Semiconductor Industry: Wafer Fab Exhaust Management

Semiconductor Industry Wafer Fab Exhaust Management

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Semiconductor Industry Wafer Fab Exhaust Management J. Michael Sherer

Boca Raton London New York Singapore

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

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Published in 2005 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2005 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 1-57444-720-3 (Hardcover) International Standard Book Number-13: 978-1-57444-720-0 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

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Preface I would like to dedicate this book to my spouse Mary, our daughters Katie, Allison, and Nicole, and my parents Frank and Kerin. I would like to thank my numerous friends in industry (you know who you are!). Special thanks to Vicki, Gregg, and Joel for their support and interest in this book. I am very grateful for the many blessings in my life from my Lord and Savior Jesus Christ. In my freshman year of college, I attended Northern Arizona University (NAU). During Christmas break, my father and I had a conversation about what major I would have at NAU. My thought, which I shared with my Dad, was to be a chemical engineer. My father mentioned to me that if I wanted a chemical engineering degree, it would be beneficial to go to a university which has a chemical engineering program (reason he said that was NAU did not). Being of somewhat sound mind, I said, yes, it would be good!! The next year I was at the University of Arizona. Sometimes we know where we want to go, but are not sure where to get ideas and solutions to get there (even if it may be obvious). My hope is this book will help you find some ideas and solutions to your wafer fabrication (fab) exhaust management questions (wafer fab exhaust management can be considered exhaust systems, point-of-use devices, and centralized abatement equipment). Wafer fab and semiconductor manufacturing will be used interchangeably. The good news is that chemistry and physics principles apply to wafer fab exhaust management. Whenever there has been a question on what is occurring, using chemistry and physics has been helpful in understanding the situation and determining solutions. Exhaust (airflow) reduction will not be discussed in this book. Exhaust reduction (with related energy reduction) is an area in which the semiconductor industry is interested. There are technical efforts and products being developed to reduce exhaust in some processes such as wet benches, ion implant, and gas cabinets. This book begins with semiconductor manufacturing facility layout, support facility operations, and semiconductor process equipment. The next chapter reviews exhaust types and challenges. Chapter 3 details exhaust point-of-use devices, which are used close to the process equipment, and exhaust line requirements between process equipment and the centralized exhaust system. Chapter 4 provides information on centralized wet scrubbers for a centralized acid exhaust system and a centralized ammonia exhaust system, and Chapter 5 provides information on centralized equipment to control volatile organic compounds. Examples are used to better understand the discussion points. Chapter 6 discusses emergency releases. After understanding the important factors of exhaust management, Chapter 7 lists some examples of whole-fab exhaust management situations. These details can be used to help the reader see how the previous chapters can be used in a semiconductor manufacturing facility.

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The majority of this book will be for silicon wafer fab facilities, which is the largest percentage of facilities today. There will be some information provided for compound (III-V) semiconductor wafer fab facilities and silicon wafer manufacturing facilities; these facilities do have exhaust management challenges. Throughout the world there are numerous wafer fab layouts and exhaust system approaches. I have been fortunate to visit dozens of silicon and compound semiconductor wafer fabs, and a few wafer manufacturing plants throughout the world. The data shared cannot be reproduced here, as these data are confidential to companies. This book will discuss wafer fabs in general and is not written to explain all designs and practices employed. However, the technical information and solutions presented henceforth can be utilized in any wafer fab.

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The Author Mike Sherer was born to wonderful parents Frank and Kerin in Dearborn, Michigan in 1958. Mike has two supportive sisters Jayne and Sue. He received a Bachelor of Science in Chemical Engineering from the University of Arizona in 1981, and a Master of Science in Chemical Engineering from the University of Rochester, NY in 1986. Mike married Mary, the love-of-his-life, in 1982. Mary has two wonderful parents John and Jean. Mary and Mike have three loving daughters: Katie, Allison, and Nicole. Mike worked for Eastman Kodak in Rochester, New York from 1981 to 1990 in the environmental field. He spent two and a half years (1990 – 1992) in environmental consulting and a short stint in government. Mike’s next position was at a Motorola semiconductor division located in Phoenix, Arizona from 1992 to 1999; he was a senior environmental engineer, and then international environmental, health, and safety engineering manager. Mike worked at ATMI from 1999 to 2004. Mike is the principal for Sherer Consulting Services located in Gilbert, Arizona. Mike has had the opportunity to walk through dozens of wafer fabs worldwide, conducting numerous fab exhaust management reviews. He has been involved in organizations such as Semiconductor Environmental, Safety, and Health Association (past President); Semiconductor Equipment & Materials International; International SEMATECH; Semiconductor Industry Association; and American Institute of Chemical Engineers. Mike enjoys spending time with his family, taking part in church activities, exercising, and fishing. Mike has retired from basketball and has been known to say a few puns.

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Contents Chapter 1 1.1 1.2 1.3

Wafer Fab Layout ............................................................................................1 Facility-Support Operations .............................................................................3 Semiconductor Process Equipment .................................................................3 1.3.1 Manufacturing Facilities for Silicon Wafers .......................................3 1.3.1.1 Chemical Wafer-Thinning Baths ..........................................3 1.3.1.2 Solution Baths of Hot Nitric Acid .......................................4 1.3.1.3 Other Chemical Baths ..........................................................4 1.3.1.4 Large Storage Tanks for Acid Solution................................5 1.3.2 Silicon Wafer Fabs ...............................................................................5 1.3.2.1 Chemical Vapor Deposition Processes .................................5 1.3.2.2 Etch Processes ......................................................................6 1.3.2.3 Ion Implantation....................................................................7 1.3.2.4 Wet Benches (Hoods) ...........................................................7 1.3.2.5 Spray Processes ....................................................................8 1.3.2.6 Photolithography...................................................................9 1.3.2.7 Chemical Mechanical Planarization (Polishing) ..................9 1.3.2.8 Furnace Tube Cleaners .........................................................9 1.3.2.9 Gas Cabinets .........................................................................9 1.3.2.10 Bulk Chemical Distribution................................................10 1.3.2.11 Plating .................................................................................10 1.3.2.12 Other Silicon Wafer Processes Not Discussed in This Book............................................................................10 1.3.3 Compound Semiconductor Wafer Fabs .............................................10 1.3.3.1 Metal Organic Chemical Vapor Deposition .......................10 1.3.3.2 Compound Semiconductor Wafer Etch ..............................11 1.3.3.3 Equipment Cleaning ...........................................................11 1.3.3.4 Other Processes...................................................................11

Chapter 2 2.1

Semiconductor and Wafer Manufacturing Facilities ...........................1

Exhaust Types and Challenges ..........................................................13

Exhaust Types ................................................................................................13 2.1.1 Centralized Acid Exhaust System .....................................................13 2.1.2 Centralized Ammonia Exhaust System .............................................14 2.1.3 Centralized Volatile Organic Compound Exhaust System................14 2.1.4 Centralized General, Heat Exhaust System.......................................14 2.1.5 Centralized Pyrophoric Exhaust System ...........................................14 2.1.6 Centralized Exhaust System for High Acid Concentration ..............15 2.1.7 Other Centralized Exhaust Systems ..................................................15 2.1.8 Individual Exhaust to Outside ...........................................................16

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2.2

Exhaust Management Challenges..................................................................16 2.2.1 Ammonia ............................................................................................16 2.2.2 Bends..................................................................................................18 2.2.3 Corrosion and Other Damage ............................................................18 2.2.4 Diameters for Exhaust Line or Exhaust Ducts..................................19 2.2.5 Explosions and Fires..........................................................................19 2.2.6 Fluorine ..............................................................................................19 2.2.7 Gaskets, O-Rings, Connections, and Fittings....................................20 2.2.8 Hexamethyldisilazane ........................................................................21 2.2.9 Industrial Wastewater Piping Vent .....................................................21 2.2.10 Junior Size It ......................................................................................21 2.2.11 Kinds of Materials for Exhaust Lines and Exhaust Ducts, Etc. .......21 2.2.12 Air Leaks............................................................................................22 2.2.13 Maintenance .......................................................................................22 2.2.14 Not Enough Exhaust ..........................................................................22 2.2.15 Oil Mist ..............................................................................................23 2.2.16 Perfluorocompounds (Global Warming Gases) .................................23 2.2.17 Quality of Installation ........................................................................24 2.2.18 Restrictions (Particulate Deposition, etc.) .........................................24 2.2.19 Silane ..................................................................................................24 2.2.20 Toxic Gases ........................................................................................24 2.2.21 Ultrapure Water and Other Water Supplies .......................................24 2.2.22 Vertical Tube Cleaners and Other Spray Processes ..........................25 2.2.23 Wet Benches (Hoods) ........................................................................25 2.2.24 “X-cess” Negative Static Pressure of Exhaust ..................................26 2.2.25 “Y” Heat Exhaust Lines ....................................................................26 2.2.26 Zero Unplanned Downtime................................................................27

