns about the ability to clean the package between reuse cycles. Parts that will not be cleaned in-house require an entirely different packaging strategy from parts that will be cleaned in-house. Parts that will not be cleaned in-house are sometimes referred to as direct-line-feed parts. These are generally double-bagged for cleanroom applications. The procedures by which direct-line-feed parts are introduced into the process area also can be complex. Parts to be cleaned in-house generally do not need to be shipped in special cleanroom packaging (such as double bagging), but the packaging should be designed to protect the parts from damage in transit, preserving the cleanliness achieved at the supplier.

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The design of the parts must consider the need for packaging. For example, custommolded containers can be vacuum formed from relatively inexpensive materials that are suitable for a single-use application. Design for ease of packaging is also an important consideration within a process. Intermediate packaging is needed to facilitate product flow from process step to process step. Occasionally, this in-process packaging serves as cleaner basket inserts, further complicating the design. One recommendation for in-process packaging is to consider the aspect of reusability. Packaging that can be reused many times during the life of a project can result in significant savings to the project. A good example is the use of wafer boats in both semiconductor and disk manufacturing applications, a very well accepted practice. These packages are so thoroughly integrated into their manufacturing processes that they travel through the process from the beginning to the end. Thus, we can simplify this discussion. Some packages will be used once and discarded. Here the only considerations are that the packaging meets the initial cleanliness requirement and does not unduly contaminate the product being shipped. This is the most common consideration for parts to be cleaned in-house. In some cases where the supplier is close to the customer, it makes better sense to ship in reusable shipping containers. Here in addition to the initial cleanliness concern, the ability of the process to restore the material to its cleanliness over the life of the project is a consideration. Probably the most demanding packaging problem for parts is for those to be used as-is, without any in-house cleaning by the customer. These are typically parts such as motors or other assemblies containing lubricated components that might be damaged by cleaning. Good examples include lubricated disks for data storage applications or coated lenses or reflectors for aerospace applications. These parts must be packaged ready to enter the contamination- or ESD-controlled work area. Quite often, direct-line-feed parts for contamination-controlled applications must be double-bagged for direct entry into the contamination-controlled workplace. In the packingout portion of a double-bagging operation, contamination-sensitive parts are enclosed within an inner container meeting all cleanliness requirements. This may be a tape-sealed box, vacuum-formed container, or heat-sealed bag. While still in the cleanroom packaging station, this will then be placed into a second container, either alone or with several other sealed inner bags, and the outer container will also be sealed. The packing-out portion of the process is fairly straightforward and most users have little difficulty implementing it correctly. Where users have difficulty is in the receiving end of the depackaging process for direct-line-feed parts. Figure 5.11 illustrates a depackaging area for direct-line-feed parts. To understand how this workstation is used, it is important to have an understanding of the cleanliness condition of the various surfaces in the package. The outer surface of the outer bag is dirty, because it has been handled outside the clean environment. The inner surface of the outer bag is still clean, because it was sealed inside the clean environment. The outer and inner surfaces of the inner bag (which may be a box) are clean because the inner bag has only been handled within the clean environment. The object is to figure out how to remove the outer bag without contaminating its clean contents. The workstation shown in Figure 5.11 provides a way to accomplish this task. Here we illustrate a workstation in a cleanroom next to a double-door pass-through. The doublebagged parts can be placed in the pass-through and loose contamination can be blown off using ionized, filtered compressed air. The outer door of the pass-through is closed and

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Inner package operator (never touches outer bag)

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Outer package operator (never touches inner bag)

Pass-through Clean-Side Operation

FIGURE 5.11

Dirty-Side Operation

Depackaging station for double-bagged parts.

after a brief purge cycle, which allows for the blown-off contamination to be swept away, the inner door of the pass-through can be opened. Keep in mind that the outer surface of the outer bag is still not clean: The blow-off removes only gross contamination such as paper particles. Thus, care must be taken at this point to prevent the contamination on the outer surface of the outer bag from contaminating other surfaces. One way to manage this is to divide the workstation into a clean half and a dirty half. The parts, still in their dirty outer bag, are placed on the dirty side of the depackaging workstation. At this point, some choose to wipe the exterior of the dirty outer bag. The outer bag is cut open, often using a scalpel or a pair of surgical scissors. The bag is spread open without touching its contents. What happens next, getting the clean inner bag out of the dirty outer bag, can be done in several ways. If a single operator performs the depackaging operation, he or she should wash, wipe, or change his or her gloves. The operator can then reach into the opening of the outer bag and remove the clean inner bag. The clean parts are moved to the clean side of the depackaging station. Alternatively, if a pair of operators work together, a second person, who has never touched the dirty outer surface of the outer bag, reaches into the outer bag and removes the clean inner bag. Again, the clean inner bag is placed only on the clean side of the depackaging station. Early Supplier/Purchasing Involvement Modern manufacturing methods have introduced considerations that also affect design. Many manufacturers have begun to focus on a limited supplier base. This provides a benefit, since a long-term relationship with a supplier allows for technology transfer that benefits both the customer and the supplier. Conversely, both suppliers and customers can become too comfortable about these long-term relationships. This can result in suppliers who become unresponsive and customers who become undemanding. Contamination and ESD issues especially can suffer under this strategy. Contamination and ESD control requirements are dynamic. They tend to evolve to ever-moredemanding specifications. Suppliers and customers who become unnecessarily comfortable with their relationship can quickly fall behind the state of the art. Of course, this sharing of expertise is a two-way street. Many suppliers try to stay on the cutting edge of technology. Thus, it is important for customers to listen to their suppliers, especially where the supplier may have established a technology alliance with one of their

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secondary suppliers, who may be completely unknown to the customer. In some cases, a supplier may be so experienced and specialized in their area that they themselves own and use proprietary processes that are unknown to the customer. There is, of course, a danger in relying on proprietary processes without adequate documentation. The dilemma is that the supplier may have a genuine and justifiable need to maintain secrecy. On the other hand, the customer needs to be able to rely on the quality of the products produced by these proprietary processes. This is one area where a systematic approach to contamination and ESD control can be especially beneficial. It is not necessary to know the process by which a part is produced. However, it is reasonable to expect to see objective data describing the important performance statistics, such as ionic contamination burden, particle contamination burden, and nonvolatile residue on the products, measured using agreed-upon analysis methods. 5.4.5

Design-for-Cleanability Management Considerations

The organization under which a design for cleanability strategy is implemented is a critical factor in assuring success. Management must lose their territorial nature. Success in design is not diminished by cooperation. All too often, the sharing of credit is viewed as diminishing prestige. Management must be willing to accept that the cleaning process engineer, materials engineer, and packaging engineer can provide valuable contributions to ensure the success of a project. Among the elements of a project important to success with respect to contamination or ESD control are the following: ●

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5.5

Management must be willing to accept feedback of cleaning problems to the part/ assembly designer. Management must be sensitive to the correlation of contamination or ESD-related yield and reliability failure rate to the extent of DFC enforcement. There must be close interaction between design and CC/ESD control engineering functions. One approach that has been implemented with success is to assign responsibility within the design function to a person acting as a contamination control or ESD control coordinator. CC and ESD control function must have approval authority in the form of sign-offs on drawings prior to their release. Creation of a mindset in the design and development engineering community that cleanability is an important design objective.

PROCESS DESIGN GUIDELINES

Equally important to the design of materials is the design of processes. Indeed, the selection of materials and of processes by which they are produced are intimately interconnected. Among the considerations are the following: ● ● ●

Use water-soluble cutting fluids. Minimize work in progress and implement continuous-flow manufacturing. Implement rinsing between successive machining steps to reduce the concentration of cutting fluids on surfaces.

PROCESS DESIGN GUIDELINES ● ● ● ●

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Install appropriate aqueous cleaners and drying systems. Carefully consider parts handling after final cleaning. Use appropriate clean or ESD safe packaging. Use water-soluble solder pastes and fluxes.

5.5.1

Use of Water-Soluble Cutting Fluids

Cutting fluids are essential. However, traditional hydrocarbon oil–based cutting fluids can introduce cleaning problems. First and foremost, they are not easily removed by waterbased cleaning processes. In general, the only cost-effective way to remove them is through the use of organic solvents. Unfortunately, the cleaning process can then expose the worker population to chemicals that are considered to be hazardous to health. Solvent cleaners often have elaborate mechanisms, such as multiple condenser coils, high sidewalls (called freeboards), or loading–unloading chambers to minimize exposure of workers to the cleaning chemicals. Water-soluble cutting fluids are an increasingly attractive alternative to hydrocarbonbased cutting fluids. Water-soluble cutting fluids often are oil-in-water emulsions stabilized by surfactants and fortified using other chemicals, such as antioxidants. This formulation can provide several functional advantages. The surface wetting of the cutting fluid can often be optimized. This can improve lubrication and cutting rate for the process. In addition, the higher heat capacity and heat of vaporization of the water in the formulation can provide cutting-rate advantages over pure hydrocarbon cutting fluids. Of course, there can also be disadvantages. There may be a limit to the lubricity of the emulsion compared with a hydrocarbon-based cutting fluid. The presence of water in the formulation may lead to adverse chemical reactions, even when additives such as antioxidants are used. The high volatility of the water phase may lead to significant evaporation loss, changing the properties of the coolant and decreasing its usable life. Regardless of these potential problems, substitution of water-soluble cutting fluids should always be considered when establishing a new process. Specifically, the benefits that drive the choice of water-based cutting fluids include the following: ●

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They rinse off easily in the wet state. Even in the dry state, they typically do not form hard-to-remove residues, such as lacquers, that can form from dried-on hydrocarbonbased cutting fluids. The detergent can often be chosen primarily based on cost, ease of removal during the rinsing step, and ease of disposability. In fact, with many water-soluble cutting fluids, the stabilizing emulsifying agent acts as a detergent in subsequent cleaning and rinsing processes. Moving to water-soluble cutting fluids eliminates the need for solvent dips. This can be an important consideration where multiple-step machining processes require the use of immersion rinse tanks between multiple processing steps to prevent the parts from drying out.

The two biggest challenges to implementation of water-based cutting fluids and their water-based cleaning are how to handle reactive metals and how to achieve the lubricity of heavy lubricants such as mineral oils.

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5.5.2 Minimizing Work in Progress by Implementing Continuous-Flow Manufacturing One of the key factors in optimizing the cleanliness of parts is to minimize the time during which the parts are in contact with cutting fluids, mold release compounds, and other contaminants. One strategy to achieve this end is to implement a “pull” versus a “push” system. There are a number of considerations favoring implementation of a continuous-flow manufacturing approach to minimize work in progress. The adhesive force between contaminants and parts surfaces increases exponentially with time. Thus, a process that minimizes the time that a part is left with contamination on its surface facilitates easier cleaning and improved part cleanliness. Another advantage to using a manufacturing process that minimizes work in progress is that it is easier to locate parts of various vintages if they are not all jumbled together in the process. This becomes a consideration where there have been significant process improvements at suppliers. Older vintage parts not having these process improvements are easier to identify if they are still on the shelf in their original packaging. The older parts may be candidates for rework by the supplier. 5.5.3

Rinsing After Machining

One way to minimize the time that contamination remains on the surface of parts is to implement rinsing after each step in the machining process. Two types of rinse processes are used: static rinse and spray rinse. In most machining operations, the parts are rinsed off while still on the machine, using a spray of the cutting fluid. The parts are then removed from the machine tool and kept wet between machining steps in either a spray rinse tank or in an immersion rinse tank to minimize drying. It is important to monitor rinse tank liquid quality and change it periodically on some rational basis. With a static immersion tank, monitoring the rinse liquid quality can be as simple as checking the color of the liquid in the tank. The closer the color of the rinse liquid in the tank to the color of the cutting fluid, the higher is the concentration of cutting fluid in the rinse tank. This may adversely affect cleanliness of the parts at the end of the final cleaning process. If water-soluble coolants are used, intermediate-rinse cleaners can be water based as well. 5.5.4

Parts Handling After Final Cleaning

After final cleaning, several factors should be considered to preserve the cleanliness achieved in the final cleaner. The level of precautions that need to be taken must reflect how the parts will be used by the customer. For example, if direct-line-feed, parts are being used, they should be handled in a cleanroom with the same classification as the customer’s cleanroom. In addition, personnel handling the parts should use the same quality of gloves, cleanroom garments, and so on, as those used by the customer. Conversely, if the parts are to be cleaned by the customer, an equivalent cleanroom may be unnecessary, but precautions to preserve the cleanliness of the parts still need to be observed: ● ●

●

Handle the parts with gloves, never with bare hands. Monitor and control the environment so that it is no dirtier than the final drying chamber in the cleaner. Cover parts during storage and transport.

PROCESS DESIGN GUIDELINES ●

219

Packaging materials should meet cleanliness requirements. Care should be taken in handling the packaging material to preserve its cleanliness.

5.5.5

Soldering and Flux Removal

The decision to clean after soldering depends on the type of solder and flux used and on the reliability of the circuit to be produced. No-clean fluxes have relatively small amounts of contaminants that induce corrosion. If these are used on products that do not require very high reliability, the decision may be made not to clean after soldering. Conversely, corrosion problems may be too severe for high-reliability electronics, and cleaning after soldering will be required [7]. The need to clean after soldering traditionally was a driving force in the use of organic solvents to remove flux residues. The ban on CFCs has sped the development of solders with no-residue flux, no-clean flux, or water-clean flux. The selection of a solder flux system and the subsequent cleaning process depends on the amount of solder flux residue that can be tolerated on the part. For some customers, such as the military or NASA, solder specifications may limit the choice of flux system or post-solder cleaning. Printed circuit boards are often cleaned by immersion in a bath containing 75% DI water and 25% isopropyl alcohol. This is effective at removal of ionic contamination associated with solder residues. The problem may become more complex in the future as lead-free solders are phased in. 5.5.6

Clean–Then Assemble vs. Assemble–Then Clean

Processes normally are planned to follow the sequence of cleaning all the parts and then assembling them, very often in a cleanroom. This has the potential to allow a significant amount of recontamination of the assembled product, due to accumulation of assembly debris, handing debris, and general in-process contamination. In addition, other factors must be considered in the use of the clean–then assemble strategy. Among these are the following: ● ● ●

●

●

The capital cost premium of the cleanroom facility over a factory workplace The operating cost premium of a cleanroom over a factory workplace The productivity loss of workers imposed by the cleanroom change and special cleaning procedures The increased number of parts baskets and cleaning inserts needed to clean each component separately versus cleaning several parts as a unit after assembly The increased contamination monitoring required to maintain certification

The latter point about the increased cost to monitor and certify a cleanroom over an ordinary factory environment is worth further consideration. Cleanrooms are monitored for a wide variety of parameters and can include air velocity, temperature and direction, airborne particle concentrations, airborne molecular contamination, witness plate monitoring, audits, white glove inspections, and tape tests, among others. In addition, the cleanliness of individual piece parts going into the cleanroom is usually under some level of inspection and control. However, none of the measurements addresses the primary concern: the cleanliness of the finished assembly. Thus, it is worthwhile to consider the feasibility and possible

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

advantages of adopting a policy of assemble–then clean for one or more subassemblies entering the cleanroom. Several methods may be used to evaluate the effect on cleanliness of the two alternative strategies. Obviously, if an existing parts cleanliness measurement method is available for the individual components, it can be applied to finished assemblies as well. In addition, contamination generation by finished assemblies due to heating, vibration, airflow, or other stimulus that the part may experience in normal use could be used as an evaluation method. In each case, the more like the actual conditions of use, the more meaningful the results.

5.6

CLEANING PROCESSES

The key to the development and implementation of effective cleaning processes is the ability to assess quantitatively the cleanliness of the product produced by the cleaning process. The acceptance criteria for a cleaning process can be based on a number of criteria, choosing among approaches to specifying the cleanliness level and among the various classes of contaminants to control. Cleanliness can be specified based on any of the following approaches: ● ● ●

● ●

Material-dependent generic values Benchmarks using comparable competitive parts The cleanability characteristics of the part, as determined by multiple extraction measurements Receiving inspection data for lot certification Vendor process capability (source inspection)

In many instances, several of these approaches are employed sequentially. Early in a development program, no parts are available, as they exist only in the form of drawings. It is desirable for the prototype parts to be made by candidate suppliers with the understanding that the final production parts must confirm to a quantitative cleanliness requirement. Thus, the first iteration of drawings could include a cleanliness callout based on knowledge of the material of the surfaces, the surface area, and to some extent, the fabrication methods. In the absence of experience with a particular material type, it is also possible to estimate the desired cleanliness level based on analysis of competitive parts, a process usually referred to as benchmarking. After initial prototype parts are received, they can be studied for their cleanability characteristics by the use of multiple extraction. At that point an analytical basis for a cleanliness-level callout can be used for the next iteration of the part drawing. During early production, parts can then be sampled as received to develop statistical process control data. Simultaneously, source inspection data can be collected at the supplier’s location. These two data sets can then be used to establish cleanliness acceptance criteria that are based on process capability. The cleanliness specification can also be based on one or more of the following: ● ● ● ● ●

Particles Ionic contamination Organic contamination Viable contamination Magnetic contamination

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Within each of these categories of contamination, several methods of measuring may be chosen. These methods vary from relatively simple techniques that can be utilized easily by any supplier (e.g., densitometry, Ionograph) to complex methods that only the most sophisticated supplier has available in-house (e.g., electron microscopy, TOF-SIMS). A more complete discussion of the analysis options is available in Chapter 3. 5.6.1

Particles in Liquid Baths

Particles can be removed from surfaces or deposited on surfaces in liquid baths, including cleaning baths. The mechanism controlling particle adhesion in fluid baths is influenced by the particle–surface–liquid chemistry of the system in addition to particle transport mechanisms. That is, liquid flow may direct particles toward surfaces, but adhesion of the particle to the surface depends on the chemistry of the system. Specifically, particles in liquids take on a charge depending on the material the particles are made of. For example, particles from a broken silicon oxide–coated wafer will take on a negative charge. This will prevent particle deposition on surfaces that carry a similar negative charge, such as an intact silicon oxide–coated wafer. By contrast, particles from a silicon nitride–coated wafer will take on a positive charge in DI water. These particles will be attracted to the negatively charged silicon oxide wafer [8]. This phenomenon of particle charging in liquid baths can be measured. The measured charge is called the zeta potential. When a particle is immersed in a liquid such as water, which is rich in both positive and negative ions, the atoms on the surface of the particle tend to attract either positive or negative charge to their surface. In turn, this cloud of charge produces a second cloud in the fluid charged to the opposite polarity. The resulting charge cloud is often referred to as a charge double layer. The zeta potential of positive or negative charge surrounding a particle can be measured by placing particles in an external electrical field and noting the direction and speed of their migration. The resulting double layer of charge has some unusual effects. Materials with similar charge will repel each other. Thus, an oxide-coated silicon wafer would not be expected to be contaminated by oxidecoated particles in a process bath, due to mutual repulsion [9]. 5.6.2

Boundary Layers

One of the limitations for cleaning processes is the presence of a fluid boundary layer. The boundary layer is a thin film of stagnant fluid attached to a surface to be cleaned. Fluid flow within the boundary layer is essentially zero at the surface and increases to the free stream velocity at the edge of the free stream flow. The fluid flow within the boundary layer increases gradually as the free stream is approached. Contaminants hidden in the boundary layer are shielded from the cleaning process. The thickness of the boundary layer in immersion cleaners varies depending on the degree of fluid agitation or, in the case of ultrasonic cleaners, the frequency of oscillation. The faster the fluid moves over the surface, the thinner the boundary layer. In ultrasonic cleaning, the higher the frequency, the thinner is the boundary layer [10]. Boundary layer thickness is also important in spray cleaning. The higher the fluid velocity in the spray, the thinner is the boundary layer. 5.6.3

Ultrasonic Cleaning

One of the most popular cleaning processes, ultrasonic cleaning, can be very effective, but a number of performance features of ultrasonic cleaning need consideration. The tendency

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

of the ultrasonic energy to damage parts is a consideration in the selection of frequency and power density in the cleaning tank and can influence equipment and process design. Low-frequency ultrasonic cleaning (less than about 200 kHz) is relatively unidirectional. As frequency increases above 200 kHz, ultrasonic cleaners tend to be come more directional. Very low frequency ultrasonic cleaning (less than 30 kHz) can produce subharmonics that can be heard by workers and can be a source of irritation. Very low frequency ultrasonic cleaners can result in severe damage to surfaces. Ultrasonic cleaners with frequencies in the frequency range 30 to 70 kHz tend to produce less mechanical damage to parts and usually operate quieter than do low-frequency ultrasonic cleaners. The susceptibility of materials to ultrasonic damage is complex. For example, nonporous metals with high surface hardness (e.g., T6 hardness in aluminum) are not often damaged by ultrasonic cleaning below 200 kHz. Conversely, porous cast aluminum can easily be damaged by 68 kHz and lower frequencies. Mechanical damage to delicate structures also is well documented. Good examples include breakage of wire bonds, delamination of adhesive bonds, and changes to shapes of fine metal parts. Some strategies have been implemented to try to minimize the damage from ultrasonic cleaners. The theory behind these is that the damage occurs because of standing waves in the ultrasonic tank that stay in fixed locations with respect to the parts being cleaned. One technique is to vary the frequency of the ultrasonic energy. The frequency cannot be varied far from the center of resonance or the power level in the ultrasonic tank drops markedly. For example, in sweep frequency ultrasonic energy operated at a resonant frequency of 47 kHz, the sweep is less than 2 kHz. In a second technique, undulation, the parts are moved slowly in the tank during the cleaning process. Undulation is often used together with sweep frequency to reduce damage. One final consideration is the damage that occurs when parts are drawn through the fluid–air interface. The energy density at the interface is higher than within the bulk of the cleaning fluid. Drawing a damage-sensitive part through the fluid–air interface while the ultrasonic energy is running can result in severe parts damage. Thus, many processes are designed to turn off the ultrasonic energy as parts are passing through the interface, often referred to as employing the quiet interface. Cleaning Efficiency of Ultrasonic Cleaners Ultrasonic cleaners appear to work by two principal mechanisms: cavitation and acoustic streaming. Cavitation is a process in which the constructive interference of sonic energy causes the formation of rarified bubbles in the cleaning fluid. When these microscopic bubbles implode (due to the passage of the rarefaction energy, the moving sound wave) they produce microscopic jets of liquid that can impinge on the surface of parts to be cleaned. These high-velocity jets remove particles from surfaces and convey cleaning chemicals to organic and inorganic chemical contamination on the surface. In the second mechanism, acoustic streaming, bulk movement of fluid occurs. Contaminants removed from surfaces that are carried away by acoustic streaming are prevented from reattaching themselves to the surface. Cavitation and acoustic streaming work together in all forms of ultrasonic cleaning, but the relative contribution of each is a function of frequency. At low ultrasonic frequencies, cavitation is very strong and dominates the cleaning process. At high ultrasonic frequencies, cavitation bubbles are very small, but acoustic streaming velocities can be very high. Thus, at high frequencies, acoustic streaming dominates the cleaning process and less cleaning occurs due to cavitation.

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The way that cavitation and acoustic streaming combine to clean is important to understand. We consider cavitation first. When acoustic waves combine constructively, the resulting decrease in pressure creates a localized bubble. In properly degassed solutions, this bubble consists almost entirely of solvent vapors. When the ultrasonic pressure waves separate, the localized pressure drops and the bubble collapses. When this occurs, a microscopic jet of liquid is formed, jetting from the bubble wall into the volume of the bubble. This high-velocity jet scours the surface of parts with which it comes in contact, knocking loose material from the surface. This cavitation action takes place preferentially at discontinuities on surfaces. Discontinuities can be scratches, pinholes in paint, and preexisting pits, among other features. For this reason, ultrasonic erosion that is so common below 70 kHz is almost always associated with these types of surface features. The second mechanism of contamination removal in ultrasonic cleaning is acoustic streaming, in which contamination is carried away from surfaces into the bulk of the liquid so that the contaminants cannot redeposit on the surfaces. Acoustic streaming cannot penetrate the boundary layer of motionless fluid that surrounds all of the surfaces in the ultrasonic tank. Particles dislodged from the surface by cavitation action are not swept away from the surface and become reattached. At high frequencies (200 kHz), acoustic streaming is highly directional, so the orientation of the part to be cleaned becomes critical. At low ultrasonic frequencies, the acoustic streaming is randomized and not highly directional. The thickness of the boundary layer surrounding the parts is a function of the ultrasonic frequency in the tank. The higher the ultrasonic frequency, the thinner the boundary layer. This is illustrated in Figure 5.12, where the boundary layer thickness is plotted as a function of frequency. The size and number of cavitation bubbles produced in ultrasonic cleaning is a function of a large number of parameters. Frequency is the most important. As frequency increases the number of cavitation bubbles increases but the size of the cavitation bubbles decreases. Other factors affecting the degree of cavitation action include the vapor pressure of the liquid, the temperature of the liquid, the amount of dissolved gas in the liquid, the surface tension of the liquid, and the presence of contaminants in the liquid. At any fixed frequency and 5 4.5

Thickness (μm)

4 3.5 3 2.5 2 1.5 1 0.5 0 0

200

400

600

800

Frequency (kHz)

FIGURE 5.12 Relationship between frequency and boundary layer thickness for room-temperature water.

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

power level, the greater the number of bubbles produced, the smaller the bubbles will be, and vice versa. As the surface tension of the liquid decreases, the ease of bubble formation increases. This results in an increase in the number of bubbles but a decrease in bubble size. Similarly, as the temperature of the liquid gets closer to the boiling point of the liquid, the ease of bubble formation increases. Thus, at higher liquid temperature, the number of cavitation bubbles increases and the size of cavitation bubbles decreases. Dissolved gases have an adverse effect on cleaning, because ultrasonic energy is expended in generating gas bubbles and moving them around. Similarly, particles in the liquid decrease cleaning efficiency because energy is lost in moving the particles around. To emphasize these points and their effects on cleaning performance: ●

●

●

●

●

Liquids with high vapor pressure tend to produce larger numbers of smaller cavitation bubbles than do liquids with low vapor pressure. This decreases cleaning intensity and also the amount of damage that occurs. As you increase the temperature of the liquid, it becomes easier to form bubbles and the liquid tends to produce larger numbers of smaller bubbles. Dissolved gases in the liquid will be coalesced into bubbles by the ultrasound, consuming energy, and decreasing cleaning action. For this reason, many ultrasonic cleaning processes include an initial degassing step prior to initiating the cleaning of parts. The higher the surface tension of the liquid, the smaller the number of bubbles produced. However, the small numbers of large bubbles produced can result in parts damage. If the liquid is not free of contamination, especially particles, some of the energy will be consumed in moving particles around and will thus not be able to contribute to cleaning.

Figure 5.13 illustrates a comparison of 40-kHz ultrasonic versus 400-kHz megasonic cleaning. The material is a bare A300 cast and machined part: a soft aluminum alloy. The 4.5 4

Log concentration

3.5 3 2.5

40 kHz Ultrasonic 400 kHz Megasonic

2 1.5 1 0.5 0 1

2 3 Log size squared

4

FIGURE 5.13 Comparison of 40-kHz ultrasonic versus 400-kHz megasonic cleaning for a soft aluminum alloy; 40-kHz ultrasonic extraction is followed by a liquid-borne particle count.

CLEANING PROCESSES

225

1. 00 0. 90 0.40 V/min

Fraction Remaining

0. 80 0. 70

0.25 V/min

0. 60 0. 50

0.15 V/min

0. 40 0. 30

0.10 V/min

0. 20 0. 10 0. 00 1

2

3 4 5 Time (min)

6

FIGURE 5.14 Tank cleanup rate as a function of volume fraction recirculation rate through theoretical 100%-efficient filters.

parts were measured using 40-kHz ultrasonic extraction followed by liquid-borne particle count. Megasonic cleaning is not as efficient at removal of small particles (less than 25 m). Ultrasonic cleaning tends to cause more erosion damage, as evidenced by relatively larger numbers of large particles (larger than 25 m). Several other factors can affect cleaning efficiency. Ultrasonic energy can be absorbed by many polymers. For this reason, plastic containers are not used as cleaning containers in ultrasonic cleaning processes. Plastic coatings, liners, or spacers may be used when metalto-metal contact between cleaning inserts and the parts being cleaned causes, or can potentially cause, physical damage. Excessive mechanical agitation can cause the ultrasonic cavitation to collapse completely. This introduces a problem. To remove contamination from a fluid, it must be recirculated through filters and reintroduced into the ultrasonic cleaning tanks. If the recirculation rate is too small, a fraction of the tank volume, cleanup times will be prolonged and the tank will continue to accumulate contamination with each successive load to be cleaned. The effect of recirculation rate on cleanup time is illustrated in Figure 5.14. Conversely, too high a recirculation rate can cause the cavitation to collapse completely. Most manufacturers of ultrasonic cleaning tanks recommend a maximum recirculation rate of no more than 25 to 40% of the tank volume per minute. It is equally important to control the way the fluid is reintroduced into the tank. For example, if the return fluid is allowed to jet into the tank, it will cause extensive stirring, causing the ultrasonic cavitation to collapse, even at recirculation rates as little as 5% of tank volume per minute. It is best to use diffuser screens to reduce the velocity of the returning fluid to prevent this collapse from occurring. 5.6.4

Spray Cleaning

Spray cleaning and rinsing are second in popularity to ultrasonic cleaning and rinsing. In spray cleaning, particle removal is entirely by shear stress, and removal forces are therefore

226

Log Concentration

GETTING CLEAN PARTS AND GETTING PARTS CLEAN

5

As Received

4

Fan, Low Foam, 120 °F, 2000 psi Fan, High Foam, 140 °F, 1500 psi Fan, High Foam, 120 °F, 2000 psig

3

Fan, High Foam, 120 °F, 1500 psi

2

Solid, High Foam, 120 °F, 1500 psi Solid, Low Foam, 120 °F, 2000 psi

1

Solid, No Foam, 120 °F, 1500 psi 0 1

2 3 Log Size Squared

4

Solid, No Foam, 120 °F, 2000 psig

FIGURE 5.15 A300 cast and machined aluminum part as received vs. after being cleaned by highpressure water.

proportional to the square of the particle diameter. As a result, finer particles are increasingly difficult to remove by spray. (Keep in mind that particle adhesion forces in the van der Waals regime typically scale as particle diameter; thus, for most conventional cleaning processes, the cleaning efficiency, i.e., ratio of particle removal to particle adhesion forces, decreases with particle size.) Depending on the nature of the liquid being sprayed, varying degrees of organic and inorganic contamination removal also is afforded. However, the use of surfactants in sprays is found to interfere with cleaning efficiency. This is believed to be due to the formation of foam on the surface of the parts being cleaned. Foam on the surface acts as a cushion and disperses some of the fluid velocity, decreasing cleaning effectiveness. Since spray cleaning is due almost entirely to shear stress, maximizing the fluid velocity on the surface increases cleaning efficiency. For this reason, with a given volume of fluid, solid jets are more efficient than fan jets. In addition, the stream of liquid should not breakup into discrete droplets before striking the surface. Spray breakup prior to contact with the surface decreases cleaning efficiency. Solid jets of liquid impinging on a surface are more effective than aerosolized droplets. Some of these points are illustrated in Figures 5.15 and 5.16. Figure 5.15 shows the cleanliness of a cast and partially machined A300 aluminum part along with the cleanliness after eight different high-pressure spray cleaning processes. These are shown together to illustrate the remarkable effectiveness of spray cleaning with high-pressure water by comparing the as-received cleanliness versus the after-cleaning cleanliness. The after-cleaning data are shown separate from the as-received data in Figure 5.16 to illustrate the differences among the various processes tried. Results of analysis of these data are shown in Table 5.1. These data show that 2000 psig is more effective than 1500 psig, solid jets are more effective than fan jets, and no surfactant is superior to low-foaming surfactant, which is in turn superior to high-foaming surfactant, as measured by particle concentrations left on the parts after cleaning.

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4 Fan, Low Foam, 120 °F, 2000 psi Fan, High Foam, 140 °F, 1500 psi Fan, High Foam, 120 °F, 2000 psig Fan, High Foam, 120 °F, 1500 psi Solid, High Foam, 120 °F, 1500 psi Solid, Low Foam, 120 °F, 2000 psi Solid, No Foam, 120 °F, 1500 psi Solid, No Foam, 120 °F, 2000 psig

log Concentration

3

2

1

0 2 3 Log size squared

1

FIGURE 5.16

4

Parts cleaned by high-pressure water.

TABLE 5.1 Comparative Spray Cleaning Performance of Various Parameters by High-Pressure Water Particle Size Variable

Level

5 m

9 m

15 m

25 m

50 m

Pressure

2,000 1,500 Solid Fan No surfactant Low High

4,462 4,350 20,871 6,725 1,325 5,350 5,475

806 1,118 412 1,512 300 900 1,325

187 338 126 400 92 185 387

21 21 9 33 6 17 30

1.2 2.5 0.7 3.0 0.5 0.5 3.2

Jet Surfactant foam

Spray cleaning and rinsing is often combined with ultrasonic rinsing and drying in the same process. Ultrasonic energy and high-frequency megasonics, with frequencies in the 1-MHz range, offer the unique benefit of being able to remove with nearly 100% effectiveness even the submicrometer particles that spray cleaning tends to be less effective at removing. However, ultrasonic cleaning below 80 kHz has a strong tendency to generate erosion-wear debris, which tends to consist of large particles. Thus, when used in combination, ultrasonic immersion cleaning is usually performed first, followed by spray cleaning to remove the large particles generated by low-frequency ultrasonic erosion. In the late 1970s and through the 1980s, spray cleaning using solvents was popular [11]. Nonflammable, high-volatility solvents, especially CFCs, were the most popular for solvent spray applications. Today, with the ban on the use of CFCs, water is largely being used in place of organic solvents. In general, fluid velocity is directly proportional to spray pressure. Many low-pressure spray processes are used in which the pressure is 90 psig or less. Higher spray pressures are often used in spin-rinse dryers. A spin-rinse dryer for cleaning

228

Removal Efficiency

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Low

~1500

High

Pressure (psig)

FIGURE 5.17

Characteristic S-shaped cleaning efficiency curve for spray cleaning.

and drying a cassette of parts usually operates at less than 400 psig; one for single wafers or disks can typically operate at up to 3000 psig. High-pressure spray cleaning for precision cleaning of metal parts typically ranges up to about 10,000 psig [12]. Figure 5.17 shows the characteristic S-shaped removal efficiency curve resulting from high-pressure spraying. Note the transition at about 1500 psig. This usually represents the demarcation point between low- and high-pressure cleaning efficiency regimes. High-pressure spray cleaning is so effective at removing contamination from surfaces that the way in which particle contamination cleanliness measurements are taken must be considered carefully. Many soft metal alloys and composite polymers are so clean after high-pressure spray cleaning that all of the energy of subsequent ultrasonic immersion is expended in working the surface and production of erosion debris. Ultrasonic extraction tends to skew the cleanliness measurement results. Thus, many high-pressure spray-cleaned parts must be measured using spray extraction rather than ultrasonic extraction. In summary, the characteristics of spray cleaning are as follows: ● ● ● ● ●

● ● ● ● ●

Pressures between 3000 and 10,000 psig show the highest cleaning efficiency. Volumetric flow rate is as important as spray pressure. Particles and films are removed by shear stress. Removal force is proportional to the square of particle size. The use of surfactants in the sprayed liquid decreases cleaning efficiency; presoaking in detergent solutions is recommended where detergents are needed. The cleaning efficiency curve is S-shaped. Cleaning manifolds must generally be custom designed for each part. Solid jets perform best. Fluid temperature is not an important factor. Surface damage due to erosion is far less than observed with ultrasonic immersion cleaning.

5.6.5

Spin-Rinse Dryer Cleaning

Spin-rinse dryers are an attractive cleaning alternative for rotationally symmetrical parts. Where surfaces are relatively smooth, fan nozzles are more efficient than needle jets. Where surfaces are not flat, needle jets can be more effective than fan jets. In the spin-rinse dryer,

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the shear stress cleaning action of the spray is enhanced by the increased velocity of the fluid over the surface of the part due to the high-velocity rotation of the part. High pressure has been shown to improve part cleaning dramatically. Parts that are not perfectly rotationally symmetrical can be cleaned on rotors designed with counterweights built in to provide balance. Spin-rinse dryers are available that operate both on one part at a time and in batch mode. Spin-rinse dryers are among the most energy efficient drying processes. Drying is accomplished by high-speed rotation of the substrate being cleaned. Heated and filtered dry nitrogen is often used as a drying gas. High-speed rotation during part drying has been observed to lead to electrostatic charging of surfaces. The source of this electrostatic charging has not been proven analytically. The most likely explanation is that some part charging occurs when the liquid is mechanically spun off the surface. It is doubtful that friction between the air and the part contributes significantly to this direct charging. It is more likely that collision between aerosolized droplets recirculating in the high-velocity airflow induced by the high-speed rotation of the part and the spinning part is contributing to charging. Air ionization of heated filter gas pumped through the chamber during drying can aid in reduction of the level of charge on the dried parts. Figure 5.18 shows how spin-rinse drying compares with other cleaning processes for a fully e-coated aluminum part. Parts were cleaned, dried, and then measured using 40-kHz ultrasonic extraction followed by liquid-borne particle counting. Freon ultrasonic cleaning at 40 kHz is effective at removing particles larger then 5 m in diameter, but not at 5 m. High-pressure spray is effective at all particles sizes 5 m and larger. Spin-rinse drying is equal to high-pressure spray in cleaning. The combination of high-pressure spray followed by ultrasonic immersion cleaning improves cleanliness at small particle sizes (less than 25 m) over either high-pressure spray or ultrasonic cleaning alone, but at the sacrifice of an increase in large particles due to ultrasonic erosion of the high-pressure spraycleaned part.

4

Log Concentration

3.5 3 As Received

2.5

Freon Ultrasonic 2

High-Pressure Spray Spin-Rinse Dry

1.5

HPS + US

1 0.5 0 1

3 2 Log Size Squared

4

FIGURE 5.18 Comparative cleaning performance for a fully e-coated aluminum part: 40-kHz ultrasonic extraction followed by a liquid-borne particle count.

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

5.6.6

Vapor Degreasing

Vapor degreasing continues to be an important cleaning process, despite the current unavailability of CFCs, which previously were one of the dominant chemicals used in vapor degreasing. Today, several other chemical systems have emerged as suitable replacements. In vapor degreasing, the cold part is immersed in the hot vapors over a boiling bath of liquid. The solvent condenses on the cold parts and drips off. Vapor degreasing is very efficient at removing organic contaminants and can be effective at flushing away large particles, but not small particles. Depending on the solvent chosen, it can also be effective at removing the ionic contaminants associated with solder residues. Alcohol vapor degreasers which boil a water–alcohol azeotrope can be effective for removal of flux residues. Hydrofluorocarbons (HFCs), alcohols, acetone, and even water are used as vapor degreasing agents today with great success. Unfortunately, the clock is ticking and the time period for the use of HFCs is running out. Many regional authorities are also increasing restrictions on the use of volatile organic compounds, which continue to contribute locally to photochemical smog and globally to global warming. 5.6.7

Chemical Cleaning

Exceptionally effective chemical cleaning methods that are more often thought of as surface treatment methods are electropolishing and bright dipping. Electropolishing is normally thought of as a surface treatment for stainless steels, but can also be used on aluminum, brass, and other alloys. In electropolishing, the roughness of the surface is reduced. This eliminates hiding places for particles and chemicals, making subsequent cleaning processes more efficient. However, there is a note of caution about electropolishing and bright dipping. Careful and thorough rinsing is mandatory to eliminate electropolishing chemicals and substrate residues. Dimensional tolerances are also a critical consideration. Alkaline etch has also been used successfully as a chemical cleaning process. Alkaline etch is generally effective at removal of residual soaps and organic contaminants prior to subsequent plating processes. Again, a note of caution is appropriate. Alkaline etch can produce a heavy oxide film on reactive metals such as aluminum. This oxide film may not be tightly adhered and can result in subsequent particle shedding problems if not properly sealed or removed. 5.6.8

Solvent Cleaning

The most widely used solvent in precision cleaning is water. Pure water has a high surface tension, about 70 dyn cm, and thus is a poor wetting agent for hydrophobic surfaces. Since many of the contaminants being cleaned using water are hydrophobic, modifying chemicals are needed to make water suitable for wetting hydrophobic surfaces. Chemicals available include simple surfactants, formulated detergents, solvents such as alcohols, and emulsionbased hydrocarbons or other organic chemicals. These are added in varying quantities, depending on the chemistry of the system, to reduce the surface tension of the mixture to typically 35 dyn cm or less. Water is a low-cost solvent compared with other alternatives. However, the chemicals added to improve wetting can increase the direct cost and can complicate waste disposal. One of the biggest advantages of the use of water as a solvent is its widespread acceptance as an alternative to CFCs for precision cleaning. It has been estimated that 90 to 95%

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of all precision cleaning can be done using water. As a consequence, the variety of equipment available as water-based cleaners is wider than for any other solvent. One of the biggest disadvantages of water comes in drying. Because of the relatively high boiling point and heat of vaporization of water, costs associated with its drying process are relatively high. The high cost of drying has led to the development of several alternatives to hot-air drying. One of the workhorse chemicals in the cleaning process industry has been normal methyl pyrrolidinone (NMP). When used at elevated temperature (about 170°F) NMP shows remarkable ability to dissolve polymers, such as polycarbonates, polyacrylates, and polyurethanes. However, it has no tendency to dissolve polyolefins. This chemical selectivity can be used to advantage in product and process design. The primary drawbacks include the cost of heating the chemical, some safety concerns about burns from splashes, and the rather strong odor. Although low in toxicity, many find the odor offensive and cannot tolerate working around it for extended periods of time. A few critical (strategic) applications have been granted exceptions to the international agreement to eliminate ozone-depleting chemicals. These chemicals include chlorinated hydrocarbons such as 1,1,1-trichloroethane and chlorofluorocarbons (CFCs). For those applications that have not been granted such an exception, alternative solvents are required. Fortunately, several alternatives are currently available among them, are hydrofluorocarbons (HFCs). HFCs are attractive because they have relatively low toxicity, are chemically stable, and are nonflammable. They are low in viscosity, low in surface tension, and have a low boiling point, all of which are ideal properties for cleaning complex parts. HFCs are also among the few solvents that can effectively dissolve the perfluorinated lubricants used extensively in the disk drive and aerospace industries. One drawback is the high cost of HFCs; a second is their high global warming potential. HFCs released into the atmosphere will remain there for hundreds to thousands of years. Several solvents based on natural plant products are also receiving widespread acceptance for niche processes. Among these chemicals are terpenes derived from pine trees and limonenes derived from citrus fruits. All have zero global warming potential, zero ozone depletion potential, low toxicity, and are completely water soluble. The primary disadvantage of the use of these chemicals is the odor, which at low concentrations or for short exposure times are pleasant but with prolonged exposure can become annoying. 5.6.9

Mechanical Agitation Cleaning

Mechanical agitation cleaning processes (undulation and sparging) are immersion cleaning processes that do not employ ultrasonic agitation. Generally, parts are immersed in a bath and either undulated, rotated, or sparged. In sparging, the parts are generally held stationary in the liquid and are showered from below with jets of liquid containing air bubbles. All of the mechanical agitation cleaning processes have relatively low particle removal efficiencies, but can be moderately effective for removal of organic or inorganic contaminants, depending on the chemistry of the bath. Advantages include relatively low equipment cost, low maintenance, and a reduced tendency to cause damage to parts being cleaned. 5.6.10

Manual Cleaning

Swabbing and wiping are remarkably effective cleaning processes. It has been estimated that the particle removal efficiency of wiping is equivalent to high-pressure spray at several thousand pounds per square inch. Thus, if the surfaces to be cleaned are not too geometrically

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complex, wiping may be an excellent choice. Indeed, nearly every high-technology manufacturer depends on wiping and swabbing to prepare their tools and workstations for use in processing. In the aerospace industry, many of the structures that must be cleaned are so large that manual wiping is often the only cost-effective alternative. Several cleaning techniques have been evaluated for their ability to achieve MIL-STD-1246 cleanliness levels. Vacuum brushing was not found to be capable of achieving level 300 cleanliness. Solvent flushing was rejected for large surfaces as likely to involve the use of large quantities of solvent having cost and environmental impacts. Thus, the cleaning methods of choice for large, complex surfaces are limited to wiping and swabbing with solvents. Multiple solvent wipes typically are capable of achieving level 100 cleanliness. A primary consideration is the selection of tools provided for personnel who will perform the wiping operation. In addition to having the correct wipers, a variety of swabs may be needed to get into hard-to-reach spaces. Selection of solvents for large complicated structures must consider the variety of materials used in the assembly. Manual wiping can also result in prohibitive cost in high-labor-rate countries such as Japan or Singapore. As a result, manual wiping operations are frequently subcontracted to lower-labor-rate countries such as China. 5.6.11

Specialty Cleaning

Plasmas have often been described as a fourth state of matter. A gas, such as oxygen, is exposed to high-energy radio-frequency radiation in a low-pressure chamber. This causes the oxygen molecules to dissociate into ions, which because of their charge, can be accelerated toward a surface. When these impinge on the contaminants, the contaminants undergo chemical reactions. Many of the reaction products produced are harmless gases which are pumped away by the chamber vacuum system. Plasma cleaning is most effective for removal of thin layers of low-molecular-weight organic contaminants. It is ineffective at particle removal or removal of inorganic contaminants. The equipment is relatively expensive and requires a good deal of maintenance. In addition, it is a batch process. Ultraviolet (UV) light, either alone or in combination with ozone, can be an effective cleaning process for removal of organic contaminants. In ultraviolet cleaning, high-energy (short-wavelength) light illuminates organic molecules on a surface. The ultraviolet energy is absorbed by organic contaminants: The absorbed energy breaks chemical bonds and produces volatile reaction products. Because the reaction products are volatile, no residues are left on the surface. This can be enhanced by illuminating with a high-powered UV laser. Both UV and laser UV cleaning are ineffective in removing particles or ionic contaminants. Both are line-of-sight cleaning processes: Only areas illuminated are cleaned; areas in shadows will not be cleaned at all. In UV ozone cleaning, the ultraviolet light is used to convert oxygen into ozone. The ozone diffuses around the part. The ozone promotes reactions in organic contaminants that produce volatile reaction products, just as in UV-only cleaning. Just as with UV cleaning, UV ozone cleaning is not effective at removal of particles or ionic contaminants. However, the ozone is free to diffuse in the bath, so UV ozone cleaning is not line-of-sight. Figure 5.19 is a typical phase diagram showing the region occupied by supercritical fluids. A supercritical fluid exists when the pressure is greater than the critical pressure and the temperature is greater than the critical temperature. Table 5.2 shows typical physical properties of supercritical fluids, density, diffusivity, and dynamic viscosity compared to gas or liquid phases of the same material.

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SUPERCRITICAL FLUID 73 (Critical Pressure)

LIQUID Critical Point

SOLID Pressure (atm)

Triple Point

4 GAS

203

305 (Critical Temperature)

Temperature (K)

FIGURE 5.19 Simplified phase diagram for carbon dioxide. At any combination of temperature and pressure above the critical pressure and critical temperature carbon dioxide has the properties of a supercritical fluid.

TABLE 5.2 Physical Properties of SCFs Property Density Diffusivity Dynamic viscosity

Symbol

Units

Gas Phase

Liquid Phase

Supercritical Fluid Phase

␦ D 

g/cm3 cm2/s g/cm  s

0.001 0.00005–0.00035 104

0.8–1 5  106 102

0.2–0.9 103 104

Supercritical (SC) fluid cleaning is an interesting niche cleaning process. SC fluids have unusual properties: They have the density of a liquid and the viscosity of a gas. SC fluids are best suited for removal of organic contaminants. The selectivity of the SC fluid in removing organic compounds can be adjusted by adjusting the temperature and pressure of the SC fluid. The most common of the SC fluids used for precision cleaning is carbon dioxide (CO2). SC (CO2) has zero ozone-depleting potential and zero global warming potential, since the carbon dioxide used comes from the atmosphere to begin with. Using SC CO2 has an advantage over the use of other solvents in that no solvent residues remain on the parts after cleaning, so an additional drying step is not required. The drawback to SC CO2 cleaning is that the operating conditions require high pressures, so the equipment can be heavy and expensive. In addition, SC CO2 cleaning is a batch process. Also, removal efficiencies for particle and inorganic contaminants are low. Carbon dioxide snow is another niche cleaning process that has found several applications. In the CO2 snow cleaning process, liquid CO2 is sprayed through specially designed nozzles where heat of expansion freezes out tiny solid particles of dry ice. The expanding gas accelerates these particles so that they can be used like a spray. The snow is relatively effective at removing particles and low-molecular-weight oils. It is less effective at removal of inorganic contamination and higher-molecular-weight oils and greases.

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Several problems were noted with early designs of CO2 snow cleaners. One was the tendency to freeze the parts being cleaned. The chief disadvantage created was the tendency of the parts to condense moisture from the atmosphere. In a few cases, differential expansion induced mechanical problems, particularly with rigid epoxy bonds. Also, the friction between the high-velocity snow particles and the surface being cleaned led to electrostatic charging of the parts. Several of these problems have been overcome by more advanced designs. In advanced CO2 snow cleaners an auxiliary heated air supply is added to reduce the cooling affect of the snow. Addition of in-line air ionizers to this auxiliary heated air supply have greatly reduced the tendency for electrostatic charge to be generated. One lasting drawback of these snow cleaners is the relatively small area over which they operate. Basically, they form a pencil-thin spray of snow. They are a line-of-sight cleaning process. If the area to be cleaned is reproducible from part to part, fixtures can be designed to automate the cleaning process to some degree. Another niche cleaning process, similar to CO2 snow, is argon–nitrogen snow cleaning [13]. This approach is particularly attractive because the cleaning medium is non-volatile, as in CO2 snow cleaning, and the cleaning chemicals are particularly inert.

5.7

DRYING PROCESSES

No discussion of cleaning processes is complete without a discussion of drying processes. This is especially true given the prevalence of water-based cleaning processes, with the associated difficulties of drying up water residues. If performed incorrectly, water-based cleaning can result in water spots, visible stains, and other defects, such as poor paint adhesion. 5.7.1

Spin-Rinse Drying

Spin-rinse dryers are among the most energy-efficient drying processes available. In the spin-rinse dry process, the rinse water is turned off and the parts are spun at high speed. The rotating parts induce high-velocity airflow over their surface, increasing the rate of evaporation of the cleaning fluid without the need to supply heat. During the drying process, compressed dry air or dry nitrogen is usually supplied to the chamber. 5.7.2

Forced-Air Drying

Forced-air drying is one of the most common implemented in the age of water-based cleaning. Forced-air drying works by a combination of physical removal of liquid and evaporation. The relative contribution of physical removal and evaporation depends on the type of cleaner and how well the parts can be oriented to facilitate the water being chased off the parts. Several types of forced-air dryers are available. Air knives are popular in many industrial applications. Air knives usually are a sheet of air formed by a continuous slot in a pipe. Air is supplied by a blower or compressor. Relatively high volumes of air are required, so HEPA filtration is often not supplied. In addition, little compressive heating occurs; so heaters must be added if heating is require to assist the air drying. These types of air knife dryers are often seen in plating and coating lines, where cleanrooms are less of a concern. Air drying manifolds often are built into the drying chambers of precision cleaning equipment. The manifolds are usually fitted with air nozzles to create high-velocity air in

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the drying chamber without the need to use large volumes of heated HEPA-filtered air from the clean air system. Convection ovens are a third form of forced-air drying. In a convection oven, additional air might be introduced to the convection to assist in drying, to maintain a positive pressure with respect to the ambient, and to provide clean air. One interesting approach to forced-air drying is found in the medical industry. Cleanable laboratory equipment, patient care items, and lab equipment are often cleaned in industrial dishwashers. Many of the items to be cleaned are shaped like deep bottles. To ensure that these get cleaned, they are held in place in the cleaner basket on hollow tubes. The bottom of the tube can have a flare that acts like a funnel to capture water and later during drying, captures air and directs it up into the item being cleaned. The key to making this work is to make certain that the inlet to the hollow tubes pass directly over cleaner fluid or air jets. 5.7.3

Vacuum Drying

Vacuum drying is popular as a final drying step after forced-air drying [14]. Often, parts have structures that cannot be dried by forced air alone; small-diameter hollow tubes are a good example. There are several considerations in vacuum drying. It is a relatively slow process and is also a batch process. As a consequence, it may be necessary to consider the need for multiple vacuum dryers to support constant throughput in a cleaning process. Pressure control is also an important consideration. If the evaporation rate of the liquid is too high, the temperature of the part will drop, due to evaporative cooling. Where water is being dried, too high an evaporation rate can actually result in the formation of ice. This can result in part damage, because the density of ice is less than that of water, or can lead to incomplete drying. Pressure control is also important from an electrostatic discharge point of view. If the pressure in the vacuum dryer drops below the Paschen limit, the air will become conductive. Any charged structures in the vacuum dryer will then electrostatically discharge to the nearest convenient point. Both the product and the vacuum chamber can be damaged unless precautions are taken. The most common precaution is to limit the pressure in the chamber to greater than about 3 mmHg, above the Paschen limit. A second precaution is to turn off all the power to heaters and other electronic components in the interior of the vacuum chamber as the Paschen limit is approached, and turn the power back on when the pressure in the chamber has become so rarified that the air is again a good insulator. 5.7.4

Adsorption Drying

Adsorption drying (including mopping) can also be effective where contaminants from the absorbent are not a concern. Common absorbents include activated carbon, clay, and silica gel. In this process the part is placed in the absorbent until all of the moisture from the part has been absorbed. The part is then removed and the dust is blown off. The absorbent can then be regenerated and reused. 5.7.5

Chemical Drying

Chemical drying utilizes various chemicals to dissolve or displace cleaning fluid from the part. The most common chemical dryers in use today use alcohols as absorbents for water. The alcohols are recycled through absorbent beds or distilled to eliminate the water and recycle the alcohol. The alcohol-dried part can then be air dried more efficiently because of

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the lower boiling point and heat of vaporization of the residual alcohol on the part. Many other classes of chemicals can be used instead of alcohols. The main problem with most of these chemical drying agents can be flammability, global warming potential, or ozone depletion potential of the drying agents. Toxicity may also be a concern.

5.8

COST OF CLEANING

No discussion of cleaning processes or equipment would be complete without a discussion of the costs associated with cleaning. Investment costs include the cost of the cleaning equipment and of the utilities and facilities modifications to support it. Where auxiliary purification equipment is required, such as DI water processing facilities or solvent recovery facilities, these must be factored into the investment cost. Ongoing costs include those of processing and maintenance labor. Many equipment manufacturers will recommend a list of preventive maintenance parts, which should be considered an ongoing cost. Operating costs must also consider cost of utilities, consumables used in daily operations and maintenance, and so on. The cost of waste disposal and solvent replacement must be included in the analysis. Cleaning is not a value-added activity. It adds nothing to the functionality of the product. It does influence quality and reliability but should not be relied upon to achieve these. Cleaning only removes contaminants that should have been kept off the surface to begin with. Thus, emphasis should be on contamination control in product design and manufacturing, with only minimal cleaning as necessary.

5.9

VENDOR PROCESS CONTAMINATION CHECKLIST

The vendor process contamination checklist is a useful device for conducting qualification surveys [15]. It can also prove useful for investigating problems identified at vendors who were previously qualified. The checklist is designed principally to get simple yes/no/not applicable answers. In some questions, there are prompts for additional details, obtained using follow-on questions. Focusing on vendor process has many benefits. First, vendors tend to produce a few, highly specialized parts that they specialize in. For this reason, their in-process and final cleaning processes also tend to be highly specialized and often are optimized for the materials and geometries of the parts they clean. Because these cleaning processes are dedicated and optimized, it is often easier and less expensive for the supplier to achieve the ultimate part cleanliness than it is for the customer. Many subassemblies cannot be immersion cleaned, leaving out the entire spectrum of the most effective and usually lowest-cost cleaning options after assembly. This means that the responsibility for optimizing the cleaning of parts and subassemblies rests almost entirely with the supplier. Examples of parts that cannot be cleaned after assembly include bearing assemblies, gearboxes, and other lubricated subassemblies, filters and absorbent pouches, motors, pneumatic cylinders, and optical components or subassemblies that contain them, such as optical encoders. In general, the customer cannot afford to have the wide variety of cleaning processes needed to do the specialty cleaning needed to achieve ultimate cleanliness of all parts. In most cases, the best the customer can hope to accomplish is to remove the shipping and

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handling debris. The vendor process contamination checklist focuses on eight general subject categories: 1. 2. 3. 4. 5. 6. 7. 8.

Receiving/inspection at the vendor Manufacturing Degreasing/cleaning/drying Final inspection Storage Shipping Employees Management

The receiving inspection process at the vendor is the proper place to begin the survey. Starting at receiving inspection allows one to follow the raw materials, piece parts, and subassemblies through the vendor’s process in a systematic manner. The manufacturing process can include cleaning of the as-received raw materials, piece parts, and subassemblies. It can also include intermediate and final cleaning. The manufacturing steps can include machining, plating, and coating processes and assembly operations. Vapor degreasers are covered in some detail. This is so because halogenated hydrocarbon solvents are still used in some industries. Rapid advances in the development of solvents with reduced global warming potential and ozone depletion potential brings new substitute solvents on the market on a regular basis. The effect these replacement chemicals can have on vapor degreasers should be reexamined periodically. Final inspections should be made taking into account the types and quantities of contaminants that could be on the finished product. In addition, process capability will determine the types and frequency of final inspection required. Procedures used to protect product from subsequent recontamination during storage are important. This must include careful selection and ongoing verification of the packaging materials. Apart from these process-specific questions, there are some that pertain to operator training and management attitudes. These include questions about the training for operators and managers, how corrective actions plans are developed and implemented, and how problems are communicated, both within the vendor’s organization and to customers. 1. Receiving/inspection at the vendor Are all incoming materials raw (stock) materials? YES NO Are some of the parts supplied by a secondary vendor that will be assembled at the primary vendor? YES NO Do stock materials receive at least a cursory visual inspection for gross contamination? YES NO How is the quality of incoming stock controlled? If some parts are received for assembly, do the parts have customer cleanliness specs incorporated on them? YES NO N/A ●

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If they do, are they being finally inspected at the secondary vendor? YES NO N/A If they do, are they being inspected for cleanliness by the primary vendor? YES NO N/A Does either the primary or the secondary vendor have a “black box” operation in the process? YES NO N/A Is inventory managed properly [i.e., on a first-in-first-out (lot integrity) basis]? YES NO 2. Manufacturing Is the manufacturing process fully documented with the customer? YES NO Briefly outline/sketch the steps in the manufacturing process. How many parts are produced per day? Is continuous-flow manufacturing being practiced? YES NO On average, how long is the part being made to wait between successive operations? MINUTES HOURS DAYS Are products for other customers being manufactured in the same area? YES NO If so, is there any chance of cross-contamination? YES NO N/A Is machining debris being removed effectively (across all size ranges) between stages? YES NO N/A If so, state how. Are the intermediate cleaners dip/rinse tanks or spray units? STATIC TANKS SPRAY CLEANERS N/A If dip type, how often is the cleaning fluid replaced? ONCE A DAY TWICE A DAY OTHER (explain) Has the nature of the cutting tool (hard/soft, sharp/dull) been correlated with the amount of machining chips/fines generated? YES NO N/A Have tool-bit changes been looked at as a means of reducing machining debris generation? YES NO N/A During machining, are the parts spray-cooled or flood-cooled? SPRAY FLOOD N/A Are all the coolants being used compatible with the downstream cleaning process? For example, if the cleaner is water-based, are all coolants and other process fluids water soluble? YES NO N/A ●

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If oil-based coolants or lubricants are in use, has the vendor explored the use of water-soluble replacements? YES NO N/A Are manufacturer’s safety data sheets (MSDSs) for all the chemicals used in the manufacturing process documented with the customer? YES NO If not, provide MSDSs for all chemicals used in manufacturing. If the process fluids dry on the part surface, do they appear to form a lacquer by means of polymerization reactions? YES NO N/A Are air particle counters (APCs) used to monitor the quality of the manufacturing environment? YES NO If so, at what intervals? DAILY WEEKLY MONTHLY N/A Explain how the APC data are used. Is any deburring done? YES NO If so, explain how. Is any deflashing done? YES NO Is any hand wiping of the surface being done? YES NO Is any oil being added (on bolt threads, etc.)? YES NO If so, explain how it is being removed. In situations where magnets are involved, are steps being taken to prevent magnetic contamination/cross contamination? YES NO Explain. Are there any “black box” operations at the vendor? YES NO If parts undergo a surface treatment such as passivation, polishing, or epoxy coating, are these processes vended out to a secondary supplier? ALL SOME NONE Are plating/coating processes at the primary vendor certified and qualified by the customer? YES NO N/A If surface plating/coating is done, are inspection procedures in place for porosity, adhesion, and so on? YES NO N/A If so, according to what specification?

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If soldering is involved, are water-soluble fluxes used? YES NO N/A If soldering is done, are water-soluble solder pastes used? YES NO N/A If soldering is done, are no-residue or low-residue fluxes used? YES NO N/A

The following questions specifically address organic contamination. Are air filters being used that were tested with DOP (dioctyl phthalate)? YES NO N/A Are any plasticizers (e.g., phthalate esters, sebacates, adipate esters, chlorinated phthalate esters) being used? YES NO Are any antioxidants (e.g., amines, aromatic phenols) used in the process? YES NO Are common epoxy hardeners (e.g., amines) used? YES NO Are fire retardants (e.g., chlorinated phosphate esters, halogenated aromatic compounds) used in the process? YES NO Are long-chain hydrocarbon oils removed by solvents? YES NO N/A If lubricants are used, are the hydrocarbons removed by a solvent? YES NO N/A If the product contains rubber, is the rubber vulcanized (sulfur-cured)? YES NO N/A If the part is electrophoretically coated with a conductive polymer, are organotin compounds added to the paint? YES NO N/A Are any polymer additives used that bloom? YES NO N/A Are mold release agents used? YES NO Are low-molecular-weight silicones used? YES NO Are any open-celled foams used? YES NO Are any organic materials used that outgas greater than 0.5% by weight? YES NO 3. Degreasing/cleaning/drying Has the cleaning process been documented with the customer? YES NO Briefly outline/sketch the cleaning, rinsing, and drying processes. ●

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Is the wash or rinse fluid being recycled? YES NO If so, describe the filtration arrangement. Is online monitoring of fluid cleanliness being done? YES NO Are chlorofluorocarbons or other chlorinated solvents used in cleaning and/ordrying? YES NO If so, what is the estimated consumption of the cleaning (and/or drying) medium per year (in gallons)? If so, what is the estimated recovery/recycling of the cleaning (and/or drying) medium (as a percentage of consumption)? Is hot air being used for the drying of parts? YES NO If so, is the air filtered? YES NO N/A If yes, describe the air filtration arrangement. Is the air oil-free? YES NO N/A Is there a possibility of overloading the cleaner? For example, are grossly contaminated parts being prewashed in the same system where final cleaning is done? YES NO Is there a separate prewash unit to remove gross contamination? YES NO Have cleaner operating parameters (e.g., temperature, pressure, flow rate) been optimized? YES NO Is the cleaning system water based? YES NO If so, is deionized (DI) water being used? YES NO N/A If so, is the DI water quality monitored on a continuous basis? YES NO N/A If Freon or other chlorinated solvents are being used for cleaning or drying, has a water-based system been looked at as an alternative? YES NO State why aqueous cleaning is not feasible. If aqueous cleaning is not possible, have alternative solvents been investigated? YES NO N/A What is the target date for solvent elimination? Describe how cleaner efficiency is monitored and maintained. How consistent is the cleaning process—is there significant part-to-part or lot-tolot variation? YES NO

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4. Final inspection Does the vendor have the customer’s cleanliness/dryness specification documents? YES NO If so, state the engineering change or revision level(s). Does the vendor have the specified cleanliness (or dryness) levels for the part(s)? YES NO If so, state the cleanliness (and/or dryness) specifications. Does the vendor have someone in charge of contamination/quality control? YES NO What analyses (particulate/chemical/outgassing/nonvolatile residue) are being done on the parts to ensure conformance to the customer’s cleanliness specifications? What percent of parts are being tested? If less than 100%, explain on what basis. Are final inspection data being reported to the customer? YES NO If yes, state in what form and at what frequency. Is the contamination monitoring equipment at the vendor properly correlated with the customer’s in-house test instruments? YES NO Has someone been sent by the customer to install the vendor equipment and to instruct operators on its use? YES NO Is periodic calibration and/or equipment servicing being done? YES NO If so, at what frequency? ONCE A YEAR TWICE A YEAR OTHER (Explain) Have receiving/inspection readings taken at the vendor and customer on the same batch ever differed considerably? YES NO If so, can transportation debris account fully for the discrepancy? YES NO N/A Is there any disassembly/reassembly of parts during final inspection? YES NO Are nonshedding gloves worn during inspection when handling final-cleaned parts? YES NO Is the environmental quality in the inspection area monitored? YES NO If so, how? How good is traceability? For example, if one part in a lot is bad, can the vendor go back through the process and find the reason? YES NO ●

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5. Storage How long are parts stored after final inspection before shipping? HOURS DAYS WEEKS MONTHS Is the air quality of the storage area monitored? YES NO If so, at what frequency? DAILY WEEKLY MONTHLY OTHER (Explain) Are finished parts covered with nonshedding sheets during storage? YES NO ●

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7. Employees Have operators been made aware of the need for (particulate and organic) contamination control (e.g., through video presentations)? YES NO Are operators familiar with requirements for working in a contamination-controlled environment (e.g., change procedures, good manufacturing practices, contraband substances)? YES NO Are they aware of simple, quick contamination monitoring procedures such as tape tests? YES NO Are employees encouraged to come up with their own suggestions for improving product quality? YES NO Are employees allowed to take responsibility for the parts they produce and given leeway (subject to management approval) to make improvements that they deem necessary? YES NO If the process involves cleanroom work, are operators properly trained for it? YES NO N/A Do personnel wear proper gloves and other apparel suitable to their workstation? YES NO 8. Management Are managers responsive to a customer’s cleanliness requirements? YES NO Are they willing to work with a customer when necessary to improve parts cleanliness? YES NO Are they concerned with product quality? YES NO Are they responsive to a customer’s suggestions and recommendations? YES NO How many years of association does the vendor have with the customer? Is the vendor willing to let a customer’s representatives inspect the product manufacturing operations? YES NO Is the vendor willing to let a customer’s representatives inspect the product cleaning facilities? YES NO Is the vendor willing to let a customer’s representatives inspect the product packaging procedure? YES NO Is there effective two-way communication between the customer and the vendor? YES NO ●

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5.10 CASE STUDIES: CLEANING EQUIPMENT AND CLEANING PROCESS DESIGN Case Study 1: Corrosion in a Multistage Cleaner In the early 1980s, one large hard disk drive manufacturer realized that the lack of quantitative cleanliness control over parts from its suppliers could not be tolerated using its simple ultrasonic vapor degreaser cleaning procedure using Freon 113 as the cleaning agent. Preliminary analysis of the contamination burden on incoming parts indicated that the parts were contaminated with particles, organics in the form of cutting fluids and mold release compounds, ionic contamination from handling and packaging materials, and so on. It was decided that the immediate solution was to implement cleaners that could remove all forms of contamination. The cleaning machines that resulted from this decision had 10 process tanks. The first tank contained a Freon vapor degreaser. The second tank was an aqueous detergent ultrasonic immersion cleaner. Tanks 3 and 4 were DI-water immersion rinses. Tank 5 was an alcohol immersion rinse. Tank 6 was a Freon 113 ultrasonic immersion clean. Tank 7 was a Freon 113 vapor degreaser. Tanks 8, 9, and 10 were hot-air dryers. This cleaner was reasonably effective at accomplishing its objective: removing particle, ionic, and organic contamination from incoming material where the composition of the contamination on the incoming material was unknown and expected to vary from part to part. It accomplished this by using a combination of cleaning chemistries: Freon 113, which was ideal for organic removal; water–detergent, which was ideal for ionic contamination removal; and ultrasonic agitation, which was ideal for particle removal. Unfortunately, this process ran into a fundamental materials compatibility issue. Several of the parts being cleaned were made of aluminum or were plated with zinc both of which are reactive metals. Reactive metals in contact with CFCs such as Freon 113 and the presence of a proton donor (such as water or alcohol carried into the bath from a previous tank) will react chemically. The chemical reaction results in the formation of hydrochloric and hydrofluoric acids, which then attack piece parts, the cleaner baskets, and the cleaner tank itself. As a consequence, periodically the Freon tanks would turn sour; they would contain hydrofluoric and hydrochloric acids that would attack corrosion-sensitive parts. Case Study 2: Detergent Drag-Out Figure 5.20 illustrates a second cleaner with a design problem. This cleaner has two ultrasonic immersion cleaning tanks at its front end. The first contained 2% by volume detergent in DI water solution. The second contained 1% by volume of the same detergent in DI water. The thinking was that the detergent concentration had to

Tank 1 2% detergent

Tank 2 1% detergent

FIGURE 5.20

Tank 3 Spray over bath

Tank 4 Spray over bath

Cleaner with a drag-out problem.

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accommodate the entire soil load from 24 hours of production and that the tanks would be dumped and recharged once every 24 hours. After removal from the second cleaner tank, the parts moved to the rinse tanks. In the first and second rinse tanks, the parts were spray rinsed over full tanks of DI water. Thus, the detergent dragged out of the wash tanks on the parts was partially rinsed away, but a significant percentage of the dragged-out detergent solution ended up rinsed in each successive ultrasonic immersion rinse tank. The parts were then immersed in the rinse tank and rinsed ultrasonically. The remaining steps were forced air drying and vacuum drying. The high detergent concentrations in the wash tanks, combined with spray over the tank, predisposed this cleaning system to produce parts at the beginning of the drying process that were still contaminated with residual detergent. Parts would emerge from the drying process contaminated with dried-on detergent residues. Visual inspection of these cleaning systems showed detergent residues on the hot surfaces of the drying tanks at the entrances to the vacuum dryers. Case Study 3: Versatility Not Optimized Here is an example of a cleaning process that is integrated into its up- and downstream manufacturing processes. In the upstream process, parts to be machined were held on a vacuum fixture. After machining, a cover fixture was applied to hold the parts in place. The vacuum fixture with its cover was then transferred to the cleaning process, which was a single-chamber cleaning and drying process. The same vacuum fixture was used in the inspection process after the cleaning process, so the process was fully integrated. The vacuum fixture was placed in the cleaner with its cover fixture in place. In the cleaning chamber, a fluid supply hose was connected to the vacuum fixture. The process chamber was then filled with a detergent–DI water solution and the residual air inside the vacuum fixture was blown out through the top of the fixture with detergent–DI water through the fluid supply hose. This would produce a tremendous amount of foam. The foam was always contaminated with the residue from the machining process, which had been drawn into the fixture during the machining process. The parts were than cleaned ultrasonically. The next step in the process was a quick dump to get rid of the detergent–water solution. The surface of the liquid in the tank was coated with a thick layer of foam; so when the initial quick dump was performed, the foam would coat the parts in the tank. This would redeposit machining residue. The next step was rinsing. The tank was refilled and the parts were rinsed ultrasonically. During this step the process chamber was continuously overflowed to drain. Finally, the parts were dried. The initial drying was forced hot air. The final drying was done off-line in a separate convection oven. The step following cleaning included microscopic inspection of critical surfaces on the parts while still on the same vacuum fixture. A significant number of parts were found visually to be contaminated. The contaminants were identified as machining residue. The process equipment in this case, despite the fact that it only has a single process chamber, has remarkable flexibility and is fully programmable. The contamination concentration in the tank during the process was studied by manually drawing samples out of the tank using a pipette and measuring using a turbidimeter. A quite precise picture of what was happening in the original cleaning process was developed. These data were then used to design a new cleaning process. The main problem was the foam on the surface of the cleaning fluid after the initial wash. When the quick dump was done at the end of the ultrasonic cleaning process, particle-laden foam would deposit on the parts. This particle-laden foam would then place an additional

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burden on the rinsing process. Simple changes were implemented in the cleaning process. The most effective was to continue pumping detergent water solution into the tank in the beginning of the cleaning step to top-skim the particle-laden foam off the surface of the tank when the tank was filled for the wash step. The second change was to optimize the rinse time; the rinse time was optimized based on the turbidimeter measurements. These simple changes were remarkably effective and the number of parts rejected for contamination in the subsequent inspection dropped to nearly zero. Case Study 4: Sprayed DI Water and ESD In another cleaning application, work in process was being cleaned between two particular process steps using an atomizer cleaner. This atomizer cleaner had been installed and had eliminated a labor-intense manual cleaning process that was not a capable process. Figure 5.21 illustrates the cleaner head. A highvelocity stream of air strips particles of DI water from a hypodermic needle, forming a fine spray moving at very high velocity. This proved to be very effective at removal of submicrometer abrasive particles and lapping residue from the surface of the part. When this process was successfully implemented initially, the product being cleaned did not have a high degree of electrostatic discharge sensitivity. Then the product moving through the line changed. The new product coming though the line was very sensitive to electrostatic discharge. When the new product was cleaned using this cleaning process, many of the parts failed due to electrostatic discharge. The problem here is that the highvelocity shear of the filter air over the filtered DI water produced charged droplets. When the product being cleaned had relatively little ESD sensitivity, the charged droplets were not a problem. However, when the new product with significant ESD sensitivity was cleaned, the charge on the droplets became a serious problem. Virtually all of the ESDsensitive parts cleaned using this spray cleaner were destroyed due to ESD. Eventually, this particular cleaning process had to be abandoned as the company moved to 100% production of the ESD-sensitive product. Case Study 5: Vacuum Drying ESD problems in cleaning processes can manifest themselves in several other ways. In the cleaner in this example, the forced-air drying chambers High-velocity air in

Deionized water in

Charged water droplets out

FIGURE 5.21

Atomized spray cleaner head.