Chapter 3 3.1

Point-Of-Use Devices and Exhaust-Line Requirements ...................29

Point-Of-Use Device Information..................................................................29 3.1.1 Main Reasons for the Use of Point-Of-Use Devices ........................29 3.1.2 Point-Of-Use Device Technologies ...................................................31 3.1.2.1 Point-Of-Use Wet Scrubbing Systems ...............................32 3.1.2.2 Point-Of-Use Oxidation Systems .......................................32 3.1.2.3 Point-Of-Use Cold Bed Systems........................................33 3.1.2.4 Point-Of-Use Hot Bed Reactors.........................................34 3.1.2.5 Point-Of-Use Reactor Systems...........................................34 3.1.2.6 Point-Of-Use Particulate Removal Systems.......................34 3.1.2.7 Point-Of-Use Recycle or Reclaim Systems .......................35 3.1.3 Purchasing Point-Of-Use Device.......................................................35 3.1.3.1 Understanding Semiconductor Fab Processes, Materials, and Environmental, Health, and Safety..............................35 3.1.3.2 Understanding Point-Of-Use Device Technologies and Keeping the Customer No. 1.......................................35 3.1.3.3 Understanding Site-Specific Information ...........................35

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3.1.3.4

3.2

3.3

Propose the Best Point-Of-Use Device Technology Considering All Factors......................................................36 3.1.3.5 Point-Of-Use Device Inlets and Number of Process Exhausts ..............................................................................36 3.1.3.6 Emissions and Destruction and Removal Efficiency Data .....................................................................................37 3.1.3.7 Standards, Guidelines, and Guides.....................................37 3.1.3.8 Quote Specifications ...........................................................37 3.1.3.9 Warranty..............................................................................38 3.1.3.10 Identifying Competent Local Maintenance Support and Training ........................................................................38 3.1.3.11 Spare Parts ..........................................................................38 3.1.4 Point-Of-Use Device Installation.......................................................38 Surrounding Exhaust-Line Requirements......................................................39 3.2.1 Processes to Look Out for .................................................................40 3.2.2 Outlet Connection of Process Pump..................................................40 3.2.3 Keep It Straight, Keep It Close, and Keep It the Same Diameter....41 3.2.4 Minimize Connections .......................................................................41 3.2.5 Install Right Exhaust-Line Construction Materials...........................41 3.2.6 Pitch Exhaust Line Coming from Point-Of-Use Device That Uses Wet Scrubbing Section .....................................................41 3.2.7 Keep Static Pressure in Exhaust Negative ........................................42 Examples of Point-Of-Use Devices for Selected Processes .........................42 3.3.1 Oxide and Polysilicon (Poly) Chemical Vapor Depositions .............42 3.3.2 Doped Oxide and Poly-Chemical Vapor Depositions .......................45 3.3.3 Nitride and Oxynitride Chemical Vapor Depositions .......................49 3.3.4 Tungsten Chemical Vapor Depositions..............................................51 3.3.5 Tungsten Silicide CVD ......................................................................53 3.3.6 LPCVD Nitride ..................................................................................53 3.3.7 Low-Pressure Chemical Vapor Deposition of Poly-Oxide (Undoped or Doped) Using Silane ....................................................53 3.3.8 Epitaxial Deposition...........................................................................54 3.3.9 Other Examples of Chemical Vapor Deposition Processes ..............56 3.3.9.1 Titanium Nitride Chemical Vapor Deposition ...................56 3.3.9.2 Low-K Chemical Vapor Deposition ...................................57 3.3.9.3 Furnace Process Using Ammonia ......................................58 3.3.10 Alumiylum Etch.................................................................................59 3.3.11 Oxide, Poly, Nitride Etch Processes..................................................59 3.3.12 Tungsten Etch.....................................................................................61 3.3.13 Ion Implant.........................................................................................61 3.3.14 Wet Benches.......................................................................................61 3.3.15 Spray Processes Using Acid Solutions and Ammonium Hydroxide-Hydrogen Peroxide Solution.......................63 3.3.16 Cylinder and Other Storage Container Gas-Line (or Vent Gas) Purges ..........................................................................64 3.3.17 Metal Organic Chemical Vapor Deposition.......................................65 3.3.18 Gallium Arsenide Etch.......................................................................67

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3.4 3.5

3.6

Fab Examples of Point-Of-Use Devices........................................................67 Developing a Point-Of-Use Device and Surrounding Exhaust-Line Strategy...........................................................................................................73 3.5.1 Develop Overall Strategy...................................................................73 3.5.2 Example of Process Exhaust Management and Point-Of-Use Device Strategy (This Fab Does Have Both Centralized Acid Exhaust System and Centralized Ammonia Exhaust System)..........74 “Development Opportunities” for Point-Of-Use Devices .............................76 3.6.1 More Process to Point-Of-Use Device Interface...............................76 3.6.1.1 Water Usage........................................................................76 3.6.1.2 Fluoride Wastewater Segregation .......................................77 3.6.2 Understand Cost of Ownership .........................................................77 3.6.3 Better-Performing Inlets for Point-Of-Use Devices..........................77

Chapter 4 4.1 4.2

4.3

4.4 4.5 4.6 4.7

Centralized Scrubbers ........................................................................79

Introduction ....................................................................................................79 General Design and Operation ......................................................................80 4.2.1 General Design Information ..............................................................81 4.2.1.1 Distribution of Airflow .......................................................83 4.2.1.2 Distribution of Scrubbing Solution ....................................84 4.2.1.3 Designing Centralized Scrubbers and Fans for Installation.....................................................................85 General Operation Information......................................................................85 4.3.1 Ammonia – Centralized Acid Scrubber Enemy No. 1......................86 4.3.2 Acid Mists ..........................................................................................88 4.3.3 Fluorine ..............................................................................................91 4.3.4 Volatile Organic Compounds .............................................................92 4.3.5 Source of Makeup Water ...................................................................94 4.3.6 pH of Scrubbing Solution ..................................................................96 4.3.7 Biogrowth and Scaling.......................................................................98 4.3.8 Preventive Maintenance .....................................................................99 4.3.9 Wastewater Discharge from Centralized Scrubbers ..........................99 Acid Scrubbers for Large Storage Tanks of Concentrated Acid Solution....99 Centralized Abatement Systems for Higher Inlet Concentrations of Acid Compounds, Chlorine, and Acid Mists .......................................................100 Centralized Scrubbing System for Oxides of Nitrogen ..............................102 Specifying a Centralized Wet Scrubber.......................................................103 4.7.1 Centralized Acid Scrubbers for Fab and Acid Scrubber for Large Storage Tank of Acid Solution ........................................103 4.7.2 Centralized Ammonia Scrubber for Fab..........................................106 4.7.3 Centralized Acid Scrubbing System for High Inlet Concentrations of Acid Compounds, Acid Mists, and Chlorine.....................................................................................108 4.7.4 Centralized Scrubbing System for Oxides of Nitrogen ..................110

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Chapter 5 5.1 5.2

5.3 5.4 5.5 5.6

Introduction ..................................................................................................113 Centralized Equipment Technologies to Control Volatile Organic Compounds...................................................................................................114 5.2.1 Centralized Oxidation Systems........................................................115 5.2.1.1 Regenerative Thermal Oxidation Systems .......................115 5.2.1.2 Recuperative Catalytic Oxidation Systems ......................117 5.2.2 Centralized Capture-Oxidation Systems..........................................119 5.2.2.1 Rotor-Concentrator, Recuperative Thermal Oxidation Systems ............................................................120 5.2.2.2 Rotor-Concentrator-Recuperative Catalytic Oxidation Systems ............................................................123 5.2.2.3 Fluidized-Bed, Thermal Oxidation Systems ....................126 5.2.3 Centralized Capture and Recovery Systems ...................................129 5.2.3.1 Systems of Fixed-Bed Carbon with Steam Regeneration .....................................................................129 5.2.3.2 Fixed-Bed Systems and Recovery of Liquid Volatile Organic Compounds............................................129 5.2.3.3 Fluidized-Bed Systems and Recovery of Liquid Organic Compounds .........................................................130 5.2.4 Other Centralized Equipment Systems for Control of Volatile Organic Compounds.........................................................................130 Examples of Selection Criteria for Centralized Equipment to Control Volatile Organic Compounds .......................................................................130 Location of Fans ..........................................................................................138 Testing of Centralized Equipment to Control Volatile Organic Compounds...................................................................................................138 Specification to Supplier of Centralized Equipment to Control Volatile Organic Compounds.....................................................................................138

Chapter 6 6.1 6.2

6.3 6.4

Centralized Equipment to Control Volatile Organic Compounds...113

Emergency Releases.........................................................................141

Introduction ..................................................................................................141 Controlling Releases from Gas Cylinders or Other Vessels .......................141 6.2.1 Restricted Flow Orifice ....................................................................142 6.2.2 Gas Storage at Low Pressure or Subatmospheric Pressure ............142 Silane Emergency Release...........................................................................142 Reduction of Emergency Release of Concentrations of Stack Discharge......................................................................................................142 6.4.1 Dilution Used for Emergency Release ............................................142 6.4.2 Abatement Used for Emergency Release ........................................143 6.4.2.1 Dry Resin Abatement System for Emergency Release ..............................................................................143 6.4.2.2 Wet Scrubber for Emergency Release Using Water as Scrubbing Solution.......................................................144

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6.4.2.3

Chapter 7 7.1 7.2

7.3

7.4 7.5

Wet Scrubber for Emergency Release Using Water as Scrubbing Solution, Followed by a Submicron Filter for Acid Mist ....................................................................145

Exhaust Management and Air Abatement Equipment Examples ......149

List of Examples ..........................................................................................149 Example No. 7-1 ..........................................................................................150 7.2.1 Fab Personnel Needed to Understand Technical Information Surrounding Issues...........................................................................151 7.2.2 Develop Strategy for Exhaust Systems, Point-Of-Use Devices, and Centralized Abatement Equipment for Fab ..............................152 7.2.3 Three Actions Combined and Project Plan Developed for These Actions .............................................................................153 7.2.4 Project Plan for Action on Centralized Equipment to Be Installed to Control Volatile Organic Compounds, Due to Requirements of Local Air Regulatory Agency ..............................159 7.2.5 Project Plan for Odors Reported at the Clean-Water Production Plant Near Storage Tank of Concentrated Hydrochloric Acid Solution .............................................................161 Example No. 7-2 ..........................................................................................161 7.3.1 Fab Personnel Need to Understand Technical Information Surrounding Issues...........................................................................162 7.3.2 Project Plan and Solutions for Sources of Sporadic Visible Emissions from Stacks and Odors...................................................163 7.3.2.1 Ammonia Project Plan, Implementation, and Solutions.....163 7.3.2.2 Project Plan, Implementation, and Solutions for Fluorine and Other Possible Odor Sources......................163 7.3.3 Project Plan for Centralized Acid Scrubbers...................................164 7.3.4 Project Plan and Solutions for Higher-Than-Expected Emissions of Carbon Monoxide and Oxides of Nitrogen from Centralized Equipment to Control Volatile Organic Compounds.........................................................................165 7.3.4.1 Carbon Monoxide Emissions ...........................................166 7.3.4.2 Emissions of Oxides of Nitrogen.....................................166 7.3.4.3 Air Permit Limitations on Emissions for Centralized Equipment to Control Volatile Organic Compounds .......................................................................166 Example No. 7-3 ..........................................................................................166 7.4.1 Fab Process Investigation and Solutions .........................................167 Example No. 7-4 ..........................................................................................169 7.5.1 Phase 1 .............................................................................................169 7.5.1.1 Centralized Acid Scrubbers and Centralized Ammonia Scrubbers .........................................................170 7.5.1.2 Centralized Equipment to Control Volatile Organic Compounds .........................................................171 7.5.2 Phase 2 .............................................................................................173

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7.6

7.7 7.8

Example No. 7-5 ..........................................................................................177 7.6.1 Phase 1 .............................................................................................177 7.6.1.1 Centralized Acid Scrubbers ..............................................177 7.6.1.2 Phase 2 ..............................................................................178 Example No. 7-6 ..........................................................................................180 Example No. 7-7 ..........................................................................................181

Index......................................................................................................................185

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List of Acronyms ACRONYMS AND TERMS BCD: bulk chemical distribution or dispensing BOE: buffered oxide etch CMP: chemical-mechanical planarization or chemical-mechanical polishing CVD: chemical vapor deposition DRE: destruction and removal efficiency EPI: epitaxy or epitaxial HAP: hazardous air pollutant HMDS: hexamethyldisilazane IDLH: immediately dangerous to life and health LFL: lower flammable limit LPCVD: low-pressure chemical vapor deposition MOCVD: metal organic chemical vapor deposition NMP: n-methyl pyrrolidone PECVD: plasma-enhanced chemical vapor deposition PFC: perfluorocompound (as defined by semiconductor industry) POU: point-of-use ppmv: parts per million by volume RO: reverse osmosis RP: reduced pressure SACVD: subatmospheric chemical vapor deposition SC1: standard clean 1 TEOS: tetraethylorthosilicate TMAH: tetramethyl ammonium hydroxide UPW: ultrapure water VOC: volatile organic compound

CHEMICAL SYMBOLS AND FORMULAS AlGaAs: aluminum gallium arsenide GaAs: gallium arsenide HCl: hydrogen chloride HF: hydrogen fluoride Hg: mercury InP: indium phosphide InGaP: indium gallium phosphide NH3: ammonia

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NH4Cl: ammonium chloride NH4F: ammonium fluoride NH4HF2: ammonium bifluoride NH4NO3: ammonium nitrate

REFERENCES 1. Perry, R.H., Green, D., and Maloney, J. Perry’s Chemical Engineer’s Handbook, 6th ed., McGraw-Hill, New York, 1984, pp 3-64 and 3-70.

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1

Semiconductor and Wafer Manufacturing Facilities

1.1 WAFER FAB LAYOUT Wafer fabrication facilities (referred to as “fabs” throughout this book) are generally constructed with a few levels, with the main two levels being the fab level and the subfab level (the level below the fab). The fab level has the wafer processing equipment. The subfab (and any subfloors and other spaces) has exhaust ductwork, wastewater drains, chemical piping, gas piping, process support equipment, etc. This subfab space can be fairly congested. In older fabs, chases are used. Chases are on the same level as the wafer process equipment and contain some of the same equipment as the subfab. Some wafer fabs have both chases and subfab. It is important when understanding the exhaust management of the fab to review its design and where the exhaust ductwork is located. For example, some fabs have exhaust ductwork below the process equipment, and some fabs have the ductwork above the process equipment; many fabs have both. Wafer fab layouts of process equipment can vary from one fab to the next. The current layout tends to be a very large, open-room format where similar process equipment is located together. Examples of similar equipment areas are: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Epitaxy or epitaxial (EPI) reactors Chemical vapor deposition (CVD) Diffusion Dry etch Wet etch Ion implantation (implant) Metals Photolithography (photo) Chemical-mechanical planarization (CMP; also called chemical-mechanical polishing) Equipment parts cleaning and furnace tube cleaning (a few areas possibly dedicated to cleaning parts) Bulk chemical (liquid) distribution (BCD; also called bulk chemical dispensing) Gas distribution room, usually for pressurized toxic and hazardous gases; also called gas bunker Bulk gas distribution (pads) for gases such as nitrogen and hydrogen 1

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2

Semiconductor Industry: Wafer Fab Exhaust Management

It is important to note that any of Items 10 to 13 listed above may be classified as facility-support operations, not wafer fab operations. Semiconductor processes in areas listed above will be discussed later in this chapter. In a subfab design, the exhaust ducts (also called duct headers, duct laterals, or centralized exhaust ductwork) are generally installed below the floor of the fab level and overhead of persons walking in the subfab. Owing to changing production needs (technology change) in a fab, the centralized exhaust ductwork for each type of centralized exhaust system should be constructed with a larger capacity of the rate of airflow to allow for additional process exhaust equipment to be installed. The types of separate centralized exhaust systems that should be considered when a fab is constructed or retrofitted are: • • • •

Acid Ammonia Volatile organic compound (VOC) General and heat

Other centralized exhaust systems for fab support areas are: • •

Pyrophoric gas cylinders (e.g., silane). Toxic gas cylinders (arsine, phosphine, etc.)

Other centralized exhaust systems that can be found in a few wafer fabs are: • • •

Pyrophoric process Gas-specific reclaim or abatement (perfluorocompounds [PFCs]; hydrogen; etc.) High-concentration acids (hot nitric acid solution baths; hot aqua regia solution baths; etc.)

The exhaust types listed above will be explained in detail in Chapter 2. The names of these exhaust types can vary from fab to fab, even in the same company. Point-of-use (POU) devices (also called exhaust conditioners) are used near the process equipment to remove contaminants in the exhaust line before the exhaust enters the centralized exhaust system. These POU devices will also be discussed further in Chapter 3.

1.2 FACILITY-SUPPORT OPERATIONS Exhaust management is more than the exhaust systems, POU devices, and centralized abatement equipment. It is essential to review the impact of exhaust management. For example, if a POU device containing a wet scrubbing system is used on a high concentration fluorine-emitting process, the wastewater discharge from this POU device can contain high fluoride concentrations. The impacts of this wastewater on the wastewater treatment plant and site discharge limit of fluoride wastewater need to be assessed.

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Semiconductor and Wafer Manufacturing Facilities

3

Typical facility-support operations are: 1. Boilers, diesel generators, cooling towers, and chillers 2. Clean-room air system, to ensure very clean air at proper temperature and humidity 3. Centralized scrubbers with fans, etc. 4. Centralized VOC control equipment with fans, etc. 5. Wastewater treatment plants: neutralization, fluoride (if needed), CMP (if needed), etc. 6. Clean-water production plant such as for ultrapure water (UPW) 7. Numerous other mechanical devices such as fans, pumps, etc. 8. Maintaining centralized exhaust systems in the fab and outside the fab 9. Maintaining facility piping, including incoming water, UPW, wastewater, waste organic solvent, etc. 10. Waste handling 11. Maintaining buildings, offices, etc. There are other functions, which may include some of those listed in the wafer fab layout section discussed previously. As mentioned earlier, it is essential to review all impacts from exhaust management decisions (examples of these will be given later).

1.3 SEMICONDUCTOR PROCESS EQUIPMENT This section is not intended to be a semiconductor process course but a general overview. Semiconductor process equipment that impacts exhaust management will be briefly explained.