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contained PVDF tubes outfitted with multiple stainless steel blow-off nozzles. The air to these nozzles was filtered and heated. In order to blow water off the parts and heat them sufficiently, the part-to-nozzle distance was quite precisely controlled and the spacing was very close (less than 1 cm). The parts being dried had numerous structures that could not be dried by forced-air drying alone. Thus, this cleaning equipment was equipped with an inline vacuum drying chamber, which could accept one basket of parts at a time. The cleaner had significant ESD damage problems. A careful ESD survey showed that the surface of the PVDF manifold would charge to thousands of volts. This would in turn induction-charge the stainless steel blow-off nozzles. Microscopic examination of the damaged parts using a SEM revealed that the parts were being damaged by a charged-device mechanism. Several changes were needed. The parts going into the cleaner were shorted using shorting clips. This ensured that all of the input and output connectors were shorted together. The PVDF manifold was replaced with a stainless steel manifold. The air supply to the forced-air drying chamber was fitted with in-line air ionizers. These actions reduced but did not eliminate the ESD failures. Further investigation revealed the cause. The product contained a structure that could not be grounded, so the shorting clip was only partially effective. The ungrounded portion of the product would become charged in the drying chamber even though the charge levels in the drying chamber had been reduced from thousands of volts to hundreds of volts. Then the charged parts would move into the vacuum dryer. In the vacuum dryer, the pump-down was not being regulated properly. The vacuum should have been controlled so that it never dropped to the point at which air becomes conductive. Unfortunately, the solenoid valve provided was defective. The charged parts were exposed to a conductive atmosphere, causing them to be damaged by electrostatic discharge. Repairing the solenoid valve corrected the vacuum problem and eliminated the ESD damage problem completely. This cleaner also illustrates a design problem when vacuum drying is chosen. The vacuum dryer in this cleaner was in-line. The vacuum-drying cycle required 24 minutes, exclusive of basket move times. By contrast, the forced-air dry step only took 8 minutes. When parts were in the vacuum dryer, the remainder of the cleaner stood idle. No consideration had been given to balancing the process. The capacity of the cleaner was not optimized. In addition, hot, wet parts were exposed to the air, which resulted in oxidation of the parts. Parts wet with detergent solution would often be found to be stained after cleaning. The stains were caused by absorption of detergent by the newly formed oxide coating on the surface of the parts, which would then discolor in the heat of the drying process. This company anticipated a 10-fold increase in production in the future. Additional cleaners would be required to support this increased production schedule. If the original cleaner was cloned, with its single vacuum dryer, three additional cleaners would be required. If, on the other hand, a balanced design for the new cleaner was adopted, a single cleaner with three vacuum dryers would suffice. At a cost of $600,000 each, it is clear that a balanced design would save capital cost, reduce floor-space requirements, and reduce plumbing for supply and waste lines with no sacrifice in product quality. Case Study 6: Optimized Cleaner This example is illustrated by the cleaner shown in Figure 5.22. Several interesting design examples come from this cleaner, which was designed specifically to be used in cleanrooms. Its materials of construction and design principles permit this type of installation. The first step in this cleaning process was a detergent–water ultrasonic cleaning tank, shown in Figure 5.23. The cleaner was designed not only to support a specific process but

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FIGURE 5.22

Typical in-line cleaner.

Liquid level when full Tank Cleaning Spray Manifold (runs when tank is empty)

Weir

Immersed ultrasonic transducer – not welded and side mounted for ease of removal UnderTransducer Cleaning Spray Manifold (runs when tank is empty)

FIGURE 5.23

Tank drain (open for cleaning and refill of cleaning tank)

Overflow drain (always open)

Ultrasonic cleaning tank, stage 1, in the optimized cleaning process.

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also to optimize the performance of the cleaner itself. The first cleaning tank was equipped with ultrasonic transducers that were conduit mounted through the side of the tank (i.e., the transducers were not welded to the floor of the cleaning tank) placed above the floor of the tank. The floor of the tank was not flat, but slaned in two directions so that liquid would flow to the corner drain. The bottom of the tank was equipped with spray nozzles that would flush the floor of the tank during dump and recharge. The surfaces of the transducers were tilted to prevent particles from settling on the surfaces and to minimize the tendency of particles to erode the transducer surfaces. None of the walls of the tank were perfectly perpendicular. The last two features were done to minimize the formation of standing waves in the tank and hence to minimize the tendency to erode parts. The tank had a weir on one side. The tank was always filled to the top edge of the weir with a detergent–water solution. The detergent concentration in the water was 500 ppm by volume. Each time a basket of parts was placed in the tank, some water from the tank would spill over the weir and go to the drain. Each time a basket of parts was removed from the tank, the water level in the tank would drop and the tank would be refilled to the edge of the weir with fresh 500-ppm detergent–water solution. The detergent concentration was high enough to minimize the surface tension of the DI water, thereby optimizing wetting of surfaces to be cleaned. However, studies had previously shown that detergent drag-out must be avoided to minimize contamination. Therefore, the detergent concentration was held as low as possible to minimize drag-out. This would, in turn, minimize the contamination holding capacity of the wash tank. This made it necessary to replenish the tank repeatedly with a small quantity of fresh detergent solution with each basket of parts. The tank was also equipped with a recirculation system that filtered the tank contents. This ran continuously at approximately 20% of the volume of the tank per minute. The return from the recirculation system was behind a diffuser, to minimize collapse of the ultrasonic field in the tank. Some parts cleaned in this tank were more damage sensitive than others. Every part put into the tank had a unique cleaner basket. The damage-sensitive parts were placed in cleaner baskets with a signal flag. When the optical sensor on the cleaner load station saw this flag, it would automatically reduce the power by 50% to minimize parts damage. Finally, the cleaner would dump and recharge itself on a programmed basis. The cleaner could be programmed to dump after a predetermined number of baskets or after a predetermined time. The cleaner was also equipped with a spray manifold around the top. After the dump, the top and bottom spray manifolds would rinse the interior of the tank automatically. The tank would then automatically refill itself with a hot detergent–DI water solution. Parts removed from the cleaner were then rinsed in a two-stage spray rinser. The spray was over drains, so detergent residues did not accumulate in any rinse tank. In this cleaner, the rinse water in the second spray rinse was so free of detergents that to conserve water it was held in a sump and used as the source of rinse water for the first chamber. The drying process was entirely forced air. Three chambers were used. The first two chambers used forced air at 90°C. Air was filtered and then reionized using bipolar corona discharge air ionizers. Two drying chambers were used, as were two rinse chambers, to balance the cleaning machine throughput. The third drying chamber was a cool-down chamber. Cleaning was a true just-in-time process. Manufacturing wanted to be able to handle the parts as soon as they came out of the cleaner. This was made possible through the use of the third drying chamber, which cooled the parts using filtered room-temperature ionized air.

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5.11 DETAILS ON THE CLEAN–THEN ASSEMBLE AND ASSEMBLE–THEN CLEAN PROCEDURES Processes normally are planned to follow the sequence of cleaning all of the parts and then assembling them, very often in a cleanroom. This has the potential to allow a significant amount of recontamination of the assembled product, due to accumulation of assembly debris, handling debris, and general in-process contamination. In addition, other factors must be considered in the use of the clean–then assemble strategy. Among these are: ● ● ●

●

●

The capital cost premium of the cleanroom facility over a factory workplace The operating cost premium of a cleanroom over a factory workplace The productivity loss of workers imposed by the cleanroom change and special cleaning procedures The increased number of parts baskets and cleaning inserts needed to clean each component separately vs. cleaning several parts as a unit after assembly The increased contamination monitoring required to maintain certification

The latter point about the increased cost to monitor and certify a cleanroom over an ordinary factory environment is worth further consideration. Cleanrooms are monitored for a wide variety of parameters and can include air velocity, temperature, and direction, airborne particle concentrations, airborne molecular contamination, witness plate monitoring, audits, white glove inspections, tape tests, and so on. In addition, the cleanliness of individual piece parts going into the cleanroom is usually under some level of inspection and control. However, none of the measurements addresses the primary concern: the cleanliness of the finished assembly. Thus, it is worthwhile to consider the feasibility and possible advantages of adopting a policy of assemble–then clean for one or more subassemblies entering the cleanroom. Several methods may be used to evaluate the effect on cleanliness of the two alternative strategies. Obviously, if an existing parts cleanliness measurement method is available for the individual components, it can be applied to the finished assemblies as well. In addition, contamination generation by the finished assemblies, due either to heating, vibration, airflow, or other stimulus that the part will experience in normal use, could be used as evaluation methods. In each case, the more like the actual conditions of use, the more meaningful the results. When developing a strategy for achieving cleanliness of parts and assemblies, many factors must be considered. Among them are those associated with quantitative measures of cleanliness, including particle contamination, ionic contamination, volatile organic contamination, and viable contamination. Fortunately, the quantitative measurement and specification of contamination is a fairly mature area of applied technology. A number of quantitative methods can be used to estimate cleanliness of parts. Among the most mature of these is the International Disk Drive Equipment and Materials Association (IDEMA)’s standard procedure for particles [16]. The validity of this approach to measurement of particle cleanliness has been demonstrated repeatedly [17–19]. Similarly, there are generally accepted IDEMA methods for quantification of extractable anions [20] and cations [21], which are adaptations of widely used ASTM [22] and EPA methods [23,24]. In the disk drive industry, nonvolatile residues are measured using an IDEMA technique [25] based on well-accepted and widely used ASTM standards [26–28]. In general, the selection of methods and controls used in the disk drive industry are based on a military standard [29] that

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has withstood the test of time. Most tests for viable microorganisms are modeled after a well-accepted ASTM test method [30]. Another factor that must be considered is the overall risk of recontamination of parts after they are cleaned in-house. In general, the risk of recontamination by ionic or organic contamination during transportation and assembly is quite low. This is due to several factors, the most important of which is the careful process used to qualify materials used to make the product (e.g., coatings, adhesives [31,32]) and the materials these parts come in contact with during assembly (e.g., gloves [33]). Moreover, the migration of many industries to aqueousbased cleaning has provided a great deal of protection from corrosion due to the high relative solubility in water of most contaminants promoting corrosion, (i.e., ionic contamination). The use of aqueous cleaning chemistries has driven the elimination of cutting fluids and other materials that previously required the use of solvents for cleaning [34,35]. As a consequence, organic residues that are not readily soluble in aqueous detergent cleaning have largely been eliminated. This has driven down the amount of nonvolatile residue. Most precision parts that must be free of bacterial contamination (e.g., spacecraft, medical devices) are designed so that the product can be sterilized after assembly [36,37] using validated processes. 5.11.1

Cleaning Strategies

Two different strategies may be considered: clean–then assemble and assemble–then clean. The clean–then assemble (CTA) strategy is illustrated in Figure 5.24. In this strategy, the responsibility for achievement of the ultimate cleanliness of parts rests with suppliers. This strategy is adopted recognizing that the supplier produces a relatively small number of parts, whereas the customer receives all parts. The supplier is better able than the customer to supply a process customized to the individual piece parts. Parts are shipped by the supplier and received by the customer. Two categories of parts may be described: parts that may be cleaned in-house and parts that must be used as is. During the shipping and receiving process, all of these parts may accumulate new contamination due to packaging and handling. For parts that can be cleaned in-house, the packaging and handling debris can be removed. For parts that must be used as is, this accumulated new packaging and handling contamination passes through to the cleanroom. Parts emerging from the in-house cleaner are also delivered to the cleanroom. Any additional packaging and handling debris are then

Eliminate contamination generated during manufacture

Clean at supplier Ship

Receive

Accumulate new contamination Receive

Clean “in-house”

Use “as is”

Deliver to cleanroom

Assemble

FIGURE 5.24

Accumulate new contamination

Clean–then assemble strategy and its contamination consequences.

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passed through to the assembly process. Finally, parts are subject to assembly. Here additional contamination is likely to be generated. As can be seen from this illustration, three pathways for accumulation of new contamination exist that are not mitigated by the clean–then assemble strategy: ● ●

●

Accumulation by parts to be used as-is parts during shipping and handling Accumulation by parts cleaned in house and parts to be used as is in the subsequent handling and movement within the cleanroom Accumulation by parts due to the assembly process

Looking at this strategy, it is obvious that some parts, such as motors and bearings that contain lubricants, cannot be cleaned by conventional in-house cleaning processes, which usually involve immersion in a bath of liquid. However, there are sets of parts that could possibly be assembled outside the cleanroom and then could be cleaned prior to delivery to the cleanroom for further assembly. This is the assemble–then clean strategy. Figure 5.25 illustrates one possible application of the assemble–then clean strategy. The degree to which an assemble–then clean (ATC) strategy benefits an overall assembly process depends on the relative proportion of the subassemblies that are to be cleaned in house vs. parts that must be used as is and the relative improvement in cleanliness achieved. In a process where all parts must be used as is, there is no benefit to be derived because it cannot be implemented. Conversely, where none of the parts received must be used as is, the maximum benefit can be derived, depending on the ability to qualify the process. In most real-world situations, some portion of the parts can fall into the assemble–then clean strategy. In the assemble–then clean strategy, the process is analyzed to identify those subassemblies that could be assembled outside the cleanroom and subsequently cleaned, eliminating the contamination generated handling and assembly by those steps previously performed in the cleanroom after in-house cleaning.

Eliminate contamination generated during manufacture

Clean at Supplier Ship

Accumulate new contamination

Receive

Assemble

Use “as is”

Clean “in-house”

Deliver to cleanroom

Assemble

FIGURE 5.25 consequences.

Accumulate new contamination

One possible application of the assemble–then clean strategy and its contamination

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Qualification Methods In the course of the study, several different qualification methods have been employed. Cleaning of assemblies can introduce new failure modes of other than cleanliness or dryness degradation. In each case to be reported, careful consideration has been given to the various failure modes that might be introduced by the cleaning process: ● ● ● ●

●

●

The breakaway torque of all mechanical fasteners was measured. The sheer strength of all in use of bonds was measured. Critical dimensional positions of all components were measured. Ionic contamination levels were measured by deionized water extraction and ion chromatography. Volatile organic contamination was measured using witness plates and a combination of FTIR spectroscopy and GC/MS. Particle cleanliness was measured using a liquid particle count following one of two extraction methods: ultrasonic immersion extraction and a detergent–DI water solution or needle spray extraction with pure DI water.

The breakaway torque of all mechanical fasteners was made using instruments capable of measuring the torque, whose gage capability has been shown to be able to measure to the degree specified on the drawing. The pull and shear strengths of all adhesive bonds were measured using Instron mechanical testers, again with gages demonstrated to be capable for the measurement required. Critical dimensional properties were measured using the receiving inspection’s coordinate measurement machines. Ionic contamination levels were measured using standard extraction and measurement techniques. Volatile organic contamination was measured by either extracting the part using a suitable solvent or by placing the part in the chamber containing an absorbent cartridge and subsequently measuring the volatile component using FTIR spectroscopy or GC/MS. All of these measurements were made using the instruments used for either materials qualification or for receiving and inspection of parts. The most important measurement for these studies was the particle content of the parts after assembling the parts. Particle cleanliness was measured in ultrasonic immersion extraction in a 200-ppm solution of Triton X-100 detergent in deionized water, 1-minute extraction, and 40 kHz in a Branson DHA 1000 tank, with a 1-in. coupling fluid depth. Particles were measured by spray extraction using pure DI water (no detergent) at 50  5 psig with approximately 0.7-mm-diameter needle jets. Particle concentrations in the spray extracts were measured after ultrasonic pulse degassing, all using extinction particle count instruments with either 5- or 2-m lower detection or light-scattering optical particle counters with a 0.5-m lower detection limit. Clean–then assemble vs. assemble–then clean case studies are informative. Four case studies are explored in this section: a top cover assembly, a comb seal assembly, a voice coil motor permanent-magnet assembly, and an actuator assembly. This succession represents parts in increasing order of complexity and cleaning challenge. In every one of these evaluations, the ionic contamination and organic contamination were well within specified limits. 5.11.2

Case Studies: CTA and ATC

Case Study 1: Top Cover Assembly The top cover assembly is illustrated in Figure 5.26. The top cover assembly is a very large casting that has relatively few machine features.

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Cast, painted, machined top cover SS, e-polished screws into blind tapped holes SS particle count sample ports and Oring, into through tapped holes

15% PAN carbon fiber-filled, molded PC heat shield

Comb opening

FIGURE 5.26

Top cover assembly.

Unfortunately, during assembly every one of the machine features is used. Two stainless steel particle count sampling ports are driven into their aluminum through-holes in the top cover. A heat shield is fastened to the inside of the top cover assembly using three stainless steel electropolished screws. The heat shield is molded from a highly friable material: 15% polyacrylonitrile carbon fiber–filled polycarbonate. The heat shield is a great concern for this qualification: Experimental tests to select the optimum cleaning process had previously shown that ultrasonic immersion cleaning resulted in significant particle generation and thus was not a suitable cleaning technique. The in-house cleaner proposed for the top cover assembly used ultrasonic immersion cleaning. There was the concern that in-house cleaning of an assembly containing the heat shield would result in a dirtier part. The challenge is then to determine whether ultrasonic immersion cleaning after assemble adversely affects the following: ●

● ●

●

●

Torque of the screws used to attach the top heat shield or the particle count sample ports to the cover. Air leakage through the particle count sampling port seals Retention of water in any of the fastening holes (dryness achieved by forced-hot-air drying). Increase of detergent drag-out due to an adequate rinsing oven assembly that contains an obstruction that prevents rinse water from spraying directly on a portion of the part. Erosion of particles from the carbon-filled heat shield.

Top Cover Assembly Results ●

● ●

Screw torque for the top cover heat shield and particle count sampling ports was unaffected. No increase in air leakage around the particle count sample ports was measurable. The existing forced-hot-air dryer for the top cover part using the original cleaner basket was acceptable for drying the top cover assembly.

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TABLE 5.3 Liquid-Borne Particle Counts After Spray Extraction of Top Cover Assemblies Using Two Cleaning Strategies Particle Size (m)

Clean–Then Assemble

5 9 15 25 50

● ●

8,497 5,214 2,402 961 117

Assemble–Then Clean 2,775 1,620 956 238 25

Detergent drag-out was not increased using the existing top cover cleaner basket. Particle cleanliness (50-psig needle jet spray extraction with DI water, followed by a liquid-borne particle count) was improved significantly by the assemble–then clean vs. the clean–then assemble strategy (see Table 5.3).

Case Study 2: Comb Assembly A second case study is a comb assembly that fits into an opening in the top cover assembly. All of the components had previously been shown to be acceptable for ultrasonic immersion cleaning. In addition, the finished part could be extracted by ultrasonic immersion. Figure 5.27 shows a comb assembly consisting of a partially machined electrophoretically painted aluminum casting. An elastomeric seal is held in place by an e-coated aluminum part using nine electropolished stainless steel screws. Because of the thickness of the part, the screw holes are not blind holes. Assembly debris generated by driving the screws in the holes into the comb could become a significant contaminant.

Elastomeric seal

Comb, e-painted and machined

Electropolished stainless steel screws

9 drilled and tapped screw holes in comb (not shown)

FIGURE 5.27

Retainer, e-coated aluminum, 9 through screw holes (not shown)

Comb assembly.

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TABLE 5.4 LPC of the Comb Assembly Particle Size (m) 5 9 15 25 50

Clean–Then Assemble

Assemble–Then Clean

2,541 980 402 61 7

1,192 624 285 38 5

The challenge for the comb assembly includes the following concerns: ●

●

●

●

●

There are many screws. The torque of each screw must be measured individually because each screw’s position on the assembly may render it susceptible to ultrasonic energy that reduces the fastening strength needed for its position. A large portion of the elastomeric seal that is unsupported (more than 1  20 cm) is subject to deformation. The dryness of the subassembly, where a layer of elastomeric seal is sandwiched between two layers of electrophoretic painted parts is very important. Detergent drag-out is critical because this part will end up assembled in the final disk drive within 1 hour after cleaning. Particle cleanliness is critical.

Comb Assembly Results ● ● ● ●

Screw torque was unaffected. Elastomeric seal was not distorted. Forced-hot-air drying was effective; the existing cleaner basket was acceptable. Particle cleanliness (40 kHz, ultrasonic immersion extraction in 200-ppm detergent– DI water, followed by a liquid-borne particle count) was improved (see Table 5.4).

Case Study 3: Voice Coil Motor Assembly A more challenging assembly to qualify using the assemble–then clean strategy is the voice coil motor magnet assembly. This consists of a large number of parts assembled using adhesives or press fit parts but no screwed fasteners. Voids in the adhesives could result in pockets that retain either moisture or detergent. The voice coil motor assembly contains magnetic materials, adhesives, elastomeric materials, electroless nickel-coated parts, springs that must maintain tension, and molded plastic parts that must maintain position. The magnets must retain their magnetic force (after forced-hot-air drying). Similar to all the other parts, this subassembly must be cleanable using an existing cleaning process to minimize capital equipment cost. Figures 5.28 to 5.31 illustrate the complexity of the voice coil motor magnet assembly. VCMA Results ● ● ●

Adhesive bonds were unaffected. Position of press-fit components were unaffected. Forced-hot-air drying was effective; the existing cleaner basket was acceptable.

DETAILS ON THE CLEAN–THEN ASSEMBLE AND ASSEMBLE–THEN CLEAN PROCEDURES

Magnets (4)

259

Stops (2)

Backing plates (3)

Spacer Blocks (4)

FIGURE 5.28 Front view of the voice coil motor magnet assembly showing four nickel-coated magnets, four nickel-coated spacers, three nickel-plated backing plates, and two molded polyimide stops.

Retainer springs (2) one not visible

Latch magnet

Yoke

Housings (2)

FIGURE 5.29 Partial rear view of the voice coil motor magnet assembly showing two nickel-coated springs, a latch magnet and yoke, and two molded polyimide housings.

●

Particle cleanliness (40 kHz, ultrasonic immersion extraction in 200-ppm detergent–DI water, followed by a liquid-borne particle count) was improved (see Table 5.5).

Case Study 4: Head Stack Assembly The head stack assembly is the most complicated part tested in this study. The motive for testing is to determine the effect of cleaning after rework, which may be thought of as an assemble–then clean process. For this comparison, reworked–but not cleaned assemblies were compared to reworked–then cleaned assemblies.

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Latch magnet retainer

Housings

2nd retainer spring

FIGURE 5.30 Partial rear view of the voice coil motor magnet showing a second retainer spring and a bare steel latch magnet retainer spring. The two housings are shown in Figure 5.27.

FIGURE 5.31 Front view of the voice coil motor magnet showing the locations of the 12 adhesive bonds fastening the spacer blocks, magnets, and backing plates. All other components are press-fit.

Figures 5.32 and 5.33 show the complexity of the head stack assembly. In these figures emphasis in on the rework operations rather than on a detailed list of the many materials present. Figure 5.34 shows the arrangement of the head stack assembly in the measurement beaker for ultrasonic particle extraction. It is known that 40-kHz ultrasonic cleaning can damage the delicate flexure assembly, head attachment adhesive, and wire bonds for the magnetic recording heads. However, this portion of the he ad suspension is not touched during rework. Thus, the liquid level in the beaker is adjusted to immerse up to and including the swage hole and all other portions of the actuator assembly touched during rework.

DETAILS ON THE CLEAN–THEN ASSEMBLE AND ASSEMBLE–THEN CLEAN PROCEDURES

TABLE 5.5 LPC of Voice Coil Motor Magnet Assembly Clean–Then Assemble

Assemble–Then Clean

Particle Size (m)

Mean

Mean  4.5

Mean

Mean  4.5

2 5 10 15 25 50

39,827 24,506 12,344 4,845 1,230 355

101,354 67,819 43,000 12,704 3,425 1,067

12,345 8,223 4,530 1,121 345 92

25,478 16,454 12,003 2,311 569 245

Remove Retainer Screws (top and bottom) Remove Cartridge Bearing

Remove Cartridge Bearing Screw

FIGURE 5.32

First portion of the head stack assembly rework operations.

Remove and Replace Wire-Tack Adhesive

Remove Conformal Coat, Debond Wire, Bond New Wires, Recoat

Deswage, Replace HGA, Reswage

FIGURE 5.33

Second portion of the head stack assembly rework operations.

261

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

Beaker

Liquid level

Damage-sensitive heads, out of solution Swage holes, primary rework location

Wire tack adhesive, primary rework location

Bearing lock screw, secondary rework location

Bond pads, primary rework location

Actuator bearing hole, secondary rework location

Bracket screws, secondary rework location (top and bottom)

FIGURE 5.34

Extraction diagram for actuator assemblies.

TABLE 5.6 LPC of the Actuator Assembly Assemble (Rework) Particle Size (m) 0.5 2 5 10 15 25

Assemble (Rework)–Then Clean

Mean

Mean  4.5

Mean

Mean  4.5

116,900 35,630 21,120 10,220 3,455 823

489,500 122,400 89,230 34,450 11,209 1,650

23,040 11,020 8,056 3,045 1,122 433

54,600 23,400 19,120 7,776 2,307 745

Head Stack Assembly Results ● ● ● ●

Screw torque was unaffected. Adhesives and coating were unaffected. Force hot air/vacuum drying was effective; the existing cleaner basket were acceptable. Particle cleanliness (40 kHz, ultrasonic immersion extraction in 200-ppm detergent–DI water, followed by a liquid-borne particle count) was improved significantly (see Table 5.6).

5.11.3

Case Study Results and Discussion

All four of the case studies described here demonstrate the feasibility of the cleaning strategy assemble–then clean. In the cases of the top cover assembly and the comb assembly, only the mean value of the qualification trials is shown. The reliability of disk drives and other precision mechanical and electromechanical components is a statistical phenomenon. To the extent that cleanliness affects reliability, it is perhaps more important to know about the variability of the cleanliness of parts. For this reason, the mean plus 4.5 cleanliness data are included for the voice coil motor magnet assembly and the actuator assembly.

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TABLE 5.7 Particle Cleanliness Improvement Factors Afforded by Assemble–Then Clean vs. Clean–Then Assemble Particle Size (m) 0.5 2 5 9 10 15 25 50

VCMA

Actuator

Cover Mean

Comb Mean

Mean

Mean  4.5

Mean

Mean  4.5

— — 3.1 3.2 — 2.5 4.0 4.7

— — 2.1 1.6 — 1.4 1.6 1.4

— 3.2 3.0 — 2.7 4.3 3.6 3.9

— 4.0 4.4 — 3.6 5.5 6.0 4.4

5.1 3.2 2.6 — 3.4 3.1 1.9 —

9.0 5.2 4.7 — 4.4 4.9 2.2 —

The overall improvement in cleanliness afforded by the assemble–then clean strategy vs. the clean–then assemble strategy may be estimated by dividing the cumulative concentration at each size for the clean–then assemble approach by the corresponding result for the assemble–then clean approach, summarized in Table 5.7. The data in the table show that there is a significant improvement in mean particle cleanliness for all particle size ranges over all of the four types of assemblies examined. For the VCMA and actuator assemblies, there is an even greater improvement in statistical cleanliness over all size ranges, based on the higher improvement ratio for the mean plus 4.5 versus the mean value improvement ratio. The implications for the manufacturing processes are significant. The benefits include reduction in the square footage of cleanroom floor space. In most precision assembly cleanrooms, floor space costs from $300 to $500 per square foot more for acquisition than factory floor space. Operating costs for cleanrooms range from $30 to $50 per square foot per year for FED-STD-209 class 100 clean space (ISO 14644 class 5). Looking at each individual subassembly reveals cost savings associated with handling and cleaning. Top Cover Assembly Cleaning the individual pieces required one basket per top cover, one small basket for particle count ports, one small basket for seals, one large basket for heat shields, and one small basket for screws. Elimination of cleaner baskets for the smaller parts resulted in a fractional reduction in the number of baskets going through the cleaner. It did result in much less material handing, resulting in significant labor savings. Cleaning baskets could be eliminated since the top cover assembly could be cleaned and dried effectively using the existing top cover cleaner basket and cleaning machine. Comb Assembly Cleaning the individual parts required one basket for combs, one small basket for seals, one small basket for retainers, and one small basket for screws. Cleaning the assembly reduced the volume of baskets going through the cleaner by half and eliminated labor cost and handling damage. The existing cleaner basket for the comb could be used effectively for cleaning the comb assembly. Voice Coil Motor Permanent-Magnet Assembly There was an increased cost because a new cleaner basket had to be designed for the VCMA assemblies. However, there were offsetting costs due to reduction in the number of individual parts going through the cleaner. For each basket containing VCMA assemblies there had previously been approximately

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

three times as many baskets to clean the 20 individual parts cleaned previously. Thus, the major savings was a reduction in labor cost and handling, Actuator Assembly There were no significant savings by implementation of the rework cleaner. The improvement in particle cleanliness of the reworked actuators was seen as a benefit that drove decisions about the process. Cleaning after assembly can be accomplished using conventional DI water cleaners. In most cases, no new cleaner baskets must be designed, fewer individual parts are handled, and fewer numbers of cleaner baskets are required. This results in an effective increase in the capacity of the cleaners. Dryness is acceptable using existing process equipment and times. Dimensions and locations of parts were unaffected. Screw torque and adhesive bonds were unaffected. Cleanroom floor space and operating costs are reduced. Finally, the finished assemblies parts come out cleaner from a particle perspective than in the clean–then assemble approach.

5.12

PARTICLE SIZE DISTRIBUTIONS

Particle contamination is an important source of yield loss and field failures, despite the recent moves toward cleanrooms with better than class 100 performance. This is true because particle contamination can be produced by consumable supplies and packaging materials, can originate on incoming direct and indirect material, can be the result of improper cleaning and drying processes, or can be generated in situ by tooling, process equipment, material handling, and assembly. None of these sources can be adequately abated by the cleanroom or predicted by airborne particle counting used to verify cleanroom performance. A method of quantifying particles on surfaces relied on the particle size distribution is described in MIL-STD-1246. In many cases, failure mechanisms are depend on particle size. Thus, the particle size distribution, and the materials, surface treatments, and processes that affect particle size distribution may be critical to know and control. 5.12.1

MIL-STD-1246

MIL-STD-1246 is a method for specifying surface-borne particle contamination. It defines a geometric particle size distribution, where the log of the cumulative particle count, in particles per unit surface area, is plotted vs. the square of the log of the size in micrometers. The equation for this relationship is log N(x)  0.926 (log2xi  log2x) where N(x) is the number of particles per unit surface area equal to or larger than x, xi the class level, and x the particle size in micrometers. There are two ways this can be measured: directly and indirectly. Direct measurements can be made for specularly reflective surfaces such as silicon wafers, the air bearing surface of magnetic recording heads, glass surfaces of flat-panel displays, or electropolished stainless steel. These types of surfaces can be inspected visually, often using a microscope. However, the majority of surfaces are not suitable for direct inspection and must be inspected indirectly. Indirect methods remove the particles from the surface, followed by particle counting. Historically, particles were trapped on filters and inspected using microscopes.

PARTICLE SIZE DISTRIBUTIONS

265

The need for indirect inspection for the majority of surfaces was realized by IBM in the 1980s. IBM developed several methods for indirect inspection of surfaces for particle contamination, including tape lift followed by microscopy or densitometry, and undulation, ultrasonic, or spray extraction, followed by the use of a turbidimeter or liquid-borne particle counter (LPC). Part of this development effort resulted in an early attempt to correlate indirect contamination measurements to MIL-STD-1246 [38], examining ultrasonically extracted particles from plastic parts. Reasonable correlation was found. Subsequently, it was found that the cavitation erosion of erosion-sensitive plastic and metal alloys significantly affected the correlation between real particle size distributions and the model used for MIL-STD-1246, through the production of large wear particles [39]. The result is that particles extracted from eroded parts follow the MIL-STD-1246 distribution more closely, but at the expense of a relative increase of large particles on the parts vs. their as-received counterparts. Further study showed that wear-induced large particle production was not a problem for high-pressure-spray (3000 psig) water cleaning [12]. This resulted in significant deviation of the particle size distribution from MIL-STD-1246, in that there were far fewer large particles than predicted. Clearly, the cleaning process history of the part can have an effect on the accuracy of the MIL-STD-1246 particle size distribution in describing and specifying acceptable particle cleanliness. Finally, a study was reported whose aim was a direct comparison of size distributions of particles extracted from parts and measured using a LPC [18]. This study supported the earlier observations and went one step further. Not only does the prior history of the part have an effect on the particle size distribution, but the method of removing the particles from the surface also affects the size distribution. 5.12.2

Analytical Methods

Materials were extracted according to the required procedure. The resulting particle in water suspensions were analyzed using three HIAC-ROYCO Model 8000 particle counters, sampling through model ASAP samplers and counted using model HRLD-150 sensors. The ASAP sampler is convenient to use and allows samples to be analyzed rapidly. The HRLD-150 sensor is an extinction-based sensor, resolving the particle size range from 2 to 150 m. Three samples of each material were sampled a minimum of two times for each of the three particle counters. The data reported are the mean of the three samples counted with three particle counters. As shown in several previous publications, specifications should be based on a calculation considering both the average and the variability of performance [40]. However, in the study reported here, insufficient parts were examined to get a realistic estimate of variability. Therefore, only average values are reported. One method of comparing the MIL-STD-1246 size distribution with any measured distribution is to normalize the data. This process eliminates the orders-of-magnitude difference between very pure samples, such as filtered DI water, vs. particles from very dirty parts that have been extracted in their as-received condition. Normalization is done by dividing the particle concentration at each size by the concentrations at the smallest particle size, which for these data is 2 m. Data can then be plotted on a single graph for groups of comparable parts to determine the effects of materials variation on the resulting size distribution. Another way of comparing is to look at the slope of the plotted distribution, which when normalized to a starting value of 1.0 has a negative slope. The slope of the MIL-STD-1246 distribution is 0.926. Normalized distributions from real parts having slopes greater than 0.926 have fewer large particles than predicted by

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

MIL-STD-1246. Normalized distributions with slopes smaller than 0.926 have larger numbers of large particles than predicted by MIL-STD-1246. After transforming the data, linearity was checked by regression analysis. 5.12.3

Extraction Methods Tested

Several different materials, surface treatments, and prior cleaning histories were tested to determine their effect on agreement between the particle size distributions measured using LPC and the MIL-STD-1246 particle size distribution. In many cases, materials tested, already had industry-accepted extraction methods, such as IEST recommended practices. Where no industry-accepted extraction method was available, an extraction method from related material was used. The extraction methods were undulation, ultrasonic, and low-pressure spray. In undulation, the parts are immersed in a filtered DI water bath containing 0.02% surfactant. They are then undulated in an orbital shaker at 120 rpm for 10 minutes. Following this, the parts are removed from the bath and excess liquid is allowed to drain back into the bath. The resulting suspension is degassed by pulsing the ultrasonic power on and off until the suspension stops effervescing. The suspension is then analyzed by LPC. In ultrasonic extraction, the parts are placed in a beaker containing 5 to 10 mL/cm2 of surface area of DI water plus 0.02% surfactant. The beaker is then placed in a 40-kHz ultrasonic tank operating at about 150 W gall and agitated ultrasonically for 1 minute. The parts are removed and the suspension is analyzed by LPC. In spray extraction, pure 0.45-m filtered DI water is sprayed on the part through a 750-m orifice at 45 to 55 psig. This produces a well-collimated stream of water moving at over 10 m/s. The spray pattern for each part is described precisely in an attempt to minimize operator variability on the outcome of the test. After extraction, the suspension is degassed ultrasonically and counted using the LPC. Different materials and their prior treatments should be grouped according to their extraction method, because the extraction method can have a very profound effect on the resulting size distribution. Consumable supplies are generally extracted using undulation following accepted industry standards such as IEST RP5. Spray extraction is generally used for parts that are so large that excessive dilution will occur for ultrasonic immersion. Parts that suffer excessive ultrasonic cavitation erosion are also spray extracted. The effect of cavitation is well documented. Most parts can be extracted using ultrasonic immersion. 5.12.4

Results

We describe the results in the following sections. In every test, the correlation coefficient R2 in the regression analysis was at least 0.94. This indicates that the assumptions in the MIL-STD-1246 model that the log of the concentration is linearly proportional to the log of the square of the particle size are reasonable. Results are discussed as a function of the extraction method. Undulation Extraction Undulation has become the extraction method of choice for a large number of consumable supplies. This includes gloves, packaging film bags, and swabs. Wipers were also evaluated using undulation to provide another test point. The material, condition, and slope of the transformed particle size distribution are shown in Table 5.8 for materials extracted using undulation. The undulated samples in general have slopes in fair

PARTICLE SIZE DISTRIBUTIONS

267

TABLE 5.8 Slope of Transformed Particle Size Distributions for Materials Extracted Using Undulation Followed by LPC Analysis Material MIL-STD-1246 Nylon film bag Polyethylene film bag Metallized ESD bag Polyester knitted wiper Polyester knitted swab Natural rubber latex glove

Condition

Slope

Specification As received As received As received As received As received As received

0.926 0.888 0.953 0.837 1.431 0.877 1.064

1246 Nylon Bag Log Concentration per Unit Area

1 Polyethylene Bag

0 100

-1 0

ESD Bag

-2 -3

Polyester Swab

-4 NRL Glove

-5 -6 Particle size (μm)

FIGURE 5.35

Polyester Wipe

Consumable supplies.

agreement with MIL-STD-1246, indicating it may be an acceptable model for establishing particle count limit, with the exception of the polyester knitted wiper. This is shown in Figure 5.35. The polyester knitted wiper has far fewer large particles than predicted by the standard. It may be appropriate, using the principles of establishing the acceptance criteria based on process capability, to specify fewer large particles than allowed on the basis of MIL-STD-1246 for the wipers in this case. Different 1246 levels may be chosen for small particles and for large particles. It is interesting to note that all of these materials are polymers. Spray Extraction Three different parts were evaluated using spray extraction. These are parts that exhibit extreme sensitivity to erosion by ultrasonic cavitation or are so large that immersion in an ultrasonic bath sufficient to contain them would result in excessive dilution of particle counts. Part A is an extremely complex aluminum casting made of a relatively soft alloy. The part is epoxy coated and then machined, exposing bare aluminum. Part B is also a very large cast aluminum part, made of the same alloy as part A. However, part B did not require machining after epoxy coating, so no bare aluminum is exposed. Part C is a fully epoxy-coated aluminum part nearly the shape of a magnetic recording disk. Because of its

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

TABLE 5.9 Slope of Transformed Particle Size Distributions for Materials Extracted Using Spray Followed by LPC Analysis Material

Condition

Cast soft aluminum alloy, epoxy coated and partly machined (part A) Part A Part A Cast aluminum, epoxy coated (part B) Part B Part B 15% Carbon fiber–filled polycarbonate (part C) Part C

Slope

Ultrasonically cleaned

1.182

Ultrasonically cleaned twice High-pressure-spray cleaned Low-pressure-spray cleaned

1.040 1.541 1.310

Ultrasonically cleaned High-pressure-spray cleaned Spin-rinse dried

1.238 1.455 1.886

Ultrasonically cleaned

0.994

1 Log Concentration per Unit Area

1246 -1 0

100 US Cleaned

-3 US Cleaned 2X

-5

-7

HPS Particle size (μm)

FIGURE 5.36

Cleaned

Effect of the cleaning process on part A.

rotational symmetry, it can be cleaned using a spin-rinse dryer. Table 5.9 lists materials, prior history, and slope of the transformed curve using the MIL-STD-1246 approach. One-time ultrasonic cleaning of part A results in a slope slightly lower than that of MILSTD-1246. However, two-time ultrasonic cleaning of the part results in the production of additional large particles, which raises the slope of the distribution as shown in Figure 5.36. Low-pressure spray cleaning of part B results in a slope lower than that of MIL-STD-1246. Ultrasonic cleaning raises the slope, but not to the extent of part A, as shown in Figure 5.37. The entirely epoxy-coated part B resists cavitation better than part A, which is partially exposed aluminum. Part C exhibits extreme ultrasonic erosion sensitivity, as shown in Figure 5.38. Ultrasonic Extraction Because of the size of the database, ultrasonic extraction is perhaps the most interesting extraction method from a materials perspective. It is divided into several different groups illustrating how variations in surface treatments and prior history

PARTICLE SIZE DISTRIBUTIONS

269

1

Log Concentration per Unit Area

1246 100

-1 0

LPS -3

Cleaned US

-5

Cleaned HPS

-7

Cleaned Particle size (μm)

FIGURE 5.37

Effect of the cleaning process on part B.

Log Concentration per Unit Area

1 -1 0

1246

100

-3

Spin-Rinse Dryer

-5 US Clean -7 -9 Particle size (μm)

FIGURE 5.38

Effect of the cleaning process on part C.

TABLE 5.10 Slope of Transformed Particle Size Distributions for Materials Extracted Using Ultrasonic Immersion Followed by LPC Analysis Material Bare T4 hardness aluminum extruded and machined (part D) Part D Part D Part D with 15% SiC

Condition

Slope

Bare

1.347

NiP-coated Alkaline-etched Bare

1.540 1.698 1.822

affect particle contamination levels. Table 5.10 lists an extruded and machined aluminum part in T4 hardness. This part is examined as an uncoated part, as a part coated with nickel phosphide (electroless nickel) etched in strong alkali solution, and as an uncoated alloy containing 15% silicon carbide fibers. All of these had been washed using a conventional

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

1246

Log Concetration per Unit Area

1 -1

Bare 0

100

-3

NiP Coated

-5 Alkaline -7

Etch

-9

SiC Filled Particle size (μm)

FIGURE 5.39

Effect of surface treatment on part D.

TABLE 5.11 Slope of Transformed Particle Size Distributions for Materials Extracted Ultrasonically, Followed by LPC Analysis Material T4 hardness, extruded and machined aluminum alloy (part E) Part E Part E

Condition

Slope

Electropolished

1.432

Tuffram-coated Nedox-coated

1.680 1.840

ultrasonic cleaner prior to testing. All conditions for this part result in a slope greater than MIL-STD-1246. The NiP-coated, alkaline-etched, and SiC metal matrix treatments all increase the hardness of the surface, making the part less erosion sensitive, as shown in Figure 5.39. The next part was an extruded and machined aluminum alloy in T5 hardness. Three different versions were tested. Parts were either electropolished, coated with Tuffram, or coated with Nedox. Both are proprietary coating from General Magnaplate. Table 5.11 summarizes the slope of transformed particle size distributions for ultrasonically extracted materials followed by LPC analysis. All of the parts tested have slopes higher than MIL-STD-1246. Tuffram is harder than bare aluminum, and Nedox is harder than Tuffram. As the hardness of the coating increases, the degree of erosion sensitivity decreased, resulting in smaller numbers of erosion-produced large particles, as shown in Figure 5.40. Process Effects This technique may also be used to evaluate various processes. For example, it can be used to determine the effect of alternative assembly procedures. One example is the option to clean all of the individual piece parts, then assemble. The alternative is to assemble and then clean. The latter may be a desirable alternative, since if the assembly operation can be done outside the cleanroom, valuable cleanroom floor space can be reserved for other operations. Another example is to measure the parts before and after a particular operation, in this case a vibratory bowl feeding operation.

PARTICLE SIZE DISTRIBUTIONS

271

Log Concetration per Unit Area

1 1246 -1 0

100 Electropolished

-3 -5

Tuffram -7 Nedox

-9 Particle size (μm)

FIGURE 5.40

Effect of surface treatment on part E.

TABLE 5.12 Slope of Transformed Particle Size Distributions for Materials Extracted Using Ultrasonic Immersion Followed by LPC Analysis Material

Condition

Electropolished stainless steel (part F) Part F 302 Stainless machined part (part G) Part G Subassembly (part H) Part H

After cleaning

2.306

After vibratory bowl feeder After cleaning

2.450 1.238

After surfactant treatment After cleaning After shipping

1.111 1.117 0.976

1246

Log Concentration per Unit Area

0 -2

Slope

0

100 After Clean

-4 -6

After Vibratory Bowl Feeder

-8 -10 Particle size (μm)

FIGURE 5.41

Effect of the process on part F.

Table 5.12 summarizes how the analysis method can reveal how processes may result in undesirable production of large particles. Part F has a noticeable increase in the number of small particles after 8 hours of exposure to a vibratory bowl feeder, as shown in Figure 5.41. Part G has a noticeable increase in the number of large particles after surfactant treatment

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GETTING CLEAN PARTS AND GETTING PARTS CLEAN

Log Concentration per Unit Area

1 0

1246

-1 0

100 After Clean

-2 -3

After Surfacting

-4 -5 Particle size (μm)

FIGURE 5.42

Effect of the process on part G.

Log Concentration per Unit Area

1 0

1246

-1 0

100 After Clean

-2 -3

After Shipping

-4 -5 Particle size (μm)

FIGURE 5.43

Effect of the process on part H.

to reduce galling (Figure 5.42). Part H shows an increase in large particles after shipment in a polyethylene vacuum-formed shipping tray, shown in Figure 5.43. Each of these is an example that illustrates how particle size–dependent failure mechanisms can be detected by transforming particle data using the approach of MIL-STD-1246. MIL-STD-1246 provides a method for analyzing size distributions of particles extracted from the surface of parts. Transforming the size distribution data to a log of concentration per unit area versus log of the particle size square provides a nearly linear plot. Regression analysis can then provide an indication of the relative proportion of large versus small particles, which may be critical in the determination of size-dependent failure mechanisms.

5.13

TOOL PART CLEANLINESS

One of the often neglected aspects of tool maintenance is specification of the cleanliness of tool parts that need periodic cleaning. Parts that typically require periodic cleaning include shields and baffles, pallets and substrate holders, and other components found within the

REFERENCES AND NOTES

273

process chambers. One factor that has hindered the application of quantitative surface cleanliness specifications has been the separation between tool designers and cleaning processing engineering. Because of this separation, it was not recognized by tool designers that appropriate cleanliness measurement techniques were in existence. A second reason was that many companies attempted to clean the tooling parts in-house and did not provide adequate resources for cleanliness process development, including the failure to provide objective cleanliness measurement methods. Today, many companies contract out their process tool component cleaning [41]. The contaminants being removed in tool cleaning are generated in the process itself and consist largely of deposited metals or organic residues such as photoresist. Many processes are needed to remove these contaminants because of the wide range of physical properties they exhibit. Among these processes are abrasive slurry blasting, dry bead blasting, and CO2 snow cleaning for removal of bulk contamination. Once bulk contaminants are removed, various chemical cleaning processes can be utilized. These include cleaning using piranha (H2SO4 : H2O2), dilute acids and bases, and solvents such as acetone, normal methyl pyrrolidone (NMP), and others. Following cleaning, residues are removed by rinsing with copious quantities of water, followed by various drying processes [42].

REFERENCES AND NOTES 1. IEST-STD-CC1246d, Product Cleanliness Levels and Contamination Control Program. 2. R. Nagarajan, Guidelines for design of machining processes to eliminate solvent cleaning, Proceedings of the 2nd Annual CFC Elimination Conference, Summers, NY, 1990, pp. 510–517. 3. A. D. Zimon, Adhesion of Dust and Powder, 2nd ed., translated by R. K. Johnston, Consultants Bureau, Plenum Press, New York, 1982. 4. A. Robinson and N. Johnson, Ship shape, CleanTech, July–Aug. 2002, pp. 20–23. 5. M. Tate, Cleaning printed circuit boards, Advancing Applications in Contamination Control, 1999, pp. 15–17. 6. G. Schultz, Water: the deadly intruder in microelectronic packaging, Journal of Microcontamination Detection and Control, 1998, pp. 17–20. 7. K. Rayl, Extraction cleaning: the new approach to circuit board processing, Precision Cleaning, 1997, pp. 21–25. 8. D. J. Riley and R. G. Carbonell, Investigating liquid-based particle deposition and the effects of double-layer interactions using hydrophobic silicon wafer, Microcontamination, Dec. 1990, pp. 19–25, 60–61. 9. I. Ali and S. Raghavan, Measuring electrokinetic characteristics of positive photoresist particles, Microcontamination, 8(3):35–37, 58, 1990. 10. D. H. McQueen, Frequency dependence of ultrasonic cleaning, Ultrasonics, 24:273–280, 1986. 11. R. P. Musselman and T. W. Yarbrough, Sheer stress cleaning for surface departiculation, Journal of Environmental Sciences, 1987, pp. 51–56. 12. R. Nagarajan and R. W. Welker, Precision cleaning in a production environment with high-pressure water, Journal of the Institute of Environmental Sciences, July–Aug. 1992, pp. 34–44. 13. J. W. Butterbaugh, S. Loper, and G. Thomas, Enhancing yield through argon/nitrogen cryokinetic aerosol cleaning after via processing, 17(6):33–43, 1999. 14. R. Nagarajan, Aqueous cleaning and vacuum drying of hard to dry parts: a process alternative to the use of solvents, Microcontamination Conference Proceedings, San Jose, CA, Sept. 21–23, 1993, pp. 473–480.

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15. R. Nagarajan, Vendor process contamination checklist: a diagnostic and troubleshooting tool, Journal of the Institute of Environmental Sciences, 1991, pp. 45–49. 16. IDEMA Standard M9-98, Particulate Contamination Test Methods for Hard Disk Drive Components. 17. R. Nagarajan and R. W. Welker, Size distributions of particles extracted from disk drive parts: comparison with the MIL-STD-1246 distribution, Journal of the Institute of Environmental Sciences, Jan.–Feb. 1993, pp. 43–48. 18. R. W. Welker, Size distributions of particles extracted from different materials compared with the MIL-STD-1246 particle size distribution, Journal of the Institute of Environmental Sciences and Technology, 43(4):25–31 (2000). 19. R. W. Welker, Gage capabilities: the first step in achieving capable manufacturing processes, ISMSS ’92, San Francisco, CA, June 15–17, 1992. 20. IDEMA Standard M13-99, Measurement of Extractable/Leachable Anion Contamination Levels on Drive Components by Ion Chromatography (IC). 21. IDEMA Standard M12-99, Measurement of Extractable/Leachable Cation Contamination Levels on Drive Components by Ion Chromatography. 22. ASTM D4327-91, Standard Test Method for Anions in Water by Chemically Suppressed Ion Chromatography, Vol. 11.01. 23. USEPA Method 300.1, Determination of Inorganic Anions in Drinking Water by Ion Chromatography. 24. USEPA Method 300.7, Determination of Inorganic Cations in Drinking Water by Ion Chromatography. 25. IDEMA Standard M7-98, Organic Contamination as Nonvolatile Residue (NVR). 26. ASTM E595, Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment. 27. ASTM E1235-88, Standard Test Method for Gravimetric Determination of Nonvolatile Residue (NVR) in Environmentally Controlled Areas for Spacecraft. 28. ASTM D4526-85(1991), Standard Practice for Determination of Volatiles in Polymers by Headspace GC. 29. MIL-STD-1246C, Product Cleanliness Levels and Contamination Control Programs. 30. ASTM F488-95, Standard Test Method for On-site Screening of Heterotrophic Bacteria in Water. 31. IDEMA Standard M2-98, Materials Used in Hard Disk Drives. 32. IDEMA Standard M6-98, Environmental Testing for Corrosion Resistance and for Component Compatibility. 33. R. W. Welker and P. G. Lehman, Using contamination and ESD tests to qualify and certify cleanroom gloves (first in a series), Micro, May 1999, pp. 47–51. 34. R. Nagarajan, R. W. Welker, and R. L. Weaver, Evaluation of aqueous cleaning techniques for disk drive parts, Microcontamination Conference Proceedings, San Jose, CA, Oct. 16–18, 1991, pp. 312–326. 35. R. Nagarajan, Design for cleanability, in Supercritical Fluid Cleaning: Fundamentals, Technology and Applications, J. McHardy and S. P. Sawan, eds., Noyes Publications, Park Ridge, NJ, 1998, pp. 38–69. 36. J. Barengoltz, Mars Global Surveyor: Planetary Protection Plan, JPL D-12742, Jet Propulsion Laboratory, Pasadena, CA, 1995. 37. ISO 11137, Sterilization of Healthcare Products: Requirements for Validation and Routine Control—Radiation Sterilization. 38. R. Coplen, R. L. Weaver, and R. W. Welker, Correlation of ASTM F312 particle counting with liquidborne optical particle counting, Proceedings of the 34th Annual Technical Meeting of the Institute of Environmental Sciences, King of Prussia, PA, May 3–5, 1998, pp. 390–394.

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39. R. Nagarajan, Cavitation erosion of substrates in disk drive component cleaning: an exploratory study, Wear, 1992, pp. 75–89. 40. R. W. Welker and R. Nagarajan, Getting clean parts and getting parts clean, tutorial first presented at Microcontamination ’91, San Jose, CA, Oct. 1991. 41. R. Bruns, D. Zuck, and W. Warner, Measuring tool part cleanliness and its effect on process performance, Micro, May 2002, pp. 21–38. 42. D. Zuck and K. Macura, Environmentally compatible advances in semiconductor tool part cleaning, Advances A2C2, 2001, pp. 9–13.

CHAPTER 6

TOOLING DESIGN AND CERTIFICATION

6.1

INTRODUCTION

Contamination and ESD requirements for the design, development, and manufacture of high-technology products such as disk drives, flat-panel displays, and semiconductors have changed substantially in the four decades of these high-technology products. This is true because the performance, design, and materials of these and other high-technology products have evolved substantially. For example, disk drives routinely read and write at more than 1 gigabyte per square inch, flat-panel displays are growing, so that panels larger than 17 in. are considered commonplace, and state-of-the-art semiconductor design rules are now below 0.25m, requiring the use of special photoresists that are sensitive to vapor-phase chemical contamination. In lockstep with this evolution, the tolerance of high-technology products to quantities and types of contamination has decreased. As manufacturers seek higher yields to ensure competitiveness, the concern about contamination and ESD has also increased. The tolerances of high-technology products and the need for high-volume production facilities make it necessary to incorporate more and more automation into the fabrication and assembly processes. The need for high-volume manufacture of low-cost components, as in the manufacture of silicon wafers or rigid recording media, places further emphasis on the increased use of automated tooling. This combination of decreased tolerance of hightechnology products for contamination and ESD, the increased use of automation in manufacturing, and the need to maximize yield underscores the need for a formal approach to the design and certification of tooling. This is the subject of this chapter. Cleanrooms are environments in which conditions, including contamination levels, are controlled to specified limits. The selection of conditions and contamination limits are not

Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

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arbitrary but are dictated by the needs of the products or processes in the cleanroom. Requirements may include controls on: ● ● ● ● ● ●

Airborne particle concentration Air velocity, direction, temperature, and relative humidity Organic, ionic, and magnetic contamination Electrostatic discharge protection Surface cleanliness, including biological activity Process fluid cleanliness

The objective of tooling design for cleanroom use is to provide tools that do not violate the requirements of the cleanroom in which they are to be used. The purpose of this chapter is to help the engineer, designer, and tool fabricator to achieve successful designs for cleanroom tools. To accomplish this, we do the following: ● ● ● ● ●

Define the limits that tooling may need to satisfy. Guide the selection of materials, surface treatments, and finishes. Guide the selection of components. Guide the workstation layouts. Show how to demonstrate conformance to requirements (certification and verification).

6.1.1

Tooling Design Process

A seven-step procedure can be followed to design a tool that satisfies all contamination and ESD requirements. This procedure may be applied at the material, surface treatment, component, subassembly, and complete tool levels. Step 1: Identify the Needs and Define the Requirements This is by far the most important step, as starting out with poorly or undefined requirements is sure to lead to costly delays due to redesign, program schedule slippage, or at worst, complete project failure. Step 2: Identify the Alternatives This is the beginning of the conceptual design phase. During this phase, designers and engineers, often in consultation with the machine shop that will build or replicate the tooling, propose alternative approaches to solving the tool design problem. These alternatives must be evaluated from a number of perspectives, including feasibility, cost, risks of employing unproven technology, and others. Step 3: Survey the Intended Use A survey reviews the alternatives for implementation of the tools. One design consideration is the degree to which the tool will be operated independent of intervention by production, maintenance, and engineering personnel during the life of the tool. Here are a few of the questions that must be answered, as they all affect the design: ● ● ● ●

Will the tools be completely automatic, or will some operator interaction be involved? Will the material-handling system be automated or manual? Who will maintain the tool? What parts of the tool can be remotely located outside the cleanroom to facilitate ease of maintenance?

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What portions of the tool will remain under engineering control? What procedures need to be in place for operation and maintenance of the tool? Will the tool find continued use in future applications, where current requirements might evolve into more stringent control limits or new requirements might be imposed?

Considering the possible use of a tool for a future program can allow anticipation of new requirements that can be designed for in the initial implementation of the tool. For example, a production line could be planned for a family of disk drive products. Early models in the family might use components with relaxed contamination and ESD sensitivity. Later models might be planned to have components that are more sensitive to contamination and ESD. Anticipating this evolution within a product family might allow for design of tools that anticipate the future requirements. Step 4: Identify Possible Abuses and Identify the Risk to Product or Processes This process, often referred to as failure mode and effects analysis (FMEA), is often not given sufficient attention. A good example is the incorrect design of a maintenance access cover that is critical for the contamination performance of a tool. A poorly designed maintenance access panel might contain an excessive number of hard-to-use fasteners. If so, the maintenance person may neglect to reinstall all fasteners. If failure to fasten the panel correctly occurs, it could adversely affect the contamination performance of the tool. Based on FMEA, this would be taken into consideration. Step 5: Devise Test Methods Often, the performance of a material, surface finish, or device cannot be predicted on the basis of known information. In this case, test methods must be devised to evaluate the suitability of the item for the application intended. Two types of tests can be devised: functional and nonfunctional (often called objective laboratory tests). Step 6: Test the Alternatives Often, more than one alternative is available for a given application. Each alternative should be subjected to the same battery of functional and objective tests. Here it becomes important to know that the tests selected are unbiased. A test that unfairly predisposes an alternative to failure might result in a less than optimum choice. Step 7: Select the Optimum Alternative Finally, the outcome of unbiased testing results in selection of the optimum alternative. These seven steps are repeated at the materials, component, subassembly, and finished assembly level, to gradually build a tool that satisfies all contamination and ESD objectives. 6.1.2

Applications and Limitations of Tooling Design

This chapter is applicable to all phases of tooling design: (1) design, (2) certification, (3) installation, and (4) maintenance of certification. Tooling in this context includes virtually any mechanical device placed in the cleanroom. This includes: (1) fixtures, (2) workstations, (3) storage racks, (4) material-handling equipment, (5) hard automation, and (6) soft automation (robots). These guidelines may be applied to the design of equipment for use in the manufacture of semiconductor devices or cleaning process equipment. However, this document is not a comprehensive guide to design of semiconductor equipment, nor is it a guide to

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the selection and development of cleaning equipment or processes. Semiconductor processing equipment is highly specialized in design. Generally, the designers of the equipment, working with their customers, will be able to define contamination requirements. A complete set of the Semiconductor Equipment and Materials International (SEMI) standards for tooling should be reviewed for completeness [1]. The development of cleaning processes and the equipment to support cleaning processes needs to be addressed as a separate topic, outside the scope of this chapter (see Chapter 5). There are two general cases to which the guidelines may be applied: 1. Retrofitting of existing tools Tools from an old product must be adapted for use with a new product. Commercial equipment that is not normally used in cleanrooms can be modified to make it compatible with cleanroom requirements. ● ●

Retrofitting has some advantages. Modification of existing tools may involve little expense and may be very rapid. In addition, experience with the existing tool usually provides precise information on what needs to be upgraded. Commercial equipment under consideration may also be available in a cleanroom-compatible form. Or the supplier of existing equipment may be able to quickly modify the existing design, as the producer already possesses design drawings, bills of material, and so on. For example, a painted metal panel can easily be fabricated in unpainted stainless steel using an existing drawing simply by changing the material callout. 2. Design of a completely new tool. This has some advantages over retrofitting of existing tools: Freedom to relocate inherently dirty parts Choice of clean materials and components Freedom to design streamlined clean enclosures ● ● ●

6.2

CONTAMINATION AND ESD CONTROL REQUIREMENTS

The single most important step in the design and certification of a tool is to define what its contamination and ESD requirements are. The selection of what to control and what limits to impose are not arbitrary choices. They are dictated by the needs of the product and the process. The tooling engineer nearly always depends on the product design and the production engineering functions to define these requirements. Requirements may also be dictated by the manufacturer of the process equipment. The design of the cleanroom in which the tooling is to be installed may also constrain the design. Some of the more important factors that must be specified are described in Section 1.5. Electrostatic discharge control requirements are usually dictated by the product or process to be supported by the tooling. However, there are cases in tooling design where the tooling itself will exhibit ESD sensitivity. Examples include microprocessors that control the tooling, sensors that detect conditions within a tool, and communication equipment. All of these may be affected by conditions existing within the facility and must be planned for in the tooling design phase. The subject of ESD requirements is covered in more detail in Chapter 2.

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6.3

MAINTENANCE REQUIREMENTS

One of the most common deficiencies in contamination control for tooling is lack of proper documentation and conduct of maintenance procedures. The need for maintenance must be considered during the conceptual design phase. There are four concerns: ● ● ● ●

Cleaning by production operators (part of the manufacturing process) Cleaning by maintenance personnel (part of the maintenance process) Maintenance access to enclosed components (a design consideration) Cleaning by engineers (part of the engineering process)

The engineer who designs or directs the design of a tool by a contractor is responsible for describing wipe-down procedures. Engineering is also responsible for performing wipe-down procedures after the tool has been subjected to engineering changes. Wipe-down chemicals may impose a restriction on the selection of materials for a tool or workstation. For example, if a tool or workstation is to be cleaned with isopropyl alcohol (IPA) and the alcohol causes a paint to chalk or causes an ESD mat to become ineffective, the paint or ESD mat material may not acceptable for use in the tool or workstation. Similarly, the selection of wipers may effectively limit the surface finish of a tool or one of its parts. If the finish on the surface shreds or tears the required wiper, the surface finish is too rough for use in the application. It may then be necessary to change the surface to a smoother finish to eliminate this problem. In addition, tools are often made compatible with cleanroom requirements by the addition of evacuated enclosures. These normally are applied over some mechanical or pneumatic device that produces an unacceptable amount of wear-generated contamination or which requires period adjustment. Thus, maintenance access, either by operators, engineers, or maintenance personnel, will be required. This issue is so important that special emphasis is required. After all else is done, maintenance of the tooling is the most critical factor. As maintenance has often been overlooked in the past, special emphasis is focused on it. 6.3.1

(Basics of a) Wipe-Down Procedure

Wipe-down instructions that are general in nature are inadequate. General instructions such as “wipe the tool with a cleanroom wiper wetted with IPA” are inadequate. Wipe-down procedures must specifically call out critical areas that are to be wiped, how they are to be wiped, what materials are to be used to wipe, and how to know when wipe-down has been completed successfully. The wipe-down procedures should not be in a general cleanroom procedures document but within each workstation or operation procedure document. Critical areas on tools are areas that come in contact with or in close proximity to product surfaces, the operator’s hands, or cleanroom garments. Close proximity is a relative term. An object 60 cm upwind of the product in a vertical unidirectional flow is relatively closer than an object 10 cm downwind of the product. Similarly, the undersurface of workstations where operators sit is critical, as it is in close proximity to operators’ cleanroom garments and potentially, operators’ gloves, if they place their hands on their lap. Here are some guidelines on what to specify in a wipe-down procedure: ●

Only cleanroom-approved wipers, swabs, and wipe-down chemicals may be used. In order of preference: deionized (DI) water; DI water–detergent; IPA; IPA–DI water; other

MAINTENANCE REQUIREMENTS

●

● ● ●

●

● ● ●

●

281

solvents. Consult with your local contamination engineering personnel to determine what materials are considered to be acceptable. The wiper should be folded to form a pad. Wipers should not be used as a wadded-up ball of material. Wipe-down begins at the top and proceeds downward. Wipe-down begins at the back and proceeds forward. Wipers must be thoroughly wet (but not dripping wet), or shredding, linting, and tearing can occur. The wetting agent provides lubrication to prevent wiper shredding, can provide some added chemical extraction ability (which aids in cleanup of organic contaminants), and provides surface tension (which aids in particle removal). Inspect the wiper often. When the wiper becomes visibly soiled, fold it to expose a fresh surface, rewet, and wipe the offending surface again. Continue until no soil remains. Change the wiper often so that a clean wiper surface is always available. Through- and blind holes, crevices, and slots may need to be cleaned with swabs. Pay special attention to areas of workstations, and tools that come in contact with product (wear and contact transfer). Vacuum, then wipe, then vacuum again for best effectiveness.

The surface of a tool is considered to be clean when no visible residue is left on the surface and when no residue can be removed from the surface when it is wiped. This is commonly referred to as visibly clean. It can be thought of as a form of “white glove” inspection. There has always been an element of controversy about this acceptance criterion. There is often concern that the visible cleanliness criterion may be inadequate, because people know that there are many particles smaller than the eye can see. But the only way that people can see smaller particles is through the use of some form of magnification. This limits our field of view and depth of focus, so some areas may be overlooked. In addition, it requires the use of an instrument that makes inspection inefficient. The requirement that a workstation always be visibly clean is easy to inspect and easy to enforce. In addition, wiping is effective at removing contaminants too small to be seen with the unaided eye. The visible contamination is an indication that an area has not been wiped. Magnification is seldom called for to verify that a surface is visibly clean. Conversely, special illumination may be required. Special illumination can include bright light inspection, illumination using an oblique light, or illumination using an ultraviolet lamp. The latter is especially useful as many materials fluoresce (i.e., emit visible light after absorbing energy from some other source: in this case, invisible ultraviolet radiation). Not all materials fluoresce, but sufficient numbers of materials do so that they act as markers, indicating that a surface has not been cleaned adequately. 6.3.2

Maintenance Wipe-Down

Tools and workstations require scheduled and unscheduled maintenance. Maintenance activities often require access to parts of a tool that are not ordinarily exposed to a cleanroom. Maintenance may use materials that are not easily removed by the operator during standard wipe-down. That is, maintenance may require the introduction of lubricants and other materials that require special chemicals, wipers, and cleaning procedures for their removal. These need to be evaluated and specified for a comprehensive tooling installation. In addition,

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maintenance will have access to areas of the tools not normally accessible but of concern to the production operator. Hence, special cleaning procedures must be performed by maintenance. Parts of tools that require seals or evacuated enclosures probably are covered because they are a source of contamination. Several design considerations are important. Parts behind covers often need adjustment or replacement. The designer must consider ease of maintenance access. For example, sealants such as room-temperature vulcanizing silicone should not be used where access is needed, because they are difficult to remove and to replace. Enclosures should have a minimum number of easy-to-use fasteners. 6.3.3

Engineering Changes

Engineering changes inevitably occur to a tool during its lifetime. Engineering has many responsibilities when changes occur. Among these are the following: ●

●

●

Engineering must revise operator wipe-down instructions as needed by engineering changes to the tool. Engineering must revise maintenance wipe-down instructions as needed by engineering changes to the tool. Engineering changes must maintain particle count and other contamination control requirements.

For example, many chemicals are sensitive to ultraviolet radiation (light). Ordinary fluorescent lamps produce sufficient ultraviolet light that they are unacceptable for use in photolithography processes for semiconductor, flat-panel display, and magnetic recording head manufacture, among others. Where lighting is required in tools, it is important to determine if there are wavelengths that are unacceptable. Similarly, for some types of tools, vibration is a critical issue. A good example is photolithography exposure tools. Manufacturers of steppers and other extremely vibration-sensitive tools understand and specify the vibration requirements of their tools very carefully. These requirements may place restrictions on the design of nearby tools, so it is wise to ask about any vibration requirements. When products evolve, their vibration tolerance may shrink. The engineer may thus need to plan for the evolution of vibration requirements in basic tool design. 6.3.4 ●

● ● ● ● ●

● ●

Summary of Requirements

The airborne particle concentration at product locations must not exceed the stage 2 limits (state 3 limits where enclosures allow). Airflow must comply with directions and velocity. Materials must not outgas harmful vapors. Ionic contamination must be controlled. Surface wear and parts damage must be minimized by design. Materials and surface finishes must be compatible with cleaning fluids and wiper materials. Obstructions to airflow must be minimized. Air should be delivered to the product first.

GENERAL DESIGN ALTERNATIVES ● ● ●

283

Wipe-down procedures must be fully documented. Fans for evacuated enclosures must exhaust to safe places. Electrostatic charge requirements must be understood and designed for.

6.4

GENERAL DESIGN ALTERNATIVES

Three primary alternatives are available for designing a tool for cleanroom use: (1) eliminating contamination generators, (2) relocating contamination generators, and (3) enclosing and evacuating contamination generators. 6.4.1

Eliminating Contamination Generators

Materials, surface treatments, and components must be selected which are inherently clean and compatible with cleaning chemicals and processes. The following guidelines are suggested. ● ● ● ● ● ●

Surfaces must be nonshedding. Surfaces must be nonporous. Surfaces must be protected (surface treatments). Surfaces must be smooth. Fillets and outside radii must be sized generously (see Figure 6.1). Textured surface finishes should be avoided.

Electric motors, transmissions, pneumatic devices, linear actuators, and other devices are often sources of contamination. Conventional electric motors use brushes that wear and generate contamination. Transmissions, lead screws, and gearboxes are lubricated. The spinning

Fillet radius matches swab tip radius, making cleaning effective

Fillet radius too small

(a)

(b)

FIGURE 6.1 Inadequate fillet radius makes it difficult to remove contamination using a swab. (a) The fillet radius is too small to be cleaned by the swab. (b) Knowing the size of swabs available, a fillet radius is chosen that can be cleaned effectively.

284

TOOLING DESIGN AND CERTIFICATION

action in these devices will aerosolize the lubricants, causing contamination. Conventional pneumatic cylinders tend to leak compressed air and often are vented directly into the workplace. In addition, the shaft seals must be lubricated, allowing for aerosolization of the lubricant. Hydraulic devices are difficult to seal. The following guidelines apply: ● ● ● ●

Brushless electric motors are inherently cleaner than electric motors with brushes. Sealed transmissions and gearboxes are preferable to open gearboxes and transmissions. Hydraulic devices can be sources of oil spray and vapor and should be avoided. Pneumatic cylinders made especially for cleanroom application should be chosen. These are usually equipped with shaft exhaust systems and are ventilated remotely. Dual-action cylinders generally do not exhaust to the workplace.

It is impossible to predict the contamination performance in advance for all components. If no prior experience exists, new components must be tested. Several test protocols can be followed. First, the design and bill of materials for the component should be reviewed to verify its suitability for application. The components should be cleaned initially using the intended wipers and cleaning chemicals to verify their suitability. Preliminary functional tests should be done to verify that the materials of construction are not harmful to the product. If these three steps look encouraging, more formal laboratory tests can begin. Of these, particle count performance is one of the most difficult to achieve but also one of the easiest to set up and run. 6.4.2

Relocating Contamination Generators

A contamination generator that cannot be eliminated should be relocated to below the workstation, downwind from product locations. Take care to ensure that contamination in the downwind position cannot be carried to product locations by turbulence or contact transfer. Figure 6.2 illustrates some aspects of the relocation process. Also, make certain that chemical contamination is not being carried throughout the cleanroom in the recirculation airflow. An option for relocating contamination generators is to remove them from the cleanroom entirely. Power and control centers for robots are good examples of auxiliary equipment that could be removed entirely. Also, where the cleanroom architecture permits, consider mounting the tools on a bulkhead. Figure 6.3 illustrates a typical bulkhead installation for a tool. There are several distinct advantages to relocating outside the cleanroom: ● ● ●

Contamination requirements are eliminated for the relocated portions of the tool. The floor space for the cleanroom can be reduced. Maintenance is done outside the cleanroom.

In Figure 6.3 a vacuum process tool has been bulkhead-mounted. If all the equipment were located within the cleanroom, contamination control requirements would apply to all. As shown, the only portion of the tool for which contamination control requirements are applied is the loading–unloading station. This includes surface treatments and finishes, materials selection, and particle contamination. However, outgassing may still be a concern, since the relocated components are in the air return plenum. Any vapors emitted by the relocated components could be recirculated into the cleanroom. Maintenance requirements can also be relaxed. However, care may still need to be exercised if personnel from the cleanroom can

GENERAL DESIGN ALTERNATIVES

15,000 ppcf

285

300 ppcf

(a)

(b)

5 ppcf

(c)

FIGURE 6.2 Aspects of relocation of components. (a) The carbon vane vacuum pump generates a lot of contamination. The particle count operator is very unhappy about the particle concentration that he is reading on the OPC. (b) Relocating the carbon vane vacuum pump to below the work surface reduces but does not eliminate the excessive particle count. Turbulence around the operator and below the table allows the contamination to get about the work surface. This effect can be even more pronounced in cleanrooms without a raised floor. (c) Relocating the vacuum pump to below the raised floor or out a return plenum eliminates the contamination produced by the pump.

enter the return area to look after the portions of the tool located there and then return to the cleanroom, as is usually the case. 6.4.3

Enclosing and Evacuating Contamination Generators

Contamination generators that cannot be eliminated or relocated must be enclosed. Several common design problems must be addressed when designing enclosures for contamination generators: ●

Sealed enclosures are possible in only a few, limited cases because contamination generators are subject to wear and require frequent service or adjustment by maintenance

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TOOLING DESIGN AND CERTIFICATION

Cleanroom Wall

Cleanroom

Support Electronics

Loading– Unloading Station

Return Plenum or Service Aisle

Process Chamber

Exhaust Line

Roughing Pump

FIGURE 6.3 Plan view of a bulkhead-mounted tool.

●

●

●

●

or engineering personnel. In addition, contamination generators often have moving parts, making completely sealed enclosure impractical. Close-fitting but not airtight enclosures with an exhaust system to evacuate the enclosures are generally possible. These will generally have openings that allow a moving part freedom of movement. Avoid the use of bellows. Sufficient air must be drawn into enclosures to draw test smoke into any opening in the enclosure. In general, to minimize leakage, openings in vertical unidirectional flow [90  20 ft/min (0.40  0.05 m/s)] should be evacuated so that air entering the opening is four times the prevailing free stream velocity. An exception is an enclosure around a moving part. The air velocity entering the enclosure should be four times the free stream velocity or double the maximum velocity of the moving part, whichever is greater. Enclosures should be as streamlined as possible. The relative effect of cross-sectional shape on turbulence is illustrated in Figure 6.4. Fasteners should be kept to a minimum and recessed if possible. Recessed fasteners can be made easier to clean by providing self-adhesive metal or plastic film “dots” to provide a flush surface, as shown in Figure 6.5. Streamlined enclosures encourage adherence to housekeeping rules. They look like they can be kept clean, and due to their simple geometry, are easy to clean.

Alternative exhaust strategies are possible. It is often possible to exhaust contamination below a raised floor, into a return air plenum, or through a HEPA filter. However, these strategies will probably allow molecular contamination to be released into the cleanroom or tool. Where molecular contamination is a concern, the most cost-effective and safest strategy is

GENERAL DESIGN ALTERNATIVES

(a)

(b)

287

(c)

FIGURE 6.4 Effect of cross-sectional shape on turbulence in unidirectional airflow. (a) Rounded square beam, having both leading- and trailing-edge turbulence fields. The trailing-edge turbulence field generally extends from two to four obstruction widths downstream of the obstruction at a free stream velocity of 70 to 110 ft/min (0.35 to 0.45 m/s). (b) Circular beam of equal width. The leading-edge turbulence is largely eliminated and the trailing-edge turbulence is typically half that observed under square-cross-section obstructions at similar velocities. (c) Trailing-edge taper that eliminates the flow detachment and nearly all downstream turbulence.

Fastener Head

Self-Adhesive Seal

Recess for Fastener Head

FIGURE 6.5 Use of adhesive dots to cover recesses and provide a flush surface for wiping.

to exhaust using the housekeeping vacuum. Check with the facilities engineer to determine the availability and capacity of the housekeeping vacuum. In some cases, tools can be built with self-contained exhaust systems, which can be provided with chemical filtration to handle molecular contamination. Two types of vacuum systems are in use in most facilities: housekeeping and process. (These must not be confused with the very low pressure vacuum systems dedicated for use on high-vacuum processing equipment.) Housekeeping vacuum systems usually are lowvacuum (typically, less than 2 in. H2O) high-volume (typically, 50 to 500 ft3/min) systems designed to remove large volumes of contaminated air at very high velocities. Process vacuum usually is intermediate pressure (15 to 27 in. Hg) and low volume (less than 10 ft3/min) designed to operate low-pressure vacuum equipment.

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TOOLING DESIGN AND CERTIFICATION

Cylinder End Caps

Shaft

Bellows

Cylinder Body

Bellows Fixed Mount

Supply and Exhaust Fittings and Hoses

Bellows Moving Mount

To Safe Exhaust Point

FIGURE 6.6 Bellows enclosing and exhausting solution for contamination leakage around a pneumatic system shaft. The bellows requires mounting hardware on the cylinder body and shaft, as well as an exhaust.

Close-Fitting but NonContact Enclosure Cylinder Body Shaft Seal Detail

Supply and Exhaust Fittings and Hoses

Airflow To Vacuum

FIGURE 6.7 Close-fitting but not contacting contamination control solution for the shaft of a pneumatic cylinder.

One of the popular solutions to containment of contamination on components that move has been to supply bellows, usually made of polyurethane or polyethylene. These may not be the ideal solution. Bellows wear and require replacement and need mounting adapters and exhaust tubes. Figure 6.6 shows an example of a bellows applied surrounding the shaft of a pneumatic cylinder. This adds maintenance requirements and introduces mechanical complexity. In addition, part of the stroke of the cylinder is unusable because of the bellows mounting hardware. A better arrangement may be to provide close-fitting and evacuated but noncontact enclosures around moving members. An example of a pneumatic cylinder shaft is shown in Figure 6.7. This solution eliminates the bellows and the moving bellows mount but still adds a noncontact seal around the shaft, sacrificing some stroke length. In addition, the seal opening must be evacuated continuously to ensure that any contaminants generated at the cylinder-to-shaft seal are kept out of the cleanroom. Figure 6.8a exhibits a more complicated design example. This is a full-function robot that travels along a long linear path. The robot is supported by wheels below its base: wheels

GENERAL DESIGN ALTERNATIVES

289

(a) Upper Rail Upper Rail Trolley Enclosure, X-Travel Upper Rotary Joint, θ Y-Axis Seals Z-Axis Carriage

Y-Axis Seals Lower Rotary Joint, θ -Axis

X-Axis Carriage – contains blower to evacuate robot

Lower Rail (b)

FIGURE 6.8 Direct-access handling subsystem design example: (a) a very complicated cleanroom robot; (b) the contamination control features of the robot.

that are located below the raised floor and thus are unlikely to contribute to contamination in the cleanroom. Other features of this robot that are located above the raised floor can contribute to contamination in the cleanroom and should be examined in detail. Figure 6.8b identifies the features of the robot that are important for this discussion.

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Upper Rail

Upper Rail Rollers (only front rollers shown)

Airflow from Cleanroom

Airflow to Upper Rotary Joint

Upper Rail Trolley Enclosure (front half not shown)

FIGURE 6.9 Upper rail trolley enclosure contamination control system. The enclosure is mounted with about 1 mm of clearance from the upper rail. Air is drawn into the enclosure by the blower, located in the X-axis carriage, mounted under the raised floor.

Upper Rail Enclosure

a

b

Upper Rotary Joint

c

FIGURE 6.10 Upper rotary joint contamination control system. Air enters the upper rotary joint enclosure from the upper rail enclosure at point a. Air also enters the upper rotary joint from the cleanroom via the circular slot, b. Air flows from the upper rotary joint to the upper arm, c. Note that there is no seal material in the circular slot.

One of the simplest enclosures is that for the upper rail trolley, which is seen at the top of Figure 6.9. The upper rail trolley has steel wheels that roll against a steel X-axis upper rail. Because this structure is located near the ceiling in a vertical unidirectional-flow cleanroom, it is imperative that it be clean and not be lubricated. The function of the upper rail trolley enclosure is to capture contamination generated by rotation of the steel wheels and to vacuum any contamination generated by wear or fretting corrosion between the steel wheels and the upper rail (Figure 6.10).

GENERAL DESIGN ALTERNATIVES

291

Air from the upper X-axis trolley enclosure and upper rotary joint enclosure Y-column seal belt in guide

Y-column

Y–Z carriage (moves vertically on the Y-column)

Air from inside the Y–Z carriage

Air to lower rotary base

Air from cleanroom enters the Y-column along the entire length of the Y-column seal

FIGURE 6.11 The Y-column contamination control seal. The seal belt is fixed at the top and bottom and forms a close-fitting but noncontact seal. Air flows into the Y-column between the seal belt and the seal guides. The seal is lifted and replaced in the guide by the rollers carried by the Z-carriage.

The next contamination control feature is the upper rotary joint. The upper rotary joint has a close fitting but non-contact circular slot. Air is drawn into the upper rotary joint enclosure past a tortuous path, sometimes described as a labyrinth seal. Another solution that has been employed is a stationary belt in the Y–Z support columns along which the Z-carriage moves. The belt moves into and then out of the Y–Z carriage as shown in Figure 6.11, via a system of rollers mounted on each side of the Y–Z carriage. The Y-column seal is a particularly interesting design. Because the seal material is fixed at either end, spring wind-up mechanisms are not needed. The Y–Z carriage is raised and lowered by lead screws. The lead screws are heavily lubricated to prevent wear. Rotation of the lead screws aerosolizes this lubrication. Thus, the surface of the seal facing the lead screw becomes contaminated with the lubrication sprayed by the rotating lead screw. However, this lubricant does not tend to migrate to the outer surface of the seal, because the contaminated interior does not contact the exterior surface as would happen if the seal material rolled up like a window shade. In addition, the rollers that get contaminated by contact with the contaminated side of the seal are fully contained within the Y–Z carriage enclosure. Figure 6.12 illustrates the seal mechanism for the Y–Z carriage. The seal is fixed to the Z-axis actuator and travels around the Z-axis actuation mechanism. The interior surface of the seal becomes contaminated by lubricant spray from the Z-axis lead screw. However, the contaminated inner surface of the seal never contacts the outer surface of the seal, preventing migration of lubricant to a position exposed the the cleanroom. Air enters the Z-carriage from the cleanroom and is exhausted through the Y-axis columns. The lower rotary base is similar in design to the upper rotary joint. The X-axis carriage rides below the raised floor. Because the X-axis carriage resides below the raised floor, contamination control features are not required. However, the X-axis carriage contains the blower for evacuating portions of the robot above the raised floor. Many electronic enclosures are kept inside the cleanroom. These enclosures often are equipped with exhaust fans to cool the electronics within them. The normal arrangement for these cooling fans in noncleanroom applications is for the fans to take advantage of the

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TOOLING DESIGN AND CERTIFICATION

Z-axis motions

(a) Air enters the Z-carriage from the cleanroom

Z-axis motions

Air exhausts to Y-column

Air exhausts to Y-column (b)

FIGURE 6.12

(a) The Z-axis seal and guide rollers; (b) airflow for the Z-carriage.

Standard exhaust fan, blowing up

Auxiliary exhaust duct

Auxiliary exhaust fan

Electronic enclosure

Optional HEPA filter

FIGURE 6.13 Generic fix for airflow for an electronic enclosure. The optional HEPA filter is used if the auxiliary ductwork cannot be exhausted to below a perforated raised floor.

natural buoyancy of the heated air within the electronic enclosure, so these fans exhaust in an upward direction. This is highly undesirable in a vertical, unidirectional-flow cleanroom. (Despite the obvious nature of this problem, it is often overlooked.) Figure 6.13 illustrates a generic fix for an electronic enclosure to control airflow.

MATERIALS

6.5

293

MATERIALS

One of the most important decisions that must be made during the conceptual design phase is selection of materials. Materials must be selected that are going to be compatible with chemicals used in the workplace. In addition, the wear properties of the materials, alone and in combination with others, must be considered carefully. Elements of material selection are discussed by material type: ● ● ● ● ● ● ●

General guidelines for materials General guidelines for wear Metals and alloys Lubricants General guidelines for plastics Plastic composites Antistatic polymers

6.5.1

Guidelines for Materials

Surfaces must be protected from degradation by such methods as passivation, plating, coating, and other methods as appropriate on a case-by-case basis. Surfaces must not be degraded by contact with cleaning chemicals, process chemicals, and other materials used in the workplace. Surfaces must be nonshedding under normal conditions of use. For this reason, wood and wood products are inherently unacceptable. Even coated wood products covered with plastic lamination materials are unacceptable, as the plastic laminates are not inherently wear resistant. Metal-laminated wood products have also been found to be unacceptable, because damage to the metal laminates often result in exposed wood products in the cleanroom. Conversely, laminated wood products are acceptable for use in ESD-protected work areas, as long as these are not, simultaneously, cleanrooms. Coatings, sealants, and materials containing known or suspect killer chemicals are also not considered acceptable for use. Examples include silicones, organotin compounds, and organic amines. Surfaces must be smooth and of an acceptable surface roughness. Acceptable surface roughness is dictated by the ability to clean the surface to an acceptable cleanliness level. Generally, this excludes the use of open-celled foams. In general, two types of tests are used to qualify materials for use in cleanroom applications. Contamination qualification tests fall into two broad categories: functional tests and nonfunctional tests. Contamination functional tests include contact and near-contact stain. Contamination nonfunctional tests include extractable particles, anions, cations, viable organisms, and organic contaminants. In some applications, functional and nonfunctional tests for ESD properties may also be important. Functional Contamination Tests: Contact Stain and Near-Contact Stain Many materials used in tooling for cleanrooms sometimes come in contact with products or are in close proximity to, but not in contact with, products. Two types of tests are applied to evaluate the functional suitability of materials for cleanroom tooling applications for these two cases: contact stain and near-contact stain. Others may be specified, depending on the users’ functional requirements. In a contact stain test, apparatus suitable to hold the test material and product is prepared so that apparatus contribution to the test is negligible.

294

TOOLING DESIGN AND CERTIFICATION

Top cover plate Perforated spacer plate (typically 10 to 50 mils thick, made of an inert material such as Teflon™)

Substrate Material under test

Bottom cover plate

FIGURE 6.14

Typical apparatus for a near-contact stain test.

Several strips of the material under test are held against the product. The apparatus is then sealed within a polyethylene plastic bag to prevent gases from adjacent bags (usually, several materials are under evaluation simultaneously in the temperature–humidity chamber) containing test specimens from interacting. The bags are then placed in a temperature–humidity chamber for conditioning. Many different companies use this test. Typical conditions are 70 to 80°C and 70 to 85% relative humidity for a period of 4 to 7 days. At the end of the test, the chamber is returned to ambient temperature and humidity under noncondensing conditions. The product is removed from the chamber and inspected for signs of stains, discoloration, or corrosion. The material may also be subject to staining or corrosion. This may be done by unaided eye inspection or inspection using magnification. If either the material or the produce shows signs of staining or corrosion, they should be rejected. A near-contact stain test is virtually identical to a contact stain test. The primary difference is that the material under evaluation is held in close proximity to, but not in contact with, the product. Care is taken to ensure that the material under test cannot drip or sag onto the product. The material under test is usually beneath the product to prevent the sagging or dripping from contacting the product. Spacing between the material under test and the product is typically 250 to 1270 m (0.01 to 0.05 in.). Figure 6.14 illustrates a typical arrangement of materials in a near-contact stain apparatus. There is another consideration in contact and near-contact stain tests. The material being qualified may come in contact with water, isopropyl alcohol, or other chemicals, which extract damaging substances from the material. If this is the case, extracts obtained by soaking the material in appropriate solvents are used as the challenge material in the functional tests, often in the form of dried residues. Other functional tests include testing the materials for compatibility with cleaning chemicals, process chemicals, and any other materials or energy to which they may be exposed during their life in the process area. For example, paints that degrade when cleaned with wipe-down chemicals are unacceptable for use. Nonfunctional Tests: Objective Laboratory Measurements Materials qualified under functional tests are then characterized using objective laboratory tests to characterize the material. The results of these tests are then used to specify to the supplier the properties desired. The tests will quantify such parameters as extractable particles, anions, cations, organic, and viable contamination. Electrostatic charge can be considered a form of contaminant for some applications. Nonfunctional tests often become the basis for specifying the material desired, as opposed to the performance in functional tests. Materials are specified in this manner, using objective laboratory tests rather than functional contamination tests

MATERIALS

295

because materials suppliers seldom have access to product to conduct contact or near-contact stain tests. This is especially true where products needing protection from objectionable materials are under development and must be kept confidential. Extractable Particles One of the earliest tests to be applied to materials was the extractable particle test, developed originally for characterization of piece parts used in manufacture. Several extraction methods are available, including ultrasonic, spray, and undulation. In ultrasonic extraction the material is normally placed in a detergent–water solution of known particle concentration. The part is then extracted at a controlled temperature, ultrasonic frequency, sound power level, and time. The additional particles suspended in the solution after extraction are used as a measure of the cleanliness of the part. In spray extraction, pure filtered water (no detergent) is sprayed on the material using a controlled procedure. The particles added to the solution are then counted and used as a measure of the material’s particle cleanliness. In undulation extraction, the part is placed in a detergent–water solution of known particle concentration. Either the part is used to agitate the solution, or the vessel containing the part is agitated. Again, the time, temperature, magnitude, and frequency of agitation are controlled to produce a reproducible extraction. Once again, the added particles in the solution are used as a measure of the particle cleanliness of the material. In some cases, solvents are used in place of water solutions as the extraction medium. One of the most frequently used standards for measuring particle cleanliness levels of surfaces is MIL-STD-1246, which specifies the number of allowable particles per square foot of surface area. As with ISO 14644 (FED-STD-209), the allowable particle concentration is expressed as a particle size distribution. However, where ISO 14644 (FED-STD-209) specifies particles per unit volume of air, MIL-STD-1246 specifies particles per unit surface area. The two standards are thus not interchangeable [2]. Table 6.1 is an abbreviated table of allowable particle concentrations for several MILSTD-1246 cleanliness levels. The table shows the cumulative number of particles per square foot of surface area equal to or larger than the stated size. Where a cell contains the entry, n.a., control limits should not be specified. Particle area densities are so small that it becomes statistically difficult to verify compliance. A quick inspection of the table shows TABLE 6.1 Some MIL-STD-1246 Particle Size Distributions Particle Sizea Level

1 m

5 m

5 25 50 75 100 150 200 250 300 500 750 1000

3 65 471 1,802 5,058 24,264 79,970 211,158 480,901 5,564,044 45,112,298 215,774,441

1 23 166 636 1,785 8,561 28,218 74,508 169,688 1,963,292 15,918,028 76,136,745

a

10 m

15 m

n.a. n.a. 8 3 56 25 214 94 600 265 2,877 1,271 9,483 4,189 25,038 11,060 57,024 25,190 659,767 291,445 5,349,275 2,362,982 25,585,859 11,302,264

n.a., Not applicable (control limits should not be specified).

25 m

50 m

100 m

n.a. 1 7 28 78 376 1,240 3,273 7,455 86,252 699,314 3,344,854

n.a. n.a. 1 4 11 52 170 448 1,021 11,817 95,807 458,249

n.a. n.a. n.a. n.a. 1 5 16 42 95 1,100 8,919 42,658

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TOOLING DESIGN AND CERTIFICATION

the wide range of cleanliness levels in existence. The cleanest surfaces available, bare silicon wafers and glass panels for thin-film displays, are typically cleaner than level 1 at the start of production, after initial cleaning. It is also not unusual to find surfaces in the range from level 500 to 1000 after machining or other processes. Extractable Ionic Content Ionic contamination is usually extracted in DI water with no detergent. Ionic extraction methods range from a room-temperature DI water rinse to a 1-hour or longer soak in DI water at 80°C. The longer the material is in contact with the water and the higher the temperature, the greater is the tendency for the water to extract ions from the sample. Many customers prefer a high-temperature leach test to a low-temperature extraction test, because the leach accelerates slow chemical reactions to maximize the extent they contribute ions to the extract. Following extraction, samples are usually analyzed using ion chromatography for anions and atomic absorption spectroscopy (AAS) for cations. Anions of interest generally are chloride, nitrate, and sulfate, although some end users specify phosphate. Cations of interest include aluminum, copper, iron, magnesium, silicon, sodium, and zinc. Other analysis methods are also in use. Other Contamination Tests Several other contamination tests are available. Among these are nonvolatile residue (NVR), organic extractable, and viable organisms. In the NVR test the material is washed with a suitable solvent, often isopropyl alcohol, and the solvent is allowed to evaporate in a preweighed weighing dish. Isopropyl alcohol is often chosen for solvent extraction because it is used as the wipe-down chemical for tooling. The residue remaining after solvent evaporation is weighed. The resulting added mass is reported in milligrams per unit area of surface. Drawbacks of the NVR test are that it is time consuming and procedurally difficult, occasionally resulting in gross errors. However, it is can essential for responding to requirements based on MIL-STD-1246. Organic materials can be extracted from certain types of materials by various organic solvents. Again, isopropyl alcohol, which is commonly used in cleanrooms as a housekeeping chemical, might be a good starting point for extracting organic residues. In other cases it might be desirable to extract with more aggressive solvents, such as acetone, methylene chloride, or hexane to enhance recovery of hydrocarbons, soluble oligomers, plasticizers, siloxanes, or other molecules considered undesirable. After recovery of the soluble material, the samples can be concentrated by evaporation, as in the NVR procedure. However, in place of weighing the concentrate, some of it is analyzed by Fourier transform infrared (FTIR) spectroscopy, gas chromatography with mass spectrometry (GC/MS), and time-of-flight secondary ion mass spectrometry (TOF/SIMS) detection. Many organic compounds are so detrimental to products or processes that the acceptance criterion is “none detected”. Viable contaminants may be detected by contacting the surface of culture medium or by pipetting a wash from the material onto the culture medium. The medium is then incubated to develop colonies of the viable organisms, which can be identified and counted. ESD Considerations Selection of material for ESD applications, covered in detail in Chapter 2, is critical. One of the primary considerations is surface resistivity of the surfaces; a second is the durability of the surface resistivity. The resistivity of the surface might change over time due to wear, accumulation of contamination, or depletion of materials essential for imparting conductivity after cleaning with water or other solvents.

MATERIALS

297

Initial Qualification vs. the Need for Ongoing Certification During initial qualification tests, there are seldom enough resources available to determine the supplier’s ability to achieve the desired contamination performance by a controlled process. Typically, the initial functional tests and benchmark measurements to determine the levels of contaminants on the materials are done quantitatively on only one or two batches of material. The degree with which these initial batches are representative of the population at large should ideally be checked on an ongoing basis. This is especially true for the particle content of a material. Materials are subject to aging and wear. Aging may involve chemical transformations, resulting in the release of ionic or organic contamination. Aging and wear result in the production of particles. For this reason, periodic recertification of tooling materials in the application can be a wise strategy. 6.5.2

Guidelines for Wear

Wear and friction are closely related. Friction can be reduced through many factors, although universal rules of friction and wear do not exist. Wear and friction can be reduced by: ● ● ● ●

Selection of materials Elimination of sliding contacts Optimization of surface finish Use of special surface treatment (Two of the most common and useful special surface treatments are special lubricants and synergistic coatings.)

There are no precise laws of wear. In general: ● ●

Wear increases with time and load. If two dissimilar materials are in contact, the softer material will wear more than the harder, but take care in applying this generalization to alloys or composite materials.

Wear due to abrasion under low stress can be a common problem. Several approaches can be taken to minimize wear under low stress: ●

●

●

●

For wear induced by filled polymers, eliminate the abrasive filler. Avoid glass reinforcements, especially where these glass-filled polymers are in contact with soft metal alloys or other composite polymers. Determine the hardness value of abrasives that cannot be avoided. Make certain that the surface in contact with the abrasive is harder. Practical high-hardness surfaces are plasma-deposited ceramics, steels with a high fraction of carbides, anodized aluminum, electroless nickel, and nickel and chrome electroplate. Reduce contact stress. That is, control the mating pressure between mating surfaces and maximize the surface area in contact without exceeding the friction requirements.

To reduce metal-to-metal wear: ● ●

Always lubricate according to the manufacturer’s directions. Do not overlubricate, as the excess lubricant will drip, run, or spray.

298 ● ●

●

● ● ●

●

TOOLING DESIGN AND CERTIFICATION

Do not run items dry that are meant to be lubricated. If the lubricant is forbidden or incompatible with products or processes, consider the following options: – Substitute a lube that is known to be compatible with products or processes. (For example, in the disk drive industry, use lubricants similar to the lubricant used on the disk.) – Where lubricants must be substituted, consult with the equipment supplier and if reliability of the equipment is an issue, arrange for testing. – Modify parts using coatings so that they may be run dry. – Use a self-lubricating plastic. Carefully select lubricants. Dry film lubricants such as Teflon powder, graphite, or molybdenum disulfide are forbidden from use in a cleanroom. Rolling surfaces should be kept clean. A hard–soft metal couple will produce more wear than will a hard–hard couple. Metals with hardness [i.e., 60 hardness on the Rockwell hardness C (HRC) scale] are best. Avoid soft–soft metal couples: – Avoid high-copper alloys. The exception here is alloys of beryllium and copper. – Avoid aluminum alloys without a hard coat. – Avoid pure metals with the exception of nickel and chrome.

Surface fatigue can be a problem where high compressive loads are encountered. These can lead to fretting corrosion of steels and flaking of surface treatments. ● ●

Use materials of high surface hardness and high compressive strength. Avoid thin case-hardened surfaces; they flake.

For cases of nonmetal sliding wear; observe the following precautions: ● ● ●

● ●

Avoid sliding contact. Use self-lubricating plastics where practical; check materials compatibility carefully. Avoid self-mating plastics; many will friction weld. For the purpose of this discussion, a self-mating plastic wear couple is one that occurs each time the base polymer is the same in both wear members. Keep plastics within their pressure–velocity and temperature limits. Avoid self-mating of ceramics (e.g., anodized aluminum couples).

Metals and Alloys The most common metals for tooling are steel and aluminum alloys. Selection depends on the application intended. The following guidelines are suggested: ●

●

For items not subject to wear and abrasion (e.g., a tooling base plate), use surfacetreated aluminum or steel. This is an application where stone (e.g., granite) might be considered in place of metal, especially where vibration control is needed. For storage cabinets and enclosures, painted steel or anodized aluminum is usually acceptable.

MATERIALS ●

●

●

●

●

299

Where conductive surfaces are required for ESD protection, only stainless steel may be satisfactory. Under some circumstances, it may be possible to continue using cabinets that are not made of dissipative materials by adding grounded mats cut to fit the individual shelves. Another option is to store product on an unsafe insulative surface only while it is in static-shielding containers. The term stainless steel is misleading. Stainless steel alloys do corrode, but at a much lower rate than do their non-stainless steel alloy counterparts. Components subject to wear or abrasion must be stainless or tool steel. Avoid the use of hard anodization on aluminum, as the anodize coating is brittle. Another option is to use bronze or beryllium copper alloys, although these tend to be more expensive. Check reactive metals for chemical resistance. Examples of metals considered to be reactive are zinc, aluminum, magnesium, copper, and their alloys. Coatings can sometimes protect reactive metals. However, coatings may not be continuous, leaving pinholes, or may not reach the bottom of blind holes. Avoid electroless nickel over reactive metals where contact with water–process fluids can occur. Pinholes, cracks due to fastening, and holes from subsequent machining can allow galvanic corrosion to occur. The most familiar example of this occurs when electroless nickel is applied over aluminum or steel.

Table 6.2 provides some useful wear characteristic data for material combinations. Table 6.3 provides some guidance for selection of steels for tooling applications. Stainless steels are alloys of iron with chrome and nickel (for 300 series alloys) or nickel only (for 400 series alloys). Most tool surface will be made of 300 series steels. In some applications, 400 series stainless is preferred. Table 6.4 lists some common stainless steels in terms of their relative formability, weldability, machinability, hardenability, and processing cost. Aluminum alloys generally offer poor wear and corrosion resistance and are used primarily for lightweight structural components. They are used almost always with surface treatments such as electrophoretic paint, electroless nickel coat, or anodized surface treatment. Aluminum that has been electropolished or bright dipped, followed by light anodization, has been used in some lightly loaded sliding wear application for dispensing parts with some success, Table 6.5 lists some relative properties of aluminum alloys for tool applications: resistance to stress, machinability, brazing ability, cold workability, strength, weldability, and typical surface treatments.

TABLE 6.2 Wear Rates of Metals Against Themselvesa Alloy

Condition

Chrome plate

0.002 in. over stainless Annealed Annealed Annealed Cold drawn

301 316 304 303 a

Hardness, Rockwell

Weight Loss (mg/1000 cycles)

39 HRC

1.66

B 90 B 91 B 99 B 98

5.47 12.5 12.77 386.1

Test conditions: Taber Met abrader, 12.7-mm cross cylinders, 90°, 71-N (16-lb) load, 105 rpm, 120-grit surface finish, 10,000 cycles.

300

9

5 9 8 9

4340

S1 M2 420 440

5 9 6 3

4

9 4

10

3 7 8

Resistance to Heat Softening

1 9 3 8

2

3 2

10

4 4 8

Abrasive Wear Resistance

10 1 3 1

10

8 9

1

5 2 1

Toughness

7 8 4 4

1

6 1

10

7 3 6

Metal-to-Metal Wear Resistance

A rating of 10 is best; for cost, a rating of 10 is high. Source: K. Budinski, Engineering Materials, 2nd ed., Reston Publishing: Reston, VA, 1983, p. 243.

a

High speed Stainless

9 7

H13 4140

Hot work Shock resisting

5 8 9

10

O1 A2 D2

Cold work

Cemented carbides

Alloy

Tool Steel

Depth of Hardening

TABLE 6.3 Selected Tooling Steels and Their Propertiesa

3 7 6 6

1

4 1

10

3 4 5

Cost Factor

44–47 62–64 50–52 58–60

30–32

For corrosion-resistant wear parts

Good shock resistance Ex. toughness, light sections Ex. toughness, heavy sections

Use for extreme service

10 48–50 30–32

Low distortion

Good machineability

Comments

58–60 60–62 60–62

Recommended Hardness (HRC)

301

MATERIALS

TABLE 6.4 Relative Properties of Stainless Steels for Tooling Applicationsa Alloy

Formability

Weldability

Machinability

Hardenability

Processing Cost

303 304 410 416 430 440C

4 8 6 1 6 n.r.

1 8 2 1 6 n.r.

8 4 6 8 6 4

n.r. n.r. 10 10 n.r. 10

8 6 6 8 8 6

a

A rating of 10 is best. n.r., Not recommended.

TABLE 6.5 Relative Properties of Aluminum Alloys for Tooling Applicationsa Weld Alloy Hardness 1100, H4 2024,T4 6061, 0 6061, T4 6061, T6 a

Stress

Cold-work

Machine

Braze

Gas

Arc

Resistance

Strength

10 6 10 8 10

10 6 10 8 6

4 8 4 4 6

10 4 10 10 10

10 6 10 10 10

10 8 10 10 10

10 8 8 10 10

4 10 2 6 8

A rating of 10 is best.

Lubricants Special care must be taken in the selection of lubricants. Lubricants are often used to prevent catastrophic failure of tooling components. The following guidelines are suggested: ● ● ●

●

●

Never use dry powder lubricants. Avoid using graphite, molybdenum disulfide, and oils containing these. Never try to run equipment in a dry condition if the equipment is meant to be lubricated. This will help avoid accelerated failure. – Follow the manufacturer’s suggestions for lubrication. – Check the lubricants for compatibility with products and processes. Where lubricants suggested are unacceptable, try substituting one used in the product or process (e.g., disk lube) or one known to be compatible. – Note: Fomblin or other perfluorinated grease will be chemically indistinguishable from disk lube, which can present a problem when analyzing stiction failures. Mechanisms requiring lubrication will often need exhausted enclosures to vent away aerosolized lubricant or vapors that outgas from the lubricant.

6.5.3

Guidelines for Plastics

Plastics (also called polymers) are versatile and numerous. Their versatility makes them attractive for many tooling applications. However, not only are the different plastics numerous, but each may be described using several different names, which can lead to considerable confusion. The various names include the trade name, the chemical name, abbreviations, and generic

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TOOLING DESIGN AND CERTIFICATION

family names. As an example, we have Plexiglas, poly(methyl methacrylate), PMMA, and acrylic, all different names for the same familiar clear plastic. In addition to relatively pure polymers made from one type of monomer, such as PMMA, several different monomers can be linked together, polymers can be blended together, and fillers and additives can be added to create entirely new physical properties. When choosing among a variety of plastics, it is important to consider the wide range of properties that each exhibits: ● ● ● ● ●

Physical properties (e.g., transparency, fire resistance) Mechanical properties (e.g., strength, creep resistance) Chemical properties (e.g., solvent compatibility) Electromagnetic properties (e.g., conductivity or electromagnetic interference shielding) Processability (e.g., ease of machining, ease of molding)

It is unwise to assume that the generic properties of all trade-named plastics within a generic class are identical. This is true because of the wide range of performance exhibited by difference grades within the same family and because of differences in manufacturing, formulation, and so on. It is unwise to trust that processing of the plastic has not changed its properties in applications where a specific property may be critical. For example, mistakes in molding can decrease the molecular weight of a plastic, causing it to outgas excessively, and mistakes in processing, such as the use of ionizing radiation to make fluoropolymers easy to grind, can result in the production of ionic contamination. There are general guidelines to selection of plastics from a contamination point of view. Plastics should have good chemical stability for the intended application. Plastics should not be attacked by wipe-down chemicals (such as alcohols or detergents) and crack or form a white, flaky film, referred to as chalking. To ensure that this does not happen, samples of the plastic should be exposed to the chemicals in the workplace to determine their stability. Plastics should not shed or have undesirable wear characteristics. Shedding from the starting material can usually be controlled by proper material selection and initial cleaning. Porous, foamed, and highly textured surfaces can trap dirt and be difficult to clean. Such surfaces must not be used for exposed surfaces of tooling. In use, wear can become a problem. To control wear, several factors must be considered. Where plastics are used in wear applications they must be keep within their pressure  velocity limits. Self-mating wear couples are to be avoided (self-mating refers to wear couples where both surfaces are made of the same material). The outgassing properties of plastics should be considered. Flexible poly(vinyl chloride) is made to be so by the incorporation of plasticizers such as dioctyl phthalate. Many polymers sold as static dissipative are made so through the incorporation of antistatic agents such as organic amines; and some polymers, such as silicone rubber, outgas materials that are detrimental to many products and processes. Table 6.6 lists some common plastics, their abbreviations, some common applications, their advantages and disadvantages, and some common trade names and suppliers. Plastic Composites The physical properties of plastics may be modified by addition of many types of fillers. When fillers are added specifically to alter the properties of the parent polymer, the result is often referred to as a composite polymer. Some fillers are added to improve strength, others to improve durability, and still others to improve processability.

303

Epoxies

Polyamide–imide

Polyamides

High-temperature parts

Adhesives Paints Molded products Laminated products Coatings Wafer and disk containers Filter media Pipe thread tape Tubing Electrical insulation Fibers Molded and machined parts

Acrylonitrile–butadiene– styrene (ABS)

Fluoroplastics

Vacuum-formed or injectionmolded parts

Acrylates (PMMA)

Common Applications Precision-molded parts Machine Parts Clear protective screens

Acetal

Family Name

High moisture absorption Good-to-excellent wear resistance Variable chemical resistance Good-to-excellent engineering properties Good Engineering Properties Moderate-to-good chemical resistance Fair-to-moderate wear resistance Excellent thermal properties

Highest chemical resistance Poor creep resistance Generally poor wear resistance Low strength

Good engineering properties Fair-to-moderate chemical resistance Poor wear resistance Poor impact resistance Good engineering properties Fair-to-moderate chemical resistance Poor wear resistance Excellent impact resistance Excellent bonding properties Variable chemical resistance Excellent electrical properties

Good engineering properties

Advantages and Disadvantages

TABLE 6.6 Plastics, Their Applications, Advantages and Disadvantages, and Common Trade Names

(Continued)

Nylon (common name) Capron, Allied Zytel, Du Pont Vydyne, Monsanto Torlon, Amoco

Teflon, Du Pont Halon, Allied

Cycolac, Borg-Warner Lustran

Acrylic (common name) Plexiglas, Rohm & Haas Lucite Du Pont

Delrin, Du Pont

Trade Names and Suppliers

304 Vacuum formed Transparent trays and covers Films and coatings Flexible ribbon connectors High-temperature wear Applications

Polyimides

Tubing Packaging films Coatings

low and medium density (LDPE and MDPE)

Polyethylene terephthalate (PET) and glycollatemodified PET (PETG)

Parts subject to extreme wear Material-handling equipment

high density, ultrahigh molecular weight (UHMWPE)

Tubing Vacuum-formed trays Injection-molded parts

Polyethylene high density (HDPE)

Common Applications Filter housings Impact-resistant screens Molded parts

(Continued)

Polycarbonate

Family Name

TABLE 6.6

Excellent thermal properties Good-to-excellent electrical properties Moderate-to-good chemical resistance

Transparency Fair-to-moderate chemical resistance Fair wear resistance

Transparency in thin film Heat bondable Good-to-excellent chemical resistance Moderate-to-good wear resistance

Excellent chemical resistance Excellent wear resistance Machineable Extremely difficult to injection-mold

Excellent machinability, but low strength Difficult to injection mold Excellent chemical resistance Excellent wear resistance

Transparency Fair-to-moderate chemical resistance Fair wear resistance Good impact resistance Excellent machinability

Advantages and Disadvantages

Kodar, Eastman Chemical

Bapolene, Bamberger Tenite, Eastman Chemical Many others

American Hoechst Hercules

Bapolene, Bamberger Many others

Lexan, General Electric Merlon, Mobay

Trade Names and Suppliers

305

Silicone

rigid

Tubing Seals and caulks

Wire insulation Upholstery covers Decorative laminates Pipe Tanks Chemically resistant benches

Good moldability Good machinability Impact resistant Poor resistance to solvents Transparency Excellent thermal properties Good-to-excellent chemical properties Can be high outgassing; test thoroughly

High outgassing do not use

Tubing

Polyvinyl chloride) (PVC) flexible

Polyurethane (PU)

Polystyrene (PS)

Excellent chemical resistance Good engineering properties Fair-to-moderate wear resistance Good thermal properties Poor wear resistance Poor-to-fair chemical resistance Low cost Excellent wear resistance Variable chemical resistance Translucent Static dissipative Transparency

Furniture parts Submersible pumps Appliance housings Coatings Packaging materials Films Molded parts Coatings Tubing Protective bellows

Polyphenylene sulfide (PPS)

Variable chemical resistance Fair wear resistance Excellent impact resistance

Furniture parts Appliance housings Injection-molded parts

Polyphenylene oxide (PPO)

General Electric Dow

Vinyl (common name) Geon, B.F. Goodrich

Geon, B.F. Goodrich Polyvin, Schulman

Vinyl (common name)

Many suppliers

Many suppliers

Phillips

Polyfort, Schulman

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TOOLING DESIGN AND CERTIFICATION

Among these are glass, metal and carbon powders and fibers, opacifiers and whiteners, and other plastics, such as silicones and fluoropolymers. Properties that can be modified include: ● ● ● ●

Mechanical (i.e., strength, toughness, impact resistance) Electromagnetic (i.e., conductivity, EMI) Aesthetics (i.e., color, textures) Stability (i.e., mold shrinkage, UV stability)

The additives can affect the contamination performance of the resulting composite plastic. One of the most important properties that might be affected from a contamination perspective is chemical resistance. Composite plastics are often as good as or better than the parent unfilled polymer because the filler can be chemically more inert than the parent polymer. The composite formulator should be contacted to determine the chemical resistance of the composite. Chemical compatibility tests are relatively easy to perform and should be used to verify the chemical suitability of a composite. Wear factors of composites against a steel surface can vary tremendously from those of the parent polymer. In fact, one of the primary reasons for adding fillers is to improve the wear characteristics of the plastic. Table 6.7 lists a few examples of the effect on the wear rate of adding polytetrafluoroethylene (PTFE, commonly called Teflon), silicone oil, glass fibers, or carbon fibers to parent polymers.

TABLE 6.7 Typical Unfilled and Composite Plastic Compositions and Wear Factors Parent Polymer ABS

Acetal

Nylon 6/12

Polycarbonate

Polyetherimide

Polystyrene

PTFE (wt %) 0 15 0 15 0 15 18 0 0 0 18 0 0 15 13 0 0 0 15 0 15

Silicone Oil (wt %) 0 0 2 0 0 0 2 0 0 2 2 0 0 0 2 0 0 0 0 0 0

Glass Fiber (wt %)

Carbon Fiber (wt %)

Wear Factora

0 0 0 30 0 0 0 30 0 0 0 30 0 0 0 0 0 30 30 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30 0 0 0 0 0

3500 300 80 75 65 20 8 245 190 48 85 85 2500 75 40 85 4000 130 35 3000 175

Measured using thrust-washer test apparatus, against carbon steel, 12 to 16 in. finish, 18 to 22 HRC. Wear rate units  1010 in.3 min per ft-lb/hr.

a

MATERIALS

307

10 9

Relative Wear

8 7 6 5 4 3 2 1 0 10

20

30

40

50

60

70

Shaft Hardness (Rockwell C)

FIGURE 6.15

Effect of shaft hardness on wear of bearing materials lubricated with PTFE.

4

Relative Wear

3.5 3 2.5 2 1.5 1 0.5 5

10

15

20

25

30

Surface Roughness (μin. RMS)

FIGURE 6.16

Effect of shaft finish on wear of bearing materials lubricated with PTFE.

From a tooling design standpoint, the surface finish on the metal mating surface is important, as it can have an effect on the wear rate of the plastic. Figures 6.15 and 6.16 illustrate the effect of metal hardness and surface roughness, respectively, on the wear rate of polymers lubricated by the addition of PTFE. Polymers for ESD Applications Electrostatic charges on surfaces in cleanrooms can cause serious problems, including increased rate of surface contamination, interruption of the operation of microprocessors, and damage due to electrostatic discharge (ESD). In some cases, electromagnetic interference (EMI) can also affect the performance of microprocessors. Plastics can be good insulators, static dissipative, or conductive. Most polymers are inherently insulative. Some polymers are inherently static-dissipative. However, in most cases, static-dissipative or conductive plastics are made so by adding fillers to an otherwise insulative plastic.

308

TOOLING DESIGN AND CERTIFICATION

TABLE 6.8 Some Electrical Properties of Plastics Electrical Class

Surface Resistivity (/sq)

Bulk Resistivity (cm)

Insulative Static dissipative

1012 106– 1012

Conductive EMI/RFI shielding

106

103

1011 105– 1011 away charge

105

102

Application Nonconducting surfaces Surface that will gradually drain Static shielding materials EMI/RFI shielding materials

Many options are available to obtain a static-dissipative plastic that is suitable for most applications where electrostatic discharge is a concern. If transparency is not required, polyacrylonitrile (PAN) carbon fiber–filled polymers are commended. PAN carbon fibers do not shed as easily as powder carbon fillers and thus are superior for cleanroom use. Conversely, polymers modified by use of organic amines are not considered desirable because the additives that impart the electrostatic dissipative properties outgas and are often depleted in water-based cleaning processes. For applications where transparency is desired, polyurethane plastics are an attractive alternative. Polyurethane is static dissipative without the use of additives. In addition, polyurethane is available in flexible forms, making them suitable for curtains but not for rigid applications such as safety enclosures. For safety enclosures that require transparency and static-dissipative performance, special coatings have been developed. These are applied to rigid poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), or polycarbonate (PC). These are available as sheet stock in various thicknesses and can be machined, chemically welded, and then treated in the field to ensure that their ESD performance is preserved at machined joints. For electromagnetic interference the range of available materials is more limited. To protect from electromagnetic interference effects, the plastic must be made conductive instead of simply static dissipative. This lower level of volume or surface resistivity requires the use of thick metal films, metal powders or fibers, or metal-coated carbon fibers. Generally, EMI shielding is provided when bulk resistivity is less than 102 cm. A useful summary table is shown in Table 6.8.

6.6

SURFACE TREATMENTS

There are many categories of surface treatments. Some surface treatments add material to an otherwise unacceptable surface. Other surface treatments enhance the material properties of a nearly acceptable surface by taking material away. In some cases, this removal is selective in nature. Surface coatings include paints, anodized and related chemical conversion coatings, passivation, electropolishing, electroplating, and electroless plating. Most if not all of these ways of modifying the surface of a material involves the use of chemicals that may introduce other contamination issues, if not dealt with properly. Typical surfaces requiring surface treatments include ordinary steel, which must be treated to prevent rust. Stainless steel may need passivation or electropolishing, depending on grade of steel, its fabrication processes, and its intended use. For example, welding can change the corrosion resistance of an otherwise corrosion-resistant stainless steel. It may

SURFACE TREATMENTS

309

TABLE 6.9 Comparative Properties of Typical Paints Paint Type Elastomeric polyurethane No pigment, gloss No pigment, flat Pigmented Amine epoxy enamel Polyester urethane enamel Polyamide epoxy enamel a

Abrasion Weight Lossa (mg)

Impact Resistance (in.-lb)

Nil to 8 Nil to 22 Nil to 33 40–60 50–70 90–150

100–160 90–160 80–160 50–120 30–80 30–50

Data obtained using a Taber abraser, CS-17 wheel, 1000 cycles.

then be necessary to repeat passivation of stainless steel after welding to prevent it from staining. Aluminum often needs surface treatment to yield chemical and wear resistance. Requirements for surface treatment are identical to those for bulk materials, which are restated here briefly for emphasis: ● ● ● ● ● ●

Surface treatments must not outgas harmful vapors. Wear and parts damage must be minimized by design. Surface treatments must be compatible with cleaning fluids and processes. Surface treatment must be nonshedding. Porosity should be minimized. The surface must be smooth and provided with generous fillets and radii. Textured surface finishes should be avoided.

6.6.1

Paints

Paint can provide a low-cost surface finish for low-cost materials, such as high-carbon steel for cabinets. Used without coating, the steel will rust and rapidly generate contamination. Painted, it will be corrosion resistant and easy to clean provided that the painting process is controlled properly. The most important consideration for painting is to start with good surface preparation to ensure good adhesion. The surface must be free of machining and cutting oils, rust, scale, and excessive oxidation. The paint to be applied must take into consideration exposure to splashes or wipe-down by chemicals. In general, paints are unsuitable for immersion application but often are acceptable for incidental contact with chemicals. Some properties of typical paints are compared in Table 6.9. Epoxies are among the most common of industrial paints. They are available as epoxy enamels and epoxy urethanes. These two classes of epoxy paints have applications for which each is ideal. Epoxy enamels usually have good chemical–solvent resistance. They also have fair to good abrasion resistance and good adhesion. Their chip resistance is fair to good. Urethane enamels have variable chemical resistance. They can have excellent abrasion resistance and excellent adhesion. Urethane paints are available in two forms. Urethanes that cure to hard, brittle coatings are generally enamels, intermediate between epoxy enamels and elastomeric urethanes in chip and abrasion resistance. Often, these are among the most chemically resistant paints. Urethanes that cure to elastomeric coatings are intermediate in

310

TOOLING DESIGN AND CERTIFICATION

chemical resistance between epoxy enamels and urethane enamels, but have the best chip and abrasion resistance. Guidelines for paint selection are as follows: ●

●

● ●

Epoxy enamels should be used for surfaces not subject to abrasion, impact, or repeated exposure to chemicals. Urethane enamels should be used for surfaces subject to infrequent abrasion or impact but often exposed to chemicals. Surfaces subject to abrasion and impact should be painted with elastomeric urethanes. The chemical resistance should be checked for the intended application.

Paints may be applied using a variety of processes. Beside the familiar methods of brushing, spraying, and dipping, materials can be powder coated or electrophoretically coated, processes analogous to electroplating of metals. Powder coating is generally performed as a solvent-free operation. The dry paint particles are charged electrostatically, and the part to be painted is charged to the opposite polarity. This attracts the paint to the surface of the part. Paint thickness builds relatively uniformly. After coating, the paint is fused at elevated temperature. This usually results in a pore-free surface but can result in porosity in the paint below the surface. Machining after powder coating can result in exposing the porous paint. Electrophoretic painting is even more similar than powder coating to electroplating of metals. In electrophoretic painting, droplets of paint are dispersed in a conductive water solution. The paint droplets are charged to one polarity, and the part to be coated is charged to the opposite polarity. Paint accumulates on the surface to a relatively uniform thickness. Because the paint is applied in liquid form, the droplets coalesce on the surface of the part to produce a relatively uniform pore-free surface. 6.6.2

Anodizing and Related Treatments

Aluminum has many desirable properties, including light weight and easy machineability, but abrasion and chemical resistance are not among them. Fortunately, both abrasion and chemical resistance may be improved by anodizing and related surface treatments. ● ● ● ● ● ●

Anodizing is the electrochemical conversion of a metal surface to metal oxide. As metal is converted, the size of a part often increases. Oxide coating is thin at sharp corners. For chip resistance the outside corners should be radiused. The ratio of radius to coating thickness should be about 30 : 1. To improve wear resistance, electropolishing should precede anodizing.

Chromate conversion coatings are another form of chemical conversion coating frequently used for aluminum, zinc, or magnesium alloys. Unfortunately, these are very thin and do not provide abrasion resistance but are often applied to provide some corrosion protection. However, their primary use is as a surface treatment prior to the final surface finish, often to improve paint adhesion. It is not recommended that chromated surfaces be used as the final finish; they can be used as primers. Note that chromate conversion coatings can be a problem on casting alloys. Cast alloys usually have significant porosity. During the chromating process, some of the bath chemicals can get trapped in this porosity and it can be very difficult to remove the residue trapped in the pores in subsequent cleaning processes.

SURFACE TREATMENTS

6.6.3

311

Electroplating, Electropolishing, and Other Treatments

Electroplating is the process of depositing a metal film on the surface of a conductive part. When the part is immersed in a solution containing ions of the metal that is to be deposited and electric current is applied, a metal film is formed on the surface from the ions in solution. In this process, electric current is needed. The plating process tends to build up extra thickness at corners but has difficulty plating inside blind holes. In electroless plating the metal film is deposited when the part to be plated is immersed in the plating solution; no external electrical energy need be applied. Electroless plating tends to form films of uniform thickness. Thus, film thicknesses on corners and within holes tend to be uniform. As with electroplating, it is important that the bath solution be in contact with the part surface. Air bubbles trapped in corners and in blind holes can thus result in voids in the coating. There are also special surface treatments for stainless steels: passivation and electropolishing. Passivation of stainless steel is done by immersing the parts in dilute nitric or citric acid. This partially dissolves metal impurities from the surface. Stainless steels are alloys of iron with chromium or chromium and nickel. During passivation, iron becomes depleted from the surface of the part, and much of the chromium and nickel become converted into oxides. This provides an enhanced degree of resistance from corrosion over untreated stainless steel alloy. Electropolishing is essentially the opposite of electroplating: Instead of adding material to the surface, material is removed. Whereas electroplating tends to add material more rapidly to corners (i.e., peaks in surface roughness), electropolishing tends to remove material more rapidly on corners and peaks in the surface roughness. This results in an improvement in surface roughness. Many metals other than stainless steel, including aluminum, may be electropolished. 6.6.4

Cautions About Coatings

If doubt exists as to the ability of the coating to withstand its intended use, tests should be run. Electroless nickel coatings on aluminum or steel are often thin enough that the coating is not free of pinholes, whose exposure to moisture, alcohol, or other chemicals can result in electrochemical reactions. Such corrosion is often referred to as galvanic corrosion. Two dissimilar metals in contact will behave like a battery. The presence of water or other chemicals can accelerate this reaction. If the plated parts undergo assembly after coating, the plated surface may become cracked, again offering the possibility of galvanic corrosion. Coatings are almost always applied using processes that involve chemicals that promote corrosion. Residues left on parts after coating can thus generate contamination. Proper metal plating procedures specify how the chemicals should be neutralized and washed away. 6.6.5

Synergistic Coatings

Synergistic coatings were originally developed to lubricate and protect moving parts aboard manufactured satellites. Ordinary lubricants based on petrochemicals and even perfluorinated synthetic lubricants could not withstand the intense vacuum (outgassing), temperature extremes (typically, 200 to several hundred degrees Celsius in near Earth orbit), and radiation. Dry lubricants would migrate and foul delicate equipment. Properties that make synergistic coatings work in satellites in space applications make them good for tooling applications in the cleanroom. Among these properties are excellent

312

TOOLING DESIGN AND CERTIFICATION

TABLE 6.10 Wear Properties of Various Coating Materials Coating Material

Coating Wear (cm3/min)

440C, HRC 58 (ref.) Silver Hard chromium Flash chromium Hard anodize  PTFE (synergistic) Hard anodize Hardened electroless nickel (HRC 65) Hard nickel (HRC 50) Clear anodize Soft nickel (HRC 41) Tin Cadmium Electroless nickel (HRC 45) Black nickel

1 1 2 3 4 10 15 17 20 53 79 80 162 578

Block Wear (cm3/min) 3 1 2 2 5 10 4 2 19 Nil Nil Nil 2 Nil

corrosion resistance and excellent wear resistance. They should be considered for any application where exposure to harsh environments could damage the unprotected metal surface, where lubrication cannot be tolerated, and where generation of wear debris is a highly probable. Several examples can be cited where synergistic coatings may be an ideal choice. The surfaces of moving parts in vacuum equipment might be one of the best, especially where the atmosphere in the equipment can corrode or contaminate the tool. The surfaces of parts that come in contact with product repeatedly, such as preload devices and clamps, are an excellent choice to minimize debris generation. Synergistic coatings have been developed for aluminum, steel, stainless steel, copper and copper alloys, magnesium, titanium, and other metals and alloys. 6.6.6

Relative Wear Properties of Coatings

Table 6.10 summarizes the relative wear characteristics of some common surface coatings. In this test a 440C stainless steel block was rubbed against a steel shaft coated with various materials. The 440C block was HRC 58 hardness, with an RMS 16 surface finish. The wear of various coatings and the block are listed as volume loss. Hard chrome, flash chrome, and synergistic coatings provide the best wear combinations among the usual engineering materials. 6.6.7

Surface Texture and Porosity

Some control specifications include a statement prohibiting the use of porous or textured surfaces. Such a specification prohibiting roughness or texture may not be practical for the tool designer to comply with. First, surface roughness is a relative term. The cost to achieve a microscopically smooth and pore-free surface is very high and might be unjustifiable. If no way of measuring compliance is given, the tool designer has no way of knowing what is acceptable. Finally, it is often desirable to use commercial devices that are not available in alternative surface finishes.

SELECTION AND EVALUATION OF COMPONENTS

313

These guidelines provide a practical solution to the problem of specifying what is an acceptable surface roughness and porosity: ●

●

●

● ●

Surface roughness and texture should be a minimum on the surfaces of tool components that come in contact with or which could shed debris on the product. Peripheral devices (personal computers and other support equipment) that do not come in contact with or in close proximity to the product can have surface roughness and texture in their commercially available form. Acceptable surface roughness and texture on peripheral devices is a surface that can be cleaned using wipers and cleaning fluids. A surface that cannot satisfy the white glove test after wipe-down is unacceptable. A surface that shreds, tears, or snags moist cleanroom wipers is unacceptable.

Tearing and shredding of wipers can itself form a trap. In one case, the buyer of a tool tried to argue that a surface was unacceptable because it shredded a dry paper facial tissue when vigorously rubbed. However, dry facial tissues are much more fragile and likely to tear and shed paper fibers than are cleanroom wipers moistened with a cleaning fluid. It is for this reason that most companies do not allow dry wiping with paper products in their cleanrooms. Surface finish recommendations are as follows: ● ● ●

Components with a 64-in. RMS surface finish are generally acceptable. A 125-in. RMS finish is conditionally acceptable on inspection of prototype parts. Surface finishes smoother than 64in. RMS can be expensive to achieve.

In one rather amusing surface finish anecdote, tool designers asked if the surface finish on an electropolished stainless steel panel could be changed to a mat finish. When asked why this was requested the tool designer responded that it would hide fingerprints better. Clearly, this designer did not understand the underlying purpose for the surface finish.

6.7

SELECTION AND EVALUATION OF COMPONENTS

Electric motors, pneumatic cylinders, belt drives, and similar assemblies are components. Components exposed to the cleanroom must comply with requirements for chemical stability, materials, and surface finishes. In general, the following guidelines should be followed: ●

●

●

Airborne particle concentrations from components must conform to the requirements for tooling. Materials for components must comply with the requirements for production of chemical vapors. Materials that contact product surfaces must be ESD Safe and be at least as clean as the product.

Components for which no use experience exists should be subjected to contamination evaluation as separate items.

314

TOOLING DESIGN AND CERTIFICATION

6.7.1

Pneumatic Devices

Pneumatic devices are common in cleanroom automation. Pneumatic devices often rely on a facility compressed dry air (CDA) system. Most manufacturing engineers believe CDA means clean, dry air. Most contamination engineers know it means compressed, dirty air. Do not assume the CDA in a facility is clean; it rarely is. Typical contamination generation points on pneumatic cylinders include wear debris from shaft seals and aerosolized lubricants, necessary to ensure reliable operation of the pneumatic cylinder, will add to the contamination in the CDA that leaks from the pneumatic device. One important observation is that a pneumatic device may initially be tested and found to be acceptably clean, but they gradually deteriorate in contamination performance unless the device has been designed specifically for cleanroom use. The guidelines for control of contamination that may come from pneumatic devices are as follows: ●

●

●

●

● ●

Try to select pneumatic devices that have been designed specifically for the cleanroom environment. Never run a pneumatic device dry when it was intended to be run lubricated. Eliminating the lubrication will cause premature failure of the pneumatic device. The manufacturer’s specified lubricants may be considered unacceptable for the customer’s application. When this occurs, the most likely causes are unacceptable outgassing performance or a tendency to degrade in the intended environment, usually because of attack by chemical vapors in the customer’s process. Ask the customer what lubricants are acceptable. In many industries, experience with lubricant problems has resulted in selection of substitutes with which the customer has acceptable experience. In cases where pneumatic devices are to be modified for a cleanroom application, evacuated enclosures are preferable. Self-evacuating pneumatics are available. Custom-made bellows to contain the contamination are possible. However, there are drawbacks to using bellows. Among these are that they require mounting brackets, which in combination with the collapsed space of the bellows consumes linear stroke, their complex shapes make them difficult to clean, and they need an exhaust line. All of these add mechanical complexity to the cylinder.

6.7.2

Linear Motion Guides

With linear actuators and linear motion guides, it is wise to subject components to qualification tests prior to using them. Two designs that have been used with some success include the linear bearing and shaft and V-grooved wheels riding on a V shaft or groove. 6.7.3

Electric Motors

Electric motors are often generators of contamination. Contamination emerges from around electrical feed-through openings. It also appears in abundance around shaft openings and from seams in the housings. As with pneumatic cylinders, an electric motor may initially appear to be clean but periodically produce unacceptable concentrations of contamination. An example is shown in Figure 6.17, Where we see that the motor produces periodic bursts of contamination that exceed FED-STD-209 class 100 (ISO class 5) or class 1000 (ISO class 6) that last for hours, but in other intervals the motor operated at better than class 10 (ISO class 4).

SELECTION AND EVALUATION OF COMPONENTS

315

Concentration (ppcf ≥0.5 μm)

100,000

10,000

1,000

100

10

8:

1 20 0:0 :1 0 0: 8: 00 1 20 0:0 :1 0 0: 8: 00 10 20 :0 :1 0 0: 8: 00 10 20 :0 :1 0 0: 8: 00 10 20 :0 :1 0 0: 8: 00 10 20 :0 :1 0 0: 8: 00 1 20 0:0 :1 0 0: 8: 00 10 20 :0 :1 0 0: 8: 00 10 20 :0 :1 0 0: 8: 00 1 20 0:0 :1 0 0: 8: 00 10 :0 0

1

Time of day (hh:mm:ss)

FIGURE 6.17

Burst pattern of particle generation from a stepper motor. Encoder to motor housing tape seal

Shaft

Encoder housing

Motor housing

Evacuated enclosure Evacuated enclosure To vacuum

FIGURE 6.18

Contamination control fixes for a stepper motor.

The motor was carefully scanned using an optical particle counter while the body of the motor was pressurized with filtered air. Contamination was found to leak out of the motor body of the motor at the shaft opening and around the wire feed through a grommet. These two contamination sources were not surprising. More surprising was the discovery that contamination leaked from the very close seam between the encoder and motor portions of the casing. Figure 6.18 illustrates the contamination control fixes that were used to change the motor, rendering it continuously acceptable for class 10 use. 6.7.4

Process Piping and Point-of-Use Filtration

Tools should be planned to include point-of-use filtration for process fluids. The reason is that few utilities provided in cleanroom facilities are able to maintain the desired degree of

316

TOOLING DESIGN AND CERTIFICATION

cleanliness throughout the distribution system over the life of the facility. Because it is often necessary to install new tooling in older facilities, planning for possible degradation of an existing process fluid distribution system, planning the necessary filters within the tools is a good precaution. In addition, process fluid distribution contamination integrity can become compromised after the process tools have been installed. Process fluid filters within the tools thus provide a valuable backup for upsets that can occur in the process fluid distribution system of the facility. In the design of the plumbing and filtration system, the following must be considered: ● ● ●

Chemical compatibility with the process fluids Delivery pressure and flow rate Amounts and types of contamination to remove

The factors can then be used to select the materials of construction of the fluid delivery system, including size, materials selection, valves, and filters. In addition, for critical processes, this planning should include the types of process fluid monitoring sensors, local and remote alarms, and so on. Filters need to be replaced periodically. The need for this maintenance must be considered in the conceptual design phase, so the filters can be conveniently placed. When tools contain both liquids and gases, these are usually located in separate compartments. Liquids will require secondary containment and often, chemical exhausts. Gases usually require chemical exhausts. Where toxic or flammable chemicals are employed, leak detection is an important safety consideration in addition to compartment sealing. Documents should clearly indicate where filters are located. Many times tools have been found in service with empty filter cannisters, because filter cartridges were never installed as part of initial tool set-up. 6.7.5

In Situ Monitoring Equipment

Figure 6.17, which showed the burst pattern of particle generation from an electric motor, illustrates an important consideration for tooling in general. Contamination generated from a tool can appear to be acceptable for long periods of time, only to produce unpredictably large amounts of contamination. In situ (within) monitoring equipment is available to deal with this random generation of contamination. Monitoring equipment is available for all of the contamination types that may be specified for the tool. Plan for their installation in tool design. A partial list of what may be monitored includes the following: ● ● ● ● ● ● ●

Particle counting for air-and/liquid-borne particles Organic and ionic contamination in fluids Particles in vacuum chambers, ovens, and so on Organic and inorganic contamination in the vapor phase Vibration levels Pressure in compressed air or vacuum exhaust systems Electrostatic charge levels, including alarms and performance of air ionization systems

Figure 6.19 illustrates how several of these can be combined on a hypothetical tool. The subject of continuous monitoring is covered in more detail in Chapter 7.

SELECTION AND EVALUATION OF COMPONENTS

317

Air ionizer Particle counter. Body of the particle counter is mounted outside the work envelope. Inlet tube is located at the front of the work envelope between the operator and the product.

Charge monitor and ionization sensor, positioned at the back of the product work envelope Wrist strap monitor

Work envelope

FIGURE 6.19

..

Typical continuous monitoring system for a single workstation.

In situ monitoring for process equipment has several advantages. For example, the ESD or contamination state of the tool can be sensed in real time. Sensitive product can be held in protective packaging or enclosures until contamination or ESD conditions are under control. In many semiconductor processes, in situ monitoring has replaced the use of witness plates as tool qualification criteria. In addition, in situ continuous monitors have been used to optimize process cycles [3]. 6.7.6

Hand Tools

The considerations for hand tools are similar to those for end effectors on automation. Materials of choice must be wear resistant and compatible with cleaning chemicals and processes. Surface finishes that are intended to improve grip, such as knurled or deeply grooved surfaces, may tend to harbor contamination and be difficult to keep clean. The handle of the tool has an additional consideration. The materials and surface finishes for the handles must be compatible with the gloves and hands of operators. Where tools are used in static-protected workplaces, conductive or static-dissipative materials are preferred over insulative surfaces. Often, plastic-handled tools are treated by the use of topical antistatic agents or are wrapped with conductive tape. Where contamination is a concern simultaneously with ESD, such treatments may introduce greater contamination problems than the ESD problem they eliminate. Ceramic or ceramic-coated tips on hand tools are worthy of consideration. Tungsten carbide and titanium carbide have excellent wear properties. Many ceramic alloys have been formulated to be static dissipative as well, an important consideration in ESD applications. In many ESD applications, the slower discharge afforded by static dissipative tweezers is preferred over the rapid discharge provided by conductive tweezers to prevent damage to ESD-sensitive components that may have become charged inadvertently. Conveyors that are designed to eliminate friction between the bottom of a package and the transport mechanism are preferred. Several different designs are available to accomplish this: ●

Packages can be lifted at accumulation points while the conveyor transport mechanism moves continuously. This requires additional mechanism and thus introduces mechanical complexity.

318 ●

●

TOOLING DESIGN AND CERTIFICATION

Some conveyors halt the transport mechanism at accumulation points. This requires sensors to detect the presence of packages. Some conveyors use rollers that stall against the bottom of the package while the conveyor transport mechanism continues to move.

These designs have two very significant advantages. When the package stops moving, no wear occurs because it is either not in contact with the moving portions of the transport mechanism or the transport mechanism also stops. This reduces contamination from both the conveyor mechanism and the packaging material. In addition, tribocharging is reduced or eliminated because the rubbing action stops. Choice of materials for the belts or rollers in the transport mechanism that will be in contact with the package is important. Fortunately, this is another case where modern materials make the choice of materials compatible with both contamination and ESD requirements. Carbon fiber–filled plastic rollers or polyurethane belts offer both good wear resistance and can be made conductive or static dissipative.

6.8

TOOL AND WORKSTATION LAYOUT

One of the more important considerations in the design and layout of tooling and workstations is the way that airflow induced by the tooling design affects airflow and thus contamination behavior and performance of air ionizers. To appreciate the importance of airflow in tool design, the way that air flows in a cleanroom should be understood. How this is affected by installation of tooling is explored below. The discussion of tooling effect on airflow design is discussed in Chapter 4. The effects or airflow must be considered for all portions of tooling design: (1) material handling systems, including pick-and-place robots and precision-handling robots; (2) parts storage locations; (3) product fixtures; and (4) operator movements. However, with the growing importance of the use of minienvironments, some additional details for airflow considerations within tooling are included here. 6.8.1

Flow Control Enclosures, Minienvironments, and the Standard Machine Interface

Airflow in flow control enclosures and minienvironments includes important additional considerations not normally considered as part of facilities design: ● ●

●

●

One of the most important of these is the concept of the protection ratio. A second consideration, is that of induced flows, similar to the induced rotational flows, called standing recirculation zones, which are found in non-unidirectional-flow cleanrooms. An example of induced airflows within a minienvironment enclosure is shown in Figure 6.20. A third consideration is the need for the minienvironment to be opened periodically for operator intervention or maintenance access. The effect of air induction into a minienvironment due to the Venturi effect must also be considered. Air flowing past an opening creates a pressure drop. This pressure drop can induce airflow into a chamber even though the chamber is positively pressurized with respect to the ambient cleanroom.

TOOL AND WORKSTATION LAYOUT

319

Induced airflows

Induced airflows

Primary clean airflow

FIGURE 6.20 The primary clean airflow into the enclosure, shown by the dashed arrow, induces airflows within the chamber, shown by the solid arrows. These induced flows can create standing recirculation zones that increase the residence time of the contamination inside the chamber.

The protection ratio is the ratio of the contamination concentration inside the minienvironment to the contamination concentration outside the enclosure: Cpr 

Ca Cm

where Cpr is the contamination protection ratio, Ca is the contamination concentration in the ambient cleanroom, and Cm is the contamination concentration in the minienvironment. In general, the higher the contamination protection ratio, the better. Particle contamination protection ratios greater than 100 are not uncommon. However, there have been cases discovered where particle contamination protection ratios below 1 have been observed.

Case Study: Minienvironment A preliminary investigation was conducted to determine where and how to monitor minienvironments. This semiconductor factory is a fully SMIFed facility, offering several possibilities regarding where and how to sample. Our survey instruments included a field potential meter for electrostatic charge survey, a Met one 227 optical particle counter (0.1 ft3/min, 0.3-m resolution), and a PMS 0.lLPC turbo 110 (1.0 ft3/min, 0.1-m resolution). The preliminary investigation focused on Clean Track photoresist-apply tools. These tools are installed in a cleanroom with ceiling filter coverage that would suggest a FED-STD-209 class 10,000 (ISO class 7) design. The garmenting discipline in the room and level of automation may allow the room to operate as clean as class 1000. Accordingly, a properly designed fully SMIFed minienvironment would be expected to operate with a contamination protection ratio of approximately 100, allowing class 10 to class 1 performance for the photoresist-apply process. These tools include SMIF indexers for wafer input and output, photoresist-apply and developer tools, hot plates and cold plates for wafer thermal conditioning, and an exposure tool.

320

TOOLING DESIGN AND CERTIFICATION

Masks were also housed in wafer pods located within the exposure tool enclosure. All of the wafer handling within this integrated tool was automated, including: ●

●

●

Automated indexers arms for opening SMIF pods and picking and placing wafers from the wafer carriers within them. A four-axis robot (X, Y, Z and ) for handling wafers picked and placed by the SMIF arms onto thermal conditioning stations, in and out of the photoresist bowls, and in and out of the exposure tool. Automation for changing the photomasks, also contained within SMIF pods.

Electrostatic Charge Levels Our primary interest was in measuring charge levels on SMIF pods when in storage, on the automated handling system, and when placed manually on the indexers. ●

●

●

Charge levels on the exterior of pods in storage locations was consistently measured to be less than 1000 V and averaged around 250 V. Pods measured in the production area, where pods are handled by an automatic guided vehicle (AGV), averaged 2000 V. Pods measured in the manually loaded area were consistently over 1000 V, averaged approximately 5000 V, and had a high reading of 18,000 V.

We attribute the low charge on stored pods to a long storage duration; it was undoubtedly a long time since these were handled. The intermediate charge level on pods handled by the automated material-handling system in production is undoubtedly due to tribocharging by the AGV and robotic stocker. Conversely, manual handling produces high static charge levels. The general cleanroom environment was provided with overhead room ionizers. The measured discharge times directly under an ionizer set was consistent with general room performance. That is, the discharge times averaged 30 to 40 seconds from 1000V to less than 100V, with the float potential in the range 50 to 100V. Sampling of the Photo Tool Figure 6.21 shows the layout of the photoresist-apply tool. Three primary areas were sampled for airborne particles: the ambient environment of the load station during the manual loading of the wafer pods by an operator, labeled A in Figure 6.21; inside indexer 1 and the wafer transfer hard automation, labeled B in Figure 6.21; and the area immediately next to the robot as it picked and placed wafers on the hot and cold plates, labeled C. Table 6.11 summarizes the airborne particle counts in these three locations during various stages of operation. Three different particle sizes were reported: 0.1, 0.3, and 0.5 m. Ten samples were collected at each location so that both the mean and standard deviation of particle counts could be determined. Particle concentration was equal to or larger than the stated size per cubic foot of air. Particle concentrations in the ambient cleanroom with no activity at the wafer loading– unloading station are well within the class 1000 requirements. The movements of the operator to load and unload the wafer pods added only slightly to the particle counts, which remained well within class 1000 conditions. Samples taken at location B show the elevation and translation movements of the transfer tool. Particles added by this mechanism were not statistically different from the particle counts taken when there was no tool movement. These data indicate that a protection ratio in location B afforded by the minienvironment was much greater than 1.

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Wafers in pods on indexers

Hot and cold plates C Photo exposure stepper

Robot work space

A Photoresist apply and develop

Vertical, unidirectional flow minienvironment

B

Indexer 1

Wafer transfer hard automation

FIGURE 6.21 Plan view of an integrated photoresist apply–expose–develop tool. The entire portion of the tool within the dashed line was enclosed within a vertical unidirectional-flow minienvironment. TABLE 6.11 Particle Concentrations in and Around an Integrated Photoresist Tool in a Minienvironment in a Class 1000 Cleanroom Particle Size Location

Activity

A

Idle

B

Operator pick and place Idle Wafer elevate Wafer translate

C

Idle Robot move 1 Robot move 2 Robot move 3

Average Std. dev. Average Std. dev. Average Std. dev. Average Std. dev. Average Std. dev. Average Std. dev. Average Std. dev. Average Std. dev. Average Std. dev.

0.1 m

0.3 m

0.5 m

33.7 38.7 64.7 17.6 0.5 0.7 0.0 0.0 0.3 0.8 7.3 12.0 9.6 24 247 151 978 278

14.2 17.0 26.9 10.6 0.0 0.0 0.0 0.0 0.3 0.8 0.0 0.0 8.0 20.7 103 72 338 79

5.0 6.3 13.1 4.3 0.0 0.0 0.0 0.0 0.1 0.1 0.0 0.0 6.6 16.9 45 36 192 42

The particle concentrations measured at location C were a different story. In the idle condition, very few particles were measured. However, as soon as the robot began moving wafers, the particle count rose significantly. The particle counts with the robot moving continued to rise through three consecutive sets of 10 samples, as the robot continued moving the entire time. The concentration inside the minienvironment exceeded those in the ambient cleanroom environment after only 10 minutes of operation. After 30 minutes of operation, the particle

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counts were still rising. (Note: At this point our host elected to terminate processing on this tool. We sample two other identical tools at the same fabricator site and found the same result.) Safety enclosures often can act as flow control enclosures. Enclosures that do not provide HEPA filters, air ionizers, or blowers must consider the following: ●

●

The cleanliness of the interior depends on the cleanroom air supply, air ionization, and so on. Openings must be provided in the enclosure ceiling, walls, and floor for HEPA-filtered air.

Minienvironments are enclosures with HEPA filters, blowers, air ionizers, and so on. These may be fully sealed and accessed using a loading–unloading feature. Considerations for the design of the minienvironment are discussed next. Maintenance Access In many tools there is a need for operator or maintenance access, in which one or more doors or panels must be opened. This can temporarily disrupt the cleanliness within a tool. One strategy that has been employed is to increase the flow rate of the air supply fans when a door is opened or an access panel is removed, to provide additional clean air during these moments. The increase in airflow can be automated either by using a door position sensor or by looking at the pressure within the enclosure. Care must be taken to ensure that the increased air velocity by this approach does not increase turbulence within the enclosure or increase the induction of contamination into the chamber by the Venturi effect. Venturi Effect High-velocity air produces localized negative static pressures, which can cause air to leak into the enclosure. If the static pressure within the enclosure is great enough, little air induction will occur. However, if the static pressure inside the enclosure is decreased, eventually some air will begin leaking into the enclosure due to the Venturi effect. 6.8.2

Putting the Cleanroom Tool Together

Several options are available for construction of the finished tool. One approach is to assemble the tool in a shop air environment, deliver it to the site, and attempt to clean up the finished tool. This often results in difficulty in achieving the desired level of cleanliness for the tool. A second alternative is to follow the process outlined below. ●

●

● ● ●

Parts and subassemblies are precleaned and measured to assure compliance with requirements. Cleaned parts are delivered from the cleaning operation directly into a temporary portable or softwall cleanroom for final assembly of the tool. Contamination generated by the assembly process is removed as soon as possible. After assembly, testing for contamination is done before removal from the cleanroom. The clean package is prepared for shipment in the cleanroom.