1.3.1 MANUFACTURING FACILITIES

FOR

SILICON WAFERS

Manufacturing facilities for silicon wafers grow silicon ingots, cut ingots into silicon wafers, and then process these wafers. EPI deposition and processes that can be used in these facilities similar to wafer fabs will be discussed in the next section. Many of the processes used in manufacturing facilities for silicon wafers have minimal impact on exhaust systems and centralized abatement equipment. The following sections list some of the processes that can impact exhaust systems and centralized abatement equipment. 1.3.1.1 Chemical Wafer-Thinning Baths A chemical bath using a mixture of nitric acid solution and hydrofluoric acid solution (sometimes other chemicals also) is used at an elevated temperature to thin raw silicon wafers. This wafer thinning causes the generation of oxides of nitrogen due to surface reactions. If the temperature of the bath is not controlled with a cooling system, more oxides of nitrogen can be generated. Nitrogen dioxide may be seen coming from this bath, as visible emissions begin to occur at low concentrations

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Semiconductor Industry: Wafer Fab Exhaust Management

(parts per million by volume, or ppmv) of nitrogen dioxide. Visible emissions from the centralized wet scrubber may occur if stack outlet concentrations of nitrogen dioxide are high enough. Nitric acid and hydrofluoric acid emissions will also occur. If nitric acid emissions from the centralized wet scrubber are high enough, visible emissions can also occur. 1.3.1.2 Solution Baths of Hot Nitric Acid Heated solution baths of nitric acid can be used in wet benches, in addition to chemical wafer thinning baths. The temperature and concentration of the nitric acid in solution are crucial to emissions from these baths. The temperature of the solution bath of nitric acid (assuming everything else is constant) is critical to nitric acid emissions, as reflected below in vapor pressure (see Reference 1). • • •

Vapor pressure of 70% nitric acid solution at 25°C: 4.10 mmHg Vapor pressure of 70% nitric acid solution at 50°C: 16.5 mmHg Vapor pressure of 70% nitric acid solution at 70°C: 43.3 mmHg

The temperature increase of twofold, from 25 to 50°C, causes a nitric acid emission increase of about four times. The temperature increase of almost three times, from 25 to 70°C, causes nitric acid emissions to increase over 10 times. The concentration of the solution bath of nitric acid (assuming everything else is constant) is critical to nitric acid emissions, as reflected below in vapor pressure.1 • • •

Vapor pressure of 25% nitric acid solution at 70°C: 0.54 mmHg Vapor pressure of 50% nitric acid solution at 70°C: 7.25 mmHg Vapor pressure of 70% nitric acid solution at 70°C: 43.3 mmHg

The concentration increase of twofold, from 25 to 50%, causes nitric acid emissions to increase over 13 times. The concentration increase of almost three times, from 25 to 70%, causes nitric acid emissions to increase about 80 times! When reviewing each bath and its chemistries, it is important to note temperature and concentration to determine emissions and their impact. Other factors in determining bath emissions are surface area, air velocity over the surface, and if the bath is being mixed, air bubbled, etc. If nitric acid emissions are high enough from the centralized wet scrubber, visible emissions can occur. Chemical baths, especially heated ones, can cause condensation in the ductwork; duct damage can result. 1.3.1.3 Other Chemical Baths All chemical baths used in wafer manufacturing should be reviewed for exhaust impact and emissions. One such bath is ammonium hydroxide-hydrogen peroxide solution (also called SC1, which represents standard clean 1), based on ammonia emissions and its impact if exhausted to the centralized acid exhaust system due to formation of ammonium compounds (discussed in Chapter 2).

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1.3.1.4 Large Storage Tanks for Acid Solution Large storage tanks for acid solution (examples are concentrated hydrofluoric acid solution, concentrated hydrochloric acid solution, and concentrated nitric acid solution) can contain high concentrations of acid solutions and large volumes. Sometimes these storage tanks are exhausted directly by fan or by an acid scrubber with a fan (i.e., “suction” on tank), causing higher emissions from these tanks. Acid condensation in the exhaust system can also result. Eliminate this suction on the storage tank so as to reduce emissions and acid condensation in the exhaust system.

1.3.2 SILICON WAFER FABS Some of the processes utilized in silicon wafer fabs that affect exhaust systems and centralized abatement equipment are discussed in this section. 1.3.2.1 Chemical Vapor Deposition Processes CVD processes use compounds to deposit specific layers on wafers and are important to the manufacture of semiconductors. These compounds are typically delivered by gas source or liquid source. Energy can be used to separate these compounds into elements and deposit them as needed (e.g., silane and ammonia are used in plasma to form silicon nitride). The chambers, furnace tubes, bell jars, etc. used for wafer CVD are typically cleaned in situ (inside) using specific gases, or externally to the process equipment in vertical tube cleaners or wet benches. Wipe cleaning can also be used. In situ CVD chamber cleanings generally use perfluorocompound gases (e.g., nitrogen trifluoride, hexafluoroethane, octafluoropropane), chlorine trifluoride, hydrogen chloride, etc. Predevice CVD chamber cleaning can be attained by using a device prior to the CVD chamber to break apart the cleaning gas into elements for immediate use in chamber cleaning (e.g., nitrogen trifluoride in a predevice is fractured into monatomic fluorine and diatomic fluorine for chamber cleaning). Numerous CVD processes are used in a typical silicon wafer fab. Some of these CVD processes are: 1. 2. 3. 4. 5.

Low-pressure chemical vapor deposition (LPCVD) Subatmospheric chemical vapor deposition (SACVD) Plasma-enhanced chemical vapor deposition (PECVD) Atmospheric deposition EPI deposition; generally atmospheric or reduced pressure (RP)

Some examples of processes and chemicals used for different layer depositions are: * 1. Silicon EPI: dichlorosilane or trichlorosilane; arsine; phosphine; diborane; hydrogen chloride; and hydrogen. Other silicon sources such as silane can be used. 2. Silicon germanium EPI: similar to silicon EPI, plus use of germanium. * Note that there are many more processes such as titanium nitride, low-k, etc.

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3. Poly deposition: silane or dichlorosilane; chamber-cleaning gas if cleaning is done in situ. 4. Oxide deposition: silane and nitrous oxide; or tetraethylorthosilicate (TEOS); chamber-cleaning gas if cleaning is done in situ. 5. LPCVD nitride: dichlorosilane and ammonia; clean gas if in situ cleanings are done. 6. PECVD nitride: silane and ammonia; chamber-cleaning gas. 7. Tungsten CVD: tungsten hexafluoride, silane, and hydrogen; chambercleaning gas. 8. Tungsten silicide CVD: tungsten hexafluoride and dichlorosilane or silane; chamber-cleaning gas. Note that a few of the layers cited above can be doped, using inorganic dopants (e.g., phosphine and diborane) or organic dopants (e.g., triethyl phosphate and triethyl borate). CVD feed gases generally have low utilization in the deposition process, so as to deposit a uniform layer. Utilization rates for CVD feed gases of only 5 to 20% are common. Therefore, most of the feed gases are emitted from the process. This is important in developing process exhaust management and POU device strategies. Most CVD process chambers are evacuated by a process pump (example of a nonpump CVD process is atmospheric CVD, which utilizes a negative static pressure from the centralized exhaust system rather than a process pump to evacuate the chamber). This process pump can be dry (using nitrogen purge) or wet (using pump oil usually with smaller amounts of nitrogen purge than dry pumps). The wet pump’s oil needs to be removed at POU (usually by a filter) before entering the exhaust line, POU device, or exhaust lateral. By-products of in situ chamber cleaning depend on the cleaning gas and the material cleaned off of chamber walls. For example, chamber cleaning using carbonbased perfluorocompounds for a silicon oxide deposition chamber can yield silicon tetrafluoride, hydrogen fluoride, fluorine, unreacted feed gas perfluorocompounds, and additional perfluorocompounds (e.g., tetrafluoromethane created from hexafluoroethane cleaning gas). It is important to note that perfluorocompounds can also be made in cleaning the chamber with noncarbon cleaning gas (e.g., nitrogen trifluoride), from the deposits on the chamber wall after using an organic compound. An example is to use a nitrogen trifluoride chamber cleaning gas for a low-k deposition process using a silicon-containing organic compound. Furnace tubes can be cleaned outside the furnace in wet bench baths of acid solution or spray chambers of acid solution (e.g., vertical tube cleaners). These cleaning operations are usually done in an area near the fab or in the fab itself. However, this cleaning operation can also be done outside of the facility. 1.3.2.2 Etch Processes The removal of a layer or portion of a layer on a wafer is another important process. Energy can be used to separate etch feed gases into elements to etch material on

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the wafer. The etched material is considered an etch by-product (e.g., silicon tetrafluoride from an oxide etch process and aluminum trichloride from an aluminum etch process). Generally, if only certain areas of a wafer are to be etched, a photoresist will cover the areas not to be etched. The photoresist will be slightly etched during the etch process, generating low concentrations of halogenated by-products (e.g., chlorinated organic compounds from an aluminum etch process and fluorinated organic compounds from poly, nitride, and oxide etch processes). Numerous etch processes are in a typical silicon wafer fab. Some examples of these etch processes are: 1. 2. 3. 4. 5.