Cleaning of Parts and Subassemblies Parts that can be immersed can be cleaned using a variety of techniques. Among these are some cleaners encountered very commonly in the tool shop. These includes vapor degreasing, in which a part is immersed in a solvent

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vapor. The solvent vapor condenses on the cold surface of the part, extracting particles and dissolving soluble contamination. Be aware that vapor degreasing has relatively low efficiency for removing particles but can be quite effective at removing soluble materials. One drawback is that many of the chemicals previously used in vapor degreasers, halogenated hydrocarbons, may no longer be used because of their ozone-depletion and global-warming potential. Alternatives include alcohols and hydrofluoroethers. Spray cleaning is also an option. It has the advantage over vapor degreasing of being better at particle removal. The sprayed liquid is usually caught in a sump, then filtered and dewatered so that it can be used repeatedly. One of the biggest applications is to spray using water. This is often done after the part or subassembly has been immersion-cleaned in an aqueous detergent solution. Ultrasonic immersion cleaning is also a very common alternative. Laboratory-size cleaners are commonly in use. Aqueous detergent solutions are the most commonly used. It may come as a surprise to many, but soaking and hand scrubbing are very effective. All that is needed is a suitable container (e.g., sink, plastic bucket, 33-gallon plastic garbage pail), some simple tools (toothbrushes, pipe cleaners, bottle brushes), and some patience. After cleaning, rinsing and drying are critical. If allowed to dry on parts and subassemblies, residual contaminants and detergents can cause the final tool to fail contamination tests. Initial rinsing can be done with tap water, followed by a deionized water rinse to eliminate water spotting from water hardness chemicals in a city water supply. Excess moisture can then be blown off the part with compressed air or nitrogen. This can be followed by a slow bake at 50 to 60°C. Baking can be as short as 20 minutes for small uncomplicated parts to as long as 12 hours for printed wiring boards and similar very complicated subassemblies. Consult the wiring board manufacturer or the connector manufacturer for recommendation about protective coatings, pin lubrication, and so on. Of course, there are many parts that should not be immersed. These are parts where irreparable damage could occur or where the lifetime could be shorted significantly by immersion in water or solvents. Parts that usually cannot be wetted by immersion include bearings, motors, pneumatic cylinders, gearboxes, and optical encoders. These all require superficial cleaning, preferably using a damp but not dripping clean wiper followed by blow drying using ionized filtered air. Oven or vacuum oven drying can be used to finish the cleaning process. After the parts have been cleaned, they should not be handled with bare hands. Consult the customer for recommendations regarding what types of gloves to wear and what types of packaging materials to use if parts must be transported to the final assembly location. Verify surface cleanliness by direct or indirect inspection. Direct surface inspection is suitable for optical surfaces and surfaces with a smooth surface finish. Techniques include low-power binocular microscopy using oblique illumination or ultraviolet illumination. Indirect inspection may be required for complex surfaces unsuitable for direct inspection. For example, a part with many blind holes is difficult to inspect using a microscope. Indirect inspection includes two steps: (1) removal of contaminants (extraction), and (2) measurement. Extraction methods include Scotch tape lift, sticky stubs, flushing with a stream of fluid (especially good for blind holes), and ultrasonic extraction. Particle measurements include capturing the contamination on a filter and inspecting the filter, inspection of tape lift samples using densitometry, or turbidimetry and liquid-borne particle counting of flushed or extracted samples. Ionic measurements include ion chromatography, ionography, ion selective electrodes use, and spot tests. Organic measurements include gas chromatography, mass spectroscopy, infrared and ultraviolet spectroscopy, contact angle goniometry, and optically

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stimulated electron emission. The variety of cleaning and inspection methods is so great that it impossible to discuss this subject here thoroughly. Fortunately, the techniques for cleaning parts for tooling are no different than those for cleaning product parts. Refer to Chapter 5 for a more detailed discussion of this important topic. Assembly Environments After the parts have been cleaned, they are delivered to the cleanroom. If the cleaning has been done in an off-site location, the parts will come packaged in clean packaging materials such as polyethylene or nylon bags. These bags should not be removed until after the parts are in the cleanroom or an equipment pass through. Cleanrooms in this context range from small clean benches to full-size cleanrooms. Portable modular soft-wall cleanrooms are inexpensive, usually available on short lead times, and adaptable to larger or smaller tools. Clean benches and downflow units are attractive for temporary clean space, particularly in the shop. This can be further enhanced by making them part of a soft-wall cleanroom. The tool is assembled in the cleanroom. After each assembly operation, assembled items should be wiped to remove all visible contamination (do not wait until assembly is complete to remove assembly debris). Do this with the gloves, wipers, and fluids that are to be used in the final location to verify that surface finishes are acceptable and that the wipe-down procedure is adequate. Contamination generated by assembly process is removed as soon as possible. For example, if holes must be drilled and tapped, this should be done while vacuuming the drill point. Where lubricants are applied, the excess should be wiped up immediately. If left to stand for a long period of time, lubricants will gradually spread over a larger area, making cleanup more difficult. Soldering, braising, and welding processes should be accompanied by vacuuming during operation, and residual flux should be cleaned up immediately afterward. Finally, if the tool is subject to acceptance qualification tests, such as an airborne particle count test or a test for residual charge retention on surfaces, consider doing a preliminary test before removing the finished tool from the cleanroom. If problems are found, they are usually easier to correct in the shop than in the field. Final Cleanup and Packaging The area where final packaging is done is usually within the cleanroom itself. Package using clean packaging materials such as level 100 film stock and bags, specified tapes, and cleanroom-acceptable desiccants. Consult the customer for acceptable materials for this step. Some customers may require that the tooling be packaged twice. Referred to in contamination control as double bagging this is done in the cleanroom with clean packaging materials. The double-bagged tool is then removed from the cleanroom and finally packaged for shipment to the customer. Installing a Tool in a Cleanroom The first step in the installation of a tool is to remove the packaging material. If the tool has been double bagged, determine from the customer where to remove the outer packaging material. Some customers allow this to be done outside the cleanroom; other customers have tooling pass through rooms where this is done. Others will have an unpacking area in the cleanroom itself. The outer surface of the outer package is dirty because it has been exposed to the non-cleanroom environment. Care must be taken when removing the outer package to minimize contamination of the inner package for double-wrapped packaging.

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An idealized description of activities that should be followed to move a tool successfully into a cleanroom without disrupting operations is as follows: 1. Remove the outer packaging material (cardboard or wooden shipping container) on a loading dock or other non-cleanroom environment. 2. Clean all gross contamination from the exterior of the outer cleanroom package by vacuuming and wiping. Achieve visible cleanliness. 3. Move in to the change room, air lock, or temporary vestibule. 4. Remove the outer clean packaging material. 5. Vacuum and wipe to achieve visible cleanliness on the outer surface of the inner packaging material. 6. Move to the intended location in the cleanroom. 7. Erect a temporary enclosure. 8. Remove the inner packaging material. 9. After final assembly and debugging, reestablish visual cleanliness on the surface of the tool. 10. Complete the certification procedure. Some companies prefer that tooling installation and debugging be done inside temporary enclosures. These can be stick-built on location using soft plastic sheet and metal studs or erected using temporary prefabricated wall panels.

6.9

CLEANROOM CERTIFICATION OF AUTOMATED TOOLING

Stage 2 certification is the appropriate stage for certification of tooling. Several options are available for stage 2 certification. One is to send the tool to its final location for installation and certification after installation and debugging. Another is to perform a preliminary certification at the assembly location to discover any problems and fix them prior to shipping and final installation. In either case, this is usually done as a three-step process. First, the environment in which the tool is to be certified should be tested to determine if it complies with stage 1 certification criteria. In the second step, referred to as phase 1 of stage 2 certification, points of contamination generation are certified individually. If these individual points are found to generate excessive amounts of contamination, they must be fixed and recertified. Possible points of particle generation include: ● ● ● ●

Pneumatic cylinders, air fittings, valves, and switches Electric motors and solenoids, linear variable displacement transducers, and other sensors Hydraulic cylinders and their associated fittings Belt, gears, slides, guides, and hinges

Testing single points of contamination generation on a one-by-one basis, sometimes referred to as the single-axis method, makes it possible to repeat the motion or run the axis more frequently than is intended in the final application. There is an advantage to this increased use: It allows for the action to be sampled more often, giving a statistically better indication of individual axis performance. However, increased use of the single-axis mode of testing

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TOOLING DESIGN AND CERTIFICATION

will increase the amount of contamination generated over the actual use conditions. To prevent failing an axis unnecessarily, the contamination is normalized to predict the contamination that would be generated at the use rate in the final application. The increased sample frequency for the single-axis method allows for statistical comparison with the required performance because more data points can also be gathered. A worksheet that can be used for this phase 1 single-axis mode of accelerated contamination generation is shown in Figure 6.22. After phase 1 has been completed, all possible points of particle generation have been fixed. It is now possible to perform the third and final step in the three-step certification

FIGURE 6.22

Tool certification worksheet.

CLEANROOM CERTIFICATION OF AUTOMATED TOOLING

327

process. This is phase 2 of stage 2 certification: The tool is operated in its intended application, hopefully in its intended location. All individual contributors of contamination now combine, sometimes in unusual ways, due to the prevailing airflows in the installed location. 6.9.1

Statistical Requirements for Sampling

Detailed plans for sampling need to be developed. In the following sections we describe a sampling plan that satisfies the requirements of FED-STD-209 for cleanroom airborne particle count certification. Number of Sampling Locations The minimum number of sampling points must be 1, corresponding to the location of the product or where the process is to take place. In some tools, more than one sample location may be of interest, so more than one sample will be taken. The inlet to the particle counter should be positioned as close to the product as practical and oriented to capture particles that may be generated by the process without interfering with the assembly process, motions of tooling, or operators. If product spends time at several locations, (e.g., in a parts queue as well at the loading–unloading station or assembly point), each location where the product spends a significant amount of time should be sampled. Sampling Volume, Time, and Number of Samples Two sample strategies may be used for tool certification: single samples and sequential samples. For single sampling, the sample volume must be that which would be expected to contain 20 particles if the area being sampled were operating at exactly the airborne particle concentration limit. Table 6.12 assumes that the stage 2 acceptance limit for the tooling is 50% of the cleanroom class limit. If the customer requires that a percentage other than 50% be used for the tooling, the entry in Table 6.12 is divided by the customer’s tooling allowance percentage and multiplied by 50. Note that the minimum sample time is determined from the volume in Table 6.12 divided by the flow rate of the optical particle counter in cubic feet per minute. Long sample times are not considered desirable. First there is poor economy of effort. In addition, long sample times may be incompatible with typical processing times. When process time is much shorter than the sample time calculated as above, the sample time should be shortened to approximate the process time. Sequential sampling should then be

TABLE 6.12 Minimum Sampling Volume per Single Sample in Cubic Feet of Air as a Function of Class and Particle Size for Stage 2 Certification, Assuming That the Allowance Is 50% of Class Particle Sizea Class 1 10 100 1000 10,000 100,000 a

n.a., Not applicable.

0.1 m

0.2 m

1.14 0.11 n.a. n.a. n.a. n.a.

5.3 0.53 0.05 n.a. n.a. n.a.

0.3 m 13.3 0.13 0.013 n.a. n.a. n.a.

0.5 m 40 4.0 0.4 0.04 0.004 0.0004

5.0 m n.a. n.a. n.a. 5.7 0.57 0.057

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TOOLING DESIGN AND CERTIFICATION

used for certification when calculation results in extremely long sample times, with the sequencing interval approximating the process time. Statistical Analysis of Results A tool is considered to be certified for particle count under the following conditions: ●

●

The mean values of the samples at every sampling location are less than the stage 2 certification limit. The 95% upper control limit (UCL) of multiple sampling locations are less than the stage 2 certification limit.

The 95% UCL is computed as follows: UCL 

M  K (S.D.) L

where M is the mean of average particle count at all locations, S.D. the standard deviation of means at locations, and L the number of locations. The K factors are as follows: No. Locations, L

K Factor

2 3 4 5–6 7–9 10–16 17–29 29

6.3 2.9 2.4 2.1 1.9 1.8 1.7 1.65

In sequential sampling the data points are inspected one sampling period after another, deciding whether the tool passes, fails, or additional sampling is needed, using the control limits listed in Table 6.13. ●

●

●

●

If the concentration is higher than the upper limit in Table 6.13, the tool fails and sampling may be terminated early. If the concentration is lower than the lower limit in Table 6.13, the tool passes and sampling may be terminated early. If the concentration falls between the upper and lower control limits, sampling must continue. The test is truncated when the sample time satisfies that calculated from the volume in Table 6.12.

Sequential Sampling Assume that a tool with a process time of 2 minutes is to be certified to class 10 using a 0.5-m resolution optical particle counter with a flow rate of 0.1 ft3/min.

329

53 35 29 26 25 23 23 22 19

UCL

— — 5.8 8.8 11 12 13 14 15

LCL

0.1 m

11 7.5 6.3 5.6 5.3 5.0 4.8 4.7 4.2

UCL — — 1.3 1.9 2.3 2.5 2.7 3.0 3.1

LCL

0.2 m

Class 1

4.5 3.0 2.5 2.3 2.1 2.0 1.9 1.9 1.7

UCL — — 0.5 0.8 0.9 1.0 1.1 1.2 1.3

LCL

0.3 m

1.5 1.0 0.83 0.75 0.70 0.67 0.64 0.63 0.56

UCL — — 0.17 0.25 0.30 0.333 0.36 0.41 0.42

LCL

0.5 m

Particle Size for:

Sequential Sampling Control Limits for Stage 2 Certificationa

113 75 63 56 53 50 48 47 42

UCL — — 12 19 22 25 27 30 31

LCL

0.2 m

45 30 25 23 21 20 19 19 17

— — 5.0 7.5 9.0 10 11 12 13

LCL

0.3 m UCL

Class 10

15 10 8.3 7.5 7.0 6.7 6.4 6.3 5.6

UCL

— — 1.7 2.5 3.0 3.3 3.6 4.1 4.2

LCL

0.5 m

150 100 83 75 70 67 64 63 56

UCL

— — 17 25 30 33 36 41 42

LCL

0.5 m

Class 100

a If the particle count exceeds the UCL, the sampling point fails and sampling is terminated. If the particle count falls below the LCL, the sample point passes and sampling is terminated. If the particle count remains between the ULC and LCL, sampling continues until the full sampling period is complete.

10 20 30 40 50 60 70 80 90

Percent of Time

TABLE 6.13

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TOOLING DESIGN AND CERTIFICATION

TABLE 6.14 Sequential Sampling, Class 10, at 0.5 m, 0.1 ft3/min, Limits, and Counts in Particles/ft3 Time Min. 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Failing Early

Passing Early

Full Duration

%

Limit

Count

Limit

Count

Count

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

25 15 11.7 10.0 9.0 8.3 7.9 7.5 7.2 7.0 6.8 6.7 6.5 6.4 6.3 6.3 5.9 5.6 5.3 5.0

6 7 10 8 6 7 5 7 5 7 8 — — — — — — — — —

— — 0.0 0.0 1.0 1.7 2.1 2.5 2.8 3.0 3.2 3.3 3.5 3.6 3.7 4.1 4.1 4.2 4.2 4.3

2 2 3 2 3 2 3 3 2 — — — — — — — — — — —

6 7 6 8 4 6 5 7 5 6 5 6 6 5 4 5 5 5 5 5

1. In Table 6.12, find the row of sample volumes for class 10. 2. Look across to the column under 0.5 m. Note that the sample volume is 4.0 ft3. 3. Divide the sample volume, 4.0 ft3, by the sample rate of the optical particle counter, 0.1 ft3/min. The minimum sample time is 40 minutes. 4. Compare the sample and process times. The sample time (40 minutes) is very much greater than the process time for the operation (2 minutes). 5. Set the sample time to 2 minutes to match the process time. Table 6.14 presents particle count data for three hypothetical examples: a sequential sample that failed early, a sequential sample that passed early, and a sequential sample that required the entire 40 minutes in which to make a decision. Certification of Cleanroom Tooling Several alternate approaches have been proposed for certification of tooling. One was the closed chamber method [4], which was able to estimate the rate of contamination generation from a tool housed within a closed chamber, sampling from the chamber at a single sampling point. There were two major drawbacks to this method. First, the chamber in this method did not contain mixing fans, so the contamination distribution in the chamber could not be expected to be uniform. Thus, the contamination production rate from the tool could only be estimated. Second, the location on the tool where the contamination originates is not determined by this method. The location from which the contamination originates from the tool can be especially important in a unidirectional-flow cleanrooms, because locally very high contamination concentrations can result.

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An improvement on the closed chamber method was developed in which the closed chamber was well stirred. [5]. This method allowed for a significant improvement in the estimation of contamination generation rate when using a single sampling point. However, it still suffers from the drawback that it underestimates the influence of contamination from a highly localized source on a specific contamination-sensitive location in a unidirectional-flow cleanroom. A more systematic approach has sometimes been used in which the contamination contributed by each potential source is measured in a unidirectional-flow environment with the sampling apparatus positioned to maximize the probability of detecting the contamination. For example, in an X, Y, Z, , robot, each axis would be exercised individually. Each source is fixed to correct out-of-specification conditions. The tool is then exercised through its intended application, and the sampling points are fixed at intended product locations. The advantages of this approach is that the location of specific contamination sources have previously been minimized. Sampling at intended product locations while the tool operates in its intended application then measures the impact on the product directly. This technique is probably the only practical approach for tools that operate over long distances, such as automated storage and retrieval systems. Maintenance of Certification Unless materials or components are changed, the materials from which a tool is initially built will generally continue to conform to their organic and ionic contamination requirements. Thus, ionic and organic contamination levels seldom need to be monitored on a routine basis. Conversely, particle contamination, especially magnetic particle contamination, can change over the life of a tool. As a consequence, general particle and magnetic particle contamination must be monitored frequently. The frequency of monitoring must be determined by the fragility of the contamination performance and the risk to customer product. Thus, tool certification for cleanroom use is an ongoing process. Materials and components in a tool wear at unpredictable rates. Initial testing will not be long enough to capture the gradual deterioration of the particle count performance of the tool. Tooling must be monitored routinely throughout its life for changes in contamination levels produced. The contamination recertification or monitoring plan must require recertification after each of the following activities: ● ● ● ●

Routine scheduled maintenance Unscheduled tool repair or maintenance Tool modification Tool relocation

In the absence of any of the above, the tooling should be recertified at least once every week or as often as the customer requires. But recall our experience with stepper motors, which produced sporadic bursts of contamination. The sporiatic bursts are not rare. As a consequence, there may be a need to monitor and hence verify continuously. Installation of a continuous monitoring system accommodates each of these requirements, since it sees all maintenance, engineering, and so on. 6.9.2

Analytical Equipment and Methods

Analytical equipment necessary to qualify materials, surface finishes, and treatments are necessarily complex. It is advised that the support of a materials laboratory be obtained

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TOOLING DESIGN AND CERTIFICATION

early to define the test methods needed for each item under consideration for use in tooling. Often, a customer has a lab that can perform qualification tests on materials and surface treatments. The types of equipment needed for qualification and certification are described briefly in the following sections. Flow Visualization and Measurement Airflow can be visualized using many different techniques. Dry ice or liquid nitrogen–condensed fogs can produce dense, persistent fogs that when viewed as illuminated from behind reveal subtle flow problems. However, care must be taken when using dry ice, as it can be a source of oily contamination. It can contaminate a production cleanroom. Other foggers, which aerosolize water by various methods, are commercially available, but these tend to wet surfaces. Alternatively, a liquid nitrogen fogger that is acceptable for use in a cleanroom during full production is MSP Corporation’s Model 2000 cleanroom fogger. Flow visualization is performed: ● ● ● ●

● ●

To verify the proper direction of airflow through tools. To locate areas of flow stagnation and/or standing recirculation. During tool design and build. After installation in the cleanroom in somes cases. Airflows in a room may be altered by tool installation. To verify that evacuated enclosures are functioning correctly. To confirm proper sealing of bulkhead-mounted tools.

Air velocities are measured using hot-wire or hot-film anemometers, platinum resistance velocity transducers, or rotating vane anemometers. Hot-wire or hot-film anemometers and platinum RTD velocity sensors are especially useful because their small size allows measurements to be made within confined spaces in the tool. Particle Counters A variety of optical particle counters are available. Used primarily for tool certification are stand-alone OPCs with build-in vacuums, display, printers, and RS-232 interfaces. These are available from a large number of suppliers (e.g., Met One, a division of Pacific Scientific; PMS; Climet). Other companies make optical particle counters. Stand-alone optical particle counters are especially useful during the phase 1 portion of certification to locate and eliminate points of contamination generation. Modular optical particle counters that can be integrated into continuous monitoring systems can be especially useful where the need to continuously verify performance of the tool is needed. Two types are available: ●

●

Electronically multiplexed systems. These are miniaturized particle counters that do not contain vacuum pumps, flow meters, or displays. They are powered from and report back to a centralized computer, which monitors their operation continuously. Pneumatically multiplexed systems. These consist these of a single optical particle counter sequentially monitoring many sampling points using a manifold or tube bundle sampling system. Again, the particle counter reports back continuously to a central computer.

Within the general category of particle counters are many variations. Condensation nucleus counters (CNCs) typically provide no size information but report the total particle concentration

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with a lower detection limit typically in the range 0.002 to 0.005m. Optical particle counters (OPCs) typically have lower detection limits, from about 0.05 to 0.5 m. The lower the detection limit of the airborne optical particle counter, the greater the cost. In addition, there are particle counters for sampling in vacuums and ovens. A recommendation about multiplexing: Electronically multiplexed systems are preferred over pneumatically multiplexed systems. Pneumatically multiplexed systems sample each location sequentially. Electronic systems sample all locations simultaneously. Liquidborne optical particle counters are also available. These are especially useful for measuring particles extracted from the surface of parts or for continuous monitoring of process fluid streams or cleaning baths. They also can be used as in-line monitors for process fluids entering a tool. Witness Plate Monitoring A witness plate is a bare, unpatterned silicon wafer whose surface can be inspected using a microscope or automatic surface scanner. Witness plates are used in the semiconductor manufacturing industry to monitor the performance of process equipment in areas not normally accessible to conventional particle counters. Witness plates find use in monitoring ambient air in precision manufacturing environments. Manufacturers of silicon wafer surface scanners include PMS and Tencor. QC-Optics has developed a surface scanner especially designed for looking at rigid disks. The principle of a witness plate can be extended to any part that is used as a surrogate for shippable product. That is, the witness plate might be a magnetic recording disk, CD-ROM, or piece of glass in a flat-panel display factory. The quantity of contamination collected on the plate is measured directly or indirectly. Samples of the materials collected can be analyzed to determine their origin. Analysis Methods Chemical composition of contamination collected at a tool can be a valuable clue to the source of the contamination. A wide variety of sampling methods are available: ● ● ● ●

Pick samples using tweezers, dissection probes, or wipes or swabs. Sample with tape, sticky stubs, or adhesive-coated films. Collect airborne particle samples on open-face filters. Filter extracts from parts or witness plates.

Contamination collected by the methods above may then be analyzed to identify their origin. This can be done starting from the simplest, easiest-to-perform tests, adding in more sophisticated analyses as needed. ●

●

Start with binocular light microscopy. The vast majority of contaminants can be identified quickly, and the process by which they are generated can often be deduced by state of matter, size, shape, color, and translucency. Use the results of low-power light microscopy to guide the selection of additional analysis. For example, the sample might be divided and submitted to SEM/EDX analysis for metals and ceramics. The remaining portion of the sample can then be subjected to infrared or Raman analysis to identify the organic materials.

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Additional analysis methods can then be tried: ●

●

●

●

●

Scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDX) can provide high depth of field images and elemental composition information. Infrared (IR) spectroscopy is useful in identifying organic materials such as polymers and lubricants, where the energy dispersive x-ray analysis from the scanning electron microscope might only report carbon and oxygen. IR spectroscopy generally is useful for samples in the diameter range 20 to 50 m, but a sample can be tenths of 1 m thick. Raman microprobe analysis can be used for identification of smaller bits of organic contaminants. Raman microprobe analysis is limited by the spot size of the laser, which is typically down to as little as 1 m in diameter. Gas chromatography/mass spectrophotometry (GC/MS) is used to analyze volatile organic matter. Time-of-flight secondary ion mass spectrophotometry can analyze even smaller quantities of matter than GC/MS. Spot tests and ion chromatography of water extracts are especially useful for corrosive anionic content. Cations can be analyzed by cation chromatography or by atomic absorption spectroscopy.

REFERENCES AND NOTES 1. Semiconductor equipment standards generally focus on interface specifications. See www.semi.org for details. 2. IEST-STD-CC1246D, Product Cleanliness Levels and Contamination Control Program. 3. R.J. Bunkofske, Using real-time process control to enhance performance and improve yield learning, Micro, Feb. 2000, pp. 49–57. 4. G.C. Roger, and L.G. Bailey, A closed-chamber method of measuring particle emissions from process equipment, Microcontamination, 5 (2): 42–47, 66–67, Feb. 1987. 5. R.P. Donovan, B.R. Locke, and D.S. Ensor, Measuring particle emissions from cleanroom equipment, Microcontamination, 5 (10): 36–39, 60–63, Oct. 1987.

ADDITIONAL READING Many trade and technical organizations provide useful information for contamination control. In addition, some companies provide particularly helpful guidebooks. The following lists are not meant to be comprehensive, simply representative. ●

●

●

The publications Modern Plastics and the Modern Plastics Encyclopedia, published by McGraw-Hill, are good sources of general information. LNP Engineering Plastics, Inc. produces a binder, A Design Guide for Molders, Designers and Engineers, that is full of property and design information and is especially strong for composite plastics. CleanRooms, a publication of PennWell Publishers, is an excellent source of information for the general cleanroom environment.

ADDITIONAL READING ●

●

●

335

Precision Cleaning, a publication of Witter Publishing Corporation, is a good source of information on cleaning chemicals and equipment. Micro magazine, a publication of Canon Communications, is a good source of technical and trade information, with a focus on the semiconductor and electronic industries. Evaluation Engineering, a publication of Nelson Publishing, provides a focus on electrostatic discharge and electromagnetic interference issues.

In addition, there are technical societies that publish standards that generally receive international recognition: ● ●

The Institute of Environmental Science and Technology, Mount Prospect, Illinois The Electrostatic Discharge Association, Rome, New York.

CHAPTER 7

CONTINUOUS MONITORING

7.1

INTRODUCTION

It is often difficult to justify the large capital investment necessary to install a continuous contamination or electrostatic charge monitoring system. Often this is the result of faulty historical data, where sampling practices precluded obtaining a true characterization of the workplace. There is a fear that hasty installation of a system will result in placement of sensors in locations where they are not needed. Finally, there often are questions about the types of sensors that should be used, the resolution required, and other technical concerns that make decision making difficult. To overcome these difficulties, a method is needed that will permit one to determine objectively where and what type of continuous monitoring system is needed. Several examples will illustrate a method that can be used to determine if and what type of continuous monitor should be used. We begin with a description of the method for choosing airborne particle monitoring. The first step in this method is installation of sampling hardware on workstations that conforms to the requirements for critical and busy sampling. Data are collected to determine if the traditional sampling method has determined an accurate measure of the conditions at the workstation. Thereafter, sampling may continue using a manual optical particle counter, electrostatic charge monitor, or other workstation monitor, with a modified sampling protocol to collect comparative data. Sampling may also continue using the previous protocol to provide control data. Data collected with the new protocol are then compared with the historical database using the old protocol and the historical database. Generally, this uncovers a number of sample points where the old particle counting protocol grossly underestimates the particle concentrations or static charge levels present. The new data are used to identify workstations that are out of compliance with contamination acceptance limits. An attempt can then be made Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

336

INTRODUCTION

337

to isolate and correct those items found to be contributing to the unacceptable conditions. Workstations that can be brought under control and maintained using a reasonable manual sampling frequency do not need continuous monitoring. Workstations that repeatedly show unacceptable conditions under manual sampling are candidates for continuous monitoring. The data collected manually are examined for evidence of burst, trend, and periodic contamination behavior. In addition, the results of modified manual sampling allow for the selection of contamination sensors with the optimum resolution, avoiding unnecessary costs associated with selecting sensors with unneeded resolution. The final examples will illustrate the evaluation of the need for continuous horizontal flow monitoring in a vertical unidirectional flow cleanroom and electrostatic charge monitoring. 7.1.1

Approaches to Monitoring

Traditionally, a variety of approaches have been taken to measure contamination or electrostatic charge in manufacturing areas. For example, it is universally recognized that a cleanroom must be positively pressurized with respect to the general factory environment to prevent the intrusion of contamination from uncontrolled adjacent factory areas. In most cleanrooms, a pressure gage or inclined tube manometer would be installed permanently on an outside wall. Once per day or once per shift the pressure would be read and the reading would be recorded. In this way, the cleanroom would be audited on a sampling basis. Other examples can be sited. Rotating vane or hot-wire anemometer measurements are taken near the face of HEPA filters to verify linear discharge velocities, verifying that the room air recirculation system was functioning correctly. This would involve taking a large number of measurements and so would be done infrequently. Often, such a survey would only be taken as part of an annual room certification. Room air velocities at workstation level would be collected for a smaller number of sampling points and are often surveyed manually less often than once a week. In cleanrooms with unreliable recirculation fan systems, airflow problems might go undetected for weeks at a time. Surveys for ESD compliance are similarly troubled by inadequate data from manual sampling. Most ESD-protected work areas are surveyed manually. These surveys are time consuming and occur infrequently. In addition, the duration of data collection in each survey can be quite short, so only a snapshot of charge generation and electrostatic discharge can be obtained. As with contamination manual sampling, activity in the area being surveyed often changes during the survey, so the data can be distorted further. Some environmental conditions were recognized to be more critical than room pressure or air velocity (e.g., relative humidity) and would be checked at least once per shift or as often as once per batch (e.g., the starting pH of a bath). For critical contamination parameters, often a continuous monitoring system would be build into the process equipment or its dedicated environmental enclosure (e.g., temperature in a stepper). The continuous monitoring system could easily be justified, because a clear link between the process parameter and yield can be made. Airborne particle contamination has long been considered an important factor to measure and control. As a consequence, many manufacturing processes would require that airborne particle measurements be taken every day or every shift. However, the traditional methods used for sampling airborne particle contamination often produce erroneously low particle count results. In addition, the infrequency of particle count measurement makes it difficult or impossible to correlate with yield. These erroneous data are often used to justify minimizing the frequency of the manual survey and have been cited as evidence that automated continuous contamination monitoring is not justifiable.

338

CONTINUOUS MONITORING

Prior discussions of continuous contamination monitoring systems have tended to focus on the data management software [1] or make the tacit assumption that a system will be bought [2], without a discussion of how to justify acquisition of a system to skeptical management. Occasionally, clever methods have been developed for reducing the cost per sample point of a continuous monitor [3] but still omitted discussing a method to demonstrate their necessity. 7.1.2

Traditional Airborne Particle Measurements

In the traditional approach to monitoring airborne particle contamination, an operator moves a conventional self-contained optical particle counter to the workstation and places an isokinetic sample probe at a convenient location on the workstation, usually held in place by a stand. The conventional self-contained optical particle counter usually contains a vacuum pump, power supply, display, and often a printer. Inclusion of these features often results in a large and heavy particle counter. As a consequence, conventional particle counters are mounted on a lab cart to facilitate moving about the cleanroom. This is conspicuous to production personnel. In addition, the isokinetic probe and its stand are frequently bulky and difficult to locate close to the product or process. As a consequence, the probe is often placed arbitrarily on the workstation in a location driven more by convenience than by any other consideration. Production personnel at workstations almost invariably stop all activity and move away when a particle count sample is to be taken. This results in the elimination of actions that may be generating contamination during normal production, thus lowering the particle count in the sample. More often than not, sampling via a conventional approach is only able to obtain contamination associated with the cleanroom or clean bench. This is often described as cleanroom idle sampling, sampling in which the contribution of equipment and personnel is not included in the total. Contamination associated with materials handling, load–unload operations, personnel-generated contamination, and the like are seldom included in such sampling. Improper censoring of data is often found to take place. The particle count operator observes the rate of the particle count. As long as the counts arrive at a relatively steady rate, the count is allowed to proceed. However, the particle count operator will almost invariably terminate the count if a sudden burst of particles is observed to occur, especially if the particle count operator can associate the burst with some undesired activity such as someone walking by. Improper data censoring will be repeated as often as necessary until an acceptable result is obtained. Quite often the only count considered acceptable is one that is below the class limit for the area being sampled. These two factors, sampling during workstation idle periods and improper data censoring, result in a historical particle count database which makes the cleanroom and its workstations appear to be well in control with respect to particle count. But adverse actions are generally taken using such data. First, there is a tendency to reduce the manual particle sampling frequency to reduce the labor cost associated with particle count sampling. It is difficult to justify sampling more often when the data indicate that the areas are in control from a particle count perspective. It is not uncommon to see the facilities monitoring points divided into two or four subgroups, cutting in half or quartering the sampling frequency. In extreme cases of very large assembly operations, this may be carried to the extreme that each sample location is visited only once per month. Second, given the apparent compliance with airborne particle limits, all appears to be in control, and

INTRODUCTION

339

switching from a manual sampling protocol to a capital-intensive continuous monitoring system simply cannot be justified. To correct the historical database and develop a more accurate description of the work area, a new sampling strategy must be developed. In the early stages of implementation, this strategy should be designed to minimize the expenditure of capital. The strategy must also deal with the chief factors affecting the accuracy of the particle count: sampling the wrong place at the wrong time and improper data censorship. 7.1.3

Critical and Busy Sampling

Here we introduce and define critical and busy sampling: ●

●

●

Critical location: a location as close to the product or process as possible, without physically interfering with the movement of the product, the people, or the process equipment Busy period: a period during actual manufacturing operations, especially when the product is exposed Critical and busy sampling: sampling that satisfies the requirements of critical locations and busy periods

The critical location often places the inlet to the particle counter in a place where laminar airflow does not exist. This works to great advantage, since the bulky isokinetic probe can be eliminated, allowing greater freedom in placement of the inlet near the product. The tubing for the inlet to the particle counter should then be fixed to the workstation with brackets, tie wraps, and other means. This ensures repeatability of the sample location and protects against the tubing getting loose to interfere with the process. Hardware needed to implement critical and busy sampling costs only a few dollars per workstation and takes only minutes to install. The particle counter outlet end of the tube should then be terminated at some point on the workstation that allows the particle count operator to attach the conventional particle counter to the sample tube, without disturbing the process. This allows for sampling without stopping the process, referred to as busy period sampling. Figure 7.1 shows a critical and busy sampling tube installed on a workstation. 7.1.4

Modified Data Collection Protocol

Once this low cost, critical, and busy sampling hardware is in place, a new data collection protocol must be adopted. In the new protocol, data censoring is not allowed. The operator observes and records the activity at the workstation during each sample. ●

●

If no product is in production and the workstation is unoccupied, the sample is labeled as taken during stage 1 operation, or a cleanroom idle sample. If product is being processed but no production personnel are present, the sample is labeled as taken during stage 2 operation, or cleanroom and process tooling, but no personnel. This rule needs further discussion, covered below. – If the sample point is at a loading and material-handling location on the workstation, table, or cart, but the product is processed inside a tool or enclosure and no personnel are present, the sample is labeled as a stage 1 sample. – If the sample point is inside the tool and contamination from personnel is isolated from the sample or product location, the sample is labeled as a stage 2 sample.

340

CONTINUOUS MONITORING

FIGURE 7.1 Critical and busy sampling tube installation on a workstation. ●

●

If product, people, and tooling are present, the sample is labeled as a stage 3 operation, fully operational and fully populated. Again, this rule requires further discussion. – If the process is within a tool or enclosure that effectively prevents contamination or electrostatic charge generated by the operator from getting on the product, the sample is labeled as a stage 2 sample. If the inlet to the particle counter is scraped, or the tubing is bumped or otherwise disturbed to invalidate the count, the sample is so annotated. Similarly, if charge sensors are disturbed in a manner that alters their readings in a way not representative of actual use conditions, these disturbances are noted. These occurrences indicate the need to correct installation of the critical and busy sampling hardware to assure the highest-quality data.

By eliminating the option for data to be censored, we eliminate rejection of otherwise valid data. In addition, by labeling the stage of operation for each sample, it is possible to diagnose possible sources of the contamination. For example, if stage 1 particle counts are a significant fraction of stage 3 counts and the stage 3 counts are out of specification, the facility probably would be a fruitful place to begin searching for the source of contamination. 7.1.5

Ongoing Use of Critical and Busy Sampling

When a sample location is identified as out of specification with respect to contamination or electrostatic charge, a second stage of investigation is initiated. For example, a standalone particle counter may be used like a Geiger counter, sniffing out the individual particle generation points. If these can be located, fixed, and kept under control by sampling manually at a tolerably low frequency, continuous monitoring is not justifiable. However, the critical and busy sampling hardware and protocol should continue to be used. What if workstations are identified that require continuous monitoring? In this case the continuous monitoring system is connected to the same critical and busy sampling hardware. Whenever an alarm is signaled, the manual sampling equipment is brought back to the location and is used again in Geiger counter mode.

INTRODUCTION

341

TABLE 7.1 Traditional vs. Critical and Busy Sampling in a Class 10,000 Mixed-Flow Cleanroom (particles/ft3 0.5 m) Workstation and Line 1A 2A 3A 4A 5A 1B 2B 3B 4B 5B

7.1.6

Traditional Sampling

Critical and Busy Sampling

Average

Std. Dev.

Average

Std. Dev.

325 458 325 452 675 236 601 266 301 425

79 85 45 250 201 125 322 64 102 211

456 531 357 694 628 288 908 254 321 623

84 139 38 242 165 159 404 52 125 364

Case Studies: Traditional vs. Critical and Busy Sampling

Case Study 1: Workstations in a Class 10,000 Ballroom Table 7.1 shows the results of sampling two sets of data collection workstations in a class 10,000 ballroom-style cleanroom. Data listed are the average and standard deviation of particle concentration, in particles 0.5mm in diameter and larger per cubic foot. All workstations had previously been found to comply with the airborne particle count requirements of a class 10,000 cleanroom using the traditional manual sampling protocol. The particle count increases slightly using the critical and busy sampling protocol but not enough to change the conclusion that all workstations are in compliance with class 10,000. There are only slight differences between stations on line A vs. line B. Results similar to that in Table 7.1 are often found in mixed-flow cleanrooms. The general contamination in the room is dominant over the contamination generated at the individual workstation. Data like these indicate that a continuous monitoring system would not be necessary for these workstations. Plotted in Figure 7.2 are the data from Table 7.1, plotting airborne particle concentration in particles 0.5 m in diameter and larger per cubic foot of air. The tick marks on the vertical bars represent the average particle concentration. The upper and lower ends of the bars represent the mean 3 standard deviations respectively. The bars are labeled along the X-axis to indicate the sample location number and traditional vs. critical and busy sampling protocol. Case Study 2: Class 100 Unidirectional Flow Benches in a Class 10,000 Ballroom Table 7.2 shows comparative data for two identical sets of class 100 workstations in the same cleanroom as in Case Study 1. These class 100 workstations are located under vertical unidirectional flow units, effectively isolating each from the others. None of the nine workstations from either line A or line B exceeds class 100 when sampled using the traditional approach. Conversely, the average and standard deviation of the particle count increase for all 18 workstations when sampled using the critical and busy protocol. In seven of the 18 cases, critical and busy sampling shows workstations far dirtier than class 100. Also of interest is a comparison of workstation 6 on line A vs. its identical counterpart on line B. The line B station is almost 10 times dirtier. Plotted in Figure 7.3 are some of the data from Table 7.2 to illustrate the differences in results obtained using traditional vs. critical and busy sampling protocols. The Y-axis lists the airborne particle concentration in particles per cubic foot of air 0.5mm in diameter and

342

CONTINUOUS MONITORING

Mean plus 3 S.D. Mean - 3 S.D. Mean

. 1600

Particles/ft3 ≥ 0.5 μm

1400 1200 1000

. .

800 600

.

.

.

400

.

.

.

.

.

200

5

&B ,5 C

., Tr ad

4 .,

&B ,4 C

Tr ad

&B ,3

.3

C

Tr ad

2

&B ,2

.,

C

Tr ad

&B ,1 C

Tr ad

.1

0

Sample Type, Work Station

FIGURE 7.2 10,000 room.

Comparison of traditional with critical and busy sampling in a mixed-flow class

TABLE 7.2 Average and Standard Deviation of Particle Counts, Class 100 Unidirectional Flow Workstations in a Class 10,000 Ballroom (particles/ft3 0.5 m) Workstation and Line 6A 7A 8A 9A 10A 11A 12A 13A 14A 6B 7B 8B 9B 10B 11B 12B 13B 14B

Traditional Sampling

Critical and Busy Sampling

Average

Std. Dev.

Average

Std. Dev.

2 14 12 6 2 6 2 1 3 3 10 8 5 5 4 2 1 3

2 4 5 5 3 4 2 2 2 2 4 4 4 2 3 2 2 2

27 180 238 292 28 52 11 10 31 258 293 153 223 36 56 19 44 10

39 114 169 151 40 31 9 55 28 200 88 116 47 17 52 9 20 6

INTRODUCTION

.

Mean plus 3 S.D. Mean - 3 S.D. Mean

Particles/ft3 ≥ 0.5 μm

1000

.

100

.

10

.

343

.

.

. .

.

.

Tr ad

.6 A C &B ,6 A Tr ad .. 7A C &B ,7 A Tr ad .8 A C &B ,9 A C &B ,8 A Tr ad ., 9A Tr ad ., 10 A C &B ,1 0A

1

Sample Type, Work Station

FIGURE 7.3 Comparison of traditional with critical and busy sampling protocols on measured contamination in class 100 clean benches.

larger. In Figure 7.3 the particle concentrations are plotted on a logarithmic scale (unlike Figure 7.2) to accommodate the broad data range. The tick marks on the vertical bars represent the average particle concentration. The upper and lower ends of the bars represent the mean 3 standard deviations, respectively. The bars are labeled along the X-axis to indicate the sample location number and traditional vs. critical and busy sampling protocol. Case study 2 illustrates two common results of using critical and busy sampling in unidirectional flow work areas: ●

●

The emissions from the individual workstations are evident because the effects of the mixed flow cleanrooms are eliminated. Differences between pairs of otherwise identical workstations can be detected.

Case Study 3: Class 100 Hoods in a Class 1000 Room This case study is for a set of operations under class 100 vertical unidirectional flow units located in a class 1000 clean room. Here we show average values, omitting standard deviations, due to the limited sample size of the survey. All 20 workstations sampled using the traditional approach easily meet class 100. The boast in this facility was that most of the workstations would also meet or be better than class 10. The critical and busy samples indicate that most do not even satisfy class 100 requirements, as shown in Table 7.3. The worst-case discrepancy is found in location 18, where class 1000 was exceeded. Note that when using the critical and busy sampling approach, there are sufficient particles available to allow use of a 0.5-m resolution 0.1-ft3/min optical particle counter at nearly every workstation. If the data collected using the historical approach were used, the particle counter chosen would probably have been either a 0.3- or 0.1-m resolution particle counter, greatly increasing the cost of continuous monitoring. The data in Table 7.3 may also be plotted. Figure 7.4 is a plot of traditional vs. critical and busy sample averages for the fully automated workstations. It illustrates an important feature of the critical and busy sampling approach in workstation 7. To sample using the

344

CONTINUOUS MONITORING

TABLE 7.3 Average Obtained by Traditional vs. Critical and Busy Sampling in Class 100 Hoods Installed in a Class 1000 Cleanroom (particles/ft3 0.5 m) Location 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Traditional Average

Critical and Busy Average

Location Type

5 9 5 5 1 5 5 33 11 7 12 5 10 10 27 12 3 14 26 5

198 77 31 292 507 326 977 36 36 489 12 70 407 155 499 254 78 1258 224 56

Hybrid automation Hybrid automation Fully automated Hybrid automation Hybrid automation Hybrid automation Hybrid automation Fully automated Fully automated Hybrid automation Fully automated Fully automated Hybrid automation Hybrid automation Hybrid automation Hybrid automation Fully automated Fully automated Hybrid automation Hybrid automation

traditional protocol, the operator had to open the doors to the work cell. The safety interlock would stop the machinery inside, eliminating its contribution to contamination. The critical and busy sampling hardware was mounted so that the operator could connect to the sample tube without having to open the enclosure. Thus, the machinery would continue operating, allowing its’ contribution to be detected. Figure 7.5 is a plot of traditional vs. critical and busy sampling in hybrid workstations, where an operator and automated tooling work together. Comparison of Figures 7.4 and 7.5 illustrates a fairly widely held belief in a way seldom so clearly demonstrated: People are a major contributor to contamination. Case Study 4: Class 100 Unidirectional Workstations Ten different class 100 workstations were monitored using critical and busy sampling hardware [4]. These data were also compared to results obtained by traditional monitoring. Sampling was of sufficient duration that the percent compliance could be calculated. Percent compliance is the percent of time that a workstation is monitored that it is below its particle count limit. A high percent of compliance is considered to be good. Workstations with very low compliance are highly likely to be detected in a traditional once-a-week particle sampling protocol. This is illustrated in Figure 7.6. Case Study 5: Extended-Duration Manual OPC Monitoring In this study a manual optical particle counter was used to sample a workstation using critical and busy sampling hardware for several hours. The data were collected once per minute in a class 100 clean hood located within a class 10,000 ballroom. The particle count operator observed and recorded the activities at the workstation but did not interfere with the actions of the production operators

INTRODUCTION

345

Traditional, Ave. C & B, Ave. Particles/ft3 ≥ 0.5 μm

10,000 1,000 100 10 1 1

2

3

4

5

6

7

Location

FIGURE 7.4 Comparison of critical and busy sampling with traditional sampling protocols for fully automated workstations. Traditional, Ave. C&B, Ave

Particles/ft3 ≥ 0.5 μm

1000

100

10

1 1

3

5

7 Location

9

11

13

FIGURE 7.5 Comparison of traditional with critical and busy sampling protocols for workstations containing automation and the presence of people.

in any way. Table 7.4 is a partial summary of the particle concentration data, averaged to the nearest 5 particles/ft3 0.5 m and the particle count operator’s notes of the activities in the workstation. The results are also plotted in Figure 7.7. The particle count operator’s notes provide a very clear understanding of what is happening at the workstation. Wipe-down is a relatively messy process since it stirs up large amounts of contamination. Setup and waiting for work in progress (WIP) generate little contamination. Assembly and, especially, soldering generate large quantities of airborne contamination. The arrangement of the items on the workstation is not fixed. For convenience, the second operator moved the solder fixture and fume extraction system, with disastrous

CONTINUOUS MONITORING

Traditional

C&B

% Compliance

Particles/ft3 ≥ 0.5 μm

1,000

100 80

100

60 40

10

20 1

% Compliance

346

0 A

B

C

D

E F G Location Code

H

I

J

FIGURE 7.6 Comparison of critical and busy sampling with traditional sampling. In this example we see that percent compliance, the percent of time that a location complies with particle count requirements, must be viewed separately from the average particle count performance of workstations.

results. During soldering, the first operator averaged 370 particles/ft3 0.5 m: the second operator averaged 746 particles/ft3 0.5 m. This case study illustrates when a continuous monitoring system may be justifiable. Some flexibility in workstation layout must be provided to accommodate the reach and comfort of the operator. A continuous monitoring system might be a useful tool to keep particle counts under control after such rearrangements.

7.1.7

Trend, Cyclic, and Burst Patterns of Particle Generation

In addition to the average particle concentration prevailing at a workstation, we must be concerned with trend, cyclic, and burst patterns of particle generation [5]. Sampled over a long duration, the average particle concentration may appear to be within control limits. Looking at the data in more detail may reveal unwanted particle concentration behaviors. Upward trends in particle count are considered undesirable because they may, at some future moment, exceed the control limits. Examples of an upward trend are observed where workstations gradually become dirty between deep cleaning intervals. Since the rate at which workstations become contaminated is not perfectly constant, it is seldom easy to predict when the next deep cleaning should be scheduled. This is an example where a continuous monitoring system may provide a useful benefit. Cyclic patterns of particle generation are a special case of burst pattern, where the bursts have a repeatable pattern. It is usually easy to associate these patterns with specific activities on the workstation. If associations can be established, it is often easy to develop and implement fixes. Experience has shown that a cyclic patterns of particle generation can usually be controlled adequately using manual monitoring and the critical and busy sampling hardware. Random bursts of contamination are observed in nearly every cleanroom. These can be associated with sudden, catastrophic events. A good example is shedding from an electric motor, illustrated in Figure 7.8. This stepper motor was monitored continuously for over a week. The counts downwind of the motor started out in the range 15 to 30 particles/ft3 but cleaned up within a short time to 1 to 3 particles/ft3. Two large bursts are seen. Each sample is the average over a 10-minute duration, collected at 0.1 ft3/ft3. Averaged over the 7 plus days, the electric motor produces only 16 particles/ft3. The second burst exceeded class 100 for 25 hours. With a once a week manual sampling plan,

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TABLE 7.4 Extended-Duration Manual Particle Monitoring Data (particles/ft3 0.5 m) Time

Count

1150 1152 1154 1156 1158 1200 1202 1204 1206 1208 1210 1212 1214 1216 1218 1220 1222 1224 1226 1228 1230 1232 1234 1236 1238 1240 1242 1244 1246 1248 1250 1252 1254 1256 1258 1300 1302 1304 1306 1308 1310 1312 1314 1316 1318 1320 1322 1324 1326

2 1 6 3 4 2 0 1 20 15 135 255 600 125 90 125 360 425 250 35 50 100 255 325 385 100 35 60 125 175 325 475 100 65 25 175 225 375 400 100 65 35 90 55 35 25 45 75 125

Observations Lunch break, room empty Lunch break, room empty Lunch break, room empty Lunch break, Lunch break, room empty Lunch break, room empty Lunch break, room empty Lunch break, room empty Operators returning Operators returning Operator wiping down Operator wiping down Operator wiping down Operator wiping down Set-up Set-up Assemble Solder Solder Set-up Set-up Assemble Assemble Solder Solder Set up Set up Set up Assemble Assemble Solder Solder Set up Set up Set up Assemble Assemble Solder Solder Set up Set up Supervisor interrupts Supervisor interrupts Supervisor interrupts Supervisor interrupts Supervisor interrupts Second operator replaces first Wipe down Wipe down

Time

Count

Observations

1328 1330

325 125

1332 1334 1336 1338 1340 1342 1344 1346 1348 1350 1352 1354 1356 1358 1400 1402 1404 1406 1408 1410 1412 1414 1416 1418 1420 1422 1424 1426 1428 1430 1432 1434 1436 1438 1440 1442 1444 1446 1448 1450 1452 1454 1456 1458 1500

80 65 150 225 555 985 65 25 125 225 750 625 25 15 125 155 455 625 25 55 125 250 1250 955 50 35 225 175 655 475 25 15 25 35 65 55 225 350 875 1120 25 15 30 25 10

Wipe down Operator adjusts fume extractor Set up Set up Assemble Assemble Solder Solder Set up Set up Assemble Assemble Solder Solder Set up Set up Assemble Assemble Solder Solder Set up Set up Assemble Assemble Solder Solder Set up Set up Assemble Assemble Solder Solder Waiting for WIP Waiting for WIP Waiting for WIP Waiting for WIP Set up Set up Assemble Assemble Solder Solder Waiting for WIP Waiting for WIP Waiting for WIP Waiting for WIP Waiting for WIP

CONTINUOUS MONITORING

106

99

92

85

78

71

64

57

50

43

36

29

22

15

8

1400 1200 1000 800 600 400 200 0 1

Particles/ft3 ≥ 0.5 μm

348

Time

FIGURE 7.7 Insights provided by extended-duration manual sampling using critical and busy sampling hardware. Between minutes 50 and 57 a new operator arrived at the workstation and changed the layout of the soldering station. The operator did not change the position of the solder fume extractor, causing an increase in particle count.

Particles/ft3 ≥ 0.5 μm

10,000 1,000 100 10 1 0

FIGURE 7.8

200

400

600 800 Sample Number

1000

1200

Burst pattern of particle generation behavior from a stepper motor.

the chance of detecting this burst is only one in seven. The first burst, with a duration over class 100 for 5 hours, has only a 1 in 37 chance of being detected, sampling once a week.

7.1.8

Case Studies: Other Applications of Continuous Monitoring

Case Study 6: Continuous Electrostatic Charge Monitoring Magnetoresistive (MR) heads are among the most ESD-sensitive devices in existence. Modern static safe facilities thus required many ESD protection tools to allow for safe manufacture. Among the more important tools provided for these static-safe work areas are air ionizers. The performance of air ionizers traditionally has been measured using charged plate monitors. During weekly audits the charged plate is used to measure discharge times and float voltages. In this procedure the ESD technician places the sensor of the charged plate as close to the intended product location as possible. It is occasionally found that the air ionizer has drifted out of balance and needs service. This often consists merely of cleaning the emitter points on the air ionizer. Occasionally, simply cleaning the emitter points is inadequate, and the ionizer must be balanced manually. One of the less well understood features of air ionizer performance is that they interact with their environment. That is, grounded objects on the workstation below the ionizer tend

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349

Min − Max + Maximum Volts per 10-Minute Sample

80 60 40 20 0 −20

0

100

200

300

400

500

600

Sample

FIGURE 7.9 Charge levels in an ESD-protected work station in a cleanroom.

to drain charge to ground. The polarity and amount of charge drained off is a function of the distance to and position below unshielded emitter points on the ionizer. Relocating objects on a workstation can thus change the balance of the ionizer. This can occur frequently in a development site, where tooling and workstations are often changed due to changes in products or process flow. To characterize these changes more fully, an electrostatic charge monitor equipped with a 20-pF plate was installed at a workstation for 4 days. Each 10-minute sample was scanned for the maximum positive and negative voltage swing. Figure 7.9 shows the variation in float potentials measured for one workstation in a development cleanroom. The layout of the workstation was observed on a shift-by-shift basis and any changes were noted. Inspection of Figure 7.9 shows the larger variations observed in the workstation during the first shift (observations 0 through 50, 160 through 210, etc.) than at other times of the day. The development cleanroom was being used for production on first shift, engineering on second shift, and was practically empty at all other times. Note however, that an engineering experiment was performed on the second shift of the third day. In this experiment, a tall measurement stand was placed on the workstation nearly directly under the ionizer. The workstation was rearranged to accommodate the stand. However, when the stand was taken away at the end of the second shift, the workstation was not returned to its original layout. Hence on day 3 the balance in the workstation has swung to a strong positive imbalance.

Case Study 7: Continuous Airflow Monitoring In this example a very large cleanroom was equipped with 54 modular flow units. Once a week a cleanroom technician would do a velocity survey, measuring the linear air velocity discharged from the filters in each module. Approximately every other week, at least one of the modules would be found to have very low or no air velocity. The problem this creates in a vertical unidirectional flow cleanroom is unwanted horizontal airflow. Clearly, discovering this once a week is undesirable, but how would one design a cost-effective continuous monitoring system to monitor for the condition? The answer lies in the design of the cleanroom, as shown in plan view in Figure 7.10. The design of the cleanroom lent itself to definition of four airflow zones, labeled A, B, C and D in Figure 7.10. These zones were supplied air from 10 to 16 flow modules. It was recognized immediately that airflow from any module would change the horizontal airflow through the restricted areas defined by the return plenums. To provide a module flow monitoring system, five hot-wire anemometers were installed in the restricted locations

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Returns 2

1 A

C

B

3

5

4

Service Core and Return Change room D

FIGURE 7.10

Plan view of a horizontal flow–monitored vertical laminar flow cleanroom.

numbered 1 through 5 in Figure 7.10. Hot-wire anemometers are frequently used to measure vertical airflow in cleanrooms. In this application, though, they were mounted to monitor horizontal flow rather than vertical flow. After installation of the horizontal flow monitor, no imbalance condition went unnoticed for longer than a single shift. Of course, the flow monitor would not tell the cleanroom technician which module had failed. But the monitor would tell which intersection between zones was out of control. The technician would then go to the out-of-control intersection and determine the direction of the horizontal flow. This would then determine in which zone a module had failed, allowing the technician to survey and identify the failed unit quickly. 7.1.9

Summary and Conclusions

An effective method has been developed to permit assessment of the need for continuous contamination monitoring. Its use has been demonstrated for sampling and measurement of airborne contamination, for monitoring voltage balance in a static-safe work area, and for monitoring the function of flow modules. This method optimizes the placement of sample points to allow a correct characterization of the workplace to be made. The data obtained allow for the selection of a particle counter or other sensors using a lowest-cost strategy.

7.2

CONTINUOUS CONTAMINATION MONITORING

To determine the best system for continuous monitoring of particles in a cleanroom it is necessary to understand the two types of continuous particle monitoring systems.

7.2.1

Electronically Multiplexed Monitoring

This involves the use of a dedicated particle counter or particle sensor at each specific location. Every event would be detected and counted. There are no gaps in the particle counting data. Particles are monitored in particles per cubic foot or particles per cubic meter. This

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system is best used at critical locations where events can happen at any time. Critical, highly specialized, or very sensitive operations are among the best candidates for electronically multiplexed systems. The selection of sampling rate, lower particle size detection limit and types of particle sensors that can be used is enormous. The list includes relatively inexpensive 0.1 cfm, 0.5 m resolution sensors, through more expensive condensation nucleus counters capable of resolving 0.02 m, particle sensors inside vacuum systems, and in other harsh conditions such as within high temperatures, corrosive environments, and so on. Electronically multiplexed monitoring advantages are as follows: ● ●

● ● ● ●

●

●

Continuous detection and recording of all events Optimized for critical or sensitive monitoring at tailorable lower detection limits, flow rates, and for hostile or exotic environments Good for equipment monitoring for failure and predicting unscheduled maintenance Ability to correlate events with yield loss to high degree Immediate notification or alarming of yield-destroying excursions Allows for integration of a wider variety of sensors sampling liquids, vacuums, pressures, velocities, ESC sensors, and so on. Immediate feedback to operators and technicians when procedures are not being followed or when processes are out of control Immediate feedback after shutdown and evacuation if the area and its processes are within specification

7.2.2

Pneumatically Multiplexed Particle Monitoring

Pneumatically multiplexed particle counting systems are sometimes referred to as sequential particle monitoring systems. This type of particle monitoring system uses a single particle counter to monitor multiple points. Sampling of multiple sampling locations is accomplished by adding a sequential manifold sampler that connects the particle counter to several different sample tubes. Each tube is sampled in a programmable sequence. Once a tube is sampled, the manifold switches to the next tube to be sampled. During this change the particle counter stops counting until the change is over, then delays to allow any air from the previous sample to be purged. Air is being pulled through all sample tubes continuously via a blower. This avoids any “air hammering” which may free particles in the sample tubing from the start and stop of the airflow. Particles are monitored in particles per cubic foot or particles per cubic meter. The applications for a pneumatically multiplexed system are those that can tolerate the gaps in the data, such as mixed flow cleanroom. These are generally more contamination tolerant processes. Pneumatically multiplexed particle monitoring systems are relatively cheaper than electronically multiplexed particle monitoring systems, on a sensor by sensor basis. Therefore, there will always be a struggle between which type of system to choose. Fortunately there is a relatively straightforward way to choose between them. This decision must be made on the basis of the particle concentration frequency distribution, the probability that out of conformance particle concentrations will be detected and the consequence of failure to do so. The particle concentration frequency distribution is obtained during the extended manual use of the critical and busy sampling hardware. The probability of detecting out of conformance

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particle concentrations was illustrated in Figure 7.7, showing burst pattern of particle behavior. The decision can be made using Monte Carlo analysis to determine if a pneumatically multiplexed system can prevent unwanted yield loss.

7.3

CONTINUOUS MONITORING OF MANUFACTURING

Continuous monitoring in manufacture must consider many different factors. The parameters monitored are dictated primarily by the requirements of the product and the production process or may reflect prior experience with the operation of a particular cleanroom. Each manufacturer will monitor different factors, in many cases to unique control limits. Thus, a discussion of what parameters to monitor for manufacturing must necessarily be general. All parameters discussed will not necessarily be monitored in every facility. In this discussion we consider parameters that affect (1) the quality of air, (2) process fluids (such as DI water or compressed gases), and (3) the cleanliness of surfaces and electrostatic charge. The reasons for monitoring include both contamination control and concern for worker health and safety.

7.3.1

Air Quality

Parameters that affect the quality of the air in a cleanroom are discussed below. Airborne Particle Concentration There are two generic approaches to monitoring airborne particles. One uses a network of individual particle counters connected to a central monitoring system. This is often referred to as an electronically multiplexed continuous particle monitoring system or a real-time particle monitoring system. The second approach uses a single particle counter that sequentially samples a number of different locations in the room using a manifold of sample tubes. The latter approach is often referred to as a pneumatically multiplexed or particle monitoring system. The choice between the two systems depends on the type of cleanroom (mixed flow vs. unidirectional flow) and the probability that unpredictable bursts of excessive contamination will adversely affect the product or process. In a mixed-flow cleanroom, sample locations nearby one another tend to exhibit relatively uniform and constant particle concentrations. The gaps in time between each count are tolerable under such conditions. In addition, because airborne particle concentration tends to be relatively constant compared to that in unidirectional flow cleanroom, sequential sampling particle counters are often recommended for mixed-flow cleanrooms. Conversely, in unidirectional flow environments, particle concentrations are highly dependent on the activity at each sample location, and the contributions from nearby workstations are often completely undetectable. In addition, the activity in the individual workstations results in particle counts that are highly variable in time. Here real-time continuous monitoring system is favored. The selection of lower particle size resolution and volume flow rate for the airborne optical particle counter is a function of the class of cleanroom. Following the statistical requirement of FED-STD-209, a minimum of 20 particles should be counted to obtain a 95% confidence level in the particle concentration. Thus, the resolution and flow rate of the particle counter are selected to ensure that 20 particles would be sampled if the room operates exactly at its class limit. Two examples will serve to illustrate this principle.

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Example 1 Assume a class 100 cleanroom with an average process time at each assembly operation of 2 minutes. For a fully operational cleanroom, we expect 100 particles/ft3 0.5 m in diameter and larger. Thus, we want to use a particle counter where we expect to sample 20 particles in 2 minutes. Twenty particles are expected to be found in two-tenths of a cubic foot of air. In this case, a 0.5-m resolution 0.1-ft3/min particle counter is adequate. Example 2 Assume a class 10 minienvironment with a process time of 4 minutes. For the fully operational state, we expect 10 particles/ft3 0.5 m in diameter, 30 particles/ft3 0.3 m in diameter and larger, and 75 particles/ft3 0.2 m in diameter and larger. For this case, a 0.5-m resolution 0.1-ft3/min particle counter would require 20 minutes to sample 20 particles and thus is inadequate. A 0.3-m resolution 0.1-ft3/min particle counter would sample 20 particles in 6.6 minutes and might be considered marginally adequate. A better choice would be to use either a 0.5-m resolution 1.0-ft3/min particle counter (20 particles in 2 minutes) or a 0.2-m resolution 0.1-ft3/min counter (20 particles in 2.66 minutes). Temperature and Relative Humidity The temperature and relative humidity in a cleanroom are dictated by the process or, in the absence of process factors, by the need to provide a comfortable workplace for the operators. Cleanrooms are generally considered comfortable when the temperature is in the range from 20 to 22°C (approximately 68 to 72°F) and the relative humidity is in the approximate range 35 to 55%. Temperature probes with an accuracy of 0.2°C and relative humidity sensors with an accuracy of 2% are generally considered adequate for monitoring cleanroom conditions that are dictated by comfort. Some processes require tighter control than others over temperature and humidity. An example is photolithography. Often, photolithography tools are housed in a special enclosure called a minienvironment so that the tighter temperature and humidity control need not be applied over the entire cleanroom. Sensors for these more highly controlled areas have correspondingly greater precision and accuracy than those used in the cleanroom in general. Room Pressurization Cleanrooms are pressurized with respect to the ambient factory environment to ensure that contaminated air from the factory does not enter the cleanroom. Indeed, within a given cleanroom there are likely to be several areas where differential pressure may be monitored. For example, the change room will be at a slightly lower pressure than the cleanroom but still positively pressurized with respect to the factory. In addition, service cores in the cleanroom should be negatively pressurized with respect to the process areas. Minienvironments are usually at a positive pressure with respect to the cleanroom. Equipment contamination enclosures are often at a negative pressure with respect to the cleanroom. A typical cleanroom will operate at between 0.05 and 0.1 in. of positive water pressure with respect to the factory, service cores, and the interiors of evacuated enclosures. Minienvironments often are as much as 0.2 in. H2O pressurized with respect to the cleanroom. Minienvironments can be equipped with intelligent differential pressure controllers, which adjust fan speeds to maintain the positive pressure regardless of door positions. Air Velocity and Direction Many cleanrooms have multiple fan centers or may be designed to use fan-filter units. Occasionally, it is found that undesired horizontal airflows exist, due to imbalance in the cleanroom air system. The flow imbalance can be caused by failure of individual fans, adjustment of fans, improper opening or closure of dampers, collapse of

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ductwork, and other facilities problems. The effect of these airflow problems can be monitored using hot-wire anemometers mounted on the walls in openings that tend to amplify horizontal flows. A few strategically placed anemometers allow one to monitor the balance of even relatively large facilities. Typical horizontal flows in laminar flow cleanrooms are limited to no more than about 0.15 m/s (about 35 ft/min). When rooms go out of balance, it is not unusual to encounter horizontal flows of several hundred feet per minute. Airborne Molecular Contamination Airborne molecular contamination (AMC) can be categorized roughly as inorganic acids and bases and organic acids and bases. These compounds, if present at high enough concentrations, can severely corrode or react chemically with sensitive product surfaces. Types of failures attributed to AMC in semiconductor factors include defects in photoresist, unintentional doping, corrosion, adhesion failure of films, and uneven film growth in epitaxial deposition or oxidation [6,7]. Fortunately, realtime and near-real-time monitoring systems are available that can monitor down to low-ppb levels. Even lower concentrations can be detected using sample concentration techniques, specialized detectors, and other methods, within tens of minutes. One type of continuous monitor used for monitoring airborne molecular contamination is the surface acoustic wave sensor, also referred to as a SAW device. SAW devices are a special form of quartz crystal microbalance (QCM). QCMs have been used since the early 1970s as particle detectors. A typical design for a QCM is illustrated in Figure 7.11. Electrical charge is applied to electrodes on either side of the quartz crystal. The thickness of the quartz crystal changes in response to the applied electrical potential because of the piezoelectric effect. Typical quartz crystals will resonate at a few megahertz. In operation, a pair of well-matched crystals are used; one is exposed to an aerosol-laden flow while the other is kept clean. The frequency of oscillation of the two crystals are compared producing a difference signal, oscillating at a few thousand hertz. The change in beat frequency is then a measure of the change in resonance of the exposed crystal imposed by the mass loading of the exposed crystal. The sensitivity of these quartz crystal microbalances is tens of micrograms per square centimeter. Coating the surface of the crystal with selective absorbant can turn a QCM into a real time AMC monitor. In a surface acoustic wave sensor, the electrodes are arranged on the same side of the crystal, as illustrated in Figure 7.12. Here the oscillations are across the surface as opposed to through the thickness of the crystal. Again, as mass is applied to the crystal, the resonant frequency changes. However, in the SAW device the resonant frequency is hundreds of megahertz. The SAW device has a corresponding greater sensitivity than a QCM and is capable of typically measuring 0.2 ng/cm2. One of the more interesting features of SAW devices is that absorbant coatings can be applied to their surfaces. The chemistry of the coatings can then impart some chemical selectivity to the sensor, providing an AMC monitor that is tuned to specific classes of chemicals.

~V

FIGURE 7.11

Electrode geometry in a quartz-crystal microbalance.

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355

~V

FIGURE 7.12

Electrode geometry in a surface acoustic wave device.

Other Factors There are also other factors that can have an effect on air quality in the cleanroom. Among these are the following: ●

●

●

●

Plenum pressures. Steady plenum pressures are important to air quality, indicating continued fan operation and ensuring that pressure surges do not damage filters and other vital components. Plenum pressure monitors can be used in place or in combination with horizontal flow monitors to monitor room performance. Fan-filter units. Many modern facilities are equipped with fan-filter units. There may be hundreds or thousands of these units in minienvironments and the facility. Their continuing operation is essential to air quality. Several approaches to monitoring fanfilter units are currently under development. Organic, acid, and general gas exhaust. Monitoring the pressure in exhaust systems is a convenient and low-cost way to ensure the gases that they are intended to remove from the process do not escape into the cleanroom to degrade air quality. In many cases, the materials being exhausted, if released into the air in the cleanroom, can pose potential health and safety risks. Outside air quality. In some cases, outside air quality has a perceptible effect on air quality in the cleanroom. In these cases, weather stations can be installed outside the building to assist in management of the makeup-air system.

7.3.2

Process Fluid Purity

Process fluids used in manufacturing include deionized water, compressed gases, detergents, and wipe-down chemicals. In the case of detergents and wipe-down chemicals, these are usually handled as bottled chemicals. The best way to control cleanliness of these types of chemicals is to sample them periodically using a qualified test lab and appropriate analysis methods to ensure batch purity. On the other hand, DI water and compressed gases are usually distributed from a central bulk storage location using a delivery system. The purity of the fluid is thus not only source dependent but also delivery system dependent. Process fluid purity is dependent on the performance of the delivery system. The flow variables that are essential to monitoring a process fluid delivery system are discussed below. Liquid-Borne Particle Concentrations There are two primary cases where process fluids can be monitored for liquid-borne particles: in-line and in the process. In-line monitors are available for DI water and other liquids and for compressed gases. In-line particle analyzers for liquids generally depend on line pressure to force the liquid through the sensor, suppressing bubble formation. In-line particle analyzers for particles in compressed

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gases generally make use of pressure diffusers to lower the gas pressure to the operating pressure of the particle counter. A particle counter’s lower resolution limits range from 0.05 to 2 m at flow rates capable of measuring tens to hundreds of particles per liter, adequate resolution for most applications. Grab samples can also be taken from within process tanks, drains, and other chambers where liquids are no longer at line pressure. In these cases, sampling must rely on sampling pumps to pass the samples through the particle counter. For particles in air, this reverts to standard airborne optical particle counters. However, for particles in liquids, syringe pumps or samplers that push the sample through the sensors under pressure are needed to avoid bubble formation. 7.3.3

The Value of 100% Sampling

Choosing the sensor for a continuous monitoring system particle counter is often confusing. This is because there are so many factors to consider. The factor most often considered pivotal in the choice is the lower size detection limit. However, factors other than the lower size detection limit of the sensor should be considered when selecting a sensor. For example, one should also consider the sampling rate of the sensor, which defines both the volume sampled and the detection probability for contaminants or charges. The detection probability for a particle counter serves as an illustrative example. For a particle counter, the lower detection limit is defined as the probability that a particle which is equal to or larger than the sensor’s lower size detection limit will be counted by the particle counter. This is a function of the fraction of the volume that the particle must pass through in order to be detected. Thus, if a particle counter samples 100 mL/min but can sense particles passing through only 50% of this volume, the detection probability is expected to be 50%. We illustrate using a comparison between two possible sensors: one features 90% detection at 0.05 m size and the other features 50% detection at 0.1 m size. For both particle counters, the detection efficiency is essentially 100% at 0.2 m and larger sizes. The 0.05-m lower detection limit sensor has a sampling rate of 100 mL/min, but can detect particles passing through only 0.25% of that volume. The detection probability is 1/400 of the volume sampled. Thus, the detection probability is 0.25%, or 0.0025. The 0.1-m lower detection limit sensor also has a sampling rate of 100 mL/min and can detect particles passing through 100% of that volume. In detection probability terms this means that the sensor sees 100% of the volume sampled. Thus, the detection probability is 100% or 1.0. Now, let us see how these two factors, detection efficiency and detection probability, combine to affect the data reported by the two different sensors. In the first case, the 0.05-m resolution sensor has 90% detection efficiency at 0.05 m but only 0.25% detection probability. Multiplying these two factors together gives the result that the probability of detecting a 0.05-m-diameter particle is (0.9)(0.0025)  0.00225, or 0.225%, at the lower detection limit. In the second case, the 0.1-m resolution sensor has 50% detection efficiency at 0.1 m but 100% detection probability. Multiplying these two factors together gives the result that the probability of detecting a 0.1-m-diameter particle is (0.5)(1.0)  0.5, or 50%, at the lower detection limit. Clearly, the 0.1-m resolution particle counter is more likely to detect particles at its lower detection limit than is the 0.05-m resolution particle counter. This problem becomes more obvious when we consider particles that are above the 100% detection efficiency limit. The 0.05-m lower detection limit particle counter has 100% detection efficiency for 0.5-m-diameter particles but a 0.25% detection probability. Multiplying these two factors

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together gives the result that the probability of detecting a 0.5-m-diameter particle is (1.0) (0.0025)  0.0025, or 0.25%. The 0.1-m resolution sensor has 100% detection efficiency at 0.5 m and a 100% detection probability. Multiplying these two factors together gives the result that the probability of detecting a 0.5-m-diameter particle is (1.0)(1.0)  100%. Flow Rates For many processes, flow rate is a critical parameter. For example, flow rate through the recirculation filters and makeup DI water filters are critical to the operation of DI water cleaners. Often, these are set using a rotating ball flow meter, which must be visually checked periodically, but are not monitored continuously. In cases where visual checks prove inadequate, flow rate can be monitored continuously. Filter Differential Pressure Pressure gages are often installed across the media of inline filters. These are used to check for filter leakage (low differential pressure) and for filter clogging (high differential pressure). As with in-line flow meters, differential filtration pressure gages are seldom monitored continuously. Where experience shows that visual inspection is inadequate, continuous differential pressure monitoring is called for. Conductivity and Specific Ions Deionized water, alcohol, and other fluids can become contaminated with ionizable contamination, which increases the conductivity of the fluid. In these cases, fluid conductivity meters or in-line ion-specific electrodes prove to be costeffective continuous monitoring methods. Nonvolatile Residue In some cases, DI water and other solvents can become contaminated by materials that do not ionize, so they are not detectable using conductivity sensors or do not scatter sufficient light to be detected by liquid-borne particle counters. In many of these cases, an in-line nonvolatile residue (NVR) monitor can provide the desired protection where LPCs or conductivity monitors fail to detect the contaminants. The NVR monitor samples fluid under pressure and atomizes the fluid using a nebulizer. The volatile solvent is evaporated away, leaving condensation nuclei which are too small to be seen with a conventional optical particle counter (OPC). The aerosol is first passed through a particleconditioning chamber (an evaporation condensation apparatus) to grow the condensation nuclei to a size detectable by the OPC. This detector is capable of reporting NVR down to 1 ppb in near real time (tens of seconds). Unfortunately, these sensors are too expensive per sample point to be practical for a continuous monitoring system. A practical multiple-point sampling system is needed, but at the current time a satisfactory unit does not exist. Moisture Content and Dew Point Compressed air and other gases must often be monitored for condensable vapors; especially water vapor. The requirement for compressed air is a water vapor content so low that it requires a dew point analyzer. The dew point of a gas is the temperature at which liquid water condenses out of the compressed gas. A common dew point requirement is –40°F. Total Organic Carbon Total organic carbon (TOC) is a method that can be used to monitor the purity of DI water for the presence of organic molecules that do not ionize, are not soluble, and are present above their critical Micelle concentration. If the organic contaminant does not ionize, it is undetectable by a conductivity monitor. If it is insoluble and below its critical micelle concentration, it will not form droplets and so will be undetectable

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by LPCs. TOC analyzers provide near-real-time low-ppb-sensitivity detection and monitoring for organic molecules. Composition Often, cleaning equipment is designed to meter detergent automatically into the fresh DI water put into the cleaning tanks. Metering pumps often perform unreliably. In-line UV absorbance detectors can be added to a monitoring system to ensure that the detergent solution concentration entering the wash tanks is correct. Composition of other fluid streams can be monitored using ion-specific electrodes, electrochemical analyzers, and a variety of other sensors. Ultrasonic Tank Performance Two processes take place to remove contamination in ultrasonic cleaning: cavitation and acoustic streaming. The primary cleaning mechanism is cavitation. Cavitation is the formation of tiny rarified bubbles in the cleaning fluid due to the constructive interference of ultrasonic pressure waves. When the cavitation bubbles collapse, they form a tiny high-velocity jet of liquid that removes contamination from the surface. The contaminants are then carried away from the surface by acoustic streaming, the secondary and weaker cleaning mechanism. If the sound pressure level in the liquid in the tank is too low, no cavitation bubbles are formed and the cleaning efficiency of the tank is low. Conversely, if the sound pressure level in the liquid is too high, significant damage to the surface of the parts can occur, especially if the parts are made of erosion-sensitive materials such as soft aluminum alloys and many polymers. A sensor is available for monitoring the performance of ultrasonic baths.