Poly etch: chlorine, hydrogen bromide, and perfluorocompounds Nitride etch: perfluorocompounds Oxide etch: perfluorocompounds Aluminum etch: boron trichloride and chlorine Tungsten etch: sulfur hexafluoride

Examples of perfluorocompounds used in the etching process are carbon tetrafluoride, trifluoromethane, hexafluoroethane, nitrogen trifluoride, and sulfur hexafluoride. It is important to note that perfluorocompounds are defined by the semiconductor industry as a group of compounds, but it is not a strict chemistry definition (e.g., trifluoromethane). Use of perfluorocompounds can also be used to clean the etch chamber in situ. 1.3.2.3 Ion Implantation Ion implantation utilizes gas, liquid, or solid sources to introduce dopants into selected areas of the wafer surface. Photoresist can be on the wafer. Typical gas sources for ion implantation are arsine (arsenic doping), phosphine (phosphorous doping), and boron trifluoride (boron doping). Examples of other dopants are antimony and silicon. 1.3.2.4 Wet Benches (Hoods) Silicon wafer fabs use various wet benches for cleaning wafers, removing specific materials (e.g., oxide), removing photoresist, etc. The wet benches have one or more baths containing acid solutions, base solutions, organic compounds, clean water, or chemical mixtures. Sometimes baths will have a gas introduced such as ozone. These baths can be heated or agitated. An example of a common wet bench is a standard clean bench, which can have baths containing sulfuric acid-hydrogen peroxide solution, ammonium hydroxide-hydrogen peroxide solution, hydrochloric acid-hydrogen peroxide solution, hydrofluoric acid solution, and clean water. As mentioned in an earlier section, the temperature and concentration of the acid compounds in solution are crucial to emissions from these baths. An example of this is presented next. The temperature of the solution bath for hydrochloric acid (assuming everything else is constant) is critical to hydrogen chloride emissions, as reflected below in vapor pressure.1

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• • •

Vapor pressure of 20% hydrochloric acid solution at 25°C: 0.32 mmHg Vapor pressure of 20% hydrochloric acid solution at 50°C: 2.21 mmHg Vapor pressure of 20% hydrochloric acid solution at 70°C: 8.5 mmHg

The temperature increase of twofold, from 25 to 50ºC, causes hydrogen chloride emissions to increase almost seven times. The temperature increase of almost threefold, from 25 to 70ºC, causes hydrogen chloride emissions to increase almost 27 times. The concentration of the hydrochloric acid solution (assuming everything else is constant) is critical to hydrogen chloride emissions, as reflected below in vapor pressure.1 • • •

Vapor pressure of 10% hydrochloric acid solution at 70°C: 0.35 mmHg Vapor pressure of 20% hydrochloric acid solution at 70°C: 8.5 mmHg Vapor pressure of 30% hydrochloric acid solution at 70°C: 208 mmHg

The concentration increase of twofold, from 10 to 20%, causes hydrogen chloride emissions to increase about 24 times. The concentration increase of threefold, from 10 to 30%, causes hydrogen chloride emissions to increase about 594 times! Both of these examples show the significance of concentration and temperature on emissions from chemical baths. A bath which impacts exhaust systems and centralized abatement equipment is the ammonium hydroxide-hydrogen peroxide solution bath, due to emission of ammonia. Ammonia will combine with acid compounds to form ammonium compounds. Ammonium compounds will be discussed further in Chapter 2. Wet bench design affects exhaust management. Each bath that contains certain solutions (e.g., ammonium hydroxide solution, heated aqua regia solution [a mixture of hydrochloric acid solution and nitric acid solution]) needs to be completely segregated from the centralized acid exhaust system into its own centralized exhaust system with centralized wet scrubber, or abated POU before entering the centralized acid exhaust system. Many wet bench bath exhausts are partially segregated or not segregated at all. (Note that it is important to review exhaust segregation when purchasing a new wet bench.) It can appear that the wet bench exhausts are segregated properly; however, chemical “cross talk” can occur above the baths between the individual bath exhaust sections in a wet bench. It is important to review each wet bench for proper exhaust segregation above each chemical-containing bath (and subsequent clean-water bath if applicable), especially for preventing the mixing of ammonia and acid compounds in the exhaust. 1.3.2.5 Spray Processes Spray processes are used to spray similar chemicals that can be found in a wet bench onto the wafers in a chamber. Many of the spray processes require one chamber or two chambers. The chemicals are usually fed by BCD near the spray chamber.

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1.3.2.6 Photolithography Photolithography is generally made up of the following process steps on the wafer: 1. 2. 3. 4. 5.

Vapor prime (e.g., hexamethyldisilazane [HMDS]) Photoresist coating (a mixture of liquid organic compounds and solids) Soft bake (evaporates organic compounds and leaves solids on wafer) Mask alignment and exposure (very low emissions) Develop (organic solvent for negative photoresist and organic base solution — e.g., tetramethyl ammonium hydroxide [TMAH] in water — for positive photoresist) 6. Hard bake (very low emissions) Usually there is an edge bead removal step that occurs for the photoresist that appears on the edges of the wafer after the photoresist coating. 1.3.2.7 Chemical Mechanical Planarization (Polishing) CMP is used to planarize wafer surfaces containing oxide, tungsten, copper, etc. Some of these CMP slurries use ammonium hydroxide solution, which emits ammonia that needs to be considered for exhaust management. 1.3.2.8 Furnace Tube Cleaners Furnace tubes are typically cleaned externally to the furnace. This cleaning can be done either in a horizontal tube cleaner (with acid solution baths for furnace tube immersion) or spray-type vertical tube cleaners. The horizontal tube cleaners emit compounds such as nitric acid and hydrofluoric acid from open baths. Vertical tube cleaners spray acid solutions (like nitric acid solution and hydrofluoric acid solution) in a large chamber; acid mists can be entrained in the exhaust. 1.3.2.9 Gas Cabinets Semiconductor processes utilize several gases which can be fed by pressurized gas cylinders or subatmospheric gas cylinders. These gas cylinders, especially for toxics and pyrophorics, are contained in ventilated gas cabinets. The scenario of an accidental release (leak) from the gas cylinder is usually reviewed before installation, to ensure the accidental release is below one-half the concentrations immediately dangerous to life and health (IDLH). Accidental releases are also referred to in this book as emergency releases. Reducing emergency releases below one-half IDLH concentrations at stack discharge will be discussed further in Chapter 6. Examples of gases that may be reviewed for emergency release are: 1. 2. 3. 4. 5.

Arsine Phosphine Silane Hydrogen chloride Boron trichloride

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6. 7. 8. 9.

Boron trifluoride Ammonia Dichlorosilane Tungsten hexafluoride

It is important to note that silane, as a pyrophoric gas, may be stored in an area by itself. The designs used for silane gas cylinder storage are to ensure that silane is well ventilated, allowing for silane to be adequately diluted or to burn safely if a leak occurs. When gas cylinders and other gas storage vessels are changed out, feed lines are usually purged with nitrogen. These line purges need to be evaluated to assess the need for POU devices. 1.3.2.10 Bulk Chemical Distribution Many fabs feed acid solutions, base solutions, organic solvents, etc. from a BCD area to the process equipment in the fab. The BCD cabinet can exhaust any compounds emitted from the containers and the feed lines. 1.3.2.11 Plating Plating emissions are generally low. Cyanide plating can be an issue if acidic material somehow is accidentally introduced into the cyanide solution, creating hydrogen cyanide gas emissions. This introduction of acidic material could occur by an operator inadvertently placing acidic materials into a plating bath, or could result from acid condensation from ductwork flowing into a plating bath. 1.3.2.12 Other Silicon Wafer Processes Not Discussed in This Book Numerous other wafer fab processes are not discussed in this book because of their minimal or no impact on exhaust management. Some of these processes are: 1. 2. 3. 4. 5. 6.

Steppers Clean-water wafer rinse operations Evaporators Sputterers Ashers Furnaces using inert gases or low flows of process gases (e.g., hydrogen, hydrogen chloride, chlorinated organic compounds, oxygen) that do not affect exhaust management to any extent 7. Test and assembly operations

1.3.3 COMPOUND SEMICONDUCTOR WAFER FABS 1.3.3.1 Metal Organic Chemical Vapor Deposition MOCVD is the deposition of Group III and Group V compounds. Some of these deposition layers are gallium arsenide (GaAs), indium phosphide (InP), aluminum

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gallium arsenide (AlGaAs), and indium gallium phosphide (InGaP). Examples of materials used in the deposition step are: 1. 2. 3. 4. 5.

Trimethylgallium Trimethylaluminum Arsine Phosphine Hydrogen

Gallium nitride is also an MOCVD process, as it can use trimethylgallium and ammonia. It is important to note that other CVD processes similar to those of silicon wafer fabs are used in compound semiconductor (examples are nitride CVD and oxide CVD). 1.3.3.2 Compound Semiconductor Wafer Etch Whole-wafer surface etch and trench etching occur on compound semiconductor wafers. Different surfaces can be etched after MOCVD, nitride CVD, and oxide CVD, for example. Some gases used for these etch processes are: 1. 2. 3. 4. 5. 6.

Hydrogen chloride Boron trichloride Chlorine Silicon tetrachloride Hydrogen bromide Perfluorocompounds

1.3.3.3 Equipment Cleaning Process equipment previously mentioned before can be cleaned in situ or externally to equipment. Some of the chemistries used for cleaning externally are ammonium hydroxide-hydrogen peroxide solution and aqua regia solution (a mixture of concentrated hydrochloric acid solution and concentrated nitric acid solution). Due to spontaneous reaction with air for some materials (e.g., phosphorus deposited in exhaust lines) and the toxic nature of others (arsenic), extreme care is needed during cleaning and removing parts to be cleaned. 1.3.3.4 Other Processes Similar processes (e.g., use of wet benches) are used in the compound semiconductor fab as in a silicon wafer fab. Refer to the silicon wafer fab sections in this chapter for information on these similar processes.