7.3.4

Cleanliness of Surfaces and Electrostatic Charge

Three primary areas of concern are discussed below. ● ● ●

Surface contamination rate increased due to electrostatic charge Work station, tool, and operator grounding Air ionizer status

Surface Contamination Rate Increases Due to Electrostatic Charge Currently, no real-time sensors are available for real-time monitoring of an accumulation of particle contamination on surfaces from the air. In addition, surface contamination rates can be dominated by contact transfer. In contact transfer the contamination comes from the hands and gloves of people working in the area, work surfaces, and packaging materials. Surface contamination rates can be monitored indirectly using several real-time airborne contamination monitors, such as optical particle counters and airborne molecular contamination monitors, as described above. In addition, particle contamination rates to surfaces are strongly affected by the electrostatic charge levels on surfaces. Therefore, one way to monitor surface contamination rates indirectly is to monitor air ionization performance and surface charge levels as well as airborne particle contamination monitoring. Surface contamination rates vary in proportion to the average charge level on a surface for small particles. In general, the surface contamination rate increases in direct proportion to the average charge level on surfaces. At low surface charge levels, surface contamination rates by particles larger than about 3 m are largely unaffected by surface charge levels. However, as surface charge levels increase, the maximum particle size at which there is no effect on the surface contamination rate shifts to larger particle sizes.

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359

Two factors are used to describe the charge levels on surfaces: the rate at which static electric charge is dissipated or shunted to ground and the average charge level on the surfaces. Three types of materials are commonly used in cleanrooms and ESD-protected work areas: ● ● ●

Conductive: materials with surface resistivities below 106 /sq Dissipative: materials with surface resistivities in the range 106 to 1012 /sq Insulative: materials with surface resistivities above 1012 /sq

Conductors and static dissipative materials, if grounded, will discharge very rapidly to near zero volts. Insulative materials cannot be grounded, and in a cleanroom void of air ions will discharge only very slowly. Even conductive and dissipative materials will remain charged for a long time if they are not properly grounded. The charge levels on surfaces can be characterized using a field potential meter. If high charge levels are found, it may be necessary to reionize the air in the cleanroom to help neutralize these charges. The discharge time and float potential (average voltage) in a cleanroom equipped with air ionizers can be monitored with a charged plate monitor. Miniature charged plate sensors and low-cost monitoring systems are available to provide monitoring systems for cleanrooms and ESD-safe work areas. In addition, these real-time monitors can be equipped with specialized sensors that allow monitoring of charge levels on tools, work surfaces, products, people, and other surfaces. Thus, one can monitor factors, that directly affect surface contamination rates. Work Station, Tool, and Operator Grounding In addition to monitor charge levels, monitors are available to monitor the grounding status of many things in disk drive manufacturing (e.g., workstation mats and floor). The performance of the wrist strap grounding system may also be monitored by measuring the resistance of the operator’s skin. Air Ionizer Status Several designs of air ionizers have self-balancing circuits. These have limited ability to compensate for emitter wear, contamination accumulation, and changes in workstation layout. When any of these factors drive the self-balancing ionizer out of control, they can activate the alarm. Alarms can be monitored continuously for most types of air ionizers. 7.4 EVALUATION OF IN SITU MONITORING IN AN AQUEOUS CLEANING APPLICATION The dominant cleaning process used in the cleaning of individual piece parts or subassemblies in the disk drive and other precision assembly industries is aqueous cleaning. This process often involves initial cleaning by immersion in an ultrasonically agitated, deionized (DI) water–detergent mixture, followed by rinsing in multiple, consecutive, ultrasonic tanks of increasingly pure DI water. We explore the feasibility of the use of an in situ liquid-borne optical particle counter to monitor such a cleaning process. Some of the variables affecting particle counts are explored. Preliminary correlation with off-line measurement of piece part cleanliness using two different liquid-borne particle counters are also explored. Possible management strategies enabled by the use of the particle counter as an online real-time in situ particle monitor (ISPM) are discussed. Several approaches can be taken to monitor and control particle contamination on hightechnology products that are subject to cleaning. Among these are periodic sampling of liquids

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from the cleaning baths and periodic measurement of parts using direct or indirect particle measurement techniques. Periodic parts measurements are well supported by work reported by Nagarajan [8] and Gouk [9]. The approach of periodic sampling of parts from production has historically satisfied the needs of the user. Periodic sampling of the bath fluids or parts from the bath suffer from several drawbacks, among which are the following: ● ●

●

●

Manual bath sampling may interrupt production and can result in bath contamination. Parts sampled from ongoing production may undergo recontamination in handling or packaging prior to analysis. Bath sampling and parts sampling are periodic and may not truely represent bath condition at all times. Both techniques require off-line laboratory analysis, which may introduce procedural errors and may involve a delay in obtaining results.

These drawbacks result in possible loss of data integrity and are unable to capture batches of parts that do not conform to particle cleanliness requirements on a real-time basis. Knowledge of particle cleanliness for individual samples is excellent, but knowledge of the statistical cleanliness is poor, since sampling is infrequent. This makes it difficult to implement statistical process control. These difficulties were well articulated by Vargason [10], who also described a multipoint ISPM for semiconductor acid processing baths. Hess [11] described the application of an ISPM for rapid optimization of a semiconductor cleaning bath. Later, Hess [12] described the application of the ISPM to a second semiconductor cleaning system, where an attempt was made to show correlation with direct surface inspection using a wafer scanner. The results showed an apparent negative correlation between the counts in the bath reported by the ISPM and surface counts of particles on the wafers. The implications of this result are beyond the scope of this discussion. Although these studies address many of the issues surrounding the application of ISPMs to monitoring process baths, all are focused on baths for processing semiconductor wafers. Knollenberg [13] reported on the use of an ISPM for monitoring particles in a cleaner for head stack assemblies, an important assembly for a hard disk drive. In this study the authors were able to show a strong positive correlation between particles in the final rinse overflow (i.e., sampling after the weir) and the residual particles ultrasonically extracted from the assembly and measured using a liquid-borne optical particle counter (LPC). They were also able to show several applications of the ISPM for optimizing the performance of the cleaner and for monitoring for equipment failure modes. The study does not, however, address a far more complex problem, in that the cleaner is dedicated to cleaning a single part type. The objective here is to determine the feasibility of using an ISPM to monitor the performance of a cleaner wherein several different types of parts of variable incoming cleanliness levels are being cleaned. Further complicating this problem is the fact that the arrival rate, sequence of baskets, and number of parts per basket was variable. This is considered an extreme challenge for the application of an ISPM, but one worthy of evaluation. 7.4.1

Description of Experiment

Cleaning Process The cleaner consists of five consecutive tanks. The first two tanks are prewash and wash tanks, and the final three are rinse tanks. The tanks are made of stainless

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TABLE 7.5 Description of the Cleaning Process Tank Parameter

1

2

3

4

5

Tank name Weir Fluid

Prewash Single sided DI water

Rinse 1 Four sided DI water

Rinse 2 Four sided DI water

Rinse 3 Four sided DI water

Temperature (°C) Ultrasonic frequency (kHz)

45  7  3 40

Wash Single sided DI water  0.02% nonionic surfactant 45  7  3 40

45  7  3 75

45  7  3 75

45  7  3 75

steel, with a capacity of approximately 80 L and equipped with immersible ultrasonic transducers. All power generators were 1000-W units, operating at 95% of full power in the sweep frequency mode. The system was loaded robotically, although for certain tests described below, the cleaner was operated in the manual mode. Table 7.5 describes the important operating statistics for the system. (The system also included two forced-hot air dryers, whose particle performance was not measured in this study.) The system was operating in the following fashion. When baskets of parts were in each tank, the recirculation system was off but the ultrasonic power was on for a duration of 175 seconds. When baskets were entering or leaving the tanks, ultrasonic power was turned off, but the recirculation system was on, causing a relatively large volume of water to overflow the weir. If the arrival rate of baskets was continuous, the recirculation was for a total of 35 seconds. If parts arrived at longer intervals, the recirculation system would remain on for longer periods of time. All baths were fed continuously, independently, with approximately 4 L/min of fresh water. Approximately 4 L/min was drained continuously from each tank. The filtration system for recirculation and makeup water was via 5- and 0.2-m filters. The operating sequence, turning the ultrasonic power and recirculation pumps on and off, had a noticeable effect on the particle count signature, as shown in the results below. In Situ Particle Monitor and Its Installation The in situ particle monitor consists of a PMS Model 900 CLS sampler equipped with a 0.3-m resolution sensor. The output from the instrument was collected on a portable computer using PMS Pharmacy View software, although PMS Facility View software could have been used with the same results. The in situ particle monitor sampler draws a sample through the sample burette and an overflowing reservoir. In this installation, approximately 2 m of 3-mm-inside diameter Teflon tubing connected the tank to the particle counter. The internal volume of the tubing was approximately 13 cm3. The total sample collected was approximately four times the volume contained within the sample tubing, assuring adequate sample flushing. Particles were reported as particles per cubic centimeter equal to or larger than 0.3, 0.5, 0.7, 1, 2, and 3 m in diameter. Samples were collected in three different locations: (1) in the overflow weir from tank 4 (rinse tank 2), (2) from tank 4 (rinse tank 2), and (3) from tank 5 (rinse tank 3). The desire was to obtain a comparison between the results measured in the weir for tank 4 and within tank 4, and also to compare results for sampling within tank 4 and within tank 5.

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TABLE 7.6 Basket Count for Parts Groupings and Sampling Condition Sample Location In tank 4 weir In tank 4 In tank 5

Bare Aluminum

Plastic

Other

7 11 7

6 13 9

29 98 53

Particle Measurement Process The operation of the ISPM was not triggered by the arrival of baskets in any particular tank. That is, the cleaner and ISPM operated independently. As a consequence, the sequence of events for the ISPM when a basket of parts entered the tank being monitored was uncontrolled. This may affect the peak value for any basket load. The sample process begins by drawing liquid from the sample tube through the sample burette and its attached overflow reservoir until the liquid reaches a present limit. Pressure is then applied to force the sample liquid through the sensor to suppress bubble formation. At completion of the preset sample time, the liquid remaining in the sample burette and overflow reservoir is expelled to drain. Pressure in the apparatus is vented to the atmosphere to prepare for the next sample. Parts Cleaned During This Study Twelve different parts were cleaned during this study. These parts included large-surface-area electrophoretically painted cast and machined aluminum parts; bare aluminum parts; cast plastic parts; stainless steel parts; and parts consisting of a combination of stainless steel and elastomeric plastic. The arrival frequency of these parts varied according to their size and consumption. Three generic descriptions can be used to combine these 12 different parts into sets: bare aluminum parts, plastic parts, and others. Table 7.6 summarizes the basket count for these three groups of generic part types for the three sampling conditions: sampling in the weir from tank 4, sampling within tank 4, and sampling within tank 5. Clearly, the “other” category represents the vast majority of baskets evaluated in this study. Within this category, one part number represents 111 of the 180 basket loads. This might permit a detailed description of the behavior of the particle counts in the cleaner. The type and number of parts entering the cleaner were recorded. This allows a unique association of each peak recorded by the ISPM with the type of part being cleaned. 7.4.2

Experimental Results

Effect of Sample Inlet Location The study began with the sample inlet positioned in the liquid overflowing the sides of tank 4 into its weir (rinse tank 2). Figure 7.13 shows a representative plot of the particle counts recorded during a period of production in which parts were cleaned. This chart shows the cumulative particle counts in three size ranges vs. time of day. The horizontal bars above the particle count traces indicate the approximate times for entry into and exit out of tank 4 for 11 different baskets of parts. One feature is striking. Each basket of parts is apparently reported as two separate peaks or as a peak preceded by a plateau. This can be explained by understanding the operation of the cleaner. As parts enter each tank, the recirculation/filtration system is operating. Thus, a large volume of liquid from the tank is overflowing the weir to the inlet to the particle counter. Thirty-five seconds after basket entry, the recirculation pump is turned off, so the flow rate overflowing the weir is that of the makeup flow rate, about 4 L/min. At the end

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10,000 1,000 100 10

17:07

16:55

16:44

16:33

16:21

16:10

15:58

15:47

15:36

15:24

15:13

15:02

1 14:48

Cumulative Particles/ft3

100,000

Time of Day ≥ 0.5 μm

≥ 0.3 μm

FIGURE 7.13

≥ 0.7 μm

Basket in tank 4

Particle counts: January 25, first shift, tank 4 weir.

Cumulative Particles/ft3

100,000 10,000 1,000 100 10

11:57

11:46

11:35

11:25

11:14

11:03

10:52

10:42

10:31

10:21

10:10

9:49

9:38

9:28

9:17

9:06

1

Time of Day ≥ 0.3 μm

FIGURE 7.14

≥ 0.5 μm

≥ 0.7 μm

Basket in tank 4

Particle counts: January 28, first shift, sample inside tank 4.

of the cycle the ultrasonic agitation is turned off and the recirculation pumps are turned on again, increasing the flow rate from the tank over the weir. Thus, we see an initial peak in particle counts when baskets first enter the tank (high overflow rate), a lower particle count during ultrasonic cleaning (low weir overflow rate), and finally, a high particle count after ultrasonic agitation is turned off (high weir overflow). The rate of arrival of particles into the sampling location is thus affected by the operating cycle of the cleaning equipment. Relocation of the sample inlet to inside tank 4 eliminates this double-peak effect, as shown in Figure 7.14. Horizontal bars indicate the periods in which baskets were in tank 4. A less ambiguous peak is recognizable. This allows a clearer interpretation of the particle count history during the cleaning process. A statistical comparison can be made of the location of the sample inlet outside the weir of tank 4 vs. inside tank 4 (Table 7.7). These results show that sampling outside the weir significantly reduces the peak particle counts vs. sampling within tank 4 for four out of five parts. The opposite result occurred for the small bare stainless steel parts: the particle count obtained in the weir is greater than that obtained in

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TABLE 7.7 Comparison of ISPM Results Sampling in the Weir from Tank 4 or Sampling Within Tank 4 (particles/cm3  0.3 m)

Location Tank 4 weir Within tank 4

Mean Std. dev. Mean Std. dev.

Painted Aluminum Casting

Bare Aluminum

Large Bare Stainless Steel

Stainless Plus Elastomer

Small Bare Stainless Steel

16,433 3,831 18,400 6,184

34,789 7,959 57,903 13,196

12,506 4,535 14,037 7,722

7,483 3,998 17,229 9,905

26,641 5,182 14,200 5,986

TABLE 7.8 Comparison of ISPM Results Sampling Within Tank 4 or Tank 5 (particles/cm3 0.3 m)

Location Tank 4 Tank 5

Mean Std. dev. Mean Std. dev.

Painted Aluminum Casting

Bare Aluminum

Large Bare Stainless Steel

Stainless Plus Elastomer

18,400 6,184 19,280 5,198

57,908 19,186 77,055 9,332

14,037 7,722 13,366 3,479

17,229 9,905 13,553 9,020

the tank. This result may be due to the small number of batches of these parts in the sample. Using Student’s t test, only in the case of the bare aluminum and the small bare stainless steel parts are the comparisons statistically meaningful at the 95% confidence level. Table 7.8 summarizes the mean particle counts obtained for comparable baskets measured inside tanks 4 and 5 for four parts with a high number of baskets. These results show a surprise. In two cases the outcome is as anticipated: The average particle count in tank 4 is greater than the particle count in tank 5. For the other two parts, the reverse is evident: The particle count in tank 5 is higher than in tank 4. This may be an indication that the painted aluminum castings and bare aluminum parts may require longer rinse times. Again, using Student’s t test, the difference between particle count sampled within tanks 4 and 5 are statistically meaningful only for the bare aluminum parts. Effect of Part Arrival Rate and Sequence Note that the arrival rate and sequence of baskets of parts is uncontrolled. As a consequence, a relatively clean part, such as the large bare stainless steel parts, may be preceded by a relatively dirty bare aluminum part or a relatively clean plastic part. If this occurs over an interval too short for the rinse tank to recover to baseline, the starting value for each basket will be highly variable. This unpredictability of the baseline when a basket enters a rinse tank has a significant influence on the peak particle count achieved by any given basket of parts. One possible way to correct for this is to subtract the lowest value of the baseline preceding entry of a basket from the peak value for that basket, regardless of whether or not the baseline has fully recovered. For the purpose of this analysis, peak values sampled in tanks 4 and 5 are combined and compared to peak values corrected for the lowest value in the preceding 5 minutes of cleaner operation. The results are shown in Table 7.9. In every case the mean value for a type of part is reduced by correcting for the preceding baseline. However, the amount of correction remains variable as the type of part arriving in

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TABLE 7.9 Peak for Each Part Type Sampled in Tanks 4 and 5 Compared to Peak Corrected for Preceding Baseline (particles/cm3 0.3 m)

Condition Peak value

Peak corrected for baseline a

Mean Std. dev. COVa (%) Mean Std. dev. COV (%)

Painted Aluminum Casting

Bare Aluminum

Large Bare Stainless Steel

Stainless Plus Elastomer

Small Bare Stainless Steel

18,695 5,857 31 13,519 5,942 44

64,665 18,607 29 64,174 19,227 30

13,778 6,378 46 8,758 4,247 49

15,595 9,434 60 6,526 3,036 47

14,211 5,986 42 12,294 3,745 30

COV, coefficient of variance.

TABLE 7.10 Peak for Each Part Type in Tanks 4 and 5 Compared to Peak Corrected for the Preceding Fully Recovered Baseline (i.e., less than 1000 particles/cm3 0.3 m)

Condition Peak

Peak corrected

Mean Std. dev. COV (%) Mean Std. dev. COV (%)

Painted Aluminum Casting

Bare Aluminum

Large Bare Stainless Steel

Stainless Plus Elastomer

18,695 5,857 31 18,636 5,170 27

64,665 18,607 29 73,171 9,881 14

13,778 6,378 46 11,035 4,413 40

15,595 9,434 60 6,073 1,073 18

the preceding basket is not controlled. Hence, the starting point for the correction is variable. As a consequence, the coefficient of variability (the standard deviation times 100 divided by the mean) is not always improved for this method of baseline correction. This indicates that subtraction of the lowest value preceding a peak is not a feasible way to correct for variable baseline in this study. To determine if baseline correction might eventually be of value, uncorrected peak values were compared to peak values that occur only after the cleaner has had an opportunity essentially to fully recover the baseline, that is, to less than 1000 particles/cm3 0.3 m. This comparison is shown in Table 7.10 for four types of parts. Correcting the peak value for a fully recovered baseline improves the coefficient of variability of all four of these types of parts. The comparisons are statistically meaningful using Student’s t-test for bare aluminum, large bare stainless, and stainless plus elastomer parts. In the management of the cleaner using an ISPM, allowing the cleaner to recover to a lower baseline would be beneficial. This could be accomplished by allowing a longer delay time between baskets, modifying the cleaner function to allow more rapid recovery or a combination of the two. Effect of Basket Load (Fill Level) In general, each basket would contain one or more inserts of parts. In general, each insert would be completely full. In a few cases, more than one type of part insert would enter the cleaner as a basket load. By noting the number and type of insert in each basket load, it is possible to determine if basket loading has a meaningful effect on the uncorrected peak value for the basket. The results are summarized in

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TABLE 7.11 Effect of Basket Load, as Measured by Number of Insertsa (particles/cm3 0.3 m) Bare Aluminum

Large Bare Stainless Steel

Stainless Steel Plus Elastomer

Number of Inserts

Mean

Std. dev.

Mean

Std. dev.

Mean

Std. dev.

1 2 3 4 5 6

28,359 39,099 70,071 74,382 68,563 79,744

8,747

12,845 17,152 n.a. n.a. n.a. n.a.

4,354 10,371 n.a. n.a. n.a. n.a.

5,440 11,255 17,180 n.a. n.a. n.a.

b

a b

b

8,578 b

13,308 13,013

8,946 6,242 n.a. n.a. n.a.

n.a., Not applicable. Only a single basket contained this number of inserts.

Table 7.11. The large painted aluminum castings do not appear in this analysis, as only one insert is cleaned at a time. These data show that the basket fill level has a significant effect on the uncorrected peak value. In management of the cleaner using the ISPM, basket fill level is clearly a variable that should be controlled. Correlation with Laboratory Measurements For select baskets, parts were sampled and taken to the off-line contamination lab for LPC analysis. The purpose of this portion of the study was to determine if correlation could be established between particle counts in the cleaner and particle counts using the off-line extraction and liquid-borne particle counters in the lab. The off-line extractions were done using a Branson DHA 1000 ultrasonic tank. The tank was always filled with approximately 4 L of clean DI water to which was added a few drops of dishwashing detergent. Parts were placed in proscribed Pyrex glassware for extraction, containing a proscribed quantity of filtered DI water and 0.02% of a surfactant solution. Parts were extracted for 1 minute, after which the LPC counts were taken immediately. The DHA 1000 ultrasonic tank is rated at 150 W and is filled with approximately 4 L (1 gallon) of water. By contrast, the in-line cleaner has ultrasonic generators set to output 950 W into approximately 80 L (20 gallons) of water. Thus, the energy density in the DHA 1000 is nominally 150 W per gallon, whereas the cleaner ultrasonic tanks are nominally 48 W per gallon. Particle removal in the laboratory extraction might thus be expected to be significantly greater than in the in-line cleaner. Two different particle counters were available for LPC analysis. All parts were measured using a HIAC/Royco Model 8000A particle counter, sampling with an ASAP sampler and measured using an HRLD-150 sensor. Cumulative particle counts in greater than 2, 3, 5, 9, 15, 25, and 50-m sizes were recorded. In addition, the remaining extract from some parts were counted using a PMS LPS particle counter through a CLS600 sampler and measured using a IMOLV 0.5 sensor. Cumulative particle counts greater than 0.5, 0.7, 1.0, 2, 3, 5, 10, 15, 25, and 50 m were recorded. This allowed a three-way comparison of the LPCs, as the particle sizes for the three instruments overlap as shown in Table 7.12. As can be seen in the table, the ISPM overlaps with the LPS in five size channels, ranging from 0.5 to 3 m and with the 8000A in two size channels, 2 and 3 m. The LPS overlaps with the 8000A in six size channels, ranging from 2 to 50 m. The ISPM used in this test overlapped with the HIAC/Royco 8000 in only two size channels. This is unfortunate,

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TABLE 7.12 Common Size Ranges for the Three LPCs Used in This Study Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 10 15 25 50

PMS ISPM

PMS LPS

HIAC/Royco 8000

Yes Yes Yes Yes Yes Yes No No No No No No

No Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes

No No No No Yes Yes Yes Yes No Yes Yes Yes

TABLE 7.13 Bare Aluminum Part Cumulative Particle Size Distributions as Measured by the ISPM, PMS LPS, and HIAC Royco 8000A (particles/cm3) Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 (H/R) or 10 (LPS) 15 25 50

ISPM

PMS LPS

HIAC Royco 8000A

76,010 21,156 5,214 1,193 58 16 — —

— 7,752 3,716 2,030 128 21 3 1

— — — — 483 279 66 7

— — —

0 0 0

1 0 0

as the off-line parts cleanliness measurements are taken using the HIAC/Royco instrument rather than the PMS LPS. Bare Aluminum Parts Two batches of bare aluminum parts were sampled for laboratory analysis. The particle size distributions are listed in Tables 7.13 and 7.14. A basket load of six inserts of bare aluminum parts contains 1920 parts in 80 L of water. This corresponds to 24 parts per liter. Conversely, the off-line measurements use only 5 parts per liter of water for measurement. Thus, the particle burden in the cleaner should be nearly fivefold higher than in the off-line laboratory measurement. This may partially compensate for the fact that the nominal ultrasonic energy density in the off-line extraction is approximately threefold greater than that in the cleaner. First, compare the results obtained for the ISPM in the cleaner vs. the LPS in the offline laboratory measurements. The particle counts for the ISPM in the 0.3- and 0.5-m channels exceed the 12,500-particles/cm3 coincidence limit. The counts in these channels thus underestimate the true particle concentrations in the rinse tank. In addition, exceeding

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TABLE 7.14 Bare Aluminum Part Cumulative Particle Size Distributions as Measured by the ISPM, PMS LPS, and HIAC Royco 8000A (particles/cm3) Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 (H/R) or 10 (LPS) 15 25 50

ISPM

PMS LPS

HIAC Royco 8000A

65,217 18,066 4,054 878 47 4 — —

— 11,819 7,472 4,526 298 41 5 2

— — — — 1,014 603 147 12

— — —

1 1 0

1 0 0

the count coincidence limit can result in counting several small particles as a single larger particle, which can then appear in a larger channel. This may explain the observation that the ISPM reports a greater number of particles in the 0.5- and 0.7-m channels, but fewer in the 1-, 2-, and 3-m channels than the LPS reported in the off-line size measurements for the bare aluminum parts. Particles may have been shifted to larger channels (0.5 and 0.7 m), distorting the size distribution. If comparison is limited to particles larger than 1 m, it would appear that the ISPM reports lower particle counts than the LPS, but the disagreement is small (less than a factor of 3). Considering the differences in particle counters, parts per liter loading, and power density between these two comparisons, this is reasonable agreement. Turning to a comparison between the laboratory measurements using the LPS and the HIAC/Royco, we see that the LPS counts less than the HIAC/Royco instrument by a factor of 4 to greater than 30. This is somewhat surprising, in that an earlier study by the author indicated good correlation between the LPS and the HIAC/Royco 8000 A using the same sensors. Finally, comparing the ISPM to the HIAC/Royco, we see the HIAC/Royco measurement is much greater than the counts over channels of comparable size. The particle concentrations measured in the cleaner by the ISPM exceed the count coincidence limit for the 0.3- and 0.5-m channels. Thus, particle concentrations in the 0.3- and 0.5-m channels are probably underestimated, and those in the 0.7- and 1-m channels are probably overestimated. Again, limiting our comparison to particles larger than 1 m, the ISPM reports 5 to 10 times fewer particles than the LPS. Comparing the LPS to the HIAC/Royco laboratory measurements, the HIAC/Royco reports significantly larger numbers of particles. Finally, comparing the ISPM to the HIAC/ Royco, we find that the ISPM numbers are 20 and 150 times smaller in the 2- and 3-m channels, respectively. Since these ISPM measurements have been compromised by grossly exceeding the coincidence count limit, the particle size distributions may not be the most reliable to use to establish correlation with laboratory measurement of parts cleanliness. Large Bare Stainless Parts Four batches of large bare stainless parts were sampled and compared using the three techniques, as shown in Tables 7.15 to 7.18. The basket loading in the cleaner corresponds to approximate 0.75 part/L; that in the laboratory measurement

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TABLE 7.15 Large Bare Stainless Part Cumulative Particle Size Distributions as Measured by the ISPM, PMS LPS and HIAC Royco 8000A (particles/cm3) Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 (H/R) or 10 (LPS) 15 25 50

PMS LPS

ISPM

HIAC Royco 8000A

11,014 (29,482) 3,411 (9107) 756 (2018) 114 (304) 11 (29) 4 (11) — —

— 5,093 1,773 878 58 16 6 2

— — — — 169 111 41 11

— — —

1 0 0

3 0 0

TABLE 7.16 Large Bare Stainless Part Cumulative Particle Size Distributions as Measured by the ISPM, PMS LPS, and HIAC Royco 8000A (particles/cm3) Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 (H/R) or 10 (LPS) 15 25 50

ISPM

PMS LPS

HIAC Royco 8000A

19,239 (51,368) 5,033 (13,438) 1,159 (3,094) 218 (582) 17 (45) 7 (19) — —

— 5,486 2,016 1,062 101 31 10 3

— — — — 285 193 76 17

— — —

1 0 0

4 0 0

is 2 parts/L. Thus, it would be expected that the particle burden in the cleaner is 2.67 times smaller than in the off-line parts measurement. The comparison among the particle counters is therefore expected to be a more reliable indicator if the ISPM can be used to correlate to off-line laboratory measurement of piece part cleanliness. The ISPM counts fewer particles than the LPC. However, if we multiply the ISPM data by 2.67 (numbers in parentheses), the disagreement between the two particle counters decreases. The agreement between the LPS and the HIAC/Royco is still unsatisfactory. Finally, the ISPM data underestimate those from the HIAC/Royco by a factor of 5 and 10 for the 2- and 3-m channels, respectively, when using the correction factor of 2.67 for the different part loading in the two cases. Correlation of ISPM to LPS and HIAC/Royco Table 7.19 results show the correlation between the three different techniques used in this study. Inspection of these data allow the prediction that correlation does not exist between either the raw or corrected ISPM data and the off-line measurements taken using the HIAC/Royco particle counter (R2  0.02).

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TABLE 7.17 Large Bare Stainless Part Cumulative Particle Size Distributions as Measured by the ISPM, PMS LPS, and HIAC Royco 8000A (particles/cm3) Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 (H/R) or 10 (LPS) 15 25 50

PMS LPS

ISPM 12,890 (34,416) 3,764 (10,050) 952 (2,542) 193 (515) 20 (53) 6 (16) — — — — —

— 5,020 1,903 983 55 15 5 2 1 0 0

HIAC Royco 8000A — — — — 135 84 31 9 2 0 0

TABLE 7.18 Large Bare Stainless Part Cumulative Particle Size Distributions as Measured by the ISPM, PMS LPS, and HIAC Royco 8000A (particles/cm3) Particle Size (m) 0.3 0.5 0.7 1 2 3 5 9 (H/R) or 10 (LPS) 15 25 50

PMS LPS

ISPM

HIAC Royco 8000A

10,379 (27,712) 3,188 (8,512) 817 (2,181) 163 (435) 13 (35) 3 (8) — —

— 5,265 1,865 601 83 25 8 3

— — — — 196 128 51 14

— — —

1 0 0

3 0 0

Additional Observations Additional tests were run to determine the effect of the basket on the peak value during a rinse cycle. These tests indicate that some baskets for bare aluminum parts could have a significant influence on peak particle counts. 7.4.3

Management Using ISPM

Several objectives can be defined for use of the ISPM to assist the management of cleaners. One of the most important of these is identification and isolation of batches of parts that do not meet statistical process control criteria for quality control requirements. Another objective is to collect data describing the cleanliness of parts in an automated fashion. In this study, parts were cleaned that had a diverse range of surface cleanliness. In addition, the arrival rate and composition of baskets of parts were uncontrolled. This led to difficulty in interpretation of peak contamination values in rinse baths. This also led to difficulty in correlation of particle counts in the rinse bath to off-line laboratory measurements of parts cleanliness.

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TABLE 7.19 Correlation Between the ISPM and HIAC/Royco Off-Line Parts Measurements (particles/cm3 2 m) ISPM Part Type Large bare stainless

Painted casting

Stainless plus elastomer

Plastic

Raw

Corrected

HIAC/Royco

11 17 20 13 9 26 28 25 24 16 33 10 7 57 21

29 45 53 35 34 99 106 98 91 61 125 9 6 51 12

169 285 135 196 484 419 565 647 673 504 394 177 262 261 46

Several improvements can be suggested to improve the value of the use of an ISPM to evaluate this cleaner: ●

● ●

The cleaner cycle should be improved to get closer to the baseline for full operation. That is, the recirculation time and flow rate should be increased to allow the baseline particle counts to more nearly approach 1000 particles/cm3  0.3 m between baskets. The loading of baskets should be controlled more closely. The cleanliness contribution of inserts needs to be controlled to prevent this from influencing the outcome of tests.

In this application, the operating cycle of the cleaner has a measurable influence on the results obtainable from the ISPM. First, the time for the cleaner to recover to baseline particle counts is significantly longer then the arrival interval for baskets of parts under fully loaded conditions. The arrival sequence of baskets is unregulated, so baskets of relatively dirty parts may immediately precede the arrival of baskets of relatively clean parts. This, in combination with the long bath recovery time, interferes with the interpretation of cleaner performance. 7.4.4

Conclusions

In situ particle monitoring is a well-accepted technique in the semiconductor industry for vacuum processes. It has been demonstrated for wet process baths in applications in the semiconductor industry and has also been shown to be feasible in the disk drive industry for head stack assemblies, where the identity of parts and contamination load in the cleaner are relatively tightly controlled. The manufacture of disk drives represents a different problem. The range of cleanliness of parts, basket loading, and sequence of arrival of parts in the

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cleaner are more or less random. As a consequence, for use of the ISPM to provide full benefit as a process control tool, control of cleaner performance requires modification of the management strategy for cleaners. 7.5

ANTENNAS FOR ELECTROSTATIC CHARGE MONITORING

Several types of antennas are available for electrostatic charge monitoring. The choice of antenna depends on the purpose of charge monitoring. There are three types of antennas— a disk-shaped antenna, a telescoping antenna, and a 10-pF charged plate antenna—and two ways of connecting them to a charge monitor. Antennas can be connected to the charge monitor using a single or double banana jack, which places the impedance-matching resistor inside the charge monitor, or using a microdot connector, which places the impedancematching resistor in the antenna mount. The telescoping antenna is a good general area sensor. The disk antenna is intended for monitoring more localized charge, such as on a product. The 10-pF charged plate is intended specifically for monitoring air ionizer float voltage. Connecting a telescoping antenna or a disk antenna to the charge monitor using a single banana plug allows the wire to act as part of the sensor, making it possible to monitor electrostatic charge over a relatively large area of a workstation. The double banana plug is used to connect the charged plate to the monitor, thus providing a ground plane for the floating surface of the charged plate. Connecting to the microdot connector eliminates the ability of the wire to detect electrostatic charge, providing for highly localized charge detection. Teflon on Antennas for Use in Static-Safe Work Areas Many insulators are found in static-safe work areas in addition to the Teflon used on the bases of antennas. How does one determine if an insulator is suitable for use in a static-safe work area? To answer this question, one must recognize that the majority of static-safe work areas are equipped with air ionization to control tribocharging of insulators and floating (i.e., ungrounded) conductors. These ionizers are installed and balanced to achieve discharge times and float potentials at critical product locations on workstations. For example, MR-critical workstations in which the head wires are attached to the nonshunted MR head but are not yet bonded to the bond pads on the flexible circuit assembly will typically require discharge from 1000 V to less than 20 V in under 10 seconds, with the float potential limited to less than 20 V. If a surface becomes charged under normal conditions of use, it is considered acceptable if the ionization is capable of discharging it to below the float potential in an acceptable time. It is for this reason that Teflon is considered to be acceptable for nearly all ESD-safe work areas, including MR-critical areas.

REFERENCES AND NOTES 1. D. Pariseau, The future of cleanroom monitoring systems, Cleanrooms, Jan. 1995, p. 39. 2. J. Livingston, R. Bower, R. Pochy, and L. Branst, Using an automated clean room monitoring system to maximize contamination control, Micro, Oct. 1997, p. 113. 3. B. Fardi, An evaluation of a cost effective and efficient airborne particle monitoring system, Proceedings of the 38th Annual Meeting of the Institute of Environmental Sciences, Nashville, TN, May 3–8, 1992, p. 38.

REFERENCES AND NOTES

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4. C. F. Query, Continuous monitoring in clean rooms: a guide for the first time user, Proceedings of the Asia Pacific Magnetic Recording Conference, Singapore, July 29–30, 1997. 5. T. J. Bzik, Statistical management and analysis of particle count data in ultraclean environments, Proceedings of the Microcontamination Conference, San Jose, CA, Nov. 20–22, 1985, p. 93. 6. M. Caminzind, Airborne molecular contamination in cleanrooms, Cleanrooms, 12(1): 1–5, 1998. 7. D. A. Hope, and W. D. Bowers, Measurement of molecular contamination in a semiconductor manufacturing environment using surface acoustic wave sensor, Productronica ’91, Munich, Germany, Nov. 1997. 8. R. Nagarajan, and R. W. Welker, Size distributions of particles extracted from disk drive parts, Journal of the Institute of Environmental Sciences, Jan.–Feb. 1993, pp. 43–48. 9. R. Gouk, Optimizing ultrasonic cleaning for disk drive components, Precision Cleaning, Aug. 1997, pp. 13–17. 10. R. Vargason, Liquid multiport system provides automatic real-time monitoring of wet-process station liquids, Microcontamination, Sept. 1990, pp. 39–41. 11. D. Hess, S. Klem, and J. M. Grobelny, Using in situ particle monitoring to optimize cleaning bath performance, Micro, Jan. 1996, pp. 39–45. 12. D. Hess, K. Dillenbeck, and P. Dryer, Comparison of surface monitoring and liquid in situ particle monitoring (for an HF and DI water rinse hood), Proceedings of the 43rd Annual Technical Meeting of the Institute of Environmental Sciences, Los Angeles, CA, May 4–8, 1997, pp. 321–324. 13. B. Knollenberg, and K. Edwards, Use of in-situ particle monitors in HSA aqueous cleaning processes, presented at the IDEMA Microcontamination Conference, Santa Clara, CA, Mar. 10, 1998, pp. 39–51.

CHAPTER 8

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

8.1

INTRODUCTION

Many different consumable supplies and materials are used in cleanroom and ESD-protected work area applications. Among these are disposable shoe covers, gloves and finger cots, wipers, swabs, disposable face masks and hairnets, hand creams and lotions, and disposable cleanroom garments. Selection of consumable materials must take into consideration the requirements of the product or process, cost or reusability, and waste disposal. The functional structures of electronic and electromechanical devices continue to contract. Concurrently, the ability of such devices to survive contamination and ESD diminish as well. Moreover, the process by which consumables are qualified for use in cleanrooms and ESD-safe areas by manufacturers of contamination and ESD-sensitive components has not yet been entirely standardized. The literature on the performance of consumable supplies during use in these areas is sparse. In this chapter we explore the issues associated with selection of consumables, including qualification testing. We also discuss the need for ongoing lot qualification testing. Tests for cleanrooms consumables include tests to qualify them for use with a particular product or process and ongoing lot certification tests. The test methods for qualification can be further broken down into functional tests and materials qualification tests. Ongoing lot certification tests may rely on test methods that differ from those used in materials qualification tests. Materials qualification tests will probably examine a large number of contamination and ESD performance parameters. Conversely, only a single parameter may be found to be indicative of the inherent variability of the material. In this case, ongoing lot certification tests may consider only a single parameter. A good example of this is cleanroom gloves. Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

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375

TABLE 8.1 Some Costs Associated with Cleanroom Operation Item Clothing

Housekeeping

Swabs and wipers Gloves

Case 1

Case 2

Jump suit, changed 2.5 times per week; hood, changed 3.5 times per week; knee-high booties, changed 1 time per week; cost  $0.67/operator/day Daily vacuuming, floor mat change; weekly wet mopping; $0.66/operator/day Cost  $0.52/operator/day 4 to 8 pairs of natural latex gloves per day; cost  $2.32/operator/day

Frock, changed 2.5 times per week; hood, changed 3.5 times per week; no shoe covers; cost  $0.57/operator/day Daily vacuuming, floor mat change; weekly wet mopping; $0.66/operator/day Cost  $0.52/operator/day 4 to 8 pairs of nitrile gloves per day; cost  $1.02/operator/day

Considering that the cost of consumables is significant and one of the easiest to measure of all expenses, much published work would be expected. This is not the case. There are relatively few studies available in the literature that examine the cost of consumable supplies. A 1994 [1] study illustrated at least one company’s cost estimate, as shown in Table 8.1. In addition to their high relative cost, consumables, particularly swabs, wipers, and gloves, are among the most likely sources of contamination, due to the high probability of contact transfer. Based on this financial analysis, gloves represented 40 to 75% of the consumable costs for this manufacturer. We will thus use gloves as an example of how to control contamination and costs associated with consumables. 8.2

CLEANROOM AND ESD GLOVES

Many types of gloves and finger cots are worn in cleanroom and ESD-safe work areas. The most popular are dipped film gloves, which provide a continuous barrier protecting the cleanroom environment and products from contamination by the wearer. Other types of gloves include knitted and sewn fabric gloves, some of which are also available with barrier film gloves. Still other types of gloves are intended for specialty applications, such as chemical safety gloves and gloves to provide personal protection from heat or cold. Here the discussion is limited to dipped barrier film gloves, finger cots and glove liners intended for product protection from contamination or ESD, and glove liners intended to provide comfort for the wearer. Glove Liners and Finger Cots One company using barrier film disposable cleanroom gloves sought a way to reduce their cost of ownership. In place of disposable cleanroom gloves, they used glove liners with the fingertips of the thumb and two fingers covered by disposable finger cots. This resulted in the arrangement shown in Figure 8.1. They also set up an in-house glove liner laundry operation using a commercially available cleanroom washer and dryer to maximize the benefit of this approach. The return on investment for implementing this strategy was less than six months. Many different parameters can be considered when selecting a glove. Among these are mechanical properties such as length and thickness, the absence of pinholes, and puncture, wear, and tear resistance. Contamination considerations fall into two broad categories: functional tests and nonfunctional tests. Contamination functional tests include tests for

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FIGURE 8.1 Clever strategy to reduce the cost of ownership for gloves. This company elected to wear disposable finger cots on the thumb and two fingers rather than a launderable glove liner. The company elected to launder the glove liners in-house to reduce the cost further.

contact and near-contact stain. Contamination nonfunctional tests include tests for extractable particles, anions, cations, viable organisms, and organic contaminants. In some applications, functional tests and nonfunctional tests for ESD properties may also be important. Here we concentrate on functional and nonfunctional contamination and ESD tests appropriate for the qualification of cleanroom consumables.

8.3 8.3.1

FUNCTIONAL VS. NONFUNCTIONAL TESTING Functional Materials Qualification Tests

As with any material used in cleanrooms, gloves or their extraction products are sometimes in contact with products or are in close proximity to, but not in contact with, products. Two types of tests are used to evaluate the functional suitability of glove materials for cleanroom applications: contact stain and near-contact stain. Other types of tests may be specified, depending on the users’ functional requirements. In a contact stain test, apparatus suitable to hold the test material and product is prepared so that apparatus contribution to the test is negligible. Several strips of the material under test are held against the product. The apparatus is then sealed within a polyethylene plastic bag to prevent interaction with gases from adjacent bags containing test specimens. The bags are then placed in a temperature–relative humidity (TRH) chamber for conditioning. Many companies use this test. Typical conditions are 70 to 80°C and 70 to 85% relative humidity for a period of 4 to 7 days. At the end of the test the TRH chamber is returned to ambient temperature and humidity under noncondensing conditions. The product is removed from the

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377

chamber and inspected for signs of stains, discoloration, or corrosion. This may be done by unaided eye inspection or by inspection using magnification. A near-contact stain test is very similar to a contact stain test. The primary difference is that the material under evaluation is held in close proximity to, but not in contact with, the product. Care is taken to ensure that the material under test cannot drip or sag onto the product. The material under test is beneath the product. Spacing between the material under test and the product is typically 250 to 1270 m (0.01 to 0.05 in.). There is one other consideration in contact and near-contact stain tests. The material being evaluated may come in contact with water, isopropyl alcohol, or other chemicals that extract damaging substances from the material. If this is the case, extracts obtained by soaking the material in appropriate solvents are used as the challenge in functional tests, often in the form of dried residues. These residues are often placed in a clean aluminum weighing dish or on a piece of clean aluminum foil for the near-contact stain test. 8.3.2

Nonfunctional Testing: Objective Laboratory Measurements

Materials qualified under functional tests are then characterized using objective laboratory tests to characterize the material. The results of these tests are then used to specify the desired properties to the supplier. The tests will quantify such parameters as extractable particles, anions, cations, organic, and viable contamination. Electrostatic charge can be considered a form of contaminant for some applications. This is done because consumables suppliers seldom have access to their customers’ products to conduct stain tests. This subject is discussed in greater detail in Chapter 3. Extractable Particles for Gloves One of the earliest tests to be applied to gloves was the extractable particle test, developed originally for characterization of metal parts or rigid polymeric parts. This method used extraction of material using 40-kHz ultrasonic extraction to disperse particles into a liquid for subsequent analysis using turbidimetry or liquid-borne particle counting. It was soon discovered that 40-kHz ultrasonic extraction was unsuitable because natural rubber latex was extremely sensitive to damage by ultrasonic extraction. A material that is unsuitable for ultrasonic extraction, either for cleaning or cleanliness measurement, will show a multiple ultrasonic extraction curve in which each successive measurement increases, as shown in Figure 8.2. As a consequence, ultrasonic extraction was replaced by the orbital shaker method to remove particles. To test gloves, the gloves are filled with filtered deionized (DI) water containing approximately 200 ppm by volume of a surfactant and dropped into a beaker containing the same solution. This is oscillated for 10 minutes, after which the shaker is turned off, the glove retrieved, and its liquid contents drained back into the beaker. After 10 minutes of oscillation, considerable air can be entrained into the liquid as tiny bubbles. Typically, liquid-borne particle counters (LPCs) will count air bubbles as if they are particles. A procedure then had to be developed to degas the resulting suspension. Two different procedures for degassing are available. One uses ultrasonic degassing. The beaker containing the suspension is immersed in an ultrasonic tank. The power to the tank is pulsed on and off rapidly. This procedure is repeated 10 to 20 times until the suspension no longer effervesces. An alternative procedure [2] allows the suspension to stand, undisturbed, for 20-minutes. The 20-minute stand typically results in a five- to 10-fold reduction in particle count vs. ultrasonic degassing. [3]. Following degassing the suspension is counted using a liquid-borne optical particle counter. The current practice is to count using a 0.5-m-resolution particle counter.

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Parts with extreme ultrasonic (u/s) erosion sensitivity – not suitable for u/s cleaning or extraction Multiple extraction number Initial cleanliness value 1

2

3

4

Parts with normal ultrasonic (u/s) erosion sensitivity – suitable for u/s cleaning or extraction

FIGURE 8.2 Multiple ultrasonic extraction of parts that are extremely erosion sensitive compared with normal parts.

Extractable Ionic Content Ionic contamination is usually extracted in DI water with no detergent. In one company’s method the glove is turned inside out, filled with DI water, and placed on a hot plate at 80°C. This is often referred to as an outside-only leach. Another procedure calls for pieces of the glove of known surface area to be immersed in 80°C water for 1 hour. This is an inside and outside leach test. Ionic extraction for shorter times (typically, 10 minutes) at ambient temperature is often called extraction to differentiate it from leaching. Following leaching or extraction, samples are analyzed using anion chromatography for anionic species and atomic absorption spectroscopy for cations. Anions of interest generally are chloride, nitrate, and sulfate, although some end users specify phosphate as well. Cations of interest include aluminum, copper, iron, magnesium, silicon, sodium, and zinc. Other Contamination Tests Several other contamination tests are available. Among these are nonvolatile residue (NVR), organic extractables, and tests for viable organisms. In an NVR test the glove is washed with a suitable solvent, often isopropyl alcohol, and the solvent is allowed to evaporate in a preweighed weighing dish. The resulting added mass is reported in milligrams per square foot of surface area (or in mg per square 0.1 meter). Drawbacks of the NVR test are that it is time consuming and procedurally difficult, occasionally resulting in gross errors. It can be shown for barrier film gloves that there is a direct linear correlation between the cumulative particle count greater than 0.5 m per unit area and the NVR results, as shown in Figure 8.3. The strong correlation between results of NVR and LPC tests indicates that the NVR test may be redundant for these types of gloves.

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379

0.2

mg/ft2

0.15 0.1 0.05 0 0

2000

4000

6000

8000

10,000

12,000

Particle/cm2 ≥ 0.5 μm

FIGURE 8.3 Correlation between liquid-borne particle count and nonvolatile residue mass for gloves.

Organic materials can be extracted from certain types of cleanroom consumables by various organic solvents. Isopropyl alcohol, which is commonly used for workstation wipedown in cleanrooms, might be a good starting point for extracting organic residues. In other cases, it might be desirable to extract with more aggressive solvents, such as acetone, methylene chloride, or hexane, to enhance recovery of hydrocarbons, soluble oligomers, plasticizers, siloxanes, or other molecules considered undesirable. After recovery of the soluble material, the samples can be concentrated by evaporation, as in the NVR procedure. However, in addition to weighing the evaporated residue, a portion should be analyzed by Fourier transform infrared spectroscopy, or in the case of extremely complex mixtures, gas chromatography with mass spectrometry detection. Many organic compounds are so detrimental to products or processes that the acceptance criterion for these compounds is “none detected”. A good example is silicon oil. Viable contaminants may be detected by contacting the surface of a culture medium or by pipetting a wash from the consumable onto the culture medium. The medium is then incubated to develop colonies of the viable organisms, which can be identified and counted. 8.3.3

ESD Considerations in Glove Selection

Selection of glove material for ESD applications is critical. Nitrile is widely recognized as a glove material suitable for use in the manufacture of products with extreme sensitivity to ESD. PVC gloves are also static dissipative, but are made pliable through the incorporation of plasticizers; the plasticizers also impart static dissipative properties. Unfortunately, these plasticizers can interfere with the performance of disk lubricants and under extreme conditions can interfere with film adhesion in plated products. Thus, PVC gloves may be unacceptable for contamination reasons. Gloves with suitable ESD properties can also be made from polyurethane. Polyurethane has several desirable properties. First, polyurethane is very strong, so the gloves can be made very thin. This leads to improved dexterity. Second, polyurethane gloves can be very puncture and tear resistant. Figure 8.4 illustrates a static-dissipative polyurethane glove. Polyurethane gloves are also available in a conductive version, filled with carbon to achieve conductivity, as shown in Figure 8.5. Specifying the ESD Performance of a Cleanroom Glove The ESD performance of a cleanroom glove can be specified using a number of different parameters. Among these are bulk and surface resistivity, discharge time, residual charge retention, and tendency to tribocharge. Bulk and surface resistivity are classical methods for specifying conductive properties of materials. These are often important in the selection or qualification of materials for use in a static-safe workplace. Discharge time is an important parameter, since it is

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FIGURE 8.4 Static-dissipative polyurethane glove.

FIGURE 8.5 Carbon-filled polyurethane conductive glove.

the arrival at a safe voltage level that often determines the material’s suitability for use in a given application. Residual charge is especially important when gloves or other consumable supplies are made from laminated or composite materials of construction for the gloves. If the continuous-phase material in contact with the external environment is highly insulative compared with the filler bulk of the laminate or composite structure, significant charge retention can occur.

INITIAL QUALIFICATION VS. THE NEED FOR ONGOING LOT CERTIFICATION

381

Of the ESD qualification methods, the tendency to tribocharge (i.e., to acquire and/or impart a charge when rubbed against or separated from a dissimilar material) is by far the most controversial. The repeatability and appropriateness of tribocharge testing is so in question that “no one test currently available can predict general tribocharging properties for a specific material” [4]. Since at present there is no agreed-on standard for tribocharge testing of materials, attempting to specify consumables from the standpoint of tribocharge properties is a difficult prospect at best. This leaves us with the need to test consumables from the standpoint of their bulk and surface resistivity, discharge time, and residual charge retention. Bulk and surface resistivity tests are reliable, as they are based on currently accepted test methods. Discharge-time tests are useful in that they are based on accepted test standards and reflect the expected performance of materials in their intended application. Residual charge retention tests are based largely on the experience with packaging materials and are appropriate for gloves constructed of laminated or composite structures. Bulk or surface resistivity can be measured using a number of different standards. Standards considered particularly appropriate are those of the EOS/ESD Association [5]. It is interesting that a direct correlation can be established between bulk or surface resistivity and discharge time. Discharge time is also covered by standard test methods, including FED-STD-101C [6]. Discharge-time performance has become an industry norm in the specification of gloves for use in the manufacture of hard disk drives. Discharge times are measured for a person holding his or her hand on a 20-pF charged plate while standing on an insulated surface. The plate and operator are charged to some starting voltage and the time to discharge to a target voltage is measured. The most generous disk drive discharge requirement is from 1000 V to below 100 V in less than 5 seconds. The most demanding requirement is for discharge from 1000 V to below 10 V in less than 500 ms. 8.4

GLOVE USE STRATEGIES

Many different strategies for the use of consumables and glove liners influence testing considerations. The choice of glove liners is end-use dependent. Some companies use glove liners as gowning consumables: Personnel wear glove liners as they put on their cleanroom garments and place them in a laundry bin after use, just prior to donning a pair of cleanroom gloves. Some people continue to wear the glove liner and wear a pair of cleanroom barrier film gloves over them to enter the cleanroom. Industries that require manual dexterity often prefer a half-finger glove liner. In most industries, the use of a glove liner while in the cleanroom is at the choice of the wearer; many people will choose not to wear a glove liner. All of these choices affect the ESD test strategy. Full-finger glove liners (Figure 8.6) made of materials that are insulators might interfere with the ESD performance of gloves during use. Half-finger glove liners (Figure 8.7) made of insulative materials may not interfere with ESD performance of the gloves since the finger tips are in contact with the glove material. Finally, a full-finger glove liner made to be static dissipative may afford some advantage over a full-finger glove liner made of insulative material. 8.5 INITIAL QUALIFICATION VS. THE NEED FOR ONGOING LOT CERTIFICATION During initial qualification tests of consumables, there are seldom enough resources available to determine the suppliers’ ability to achieve the desired contamination performance

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FIGURE 8.6 Full-finger glove liner. This glove liner has silver in the threads. Silver ions tend to sterilize the glove liner when it is laundered.

FIGURE 8.7 Half-finger glove liner. These are particularly useful in precision assembly operations because there is no layer of cloth between the fingertip and the barrier film glove, improving dexterity.

by a controlled process. Typically, the initial functional tests and benchmark measurements to determine quantitatively the levels of contaminants on the consumables are done on only one or two batches of consumables. This process is often called the first article qualification process. The degree with which these initial batches are representative of the population at large should ideally be checked on an ongoing basis.

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383

The most cost-effective way to perform this ongoing testing is to choose the lowest-cost test or tests that are likely to detect the greatest variability in the consumables. It is the author’s experience that viable contamination is almost always one or fewer colony-forming units per glove. Most suppliers are easily capable of meeting their published claims for anions, cations, and organic extractable materials, even if they are not testing for these variables on a regular basis. Conversely, it is the authors’ observation that many consumables are highly variable with respect to particle counts. This is especially true of gloves and plastic packaging film. Since the LPC test is also relatively inexpensive, it seems logical that a preliminary lot screening plan begin with testing for extractable particles using the LPC method. 8.6

GLOVE WASHING

Cleanroom gloves are among the most expensive of cleanroom consumables. In addition, due to the nature of their use, they are also among the most likely sources of contamination, due primarily to contact transfer. Many consumers specify the cleanliness of gloves they use on an as-received basis. This is proper. However, few companies know that the cleanliness of their cleanroom gloves can be improved by glove washing and that the cleanliness of the glove can gradually degrade in the cleanroom during use. To explore this subject, the contamination behavior of natural latex cleanroom gloves is shown from a historical perspective. Also, some more recent studies examine the contamination behavior of nitrile rubber gloves. Factors to be explored include the as-received cleanliness level of gloves, the effects of glove washing and glove laundering, gradual recontamination of gloves during use, and the effects of in-situ glove cleaning. Cleanroom gloves are critical to high-technology manufacturing. Methods for qualification and certification of cleanroom gloves from a contamination and electrostatic discharge (ESD) control perspective have been discussed in detail [7]. In this section the contamination state and performance of cleanroom gloves in use are explored. Later, factors affecting ESD performance under realistic use conditions are explored. 8.6.1

Early Observations with Natural Rubber Latex Gloves

In the period from 1985 to 1987, gloves made of natural rubber latex were the subject of some scrutiny. Natural rubber latex was the material of choice in gloves for manufacture of hard disk drives (HDDs) and many other high-technology products, due to the relative lack of sensitivity to damage by ESD. During this time period it was observed that one particular brand of natural latex gloves did not leave fingerprints on HDD parts, whereas four other brands in use in the same cleanroom did leave fingerprints on HDD parts. The five different brands of gloves were taken to the contamination control lab for analysis. The first test performed was to don a pair of gloves and touch the surface of a clean microscope slide. Several pairs of each glove were sampled this way. The microscope slides were then inspected at 160  magnification using dark-field light microscopy. It was found that gloves leaving fewer than 500 particles/cm2  2.0 m did not leave a visible fingerprint on the HDD parts, whereas gloves leaving more than 5000 particles/cm2 did leave a distinctly visible fingerprint [8]. This was the first evidence showing a correlation between contacttransferable particles and a rejectable defect on a HDD part. Recognizing that light microscopy is a tedious method for inspecting glove cleanliness, a correlation was established between light microscopy of glove particle extracts and liquid-borne particle counts of glove extracts [9]. Although not reported at the same time, a

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correlation was also established among microscopic counts of particles deposited on microscope slides, LPC count, and particles counted on filters [8]. The liquid-borne particle count procedure required the development of a particle extraction method. Preliminary tests showed that ultrasonic extraction was not an acceptable method, due to the extreme erosion sensitivity of natural rubber latex. The extraction procedure that was developed was as follows: 1. 2. 3. 4.

Put 300 mL of deionized (DI) water plus 0.02% by volume detergent in the glove. Place the glove in 500 mL of DI water plus 0.02% detergent in a 2-L beaker. Shake orbitally at 120 rpm for 10 minutes. Remove the glove from the beaker; drain the water from the glove into the water in the beaker. 5. Degas ultrasonically by pulsing the power on and off rapidly. 6. Count immediately using an LPC. Both the inner and outer surfaces of the gloves were measured. The gloves in use were hand-specific rather than ambidextrous. It had been observed that operators would invert a glove and wear it on the opposite hand when one or the other hand glove was unavailable, therefore making the inside of the glove nearly as important as the outside of the glove from a contact transferable contamination perspective. In addition, some preliminary tests were conducted to determine anion concentrations levels on the gloves. In the anion determination procedure, gloves were filled with pure 18 M  cm DI water, sealed with a twist tie and placed on a hot plate at 60 to 80°C for 1 hour. After allowing the sample to cool to room temperature, it was analyzed by ion chromatography. It was found that natural rubber latex gloves of the time period (1985–1987) averaged over 8000 particles/cm2  0.8 m and 2 to 10 g/cm2 anion contamination, dominated by chlorides and sulfates [10]. 8.6.2

Glove Washability

The next logical problem was to determine if hand washing could clean the gloves. It seemed that if the particles were so loosely held that they could be removed by contact, then simply washing them by rubbing gloved hands together under a running stream of water might be an effective way of removing particles: Water washing should also be effective at reducing ions. For this portion of the study a Hamamatsu C1515 wafer surface inspection system was used. The sensitivity of the C1515 was adjusted so that its 50% detection efficiency was approximately 5 m. A fresh pair of gloves was donned and the surface of a bare silicon wafer was touched. The resulting fingerprint contained an average of 175 particles/cm2. The gloves were then washed in a running stream of DI water and dried in a conventional (not HEPA-filtered) hand dryer. A second touch of the silicon wafer then produced an average of 3 particles/cm2  5 m in diameter. These experiments established that the cleanliness of the gloves could be measured using contact transfer followed by microscopy or wafer inspection. They also showed that measurements of extracts from gloves using an LPC proved that glove washing was effective in reducing contact transfer. An important question remained: What happens to gloves during use for manufacturing operations? It was decided that the measurement of glove cleanliness during use should be done using gloves worn during actual manufacturing operations as opposed to measuring simulated soils. For the latter experiments a new form of LPC extraction was chosen in which only the outside surface of the glove was measured, as the contamination

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TABLE 8.2 Results of Natural Rubber Latex Gloves Tested as Received and After 2 Hours Use in Manufacturing Glove Condition

Particles/cm2  0.8 m

Percent That Leak

As received After 2 hours use trial 1 trial 2

8520

0

8270 8396

70 57

accumulated by the inside of the glove where it was in contact with the operator’s skin should not pose a risk of contaminating HDD parts. The extraction procedure was as follows: 1. Clean a 500-mL beaker and fill with approximately 350 mL of DI water plus 0.02% detergent. 2. Degas the solution ultrasonically. 3. Using an LPC, count approximately 50 mL from the 500-mL beaker. Use this value as the LPC blank. 4. Turn the glove inside out. (This happens naturally when the glove is removed.) 5. Fill the inverted glove with the remaining 300 mL of detergent water solution from the 500-mL beaker. Seal with a twist tie. 6. Undulate for 10 minutes in a 2-L beaker containing 500 mL of detergent water solution. 7. Pour the water from inside the glove back into the 500-mL beaker. 8. Degas ultrasonically by pulsing the power on and off rapidly. 9. Count immediately using an LPC. The results of this experiment were somewhat surprising. The test was repeated to verify the outcome and the results are shown in Table 8.2. Clearly, something was wrong. After 2 hours of use in manufacturing the gloves were as clean as gloves in the as-received condition. This problem was resolved when the number of gloves that leaked after pouring the detergent–water solution into them was factored in. Gloves that leaked at the fingertips due to the presence of pinholes were not LPC tested. It was therefore quite likely that the few gloves that did not leak were gloves that had been changed sometime during the 2-hour work period and had little opportunity to shed or gain particles. The more disturbing observation was that a fairly significant percentage of the operators were wearing gloves with unnoticed pinholes in the fingers: clearly undesirable from a contamination perspective. To get a better indication of the glove recontamination rate, we returned to the lab and measured contact transfer to silicon wafers using the C1515 wafer surface inspection system. In this procedure gloves were washed and measured using the wafer inspections system. The operator then walked around the cleanroom touching objects on the work surfaces for approximately 5 minutes. Contact transfer to the silicon wafer from the gloves was measured again. This process was repeated for an additional 15 minutes of touching the cleanroom work surfaces, tools, oscilloscopes, and so on. Contact transfer was measured again. Finally, the gloves were again washed in running DI water and the amount of contact transfer was measured again. The results of this experiment are shown in Table 8.3. The results in the table show that the gloves did not remain clean after washing. Within 20 minutes, the contact transfer rate of the glove had returned to approximately half of its rate in the as-received

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TABLE 8.3 Results of Washing, Recontamination, and Rewashing Natural Rubber Latex Gloves Glove Condition As received After washing After 5 minutes in cleanroom After 20 minutes in cleanroom After rewashing

Contact Transfer (particles/cm2  5 m) 175 3 50 90 3

condition. Clearly, glove washing would provide an initial benefit in reduction of contact transfer, but it was evident that this benefit was only temporary. Conversely, rewashing the gloves was shown to return the gloves to a clean state. Three options were considered to keep gloves clean during use in manufacturing. One option would be for the operator to return to the glove station and rewash the gloves. This option was rejected as having too negative an impact on productivity. A second option would be to wipe the gloves with a cleanroom wiper wetted with isopropyl alcohol. This option was not tested because at the time there was no low-cost premoistened disposable cleanroom wiper on the market. The alternative, to use the available knitted or woven cleanroom wipers, was rejected as too difficult to control and too expensive. A third option, to provide operators with workstation sticky mats, was evaluated and eventually adopted. The effectiveness of the workstation sticky mat at reducing contact transfer needed to be demonstrated. A standard cleanroom floor mat was to cut an approximately 6-in. square and taken to approximately 20 operators who had washed their gloves and worn their gloves for 2 hours in the manufacturing process. A set of clean microscope slides were used to collect the contact transfer samples. The particles transferred to the microscope slides were then measured using dark-field microscopy and recorded at two different sizes, 2 and 10 m. The procedure was as follows: 1. 2. 3. 4. 5.

Touch a microscope slide with a freshly washed glove. Touch the slide with a washed glove (different finger) after touching the sticky mat. Touch the slide with the glove after 2 hours of use. Touch the mat with a different finger. Touch the slide with cleaned finger.

Table 8.4 shows the results of the sticky mat experiment. The freshly washed gloves transfer no particles larger than 10 m to the clean glass microscope slide. Conversely, they transfer 120 particles/cm2  2 m in diameter. (Recall that even the cleanest unwashed natural rubber latex glove transferred about 500 particles/cm2  2 m in diameter.) This is reduced by about 40% after touching the sticky mat. The washed gloves after 2 hours use in the cleanroom transfer, on average, 5 particles/cm2  10 m in diameter. After touching the sticky mat, no particles  10 m are transferred to the microscope slide, indicating that the sticky mat is 100% effective for removing newly accumulated large particles from the gloves. After 2 hours of wear in the cleanroom, the gloves transfer an average of 300 particles/cm2  2 m in diameter. Sticky mats were implemented on all of the manual assembly workstations. The effectiveness as a method for reducing contact transfer of particles, especially those larger than 10 m,

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TABLE 8.4 Effectiveness of a Sticky Mat for In Situ Cleaning of Natural Rubber Latex Gloves Contact Transfer (particles/cm2  stated size) Glove Condition

Particle Size (m)

Before Sticky Mat

After Sticky Mat

10 2 10 2

0 120 5 300

0 70 0 200

Washed After 2 hours

TABLE 8.5 Typical Particle and Ionic Cleanliness Levels for Nitrile Gloves as Received and After Washing Parameter Particles/cm  0.5 m Anions (g/cm2) 2

As Received

After Glove Washing

1000–4000 1–4

25–100 0.01–0.10

had been documented. Two other effects were noticed. One was that the sticky mat would tear the finger tip off a glove if it had a pinhole or tiny tear in the fingertip. The second was the reaction of the assembly operators. The operators remarked that the presence of the sticky mat on the workstation was a constant visual reminder that gloves must be kept clean. 8.6.3

Nitrile Glove Performance

Since 1991, with the introduction of magnetoresistive (MR) heads, there has been a gradual change in the HDD industry. The extreme sensitivity of MR heads to ESD made continued use of natural rubber latex unacceptable. Nitrile rubber has replaced natural rubber latex as the preferred glove material. With the introduction of nitrile, the washing, recontamination, and in situ cleaning properties were again of interest. Nitrile gloves were far less likely to develop pinholes in use than natural rubber latex gloves. Glove washing tests showed remarkable improvements, as shown in Table 8.5. As can be seen, glove washing makes a dramatic improvement in both particle and anion cleanliness levels of the gloves vs. their as-received values. The tendency of gloves to become recontaminated was also studied. First the lot was measured for particles and anions as received and after washing. In this study, two drying options were evaluated, drying with a conventional heated hand dryer and drying with a HEPA filtered cleanroom version of hand dryer. Gloves were retrieved and the outside surfaces were measured after 0.5, 1, 2, 4, and 8 hours of total wear time. Five operators at each of five different manual assembly operations participated in the study. Table 8.6 summarizes the results of the glove washing study. The results in the table show a factor of 5 improvement in particle cleanliness of the gloves and nearly a factor of 10 improvement in anion cleanliness. Note that the initial cleanliness is considerably better than observed previously for natural rubber latex gloves in the 1985–1987 time frame. Also note that there is no apparent difference in cleanliness using a standard heated hand dryer vs. using a HEPA-filtered hand dryer. Table 8.7 shows what happens to glove cleanliness as a function of the time they are worn in the cleanroom. Data for all workstations were averaged together as there did not appear to be a workstation dependence on recontamination rate. The results in the table show that from a particle contamination perspective, the gloves are as dirty after 1 to 2 hours of wear as they were as received. In addition, there is a steady buildup of particle contamination. Also note that none of the gloves leaked, indicating that there is no problem of

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CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

TABLE 8.6 Nitrile Glove Wash Performance for Particles and Anions Particles/cm2  0.5 m

Anions (g/cm2)

Condition

Mean

Std. Dev.

Mean

Std. Dev.

As received Washed regular dry HEPA dry

1120

400

0.940

0.055

200 190

10 10

0.095 0.090

0.005 0.010

TABLE 8.7 Effect of Wear Time on Particle and Anion Cleanliness of Nitrile Gloves Particles/cm2  0.5 m

Anions (g/cm2)

Time (h)

Mean

Std. Dev.

Mean

Std. Dev.

0 0.5 1 2 4 8

195 700 850 1500 1475 2250

10 600 350 450 475 475

0.090 0.045 0.180 0.050 0.115 0.185

0.050 0.015 0.190 0.010 0.135 0.170

development of pinholes while the nitrile glove is worn during manufacturing. By comparison, the anion contamination levels show an increase, but increased to no more than 20% of the as-received values, even after 8 hours of wearing. The buildup of anion contamination is not continuous, suggesting that the variability of the anion contamination level may be the inherent variability of the anion content of the gloves rather than a time effect. Clearly, additional study is required to determine if the gloves are becoming recontaminated, if anions are diffusing to the surface, or if new surface area is being exposed, allowing for anions to be extracted that were previously unavailable. 8.6.4

Glove Washing Conclusions

Gloves are among the most important of cleanroom consumables. Due to the nature of their use, they are also among the most important sources of contamination, due primarily to contact transfer. The cleanliness of cleanroom gloves can be improved significantly by glove washing. In glove washing operators puts their gloves on and then wash their gloved hands. The results are a decrease in particle and ionic contamination by one to three orders of magnitude. Unfortunately, the gloves do not remain clean during use. A small but significant increase in ionic contamination levels occurs. More important, the gloves become recontaminated by particles to a level equal to or greater then the as-received level within 1 to 2 hours. In situ glove cleaning can correct the recontamination of gloves by particles.