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2.1 EXHAUST TYPES There are several different types of centralized exhaust systems (also called house exhaust systems) that can be used in a wafer fab. The types of centralized exhaust systems are described in the sections that follow. The processes (with point-of-use [POU] devices if used), with their corresponding emissions, need to be reviewed to decide which types of centralized exhaust systems need to be installed for a new wafer fab or for retrofit of an existing wafer fab. Many wafer fabs do not have a separate centralized acid exhaust system and a separate centralized ammonia exhaust system; this lack of exhaust separation is crucial in making exhaust management decisions.

2.1.1 CENTRALIZED ACID EXHAUST SYSTEM The centralized acid exhaust system usually has the largest rate of airflow, which could be hundreds of thousands of cubic feet per minute. The centralized acid exhaust system can contain a mixture of numerous gases, mists, and particulates, depending on what processes are ducted to it. Typical process emissions that are exhausted to a centralized acid exhaust system are acid compounds (e.g., hydrogen chloride, hydrogen fluoride, nitric acid, and sulfuric acid), chlorine, fluorine, silicon tetrafluoride, perfluorocompounds (PFCs), etc. In most wafer fabs, centralized wet scrubbers (called centralized acid scrubbers) are utilized to abate certain emissions in the centralized acid exhaust system before discharging them into the atmosphere. Exhaust management is crucial to ensure that incapables, pyrophorics, particulategenerating compounds, toxics, etc. are reviewed before discharging these contaminants into the centralized acid exhaust system, as these emissions can significantly affect the centralized acid exhaust system and may not be removed efficiently by centralized acid scrubbers. It is essential that no ammonia or very small amounts be emitted into the centralized acid exhaust system, so as to prevent formation of ammonium compounds (more on this later). In general, volatile organic compounds (VOCs) should be minimized into the centralized acid exhaust system (VOCs can be a food source for biogrowth in centralized acid scrubber). Hexamethyldisilazane (HMDS) is sometimes emitted to the centralized acid exhaust system to keep it out of the centralized exhaust system for VOCs, as it can significantly impact some centralized equipment technologies for VOC control (deposition of silicon dioxide). Note that hydrolysis of HMDS can produce ammonia and silicon compounds; therefore it may be beneficial to exhaust HMDS to the centralized ammonia exhaust system (if the fab has this centralized exhaust system). 13

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2.1.2 CENTRALIZED AMMONIA EXHAUST SYSTEM The centralized ammonia exhaust system is utilized for process exhausts containing ammonia only. The rate of airflow (typically 5000 to 30,000 ft3/min) is much lower than that of the centralized acid exhaust system and is usually lower than that of the centralized VOC exhaust system. Acid compounds, fluorine, and chlorine should not be emitted to the centralized ammonia exhaust system. Hexamethyldisilazane (HMDS) is sometimes emitted to the centralized ammonia exhaust system to keep it out of the centralized VOC exhaust system, as it can significantly impact some centralized equipment technologies for VOC control (deposition of silicon dioxide). This makes some sense as HMDS generates ammonia and silicon compounds when hydrolyzed. Other organic compounds are not usually emitted to the centralized ammonia exhaust system.

2.1.3 CENTRALIZED VOLATILE ORGANIC COMPOUND EXHAUST SYSTEM The centralized VOC exhaust system is typically 5000 to 60,000 ft3/min of air from wafer fab processes emitting VOC. Only VOC emissions are recommended for the centralized VOC exhaust system. Because of construction materials for the centralized VOC exhaust system (typically metal) and their impact on centralized VOC control equipment, acid compounds, base compounds, fluorine, chlorine, etc. are discouraged from being emitted to a centralized VOC exhaust system. In addition, other emissions (e.g., hydrides) should not be emitted to a centralized VOC exhaust system because of deposition in the exhaust duct and formation of oxidation byproducts if thermal oxidation or catalytic oxidation is used for the centralized VOC control equipment. Nitrogen-containing and silicon-bearing organic compounds should also be reviewed for possible formation of by-product and their impact in the centralized VOC control equipment. For example, nitrogen-containing organic compounds can produce oxides of nitrogen in a thermal oxidizer or catalytic oxidizer, and silicon-containing organic compounds can oxidize (if oxidation control technology is used) into silicon dioxide and possibly plug the centralized VOC control equipment. One such organic compound is HMDS, which is a nitrogen-containing and silicon-containing organic compound.

2.1.4 CENTRALIZED GENERAL

AND

HEAT EXHAUST SYSTEM

This centralized exhaust system is utilized to exhaust generally noncontaminated air, which includes heat exhaust. In some cases air with very low concentrations of contaminants is emitted to a centralized general, heat exhaust system. A centralized abatement system is not used, as this centralized exhaust system should have little or no emissions by design.

2.1.5 CENTRALIZED PYROPHORIC EXHAUST SYSTEM Centralized pyrophoric exhaust system is rarely found in the semiconductor industry. This centralized exhaust system can be used instead of POU devices on certain processes that use pyrophoric gases (e.g., silane), compounds considered slightly

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pyrophoric (e.g., dichlorosilane), and flammable gases (hydrogen). Sometimes centralized wet scrubbers or other centralized abatement technology is attached to the end of the centralized pyrophoric exhaust system, as these processes may have other emissions (e.g., silicon tetrafluoride, hydrogen fluoride, phosphine, ammonia, perfluorocompounds). The centralized pyrophoric exhaust system typically has particulate deposition and sometimes corrosion, so maintenance can be high for this type of system. Due to this high maintenance and other concerns, a few wafer fabs have decided to install POU devices on these processes and discontinue use of the centralized pyrophoric exhaust system.

2.1.6 CENTRALIZED EXHAUST SYSTEM

FOR

HIGH ACID CONCENTRATION

This centralized exhaust system is very rare but important to note. Processes emitting high acid concentrations (e.g., hot aqua regia solution, hot nitric acid solution, acid mists entrained in exhaust during a process operation such as spray processes) can cause acid mists to form in standard centralized acid scrubbers, possibly causing visible stack emissions. In addition, odors and equipment corrosion can be found outside of the fab. These processes emitting high concentrations of acid can be segregated into a separate exhaust system and controlled with a specialized centralized abatement system. There usually need to be several of these processes to justify installing a separate centralized exhaust system, as a few of these processes can be controlled by POU devices and then exhausted to a centralized acid exhaust system. These acid mists can be seen as visible emissions coming from standard centralized wet scrubbers, and can deposit on surfaces near the discharge stacks causing corrosion. These acid mists are generally visible at around 1 part per million by volume (ppmv) and above. Some of these emitting processes of high acid concentration may have process by-products (e.g., chlorine and nitrosyl chloride from hot aqua regia solution) that are emitted also, which may need to be abated. An example of a centralized abatement system that has been successfully used for a centralized exhaust system for high acid concentration is a vertical countercurrent packed-bed wet scrubber (with a scrubbing solution of high pH) followed by a submicron filter for acid mist.

2.1.7 OTHER CENTRALIZED EXHAUST SYSTEMS In some instances there is a need to have a separate exhaust system for a particular purpose. Some examples of this are: 1. Centralized abatement of perfluorocompounds (PFCs) or recycle exhaust system: used to capture PFCs from many processes, and then recycle or abate them. 2. Centralized hydrogen reclaim system: hydrogen from epitaxial (EPI) processes is reclaimed and then generally reused in the EPI processes. Other uses of this hydrogen are possible. 3. Centralized process exhaust system for oxides of nitrogen: a separate centralized exhaust system for processes that emit high concentrations of

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oxides of nitrogen, due to a required special design of a centralized abatement system. This type of centralized exhaust system is found usually in a silicon wafer-manufacturing facility. Other compounds, such as nitric acid and hydrogen fluoride, may also be in this centralized exhaust system, so design of the centralized abatement system should consider this. 4. Centralized aggregate exhaust system for ion implanters: this exhaust system combines process pump exhausts from ion implanters to abate compounds of interest (e.g., arsine and phosphine). 5. Exhaust system for silane gas cabinets: allows for safe release of silane in case of emergency release. 6. Exhaust system for toxic gas cabinets: in case of emergency release, usually with abatement system or with dilution system. Some of the above centralized exhaust systems and corresponding centralized abatement systems will be discussed in later chapters.

2.1.8 INDIVIDUAL EXHAUST

TO

OUTSIDE

Individual exhaust systems can be found in a wafer fab that do not connect to centralized exhaust systems and are usually exhausted to the atmosphere (sometimes through a POU device). Some examples of these are: 1. EPI exhaust: for safely venting hydrogen from an individual EPI process 2. Pyrophoric exhaust (for venting a process exhaust that has a pyrophoric compound [and most likely others which need to be reviewed]). 3. Chemical storage vent

2.2 EXHAUST MANAGEMENT CHALLENGES This section discusses the ABCs of exhaust management, describing what the challenges are and how they affect exhaust management. More detailed solutions will be described in later chapters.