8.7

ESD PERFORMANCE OF GLOVES

The functional structures of electronic and electromechanical devices continue to shrink. As they do, their ability to withstand the effects of contamination, electrostatic overstress (EOS), and ESD diminish as well. Cleanroom gloves are critical for contamination control, and where

ESD PERFORMANCE OF GLOVES

389

ESD-sensitive products are being handled, the ESD performance of gloves is equally important. The selection criteria for qualifying and controlling cleanroom gloves for contamination and ESD properties were explored in section 8.3.3 [7]. In Section 8.6.2 the effects of glove washing and use on glove contamination levels was explored in detail [11]. The ESD performance of gloves and glove liners under realistic use conditions is explored in this section. Among the devices most sensitive to ESD are magnetoresistive (MR) and giant magnetoresistive (GMR) heads, solid-state lasers, and semiconductor devices with gate widths of less than 0.35 m. These products have design and performance characteristics that demand an aggressive and comprehensive ESD control program to deal with their high degree of ESD sensitivity. Walker expressed the need for such a comprehensive ESD control program as early as 1983 [12]. As a further example of the need for a comprehensive ESD program, Hansel [13] pointed out the need for involvement of operating personnel in the complete solution. Other authors have described the need for ground and charge monitoring systems [14,15], selection of materials for ESD applications [16,17], selection and management of air ionizers [18,19], selection of fabrics for construction of cleanroom garments [20], and even ESD from pressuresensitive adhesives [21]. More recently the need for a comprehensive ESD control program for magnetoresistive heads was described [22]. However, none of these articles focuses specifically on the issue of selection and performance of gloves with regard to ESD control. This is somewhat surprising given that gloves are an integral element in the generation of electrostatic charge and transfer of charge to and from products in manufacturing operations. The performance of gloves must be considered from the perspective of the entire ESD control system, especially where a static-safe workstation is located within a cleanroom. When gloves are used in a cleanroom application, the type of cleanroom garment, wrist strap, monitor, footwear, glove liner, and other factors have to be included in the evaluation of the performance of the glove in control of electrostatic discharge. All these components make up the comprehensive ESD control system and must work together to ensure satisfactory performance. 8.7.1

Materials Selection for ESD Properties

Selection of glove material for ESD applications is critical. Nitrile is widely recognized as a glove material suitable for use in the manufacture of products with extreme sensitivity to ESD. PVC gloves are also static dissipative but are made pliable through the incorporation of plasticizers. The plasticizers also impart static-dissipative properties. Unfortunately, these same plasticizers can interfere with the performance of disk lubricants and under extreme conditions can interfere with film adhesion in plated products. This can render them unsuitable for use in contamination-sensitive applications. Natural rubber latex, although widely used in high-technology manufacturing, has undesirable properties from an ESD standpoint. Attempts have been made to alter the performance of natural latex through surface treatments, but these are found to be unacceptable, as they can be removed in wipe-down or cleaning processes and can introduce unacceptable forms of contamination. Polyurethane has many desirable properties, such as good barrier properties, low surface resistivity, and excellent wear resistance. Unfortunately, gloves made of polyurethane are comparatively expensive. 8.7.2 Specifying the ESD Performance of Cleanroom Gloves and Glove Liners The ESD performance of a cleanroom glove can be specified using a number of different parameters. Among these are volume and surface resistance, discharge time, residual charge

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CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

retention, and tendency to tribocharge. Volume and surface resistance are classical methods for specifying conductive properties of materials: These are often important in selection or qualification of materials for use in a static-safe workplace. Discharge time is an important parameter, since it is the time to arrival at a safe voltage level that often determines a material’s suitability for use in a given application. Residual or capacitance charge potential is especially important in laminated or composite structures, where the continuous-phase material in contact with the external environment can be highly insulative compared with the bulk of the laminate or composite structure. Of these, the tendency to tribocharge, that is, to acquire and/or impart a charge when rubbed against or separated from a dissimilar material, is by far the most controversial. The repeatability and appropriateness of tribocharge testing is so in question that “no one test currently available can predict general tribocharging properties for a specific material” [4]. Since there is no agreed upon standard for tribocharge testing of materials, attempting to specify gloves from the standpoint of tribocharge properties is, at best, a difficult prospect. This leaves us with the need to test gloves from the standpoint of their resistance, discharge time, and residual charge retention. Resistance tests are reliable, as they are based on currently accepted test methods. Discharge-time tests are useful in that they are based on generally accepted test methods and reflect the expected performance of materials in their intended application. Residual charge retention tests are based largely on the experience with packaging materials and are appropriate for gloves constructed of laminated or composite structures. Resistance can be measured using a number of different standards. Standards considered particularly appropriate are those of the Electrostatic Overstress Electrostatic Discharge (EOS/ESD) Association [5] or the Federal Standards [6]. A direct correlation can be established between resistance and discharge time. Discharge-time performance has become an industry norm in the specification of gloves for use in the manufacture of hard disk drives. Discharge times are measured for a person standing on an insulative plate while holding his or her hand on a 20-pF charged plate. The plate and operator are charged to some starting voltage, and the time to discharge to a target voltage is measured. The most generous disk drive discharge requirement is from 1000 V to less than 100 V in less than 5 seconds. The most demanding requirements are for discharge from 1000 V to less than 10 V in less than 500 ms. Many different strategies for the use of gloves and glove liners influence testing considerations. The choice of glove liners is end-use dependent. Some companies use glove liners as gowning gloves: People entering the cleanroom wear the glove liner as they put on their cleanroom garments and discard them in a laundry bin after use, just prior to donning a pair of cleanroom gloves. Some personnel continue to wear the glove liner and wear a pair of cleanroom gloves over them to enter the cleanroom. Industries that require manual dexterity often prefer a half-finger glove liner. In most industries the use of a glove liner while in the cleanroom is at the choice of the wearer: Many people will choose not to wear a glove liner. All of these choices affect the test strategy. Full-finger glove liners that are made of insulating materials might interfere with the ESD performance of gloves during use. Half-finger glove liners made of insulative materials may not interfere with ESD performance of gloves since the fingertips are in contact with the glove material. Finally, a full-finger glove liner made to be static dissipative may afford some advantage over a full-finger glove liner made of insulative material. These tests were designed to determine the effect of glove and liner use strategy on discharge time in a practical application. That is, the time to discharge a charged surface using a number of glove and liner combinations was measured. The experimental design considers

ESD PERFORMANCE OF GLOVES

391

the selection of material for the glove, the choice of glove liner, and examines the issue of glove cleanliness and relative humidity. The goal of the study was to determine the effects of these variables on glove performance in cleanroom environments, where the wearer will be wearing a cleanroom garment, a wrist strap, and so on. 8.7.3

Testing Considerations

All gloves in this report were conditioned to and tested at 23  2°C (72  3°F) and 50  5% or 12  3% relative humidity. Gloves were conditioned for a minimum of 48 hours at each of these conditions prior to testing. Five different gloves considered suitable for use in cleanrooms were tested in this study. Three different types of nitrile gloves were tested to study the effect of chlorination on discharge performance. Poly(vinyl chloride) and natural rubber latex were also tested: ● ● ● ● ●

Nitrile gloves, unchlorinated Nitrile gloves, chlorinated on the inside Nitrile gloves, chlorinated on both sides Poly(vinyl chloride) (PVC) gloves Natural rubber latex gloves

The three sets of nitrile gloves were provided by the Ansell Critical Environment development group in Coshocton, Ohio. The PVC gloves were from Oak Technical Products and the natural latex glove was the CR100 glove. Four different glove liner conditions were tested with the gloves: ● ● ● ●

No glove liner Full-finger insulative Berkshire glove liner Half-finger insulative Berkshire glove liner Full-finger X-static static-dissipative glove liner

These combinations were selected to be representative of those in common practice in high-technology manufacture. Gloves were tested in three different states: ● ● ●

Directly out of the original package After a deionized (DI) water wash After contamination using magnesium silicate as a model contaminant (gloves were dusted off prior to test, so they were not visibly contaminated)

Tests were run alternately while wearing a wrist strap and when not wearing a wrist strap. The tester stood on a Teflon isolation plate during the test. This was to ensure that discharge was limited to through the charge monitor through the wrist strap. In the case where no wrist strap was worn, discharge was solely through the charge monitor. The tests were run while the gloved hand was in contact with a NOVX Series 5000 monitoring system. Data were recorded using NOVX Data Acquisition Software. This facilitated

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CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

recovery of discharge times from applied voltages of 1000 V to targets of 100, 50, 20, and 10 V. Because the NOVX instrument has a 100-G resistance in the path to ground through the electrometer, even an insulative material in contact with the 20-pF plate will appear to discharge some voltage eventually. As a consequence, natural latex gloves, normally considered an excellent insulator, will discharge relatively rapidly under these test conditions. A minimum of three gloves were tested under each test condition. The procedure was as follows (the subject wore the appropriate glove and liner): ● ● ●

●

●

The subject stood on an insulative sheet to ensure preservation of charge. The operator put a hand on the charged plate. Normal hand pressure was applied. A 20-pF picofard charged plate monitor and the operator were charged to greater than 1200 V. The operator discharged by plugging the wrist strap in a ground or where no wrist strap was worn, by touching the input to the the monitoring system. The discharge times from 1000 V to 100, 50, 20, and 10 V were recorded.

8.7.4

Factors That Affect the ESD Performance of Gloves

The primary objective was to determine which conditions had a dominant influence on discharge time. To this purpose it is possible to combine similar test conditions (straight out of the package vs. water washed vs. subsequently recontaminated) type of glove (three types of nitrile gloves with PVC, since they are all dissipative), or other factors. This combining of variables yields a clearer interpretation of the outcome of the tests. Wearing or Not Wearing a Wrist Strap One test variable included was wearing or not wearing a groundable wrist strap. The discharge time of each glove when wearing or not wearing a grounded wrist strap is summarized in Table 8.8. The results in the table are averaged over all glove liners. In all cases, not wearing a wrist strap interfered with the discharge time performance. In addition, natural latex gloves are not capable of discharging to less than 50 V in less than 10,000 ms (10 seconds), even while grounding through a wrist strap. For products with extreme ESD sensitivity, such as MR heads, discharge to less than 50 V will be required. The degradation of discharge performance to less than 50 V indicates that wearing a wrist strap is mandatory. Since not wearing a wrist strap degrades the performance of all glove and glove liner combinations, in the remainder of our discussion we include only tests in which a wrist strap was worn. Relative Humidity Relative humidity in the test environment had no effect on discharge times for an insulative natural rubber latex glove. Conversely, the effect of relative humidity could be measured for the three types of nitrile gloves and the PVC glove. The effects of relative humidity on discharge time for these four types of dissipative gloves are shown in Table 8.9. Lower relative humidity tends to increase discharge time for all three types of nitrile gloves, especially for discharge to 20 V or less. The effect is most pronounced for unchlorinated, less noticeable for inside-only chlorinated, and least noticeable for inside and outside chlorinated. The effect of reduced relative humidity on PVC is consistent but very small. In no case does glove type or relative humidity fail to meet even the most demanding requirement: 1000 V to less than 10 V in less than 500 ms, regardless of relative humidity.

393

ESD PERFORMANCE OF GLOVES

TABLE 8.8 Discharge Time from 1000 V to 50 V for Various Glove Materials as a Function of Using or Not Using a Wrist Strap Glovea

Wrist Strap Use

Discharge Time (ms)

Yes No Yes No Yes No Yes No Yes No

77 10,000 71 10,000 63 10,000 65 10,000 10,000 10,000

Nitrile unchlorinated chlorinated inside chlorinated inside and outside Poly(vinyl chloride) Natural latex a

Gloves fresh out of the bag at 50% RH averaged over all liner conditions and wash conditions.

TABLE 8.9 Effect of Relative Humidity on Discharge Times (ms) from 1000 V to Below the Target Voltage for Static-Dissipative Gloves Target Voltage Glovea Nitrile unchlorinated chlorinated inside chlorinated inside and outside Poly(vinyl chloride)

Relative Humidity (%)

100 V

50 V

20 V

10 V

50 12 50 12 50 12 50 12

51 55 48 58 47 42 35 36

71 83 63 87 62 56 44 45

105 182 92 162 90 88 56 58

169 394 126 237 150 173 65 67

a

Unwashed gloves straight out of the bag (i.e., DI water wash and in-use contamination are not included).

Wearing a Glove Liner Averaging together all gloves and wash conditions, the effect of the choice of glove liner is as shown in Table 8.10. As the target discharge voltage decreases, the discharge time increases. In no case is the average discharge time for a glove–liner combination greater than 300 ms. Clearly, all glove–liner combinations will meet even the most demanding disk drive manufacturer’s requirement (1000 V to less than 10 V in less than 500 ms). It is not surprising that a bare hand inside a glove affords the fastest discharge times. Human skin resistance is typically modeled as a 1500- resistor. This is well below the 105  lower limit for dissipative materials. The X-static liner performs nearly as well as a bare hand except for low-relative-humidity discharge to 20 V or less. The half-finger insulative glove liner performs well to greater than 10 V but shows a significant increase in discharge time to 10 V. At low relative humidity, the half-finger glove liner discharge times to 20 V or less are adversely affected, but not as much as for the X-static glove liner. The full-finger insulative

394

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

TABLE 8.10 Discharge Time (ms) from 1000 V to the Target Voltage as a Function of Glove Linera Target Voltage Liner None X-static Half finger Full finger

Relative Humidity %

100 V

50 V

20 V

10 V

50 12 50 12 50 12 50 12

51 41 51 52 50 48 79 50

67 53 70 76 67 70 70 72

95 75 110 153 107 134 128 130

126 116 161 270 192 259 202 225

a

All static-dissipative gloves and all wash conditions combined.

TABLE 8.11 Discharge Times (ms) from 1000 V to the Target Voltages Immediately After Putting on a Glove and Liner vs. After Wearing a Glove and Liner for 5 Minutes (12% RH) Target Voltage Glove Chlorinated inside and outside

Chlorinated inside

Liner

100 V

50 V

20 V

10 V

X-static X-static  5 minutes Full finger Full finger  5 minute X-static X-static  5 minutes Full finger Full finger  5 minutes

41 38

55 49

98 68

230 116

43 42

57 58

85 76

148 105

81 44

134 57

266 81

373 117

57 44

84 60

164 96

256 177

glove liner performs worst of all. Interestingly, the full-finger insulative glove liner shows little sensitivity to the relative humidity of the test environment. Rehydration Time An important observation was made during the 50% relative humidity tests. The full-finger insulative glove liner performance started out poor but improved rapidly with time. It is believed that this was the result of hydration of the liner material by perspiration. The timing of the effect was not measured. All data reported in Table 8.11 are for a fully equilibrated liner (i.e., stable readings). The time to achieve acceptable discharge performance may be affected significantly by the relative humidity in the test environment and, more important, by the degree that a person wearing the glove and liner sweats from the palm. The current tests were conducted by a wearer who sweats relatively heavily, although this should not affect the performance of the wrist strap or the performance of the charge monitoring system.

ESD PERFORMANCE OF GLOVES

395

TABLE 8.12 Effect of Chlorination on Discharge Time (ms) from 1000 V to the Target Voltage (50% RH) Target Voltage Condition Unchlorinated Chlorinated inside Chlorinated inside and outside

100 V

50 V

20 V

10 V

56 53 46

77 71 63

139 103 94

222 142 135

TABLE 8.13 Interaction Between Glove Type and Liner Type for Discharge Time (ms) from 1000 V to 10 V (50% RH) Liner Glove Nitrile unchlorinated chlorinated inside chlorinated inside and outside Poly(vinyl chloride)

None

X-static

Half Finger

Full Finger

145 155 111 66

164 108 145 125

206 146 150 128

372 158 135 142

More careful observations were made at 12% RH, where the effect of hydration of the glove liner should be amplified over testing at 50% RH. In the 12% RH tests the time for the glove to reach stable discharge time was found to be about 5 minutes. Table 8.11 shows the effect of immediately testing the glove and liner vs. the discharge time after 5 minutes for two types of nitrile gloves and two different glove liners. Both the X-static and the insulative full-finger glove liner discharge times are affected by wear time at 12% relative humidity, especially for discharge times to 20 V or less. However, in no case is the discharge time slower than required by the most demanding specification in the disk drive industry. Chlorinating Nitrile All liner and wash combinations were averaged together for nitrile gloves to determine the effect of chlorination. The results are summarized in Table 8.12. Of the various test results in this study, these are the most consistent. The more a glove is chlorinated, the better its performance in discharge time. Again, the effect is small but measurable. The effect is most prominent for discharge times to 20 V or less. Glove and Liner Combination Averaging together wash conditions, we can evaluate a glove’s ability to discharge to 10 V. The results are summarized in Table 8.13. The data show that chlorinating the glove has an effect on discharge time, but the effect is inconsistent. A nitrile glove that is chlorinated on both sides improved the discharge time for all glove liners. A full-finger glove liner appears to increase discharge times. However, in no case is the discharge time greater than 500 ms. Wash Condition of Gloves Three conditions were tested for all static-dissipative gloves: directly out of the package, after washing in DI water (followed by towel drying), and after light contamination using magnesium silicate. The results, listed in Table 8.14 show that a slight improvement in discharge time is afforded by glove washing. The improvement is most noticeable for discharge time to 10 V. Contamination on the surface of a glove, even though not visible, affects discharge times adversely.

396

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

Conclusions Not wearing a wrist strap causes all gloves to fail under even the most generous discharge time requirement. Low relative humidity increases the discharge time for the three types of nitrile and the PVC static-dissipative gloves. Use of a glove liner tends to increase the discharge time. For a full-finger glove liner, the effect is time dependent and the increase in discharge time decreases with time, probably due to hydration of the glove liner from hand perspiration. A nitrile glove chlorinated on both sides discharges more rapidly then a nitrile glove that is chlorinated only on the inside, which in turn discharges more rapidly than an unchlorinated nitrile glove. Glove liner performance does not seem to be affected significantly by choice of glove material, as all discharge times still meet even the most demanding disk drive company requirements. Finally, washing improves the discharge-time performance of the gloves slightly and contamination degrades the performance significantly.

8.8

GLOVE LAUNDERING

Gloves are such an important consumable supply for contamination and ESD control applications that the subject deserves thorough consideration. First, the subject of qualification tests for selecting a glove was considered. Second, consideration was given to glove cleanliness during use focusing on natural rubber latex and nitrile. Third, consideration was given to factors affecting ESD performance of gloves. Comparisons were made among natural rubber latex, nitrile, and polyvinyl chloride. In each of the previous discussions, one important material in the fabrication of barrier gloves has been left out: polyurethane. This is because the traditional glove materials offer the lowest cost per unit among barrier film gloves. Barrier film gloves made of polyurethane are typically 5 to 20 times as expensive as traditional materials on a per unit basis and thus are often dismissed as an unreasonably expensive alternative and never evaluated. However, polyurethane has unique chemical and physical properties that must be factored into the qualification and use decision. For example, polyurethane gloves can have significantly lower ionic, particle, and organic contamination than that of the other three materials. This may be critical to certain types of applications, such as in deep-ultraviolet photolithography. In addition, polyurethane exhibits faster discharge-time performance than either nitrile or PVC; this may be an important factor with giant magnetoresistive heads. Finally, polyurethane is more comfortable and durable than its lower-cost counterparts. The improved durability of polyurethane gloves brings into consideration another possible element that users might want to consider in the design of their overall glove strategy: glove laundering and reuse. Glove laundering and glove washing are not interchangeable terms. Glove laundering is the cleaning and drying of gloves as a bulk process. It is a common step TABLE 8.14 Effect of Glove Treatment on Discharge Times (ms) from 1000 V to Below the Target Voltagea Target Voltage Wash Condition

100 V

50 V

20 V

10 V

Not washed DI washed Recontaminated

45 44 62

60 56 90

86 78 166

127 102 276

a

Averaged over all glove types.

GLOVE LAUNDERING

397

in the manufacture of new gloves. Glove washing, as discussed previously, is the cleaning and drying of gloves, one pair at a time, by operators as they wear them. 8.8.1

Cost–Benefit Problem

The decision to buy in high-technology manufacturing usually includes cost vs. benefit analysis. Unfortunately, it is often difficult to show a cause-and-effect relationship between contamination or ESD performance of a supply and the yield and reliability of a product manufactured using the supply. As a consequence, it is often difficult to justify the cost of a premiumprice product such as a polyurethane glove over its lower-cost competitors, even when functional and objective laboratory data show its chemical and physical superiority. In this section we describe a strategy that allows consideration of polyurethane gloves for applications where the properties of the gloves can then be used to their best advantage. But first, consider how polyurethane compares to other glove materials in objective laboratory tests. 8.8.2

Polyurethane Glove Laboratory Properties

Extractable Ions Table 8.15 compares typical ionic contamination levels on four types of glove materials in their as-received condition as tested using standard industry-accepted methods [2]. In the as-received condition, polyurethane gloves are lower in ionic contamination than NRL, nitrile, and PVC, in most cases by between one and two orders of magnitude. The consistency of polyurethane with respect to ionic cleanliness has also been studied. The results are shown in Table 8.16, which lists the percent of lots tested and found to be below the method lower detection limit (LDL), often reported as “none detected.” Where TABLE 8.15 Extractable Ion Content of Natural Rubber Latex, Nitrile, PVC, and Polyurethane Gloves As Received (g/cm2) Ionic Extracts from Each Glove Material (g/cm2) Ion

Latex

Nitrile

PVC

Polyurethane

Sodium Chloride Calcium Nitrate

0.300 1.300 0.450 0.100

0.250 0.450 0.350 0.100

0.250 0.100 0.050

0.01

0.031 0.087 0.011 0.011

TABLE 8.16 Extractable Ion Content (g/cm2) of Polyurethane Gloves in Repeated Tests Ion Fluoride Chloride Bromide Nitrate Sulfate Phosphate Sodium Potassium Silicon Calcium

% of Lots, None Detected

Average

Standard Deviation

80 0 100 33 100 100 15 40 80 83

0.013 0.05 0.01 0.023 0.03 0.005 0.04 0.05 0.06 0.02

0.015 0.03 0.011 0.033 0.04 0.000 0.04 0.08 0.02 0.00

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CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

TABLE 8.17 Comparative Particle Counts for Different Glove Materials As Determined Using IES RP5.2 (particles/cm2  0.5 m) Glove Material Statistic Average Standard deviation

Natural Rubber

PVC

Nitrile

Polyurethane

1000 100

950 125

975 150

375 75

the glove extract was below the LDL, the LDL value was used in the calculation of the average and standard deviation. These data indicate that many ions are often or always below the lower detection limit of the method. The average ion concentrations are always below 0.06 g/cm2 and show good uniformity of ionic cleanliness. By contrast, natural rubber latex and nitrile gloves average in the milligram per square centimeter range, making them one to two orders of magnitude dirtier than polyurethane. Particle Characteristics Thirty-five lots of polyurethane gloves were tested using standard industry test methods and were found to average 920 particles/cm2  0.5 m in diameter, with a standard deviation of 342. This is comparable in cleanliness to NRL and nitrile gloves, as reported previously. However, this comparison between polyurethane and other glove materials is not based on tests of split lots of gloves tested at a single laboratory. There is a high degree of lot-to-lot variability in particle counts on gloves. There is also the difficulty of correlating particle count test results from different laboratories, even when they are nominally following the same procedure. To gain a better comparison between different glove materials, a split-lot comparison was run using the standard test method at a single laboratory. Cumulative particle counts  0.5 m are shown in Table 8.17.

8.8.3

ESD Performance

Many methods have been proposed for testing the performance of gloves with respect to electrostatic discharge. Among these are standard methods for measuring surface and volume resistance or for measuring discharge time [5, 6]. In recent years, customers have become aware that the glove material is only a single component in the entire ESD protection system and that the test method should include the other components. A test method for evaluating the discharge performance of the glove that included glove liners, the wearer, the wearer’s attire, and intended grounding system was devised in the mid-1980s and currently is used widely to evaluate gloves. In the test, the person conducting the test wears the gloves, liner, wrist strap, cleanroom garments, and so on, intended to be worn in actual use. While standing on an insulated plate, a person touches the surface of a charged plate; the person, glove, and plate are then grounded. The time to discharge from 1000 V to less than 100, 50, 20, or 10 V is then used to evaluate the glove’s ESD performance. This test method has been used to evaluate NRL, PVC, nitrile, and polyurethane gloves. The time to discharge is shown in Table 8.18. NRL fails to discharge. (The data are recorded as 10,000 ms, at which time the recording device timed out.) PVC discharges to all target voltages in less than 100 ms and is largely unaffected by relative humidity. Nitrile is slightly slower to discharge than PVC. Polyurethane gloves without carbon filler are intermediate between PVC and nitrile.

GLOVE LAUNDERING

399

TABLE 8.18 Discharge Time (ms) from 1000 V to Below the Target Voltage for Natural Rubber Latex, PVC, Nitrile, and Polyurethane Gloves at 50% and 15% RH Target Voltage Glove Material Natural rubber latex Poly(vinyl chloride) Nitrile Polyurethane

8.8.4

Relative Humidity (%)

100 V

50 V

20 V

10 V

50 15 50 15 50 15 50 15

10,000 10,000 35 36 48 58 31 32

10,000 10,000 44 45 63 87 40 42

10,000 10,000 56 58 92 162 51 53

10,000 10,000 65 67 126 237 60 61

Chemical Contamination

Of particular concern in high-technology industries is the presence of chemical contamination: chemicals not otherwise detected by tests for ions. These can either be in liquid form that can contaminate by contact transfer or in vapor-phase form, often referred to as airborne molecular contamination. Of particular concern in this regard are phthalate esters, silicones, and amines, either organic or inorganic. Traditionally, material is extracted with water or a solvent, the solvent is evaporated to concentrate the extract, and the concentrated residue is weighed and analyzed by a Fourier transform infrared (FTIR) spectroscope or some other instrument. In headspace analysis, the material is sealed in a glass jar and heated. The gaseous material in the headspace is analyzed using GC/MS. Polyurethane fares particularly well when these types of analyses are done. For example, silicones have never been detected in isopropyl alcohol extracts using FTIR analysis. In headspace GC/MS analysis, trace quantities of phthalate esters are found but are not quantified. Larger quantities of organic amines are found at low-ppb to sub-ppb levels.

8.8.5

Wear Characteristics

Wear characteristics of gloves were measured using a newly developed test apparatus and method. In this apparatus the wrist of the glove under test is sealed to a manifold. The glove is inflated to 7 to 10 in. H2O pressure with HEPA-filtered air. The pressure in the glove is pulsed 2.5 in. H2O at 200 to 250 Hz. The interior of the chamber surrounding the glove is ULPA filter air, from which samples are taken using a 1.0-ft3/min 0.3-m-resolution optical particle counter. Ten 1-minute air samples are collected for each glove. Three gloves of each type are sampled. Three variants of this test are run: ●

●

Oscillation. Particle shedding from the glove is measured with no abrasive challenge. This test is intended to measure shedding as the glove is stretched, typical of putting the gloves on. Abrasion. Particle shedding from the glove is measured when a lightly textured 2.25-in.-diameter cylinder is rotated at 45 to 55 rpm across the palm and finger area of the glove. This test is intended to measure shedding by the glove due to contact with dissimilar materials in the environment.

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

Particle/ft3 ≥ 0.5 μm

400

nitrile

NRL

PVC

polyurethane

1000

100

10

1 1

3

5 7 Time (min) (a)

1

3

5 7 Time (min) (b)

9

1

3

5 7 Time (min) (c)

9

9

Particle/ft3 ≥ 0.5 μm

10000 1000 100 10 1

Particle/ft3 ≥ 0.5 μm

10000 1000 100 10 1

FIGURE 8.8 Cumulative wear-generated particles due to (a) oscillation, (b) abrasion, and (c) selfabrasion, in particles/ft3  0.5 m. ●

Self abrasion. Particle shedding from the glove is measured when it is abraded by another glove of the same material type placed over the 2.25-in.-diameter cylinder. The cylinder is again rotated at 45 to 55 rpm over the palm and finger area of the test glove. This test is intended to measure shedding due to rubbing the hands together.

Further tests are under way to establish correlation with the Taber abraser, a standard method for measuring wear characteristics of films [23]. Results of wear tests on NRL, nitrile, PVC, and polyurethane are shown in Figure 8.8. In all three types of wear tests, nitrile shows the greatest amount of particle generation. For wear due to oscillation and abrasion, PVC,

GLOVE LAUNDERING

particle/ft3 ≥ 0.5 μm

6 1000 4 500

2

0

NVR (μg/cm2)

8

1500

0 1

3

5 7 9 Number of Launderings

Particle/cm2

FIGURE 8.9 and NVR.

401

Particle/cm2

μg/cm2

Effect of multiple laundering cycles for polyurethane gloves on particle extractions

natural rubber latex, and polyurethane show roughly equivalent amounts of wear. However, the self-abrasion wear protocol clearly differentiates among the four materials. Nitrile generates the most particles, followed by natural rubber latex, then PVC, with polyurethane generating the least. One would expect polyurethane to respond well to laundering based on these data, since in laundering, the principal particle generation is expected to come from the gloves contacting one another, particularly during drying. 8.8.6

Laundering Tests

Two series of multiple laundering cycles for polyurethane gloves were evaluated. In this study a sample of each laundry batch was subject to particle count analysis using the standard method. The results of these two studies are summarized in Figure 8.9. These tests showed an immediate reduction in NVR by nearly an order of magnitude. Particle counts decrease as well, but not as quickly. The reduction in particle count continued even in the tenth laundering, indicating that further laundering might be effective at further reducing particle counts. Gloves were also inspected for unraveling of the cuff bead and for holes and tears. Ninetythree percent of the gloves were judged to be wearable; the remaining 7% failed due to bead unraveling or due to small holes and tears, all located very close to the cuff bead. 8.8.7

Impact of Laundering and Reuse on Glove Cost

Over the course of the 10-wash study, 93% of the gloves survived wearing, washing, and drying. This indicates that the average glove survives 4.9 launderings (although had the study been continued beyond 10 washings, this number would undoubtedly increase). Now let us look at the advantage of laundering and reuse. Assume that the cost of a new pair of polyurethane gloves is $1.50 per pair, or $0.75 per glove. Assume that it costs $0.06 per pair to collect, inspect, launder, package, and return a pair of gloves. This corresponds to $0.03 per glove. Since each glove will be used an average of 5.9 times, the cost over the life of the glove is $0.17  5.9  $0.03  $0.897 per glove. Since each glove is used 5.9 times, this corresponds to $0.152 per glove use or $0.304 per pair per use. This is competitive with the piece cost of traditional gloves. This cost improvement is without factoring in the savings due to reduction of solid waste to dispose of gloves. Reusing the gloves an average of 5.9 times reduces the volume of solid waste by almost 85% on a rate basis.

402

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

Thus, laundering and reuse of a durable polyurethane glove eliminates some of the cost considerations from the decision. This makes it easier to make a decision on the basis of the technical merits of the glove: lower contamination, better ESD performance, and greater comfort.

8.8.8

Conclusions

Cleanroom gloves must be evaluated for individual applications. The evaluations should include functional performance as well as objective laboratory tests of the performance. In the development of an overall glove use strategy, several options must be considered, such as glove performance, acquisition cost, washing, and laundering. Glove laundering can make what appears to be an unreasonably expensive alternative appear affordable.

8.9

WIPERS AND SWABS

Cleanroom wipers and swabs are critical to manufacturing operations in high-technology industries. The primary methods for evaluating the suitability of these consumables for cleanroom applications have relied on functional and nonfunctional qualification tests, followed by application acceptance tests such as liquid absorption rate or liquid holding capacity. Some tests measure parameters that could be considered both nonfunctional and functional in nature, such as wear characteristics. Ultimately, however, the performance of a wiper must be judged on its ability to remove soils, such as particles and oily films, in its intended application. Published literature in contamination control provides many descriptions of the performance of wipers under objective laboratory tests of cleanliness and tests that might be considered both functional and nonfunctional in nature. Attempts to evaluate functional contamination performance of wipers have included dry flexing, to determine their affect on cleanroom air quality due to wiper shedding [24] and shedding during a dry abrasion test [25]. Particle shedding characteristics of dry wipers is an important consideration where wipers are used to dry surfaces after wet wiping, where dry wipers are used to soak up spills or in situations where dry wiping is preferred over wet wiping as a cleaning method. A later study concluded that liquid extraction followed by liquid-borne particle count provided a greater degree of sensitivity in the detection of a wiper that would shed particles under conditions of stress in an extremely clean environment [i.e., ISO class 5 (FED-STD-209 class 100) or cleaner] [26]. Particles shedding from liquid extracts were filtered and counted using light microscopes initially. Soon the preferred particle counting techniques for the particle suspensions were by liquid-borne optical particle counters. It was soon recognized that the LPC counting methods had its own limitations, and ultimately a technique was developed that used the scanning electron microscope to count fine particles and light microscopy to count large particles: in particular, fibers [27]. Later studies began to explore the problem of particle shedding of cleanroom wiper materials under conditions that might model conditions of realistic use: wiping while wet. Inspired by the release of a new standard [28], wipers were evaluated for moisture absorption capacity and rate, releasable particles, particles generated under conditions of wear, extractable content using water and isopropyl alcohol as solvents, and total ionic matter after high-temperature ashing [29].

WIPERS AND SWABS

8.9.1

403

Selecting the Correct Wiper or Swab

Choosing the correct wiper or swab is difficult and often confusing. There are several manufacturers of wipers and swabs intended for contamination control applications. When the wiper or swab will be used in an ESD control application, the choice is no less confusing. If the ESD application is also in a cleanroom, the options may become severely limited. There is a wide range of basic material types available. Wipers and swabs are made from natural fibers and/or synthetic materials. The most common natural fibers are cotton and cellulose. The most common synthetic materials are polyester, nylon, rayon and polypropylene, polyurethane, poly(vinyl alcohol), and others. Blends of these materials are available. Adding to this complexity are the various forms in which wipers and swabs are manufactured: ● ● ● ● ● ● ●

Woven (a pattern of one or more fibers that look like cloth) Knitted (a pattern of fibers that look like a sweater; various knits are available) Nonwoven (random orientation of fibers) Foams Wooden handles Nonconductive plastic handles Static dissipative or conductive handles

There are three overarching considerations in selecting optimum materials for wipers or swabs, discussed next. Chemical Compatibility The first question that must be answered is how the material will be used, wet or dry: with what chemicals will it come in contact. This is the issue of chemical compatibility. One needs to identify the chemicals that will contact the wiper. Based on a knowledge of the liquids, gases, and solids that the wiper will contact, a preliminary list of candidate wipers can be selected. Manufacturers’ literature can usually be relied on to screen out wipers that are obviously unsuitable from a chemical compatibility perspective. The quantity of ionic, organic, and particulate contamination that can be tolerated from a wiper must then be determined; this becomes the acceptance criterion. This should be based on the surface cleanliness requirement of the surface to be cleaned and is largely independent of the cleanliness class of the cleanroom in which the wiper or swab is to be used. Using these acceptance criteria and the list of candidate materials, it is then possible to conduct functional and nonfunctional qualification tests on the wipers. Functional and Nonfunctional Acceptance Testing Functional and nonfunctional acceptance tests are described in Chapter 3. Discussion of their application to qualification testing of materials for wipers and swabs is needed. Consider the case of wipers and swabs that will be used in a cleanroom that is also a static-safe workplace for a class 0 HMB ESD sensitive part. The choice of which functional acceptance test to use now must be made, based on how the wipers or swabs will be used. Consider how the wipers will be used. Will the parts be wiped with the wiper? If not, a contact stain test is probably inappropriate and probably should not be used as a qualification test for the wiper. Conversely, evaporated residues from the wiper may be more corrosive than the wiper itself. If there is a significant probability that the parts will be near evaporation residues left behind after wiping, a near-contact stain test using the evaporated residue

404

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

from the wiper becomes an appropriate acceptance test. Similarly, if the wiper will not be used to wipe the part, it may not be necessary for the wiper to be static dissipative or conductive. It is important that the wiper satisfy ESD requirements under normal conditions of use. But this may be satisfied when static-dissipative work surfaces are wet-wiped with the intended workstation cleaning solution. Consider how the swab will be used. Suppose that the primary application of the swab is to clean visible contamination from the leads of the ESD components. In this case, qualification using a contact stain test is appropriate. Also in this case, it is probably necessary for the swab to be groundable using static-dissipative or conductive materials. Application Optimization Application optimization is the final hurdle in selecting wipers and swabs. At this point wipers and swabs have been found to be chemically compatible with the chemicals to be used in the workplace. Functional and nonfunctional contamination and ESD qualification tests have been passed. It is now necessary to test the remaining candidates to rank them in terms of their effectiveness at accomplishing their intended task. This can often be a very subjective evaluation. The remaining candidates are used by one or more persons to determine which works best. The evaluation can be very simple. Suppose that the surface finish on the tools to be wiped is known. Samples of parts with this surface finish can be wiped or, better yet, the tools may be wiped if they are available. If the wiper tears, shreds, or leaves fibers behind it probably is not going to be acceptable for this application. Suppose that corners need to be cleaned. Visual inspection should reveal which of the candidate wipers is most effective at cleaning in corners. Working this way, the wipers are subjectively ranked as to effectivness, ease of use, and so on. Occasionally, quantitative acceptance criteria are available. For example, it may be possible to measure cleanliness of the wiped surface indirectly. This may be accomplished by flushing the surface after it is wiped and measuring the residue in the flush fluid. Case Study: Diamond Swab In the 1970s and into the 1980s, the leading-edge magnetic recording head technology was the thin-film inductive head. At the end of the head gimbal assembly (HGA; also called the head suspension assembly) production process, the parts were cleaned and inspected. If particles or smears were observed on the air bearing surfaces under magnification, the contamination was manually swabbed away using woodenhandled cotton-tipped swabs. Historically, a large percent of the air bearing surfaces would need to be swabbed. Liquid-borne particle count and ionic contamination tests were added in the mid-1980s. These instruments were used to study the contamination history of the finished HGAs from just before the final cleaner to after visual inspection. The results were rather disturbing. It was found that HGAs that had been swabbed were significantly dirtier than HGAs that had not been swabbed, from both a particulate and an ionic contamination perspective. This was sufficient reason to replace the cotton swabs. (In addition, it also launched an investigation into the source of the smears in an attempt to eliminate or minimize the need for swabbing.) A swab manufacturer was contacted for development assistance. The problem and development objectives were described. A new extraction method was developed to permit objective laboratory tests to be used in development of the new swab. A new swab, based on a polyester swab head mounted on a carbon-filled plastic handle, was developed fairly rapidly. It was more than 10 times cleaner than cotton swabs from a particle count perspective. It was more than 100 times cleaner than cotton swabs from an ionic contamination perspective.

REUSABLE AND DISPOSABLE PACKAGING MATERIALS

405

Application optimization was more time consuming than materials optimization. This was partially due to the fact that the performance of the swab was being compared to the performance of cotton swabs. The operators using the new swab were very experienced in use of a cotton swab. The operators provided feedback about their experience using the new swab, its effectiveness at removing contamination, resistance to redeposit contamination, ease of use, and so on. The swab developer was very quick to return modified head designs. Many iterations were required to optimize the application. Despite the close cooperation the application optimization took more than four times as long as the materials optimization. The swab that resulted from this extensive development effort met with very little resistance when it was introduced in the production facility.

8.10 8.10.1

REUSABLE AND DISPOSABLE PACKAGING MATERIALS ESD Considerations in Packaging

Electrostatic discharge properties are also a consideration in the selection of packaging materials. ESD considerations include shielding from electromagnetic interference (EMI), discharge time, and tribocharging. Plastic packaging materials usually are insulative materials, such as polyethylene, polypropylene, polystyrene, polyethylene teraphthalate, and others. One historic method for making polyethylene and polypropylene into packaging suitable for ESD application has been the incorporation of additives, such as aliphatic amines, which temporarily absorb moisture from the atmosphere to obtain lower surface resistivity. This approach, often referred to as the use of topical antistatic agents, has a long history of difficulties. Among these are humidity sensitivity (at relative humidities below 10 to 15% RH, they stop conducting), contamination (outgassing, contact transfer), and loss of properties after water or alcohol cleaning. Static decay tests are often used to measure the time required to discharge a given amount of charge placed on a surface (voltage decay). Many methods exist. One of the tests used most often is MIL-B-81705. Turbocharger tests are also frequently recommended. Traditional packaging materials are seldom compatible with ESD requirements. Several approaches are available to modify materials for use in ESD applications. Where the ESD-safe workplace is simultaneously a cleanroom, the competing ESD and contamination control requirements must be balanced against each other. The traditional approaches are discussed below. 8.10.2

Carbon-Filled Polymers

Carbon-filled polymers offer excellent static dissipative or conductive packaging solutions. Typically, carbon powders or carbon fibers are used. Typical fill levels are 15 to 30% by volume. The choice of carbon filler material is not trivial. Fillers that are low in ionic contaminants or sulfur compounds are needed to prevent corrosion problems from occurring. In addition, fillers that adhere strongly to the polymer are preferred to prevent sloughing of carbon particles. One commonly held belief is that carbon fill is not acceptable for use in the cleanroom under any circumstances. Often, proof that carbon-filled polymers are unacceptable for cleanroom use is the results of a crayoning test. In this test a piece of the carbonfilled polymer is used to leave a mark on a white piece of paper. This must not be accepted as definitive proof of the unsuitability of carbon-filled polymers for use in contaminationcontrolled applications. In fact, using a piece of black construction paper, it is just as easy to demonstrate crayoning with the same polymer containing no carbon filler. Objective

406

CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

TABLE 8.19 Comparative Plastic vs. Steel Wear Rates of Carbon-Filled vs. Unfilled Polymersa Carbon Filled Unfilled Wear Index

Percentage and Type of Carbon

Wear Index

Polycarbonate Polyetherimide Polysulfone Nylon 6/12 Acetal Polyether ether ketone Polyphenylene sulfide Nylon 6 Nylon 6/10 Nylon 6/6

2500 4000 1500 190 65 200 540 200 180 200

Polyester (PBT) ETFE ethylene tetrafluoroethylene

210 5000

Polyvinylidene fluoride ECTFE Ethylene chlorotrifluoro ethylene ABS acrylonitrilebutadienestyrene

1000 1000

30% PAN fiber 30% powder 30% powder 30% powder 20% fiber 20% PAN fiber 30% PAN fiber 30% PAN fiber 30% PAN fiber 10% PAN fiber 20% PAN fiber 30% PAN fiber 40% PAN fiber 30% powder 30% powder 20% polyacrylonitrile fiber 15% polyacrylonitrile fiber 15% powder

85 70 75 25 40 60 160 30 25 60 40 20 14 30 10 28 14 18

30% polyacrylonitrile fiber

100

Base Polymer

The wear index  W/PVT, where W  volume wear (in ), P  pressure (lb/in ), V  velocity (ft/min), and T  time (h).

a

3

2

wear rate measurements must be used to choose among various packaging alternatives. This is illustrated by the data in Table 8.19. The larger the wear factor, the greater the volume or mass loss of the material. In every case, the addition of carbon decreases the wear rate of the polymer vs. its unfilled state. 8.10.3

Metal Loading

Metal fillers in polymers are generally incorporated as fibers, flakes, and powders. These lower the surface and bulk resistivities of polymers far enough to impart EMI shielding properties to the polymer. 8.10.4

Topical and Incorporated Organic Agents

Topical organic agents are added to the surface of polymers. They attract moisture from the atmosphere and impart static dissipative range resistivities to the surfaces to which they are applied. Unfortunately, they have a number of serious drawbacks that limit their general usefulness. One drawback is that their static-dissipative properties depend on adsorbed moisture. At low relative humidity they lose their static-dissipative properties. Another drawback is that the topical additives are soluble in water, detergent–water solutions, and alcohols.

FACIAL COVERINGS

407

Wiping or cleaning the plastic with a topical chemical treatment with these chemicals washes the treatment away, destroying the static-dissipative properties of the cleaned surfaces. The prevalence of the use of these chemicals in cleaning systems and in wipe-down in most high-technology industries precludes the use of topically coated polymers in these industries. Many of these topical treatments have significant vapor pressure and thus form airborne molecular contaminants, which cannot be tolerated in many industries. In addition, topical surface treatments can be removed by contact transfer, causing further contamination problems. The same types of chemicals are also used as additives blended into polymers used to form film or molded packages. These survive cleaning better than surface treatments because the chemical diffuses back to the surface of the polymer after cleaning. Unfortunately, the material in the body of the polymer gradually becomes depleted and the replenishment process eventually fails. The incorporated chemical still suffers from the same humidity and contamination problems. The most popular of these chemicals for polyethylene or polypropylene are organic amides and organic amines. The most popular class of these chemicals for use in poly(vinyl chloride) is dioctyl phthalate.

8.10.5

Copolymer Blends

There are several polymers that exhibit inherent static-dissipative or conductive properties. These are added to other polymers to make alloys with inherent static-dissipative or conductive polymers without the inherent problems of particle or topical additives. Transplex is a good example.

8.11

FACIAL COVERINGS

Several options exist for controlling contamination from the mouth and nose. Disposable masks are available in a wide range of materials, including paper, spun-bonded polyolefin netting, open-cell foam, and expanded polytetrafluoroethylene (PTFE; Teflon). (Reusable face coverings or masks generally are woven or knitted fabric, usually worn attached to the head covering.) The most elaborate method of controlling contamination from the face and nose is the use of the full containment hood, which looks like a space helmet. The purpose of the face covering is to control contamination that might be generated during nose or mouth breathing, talking, or facial movements, and to prevent contacting the face or facial hairs by hands, gloves, and other objects. The effectiveness of the various forms of face covering is also variable. One factor affecting the effectiveness of the face mask is the integrity of fit against the face. This is a function of both the design of the facial covering and how it is worn. A second factor affecting the effectiveness of the facial covering is the material of construction. A third factor is the propensity of the person wearing the facial covering to generate contamination. For example, smokers tend to generate more contamination than nonsmokers, and cold and allergy sufferers tend to generate more contamination, especially if they sneeze. The effectiveness of fit of the facial covering also can be influenced by the training of the wearer. Studies have shown that an improperly fitted eyes-only hood that completely covers the nose and mouth may contribute to an increase in particle contamination over wearing no face covering at all [30].

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CONSUMABLE SUPPLIES AND PACKAGING MATERIALS

TABLE 8.20 Comparative Particle Contamination Performance of Facial Coverings (particles/ft3  0.3 m) Woven Reusable

Paper

Paper  PTFE Membrane

205 160 112 88

180 58 19 26

114 45 10 8

Head movement Talking Nose breathing Mouth breathing

Table 8.20 compares three types of facial covering in preventing particle contamination during head movement, talking, nose breathing, and mouth breathing. The three facial coverings were a paper and expanded PTFE membrane duckbill-style face cover, a similar duckbill cover made from paper only, and a woven polyester reusable cover. Head movement creates the largest concentration of particles from all three of the facial coverings tested above, suggesting that fit, abrasion of the covering against the face or head covering, or flexing of the facial cover may be more important than filtration performance. Other studies have shown similar results [31].

REFERENCES AND NOTES 1. Presentation material from the IBM Contamination Control Course, Paris, Apr. 19–21, 1994. 2. IEST-RP-CC005, Cleanroom Gloves and Finger Cots. 3. R. W. Welker, previously unpublished laboratory data. This result was obtained for used gloves. For new gloves, the difference between ultrasonic degassing and undisturbed degassing was insignificant. 4. D. Cooper, and R. Linke, ESD: another kind of lethal contaminant? Data Storage, Feb. 1977, p. 49. 5. Electrostatic Overstress/Electrostatic Discharge Association Standard S11.11-1993. 6. FED-STD-101C, Method 404. 7. R. W. Welker, and P. G. Lehman, Using contamination and ESD tests to qualify and certify cleanroom gloves, Micro, May 1999, pp. 47–51. 8. R. W. Welker, previously unpublished laboratory data. 9. R. Coplen, R. W. Welker, and R. L. Weaver, Correlation between ASTM F312 and liquidborne optical particle counting, Proceedings of the 34th Annual Technical Meeting of the Institute of Environmental Sciences, King of Prussia, PA, May 3–5, 1988, p. 390. 10. R. W. Welker, Glove selection and use, presentation material from the IBM contamination control Course, Paris, Apr. 19–21. 11. R. W. Welker, Controlling particle transfer caused by cleanroom gloves, Micro, 17(8): 61–65, 1999. 12. R. C. Walker, Implementing an ESD control program, Microcontamination, Aug.–Sept. 1983, pp. 20–24. 13. G. E. Hansel, The role of the production operator in preventing ESD damage, Microcontamination, Aug.–Sept. 1984, pp. 43–46. 14. S. C. Heymann, C. Newberg, N. Verbiest, and L. Branst, Voltage-detection systems help battle ESD, Evaluation Engineering, Nov. 1997, pp. S-6 to S-12. 15. J. C. Hoigaard, ESD test equipment and workstation monitors, Evaluation Engineering, July 1998, pp. 58–61. 16. M. Banks, Watch those electrons, ESD battle heats up, Data Storage, July 1998, pp. 61–62.

REFERENCES AND NOTES

409

17. S. L. Thompson, All about ESD plastics, Evaluation Engineering, July 1998, pp. 62–65. 18. E. Greig, I. Amador, S. H. Billat, and A. Steinman, Controlling static charge in photolithography areas, Micro, May 1995, pp. 33–38. 19. A. Steinman, How to select ionization systems, Evaluation Engineering, June 1998, pp. 62–69. 20. B. I. Rupe, Electrical properties of synthetic garments with interwoven networks of conductive filaments, Microcontamination, May 1985, pp. 24–28. 21. R. J. Peirce, and J. Shah, Potential ESD hazards from using adhesive tapes, Evaluation Engineering, Nov. 1996, pp. S-30 to S-31. 22. R. W. Welker, A comprehensive ESD control program for MR heads, presented at the Asia Pacific Magnetic Recording Conference, Singapore, July 29–31, 1998. 23. ASTM Standard Test Method D4060-90, Abrasion Resistance of Organic Coatings by the Taber Abraser Method. 24. W. J. Havel, and C. Sheridan, Modified flex test for particulate analysis of dry wipers, Proceedings of the 31st Annual Technical Meeting of the Institute of Environmental Sciences, Las Vegas, April 30–May 2 1985, pp. 80–84. 25. O. Atterbury, H. R. Bhattacharjee, D. W. Cooper, and S. J. Paley, Comparing cleanroom wipers with a dry abrasion resistance test, Micro, Oct. 1997, pp. 83–100. 26. C. F. Mattina and S. J. Paley, Assessing wiping materials for their potential to contribute particles to clean environments: a novel approach, in Particles in Gases and Liquids: Characterization and Control, K. L. Mittal, Ed., Plenum Press, New York, 1990, pp. 117–128. 27. C. F. Mattina, and S. J. Paley, Assessing wiper materials for their potential to contribute particles to cleanroom environments, Part II; Constructing the stress strain curve, Proceedings of the 37th Annual Technical Meeting of the Institute of Environmental Sciences, San Diego, CA, New Orleans, LA, May 1991. 28. IES-RP-CC004.2, Evaluating Wiping Materials Used in Cleanroom and Other Controlled Environments. 29. J. M. Oathout, and C. F. Mattina, A comparison of commercial cleanroom wiper materials for properties related to functionality and cleanliness, Journal of the Institute of Environmental Sciences, Jan.–Feb. 1995, pp. 41–51. 30. G. Sullivan, and J. Trimble, Evaluation of face coverings, Microcontamination, May 1986, pp. 64–70. 31. B. Brandt, and A. L. Wright, Analyzing particle release of cleanroom headcoverings, Microcontamination, 1990, pp. 53–99.

CHAPTER 9

CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

9.1

INTRODUCTION

People are an abundant source of contamination in many forms. Our skin and hair are continuously growing and wearing away. The surfaces of our skin and hair are covered with a wide variety of beneficial bacteria and fungi. We sweat, leaving a rich ionic broth. Our skin is lubricated with skin oils. The ordinary street clothing we wear is a source of fibrous contamination. Our shoes are carries of mud and dirt. To better understand the procedures and tools necessary for controlling contamination, including the selection of cleanroom garments and the procedures to be followed for putting them on, taking them off, and storing them between uses, we discuss briefly the sources and effects of contamination from people. In addition, the behavior of people in a cleanroom contributes to contamination. Behaviors include how to enter the cleanroom, what to do while in the cleanroom, and how to exit the cleanroom. ESD-safe work areas are similarly affected by the use of protective equipment and apparel. Behavior in the ESD-safe work area is also a concern.

9.2

PEOPLE AS A SOURCE OF CONTAMINATION

The human body is a rich source of many types of contaminants. People are a source of particles, a source of chemical and biological aerosols to the surrounding environment. Add to these street clothing fibers, terrestrial soil, and cosmetic particles and people become the major source of contamination in the cleanroom in most industries where contamination is a concern. In the past, the principal concerns have been on the control of particulate contamination and, to a lesser extent, the control of ESD. Today, the control of chemical and Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

410

PEOPLE AS A SOURCE OF CONTAMINATION

411

biological contamination has assumed greater concern in industries other than medical devices, parenteral drugs, and pharmaceuticals, where biological contaminants traditionally were paramount. One example is the result of prelaunch onboard computer failure on the Space Shuttle program. Subsequently, other semiconductor failures attributable to human chemical and biological contaminants were also reported by other investigators. The increased use of cleanrooms in the food, cosmetic, and other industries has also brought chemical and biological control into sharpened focus. 9.2.1

Skin and Hair

Human skin and hair is continuously growing and wearing away; it is often described as part of life’s great renewable resources. One of the primary forms of contamination from skin and hair is particles. It has been estimated that depending on the activity, people may generate as many 5 to 10 million particles per minute 0.5 m in diameter and larger. Skin and hair are remarkable for more than just the fact that they are renewable. They are also remarkably durable. The skin protects our bodies from mechanical damage, desiccation, and attack by microorganisms, heat, cold, radiation, and to a lesser degree, exposure to chemicals. Because skin and hair are intended to provide this protection, particles from skin and hair survive some rather surprising attempts at their removal in cleaning or high-technology processes. For example, skin flakes have been shown to survive or leave recognizable artifacts in oxidation furnaces. Protoplasmic residues released from within skin cells have been found to be remarkably resistant to mechanical scrubbing in cleaning processes. Attempts to scrub away skin and hair particles by mechanical scrubbing of a surface to knock or grind away particles often simply fractures the cells and smears their contents over the surface. This releases cell contents, many of which contain ionic contaminants that promote corrosion. The contents of skin cells can be especially sticky and difficult to remove once the cells have been broken. Experiments performed in the early 1980s showed that residues from human skin can stain the surface of a silicon wafer, even after passing through an oxidizing furnace. The ability of human cells to withstand freezing, even at liquid nitrogen temperature, makes several infertility treatments possible. These experiments also showed the remarkable resistance of human skin and hair to acids and bases [1]. Skin cells have evolved to dry out gradually as they age. As they do, they gradually undergo a process in which they harden. The outermost layer of skin is relatively hard, but also very brittle. These hard and brittle cells flake off the surface of the skin quite easily. An illustration of this process is shown in Figure 9.1. The aging process of skin is partially what provides protection from desiccation, abrasion, and to some extent, chemical attack. Skin partially achieves its protective function by being sacrificed. That is, one of the wear and chemical protection mechanisms is that the outermost layers of skin are shed. This shedding mechanism is one of the reasons why skin cells are one of the most significant sources of contamination from people. The magnitude of human skin cells shedding should not be underestimated. Skin is the largest organ on the human body. Your skin is constantly sloughing off and being replaced by new skin cells. It has been estimated that the average person sheds approximately 2 kg (4.4 lb) of skin cells during a year. Most of these skin cells are too small to be seen under normal conditions being smaller than 35 to 50 m (the smallest particle a person can see with normally corrected vision under ordinary illumination). There are conditions under which these cells are visible. Everyone has experienced seeing the light shine through a crack

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CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

Dried skin flakes, about to flake off

FIGURE 9.1 Cross section through the outermost layers of skin.

in the blinds of a partially darkened room. The beam of sunlight illuminates a cloud of particles rising through the air. Studies have shown that in the average American household, approximately 80% of the particles seen in this cloud are dead skin flakes. These particles are present all the time, but they are so small that under normal conditions of illumination, they are invisible. Under the special lighting conditions of a bright light illuminating the particles against a dark background, they become visible. Dandruff is very visible and is a conglomeration of skin flakes, producing large white flakes. These conglomerations are largely held together by oils from the scalp. One of the things you do when you wash your hair with shampoo is remove the oils. This allows the skin flakes to disperse. When they get smaller than about 30 to 50 m in diameter, they are no longer visible. The dead skin flakes are still there, but you don’t see them. The effect of skin flake shedding is also evident on our street clothing. The familiar advertising jingle “ring around the collar” was a description of soil and dirt particles on a shirt collar. The primary source of the ring was not dirt, as we normally think. What is seen are primarily embedded skin flakes. The reason that this ring appears on the collar is because of the intimate contact of the collar against the neck of the wearer. There are dead skin flakes embedded in all of our clothing and these generally go undetected. The structure of hair is quite different from that of skin despite the fact that hair and skin are chemically very similar, being made primarily of the protein keratin. The structure of the surface of hair is shown in Figure 9.2. Hair consists of two regions: the inner cortex and a surrounding cuticle. The cortex is a central bundle of fibers aligned parallel to the length of the hair. These fibers provide hair with its tensile strength when the hair is pulled. But the bonding between the individual fibers is relatively weak. Surrounding the cortex are many overlapping layers of cells, called the cuticle, arranged like the scales on a fish. The cuticle scales are very thin overlapping cells, typically five to 10 layers thick. The cuticle layer acts as armor cladding to prevent the fibers in the cortex from separating. It is also important to understand what happens to the cuticle when we brush, comb, or otherwise touch our hair. Every time we touch hair, we damage the cuticle scale. This is illustrated in Figure 9.2. The raised edges of the cuticle scale are clearly visible: There are locations on the cuticle scale where the margins are irregular and broken off. There are fragments of broken cuticle scale still clinging to the surface of the hair. This damage to human hair happens every time the hair is touched. When you brush or comb your hair, put on a hat, put on a hairnet or cleanroom hood, or fluff up your hair while looking in a mirror, you are breaking off tiny little fragments of cuticle scale. These tiny fragments are far too small to be

PEOPLE AS A SOURCE OF CONTAMINATION

413

FIGURE 9.2 Cuticle scale coating of hair fibers.

seen because they’re generally only about ½ m thick and only a few micrometers wide. They contaminate the interior surface of a cleanroom hood as soon as you put the hood on. Wearing a disposable hairnet under a cleanroom hood helps keep the interior of the hood clean, an important consideration if you have to wear your cleanroom hood 10 to 20 times between launderings. When you take your head covering off, you create an invisible shower of broken-off cuticle scale particles. When the entire cuticle has worn off the hair, it can no longer protect the fibers in the cortex. This can result in split ends, a very common annoyance for people who wear their hair very long. The important point is that the wearing off of cuticle scale and the resulting cloud of (typically) 0.5-m-thick cuticle scale particles can be a serious form of contamination. Skin and hair are also sources of chemical contamination. Skin and hair are lubricated by oils from our skin glands. These lubricating oils help reduce the wear rate of the skin and hair, so they serve a useful purpose. These same oils can wet surfaces and thus contribute to contamination. In addition, we secrete ionic contamination when we sweat. Sweat is part of our natural cooling process. Sweat usually contains sodium, potassium, and chloride ions. Sodium and potassium ions contaminating a silicon wafer can diffuse into the silicon, causing doping problems. Chloride ions promote corrosion. 9.2.2

Fingerprints

Everyone recognizes fingerprints. The most common experience that people have with fingerprints is when glassware or windows need to be cleaned. Everyone knows that when you try to clean a drinking glass or mirror surface, you can’t get a smudgy fingerprint off by wiping with a dry wiper. To clean fingerprints from a surface, a soapy water solution must be used. You need to use special glass cleaner, which is a solution of detergent in water. Fingerprints contain oils, making them difficult to remove without the use of detergents or soaps. The surface of our skin is generously supplied with sweat glands that provide cooling and oil glands that provide lubrication. The distribution of these glands on the body varies. The palms of our hands and the soles of our feet have no oil glands. Conversely, the palms of the

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CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

hands and soles of the feet have the highest concentration of sweat glands on the body. Hands are thus an inherent source of the ionic contamination associated with sweat. Conversely, the surfaces of our hands are not a primary source of oily contamination; there are no oil glands on our hands. So where does the oily fingerprint come from? The face and scalp are generously supplied with oil glands. Absentmindedly touching the face or hair adds oil to the hands. This is an important consideration for contamination control. Clearly, we must cover the hands with gloves to control the dead skin cells, sweat, and ionic contamination. The gloves must not be allowed to touch the face to prevent them from becoming contaminated with oils and all of the other contaminants associated with skin. 9.2.3

Bacteria and Fungi

In addition to the protection provided by the barrier of the outer layers of skin, skin is generously populated with bacteria and fungi. When people hear this, they quite often deny the possibility. The first argument is that they took a shower this morning. Taking a shower does remove loose bacteria and fungi along and loose skin flakes from the surface of your skin. But typically, within one-half hour the normal populations of bacteria and fungi and loose skin flakes have reemerged. For this reason, taking a shower is not an effective means of controlling the bacteria and fungi on our skin for cleanroom environments. The second reaction that people have when hearing about the bacteria and fungi on their skin is the concern that they will get sick. People normally associate bacteria with germs, and we generally make the association that germs cause disease. Fortunately, for healthy, unbroken skin this is not the case. Experiments have been done in which a live culture of Staphylococcus bacteria is spread on unbroken skin. After one-half hour the surface of the skin is tested and it is found that all of the bacteria are dead. The reason why this is true is because the bacteria and fungi that naturally populate the surface of the skin consumed the staph bacteria. Thus, we live in symbiotic harmony with the natural flora and fauna on the surface of our skin [2]. A symbiotic relationship in biology is one in which both organisms enjoy a mutual benefit. We get protection from harmful pathogens that land on our skin, and the natural bacteria and fungi get a free ride and a free lunch. These beneficial microorganisms attack and consume potential pathogens, providing a second level of protection against infection. Ordinary washing removes some of these microorganisms but does not sterilize the skin. Within minutes to hours, the microorganisms have repopulated our outer skin surface. This means, of course, that the natural shedding process includes more than just dead skin flakes; bacteria and fungi are included in the shedding. This poses an especially important problem for industries where biological contamination is a problem. 9.2.4

Spittle Droplets

When we talk we generate an invisible spray of spittle particles, consisting of internal skin flakes, digestive enzymes, salts, and occasionally, food particles. Chewing gum or eating candy increases the production of saliva. Thus, chewing gum or eating candy in a cleanroom or ESD-protected workplace increases the production of saliva and leads to producing more spittle droplets. Perhaps more important is a health concern: If you are chewing gum in a facility where chemical vapors are present, you can increase your exposure to the vapors. This occurs due to the fact that you swallow most of the additional saliva, increasing your consumption of the vapors in the production facility. This increased production of

PEOPLE AS A SOURCE OF CONTAMINATION

415

saliva also has a tendency to increase the amount of staining that occurs on the inside of a cleanroom garment if an eyes-only hood or launderable facemask is worn in the cleanroom. 9.2.5

Street Clothing

Ordinary street clothing consists of a wide variety of fibers, often used in combination to create “blends.” Among these fibers are cotton, polyester, rayon, silk, and wool. One of the characteristics of these clothing fibers is that they are very short. The fabrics, yarns, and threads spun from these short fibers therefore tend to shed: hence the dust piles on the lint filter in your clothes dryer at home. Contrast this with the fibers used to weave cleanroom garments, which are usually fairly thick and very long. The fibers in ordinary street clothing are also quite fragile. Ordinary clothing wears out with use: Pants develop shiny spots, cotton T-shirts get thin, dish towels seem to slowly dissolve. This is the result of the loss of the individual fibers—and this does not occur only in the laundry. These fibers are being shed every time the street clothing materials come in contact with something: each other, your skin, and most important, cleanroom garments. Many of the fibers from our street clothing are easy to recognize using a microscope. The most fascinating of these is cotton. Cotton has a characteristic twisted shape and in cross section looks a little bit like a dumbbell. A scanning electron microscope photograph of a cotton fiber is shown in Figure 9.3. These fibers are so easy to recognize that many microscopists use a light microscope rather than a scanning electron microscope when doing fiber microscopy for contamination identification. The fibers are large enough to be resolved at about 160 magnification. The added feature of being able to identify the color of a fiber when using light microscopy gives the light microscope an advantage when doing a fiber contamination analysis. Street clothing can present a second problem, associated with footwear. Our shoes probably are the most significant single source of contamination we try to bring into a contamination-controlled workspace. They get contaminated when we walk on tiled floors, carpeted office spaces, and especially when we walk outside. ESD footwear is also affected adversely by contamination. Most of the contamination that accumulates on footwear grounders, booties, and dedicated ESD shoes will insulate the sole of the footwear. This insulation will interfere with the ground of the person wearing the soiled footwear. As a

FIGURE 9.3

Scanning electron microscope photograph of typical cotton fibers.

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CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

consequence, it is important that footwear worn to provide grounding be checked on a frequent and regular basis. At the very least, grounding through footwear should be checked daily. Good workmanship practice requires the following precautions: ● ●

Do not allow ESD footwear grounders to be worn out of an ESD-safe workplace. Test footwear grounders every time a person enters an ESD-safe work area.

9.2.6

Other Forms of Contamination

One classical study reported that a typical application of the basic cosmetics—foundation, mascara, rouge, lipstick, and eye shadow—was measured to contain tens of billions of particles larger than 0.5 m, and makeup must be reapplied several times a day to refresh it. The concentration of particles per application of specific cosmetics is shown in Table 9.1. The data emphasize the need to exclude cosmetics (other than moisturizing creams) from cleanrooms. Conversely, it has been shown that use of cosmetics and/or lanolin-based moisturizers significantly reduces the release of skin flakes as particles, especially from the facial region of both male and female operators. This use of cosmetics and moisturizers reduces a prolific source of particulates: skin flakes and eyelashes from the upper facial region. Unfortunately, cosmetics and lotions typically are a source of sodium, potassium, other metals, silicone, lanolin, tallow, waxes, and other contaminants, making them unsuitable for use in cleanrooms. Makeup affects the performance of contamination-controlled areas in several ways. The most direct: The makeup becomes airborne and causes contamination. A second problem with makeup is that it comes in contact with and transfers to the surface of cleanroom garments. In many industries, garments are changed twice or even once per week. This means that the garments will typically be worn 10 to 20 times between each laundering. The probability that contamination will get loose in the change process is very high, allowing for contamination not only of the makeup user’s garment but of all others around the garment. Cleanroom laundries use very mild cleaning processes designed to get the garments clean but also designed not to damage the garments. More aggressive cleaning is required for garments contaminated by makeup, increasing laundering cost and probably shortening the life of the garment. Hand lotions are considered to be a form of makeup. Handling cleanroom garments while wearing ordinary hand lotion can result in contamination of the garments. Fortunately, there is a hand lotion on the market that is clean and has been shown to reduce the amount of contact transfer of ionic contamination compared with that naturally transferred from the surface of bare washed hands [4]. TABLE 9.1 Concentration of Cosmetics in a Single Application Cosmetic Lipstick Blush Powder Eye shadow Mascara Source : Ref. 3.

Approximate Particles 0.5 m in Diameter per Application 1,100,000,000 600,000,000 270,000,000 3,300,000,000 3,000,000,000

TYPICAL GOWNING PROTOCOLS

417

Colognes and perfumes are also considered to be forms of cosmetics. Because they have an aroma, many believe that they must be a source of airborne molecular contamination and that their use should be restricted. However, there is considerable controversy about whether or not the use of colognes and perfumes should be restricted. Here are a few thoughts. If we are to ban the wearing of perfume or cologne, shouldn’t we also ban the use of scented shampoos, laundry detergents, and deodorants? It can be argued that restricting the wearing of cologne or perfume in the cleanroom serves no useful purpose unless a direct link between the use of a perfume and degradation of a product or process can be found. One of the functions of a perfume is to act as an odor mask: Perfumes tend to cover up unpleasant odors but do not eliminate them. The unpleasant odor is still present in the vapor phase and potentially contributing to airborne molecular contamination. We simply don’t notice it because it is masked by a more powerful and pleasant scent. Some of the spittle we produce from our breathing and talking includes internal mucosa cells. It was shown in the early 1980s that smoking increases the production of particles in our breath, elevated considerably over that of nonsmokers. Some of this consists of an increase in the number of mucus particles, but other new contaminants are included. Drinking a glass of water does help reduce the tendency for increased production of contamination in the breath after smoking, but the effect lasts only for a fairly short time. In one high-technology company, a restriction was replaced on how long a person had to wait before returning to the cleanroom after a cigarette break [5].

9.3

TYPICAL GOWNING PROTOCOLS

It is impossible to describe all possible gowning protocols. It is possible to provide some guidance as to the types of practices commonly found. Using these guidelines as a starting point, it is then usually possible to customize a gowning protocol for any given area. A good starting point for discussion are the recommended garmenting configurations suggested by the Institute of Environmental Sciences and Technology, shown in Table 9.2. At a very minimum, the primary sources of human contamination must be controlled in a cleanroom of any class. Thus, the minimal contamination garment system would be to use a hairnet, gloves, shoe covers, face mask, and frock. This differs from the IEST recommended practice shown in Table 9.2, which recommends a hair cover, frock, and shoe cover but omits a face mask and gloves. This garment protocol also is commonly used with a clean bench in a factory air environment. This protocol also commonly is found in ISO class 8 cleanrooms, especially in the aerospace industry. The minimum garments should be used in the final stage of a cleanbuild protocol for construction of a cleanroom. The final stage of a clean build will usually be when ceiling-mounted filters or fan-filter units are being installed and thereafter. The principle is that the most significant sources of contamination must be controlled using the most convenient and lowest-cost approach. Hairnets are used to contain the hair so that it is not exposed directly to the cleanroom and the hair does not contact other things. A face mask is used to reduce spittle droplet contamination when one breathes or talks. Gloves are used to reduce fingerprints. The frock covers street clothing above the knee. Shoe covers contain the debris on street shoes. As we move into rooms cleaner than ISO class 8, additional garments are generally used. In an ISO class 7 or class 6 environment, a hood is often worn over the hairnet. Some industries will switch from a frock to a jumpsuit. In class 5 and cleaner environments, jumpsuits are considered mandatory. If a jumpsuit or coverall is worn, a knee-high bootie

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CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

TABLE 9.2 Recommended Garmenting Configurations ISO 14644-1 Air Cleanliness Classa Apparel Type

ISO Class 8

Inner suit Hair cover (bouffant) Woven gloves Barrier gloves Facial cover Hood Powered headgear Frock Coverall Two-piece suit Shoe covers Boots Special footwear Suggested frequency of change

ISO Class 7

ISO Class 6

ISO Class 5

Aseptic ISO Class 5

ISO Class 4

ISO ISO Classes 2 Class 3 and 1

AS R

AS R

AS R

R R

AS R

R R

R R

R R

AS AS AS AS AS

AS AS AS AS AS

AS AS AS AS AS

AS AS R R AS

NR R R R AS

NR R R R AS

NR R R R AS

NR R AS AS R

R AS AS R AS AS

R AS AS R AS AS

AS R AS AS R AS

AS R AS AS R AS

NR R NR NR R AS

NR R NR NR R AS

NR R NR NR R AS

NR R NR NR R AS

2/week

2/week

3/week

1/day

Per entry

Per entry

Per entry

Per entry

Source: Ref. 6, Appendix A. a AS, application specific; R, recommended; NR, not recommended.

is generally required. All of these recommendations are generally consistent with those given by the IEST as shown in Table 9.2. However, there are some points worthy of discussion. The structure of Table 9.2 will be used as a guide for this discussion. 9.3.1

Inner Suit

An inner suit is worn in place of street clothes. It generally consists of a polyester shirt and pants. The use of the inner suit was promoted widely in the early 1990s. Its primary advantage lies in the fact that street clothing worn by the workforce is excluded from entry into the change room area. IEST-RP-C003.3 describes how an inner suit should be constructed, but provides little discussion of its implementation strategy. To be optimized, the inner suit should be restricted to clean or semiclean areas of the factory. Semiclean zones are seldom defined adequately. One definition of a semiclean zone would be an area protected by an inner-suit change area, where wearing street clothes is forbidden. Wearing the inner suit outside the clean or semiclean zone should be prohibited. Going further, the semiclean zone may also restrict the use of paper products, and prohibit food, drink, and other contraband activities such as smoking. An example of an architecture using an inner suit is shown in Figure 9.4. The use of the definition of a semiclean zone avoids the need to sample the area periodically for contaminants. Consideration should be given to privacy when implementing an inner-suit garment strategy: Separate change rooms for removing street clothes and donning the inner suit will

TYPICAL GOWNING PROTOCOLS

419

Factory Environment: Arrows indicate staging of traffic flow

Semiclean zone. Innersuits only. No contraband materials or activities. Temporary offices, rest rooms, break rooms, etc.

Change Room

Cleanroom

Inner-suit change room, part of semiclean zone

FIGURE 9.4 Architectural plan for implementation of an inner-suit garment configuration.

be required for men and women. An extreme application of the inner-suit strategy is to require that employees also shower before donning the inner suit, further increasing the facility requirement. This may be an acceptable alternative where viable contamination is an extreme issue. 9.3.2

Hair Cover (Bouffant)

The hair cover or hairnet presents an interesting technical problem. One purpose of the hair cover or hairnet is to prevent hair from being loose and waving about in the cleanroom or draped over the outside of the frock or coverall. A second purpose of the hairnet is to prevent it from touching the interior of the cleanroom hood, thus minimizing contamination of a hood that must be worn many times between launderings. The IEST-recommended garment configuration shown in Table 9.2 recommends use of a hairnet in combination with a cleanroom hood for ISO classes 8 through 3. However, it downgrades this recommendation to application specific in ISO class 2 and class 1 environments, where powered headgear is recommended. In the author’s opinion, this is an unwise recommendation. A hairnet and hood should be required when using a powered headgear. This subject is discussed in more detail in section 9.3.6. 9.3.3

Woven Gloves

The IEST standard lists woven gloves as application-specific only for ISO classes 8 through 5 and is listed as not recommended for all cleaner classes. However, this recommendation should not be taken without consideration of the use of a woven glove as a gowning glove. This should also not be taken as a recommendation against wearing a woven glove as a glove liner to assure comfort of the operating personnel. The selection and use of glove liners is discussed in Chapter 8.

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CONTROLLING CONTAMINATION AND ESD FROM PEOPLE

In some industries handwashing is perceived not to provide adequate contamination control protection to complete the dressing process. At this point some type of glove will sometimes be worn while handling cleanroom garments. These are often referred to as gowning gloves, for which several options are available. One common practice is to wear a launderable glove liner. In the biotech and pharmaceutical industries, usually a pair of sterile barrier-type cleanroom gloves are worn as gowning gloves. In industries where woven glove liners are worn as gowning gloves, the usual practice is to continue to wear them under the barrier film cleanroom gloves after the gowning-in process is completed. Gowning gloves are most commonly seen in a cleanroom with a serious concern for viable contamination. The rate at which viable contamination repopulates the surface of our skin is unpredictable, and in some applications, may be considered an intolerable risk. 9.3.4

Barrier Gloves

IEST-RP-CC-003.3 lists the use of barrier film gloves as application-specific for ISO classes 8 through 5 and only recommends their use in aseptic ISO class 5 and cleaner environments. The user should take this recommendation very carefully. Contact transfer of contamination from the hands can overwhelm all other forms of contamination combined. Methods for selecting and evaluating gloves for any given application are discussed in detail in Chapter 8. 9.3.5

Facial Cover

IEST-RP-C003.3 lists the use of facial coverings as application-specific for ISO classes 8, 7, and 6. It recommends facial coverings for all other classes of cleanroom. The cleanroom operator should be conservative in application of this recommendation. The design and performance of face coverings is a consideration in their selection. Face coverings may be an integral part of the head covering. When used as a sewn-in face covering, they have the advantage that there is one less item to keep stocked in the change room. They are laundered with the head cover, reducing handling if a separate launderable face cover is used TABLE 9.3 Performance of Face Masks and Head Covers Description Snap-in washable face veil Ear loop, PTFE medium, polypropylene cover Off-the-face, PTFE medium, white face mask Surgical mask with elastic bands Beard cover Closed split shield Bottom-half face shield, safety glasses Bottom-half face shield, no safety glasses One-piece face shield No mask a

n.a., Not applicable.

Mask Stretcha

Code

Breathing

A B

650 905

95 510

C

690

9600

D

710

215

E F G

925 0 5

110 n.a. n.a.

H

20

n.a.

I

0 2900

n.a. n.a.

TYPICAL GOWNING PROTOCOLS

421

and reducing solid waste vs. use of a disposable face covering. There are also disadvantages to be considered. Most people do not want to reuse a face cover several times. Disposable face coverings can be changed with each entry to the cleanroom. One study attempted to quantify the particle containment performance of face masks and head gear [7]. In this study, particle counts in a class 10 cleanroom were measured using various combinations of face mask and head gear. These results might not seem to be too alarming. However, one must consider that the tests were performed in a class 10 unidirectional-flow environment. In a unidirectional-flow environment, the high air velocity and controlled direction of airflow will tend to dilute the particles much more than in a mixed-flow cleanroom. 9.3.6

Hood and Powered Headgear

Many different types of hoods are worn in cleanrooms today: open-face hood, open-face hood with snap-in veils, and eye-only hoods. In addition, powered headgear is also in use. Open-Face Hood The open-face hood is often considered to be the most desirable by the wearer. It does not tend to fog eyeglasses and it is cooler than a hood worn with a face mask or an eyes-only hood. In a comprehensive contamination control program, a face mask will also be worn, partially negating the openness of this style of hood. Hoods are always sized (e.g., small, medium, large). The sizes are general and do not ensure a proper fit for every person. Fortunately, hoods are generally equipped with snaps that allow wearers to customize a hood to their own head and face shape. Unfortunately, most people wearing hoods have not been trained how to use these snaps to customize the fit. Open-face hoods are often worn with a snap-in veil in place of wearing a disposable face mask. This offers the advantage that the snap-in veil is launderable, making it less expensive than a disposable face mask. There are offsetting disadvantages. The cost of laundry must be taken into consideration. Cost due to lost or damaged veils must also be considered. Eyes-Only Hood Only the eyes are visible when an eyes-only hood is worn. Another name for this type of hood is executioner-style hood. Because of the difficulty in sealing the opening around the eyes, these hoods are often worn with a paper face mask. Many people complain that this hood style is claustrophobic and that it leads to fogging of eyeglasses and goggles. The most common complaints about cleanroom hoods is that they are uncomfortable. Adjusting the fit of the hood is difficult because the snaps are in the back. In addition, for people who wear eyeglasses it is difficult to prevent a gap in the eye hole at the temple of the hood. If the hood is adjusted tight enough to minimize the gap created by the temple bar of the eyeglasses, the fit is often too tight, pinching the temple bars of the glasses to the head. Figure 9.5 illustrates an interesting innovation that gets around this problem. The sides of the hood are outfitted with slots that are sewn with overlapped flaps. The wearers adjusts the fit of the hood so that it is tight around the face before they put their glasses on. Then the temple bars of the glasses are slipped into the temple slots while looking in a mirror. This hood is also equipped with ventilated side panels. This reduces the amount of heat load and makes the hood more comfortable. The IEST-RP-C003.3 recommendations are that the use of a hood is application-specific for ISO classes 8, 7, and 6, is recommended for ISO classes 5, 4, and 3, and is applicationspecific for ISO classes 1 and 2, where powered headgear are recommended. This set of

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FIGURE 9.5 Temple slots in a cleanroom hood.

recommendations cannot be endorsed. First, a hood is often considered mandatory in the aerospace industry during spacecraft final assembly, even though this is most often done in ISO class 8 and 7 cleanrooms. Second, eliminating the use of the conventional cleanroom hood when the powered headgear is used has been shown not to be the best practice. Two options for wearing powered headgear were studied in the late 1980s. In the first alternative, powered headgear was put on in the change room in place of the regular hood. In the second option conventional cleanroom hoods were put on in the change room and the personnel entered the work area. When they arrived at their workstations they put on the powered headgear. In the second alternative the cape for the powered headgear was worn on the outside of the jumpsuit. The area where this was tested was intended to be an ISO class 3 work area. Continuous optical particle counts were available. The two alternatives were each tried for a week at a time. The protocol was switched twice, giving three snapshots of which gowning method worked best. After each trial, the powered headgear was inspected visually under ultraviolet light and hand cleaned. The results of the three trials were irrefutable [8]. ●

●

●

Airborne optical particle counts using the first protocol achieved class 3 performance. However, leaving the powered headgear in the cleanroom and wearing it over a conventional hood easily achieved class 2 performance. The powered headgear worn over a conventional hood was visibly cleaner in each of the three trials. Yield on the process line improved significantly when powered headgear was worn over a conventional hood compared with wearing powered headgear over no other garment.

9.3.7

Frock, Coverall, and Two-Piece Suit

The body covering is either a jumpsuit (sometimes referred to as a coverall or bunny suit) or a frock (sometimes referred to as a smock or cleanroom lab coat). In a frock the bottom of the suit is open at the knees. These are generally used in ISO 14644 class 8 or 7 (FEDSTD-209 class 100,000 or class 10,000) cleanrooms. In a jumpsuit the legs are completely covered. Jumpsuits are generally used in ISO 14644 class 6 (FED-STD-209 class 1000) or

TYPICAL GOWNING PROTOCOLS

423

cleaner environments. Two-piece suits are occasionally worn in place of a coverall because they are much easier to put on. People often are very relaxed when working in an ISO class 8 or 7 (class 100,000 or class 10,000) cleanroom. They assume that because the room is classified as not very clean, they don’t have to worry about generating much contamination. This is a misconception, however. Due to the airflow characteristics of ISO classes 8 and 7, contamination generated in the room remains in the room for a long time. People should be more concerned not to generate contamination in these classes of cleanrooms because of the unforgiving nature of the airflow. This is illustrated quite soundly by the practice in the aerospace industry of final assembly for spacecraft. This is generally done in ISO classes 8 and 7 high-bay cleanrooms. Because of the unforgiving nature of the spacecraft, which cannot be cleaned easily at this point in its assembly, people are always in jumpsuits. The wearers’ skin should be covered to the maximum extent possible. One area where the problem of exposed skin is particularly important is the skin at the wrist, due to the proximity to products and processes. This area is also very prone to being exposed because the cuff of the glove is not fastened to the sleeve of the frock or jumpsuit. Many different types of glove lock have been employed, including the following: ●

●

Glove locks that use tape or adhesive bands. These are difficult to use and introduce contamination from adhesive residues. Croupier sleeves, tubes of fabric with sewn in elastic cuffs at both ends. These are effective but suffer from the drawback that they introduce another article of clothing to be laundered, lost, or damaged.

During the 1990s an integral glove locking mechanism was introduced. This glove lock uses a double cuff on the end of each sleeve of the jumpsuit. It is available with elastic cuffs, knitted cuffs, or cuffs secured by snaps. An example is illustrated in Figures 9.6 to 9.8.

FIGURE 9.6 Integral glove lock mechanism sewn on the sleeve as a double cuff. The inner elastic cuff has been exposed by drawing the outer snap cuff up the forearm.

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FIGURE 9.7 A nitrile barrier film glove has been put on and secured momentarily to the inner elastic cuff by pulling the bead of the glove over the inner sleeve elastic.

FIGURE 9.8 The outer snap cuff has been drawn over the cuff of the glove, creating a labyrinth pathway. The snap has been fastened to secure the sleeve to the cuff of the glove, forming an effective glove lock.

9.3.8

Shoe Covers, Booties, and Special Shoes

Shoe covers are recommended by IEST-RP-CC003.3 for ISO class 8 and 7 cleanrooms, are listed as applications specific for ISO class 6 and 5 clean rooms, and are not recommended for

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FIGURE 9.9 Proper wearing of a disposable shoe cover. Note that the conductive ribbon is not worn in direct contact with the skin, but rather, is worn between the shoe and the sock. Footwear must be tested upon each entry into the cleanroom or ESD-safe work area.

aseptic ISO class 5 or cleaner cleanrooms. Unfortunately, this set of recommendations once again creates a misimpression about the appropriate way to select and use the various footwear options available. The section on special shoes is especially misleading, as it describes these as factory shoes and recommends that they only be used in less critical applications. In many contamination-controlled areas, street shoes must be cleaned in a shoe cleaner prior to entry. There are two generic types of shoe cleaner: those equipped with a HEPA filter, often called self-contained shoe cleaners, and those requiring an external vacuum exhaust. It is important to know which type you are using and make certain that it is set up correctly. One of the best locations for installation of shoe cleaners is in the hallway outside the entrance to the change room. Regular workers often exchange street shoes for dedicated cleanroom shoes. This is often done for safety reasons. Dedicated cleanroom shoes are available with steel toes, where safety is important. In addition, cleanroom safety shoes can also be obtained as ESD safe footwear, eliminating a major source of problems for the cleanroom or ESD-safe work area. Many street shoes do not ground properly through cleanroom footwear and/or booties. Visitors may be required to put disposable shoe covers on over their street shoes, since dedicated cleanrooms shoes will usually not be available for visitors. Figure 9.9 illustrates an example of a disposable shoe cover equipped with a grounding strap. The grounding strap is a conductive ribbon. This conductive ribbon for a disposable shoe cover generally does not incorporate a current-limiting 1-M resistor. It is therefore extremely important that the footwear be tested using a footwear tester before entering the cleanroom or ESDsafe work area. Wearing the conductive ribbon in direct contact with skin (i.e., between the skin and stocking) could expose the wearer to potentially lethal voltage. One of the best practices is to buff the street shoes, don a pair of disposable shoe covers (or change into dedicated cleanroom shoes), and then put on a pair of knee-high booties. Thus, the recommended practice of IEST-RP-CC003.3 leaves out one of the best possible clean practices.

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9.3.9

Suggested Frequency of Change

The suggested change frequency is one of the most controversial sets of recommendations to judge. The recommendations suggested by IEST-RP-CC003.3 for change frequency largely reflect the experience of the working group members but this membership is not specified in the document [9]. By way of contrast, here is an alternative set of recommendations for change frequency. ●

●

●

Disposable, launderable face coverings and hoods should be exchanged as frequently as the workforce desires. These parts of the total garment system tend to become soiled unacceptably at a rate that is highly subjective. Rely on users to tell you when they are uncomfortable. For viable contamination control, it may be necessary to change the garment on each entry, regardless of the ISO class of cleanroom. Change frequency is process dependent. Operators wearing garments in operations with a high probability of garment contamination must change their garments more frequently than others. This is exemplified by maintenance operators who work in the service core performing very dirty operations and who must periodically work in the process aisle, exposing their soiled garments to products and processes.

As stated in the recommended practice, change frequency should be determined on a case-by-case basis. However, this can be an extremely difficult task to accomplish. Case Study: Change Frequency Establishing a proper change frequency is one of the most subjective elements of an overall contamination control program. Various attempts have been made to quantify change frequency using tests such as the body box test. None have met with particular success, with one notable exception. A very dedicated engineer was given responsibility for eliminating the causes of high particle count alarms in a continuous cleanroom monitoring system. He steadfastly identified and eliminated leaky pneumatic cylinders, door slides that the bearings had failed, and many other sources. He had one workstation where particle counts failed routinely on the third shift. He could find no mechanical cause. One night he decided to observe the third-shift operation of the workstation personally. He waited and observed the display on the particle counter. The operator arrived: an extremely large man. The particle counter responded to his presence by counting very rapidly. The engineer asked the operator to step away from the particle counter. It stopped counting. Repeating this operation several times, the engineer came to the conclusion that this large man was the source of the high particle counts. He then asked the operator how often he changed his cleanroom garments. The man said, “About once every four weeks, because there are not enough garments in my size.” The laundry service was contacted and the inventory was adjusted to provide this operator with a change twice per week. The highparticle-count problem went away. 9.4

PROCEDURES FOR ENTERING A CLEANROOM

Procedures for entering a cleanroom and for what shall be worn vary according to the needs of the product within the cleanroom. It is therefore necessary to describe the general

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considerations for dressing to enter a cleanroom rather than to recommend a single procedure. This generalized procedure is as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Remove and store outerwear. Wash your face and hands. Buff your shoes. Wipe down hand-carry items. Put on a hairnet and face mask. Put on shoe covers or change into dedicated cleanroom shoes. Wash your hands. Select and put on the correct-size hood. Select and put on the correct-size frock or jump suit. Select and put on knee-high booties (if worn). Select and put on correct-size gloves. Check yourself in the mirror. Use a sticky roller or the air shower. Enter the cleanroom. Wash or wipe your gloves.

The first principle of entering a cleanroom is gradual decontamination to take care of the dirtiest items first and progressively to achieve higher and higher levels of cleanliness. This includes removing outerwear such as coats, jackets, and sweaters, and washing the face and hands. These steps are done before entering the change room. Good change room design generally provides a pre-change area and a formal gowning area. The pre-change area is often used for wipe-down, covering the hair and face, and putting on shoe covers. The inner change room is where formal cleanroom garments are put on. (For more details on change room design, see Chapter 10.) 9.4.1

Pre-Change Room Procedure

Outerwear should be removed in a locker room or break area and stored on coat racks. This provides an opportunity for personnel to store items that are not allowed in the cleanroom, such as purses and lunches, in lockers or desks. This is an especially important consideration for regions that typically experience severe weather, such as snow and sleet, where the outerwear can become heavily contaminated. In areas where such severe weather occurs, the coat rack should not be located within the change area. Where severe weather does not occur, the coat rack may be an integral part of the change room. Wearing cosmetics in cleanrooms should never be allowed. Therefore, before entering the change room, all personnel should go to the lavatory and wash their hands and faces. Prior to returning to the change room, lotion approved for use in a cleanroom or ESD-safe work area should be applied if needed. 9.4.2

Wipe-Down

Following the principle of gradual decontamination, the next step is to wipe down any article to be taken into the cleanroom. This can be simple hand-carried items or larger pieces of

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equipment. Hand-carried items are often wiped down in the change room. Larger items generally are wiped down in an equipment pass-through enclosure. Any item that you might want to use while in the cleanroom must not be under your cleanroom garments. Thus, pens, notepads, pagers, and cell phones must be wiped down and hand carried if they are to be used in the cleanroom. Wiping down simple items such as a stick pen or cell phone usually creates no resistance from the entering workforce. However, a toolbox containing hundreds of individual tools can present a daunting task for pre-entry wipe-down. In fact, it is such a daunting task that it is often done poorly or not done at all. Fortunately, there is a logical solution to this problem. The outside of the toolbox is wiped thoroughly, so that it will be clean enough to be brought into the contamination-controlled area. Then a few extra wipes are wetted, placed in a plastic bag, and put into the toolbox. In addition, a few extra pairs of gloves can be put into the toolbox, after which it is taken into the cleanroom, carried to the work area, and placed on the floor. The toolbox is opened and the tools to be used for the maintenance or engineering activity are taken out and cleaned, one at a time, and placed on the workstation. This way, only the tools to be used must be wiped down, saving time and, incidentally, usually achieving a higher level of compliance. Note that the toolbox should not be placed directly on the work surface because even though the outside of the toolbox is clean, the inside of the toolbox and the tools inside are not clean. 9.4.3

Hairnet and Face Mask

The next action is to put on a hairnet and face mask. The sequence in which one puts on the hairnet and face mask can depend on the type of face mask worn. A face mask that is held on the head using ear loops should be put on before putting on the hairnet. Conversely, a face mask that ties behind the head can entangle hair. In this case it is usually more comfortable to put on the hairnet first. Beard covers usually are gauzy, spun-bonded polyolefin materials, similar to hairnets, held to the head with elastic. With beard covers, the sequence for putting on the beard cover and hairnet is usually arbitrary. Figure 9.10 shows a typical hairnet and beard cover combination. There is some controversy about how the face mask should be worn. Previously unpublished laboratory data showed that wearing the face mask below the nose was cleaner that wearing the face mask over the bridge of the nose. This result was obtained using airborne optical particle counters measuring particle counts downwind of several subjects. This controversy is understandable because it looks unconventional. The common wisdom is that we should always wear the mask over the nose. In this face mask research it was found that wearing the face mask under the nose was actually cleaner than wearing the face mask over the nose. This observation was attributed to the inability of the face mask to make an effective seal around the bridge of the nose. The observation was that the leakage of breath around the gaps next to the bridge of the nose allowed particles from the mouth to bypass the mask. Since this leakage would also include particles from the nose, the sum of the particles leaking past the mask contributed by the nose and the mouth was greater than the particles from the exposed nose. (The evidence for this was determined in the early 1980s. Even though research had confirmed that wearing the mask under the nose was cleaner than wearing the mask over the nose, management refused to allow masks to be worn under the nose.) There are several types of paper masks. There are two different styles for these masks, depending on how they are held against the head. The sequence in which you put on a mask

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FIGURE 9.10 Typical face mask and hairnet combination. There is some controversy about how the face mask should be worn. In this illustration the face mask is shown being worn below the nose.

and hairnet depends on how the face mask is held on the head. It is difficult to put on a mask that is held on the head with ear loops after putting on a hairnet. Conversely, a mask that ties behind the head should almost always be put on after the hairnet to prevent entangling the hair when the straps of the face mask are tied. In many installations where the mask may need to be changed frequently, the mask will be worn over the outside of the hood. One of the most common styles of face mask is a pleated paper face mask. To be worn properly, the pleats on the face of the mask must be expanded so that the bottom of the mask reaches under the mask and fits under the chin; the top of the paper face mask is conventionally worn over the bridge of the nose. Usually, this style of face mask is equipped with a metal bridge piece. The metal bridge piece must be molded to the bridge of the nose to try to increase the effectiveness of the seal. One of the most common mistakes in cleanroom gowning is not wearing the mask correctly by not expanding the pleats or by not molding it to the bridge of the nose.

9.4.4

Shoe Cleaners

The next consideration is footwear. In many contamination-controlled areas street shoes must be cleaned in a shoe cleaner prior to entry. There are two generic types of shoe cleaner: those equipped with a HEPA filter, often called self-contained shoe cleaners, and those requiring an external vacuum exhaust. It is important to know which kind you are using and make certain it is set up correctly. Regular workers often exchange street shoes for dedicated cleanroom shoes. Visitors may be required to put disposable shoe covers over their street shoes, since dedicated cleanrooms shoes will usually not be available. A disposable shoe cover equipped with a grounding strap is shown in Figure 9.9. The grounding strap is a conductive ribbon. This conductive ribbon for a disposable shoe cover generally

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does not incorporate a current-limiting 1-M resistor. It is therefore extremely important that the footwear be tested using a footwear tester before entering the cleanroom or ESDprotected work area. Wearing the conductive ribbon in direct contact with skin (i.e., between the skin and stocking) could expose the wearer to potentially lethal voltage. Shoe cleaners will generally be located immediately outside the outer change room door. This is in recognition of the fact that the shoe cleaner tends to generate contamination even if exhausted or equipped with internal filters. In some facilities, the shoe cleaner is located within the change room, but as close to the outer exit door as possible. The purpose of the shoe cleaner is to minimize contamination of the change room floor. Many facilities place sticky mats on the floor very close to the shoe cleaner.

9.4.5

Handwashing

Now that the hands have touched the face, hair, and shoes, hands must be washed before handling cleanroom garments. Occasionally, handwashing stations are provided for this in an intermediate change room. In general, it takes each person about 15 seconds to wash and rinse his or her hands. Drying has two options: powered hand dryers, which usually take about 45 seconds per person, or drying using hydrophilic cleanroom wipers, which can be laundered and reused, which usually takes less than 15 seconds. The handwashing consists of a foot pedal or photoelectrically operated hand sink. In some biotech and medical operations it may be acceptable to use automated handwashing stations which dispense sterilizing chemicals as part of the washing operation. These are generally not necessary for semiconductor, aerospace, disk drive, or flat-panel display, where removal of particles, ionics, and organics are important but sterility is not. Many facilities provide lotion at this point. Use of lotion can be an important consideration for several reasons: ● ●

Lotion can correct dry skin problems, thus minimizing skin flakes. Lotion may be needed for ESD application to moisten the wrist, helping minimize ESD problems.

Case Study: Handwashing Drying hands using powered hand dryers is very slow. As a consequence, many people will skip handwashing to avoid the excess delay created when entering a cleanroom. To overcome this problem, an experiment was conducted by a large manufacturer in one of its busiest cleanrooms. In this experiment, conventional roll towel dispensers were deployed next to the handwashing stations. The dispensers were provided with special roll towels that had been woven from hydrophilic polyester fibers to render the towel absorbent but nonshedding. At first the experiment seemed to be a success. Operators were no longer observed skipping the handwashing step in the entry procedure. This was because hands could be dried in one third the time required to dry using the powered hand drier. Questionnaires were distributed to determine the operators’ reaction to the use of roll towels vs. powered hand driers. The initial reaction was very positive. However, one week later it was discovered that the roll towel dispensers had been removed and replaced by powered hand driers. When questioned about this, management for the area admitted that they had ordered the replacements. The reason they gave was that they did not like seeing the smudges and soils on the roll towels. What would you conclude from this anecdote?

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9.4.6

431

Changing into Cleanroom Garments

Now that the hands are clean, the person enters the gowning area. Two primary types of cleanroom garments are available: frocks and jumpsuits. Frocks are open at the knee: jumpsuits cover the entire leg. An example of a frock is shown in Figure 9.11. An example of a jumpsuit, also called a bunny suit or coverall, is shown in Figure 9.12. Many industries use their garments several times between each laundering. The garments must be stored between each wearing. The storage of the garments can be done in several ways. The fastest is to hang the garments. Storage in drawers, lockers, or cloth bags are also common practices but require additional time to fold the garments and often result in garments of less than optimal cleanliness. In industries reusing garments several times between each laundering, the garments are retrieved from their storage locations. In industries that launder after every use, as is common in biotech and pharmaceutical industries, fresh garments will be put on with each entry into the cleanroom. It is important to keep the outside of the cleanroom garment clean. It must not touch street clothes or the floor, and must not touch the hands of the wearer unnecessarily. The hood is applied first. Typically, the hood is grasped by the cape, which will be worn under the frock or bunny suit, and the hood is pulled over the head. The closures are fastened to achieve a snug fit around the face. Once the hood is on, the face or eye opening is checked in the mirror to be certain that all hair is tucked in. This is one area where a hairnet can be very beneficial: It is much easier to make certain that all the hair is tucked in if it was

FIGURE 9.11 Cleanroom frock. This particular frock is shown with a raglan sleeve and knitted cuffs.

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FIGURE 9.12 Cleanroom jumpsuit. This jumpsuit is made of spun-bonded polyolefin. It has elastic cuffs at the wrists and ankles. The model is wearing a knee-high bootie and an ankle-high bootie for this pose.

already secured by a hairnet. The hairnet also prevents the hair from direct contact with the inside of the hood. This helps keep the hood cleaner between launderings. Next, dressing from the top down, the person retrieves his or her frock or jumpsuit. A common mistake frequently observed at this point in the gowning process is that the frock or jumpsuit will be carried over the forearm like a dish towel. Handling like this can easily spread contamination from street clothes to the outside of the frock or jumpsuit. Putting on a frock is usually no more difficult than putting on a raincoat. However, putting on a jumpsuit without having the sleeves touch the floor can be an acrobatic feat. Here is the recommended technique: 1. Grab the ends of the sleeve that will be covered by the gloves. 2. While holding the sleeves, grab the jumpsuit at the waist and gather the legs up. 3. Step into the legs of the jumpsuit. Many find this easier to do while leaning against something stable. 4. Drop the leg gathers as each leg is inserted. 5. Insert one arm at a time in the sleeves. 6. Fasten the garment all the way to the collar.

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Next, knee-high booties are often used. This is usually done using a shoe change bench. The booties are put on one foot at a time, swinging the bootied foot over to the clean side of the shoe bench. Many booties are equipped with snaps that fasten to snaps on the legs of the jumpsuit. Others have an adjustable elastic band that snaps at the top of the legging portion of the bootie. Finally, some have elastic sewn into the top of the legging. Finally, one last check in the mirror confirms that all is in order and the cleanroom is entered. Here the procedures vary from company to company. Many companies require that the entrance to the cleanroom be through an air shower. Many studies of air showers have shown that they are of questionable effectiveness. A few companies use a sticky roller to remove contamination from the outside of the garment, in place of an air shower. If a wrist strap is to be worn, it is usually tested before the cleanroom gloves are put on. In the wrist strap test, the wrist strap is plugged in and bare skin is touched to the tester to complete the circuit. Similarly, if the footwear is to be tested, this is best done before gloves are worn. Static dissipative gloves will usually interfere with the results of the touch-andgo wrist strap and footwear testers. Next, gloves are selected and donned. Gloves are discussed extensively in Chapter 8. Finally, as a last measure, many companies wash their gloves. Glove washing is described in detail in Chapter 8. Some companies don their gloves while still in the change room. 9.4.7

Powered Headgear

The powered headgear of interest is shown in Figure 9.13. This is a full-containment hood, also known as a Stackhouse hood. This hood comes with two different styles of plastic face shields: solid face shields or split face shields, as illustrated in Figure 9.14. The split-face shield is convenient for using a microscope. Air from the cleanroom is drawn into the hood using a fan that is worn at the waist. The fan is powered by a battery pack also worn at the waist. Air is exhausted back into the cleanroom via HEPA filters. Powered headgear is rather expensive, costing several hundred dollars each. When using a hood like this, it is necessary to have more than one per person. Typically, one is being worn by the worker, one is on the rack in case the worker has to make an emergency exit from the cleanroom, and one is on a truck going back and forth from the cleanroom laundry. Beside the cost of providing three of these assemblies per person, the comfort of wearing such headgear is often questioned. Actually, such headgear is quite comfortable, partially because most of the weight is worn on the belt at the waist and partially because the air system draws air into the hood, keeping the face cool. Whether or not they will actually ever be used is easy to answer. Much of Intel used the Stackhouse hood system for many years in the manufacture of semiconductors. The Stackhouse hood system was used in sputter-load chambers at IBM for many years. The system has been used by NASA for Genesis spacecraft final assembly. 9.4.8

Footwear

Several types of footwear are used in cleanrooms. Disposable shoe covers are commonly worn in class 10,000 (CLASS 7) and class 100,000 (CLASS 8) rooms as the only type of shoe cover. Many facilities will use a disposable shoe cover during the gowning process. Shoe Covers Shoe covers are listed as not recommended for aseptic ISO class 5 and class 4 and cleaner cleanroom. Strict acceptance of this recommendation is unwise. The use

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FIGURE 9.13 Stackhouse or full-containment hood, shown here with the split face shield visor in the open position to allow working with a microscope.

FIGURE 9.14 Stackhouse hood being used by NASA in the Genesis project collector final assembly. (Courtesy of NASA/JPL/CalTech.)

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of shoe covers during the gowning in process is highly recommended for these classes of cleanrooms in addition to wearing a knee-high bootie. In cleanrooms where operators wear dedicated cleanroom shoes, visitors generally are provided shoe covers, since they are not outfitted with dedicated cleanroom shoes. As with the hairnet, the use of shoe covers helps keep the cleanroom garment cleaner when the garment will be worn several times between launderings. Dedicated Cleanroom Shoes There are also dedicated cleanroom shoes, which can be obtained as safety shoes (with steel-reinforced toes) and can also be ESD safe. An example of an ESD-safe dedicated cleanroom shoe is shown in Figure 9.15. Many of the cleanroom and ESD shoes are indistinguishable from athletic footwear. Thus, they could be worn out the cleanroom or ESD-protected workplace unnoticed. This is generally considered a bad practice, since wearing these dedicated shoes out of controlled areas can render them unsuitable for either contamination or ESD control purposes. As a consequence, these types of shoes are often provided with identification tags such as that shown in Figure 9.15. The most common type of footwear in the cleanroom is the knee-high bootie (Figure 9.12). These can also be used as an ESD-safe shoe cover, but experience has shown that not all booties have the ESD performance that they claim. Case Study: Failure of Footwear to Ground Many contamination-controlled workplaces are also ESD-safe work areas. This complicates the gowning procedure somewhat, since it may be necessary for people to ground through their footwear while working in a standing position in the cleanroom. Grounded footwear is often worn as a convenience, so ESD-sensitive hardware need not be placed in static shielding packages when carried from one ESDsafe workstation to another. However, several very notable incidents of footwear not

FIGURE 9.15 Typical dedicated work shoes. Identified as ESD safe by a special tag slipped over the shoelace. Dedicated work shoes should never be worn out of the clean area or ESD-protected work area.

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providing adequate grounding have occurred. In each case, the company using the footwear failed to notice that their people were not grounded because the company did not include a mandatory footwear ground integrity test before entering the workplace. Here are the details. Each of these companies had purchased knee-high booties advertised as ESD safe. In each of these companies, ESD events occurred that made it obvious that some part of their ESD control program was inadequate. When the footwear was tested during an audit using a portable footwear test kit, it was found that the footwear did not ground. The solution to this problem was to sew a conductive ribbon to the sole inside the knee-high shoe cover. The operator wearing the bootie would insert the conductive ribbon between their shoe and sock before zipping up the bootie. To make certain that workers were grounded, they would test the footwear before entering the workplace.

9.4.9

Shoe Cleaners and Tacky Mats

Shoe cleaners and tacky mats are often used at the entrance to a change room. Many companies place shoe cleaners outside the entrance doors to their change rooms. Some companies place their shoe cleaner inside the outer change rooms. Both strategies can be effective. There are two primary problems with shoe cleaners. The primary difficulty is getting people to use the shoe cleaner. Many people avoid using the shoe cleaner because some shoe cleaners have a strong tendency to pull their shoes off. The second problem is making the assumption that the shoe cleaner has a built-in HEPA filter. This is not always true. If you do not buy a shoe cleaner with a HEPA filter installed or do not purchase an auxiliary shoe cleaner vacuum equipped with a HEPA filter, the shoe cleaner will shower the user in shoe dust every time it is used. Case Study: Shoe Cleaning The magnitude of the problem of using a shoe cleaner without proper exhaust or filtration can not be underestimated. In a cleanroom audit at a large semiconductor manufacturer, a shoe cleaner was found in the outer change room that was not connected to an external vacuum. The shoe cleaner may have contained an internal filter system, but time did not permit it to be disassembled for inspection. Such an inspection might be meaningless in any case, because if the internal filters were installed incorrectly, were damaged, or were beyond their useful life, visual inspection would not be able to detect these conditions. So the auditor waited in the change room with a portable particle counter to see what might happen if someone used the shoe cleaner. He didn’t need to wait long. Within 5 minutes a worker entered the outer change room, washed her hands, and used the shoe cleaner. The particle counts went from approximately 5000 particles/ft3 to over 5,000,000 particles/ft3 within a few seconds. Worse yet was the effect this had on the inner change room, where two-piece jump suits were being donned for entry into the cleanroom. Particle counts in the inner change room averaged less than 100 particles/ft3. But each time a person entered the inner change room after using the shoe cleaner, the particle count would increase more than 20 ft from the entrance door. Particle counts would spike as high as 10,000 particles/ft3. The airborne particles generated after using the shoe cleaner would follow people into the inner change room. Tacky mats come in two styles. The most common are pealable tacky mats: large sheets of plastic coated with pressure-sensitive adhesive. In many installations, permanent tacky flooring is used. If the cleanroom is in an ESD-protected work area, the ESD requirements

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may limit the location of pealable tacky mats. As a consequence, locations of pealable tacky mats are not always predictable.

9.5

BEHAVIOR IN A CLEANROOM

Behavior in the cleanroom begins at the entrance to the change room. It is imperative that anyone working in a cleanroom know and follow the rules. Anyone who does not know how to dress, where to get wipe-down supplies, and so on, is almost certain not to follow the procedures correctly. Therefore, when entering a new cleanroom it is imperative to check with the cleanroom management personnel to obtain dressing items and determine what procedures are to be followed. Change rooms do not always have proper signs, so one cannot depend on signs to tell you what to do in the room. A proper cleanroom management program will have signs instructing what must be done to enter and to exit the cleanroom. However, it is seldom the case that these signs are present or, if present, are accurate. There are many general guidelines to control behavior in the cleanroom. Here are a few: ●

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● ●

●

● ●

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Handle only your own cleanroom garments. Do not touch the outside of garments that are not yours when entering. Do not touch the outside of garments worn by others either in the change room or within the cleanroom. When you do handle your cleanroom garments, try to minimize touching the outside of the garments. Thus, when you remove a frock or coverall from its package, try to limit touching the garment to that of a single grasp at the collar. When handling your hood, touch only the cape. After putting on the hood, minimize touching the outside of the hood to those touches needed to adjust the fit using the snaps. When you put on your garments, do not let the garments touch the floor. When donning a garment, if you accidentally drop it on the floor, put it in the laundry hamper and get a fresh garment. Don’t touch your face or your hair after washing your hands. Many change rooms do not have a handwashing station. If the change room you are going to use does not have a handwashing station, after you have put on your shoe covers and your hairnet, wipe your hands with a cleanroom wiper moistened with water or alcohol using the wipe-down supplies. Carefully select your gloves, cleanroom hood, frock or jumpsuit, and knee-high booties so that they are of the appropriate sized. Minimize contact of your bare hands to the outside surface of cleanroom gloves. Wash your glove thoroughly and often. If glove washing stations are unavailable, use cleanroom wipers moistened with alcohol or DI water. Remembered to step on the sticky mats anywhere you encounter them.

Just before entering the cleanroom, check yourself in the mirror. The objective is to look for hair sticking out of the eye opening on the hood and/or exposed skin and street clothes. Adjust the mask to cover your face. Make certain that all the snaps are fastened, and make sure your gloves overlap the ends of the sleeves.

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The entrance to the cleanroom may contain an air shower. There are two types of air showers in general use: cabinet style and tunnel style. Cabinet-style air showers are like small closets with two doors. One of the doors opens to the cleanroom, the other opens from the change room. Tunnel-style air showers often have no doors. Both styles of air showers have high-velocity air jets mounted on the walls whose purpose is to dislodge loose contamination from the outside of the cleanroom garment before one enters the cleanroom. Many studies have shown that air showers are relatively ineffective at removing loose particulate matter from the outside of cleanroom garments. To aid an air shower in its effectiveness, you must follow several recommendations: ● ●

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●

●

Never put more people in the air shower than it is designed to accommodate. Always use air showers equipped with bipolar air ionizers. These can be either rapid pulsing air ionizers or continuous bipolar dc air ionizers. When in the air shower, do the air shower dance! In the dance you begin wiping the surface of the outside of the cleanroom garments starting at the top of your head and working down in the systematic fashion, using a brushing action with your gloved hands. Turn continually while doing the air shower dance to ensure that the high-velocity air jets blow over all areas of the body. Never open the air shower door before all pressure in the air shower is less than that in the cleanroom. Air showers with interlocks should be tested to verify that the interlocks work correctly.

Some companies use sticky rollers in addition to or in place of the air shower. The sticky rollers are used to remove loose contamination by wiping the outside of the cleanroom garment systematically, starting at the top of the head and working down in a systematic fashion [10]. 9.5.1

Working in a Cleanroom

Once inside a cleanroom, there are several important things to keep in mind: ●

● ●

●

●

Never touch exposed skin on yourself or others. If someone offers you a hand to shake that does not have a glove covering, do not shake the hand. Be continuously aware of the fact that you are in a cleanroom. Be aware of the airflow in the room, which you should be able to perceive by looking at the locations of the HEPA filters and the air returns. Never place yourself between the source of clean air and the product or process. Be aware of the fact that you generate turbulence when you move. Step away from contamination-sensitive products and processes before you move. Be purposeful in your actions and movements.

Remember that the airflow in the cleanroom is very fragile. The airflow from a HEPA filter is usually 0.4  0.05 m/s (90  20 ft/min). This may sound like a lot of airflow, but in reality it is only about 1.4 km/h (1 mph). The average person walks at a casual pace four to six times this fast [about 6 to 9 km/h (4 to 6 mph)]. The turbulent vortices rotating off a

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moving body rotate at the velocity of the moving body. Thus, when you walk through a cleanroom, you create turbulent flows with horizontal velocities that move at four to six times the maximum velocity of air coming out of the HEPA filter. Rooms with less than 100% ceiling filter coverage will have air velocities less than this value. Therefore, the turbulence that you generate when you walk around can easily overwhelm the clean airflow from HEPA filters. When you walk in front of clean benches, stay as far away from the opening of the bench as practical. When you walk around process equipment and products, stay as far away as you can. 9.5.2

HEPA Filters

Never touch the face of a filter. If you are performing a housekeeping activity, the HEPA filter and its protective screen must never be touched. If the face of a HEPA filter or its protective screen appear to be contaminated, as can happen if water stains occur, do not attempt to clean them. Bring this observation to the attention of cleanroom management. If the filters have become contaminated, they cannot be cleaned; they must be replaced. This is true for both ceiling-mounted HEPA filters and filters mounted in flow benches. Filters in flow benches are especially vulnerable to damage because of their close proximity to people and processes. 9.5.3

Raised Floors

If you must raise perforated floor tiles, observe the limitations for the facility regarding the number of tiles that can be raised simultaneously. Always remember to place the tiles back in their original location. If you must raise a floor tile, get your work done as quickly as possible and return the floor tile to its location so that the amount of time the airflow in the room is disrupted is kept to a minimum. If product is in the area, check with engineering to verify that the airflow disruption caused by raising a floor tile will not affect production adversely. 9.5.4

Glove Awareness

Do not rely slavishly on gloves. They do get soiled and they are easy to tear. Inspect them frequently, and if you find tears or pinholes, replace them immediately. Gloves can be replaced using several different strategies. Some facilities allow extra gloves to be brought to workstations. To change gloves in these types of facilities, first step as far away from the workstation as possible. Hold your hands well below your waist and remove your gloves. Put on a new pair. Some facilities have cleanroom gloves inside the cleanroom at a central location. In these facilities you must return to the glove station to change your gloves. Some facilities do not allow changing of gloves at all while in the cleanroom. In these facilities there are several options. One is simply to put on a second pair of gloves over the damaged pair. The other possibility is to go back to the change room, remove the torn gloves, rewash your hands, and put on a fresh pair of gloves.

9.6

PROCEDURES FOR EXITING A CLEANROOM

Many companies do a good job in training their workforce on how to enter a cleanroom, but many do not do a good job of training on how to get out of a cleanroom. The most common

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areas of concern identified in audits are the procedures for exiting a cleanroom and the garment storage procedures. These deficiencies in procedures are the result of improper training rather than a lack of compliance with requirements on the part of the workforce. One of the most common mistakes arises from a misinterpretation of the statement that garments should be removed in the opposite order from that in which they are put on. If this is interpreted rigorously, the first garment removed would be the gloves. This is a mistake. The surface of the hands after wearing gloves for up to two hours in a cleanroom can be heavily contaminated with perspiration, skin flakes, and other contaminants, so clearly, one would not want to touch the outside surface of the cleanroom garments with such hands. Despite this obvious fact, one of the most common mistakes observed among people exiting a cleanroom is that they take their gloves off first. Thus, it is apparent that exit procedures must be better specified. The rule about removing the cleanroom garments in the opposite order applies only to knee-high booties, frock or jumpsuit, and hood. One time when training contamination control. I was confronted by a challenging statement: “After I have been in the cleanroom for two hours, my gloves are so dirty that I don’t want to touch my cleanroom garments!” My response was very simple. “If your gloves were that dirty, you should have changed them while you were working in the cleanroom. You should not be touching product with gloves that are so dirty that you would not want to touch your cleanroom garments.” 9.6.1

Knee-High Booties

Knee-high booties should be removed first. If they are to be stored in a bag, the soles should be placed against one another and the tops should be wrapped around the booties. If the booties are to be stored in a locker or cubbyhole, they should not be wrapped. Instead, the sole of the booties should be placed on the bottom of the locker and the tops of the booties folded gently on top. If the booties are to be stored with hanging garments, care must be taken not to have the soles in contact with the exterior of the jumpsuit above the knee. Cleanroom garments generally have snaps on the lower half of the leg. This provides a convenient way to fasten the top of the bootie so that the sole of the bootie does not touch the leg above the knee. 9.6.2

Frock or Jumpsuit

Frocks and jumpsuits are removed after the booties. If a frock or jumpsuit is to be stored in a bag or cubby hole, it should be neatly folded so that it can be placed on the clean leggings of the booties. If the jumpsuit is to be stored in a locker or bag, the garment should be rolled or folded from the bottom up so that the collar is presented to the user the next time the garment is worn. If the frock or jumpsuit is to be stored on a hanger, it should be zipped or snapped to secure it to the hanger. The booties can then be fastened to the legs of the garments and snapped in place. 9.6.3

Head Covering

The next thing to be removed is the hood. Care should be taken when removing the hood not to remove the hairnet, if possible. In addition, care should be taken not to remove the hood when standing next to someone still dressed in the cleanroom garments. Removing the hood results in a shower of hair particles that can easily contaminate the outside of another’s garments. The hair should not be shaken or tossed to fluff it up after the hood is removed. If the

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441

hood is to be hung with a jumpsuit or frock, the hood should be snapped, clean side against clean side, to the collar of the jumpsuit. Several common mistakes are committed when handling hoods. Because people are often uncomfortable in a cleanroom hood, they often remove the hood before removing the frock or jumpsuit. Clearly, this is not a wise practice. When they handle their cleanroom garments, people often stop thinking about which side is clean and which side is dirty. As a consequence, they often handle their garments by placing clean sides against dirty sides. One of the most common mistakes observed is people stuffing the hood into their jumpsuit or frock and securing it inside the sleeve. If cleanroom garments are stored in a cubbyhole or locker, the inside of the cubbyhole or locker must be cleaned regularly. This is best done when the cleanroom garments have been collected for laundering. While the cubbyhole or locker is empty, it easy to wipe the interior. 9.6.4

Hairnets, Gloves, and Disposable Shoe Covers

No additional cleanroom items should be removed while in the change room. At this point, one should exit the change room: then the hairnet, gloves, and shoe covers can be removed and disposed of. If it is not practical to exit the change room before removing the hairnet, gloves, and shoe covers, move as far away as possible from people dressed in their cleanroom garments before doing so.

9.7

RELATIONSHIP BETWEEN ATTIRE AND CLASS ACHIEVED

There is a strong relationship among the design of the cleanroom (and its expected class) and the attire and behavior of the occupants. In terms of cleanroom design, there is an expectation that there is a relationship between the number of air exchanges per hour and the class of non-unidirectional-flow cleanrooms. The effect of the attire and behavior of occupants on the performance achieved can easily be demonstrated in both unidirectional- and non-unidirectional-flow cleanrooms. Table 9.4 is based on fully operational stage 3, certifications of various cleanrooms located in the United States, Canada, Mexico, Germany, England, Singapore, and Thailand. In some cases these data were obtained through annual test and balance surveys performed over a relatively shot period of time, but conforming to the requirements of FED-STD-209D and E (the standards in effect at the time). That is, the statistical requirements for the number of sampling points, selection of particle counter size resolution and flow rate, number of samples, and so on, were satisfied. In every case, the number of sampling points required was first allocated to critical and busy locations of the room, providing at least two sampling points at each critical and busy location. For example, four sampling points would be allocated to an in-line wafer coater: two at the loading station and two at the unloading station. The area covered by the critical and busy sampling would then be subtracted from the remaining area of the room. The number of sampling points for the remaining area of the room would then be calculated and the locations would be selected as a uniform grid over the remaining area. Figure 9.16 illustrates how this sampling plan would work. Parts are contained in closed boxes unless being loaded or unloaded. The load station is critical because contamination on the exposed parts before coating will produce defects in the coating. Parts at the unloading station are also critical, because contamination on the dried coating will produce defects in the

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TABLE 9.4 Observed Relationship Between Attire and Cleanroom Performance Achieved Cleanroom Characteristics Type Non-unidirectional, grade-level floor

Unidirectional, perforated raised floor

Unidirectional, perforated raised floor in room, grade-level floor in tunnelized portion of room

Air Changes/h

Class Expected

⬃20

100,000

⬃20

100,000

⬃60 ⬃60

10,000 10,000

⬃120 ⬃120

1000 1000

⬃500

100

Frock, executioner hood, gloves

⬃500

100

⬃500

100

Frock, executioner hood, shoe covers, gloves Jumpsuit, hood, knee-high booties, gloves, full containment hood

⬃120 in room, ⬃500 in tunnel

1000 in room 100 in tunnel

Attire Jumpsuit, hairnet, gloves Jumpsuit, open-face hood, face mask, knee-high booties, gloves Frock only Jumpsuit, open-face hood, face mask, knee-high booties, gloves Frock, hood, face mask Jumpsuit, hood, face mask, knee-high booties, gloves

Frock, executioner hood, shoe covers, gloves

Class Achieved 100,000 1000

10,000 1000

1000 100

100

10

1

100 in room, 10 within tunnel

subsequent photolithography. The operator at the loading–unloading station spends time at each of two locations: in front of the workstation and the adjacent WIP cart. Sampling at each of these critical areas required three locations, whereas only one sample location would be required for each on the basis of floor space alone. This area was thus sampled with four more sample points than would be required using a uniform grid as specified in FED-STD-209. This approach to selection of sample locations will tend to reveal the effect of operator attire and discipline more clearly than will sampling based solely on a uniform grid of sample locations. For each expected class of non-unidirectional-flow cleanroom, more elaborate attire returned benefits in terms of improved airborne particle count cleanliness. In the unidirectional-flow cleanrooms, the effects of changes to attire and discipline are less apparent but still measurable. The effect of tunnelization of a unidirectional-flow cleanroom is most apparent, showing the effect of optimizing airflows by changing the room layout.

PROCEDURES FOR ENTERING AN ESD-SAFE WORK AREA

443

= Sample Location

Cleanroom Wall

Normal Unloading Operator Positions (1 Operator, 2 Positions)

Unloading Station (Not Enclosed)

Work in Progress – Parts in Boxes on Carts

Coater – Fully Enclosed

Loading Station

Normal Loading Operator Positions (1 Operator, 2 Positions)

FIGURE 9.16

9.8

Sampling location for a photoresist coating operation.

PROCEDURES FOR ENTERING AN ESD-SAFE WORK AREA

The general procedure for entering an ESD-safe work area is far less elaborate than that for entering a cleanroom. The hands should be washed with soap and water to remove oils, food particles, and hand lotion. If hand lotion is needed to correct for dry skin, only a lotion approved for use should be used because many of the chemicals in nonapproved hand

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FIGURE 9.17

A toe grounder is worn with high-heeled shoes or cowboy boots.

lotions could create shorts on sensitive electronic circuits, either by forming a conductive path or by absorbing moisture. Upon entering an ESD-protected work area, the appropriate footwear should be put on and tested. ESD footwear should be worn on both feet. Footwear includes heel grounders, toe grounders (Figure 9.17), disposable shoe covers with conductive ribbon, dedicated ESD work shoes, and cleanroom booties, among others. Then the footwear should be tested, one foot at a time, to ensure that both footwear grounders work. (Testing both feet at once could mask a bad footwear ground.) If one or both pieces fail the test for high resistance, an attempt should be made to clean them with a moist towel. After cleaning, most footwear will pass the resistance test. If the footwear fails for low resistance, it must be replaced. Next, the wrist strap is put on. Wrist straps must be worn in direct contact with the user’s skin. Wearing a wrist strap over gloves or clothing interferes with the performance of the wrist strap and the measurement made by the wrist strap tester. It is important to include skin in the test, as it is charge on the body that the wrist strap is trying to neutralize. If the resistance of the wrist strap test is too high, the skin may be too dry or the resistor in the wrist strap may be defective. If moistening the skin on the wrist does not correct an excessive resistance condition, the wrist strap cord should be tested using a conventional voltohmmeter to determine if the resistor is damaged. If the resistance is too low, the resistor is damaged and the wrist strap must be replaced. Wearing a wrist strap with resistance that is too low is a safety hazard. The footwear test and wrist strap test should be run before the wearer is wearing gloves or finger cots. Even a static-dissipative glove or finger cot can cause the test to fail. Finally, the rest of the clothes are put on: frocks, hairnets, and gloves or finger cots. 9.8.1

Behavior in an ESD-Protected Work Area

Behavior in an ESD-protected work area is very similar to the requirements for behavior in a contamination-controlled work area. People must be aware of the restrictions on their

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activities at all times. When approaching an ESD-safe area, a person must understand the requirements for grounding. If the workstation is a stand-only operation, ESD grounding footwear will be required and the floor must provide a ground path. If the person can sit at the workstation, the person must understand that grounding using a wrist strap is mandatory. Handling ESD-sensitive product places an additional restriction on normal product handling. It is the responsibility of the person handling the ESD-sensitive product to ensure that it is in the proper packaging when it is moved from workstation to workstation. Within a fully protected ESD work area this may mean no more than placing the product in a staticdissipative tray. However, in a work area where ESD-safe workstations are not separated by grounded floors, it may be necessary to enclose the ESD-sensitive product fully within a shielding package before moving it from workstation to workstation. 9.8.2

ESD-Safe Work Area in a Cleanroom

If the ESD-safe work area is located within a cleanroom, several other restrictions apply. Not only does the person entering the work area have to observe ESD requirements, he or she also has to observe contamination control requirements. This means not only complying with the restrictions for dressing in a cleanroom but also equipping himself or herself for working in an ESD-safe work area. This can complicate the procedure for entering the room. Most cleanrooms require that you wear gloves. Wearing a wrist strap on your wrist under a cleanroom garment while wearing gloves that are worn over the cuff presents a donning problem. The cord of the wrist strap must be threaded through the labyrinth seal formed by the cuff of the sleeve and the glove. This can make putting on a cleanroom garments a little more difficult. To gain entry into the protected work area, the resistance of the wrist strap must be tested. If this is done, touching the surface of the wrist strap tester with a gloved hand will cause the wrist strap test to fail. Therefore, a procedure must be developed that allows the wrist strap to be worn and tested and still accommodate the need to wear a cleanroom glove. Often this means the hand with the wrist strap is gloved and the other bare hand is used to complete the circuit on the wrist strap tester. The wrist strapcord dangling from the risk poses a hazard of damaging products and can restrict freedom of movement. One solution to this is to wear a cleanroom garment that has wrist strap slits at the waist. A wrist strap with a 10-ft cord is worn in place of a wrist strap with a 6-ft cord. The cord is fed through the slit at the waist before the garment is zipped up. The coil cord from the wrist strap is no longer dangling from the wrist, reducing the possibility of product damage by a cord dragging over the workstation. Another solution that is commonly used is to attach the ground cord to a snap on the outside of the garment rather than to a wrist strap. This works if the wrist strap test is passed by each person employing this strategy. A third solution, wearing the wrist strap on the ankle, is not recommended because a coil cord worn on the ankle might create a tripping hazard. In addition, it is more difficult to follow some rules for working in an ESD-safe work area. An important pair of rules is (1) plug in before you sit down, and (2) unplug after you stand up. If the wrist strap is worn on the ankle, it can be difficult to follow this procedure. Footwear grounders also pose a problem when the ESD-area work area is also a cleanroom. This problem is most frequently seen with knee-high booties. Several incidents have occurred where knee-high booties which had been assumed to be ESD-safe were found to be failing when tested using the footwear tester. In almost all cases this was found to be the result of the poor connection between the sole of the bootie and the nonconductive sole of the street shoe.

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9.9

GARMENTS AND LAUNDRY SERVICES

Often, there is a lack of understanding in the contamination and ESD control communities regarding the optimum garment system to be used to control particulate, chemical, and biological contamination and ESD from human operators. Personnel who are not completely aware of the latest developments in contamination and ESD control, with its wide range of design options, fabric alternatives, and laundry processes are asked to establish and supervise the garment control program. In addition, the choice of garments may be affected by the availability of an adequate facility to support the various gowning options. The interaction between the change room layout and garment selection is explored in Chapter 10. Here we explore some of the factors that must be considered in selecting cleanroom garment systems for many different applications. The requirements for cleanroom garment systems change as products evolve. This discussion provides broad guidelines for garment system selection based on currently available fabrics, garment configurations, and cleaning and sterilization technology. 9.9.1

Garment Options

The type of garment used in any cleanroom application is open to user preference. The more tightly packaged the personnel working in the cleanroom, the fewer particles they will produce and the cleaner the room will be. Many options exist in the selection of what personnel will wear: ●

● ●

●

●

● ●

Head coverings, including hairnets, open-face hoods, eyehole-only hoods, and full containment hoods Body coverings include frocks and two- and one-piece jumpsuits (coveralls) Footwear, including disposable shoe covers, dedicated cleanroom shoes, and knee-high shoe covers Facial coverings, including beard covers, face masks, surgical masks, face shields, and full-containment face shields integral to hoods Knitted, barrier palm, and barrier gloves (includes glove liners, gloves worn as gowning gloves, and finger cots) Other items, such as undergarments Ancillary equipment, usually worn for personnel protection, such as goggles, safety shields, aprons, chemical safety gloves, hot mitts, or cold mitts, all of which are available in cleanroom versions

The selection of what to wear depends on the class of cleanroom performance required, the nature of the tooling used (e.g., isolation, SMIF), and whether or not the process is aseptic. Other factors that may affect the selection of garments include how the change room is furnished, if separate change rooms are available for men and women, and whether or not air showers are used. 9.9.2

Measurements of Garment Cleanliness

One of the most enduring arguments surrounding the selection of garments and laundry services is the method of measurement of the cleanliness of laundered garments. By contrast,

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447

measurement of the ESD performance of cleanroom garments has been relatively free of controversy. One of the earliest methods for measurement of cleanliness of cleanroom garments was ASTM Method F51. In this method, a specially designed filter holder is used to vacuum loose particle contamination from the surface of the garment. The filter is then examined using a light microscope and the particles and fibers are counted. This method has a number of drawbacks. Aside from the difficulty in getting reproducible counts from microscope operator to operator, it is extremely tedious. No one wants to do this analysis on a regular basis. The rotating chamber test method, also often referred to as the Helmke drum method, was subsequently proposed as an alternative way of measuring garment cleanliness that would be an alternative to ASTM F-51 [11]. Unfortunately, in the early 1990s, round-robin tests among different laboratories showed a distinct lack of correlation, and the rotating chamber test method fell out of widespread use. The method was withdrawn by the IES as a recommended practice and was suggested as a control method to be used within a given facility for tracking garment cleanliness rather than as a method for interlaboratory comparison. Unpublished studies found that one of the principal problems with the Helmke drum was the unfortunate tendency of the drum to become electrostatically charged, even when measuring garments woven with conductive fibers [12]. This happened because the metal drum was driven by insulated plastic rollers, so the metal drum was not grounded and would not surrender charge to ground. Tests were performed with grounded metal sampling tubes attached to an airborne optical particle counter. It was observed that each time the drum sparked to the particle counter sample tube, a burst of particle counts would be observed. (It was not determined if the burst was due to particles being shed from inside the drum or to particles generated by the electrostatic discharge.) The frequency of the sparking was a function of the distance between the metal drum and the sample tube. If the metal sample tube was close to the drum, sparks would occur more frequently, raising the particle counts, than if the sample tube locations were intentionally kept quite distant from the drum. The variability in sample tube location relative to the drum may have been a factor affecting laboratory-to-laboratory reproducibility in particle counts. By grounding the drum and ionizing the air entering the drum, the electrostatic discharge effect was eliminated and the reproducibility of the counts was greatly improved. The Helmke modified drum was also used to characterize contamination shedding from desiccant pouches. The method was found to be very repeatable and could easily distinguish among new desiccant pouches and those that had become contaminated because they had been used [8]. More recent studies have shown that the reproducibility of Helmke drum results may be better than concluded earlier, especially since improvements to the method have been incorporated [6, 13]. Other methods are used for measuring the cleanliness of garments from laundry services. Among these are extracting the garments in hot deionized water to measure extractable ion contamination, and laundering the garments with a standard test panel which would later be extracted ultrasonically and the particles counted using either turbidimetry or liquid-borne particle counting. Neither of these methods has been widely adopted. Particle generation from garment systems is also measured using a dispersal chamber. The dispersal chamber is a 4  4 ft 8-ft-high HEPA-filtered closet. A clothed person enters the dispersal chamber and goes through a series of exercises. An airborne optical particle counter is used to measure particle release from garments. This test is useful for comparing

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different garment systems. Its usefulness for interlaboratory comparisons is limited, due largely to differences among people performing the exercises. 9.9.3

Selection of Fabrics

The selection of fabrics for contamination and ESD control is now reasonably well understood. For ESD control, several options are available to provide a garment that can suppress electrostatic discharge. Many disposable polyolefin laboratory garments are treated with chemicals that suppress the accumulation of electrostatic charge and facilitate fabrication of the garments. In addition, if the ESD-protected work area is not also a contamination-controlled area, cotton laboratory frocks with woven-in conductive fibers can be used. Conversely, if the work area is simultaneously a cleanroom, garments woven from monofilament polyester with conductive fibers are required. In the cleanest cleanrooms (class 10 or cleaner), garments made from expanded PTFE membranes are often chosen. Care must be taken in the selection of these garments to ensure that they have static-dissipative qualities. Most cleanrooms use fabric garments. From a contamination perspective, woven fabric garments provide superior contamination control performance to knitted fabric garments. Garments with static-dissipative properties offer an advantage in the laundering process. A significant portion of particle removal in the laundering of garments occurs in the drying step. Garments without ESD properties tend to become charged in the drying process and will not release lint and other fibrous contaminants. Thus, garments with ESD properties provide a cleanliness advantage even if electrostatic discharge is not a concern in their use. 9.9.4

Design and Construction of Garments

In general, garments should be free of pockets, foldover collars, and other features that can trap contaminants. Garments should be sewn with synthetic multifilament thread. Cut edges of fabric should be heat sealed or bound to trap fibers from the cut edges. Zippers should have a generous overflap lined with interface fabric to prevent the overflap from getting jammed in the zipper. The ends of cuffs should be provided with elastic, knit, or snaps to provide a secure fit. Finished garments should be inspected carefully, as workmanship can vary greatly. The most common workmanship problem is failure to capture loose threads from the cut edges of fabrics. Misaligned zippers and missing snaps are also common problems. Garments should also be provided with snaps to allow for fine tuning the fit. This is especially important for adjustment of the collar and the head covering. Superior garments are designed to improve wearer comfort while simultaneously improving garment performance. An example is temple slits for eyeglass wearers, which allow the temple bars of glasses to be worn outside the eye opening of the head covering. This permits the eye opening to be adjusted tight to the face without pinching the bars of the glasses. Slits can also be provided at the wrist or waist to accommodate the cord from a wrist strap. All slits must be provided with an overflap. Features that function as a glove-securing mechanism are desirable. Extralong sleeves with length-adjustment snaps are desirable. Extralong knitted cuffs make securing a glove using tape or twist ties easier. Double cuffs, usually consisting of a knitted inner cuff and an elastic or snap-secured outer cuff, allow the cuff of the glove to be trapped in between, securing the glove to the wrist via a tortuous pathway. Another option is to use a tube of

REFERENCES AND NOTES

449

cleanroom fabric provided with elastic closures at both ends. These are sometimes referred to as croupier sleeves. 9.9.5

Selection of a Cleanroom Laundry Service [14, 15]

Almost all cleanroom laundry services today use water-based detergent cleaning. Occasionally, there is a need to remove soils using dry cleaning chemicals; this service should also be available from the laundry service. A full-service laundry will also maintain an inventory of extra garments. This allows for rapid replacement of lost garments or garments that are damaged beyond repair. Extra inventory allows for handling emergencies, such as evacuations caused by chemical spills, fires, or other incidents. In addition, the laundry should be able to adjust the mix of sizes that occurs due to changes in the working population. A laundry should be able to perform minor repairs. This includes replacement of snaps and zippers and repair of minor tears and punctures of the fabric. Tears should not be repaired by darning, since this results in large numbers of loose fibers. Small tears and punctures should be repaired by application of self-adhesive patches bonded to both the inner and outer surfaces of the garments. A good laundry service will have a variety of features to look for: ● ●

●

● ●

An on-site water purification system Bulkhead-mounted washing machines, with the load-in side located in a factory-air environment and the load-out side located in a cleanroom Bulkhead-mounted drying machines, with the load-in side located in the wash cleanroom and the load-out side located in the cleanroom for folding and packaging On-site facilities for garment repair and modification On-site laboratory capability to measure DI water purity, airborne particle counts, and garment cleanliness

Some garment laundry services offer on-site support to their larger customers. In these on-site support services, the inventory at the customer’s location is managed by personnel from the laundry service. This inventory management includes making certain that there is an adequate supply of clean garments. This is an especially important consideration in facilities where evacuation of the cleanroom requires a new change of garments on an unscheduled basis. The inventory management can include sorting of garments for return to the laundry by type (i.e., hoods, jumpsuits, knee-high shoe covers, etc.) and inspection of garments for tears and stains that are not repairable. REFERENCES AND NOTES 1. E. W. Moore, Contamination of technological components by human debris, Proceedings of the 29th Annual Technical Meeting of the Institute of Environmental Sciences, Los Angeles, Apr. 19–21, 1983, pp. 324–329. 2. M. Wilson, Microbial Inhabitants of Humans: Their Ecology and Role in Health and Disease, Cambridge University Press, New York, 2005. 3. Q. T. Phillips et al., Cosmetics in cleanrooms, Proceedings of the 29th Annual Technical Meeting of the Institute of Environmental Sciences, Los Angeles, Apr. 19–21, 1983. 4. R. W. Welker and M. Schulman, Contact transfer of anions from hands as a function of the use of hand lotions, EOS/ESD Conference Proceedings, 23:288–290, 2001.

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5. Unpublished company internal report. 6. IEST-RP-CC003.3, Garment System Considerations for Cleanrooms and Other Controlled Environments. 7. P. McPherson, D. Duggan, and J. Manguray, Evaluating the particle containment effectiveness of face masks and head gear, MCRO, 3, 1998. 8. R. W. Welker, previously unreported observations. 9. By contrast, the members of committees developing standards for the ESD Association are published in every standard, lending credibility to the balance of viewpoints represented. 10. In one systematic study of sticky roller versus air shower effectiveness, the sticky roller was found to be approximately 50 times as effective as an air shower. This data has previously not been published. 11. G. E. Helmke, A tumbling test for determining the level of detachable particles associated with clean room garments and clean room wipers, Proceedings of the 28th Annual Technical Meeting of the Institute of Environmental Sciences, Los Angeles, March 25–27 1982, pp. 218–220. 12. K. Adams, and M. McSwain, Controlling ESD and particle contamination in disk drives with grounded garment systems, Advancing Applications in Contamination Control, Sept. 1999, pp. 11–14. 13. J. M. Elion, Improving the repeatability and reproducibility of the Helmke drum test method, Journal of the Institute of Environmental Sciences, May 2002, pp. 20–23. 14. P. Travis, G. Shawbar, and L. Ranta, How to choose a cleanroom laundry, Proceedings of the Annual Technical Meeting of the Institute of Environmental Sciences, Apr. 23–27, 1990 New Orleans, pp. 355–358. 15. C. W. Weber and J. M Wieckowski, The effects of variations in garment protection on cleanroom cleanliness levels, Journal of the Institute of Environmental Sciences, Nov.–Dec. 1982, pp. 13–16.

CHAPTER 10

LAYOUT OF CHANGE ROOMS 10.1

PRINCIPLES OF EFFICIENT CHANGE ROOM DESIGN

The general purpose of a change room is to provide an interface between the dirty factory environment and the cleanroom. Within the change room, personnel gradually decontaminate themselves for entry into the cleanroom. Cleanrooms are always kept at a positive pressure with respect to the factory. Good change room design is to keep the room at a positive pressure with respect to the factory but at a negative pressure with respect to the cleanroom. The design and layout of efficient and effective change rooms is an area of general weakness in contamination and ESD control practice. In general, change rooms are undersized and not designed to manage the flow of personnel traffic. They are not equipped to handle the surge of personnel traffic that occurs at critical times of the day: shift changes and breaks, when large numbers of people will be trying to use the space at the same time. Personnel using a change room should be able to move through the change process with minimal delay. Delays caused by crowding or unnecessary queues to perform transactions, such as handwashing or drying, lead both to productivity losses and to personnel skipping operations, such as handwashing. It is difficult to estimate the effect of skipped operations on degradation of cleanliness and resulting yield loss, although these effects certainly must occur. Conversely, the relationship between unnecessary queue time and loss of productivity is readily measurable. This is shown for a relatively controlled study in Figure 10.1. This chart was developed based on a time-and-motion study of the number of people using a change room and the size of the change room in terms of the number of square feet per person per shift. This study focused specifically on shift changes, when large numbers of people would be attempting to use the facility simultaneously. That is, the change room would be expected Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

451

452

LAYOUT OF CHANGE ROOMS

20 18

Excess transaction time (min)

16 14 12 10 8 6 4 2 0 0

1

2

3

4

5

6

7

8

Square foot/person/shift

FIGURE 10.1 occupancy.

Excess transaction time for entering the change room during a shift change vs. room

to be crowded because people would be entering the room simultaneously at the end of a shift and at the beginning of another shift. The study was also limited to change rooms in which people dressed following the same protocol, so complexity of change protocol should not be a factor. The protocol for these rooms was as follows: 1. 2. 3. 4. 5.

Buff shoes. Wash and dry hands. Don hood, jumpsuit, and knee-high booties or frock and shoe covers. Test footwear. Don gloves.

People tend to flow though a generously sized change room more quickly than through an inadequately sized change room. Change rooms with inadequate floor space tend to impede the movement of people though them, and the problem becomes more acute as rooms decrease in size. The relationship between the time it takes to get through the change room and the size of the room becomes more significant as the room size decreases. Using the data plotted in Figure 10.1, an estimate can be made of the loss of productivity. Assume that the operation of the cleanroom includes a morning break, a lunch break, and an afternoon break. This will result in eight periods of the day when the change room is under high occupancy: 1. Initial entry. During this period, the change room may have a double-occupancy load: persons exiting from the previous shift may be in the change room at the same time

PRINCIPLES OF EFFICIENT CHANGE ROOM DESIGN

2.

3. 4. 5. 6. 7. 8.

453

as personnel for the subsequent shift. Conversely, companies that do a ‘hand-off’ at the workstation will avoid this double-occupancy penalty. This policy, while avoiding bottlenecks in the change room, can have a productivity penalty, since two operators will be at each workstation during hand-off. Exit for morning break. Teams of operators tend to take breaks together. This behavior is encouraged by many high-volume manufacturers to ensure that operators take their mandatory breaks and to encourage team camaraderie. Entry after the morning break. Exit for lunch. Entry after lunch. Exit for the afternoon break. Entry after the afternoon break. Exit at the end of a shift.

Now assume that the change room is sized to provide 2 ft2 of floor space per person per shift. If the size of the change room could be increased to 2.5 ft2 per person per shift by eliminating unnecessary furnishings or moving some operations (e.g., relocating the shoe buff machine outside the change room), the average person will spend approximately 1.5 minutes less time during each change. This becomes a 12-minute per person per shift increase in productivity where there are eight changes. Remodeling the change room completely to increase the floor space to 4 ft2 per operator per shift saves an average of 3 minutes per change, yielding an increase in time available for production of 24 minutes per person. Subsequent observations validated this observation. In a later study, the queue time was measured by timing how long the interval was between the first person entering the change room and the last person exiting from the change room. Dividing by the number of persons using the change room, we can then estimate the average change time vs. the square footage per person. As can be seen from the Figure 10.2, there is a strong relationship between the average square footage in a change room and the average transaction time per person in a change involving a large group of people. It can also be seen that there is a diminishing return on investment: By the time the change room provides 3 ft2 or more per person, there is little improvement by providing greater space per person. The general trend shown in Figure 10.1 is supported. The more crowded the change room during shift change, the longer it takes for the workers to get through the change room. Even more startling is the trend toward very small change rooms. The productivity penalty predicted by the upward trend shown in Figure 10.1 is supported by the data in Figure 10.2 [1]. The smaller the change room in terms of square feet per person per shift, the longer the excess time required for a person to enter the cleanroom. Of course, neither of these studies considered the effect of improving the change room layout, which could only be expected to improve the transaction time. A change room should be designed to reflect the change procedure. This design should present stations where each change activity to be performed and the sequence in which they are to be performed. The actions to be performed at each station should be posted as written and illustrated instructions. Posted instructions reinforce the training for the change procedure. In addition, it becomes intuitively obvious what is to be done, because each change activity is presented in the desired sequence. The design should also try to separate personnel entering the work area from personnel exiting the work area. One of the earliest proponents of this strategy for change room design was Konrad Stokes [2]. This separation

454

LAYOUT OF CHANGE ROOMS

25

Transaction time (min)

20

15

10

5

0 0

1

2

3

4

5

6

7

8

9

Square feet/person/shift

FIGURE 10.2

Expanded study of change room occupancy and transaction time.

of personnel movement prevents collisions and blockages from developing, but introduces some special design problems. Several examples follow to illustrate these two principles. 10.2

CASE STUDIES: CHANGE ROOMS

Case Study 1: Simple Linear Change Room The change room in this case study was designed for an ISO class 5 (FED-STD-209 class 100) vertical unidirectional-flow cleanroom on a perforated raised floor with a 2-ft subfloor plenum. The change room is designed to accommodate a shift change of 140 persons (70 per shift). In this first example, the change procedure is as follows: 1. Buff shoes. 2. Remove outerwear (jackets, sweaters, etc.). 3. Deal with shoes. (Regular employees change from street shoes to dedicated cleanroom shoes; visitors put on disposable shoe covers.) 4. Wipe down equipment. 5. Wash hands. 6. Don garments: executioner-style hood, jumpsuit, knee-high booties. 7. Test footwear. 8. Test wrist strap. 9. Don gloves. 10. Wash gloves. This change room had been retrofitted from an older cleanroom, replacing a change room that was severely undersized. The general layout of the change room is shown in Figure 10.3. The shape of the change room was dictated by the layout of the tool set and process in the cleanroom. The remodeling cost of stripping out the old change room and constructing the new

CASE STUDIES: CHANGE ROOMS

455

Equipment service area To rest of cleanroom process

Glove wash area

Inner change room

Process Area

Return Plenum

Middle change room To rest of cleanroom process

Outer change room Outside landing

Ramp

FIGURE 10.3 The long narrow change room described was dictated by the shape of the process area within the cleanroom.

change room was recovered in less than 70 days, due to productivity increases, because workers could get into the cleanroom much faster than before. The top of the ramp outside the change room outermost door was provided with a shoe buff station. The shoe buff was located outside the outermost change room door to conserve valuable change room floor space and to minimize the tendency of shoe debris to become airborne within the change room. All street shoes would be expected to harbor street dust and should be buffed prior to room entry into the change room. In addition, the top of the ramp was equipped with a coat rack to accommodate suit jackets, sweaters, and other outerwear that would be uncomfortable under cleanroom garments. This is shown in Figure 10.4. After buffing shoes, the outer change room was entered. This area was an ISO class 7 (FED-STD-209 class 10,000) cleanroom with loop pile carpet on the floor and floormounted return grates under the shoe change benches. Operators working in this area were required to wear safety shoes, so shoe change lockers were provided. Benches were provided along the wall for shoe change. Visitors wore disposable ESD shoe covers. The shoe lockers divided the room in half and could be accessed by a key from either side. Division of the room into two paths allowed for separate entrance and exit paths. The shoe change area was carpeted for comfort. A floor plan of this outer change room, dedicated to the shoe change, is shown in Figure 10.5. This area includes a hairnet and face mask dispenser. A mirror is provided to assist in proper donning of the face mask and hairnet. The wipe-down station was provided with a

456

LAYOUT OF CHANGE ROOMS

In Out

Powered shoe cleaner

Coat rack and package shelf – accessible from both sides

Landing

Ramp

FIGURE 10.4 Landing outside the change room. The shoe cleaner was placed outside the change room to minimize dust generation in the change room. The coat rack and package shelf were intended for use by visitors who did not have locker space or an office for their coats, sweaters, and other items not to be taken into the cleanroom.

flip-down table, wash bottles containing isopropyl alcohol, bonded edge, polyester cleanroom wipers, swabs, and a safety waste container. (Note: Large objects were bought into the cleanroom via a separate pass-through facility.) Personnel would then pass through a door into a handwashing area equipped with five sinks and 10 hand dryers. (The old facility only had floor space for three sinks and three hand dryers, creating an unnecessarily long queue for working population of this size.) The handwashing area was on the entry side of the middle section of the change room. The exit side of the change room was the laundry disposal area of the change room. This laundry area served two purposes. It contained multiple bins for the operators to sort their hoods separate from their jumpsuits. It also provided storage for a janitor’s closet. Finally, it provided storage area for backup garments that might be needed in case of a cleanroom emergency evacuation. Details of the handwashing area are shown in Figure 10.6. This area was designed following ISO class 7 design rules but was kept at a positive pressure with respect to the stage 1 shoe change area. This area was provided with a perforated raised floor. The floor immediately after the entrance door was provided with disposable sticky mats. The

CASE STUDIES: CHANGE ROOMS

457

In Chemical waste container Out Shoe chang bench

Shoe change lockers – doors on both sides

Wipe-down supplies

Shoe change bench

Mirror

Dispenser for face masks, hair nets and shoe covers Trash cans

In Out

FIGURE 10.5 Shoe change area. This is the first area of the change room, where shoes are changed or disposable shoe covers are put on. It also is equipped with a wipe-down area and hazardous waste receptacle for disposing of solvent-laden wipers and swabs.

entrance from the handwashing area to the inner change room was provided with an automatic sliding glass door. The personnel would then pass through an automatic sliding door into the gowning area. There they could choose a new garment from the wall-mounted dispensers or could reuse a garment from the hanging garment storage. If using new garments, a trash container was provided for disposal of the packaging material. The hanging garment storage divided the room into two areas, as did the shoe lockers encountered previously. The hangers were accessible from either side. This way an operator could put his or her garment on a hanger after exiting the cleanroom on the exit side of the change room and then retrieve it from the hanger on reentry on the entry side of the change room. Occasionally, personnel would accidentally allow a garment to touch the floor. This required that the garment be placed in a soiled garment bin and be replaced by a fresh garment. There were purposely no soiled garment bins on the entry side of the room, to reinforce

458

LAYOUT OF CHANGE ROOMS

Sliding glass door

In Out

Janitor’s closet, with ladder and deep-sink

Soiled laundry hampers

Emergency garments

Out

Hand sinks and hand driers

Tacky mats

In

FIGURE 10.6 Middle change area, including the handwashing area of the entrance and the service area of the exit. The service area contains the janitor’s closet, the soiled laundry hampers, and the emergency garment closet.

the one-way flow of traffic. The operators were instructed to place any garments soiled on entry on the floor under the hanging storage. The housekeeping personnel or operators (on their next exit from the cleanroom) would then retrieve them and sort them into the soiled garment bins located in the exit side of the change room. The room was provided with a path for housekeeping personnel to perform their duties. This would require assess to both sides of the change room. A tacky mat was provided on the access path floor, since the housekeeping staff would frequently be on the exit side to access the janitorial and supplies closets and would need easy access back into the inner change room. The entry gowning room was provided with several mirrors so that personnel could check themselves frequently during the entry process. This is shown in detail in Figure 10.7. The entry from the inner change room to the glove wash area, which was considered part of the change room, was via an automatic sliding glass door. The personnel would then enter the glove area through a sliding glass door. Here they would first test their footwear and wrist straps. Then gloves would be put on and washed and dried. This area was open to

CASE STUDIES: CHANGE ROOMS

Out

459

Sliding glass door

In

Mirror

Bench for taking off booties

Bench for putting on booties Waste can

Hanging garment storage – accessible from both sides

Garment dispenser

Housekeeping access

Sliding glass door

Tacky mats

In Out

FIGURE 10.7

Stage 3 of the change room: the inner change room.

the cleanroom. If a glove was torn and had to be replaced, the glove supply was accessible without the need to exit the cleanroom. This area is shown in detail in Figure 10.8. A photograph of the glove washing station is shown in Figure 10.9. On exit from the cleanroom, personnel would enter the gowning area fully gowned. Booties, jumpsuits, and hoods would be taken off and hung for reuse another time. (There was no garbage can or mirrors in this area, as shown in Figure 10.7.) This was intended to discourage messing with the hair or removing gloves. Personnel would then proceed past the soiled garment bins to the shoe change area, where they would change back into their street shoes. Just before leaving the change room a garbage can was provided for removing gloves. A mirror was provided here in case someone wanted to comb or brush his or her hair before leaving the change room. Case Study 2: Support Outside the Change Room—Break Areas and Rest Rooms The ISO class 7 cleanroom in this case study contains forty-eight 3  5 ft ISO class 5 vertical unidirectional-flow clean benches. The room is on a grade-level floor. It is 15% covered by

460

LAYOUT OF CHANGE ROOMS

Glove driers

To cleanroom process

Wrist strap/footwear testers and glove dispensers

Glove wash

Sliding glass door

In Out

FIGURE 10.8

Glove wash area.