2.2.1 AMMONIA It is appropriate to begin this list with ammonia, as it is usually No. 1 on the exhaust management concern list. Ammonia (NH3) will react immediately with acid compounds (e.g., hydrogen chloride [HCl], hydrogen fluoride [HF], and nitric acid [HNO3]) when they come into contact in an exhaust system, forming ammonium compounds as follows: NH3 + HCl → NH4Cl NH3 + HF → NH4F NH3 + 2HF → NH4HF2 NH3 + HNO3 → NH4NO3

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The equilibrium of this reaction at room temperature is driven almost all to the product (e.g., ammonium chloride [NH4Cl], ammonium fluoride [NH4F], ammonium bifluoride [NH4HF2], and ammonium nitrate [NH4NO3]). Ammonium compounds formed when ammonia contacts sulfuric acid or phosphoric acid are not discussed, as concentrations of sulfuric acid and phosphoric acid in a centralized acid exhaust system are generally very low because of very low vapor pressures of sulfuric acid solution and phosphoric acid solution in processes used. (Note that ammonium compounds resulting from ammonia contacting sulfuric acid or phosphoric acid may be found in an exhaust duct connected to processes using these acid solutions and ammonium hydroxide solution.) Ammonia can also react with silicon tetrafluoride and hydrogen fluoride to form ammonium hexafluorosilicate. These ammonium compounds create the following exhaust management challenges. 1. Ammonium compounds can deposit in the exhaust duct where it is created and continue downstream. Ammonium compounds are generally white in color, but can be different colors if mixed with other contaminants. 2. Water molecules adhere to ammonium compounds and form submicron particles. This can occur in centralized wet scrubbers and in POU wet scrubbing systems. These submicron particles act like a gas by motion, but have minimal mass transfer from the gas phase to the liquid phase. Therefore, these ammonium compounds pass right through centralized wet scrubbers and are emitted as submicron particles. White or bluishwhite visible emissions can be observed coming from centralized wet scrubbers at around 1 ppmv and above. It is possible to observe this opacity at less than 1 ppmv, but it can be consistently found at 1 ppmv and above. Since these ammonium compound particles flow through standard centralized wet scrubbers, the removal efficiency tests generally conducted on these wet scrubbers show low removal efficiencies for acid compounds and in some cases negative removal efficiencies for acid compounds (discussed in Chapter 4). Ammonium compounds will also pass through standard POU wet scrubbing systems, typically depositing in the exhaust duct of the POU device. Therefore, do not mix gas streams containing ammonia and acid compounds! Examples of ammonia-emitting processes to be reviewed when eliminating or preventing ammonia mixing with acid compounds are: 1. Wet benches containing baths with ammonium hydroxide solution (e.g., standard clean [SC1] baths, reticle cleaners, and cleaning of compound semiconductor parts) 2. Spray processes using ammonium hydroxide solution 3. Chemical-mechanical planarization (CMP) operations using ammonium hydroxide solution 4. Wafer-cleaning (scrubbing) equipment using ammonium hydroxide solution

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5. 6. 7. 8. 9. 10.

Nitride chemical vapor deposition (CVD) process (ammonia gas) Oxynitride CVD process (ammonia gas) Low-pressure chemical vapor deposition (LPCVD) nitride (ammonia gas) Other furnace processes with ammonia gas Bulk chemical distribution (BCD) of ammonium hydroxide solution BCD of concentrated tetramethyl ammonium hydroxide solution (before dilution for process use) 11. Purge of ammonia gas cylinder line (usually very small amount of ammonia gas) 12. Vents for Industrial wastewater pipes (near ammonium hydroxide solution bath drains) 13. Cleaning of parts using ammonium hydroxide solution It is important to note that generally very low or nondetectable ammonia emissions come from use of dilute tetramethyl ammonium hydroxide solution and from the use of buffered oxide etch (BOE) solutions. The key to very low ammonia emissions from BOE solution baths is pH, which if low will keep ammonia in solution. Each BOE bath exhaust should be tested to ensure it does not significantly contribute ammonia (i.e., less than 1 ppmv in process exhaust) to the centralized acid exhaust system. It would be worthwhile to check ammonia emissions from the use of dilute tetramethyl ammonium hydroxide solution (e.g., positive photoresist developers). A “rule of thumb” can be used to determine if an ammonia-emitting process needs a POU device before exhaust enters the centralized acid exhaust system or should be segregated to a centralized ammonia exhaust system. One such rule-ofthumb is 1 ppmv or less ammonia in process exhaust, or another rule-of-thumb is no measurable ammonia in process exhaust. The choice is made by company personnel. The important thing to remember is any mixing of ammonia with acid compounds will make ammonium compounds.

2.2.2 BENDS Any bends in an exhaust line or exhaust duct can cause deposition of liquids or solids to occur. If a straight exhaust line or exhaust duct can be installed, do it! It seems to be most critical in smaller-diameter ducts (e.g., 1.5, 2, and 4 inches), which typically occur between process chamber and connection to the centralized acid exhaust system.

2.2.3 CORROSION

AND

OTHER DAMAGE

Certain gases and acid mists can cause corrosion of the exhaust line and exhaust duct or other damage, especially if moisture (condensation) is involved. It is important to install proper construction materials for exhaust lines and exhaust ducts. Typical sources of moisture (condensation) are wet benches, spray processes, fire suppression systems, and POU wet scrubbing systems. Sloping the exhaust lines and exhaust ducts and installing drains should be done in an attempt to eliminate acid solution pooling.

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Some wet benches have their exhaust outlets at floor level, including acid drains. Sometimes the acid wastewater is entrained in the ducts and drains down to the centralized acid exhaust system. Scoping the duct with a camera and light can show where the source of moisture is and where it is collecting, so it can be eliminated. Corrosion can also occur in POU devices. This occurs when the construction materials react with gases etc. from the process. Fluorine is an example. An existing POU device was operating successfully on a CVD process chamber using carbon-based PFC chamber clean. This existing CVD process equipment and clean gas were changed to nitrogen trifluoride pre-device CVD chamber clean (monatomic fluorine and diatomic fluorine are formed from breaking apart nitrogen trifluoride in a device before entering process chamber). The POU device materials were corroded away by the fluorine and the higher fluoride concentrations in the scrubbing solution.

2.2.4 DIAMETERS

FOR

EXHAUST LINE

OR

EXHAUST DUCTS

Exhaust lines for specific applications and the exhaust duct in each application in the centralized exhaust systems should be installed with minimal change in diameters, to eliminate solid deposition and to minimize pressure drop. The exhaust line diameter should be the same between process and each piece of equipment in succession. Examples are a 2-inch diameter exhaust line from the CVD process chamber to the process pump, a 2-inch diameter exhaust line from the process pump to the POU device, and a 4-inch diameter exhaust line from the POU device to the centralized acid exhaust system.

2.2.5 EXPLOSIONS

AND

FIRES

Duct explosions and fires can occur with pyrophoric gases (e.g., silane) and flammable gases (e.g., hydrogen and natural gas). Explosions and fires can also occur if pyrophoric solids (e.g., certain silicon compounds and phosphorus) build up in the ductwork, usually resulting from deposition of process by-products in the duct. A safety review should be completed on processes (especially those using pyrophoric gases and flammable gases) to ensure that gas handling is safe, any by-products generated are known, the deposition of pyrophoric solids is eliminated, proper POU device technologies are used, and codes are met.

2.2.6 FLUORINE Fluorine became a challenge when the technology of nitrogen trifluoride predevice CVD chamber cleaning was developed. Very high concentrations of fluorine and other by-products such as silicon tetrafluoride are generated from this chamber cleaning. If no POU device is used on this CVD process, fluorine can impact certain duct materials of the centralized exhaust system and emissions from centralized wet scrubbers. When the nitrogen trifluoride predevice CVD chamber cleaning was first developed, odors were found outside the fab building. Upon investigation of the source, some of the processes using the nitrogen trifluoride pre-device CVD chamber cleaning

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Semiconductor Industry: Wafer Fab Exhaust Management

did not have POU devices. These odors were likely oxygen difluoride which had formed in the centralized wet scrubbers. These centralized wet scrubbers were operating at high pH by utilizing sodium hydroxide solution. Fluorine does significantly react with moisture in air in a duct to form hydrogen fluoride; however, if a small amount of fluorine does get to the centralized wet scrubber, then it could form oxygen difluoride depending on the chemistry and pH of the scrubbing solution. Oxygen difluoride has an extremely low odor threshold; therefore a small amount can be detected. In addition to odors, some duct materials were degrading. Fluorine and water in the duct were forming hydrofluoric acid solution, and along with some effect from gas-phase fluorine, were degrading the duct. Also, some O-ring materials in the exhaust line connections of the process pump were damaged by fluorine. There is some “technical thought” that the silicon tetrafluoride from CVD chamber cleanings were causing the odors outside the fab building, instead of fluorine and oxygen difluoride. Here is some information to consider why silicon tetrafluoride may not be an odor source: 1. Silicon tetrafluoride has been a by-product from CVD chamber cleaning before the nitrogen trifluoride predevice CVD chamber cleaning was developed. Odors outside fab buildings have increased since nitrogen trifluoride predevice CVD chamber cleaning with no POU devices have been installed in fabs. 2. Odors outside the fab building also are found in some silicon wafer fabs that do not have a segregated ammonia exhaust system (i.e., ammonia and acid compounds exhaust into the centralized acid exhaust system). Ammonia is mixing with silicon tetrafluoride and hydrogen fluoride, forming ammonium compounds. The ammonium compounds are flowing through the centralized wet scrubber (only slightly scrubbed) and are being emitted. This could be an odor source. As soon as ammonia is removed from the centralized acid exhaust system (stopping the formation of ammonium compounds), the odors are reduced. 3. If silicon tetrafluoride is an odor source, the solution is still to install a POU device on the CVD process chamber that is using nitrogen trifluoride predevice CVD chamber cleaning; this is the same solution for fluorine from the same nitrogen trifluoride predevice CVD chamber cleaning. Therefore, installing a POU device in the nitrogen trifluoride predevice CVD chamber cleaning operation could be a solution for odors outside the fab. A typical nitrogen trifluoride predevice CVD chamber cleaning process that is found without the use of a POU device is an oxide CVD process using tetraethylorthosilicate (TEOS).