HEPA filters. The change room is designed to accommodate a shift change of 200 persons (100 per shift). In this case study, the change procedure is as follows: 1. 2. 3. 4. 5. 6. 7. 8.

Remove outerwear (jackets, sweaters, etc.). Wash hands in the wash room (not part of the change room). Buff shoes. Change shoes. Don garments: full-face hood and frock. Test footwear. Test wrist strap. Don gloves.

This change room was fitted into a very small area. The parts crib was already located in the area, although the door to the parts crib opened to the outside hall. Thus, operators would have to change each time they went to the parts crib. Parts and paperwork move frequently between the cleanroom and parts crib, causing a significant loss of productivity for this cleanroom operation. The area was so small that could not accommodate handwashing stations. In addition, the original break area was about 400 ft away. To better accommodate the personnel, washrooms across the hall were remodeled to provide entrance doors that

CASE STUDIES: CHANGE ROOMS

FIGURE 10.9

461

Glove washing station.

opened directly toward the change room. The location of the break room was changed so that the entrance was less than 80 ft away from the entrance to the change room. This is shown in Figure 10.10. Relocation of the breakroom permitted outerwear to be stored in the break room, reasonably close to the change room. The closeness of the break room eliminated the need to keep unnecessary items in the change room, thus reducing clutter and improving the overall cleanliness of the change room. Changing the doors to the washrooms made handwashing more convenient without the need to put handwashing stations inside the change room. This greatly encouraged compliance with the requirement for handwashing, since now the entrance to the washroom was on the way to the entrance to the change room. Putting a pass-through on the rear wall of the parts crib facilitated traffic and completely eliminated the significant number of violations in which personnel would forget to remove their cleanroom garments when going to the parts crib. The basic remodeling of the change room is shown in Figure 10.11. The shoe buffers were relocated to the hallway outside the change room entrance. After buffing shoes, personnel enter the shoe change area. This area was carpeted for comfort. Operators working in this area were required to wear safety shoes, so shoe change cubby holes were provided. Benches were provided along the wall for shoe change. The cubby holes for shoe change were located in three tiers of shelves below each bench. Visitors wore disposable ESD shoe covers.

462

LAYOUT OF CHANGE ROOMS

Restroom entrances

Break room (~ 400 feet away)

Restrooms

Offices

Parts crib

Machining area Cleanroom process area

Change room door

Parts crib door

(a) Restroom entrances Offices Restrooms Break room

Break room doors (~ 80 feet away)

Parts crib

Parts crib passthrough Cleanroom process area

Machining area

Change room door

Parts crib door

(b)

FIGURE 10.10

General area layout (a) before and (b) after remodeling.

Division of the room into two paths allowed for separate entrance and exit paths. The gowning area was divided in two by the hanging garment storage rack, providing separate entrance and exit paths. The gowning area was provided with a garment dispenser, waste receptacle, and several mirrors. A pass-through on the parts crib wall opened into the change room. Personnel then entered the cleanroom, where they test their footwear and wrist strap. Then they put on gloves. The footwear and wrist strap tester and glove dispenser were located inside the cleanroom, shown in Figure 10.12. A pass-through window was installed on the parts crib so that personnel could access crib services without the need to leave the cleanroom, as had previously been the case. They simply needed to go back out through the change room door to access the crib.

CASE STUDIES: CHANGE ROOMS

463

Shoe change benches – 3 tiers of storage underneath

Soiled garments Spare garments

Shoe buff Out Hanging garment storage

Main change room door

Out In

Parts crib In

Pass-through

Waste containers Garment dispensers

Wipe-down station

FIGURE 10.11

Change room design.

Wrist-strap footwear tester Glove dispenser

Out Sliding glass door

Change room

In

Mirrors

Waste container

FIGURE 10.12

Wrist strap and footwear test area in the change room.

Personnel would exit the cleanroom on the opposite side of the hanging garment storage, which was accessible from either side of the change room. The exit side of the change room was provided with soiled garment bins but no garbage cans. To get rid of their gloves, personnel first had to change back into their street shoes and exit the shoe change area. The waste containers were located in the hallway next to the shoe buffers, as shown in Figure 10.11. Case Study 3: Including an Equipment Maintenance Area and an Equipment PassThrough in the Change Room This case study deals with a hybrid cleanroom. It contains both ISO class 7 (FED-STD-209 class 10,000) and ISO class 5 (FED-STD-209 class 100)

464

LAYOUT OF CHANGE ROOMS

Proposed change room area

Class 100 area

Cleanroom Class 10,000 Area

Trucking Aisle – Forklift Traffic Offices, Break Rooms, Etc.

FIGURE 10.13 Overview of the design problem. The change room was very small for the number of persons to be serviced. In addition, the change room had to incorporate an equipment pass-through where wipe-down would be performed. The change room area also had to incorporate the equipment maintenance work area.

areas. The ISO class 7 area was intended for assembly. Parts from the class 7 area are moved into the class 5 area by passing through automated cleaning machines. The cleaner machines separated the class 7 area from the class 5 inspection area. The class 5 area of cleanroom contained approximately eighty 3  6 ft vertical unidirectional-flow clean benches. The room is on a 2-ft raised floor. Thus, it was necessary to provide a change room for personnel working in both class 7 and class 5 areas. The change room is designed to accommodate a shift change of 400 persons (200 per shift). In addition, maintenance personnel were required to store their garments in the same area with the garments from assembly personnel. This increased the number of hanging garment storage locations significantly. In the previous design for this cleanroom, maintenance personnel were remote from the change room. They kept their cleanroom garments in their area, and thus their garments were laundered infrequently. This led to significant contamination problems. Another design constraint was that the equipment pass-through needed to be near the equipment maintenance area. One final design problem: The main service aisle outside the change room was a forklift movement path. This complicated the placement of shoe cleaners, which are not easily portable. Figure 10.13 shows a general overview of the cleanroom and change room area. The space for this location was a rectangle, unlike the space in Case Study 2, but was very small on a square foot per person per shift basis. One of the challenges was to figure out how to provide hanging garment storage for approximately 650 garments. The usual design is to provide approximately 3 in. of hanging garment storage space for each garment. This would require approximately 160 ft of hanging garment storage. Unfortunately, the area was only 120 ft long. The solution was to divide the area into several zones, as generally illustrated in Figure 10.14. The entrance to the change room had to serve a dual purpose: accommodating both people walking both up and down the entrance ramp and accommodating equipment being installed in the cleanroom. Thus, the entry had to be custom designed, as shown in Figure 10.15. The next zone of the cleanroom was the shoe buff area. This area presented an especially complex traffic control problem and is illustrated in Figure 10.16. Figure 10.17 is an overview of the change rooms. The workforce is divided into two halves in this change room to accommodate the number of workers in the limited space. Half of the workers enter in one of the entry change

CASE STUDIES: CHANGE ROOMS

Equipment pass-through area

Double-door pass-through for equipment

465

To cleanroom

Change room

Equipment maintenance room

Ramp Personnel and equipment doors

FIGURE 10.14

Landing

Zoning for the entrance area.

Equipment wipe-down pass-through

Equipment access doors Personnel door

Landing

Ramp

Removable guardrail section

Temporary guardrail and warning signs during equipment moves

FIGURE 10.15 Detail of the equipment entry, which needed to accommodate personnel and equipment. The ramp was designed to allow removal of the guardrail to allow forklifts to raise equipment to the level of the landing. Temporary warning barriers would be placed to separate personnel from equipment when equipment was being loaded.

466

LAYOUT OF CHANGE ROOMS

Equipment pass-through and wipe-down area Shoe buff area In

Change area

Out

In

Equipment maintenance area

Soiled garment bins

Return plenum

Landing

Ramp

FIGURE 10.16 Shoe buff area. This area is enlarged by design because of the likely traffic problems. Persons entering the change room buff their shoes and enter one of two change room entrances. Persons exiting the single exit doors might need to deposit garments in the soiled garment bins. Thus, they may need to cross paths with 50% of the persons entering the change room. Meanwhile, personnel entering and exiting the equipment maintenance area participate in this confusion. Mirror

Garment Dispensers

Seats for change

Waste cans

In

Hanging garment storage – Out accessible from both sides

Waste cans

Garment Dispensers

In

Seats for change Mirror

FIGURE 10.17

Three inner change rooms.

ENTERING THE CLEANROOM

467

rooms and half of them enter in the other. All of the change options are presented in the order they are required. All exit their respective change areas to enter the cleanroom via sliding glass doors. After exiting the change room they test their wrist straps and footwear and put on cleanroom gloves. The personnel all exit the cleanroom via the central exit area. Garments are stored on hanging garment storage racks that are accessible from both sides. Booties are stored in cubbyholes underneath the hanging garment storage. This arrangement allows access to the garments during the next entry. It also allows 160 ft of hanging garment storage to be provided in only 80 ft of change room length. Another interesting feature of this change room is the soiled garment bins, located on an outside wall as part of the return plenum. Each had a swing-in door on the change room side to allow for garment disposal. Each also had a full-size door opening to the top of the ramp so that the soiled garment bins could be serviced without the need to enter the change room. The location of the soiled garment bins was shown in Figure 10.16. One final note is important. Personnel working in the equipment maintenance room could now move equipment into the pass-through and perform initial work in their street clothes. This would be done by accessing from the entrance side of the equipment passthrough. When the equipment was ready to move into the cleanroom, the personnel would dress for the cleanroom using the normal change room procedure and enter the equipment pass-through from the cleanroom side. They would then do a complete wipe-down on the equipment and move it into the cleanroom.

10.3

ENTERING THE CLEANROOM

To the maximum extent possible, change rooms should be designed to accommodate the change procedure required for the cleanroom. To design the change room it is necessary to know the change procedure specified. By way of review, the recommended entry practice is as follows: 1. Plan you trip into the cleanroom. a. Go to the washroom and remove any makeup. Wash your face and hands. b. Remove and store any outerwear and items you do not need to take into the cleanroom. c. Remove any items from your pockets that you might need to use in the cleanroom, such as cell phone, pagers, pens, and so on. d. Clean these items at the wipe-down station using the materials provided: wipers, swabs, and chemicals. 2. Do your pregowning actions. a. Put on a hairnet. b. Put on a face mask. c. Put on disposable shoe covers. 3. Wash your hands. 4. Dress in your cleanroom garments from the top down. a. Put on your hood. b. Put on your frock or jumpsuit.

468

LAYOUT OF CHANGE ROOMS

c. Put on your knee-high booties. 5. Finish dressing. a. Check yourself in the mirror. b. Use the sticky roller or air shower. c. Enter the cleanroom. d. Put on cleanroom gloves. e. Wash your gloves. This entry practice is for a cleanroom that is not designed to optimize control over viable contamination. Control of viable contamination can require additional steps.

10.3.1

Planning a Trip into the Cleanroom

The first element of the cleanroom entry procedure is to plan your trip into the cleanroom. You need to remove and store any outerwear and any other items that you do not need to take into the cleanroom. Effective change room design takes this need into consideration. The support areas surrounding the cleanroom and change room should be planned to facilitate the need for people to remove suit jackets, sweaters, raincoats, umbrellas, lunchboxes, briefcases, and so on. These storage areas should be as close as possible to the entrance to the change room. These areas are often referred to as break rooms. If they are placed conveniently close, they tend to encourage adherence to cleanroom discipline, which prohibits introduction of contraband materials and results in reduction in clutter in the change room. It is also necessary to wash up before entering the cleanroom. For most people this requires washing their hands and face to remove makeup and hand lotions that are prohibited from the cleanroom. If these washroom facilities have entrances placed convenient to the cleanroom entrance, it will encourage adherence to discipline. The second element of the cleanroom entry procedure is wiping-down. For the average person, wipe-down consists of identifying the items that you will need to use in the cleanroom. This might be a pocket pager, a cell phone, or a pen. Any item that will be used in the cleanroom must be carried into the cleanroom. Items in pockets under the cleanroom garments are not accessible while in the cleanroom because it is a violation of cleanroom discipline to reach under your garments while in the cleanroom. Thus, any item that is to be used in the cleanroom must be wiped clean in order for it to be carried into the cleanroom exposed to the cleanroom environment. Wipe-down extends to equipment and tooling to be moved into the cleanroom. These items must be decontaminated by wipe-down procedures. The most convenient way to provide for equipment access to the cleanroom is to provide a pass-through near the change room. Equipment decontamination can be done using wipe-down materials provided near the change room. After the equipment has had initial wipe-down, personnel can change into cleanroom garments and do final wipe-down before moving the equipment into the cleanroom. It is therefore necessary in the design of the change room entry to provide for a wipedown station that has a work surface for cleaning hand-carry items and to provide a wipedown station in equipment pass-through to facilitate equipment entry. It is also necessary to provide space for a waste container since the wipe-down process will generate spent wipers and swabs. Depending on the location where this work is done and the chemicals involved, this may require a hazardous waste container as well.

ENTERING THE CLEANROOM

10.3.2

469

Pregowning Actions

Pregowning actions include putting on a hairnet, a face mask, disposable shoe covers, and in some cases, a gowning glove. The sequence in which a hairnet and face mask are put on depends on the nature of the face mask and hairnet. These materials should be provided in the sequence by which they are to be put on. Disposable shoe covers or changing into dedicated cleanroom shoes should then be presented. At this point, hands should be washed before proceeding with the rest of the change. 10.3.3

Dressing in Cleanroom Garments

After the hands are washed and dried, the cleanroom garments can be put on. Dressing starts from the top and proceeds downward. This is the point in the design of a change room in which it is critical to separate people entering the cleanroom from those exiting the cleanroom. One of the most effective ways of creating the separation is to provide hanging garment storage as a divider between the entry and exit paths in the change room. This area must always provide access to clean garments, so garment dispensers must be present. This is necessary because occasionally, garments are dropped on the floor and must not be worn after they are on the floor. Replacement garments must be readily available. A waste container must always be present in this area to handle the packaging waste generated whenever a new garment is obtained. If knee-high booties are to be worn in the cleanroom, seating must be provided in the cleanroom dressing area. Similarly, seating must be provided on the exit side of the cleanroom dressing area in order to remove the knee-high booties. Several options exist for storage of the booties between wearings. Some users prefer to snap the bootie to the leg of the jumpsuit. Some companies prefer to provide a cubbyhole under the hanging garment storage. In either case, the ideal approach is to provide a space under hanging garment storage for soiled garments that are generated during the dressing in process. Alternatively, a soiled garment bin can be provided on the entry side of the change room. 10.3.4

Finishing Dressing

Now that personnel are wearing all of their cleanroom garments, it is necessary for them to check their appearance in the mirror. This check is done to verify that all of the snap enclosures are fastened correctly and to make certain that there’s no unnecessary exposed skin or street clothing after dressing. The next step in the process is to decontaminate the outside of the cleanroom garment. Many companies use an air shower for this purpose. A superior alternative to the use of an air shower to decontaminate the outside of a cleanroom garment is to use a sticky roller. Whichever method is chosen, provision must be made in the design of the change room to accommodate this step in the operation. After entering the cleanroom, gloves are put on. It is preferable to provide a glove station inside the cleanroom rather than in the change room because gloves must be changed frequently. Placing the gloves station inside the change room is inconvenient and results in a loss of productivity for operators. The final operation in the cleanroom entry procedure is to clean the cleanroom gloves after they have been put on. Thus, in the design of a cleanroom, consideration should be given to providing glove washing stations, either in the change room or located throughout the process.

470

10.4

LAYOUT OF CHANGE ROOMS

EXITING THE CLEANROOM

Generally, more procedural errors are observed as people exit the cleanroom than are observed when people enter the cleanroom. This is probably due primarily to improper training. To some degree a properly designed change room can help overcome many of the exit errors by layout and furnishings that encourage proper exiting behavior. A recommended procedure for exiting the cleanroom is as follows: 1. Exit the cleanroom. 2. Remove your cleanroom garments from the bottom up: a. Remove your knee-high booties and set aside. b. Remove your frock or jumpsuit. c. Remove your hood. 3. Exit the inner change room. Sort soiled garments into soiled garment bins. 4. Exit the middle change room. a. Remove and dispose of hairnet, gloves, and disposable shoe covers. b. Retrieve street shoes if wearing dedicated cleanroom shoes. c. Store dedicated cleanroom shoes in locker or cubbyhole. 5. Exit the outer change room. People are often taught how to exit a cleanroom but are not given adequate detailed instructions. This can be partially corrected by the layout and furnishing of the change room. After exiting the cleanroom, the personnel are in the inner change room. This is where the first exit mistake often occurs. A common instruction for exiting the cleanroom is that you remove your cleanroom garments in the opposite order in which you put them on. Most people hearing this believe incorrectly that this includes the cleanroom, gloves. One of the most common mistakes is to remove the gloves first, since these were put on last. After wearing barrier film gloves, the skin on the hands are probably covered with sweat, accumulated dead skin cells, and an enhanced complement of bacteria and fungi. One should never handle cleanroom garments with bare hands when exiting the cleanroom. The inner change room is the place where you remove knee-high booties and frock or jumpsuit, in that order, without removing anything else. The cleanroom garments are then stored if they are to be worn again. Storage of cleanroom garments is important, as improper storage degrades the cleanliness of the garments unnecessarily. The booties are removed first and set aside. A seating bench should be provided on the exit path of the inner change room as close to the exit door as possible, as shown in Figure 10.18. Placing the bench so that personnel encounter it first when they exit the cleanroom provides a reminder that the booties are to be removed first. The frocks or jumpsuits are removed second. These are hung immediately, clean side out, on an assigned hanger. If a jumpsuit is worn, the zipper must be closed to secure the garment. (If a frock is worn, the collar is snapped to secure it to the hanger.) The booties are then either placed in a cubbyhole below the hanging garment or snapped to the leg of the garment below the knee. Then the hood is removed and snapped to the collar of the frock or jumpsuit clean side against clean side. The person then exits the inner change room. The middle change room allows garments to be sorted into soiled garment bins, shown in Figure 10.19. Many facilities use snap-in

EXITING THE CLEANROOM

Out

471

Sliding glass door

In Mirror Bench for taking off booties

Bench for putting on booties Waste can

Hanging garment storage – accessible from both sides

Garment dispenser

Housekeeping access

Sliding glass door

Tacky mats

In Out

FIGURE 10.18 Inner change room from Case Study 1. Note the location of the bench and absence of a waste can in the exit path.

veils and allow these to be changed at every exit from the cleanroom. Facilities such as pharmaceutical companies with serious viable contamination concerns will often have personnel change garments on every exit as well. These are sorted into the soiled garment bins. Note in Figure 10.19 that the exiting personnel still have not encountered a waste bin. The person then exits the middle change room and enters the outer change room. Here a bench is provided for shoe change and personnel are provided a waste can for disposal of hairnets, face masks, and disposable shoe covers. A mirror could be provided in this area because people exiting this change room are completely isolated from people entering the cleanroom, but one is not shown in Figure 10.20. In change rooms where people enter and exit a single room, it is not uncommon to see someone arranging hair standing beside and using the same mirror as that used by a person checking cleanroom garments.

472

LAYOUT OF CHANGE ROOMS

Sliding glass door

In Out

Janitor,s closet, with ladder and deep sink

Soiled laundry hampers

Emergency garments

Hand sinks and hand driers

Tacky mats

In

FIGURE 10.19 Exit path through the middle change room in Case Study 1. Note that the exiting personnel still have not encountered a waste bin, reminding them not to remove their face mask, gloves, or shoe covers.

10.5

OTHER CONSIDERATIONS

The principles of change room design and layout emphasized are as follows: ●

● ●

Don’t sacrifice change room floor space. Change rooms that are too small create excess transaction time, leading to productivity loss and skipped operations. Lay out and furnish the change room in the order that the change is to take place. To the maximum extent possible, keep traffic entering the change room separated from traffic exiting the change room.

Other considerations are to control the relative pressurization of a change room to provide adequate signs to reinforce the change process. The change room must be at a negative pressure with respect to the cleanroom to ensure that air leaks from the cleanroom into the

OTHER CONSIDERATIONS

473

In Chemical waste container Out Shoe change bench

Shoe change lockers – doors on both sides

Trash cans

Wipedown supplies Shoe change bench

Mirror

Dispenser for face masks, hair nets and shoe covers

In Out

FIGURE 10.20 Outer change room from Case Study 1. This is where personnel exiting the cleanroom first encounter waste bins.

change room rather than vice versa. Furthermore, air should gradually leak out of the change room in multiple-room designs, reflecting the intent of the design. In this design, gradual decontamination of people entering the cleanroom was accomplished. Figure 10.21 illustrates this principle using the architecture in Case Study 1. One of the ways to control pressure in a step-down fashion like this is to provide adjustable dampers, which control leakage rates from each room to the factory-air environment. Most commercially available change room signs will include all steps on a single panel. This is appropriate with conventional change room designs, where all steps of the change procedure are done in a single room. However, this is inappropriate in a multiple-room change room design, where the change steps are distributed over several rooms. Further, the change steps can be distributed over several locations within a single room, further increasing the granularity of the signs required.

474

LAYOUT OF CHANGE ROOMS

Inner change room ISO Class 5 Pressure above factory air = 10 Pascal (0.04 inches of water)

Middle change room ISO Class 6 Pressure above factory air = 7 Pascal (0.03 inches of water)

Outer change room ISO Class 7

Cleanroom ISO Class 5 aisles ISO Class 4 process areas Pressure above factory air = 12 Pascal (0.05 inches of water)

Pressure above factory air = 5 Pascal (0.02 inches of water)

FIGURE 10.21

Relative room pressurization and class for the change room in Case Study 1.

REFERENCES AND NOTES 1. The data plotted in Figure 10.1 were observed in a single manufacturing facility in California that has relatively uniform requirements for dressing to enter the cleanroom. The data collected in Figure 10.2 were observed in facilities with varying garment strategies. The data plotted in Figure 10.2 were collected in Thailand, Germany, England, and locations in California, New York, and Minnesota. 2. K. H. Stokes, Cleanroom technology: change rooms—design and operation, Microcontamination, June 1987, pp. 12–18.

CHAPTER 11

PROCEDURES AND DOCUMENTATION

11.1

HIERARCHY OF DOCUMENTS AND AUDITS

Procedures and documentation are essential elements of the contamination control and the ESD control process. Procedures describe how things are done. Documents controlling procedures range from extremely high-level standards and specifications down to individual process instructions. An example illustrating the hierarchy of documents is illustrated in Figure 11.1. At each of these levels of documentation an audit can be conducted. The audit provides an assessment of compliance with requirements, helps to identify deficiencies, and can be used to guide corrective actions. There are several different approaches to audits. One of the most effective includes an operator self-audit, usually a simple visual inspection to verify appropriate levels of housekeeping or to verify the presence of items provided for protection of the workplace, such as monitoring equipment, ionizers, or ground wires. In a traditional approach, an independent quality assurance or management auditor enters the workplace and observes the conditions and activities, noting deviations. This is a form of noninstrument audit. In a third approach, a specially trained technical person surveys the work area using specialized instruments. This is termed an instrument audit. It is occasionally considered desirable to have a completely independent consultant or outside agency perform an audit, called an independent audit. The various audits can also be arranged into a hierarchical structure similar to the hierarchy of documents illustrated in Figure 11.1. A hierarchy of audits is shown in Figure 11.2. Any audit can result in reports of deficiencies and generation of corrective action plans. For a small facility, the ability of senior management to track these results is usually not a major problem. Conversely, for large facilities, which may involve evaluation of hundreds Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

475

476

PROCEDURES AND DOCUMENTATION

Hierarchy

Examples Example: ISO 14644

External Standards: International National Consensus Industry

Example: ANSI/ESD S20.20

Example: IEST- STDCC1246D

Agency Specific Standards: Corporate Government

Example: IBM

Example: NASA

Example: East Fishkill Location Tailored Standards Example: Kennedy Space Center

Example: Project Specific Contamination Control Plan Project Tailored Standards Example: Project Specific ESD Control Plan

Process Instructions

FIGURE 11.1

Examples: Build Instructions Rework Instructions Test Instructions

Typical hierarchy of standards and instructions.

(or perhaps thousands) of people and workstations, the volume of information is simply too large for management to absorb and take appropriate action. A management scorecard has been developed to facilitate collecting and processing audit data and to facilitate management understanding of both compliance and the effectiveness of corrective action.

11.2

OPERATOR SELF-CHECK

The operator self-check is the easiest audit to perform. This type of audit requires minimal instrumentation and minimal documentation. In the contamination control self-check, the person is responsible for dressing and verifying that they have completed the gowning procedure correctly. This is done by examining themselves in the mirror. In many companies, teamwork is encouraged. Where teamwork is encouraged, operators assist one another when dressing to make certain that gowning procedures are followed correctly.

OPERATOR SELF-CHECK

Hierarchy

477

Examples

Customer Certification Audits

Independent Consultant Advisory Audits

External Audits

ISO 9000 Surveillance Audits

Internal Instrument Audits

Contamination Control and ESD Control Surveys

Continuous Monitoring Systems Quality Assurance Witness of Operation Internal Visual Audits Operator Work Station SelfCheck

FIGURE 11.2

Typical hierarchy of audits and inspections.

A second part of the operator self-check occurs at the workstation. When operators arrive at the workstations they verify that the necessary contamination and ESD control features are in place. For example, they may be required to verify that the continuous airborne particle counter is functioning by looking at the power light on the local display, that sample tubes are connected, and so on. Contamination control fixes to control sources of contamination, such as covers in place or the presence of vacuum, need to be visually verified. Workstation wipe-down also includes an element of operator self-check. During the wipe-down process the wiper is examined frequently to verify that the surface does not leave a smudge or visible particles on the wiper. In addition, the visual cleanliness rule is applied to the work surface. This is usually done without the aid of magnification, but can be done using special illumination. In the ESD control self-check, operators usually must test their wrist strap and footwear. A wrist strap or footwear grounder that fails because of low resistance poses a risk of lethal electrical shock. For this reason, the operator self-check for wrist strap resistance is always documented. A volt-ohmmeter may be needed if daily checks are made of ac-powered tool resistance to ground and voltage leakage. The presence and secure attachment of ground wires is another visual verification. In this case, the individual wires are inspected by tugging on them to verify that they are attached securely. In moving equipment it is often necessary to supply a ground wire across articulated joints. The constant motion of the joint puts considerable strain on the ground wires and they fatigue rapidly. Most of the time these ground wires are light-gage multistranded wires. The individual wires break, and eventually all will fail. It is important that these be tested by tugging on them. If the multistrand wire is down to its last stand, it is better for the operator to break the last strand during inspection than to have it fail, unnoticed,

478

PROCEDURES AND DOCUMENTATION

during production. Other items can be inspected for proper aim of air ionizers and the absence of contraband material on the workstation. The greatest value of an operator self-check is to ensure that a workstation that is out of compliance with requirements is never brought into production. The self-checks are performed every time an operator begins to use a workstation. To make the operator self-check audit effective, several things must be in place: ●

●

●

When an operator identifies a condition that is out of compliance with requirements, e.g., an air ionizer that has failed, a broken ground wire, a particle counter that is no longer functioning, there must be someone available to take immediate corrective action. Any workstation that requires corrective action must be documented as to cause and corrective action performed. When an auditor finds a condition that an operator should have identified but has not reported, there must be consequences.

The best way to document these self-checks it to make them a step in the process instruction. The operator records successful completion of the visual inspection by initialing a traveler card, keying in a yes for computer-claimed data, and so on. Any finding other than compliance with requirements needs more detailed documentation than a simple checkmark. To assure that the operator self-checks are appropriate, process instructions must contain appropriate details, such as the following: ●

●

● ●

Wipe the wafer gripper with an Alpha 10 wiper wetted with IPA and blow dry with the ionized air gun. Wipe the SMIF pod nests with an Alpha 10 wiper wetted with isopropyl alcohol. Clean the groove around the elevator platform with a clean foam swab wetted with IPA and blow dry with the ionized air gun. Verify that five ground wires are present, and secure at the common point ground bus. Verify that airflow from the overhead ionizer can be felt at the product location.

Many of these instructions can be clarified using photographs. The ready availability of digital cameras has made photo documentation far more feasible than ever before. The following process instructions are vague and unacceptable: ● ●

Wipe the workstation. Check the ionizer operation.

Many institutions require the presence of quality assurance personnel during critical operations. The quality assurance “witness” of successful completion of a task is another form of visual inspection that may be documented independent of the operator record.

11.3

NONINSTRUMENT AUDITS

Noninstrument audits are usually performed by inspection personnel. These inspection personnel are often part of the quality assurance organization. These inspections are independent of the witness audit required by the critical nature of the process to be performed.

INSTRUMENT AUDITS

479

Noninstrument audits can be random or scheduled. Personnel performing the audit generally need to have special training to assure that deviations from requirements do not go undetected. In general, the quality assurance or other inspector needs to be familiar with what is expected of the operation from contamination and ESD control perspectives. This form of inspection does not require the use of measurement instruments. In this form of inspection the auditor is visually observing compliance with requirements. This form of audit cannot occur as frequently as the self-audit. It does serve as a double check to ensure that self-audits are producing meaningful results. The noninstrument audit usually includes verification that the operator self-audit has been documented and that a second inspection is made by the auditor. If an item is found to be out of compliance, there are several possibilities: ●

●

The operator failed to document successful completion of the self-check, but the workstation is in compliance. The operator documented successful completion of the self-check, but the workstation is found to be out of compliance with requirements during the noninstrument audit.

The latter case may require disciplinary action. However, it is equally likely that the process instructions are inadequate. This may include inadequate description of what the operator is expected to do during the self-check. It may also be that the procedure specified is not capable of achieving the desired result. For example, the workstation wipe procedure may need to be performed more often than specified in the process instruction in order to ensure that the workstation is always visibly clean. Noninstrument audits usually result in a written nonconformance report when a failure is identified. This form of documentation differs from the self-audit documentation. Self-audit documents are seldom checked unless quality assurance or other personnel review the document. 11.4

INSTRUMENT AUDITS

Audits using instruments provide quantitative results. Unfortunately, because they require instruments and a person who is trained to use the instrument, they cannot be performed as often as can either the noninstrument audit or the operator self-check. Instrument audits for contamination control almost always include the use of an airborne optical particle counter. In addition, the instruments to be used in the contamination control audit might include an air velocity meter and a differential pressure sensor. The auditor can also be ready with tape and microscope slides, SEM sticky stabs, viable contamination samplers, and so on. For facilities using continuous airborne particle sensors, it may also be necessary for the instrument audit kit to contain a flowmeter to verify that the continuous monitor is sampling air at the proper rate, and a zero filter, to verify that the sample lines are not contaminated. The auditor for a continuous particle monitoring system may also need a portable data station to verify particle counts at the sampling point when the central system display is not nearby. Instrument audits for ESD control almost always include a volt-ohmmeter, a surface resistivity meter, and a field potential meter. In addition, instruments for testing the wiring of ac outlets, wrist strap and footwear, relative humidity, and ionizer performance will be included routinely. Instrument audits always result in a written report, best performed using a checklist that acts as a reminder for the inspector and generates a report automatically. The frequency of instrument audits must reflect the risk of noncompliance in the work area.

480

PROCEDURES AND DOCUMENTATION

For example, in the aerospace industry the risk of using a workstation with an air ionizer that is out of balance will generally be perceived as low. As a consequence, air ionizers in the aerospace industry are generally calibrated once every six months. By contrast, the risk of having an ionizer that is out of standard in the disk drive industry is considered to be very high. As a consequence, in the disk drive industry the performance of air ionizers is usually checked monthly or more often.

11.5

INDEPENDENT AUDITS

Internal audits provide the most immediate assessment of performance. However, there is a need periodically to obtain an independent assessment using an outside independent audit authority. These independent audits have their own value, but there is a risk. If the outside audit authority does not evaluate the process based on the process requirements, but instead, bases the audit on external standards, there is a risk that the assessment will result in an audit failure. Conversely, the outside audit may overlook problems, if the auditor uses a more generous standard than that which is appropriate. This failure may be meaningless for the process if the requirements of the external authority are not in agreement with the internal requirements. The issue of independent audits has been brought to light by the need for conformance to international standards. There are often disagreements between different national standards. The International Standards Organization (ISO) has attempted to create a standard that unifies all national standards. Even within a given country, the accepted standards will not have uniform requirements. Table 11.1 shows disagreements between two external ESD control standards and an internal control standard, JPL D-1348 Rev. F [1] that illustrate this point. Most standards state that they are tailorable. It is therefore important for an independent audit to evaluate the performance in any given workplace based on the internal workplace standard to which they are expected to adhere. Independent audits can also be performed by consultants. These audits are usually advisory in nature and can be very useful. Consultants can bring a wealth of experience in performing audits in many different customers’ ESD-protected work areas or cleanrooms. This provides the independent consultant with insights that might not be available to the internal contamination or ESD control auditors. In addition, the independent consultant often can identify problems that go unnoticed by internal audits because they are so familiar to the internal auditors.

TABLE 11.1 Comparison of Some Requirements for ANSI/ESD S20.20, JEDEC ESD 625A, and JPL D-1348 Rev. F ESD Control Standards Requirement Applicability Field potential limit Ac-powered tool tip to ground When are personal grounders and smocks required?

ANSI/ESD S20.20 100 V, human body model (HBM) 2000 V at 12 in.

20 , voltage not specified When handling ESD-sensitive (ESDS) items

JEDEC ESD 625A

JPL D-1348 Rev. F

200 V, HBM

20 V, HBM

1000 V at 12 in. Not specified

200 V at 1 m

20 ,

0.020 V When within 1 m of ESDS items

When with 12 in. of ESDS items

MANAGING USE OF THE AUDIT SCORECARD

481

Audits can also be done by customers, outside certification authorities, or independent agencies. These are often done to obtain and maintain program certification. It is even wise in some cases to reverse roles. The company or agency can audit its own suppliers and customers, becoming an outside auditor in so doing.

11.6

MANAGING USE OF THE AUDIT SCORECARD

All audits are intended to identify conditions that are not in conformance with requirements. When conditions are identified that are out of conformance with requirements, they must be reported. A standard checklist or audit form ensures that inspections will not be forgotten and will ensure consistency in comparing different ESD-protected work areas. For a very small facility, this activity is very easy to manage. But for a very large facility, management of this activity can become overwhelming. One strategy that has been developed for managing very large facilities is to use an audit scorecard, a mechanism that provides senior management with an insight into the health of their processes without burdening management with the tedium of detail. One approach is to develop a scorecard based on percent nonconformance. This allows a single number to be used that describes the performance of the facility and the processes within them. Depending on the level of management involvement, the scorecard can be broken down into specific project areas, specific contributing factors (e.g., facilities, materials, personnel discipline) in any level of detail desired. By way of illustration, one way to assess the performance of personnel entering a cleanroom would be to observe an agreed-upon number of persons entering the room. They would be observed to verify that they follow the change procedure specified. Any violation that is not corrected by a coworker in the area would be marked as a failure. For example, the audit might begin by observing 10 people entering a cleanroom. If the entry procedure has nine people performing the entry change perfectly and one person who forgets to wear their gloves, the score is 90%. Working through the process, this visual audit would assess operator workstation wipedown, verification of ESD grounding requirements, and any other factors considered essential for work area control. This assessment should be done using a standardized scorecard so that all areas are assessed using equal criteria and each time an area is audited the assessments are comparable. The number of points earned is totaled. This is subtracted from the number of points available. The remainder is then divided by the number of points available, which is reported as a percentage: the percent nonconformance. The single number is then an assessment of the ability of the workplace to conform to requirements. This single number might then be used to create a Pareto chart comparing several different areas, as shown in Figure 11.3. Management usually wants to track progress, so trend charts are also useful reporting tools. An example of a trend chart is shown in Figure 11.4. Where a continuous monitoring system is available for either contamination or ESD monitors, the alarm log can also be used as an assessment of workplace compliance. The percent of time that the workstations are in alarm can be used quantitatively to assess the performance of the workstations. An example of a percent time in alarm report is shown in Figure 11.5. The beauty of these ways of reporting high-level summaries should be immediately obvious. It provides a very simple number for interpretation by management. It provides a very simple way for different departments to compare themselves vs. others within the same company. It also provides a very simple method for comparing performance in multinational

482

PROCEDURES AND DOCUMENTATION

25

% Nonconformance

20

15

10

5

0 A

F

C

B

E

D

Clean room

FIGURE 11.3

Pareto chart of percent nonconformance among seven ESD-protected work areas.

60

% Nonconformance

50 40 30 20 10 0 1

2

3

4

A

5

B

6 7 Week C

8

D

9

E

10

11

12

F

FIGURE 11.4 Trend chart of percent nonconformance for seven different ESD-protected work areas.

locations that are trying to produce the same product using the same process. Multinational corporations often faced the problem of trying to create a uniform process. The audit scorecard can provide a way to assess uniformity of processing. An independent audit does not necessarily use an outside auditing authority; it can consist of audits run internally by personnel visiting foreign sites.

TYPICAL SURVEY

Loc.

Sensor

Limit

Op. 12

OPC > 0.1 μm

< 10k p/m3

Op. 45

OPC > 0.1 μm

< 10k p/m3

Ch.Rm OPC > 0.5

< 3.5k p/m3

Op. 7

OPC > 0.1 μm

< 10k p/m3

Op.14

ESC

< 200 V

Cleaner Resistivity

483

< 18 MΩ

Op. 4

OPC > 0.3 μm

< 10k p/m3

V9

Anemometer

< 0.1 m/s

0

10

20

30

% time in alarm

FIGURE 11.5

11.7

Typical percent time in alarm report from a continuous monitoring system.

TYPICAL SURVEY

Next we look at a typical survey that might be performed during the audit of a manufacturing facility that employs controls for both contamination and electrostatic discharge. Section 1: Organization and Technical Vitality Organization The first element that must be considered in any organization is how to manage the contamination and ESD control activity. To assess these, questions must be asked about how the organization is structured: ●

●

●

●

●

●

●

●

Who has the authority to do audits, what happens when an audit identifies a deviation, who takes action when a deviation needs correction, and how does management gain insight into this process? How are contamination and ESD control responsibility and authority cascaded and embedded in the manufacturing groups across a site? Is there a single-site department or function that has contamination control authority across all processes that require contamination-controlled environments or operating procedures that must be followed? Is there a single-site department or function that has ESD control authority across all processes that require ESD-protected environments or operating procedures that must be followed? Does the contamination or ESD control authority have a reporting line independent of all the processes detailed above? How does the contamination or ESD control organization present management formally with design and development recommendations for contamination control? What types of signoffs are there: preanalysis input, process development input, and so on? What deviation authority is provided? Who approves deviations? How are deviations justified?

484

PROCEDURES AND DOCUMENTATION

Technical Vitality Technical vitality among the personnel working in the contamination and ESD control disciplines is very important. These disciplines are dynamic, for two reasons: 1. The requirements for contamination and ESD control change frequently. The contamination and ESD sensitivity of parts evolve continually to ever-less-tolerant sensitivity levels. As a consequence, the contamination and ESD control programs must follow this evolution and be revised continually to reflect the increased vulnerability of the product. 2. The technology to support contamination and ESD control evolves as rapidly as the devices that are being protected. As a consequence, technical vitality in the contamination and ESD control workforce is an essential element to ensure that the contamination control and ESD control programs are state of the art and address the needs of the product. How will the contamination control and materials science specialists in your organization interact with peers and associates inside and outside the organization? ●

●

●

●

●

Do contamination and ESD control personnel attend or present at national meetings and seminars, publish papers, and interact with other entities of your organization, such as other locations? Does the company reimburse personnel for membership in technical societies and technical subscriptions? Is the level of technical vitality achieved over the last few years adequate to maintain state-of-the-art contamination and ESD control practices for the following one to two years? What efforts are being made to be proactive with contamination control specialists outside your company? What efforts are being made to be proactive with ESD control specialists outside your company?

Section 2: Documentation The objective of all this discussion, is to be able to demonstrate that the institution has made a reasonable effort to comply with its own requirements. This can only be done through documentation. If the requirements are inadequate, they can be faulted. More detailed analysis would be needed to determine a more appropriate set of criteria for contamination or ESD control. The documentation system is concerned primarily with demonstrating compliance with requirements. Of course, careful analysis should be done in advance to determine what the exact requirements should be. For both contamination and ESD control, this can be done by estimation on a largely analytical basis, as has been shown previously. ●

● ● ● ●

●

How are contamination control and ESD control practices and behavior documented and controlled? Is there an institution-wide specification or handbook? Is the institution-wide standard tailored to each product or project? Is it online and current? What outside resources are used as reference materials? Have these outside resources been verified for applicability? Who verifies documentation?

TYPICAL SURVEY

485

Section 3: Training The most important element of the contamination or ESD control plan for a facility is the personnel training because in the end it is the performance of the people who work in a contamination- or ESD-safe workplace and have the most direct contact with the product that control the actual outcome. There are varying degrees of intimacy of contact between the personnel and the product: very intimate in the case of manual assembly of printed circuit boards and quite remote in the case of a worker who only services an automated work area by providing replenishing materials for individual robots. It can be even more remote for the engineer in the automated work area who comes in only periodically to make minor modifications. Regardless, anyone working in an ESD- or contaminationcontrolled work area must understand the requirements of that work area; and the only way to understand this is through proper training. ●

● ● ● ● ●

Are regular contamination control training sessions run for manufacturing people, engineers, managers, and customers? Are there different levels of training? Is training conducted only in the classroom, or does workstation-aided training occur? Is there a test at the end of training? What happens if a person fails? How frequently does retraining occur? What qualification is there for the trainer?

Section 4: Cleanroom or ESD-Protected Work Area Entry The change room is the interface between a cleanroom and the ambient environment. The nature of its use allows it to be a major source of contamination of the cleanroom unless discipline is adhered to strictly. Of all violations of procedure observed in contamination- or ESD-safe work areas, failure to follow the restrictions at the entry are the most common. ●

● ● ● ● ● ● ● ●

What preconditions, such as clothing suitability restrictions and handwashing, apply to personnel prior to entering the changing area? Are operators required to wash their hands before entering the change room? Is glove washing practiced? How is the change area laid out? Do operators wear dedicated cleanroom shoes? Where are outer garments stored? Do ESD restrictions apply to this work area? Is there ESD training prior to entry into the work area? What restrictions apply to bringing tools and equipment into the cleanroom?

Section 5: Garmenting Strategy In cleanrooms and ESD-protected work areas, one of the essential elements of control is the type of garments a person wears. Control over these garments is thus an essential element of the overall strategy for an effective program. The garment control strategy must include not just cleanroom garments and ESD lab coats but also gloves, face masks, wrist straps, footwear, and so on. ● ●

What types of garments are worn? What criteria were used for selection of the attire?

486 ● ●

PROCEDURES AND DOCUMENTATION

What standards are used for purchasing and testing of garments? What batch-to-batch testing is the garment supplier/laundry service using (i.e., ASTM 51, Helmke drum, etc.)?

Section 6: Facilities Maintenance A common cause of difficulties in a contaminationor ESD-controlled work area is improper facilities maintenance. This occurs largely for several reasons: ●

●

Facilities engineering personnel seldom have direct line management responsibility for production process. Their highly specialized job responsibilities seldom allow them the luxury to understand what the facility looks like and how it is used in production. Purchasing personnel seldom have direct responsibility for the facility, so they do not know what the day-to-day operations require.

As a consequence of these two problems, contractors who are asked to provide facilities maintenance activities or housekeeping activities seldom are given good guidance by the functions requesting their activities. This can lead to serious problems. For example, a facility modification in which the contractor was not instructed about the proper use of isolation sheeting (Visqueen) might put up a barrier but not recognize that it was not functioning correctly. Without proper guidance from the facilities engineering group who contracted the facility modification, this contractor has no way of knowing that the Visqueen that they erected was ineffective. Similarly, housekeeping personnel contracted by purchasing may be in adequately trained because the purchasing personnel have themselves not been trained regarding the requirements. Correcting these problems requires coordination between the central ESD and contamination control engineering activities, facilities engineering, and purchasing to ensure that these types of problems do not occur. ●

●

●

●

Are there written instructions for facilities operations in cleanrooms or ESD-protected workplaces that cover activities during operational or nonoperational periods? Is there a site contamination or ESD control signoff procedure before facilities activities can take place? Is there a mandatory requirement for recertification of an affected area by site contamination or ESD control prior to resumption of activities? Are facilities engineering or purchasing personnel trained in the requirements of contamination or ESD control awareness?

Section 7: Materials and Consumables Approvals Traditionally, one of the weakest links in the ESD or contamination control engineering activity is control over materials and consumables that are used in the process. The best way to control materials, consumables, and equipment is to have a formal certification process that results in approval that restricts the items to be purchased. Unfortunately, the mission of purchasing is to minimize costs. This purchasing mission sometimes places purchasing objectives in direct conflict with the requirements of contamination or ESD control. ●

Is there a formal approval process operating whereby all changes to materials used in manufacturing are submitted for chemical compatibility, ESD testing, and formal approval?

TYPICAL SURVEY ●

●

● ●

487

Does a similar process operate for consumable materials (i.e., gloves, swabs, wipers, etc.)? Is there is single technical focal point for coordinating consumable materials approval and sourcing? Does an approved materials list exist for each contamination- or ESD-sensitive product? Does a generic cleanroom and ESD control–prohibited materials list exist?

Section 8: In-Cleanroom or ESD-Sensitive Processes The next element of audit must focus on the processes within the cleanroom or the ESD-protected work area. This is often viewed as the easiest part of the audit to perform. ● ●

● ●

What workstation cleaning instructions are available? Are there generic cleaning instructions, or are workstation-specific instructions provided for cleaning? Are there written guidelines for cleaning the cleanroom (i.e., janitorial instructions)? Are specific audit criteria used by the operators to verify contamination and ESD controls for their workstations prior to beginning activity?

Section 9: Design Authority Who has authority for specification of cleanroom facilities? Who has authority for specification of ESD-protected work area facilities? Who has authority for selection of facilities or tooling design? Who has authority for cleaning process development and equipment selection? ● ● ● ●

Section 10: Process Design and Change Authority Is there a formal signoff procedure by contamination or ESD control personnel prior to a new process being installed? Is there a formal signoff procedure by contamination or ESD control personnel to changes to an existing process that may affect the certification of these processes? ●

●

Section 11: Tool and Equipment Design and Certification Do contamination and ESD control personnel have an influence on the design of cleanroom tooling? Are all new cleanroom workstations and tools measured for contamination or ESD generation prior to use? Are documented guidelines and procedures that describe recommended designs for cleanroom and ESD-sensitive tooling used by tooling engineers? ●

●

●

Section 12: In-House Parts Cleaning Is there a documented strategy that describes the following cleaning considerations? The use of solving cleaning systems Preferred surfactants and detergents Parts cleanliness specifications and certification processes Cleaning equipment design and process development ●

● ● ● ●

488 ●

● ● ● ● ● ● ●

PROCEDURES AND DOCUMENTATION

Is there a documented cleaner maintenance agreement that meets manufacturing operational requirements (i.e., mean time to repair, percent availability, preventive maintenance schedules, etc.)? Are cleaners monitored continuously? Are control limits and action levels monitored? How are these limits established? Is a feedback action loop prescribed? If deionized water systems are used, are these systems monitored continuously? Where in the process is the DI water monitored, and how? In aqueous cleaning processes, are surfactant concentrations monitored continuously or by grab samples? If so, how?

Section 13: Compliance Measures Are audits carried out by manufacturing to ensure compliance with control requirements and practices? Are product independent audits carried out? What is the frequency of audits? Do these audits have quantifiable assessments? If so, are trends monitored, and how are they reported? Do feedback/corrective action plans exist? ●

● ● ● ● ●

Section 14: Supplier Processes and Packaging Is there a supplier or qualification and certification procedure? Is a vendor contamination control checklist used? Does the supplier have an in-house contamination or ESD control certification program? Are suppliers cleaning processes monitored and certified? Do suppliers provide parts that cannot be cleaned in-house? Describe packaging for both supplies that are to be cleaned in-house and supplies that cannot be cleaned in-house. How is packaging material qualified? Do procedures exist for packing and unpacking parts? ● ● ●

● ● ●

● ●

11.8

CASE STUDY: BROKEN MAGNET PROCEDURE

One of the most difficult events to recover from in the magnetic recording industry is a broken magnet. The process for recovering from a broken magnet must be documented in detail: Operating personnel must be provided with detailed instructions. The personnel following the broken magnet procedure require precise training. Finally, the areas where magnets are handled and might be broken require close scrutiny. An ideal broken magnet procedure successfully eliminates broken magnet particles from the room, tooling, and operator. Unfortunately, it is virtually impossible to guarantee 100% success, so occasionally great lengths must be taken to ensure successful recovery from a broken magnet. This example

CASE STUDY: BROKEN MAGNET PROCEDURE

489

provides recommendations regarding the development of an acceptable broken magnet procedure. 11.8.1

Definition of a Broken Magnet

A broken magnet is any magnet, energized or not, capable of releasing magnetic material into the workplace. Thus, a broken magnet is any magnet for which the protective coating can no longer be expected to prevent the release of magnet particles into the workplace. Broken magnets thus include tears, scratches, and chips in the coating for the magnet, in addition to physical breakage. 11.8.2

Recommendations for the Broken Magnet Procedure

The broken magnet procedure recommended here emphasizes isolation, cleanup, and the responsibilities of two people: the person who broke the magnet, called the broken magnet operator, and an assistant. Isolation of the Contaminated Area The operator who broke the magnet announces to operators around him or her that a magnet has been broken. Other operators outside a controlled distance vacate the area. One company has established a circle with a radius of approximately 1 m as a reasonable distance to be cleared of other operators and cleaned. This was developed for a vertical unidirectional-flow cleanroom, where contamination might be dispersed over a uniform distance regardless of horizontal direction. In a cleanroom with no raised floor, where airflow is not unidirectional, it might be more reasonable to use an asymmetric distance to establish the area to be avoided and cleaned. This distance should be less than 1 m upwind of the breakage location, approximately 1 m to the sides of the broken magnet location (perpendicular to the airflow direction), and greater than 1 m downwind of the broken magnet location. In either case, the area thus defined is referred to as the magnetic contamination area. Cleanup Team The broken magnet operator, the person who broke the magnet, receives a broken magnet kit from the assistant. The assistant who provides the kit does not enter the contaminated area. The person who broke the magnet should not exit the contaminated area until cleanup is complete. The kit is contained in a zipper closure plastic bag or other suitable sealable container. The assistant retains a second sealable container for disposal of the first after cleanup is complete. That means that the broken magnet operator places the kit containing the tools and waste from cleanup in the second bag. The assistant seals the outer bag and takes it to disposal. The assistant also receives samples taken using SEM sticky stubs from the broken magnet operator. These samples are never handled with gloved hands but always with special tweezers designed for this purpose. Broken Magnet Kit The broken magnet kit should be a sealable container such as a Zipper closure plastic bag containing some or all of the following items: ● ● ● ●

A second sealable bag Several pairs of gloves A low-cost adhesive roller Artist’s gum (optional)

490 ● ● ● ● ● ●

PROCEDURES AND DOCUMENTATION

Rubber cement (optional) Tack mitt or cloth (optional) Adhesive strips or mats Voice coil or other suitable high-energy coated magnet Package of SEM sticky stub (tape) samplers Tweezers for handling sticky stubs.

Cleanup Procedures If the person who broke the magnet is still holding it or a piece of it, the glove on the hand holding the magnet is pulled off, inverting it over the magnet. The glove is then tied shut and disposed of in a broken magnet container (if available) or in a waste bag. One of the gloves from the broken magnet kit is put on. If the broken magnet kit contains a high-energy coated magnet, such as a neodymium–iron–boron magnet, it is placed in a cleanroom glove. The glove is then used to sweep the hands, gloves, and garments of the operator, tooling, workstation surfaces, and furnishings in a systematic manner to ensure complete coverage (overlapping sweeps in the direction of airflow). This procedure is often referred to as magnetic sweep. After the sweep is completed, the glove is turned inside out and tied shut. This allows the high-energy magnet to be recovered for future use. Inverting and sealing the glove contains the trapped particles. Magnets are occasionally broken in a tool, which prevents close approach of the magnet (usually defined as contact by the surface of the sweeping glove) during magnet sweep. In this case it is prudent to use some form of adhesive to trap and remove particles. Rubber cement works well for these applications. Artist’s gum also provides a suitable sticky surface. The contaminated areas of the tool are either painted with the rubber cement or dabbed with the artist’s gum. After the rubber cement cures to a rubbery state, it is pealed off. Flat surfaces should be cleaned using a sticky mat, glove, or roller. Sticky rollers suitable for cleanroom use are exceptionally effective for this step. In addition, waste material from self-adhesive seals are suitable and have no adverse outgassing problems if properly qualified. Some companies have used tacky mitts called tack cloths for this application. They are available, but it is recommended that they be qualified for inorganic and organic contamination prior to use. The flat surfaces of the work area are systematically cleaned using the chosen mat, cloth, or roller. Contamination of the broken magnet operator’s hands and garments is a serious problem, since these contaminated materials can spread contamination to other cleanroom garments and the contaminated hands can spread contamination to the tooling and workstations. It is also possible for magnetic contamination to be shed from the broken magnet operator as they exit the cleanroom, causing contamination of the cleanroom in general. Therefore, it is desirable that the broken magnet operator prevent spread of magnet contamination from their gloves and hands. Several options are available to contain magnetic contamination from the broken magnet operator. One option is to use the sticky roller and magnetic sweep a second time after cleanup has been completed. As a final step, gloves are changed again. The broken magnet operator then can use SEM sticky stubs to tape-test the area. At least four stubs are suggested for this sampling. The stubs should be analyzed to verify that a magnetically clean area now exists. One company also has the assistant provide a second set of cleanroom garments that the broken magnet operator puts on after completion of cleanup. This company has the broken magnet operator exit both the cleanroom and the change room. The potentially contaminated cleanroom garments are removed in the hallway and placed in a disposal bag for incineration.

REFERENCE

491

Documentation In every case when a magnet was damaged, a broken magnet report must be generated. This report must contain the location, time, and date where breakage occurred, and should identify the broken magnet operator. Forms for the report should be in the broken magnet kit. The report should go to quality engineering, the materials science lab, and manufacturing engineering to ensure that attention is drawn to the need for possible corrective action. Corrective action can be as simple as the need for a second (or more) cleaning of the area to restore the area to its condition prior to magnet breakage. The report can motivate engineering to take action because the design of tools or selection of materials is inappropriate to protect reasonably against magnet breakage. Other benefits can be derived from detailed incident reports. Training Training is essential, since technique and discipline are essential for successful recovery from a broken magnet. The training should include the following elements: ● ● ● ●

Operators need to understand the risks associated with broken magnets. Operators need to be trained to establish a broken magnet area. Operators should know the location of the broken magnet cleanup kit. Operators should be familiar with tools in the broken magnet cleanup kit and how to use them.

Analysis of Samples from Broken Magnet Areas It is suggested that samples collected from a broken magnet area receive a degree of priority proportionate to the degree of magnetic contamination seen in yield and reliability failures. One disk drive manufacturer faced a significant yield loss due to magnetic contamination. Desiring to protect their customers from this failure mode, they required answers about the cleanliness of areas where magnets were broken within eight hours after a broken magnet incident. The analysis of SEM samples taken after cleaning an area after a broken magnet incident should be compared to the historical record of cleanliness of the area to determine if addition cleaning is necessary.

REFERENCE 1. JPL Standard D-1348 Rev. F, Document 34906, Jet Propulsion Laboratory, Pasadexa, CA.

Index Abrasion, 399–400 Absorption, 10, 89 Acceleration factors, 122 Accumulation mode, 28 Adhesion forces, 206 Adsorption, 10, 89 Aerial density, 13 Aerosol, 10, 29–30 Airborne molecular contamination (AMC), 10, 33–34, 40, 83, 165, 413, 417 Airborne particle count, 18–23 Airflow: balancing after tool installation, 176–180 case study: continuous monitoring, 349–350 fixing problems of, 25, 39, 173, 177, 180, 186–187, 286, 289–292, 332 non-unidirectional, 166–169 Reynold’s number, 166 and standing recirculation zones, 167, 169 unidirectional,169–174, 182–183 Air ionizer, 52, 83–88 alternating current (ac), 86 blow-off guns, 87 corona discharge, 85–88 direct current (pulsed dc), 86 discharge time, 87, 111, 320 emitters, 86 float potential, 88, 112 ionizer verifier, 108, 111, 146 for non-unidirectional airflow, 168

photonic, 85 radioactive, 84 for unidirectional airflow, 174 Air pressure, 23–24 Air showers, 191–192, 433, 438 cabinet style, 438 tunnel style, 438 AMC, see Airborne molecular contamination Analysis methods: algebraic predictive modeling, 147–150 atomic absorption spectroscopy, 139–140 atomic force microscopy, 132–133 contact angle measurement, 136–137 electron spectroscopy for chemical analysis (ESCA), 139 energy dispersive X-ray (EDX), 11, 135–136 Fourier transform infrared (FTIR), 11, 129 gas chromatography/mass spectroscopy (GC/MS), 12, 139 ion chromatograph, 139 ionograph, 140 nonvolatile residue (NVR), 137 numerical simulation, 146–147 Raman spectroscopy, 129–130 scanning electron microscope (SEM), 12, 130–132, 135–136 secondary ion mass spectroscopy (SIMS), 139 statistical analysis, 150 water break test, 136 X-ray photoelectron spectroscopy (XPS), 139

Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg Copyright © 2006 John Wiley & Sons, Inc.

493

494

INDEX

Anemometer, 10 Anion, 10 Antistatic, 53, 75 Assemble–then clean versus clean–then assemble, 219–220, 252 case study: comb assembly, 257–258 case study: head stack assembly, 259–262 case study: top cover assembly, 255–256 case study: voice coil motor assembly, 258 Asymptote, 200 Audits: annual test and balance surveys, 24, 112 for ESD compliance, 106–112 formal audits, 112–114, 477 frequency of, 23–24 hierarchy of, 477 independent, 475, 480–481 instrument, 107–109, 475, 479–480 noninstrument, 475, 478–479 operator self-check, 106–107, 476–478 percent nonconformance, 482 vendor contamination checklist, 237, 483 verification audits, 113 Bacteria, 414 Behavior, 426, 437–438, 444–445, 476 Bellows, 288 Break areas, 459–463 Capillary attraction, 8 Case studies: audits and verifications, 113 broken magnet procedure, 488–491 change frequency, 426 corrosion in a multistage cleaner, 246 detergent drag-out, 246–247 diamond swab, 404 ESD chairs, 90 estimating the class of cleanroom needed, 19–20, 147–150 footwear, 99, 435 floor, 80 handwashing, 430 minienvironment, 319–322 optimized cleaner, 249–251 packaging, 103 SEM/EDX corrosion analysis, 135–136 shoe cleaning, 436–437 sprayed DI water and ESD, 248 surface treatment, 208 vacuum drying, 248–249 versatility not optimized, 247 Cathode-ray tube (CRT), 91–93 Cation, 10 Censoring of data, 338 Chalking, 17 Change room: air showers, 191–192, 433, 438

break areas, 459–463 dressing in, 426–438 efficiency of, 451–454 entrances, 455–456, 464–465 excess transaction time, 452–454 exiting a cleanroom, 439, 470 glove wash area, 460 handwashing area, 430, 458, 461 hanging garment storage, 457, 459, 463, 466 layout of, 451–474 pre-change area, 455 pressure, 474 privacy, 418–419 shoe change area, 463 shoe change bench, 457, 463 shoe lockers, 457 size of, 452–453 soiled garment bins, 458, 463 traffic, 472 Charge, 4–8, 56–68 Charged plate monitor, 108, 144–145 Charged plate verifier, 108 Charge retention, 141, 380 Chemical history, 8 Chemical reactions, 8–9 Chimney effect, 10 Cleanability: curves for various materials, 204 Hamaker constant, 4, 206 maximum cleanability potential, 205–206 surface cleanability, 205–206 Clean build protocol, 417 Cleaning: boundary layers, 221 carbon dioxide snow, 233–234 chemical, 230 design for cleanability, 216 manual, 231 mechanical agitation, 231 particles in liquid baths, 221 plasma, 232 solvent cleaning, 230–231 sparging, 222 spray cleaning and rinsing, 225–228 spin-rinse dryer, 228 supercritical fluid, 233 ultrasonic cleaning, 221–225 ultraviolet (UV), 232 undulation and sparging, 222 vapor degreasing, 230 Cleanliness level, 12, 199 and effect of part history, 8, 268 and effect of surface treatment, 207, 210, 270, 308, 311 Cleanroom: airflow, 25, 39, 174, 292, 349 air leakage, 24 air pressure, 24, 39–40

INDEX

balancing, 175–176 ballroom, 159 biological safety cabinets, 161, 188 clean benches, 161, 184–186 conventional, 166 flow control barrier, 180, 188, 318 glove box, 161 high bay, 159 horizontal unidirectional flow, 182–183 isolator, 161, 186 laminar, 166 laundry services, 396, 449 minienvironment, 160, 190, 318 mixed flow, 22, 166 non-unidirectional flow, 166 particle count, 26 pass-through, 192–193 point-of-use clean air, 187–188 portable, 160, 324 staged certification of, 30–33 tunnels, 170–173, 188–190 turbulence in, 168, 169 unidirectional flow, 13, 169, 182–183 working in a, 438–439 Close proximity, 34 Colloid, 11 Cologne, 417 Components, 313–318 electric motors, 314 filters, 315–316 pneumatic devices, 288, 314 Condensation nucleus counter (CNC), 26 Conductive material, 53 Conductivity, 53 Contact transfer, 38 Contact stain tests, 123, 293–294, 375–376 Contamination, sampling of: in air, 23, 133–134, 324–325, 338–339, 352–353 central atmospheric monitoring system (CAMS), 138 critical and busy, 11, 339–340 in air, 133–134 in liquids, 134, 255, 356, 361 on surfaces, 135, 333 organic sampling techniques, 137–138 in vacuum systems, 351 Continuous monitoring systems, 10, 24, 105, 109, 318 Cost: of cleaning, 236 of cleanroom construction, 183–184 of cleanroom operating, 184 of flooring, 78 Coulomb, 53 Critical and busy sampling, 11, 339–400 Critical location, 11 Critical operation Critical surface, 11

495

Daisy chain grounding, 82 Decay time, 53 Decontamination, gradual, 427, 451 Deionized (DI), 11 Deliquescing, 8 Decontamination of equipment, 280, 465 Densitometer, 11 Desiccants, 102 Dielectric, 53, 182 Discharge time, 87–88, 141 Doors, 191 Electroexplosive devices (EEDs), 74 Electronegativity, 62–63 Electrostatic attraction (ESA), 4–8, 67 Electrostatic discharge (ESD), 4, 141–146 Emitter materials, 87 Enclosures, 39, 190, 318 Energy, 71 Entering a cleanroom, 426–427 Entering an ESD-safe work area, 443–444 ESD control coordinator, 112 ESD ground, 54 ESD-safe (ESD-protected) work area, 54 access to, 78 chairs and stools, 90 common point, 81 facility, 88 ground-fault-interrupt circuit (GFIC), 81–82, 94, 109 hard grounds, 82 identification of, 78 through bearings, 94 workstation, 81–82 protective perimeter, 78 Event detectors, 66, 112 Exiting a cleanroom, 439–441 Expanded PTFE membranes, 448 Facilities maintenance, 486 Factory environment, 11, 417 Failure mode and effects analysis (FMEA), 278 Faraday cage, 54 Fibrous contaminants, 192 Field tests, 121 Filter: airborne molecular contamination, 165 high-efficiency particulate air (HEPA), 12, 161–165, 439 life expectancy of, 165 maximum penetrating particle size for, 162 ultralow penetration air (UPA), 13, 161–165 Filtration mechanisms: Brownian motion, 164 diffusion, 163 electrostatic attraction, 164 impaction, 163 impingement, 163

496

INDEX

Filtration mechanisms (Continued) physical absorption (physisorption), 165 sieving, 162 Fingerprints, 413–414 First article, 382 Flame ionization detector (FID), 11 Float potential, 88 Floor tiles, 439 Fogging of eyeglasses, 421 Functional ESD tests, 69, 123 Fungi, 414 Functional laboratory tests, 121–124

Ground-fault-interrupter circuit (GFIC), 81–82, 94, 109

Gage capability, 151–156 Garments: change frequency, 426 cleanliness of, 446–448 cleanroom shoes, 435 coverall, 418, 442 croupier sleeves, 423, 449 design of, 448–449 double cuff, 423–424 ESD shoes, 435, 444 executioner-style hood, 421 fabric, 448 facial cover, 418, 374, 420, 428 finger cots, 100, 374–376 footwear, 98–100 heel grounders, 425 overshoes, 98 toe grounders, 425, 444 for visitors, 99, 425 frock, 418, 431, 440 hairnets, 419, 428–429, 431 inner suit, 418–419 jumpsuit, 432, 440 knee high booties, 432 options for, 418, 446 powered headgear, 418, 421–422 protocol for, 417–426, 437, 439 shoe covers, 99, 374, 418, 424–425, 433, 435, 444 spun-bonded polyolefin, 98 stackhouse hood, 434 Gas-to-particle conversion, 27 Gauge capability, 151–156 Gloves: Barrier, 420 cleanroom, 100, 418, 439 ESD, 100, 388–396 laundering, 396–402 liners, 381–382, 393–394 lock, 423 natural rubber latex, 383–384 nitrile, 387–388 pinholes in, 385 PVC, 389 washing, 383–388 woven, 419–420

In situ contamination, 17 Insulative material, 75–77

Hand lotion, 374, 443–444 Hand tools, 317–318 Handwashing, 430 Headspace analysis, 399 Helmke drum method, 447 Hydrophilic, 12 Hydrophobic, 12 Humidity testers, 108 Hygrometers, 108

Joule, 71, 74 Laminar, see Airflow, unidirectional Laundering, 396, 401, 446–449 Light microscopy, 124–129 analytical, 125–126 graticules for, 126 low-power (binocular), 125 particle counting using, 126 as referee method, 125, 129 reticule for, 126 Liquidborne particle counter (LPC), 402, 404 Magnetic contamination, 37, 488–491 Maintenance, 322, 331, 486 Materials: antistatic, 75 conductive, 75–76, 80 cellulose, 403 cotton, 403, 415 desiccants, 102 insulative, 75–77 metals, 298–301 nylon, 403 paper, 104 plastics, 302–308, 405 polyester, 403, 415 polyethylene, 304 polyethylene terephthalate, 305 polypropylene, 304, 403 polystyrene, 305 polyurethane, 305, 399 polyvinyl chloride (PVC), 34, 305, 389, 399 rayon, 403, 415 silk, 415 silica gel, 102 stainless steel, 299 static-dissipative, 75–77, 80, 104 tape, 104 wool, 415 Maximum penetrating particle size, 12 Micron, 12

INDEX

MIL-STD-1246, 13 Mucosa, 417 Near-contact stain tests, 123, 293–294, 375–376 Non-particulate matter, 12 Nonvolatile residue (NVR), 12 Normally corrected vision, 411 Objective laboratory tests, see Analysis methods Ohm’s law, 70 Optical particle counter (OPC), 12 Organic contamination: central atmospheric monitoring system (CAMS), 138 contact angle measurement, 136 nonvolatile residue (NVR), 137 optically stimulated electron emission (OSEE), 137 sampling techniques, 137–138 water break test, 136 Oscilloscopes, 93 Outgassing, 12, 34, 405 Outlet tester, 109 Packaging, 100–103, 213–215, 324 double bagging, 192–193, 213, 214, 324 EMI shielding, 100 ESD considerations in, 405 humidity sensitivity of, 101, 405 packing and unpacking, 192–193, 214–215 pink polyethylene, 101 pseudo-holes, 102 reuseability of, 101 static shielding, 101 for tooling, 324 for transportation, 106 vacuum forming, 103 Parts crib, 462 Pass-through, 192–193, 465–466 Perfumes, 417 Permitivity, 76 Power, 70 Pressure requirements, 39–40 Productivity, 451, 453 Protection ratio, 319 Protective perimeter, 78 Proximity of exposure, 41 Relative humidity, 50, 88–90, 141, 145, 337 Residual charge, 141–142, 389–390 Resistivity, 76, 142 Resistance tests, 390 Rotating chamber test method, 447 Semiclean zone, 419 Sensitive electronic device symbols, 55 Shift changes, 452 Silica gel, 102

497

Sit-stand protocol, 78, 106 Showers, 416 Smoking, 417 Spittle particles, 414 Standing recirculation zone, 167, 169, 177, 179, 186, 191, 319 Stand only operation, 445 Static-safe workplace, testing of 141 Statistics: basic tools for, 150 gage capability analysis, 151–156 densitometry for indirect cleanliness, 153–154 liquid-borne particle counting, 155 turbidimetry for indirect cleanliness, 154 Monte Carlo analysis, 352 Sticky rollers, 80, 433, 438 Surface treatments, 207, 308–309 anodizing, 207–208, 310 conversion coatings, 208 electroless plating, 208 electroplating, 208–209, 308, 311 electropolishing, 209, 308, 311 mechanical deburring, 207 paint, 209, 309–310 passivation, 209 and porosity, 309, 312 synergistic coatings, 210, 311 and texture, 309, 312 Swabs, 104, 283, 403–404 Taber abraser, 400 Table mats, 81–82 Tacky mats, 436 Toolbox, 428 Tools and tooling: and airflow on workstations, 172, 177, 185, 188, 287 bulkhead mounting of, 173, 286 cleanliness of, 272–273 closed chamber test method, 330, 331 close proximity, 41, 280 ESD testing of, 94 grounding of, 81 installation of, 324 single-axis test method, 325 visibly clean, 42, 281 Training, 95–97 Tribocharge testing, 61, 141, 390 Triboelectric charging, 59–65, 88–90 Triboelectric series, 59 Tunnelizing, 188 Turbidimeter, 13 Turbulence, 13 U designators, 19 Ultralow penetration filter (ULPA), 13 Ultrafine fraction, 19

498

INDEX

Vacuum systems, 287 Van der Waals force, 4 Venturi effect, 191, 318, 322 Viable contamination, 13, 18, 379, 414 Visible contamination, 42, 281 Volt-ohmmeter, 108 Wear, 297–301 of coatings, 309, 312 of composites, 306 lubricants, 301 metal-to-metalwear, 297–301 Wipe-down, 40–43, 280–283

Wipers, 104, 402 Witness plate, 13 Work function, 62 Workstations, 81 grounding of, 81 isolation of, 190 layouts for, 172, 173 monitoring, 317 perforated work surfaces, 181 standard machine interface (SMIF), 430 and standing recirculation zones, 177–179, 186 storage locations, 181 Wrist strap tester, 109