2.2.7 GASKETS, O-RINGS, CONNECTIONS,

AND

FITTINGS

Proper materials are needed for materials used on exhaust line connections and exhaust duct connections (if applicable). The type of material should be selected

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based on gases used, temperature, and type of use. Since most exhaust lines and exhaust ducts are under negative static pressure of the exhaust, it is important that the connection not allow air to leak in. One location where gases could leak out and expose personnel is at the outlet connection of the process pump (there is a short distance at the process pump discharge, or exhaust outlet, where positive static pressure of the exhaust could develop before realizing negative static pressure from the centralized exhaust system). Proper connections are essential to ensure leaks in or out are eliminated. Ideally, no connections or fittings should be used; realistically, use the minimum number. It is important that exhaust lines and exhaust ducts be installed to allow for maintenance in case of plugging or corrosion (i.e., duct replacement or cleaning).

2.2.8 HEXAMETHYLDISILAZANE HMDS is used primarily for vapor prime coating prior to photoresist coating. HMDS evaporates from the wafer surface and is exhausted. Many wafer fabs have VOC control equipment. HMDS is oxidized in certain centralized VOC control equipment to silicon dioxide, oxides of nitrogen, carbon monoxide, carbon dioxide, and water. For example, silicon dioxide can plug heat transfer media of the regenerative thermal oxidizer, plug heat exchangers in recuperative thermal oxidizers, poison a catalyst in a catalytic oxidizer, etc.

2.2.9 INDUSTRIAL WASTEWATER PIPING VENT This is listed as it is one exhaust source that may be overlooked. It is common to vent the industrial wastewater piping (also called acid wastewater piping) to the centralized acid exhaust system, to provide ventilation of general fumes and for possible fumes generated from exothermic mixing reactions in the wastewater piping. If ammonium hydroxide solution is drained to industrial wastewater piping, ammonia can be entrained into this vent and then into the centralized acid exhaust system. The ammonia can react with acid compounds forming ammonium compounds.

2.2.10 JUNIOR SIZE IT Bigger is not always better! Sometimes larger systems are used to attempt to make exhaust management challenges go away, but end up making them worse or more costly. It is worth the time and resources to incorporate the right technology the first time!

2.2.11 KINDS

OF

MATERIALS

FOR

EXHAUST LINES

AND

EXHAUST DUCTS, ETC.

It is essential to specify and install the right materials for the applications. This is a widespread challenge. Consider moisture that can make acid solutions in exhaust lines and exhaust ducts (moisture could be originating from a POU device, so review that before design); consider future possible process and feed material changes; consider temperature; and proactively look at how to maintain exhaust lines and

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Semiconductor Industry: Wafer Fab Exhaust Management

exhaust ducts when maintenance is needed (how to access, how to take apart, how to clean, how fast to order replacement, etc.). Do not assume that the supplier will always provide the right materials (e.g., a centralized acid scrubber made of fiberglass needs hydrogen fluoride resistance in it; just because you told the supplier you had hydrogen fluoride in the incoming airstream does not mean they will provide the fiberglass with hydrogen fluoride resistance!).

2.2.12 LEAKS (AIR) Air leaks into process exhaust lines are a leading cause of plugging. Air (with moisture) will react with many exhaust contaminants of CVD and etch processes to produce particles, which deposit and lead to process downtime and maintenance. Check for leaks in all exhaust lines from process chamber to the first purposeful addition of air (e.g., the POU thermal oxidizing system, the centralized exhaust system, etc.). EXAMPLE 2-1: TUNGSTEN CVD PROCESS (USES TUNGSTEN HEXAFLUORIDE, SILANE, HYDROGEN, AND CHAMBER CLEANING GAS). Checks for air leaks are made from the process chamber to the process pump and from the process pump to the POU device. If the POU thermal oxidation-wet scrubber system is used, then air is added into the thermal oxidation section, so stop checking for air leak at the inlet of the POU device. If a POU wet scrubbing system is used, with no air introduction (which is common), then check for air leaks from this POU device to the connection where air is first introduced (probably the centralized acid exhaust system).

2.2.13 MAINTENANCE Maintenance is a must for exhaust systems, POU devices, and centralized abatement equipment. The questions are how often and what needs to be done. The goal is to have no unexpected process downtime and no environmental, health, and safety concerns. So, how often should maintenance be done? It really depends on the system. A possible expectation for maintenance from process chamber exhaust all the way to the connection to the centralized exhaust system is once a quarter for 4 hours. This could be accomplished at the same time that the process equipment is being maintained (note that this does not apply to all process equipment). Since centralized exhaust laterals, centralized exhaust mains, and centralized abatement equipment impact more than one piece of fab equipment, preventive maintenance may only be able to occur once a year (or longer), due to production. This is why it is essential to have an exhaust management plan to keep exhaust ducts from plugging and corroding, and to have appropriate redundancy of centralized abatement equipment with high uptime.

2.2.14 NOT ENOUGH EXHAUST Enough exhaust (flow and negative static pressure of the exhaust) is essential for process operation, POU devices, and centralized exhaust systems. Lack of exhaust

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can affect wafer quality, especially for atmospheric processes. On the outlet of a process pump, it will most likely be positive static pressure of the exhaust for a short distance until the negative static pressure of the centralized exhaust system takes over (note that it is essential to ensure that O-ring material for the exit connection of the process pump is compatible with the chemistry in the gas stream (e.g., fluorine from chamber cleaning) and the gas temperature). If the exhaust from the process pump is at higher pressure than the pressure outside of the exhaust line, then there is a possibility of contaminants leaking out of the exhaust line; this can be a larger issue if there is a blocked exhaust line after the process pump. Static pressure of the exhaust at and/or near the process pump is usually measured in case of excessive positive static pressure of the exhaust. POU devices need enough exhaust also to ensure proper operation. POU devices can cause operation issues with atmospheric processes, as atmospheric processes require a steady negative static pressure and exhaust flow. Plugging of centralized exhaust ducts can reduce the flow rate of the exhaust and provide inadequate negative static pressure to process equipment. Reduced rate of airflow and inadequate negative static pressure of the exhaust at the inlet of centralized abatement equipment can also cause issues for process equipment. This situation can be caused, for example, by plugged packing material of the centralized wet scrubbers, a plugged demister of the centralized wet scrubber, a plugged rotorconcentrator on centralized VOC control equipment, and plugged heat transfer media in a centralized regenerative thermal oxidizer.

2.2.15 OIL MIST Wet process pumps are still used in many fabs. The oil mist from these process pumps should be filtered before it enters the centralized exhaust system. Oil can deposit in the exhaust duct, impact POU devices, and in some cases be seen in centralized wet scrubbers, as drips on stacks, and as visible stack emissions. Capture and recycle programs have been utilized since some of these oils are expensive.

2.2.16 PERFLUOROCOMPOUNDS (GLOBAL WARMING GASES) Perfluorocompounds (PFCs) are relatively inert and do not affect exhaust systems directly. However, global-warming gases have been proactively addressed by the semiconductor industry. Significant research and development have been done. A voluntary agreement for PFC reduction has been signed by the semiconductor manufacturers in several countries to reduce mass emissions of certain PFC gases by the year 2010. There are certain listed PFC gases that this agreement applies to (examples are nitrogen trifluoride, sulfur hexafluoride, tetrafluoromethane, trifluoromethane, hexafluoroethane, octafluoropropane, and cyclic-octafluorobutane). It is important to note that PFCs are defined by the semiconductor industry as these compounds, but is not a chemistry definition. Nitrogen trifluoride predevice CVD chamber cleaning was developed to reduce PFC emissions. This technology development significantly reduced PFC emissions. It did create exhaust management challenges (discussed in this book).

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2.2.17 QUALITY

OF INSTALLATION

Proper installation of process exhaust lines, POU devices, POU device exhaust lines, centralized exhaust system components, and centralized abatement equipment are key to their operation (assuming the system is designed correctly for the application). A properly designed POU device can be installed improperly, causing operation challenges and excess downtime. It is worth the expense to contract with knowledgeable, high-quality contractors.

2.2.18 RESTRICTIONS (PARTICULATE DEPOSITION,

ETC.)

Particulate deposition in exhaust lines and exhaust ducts is common in the semiconductor industry. The good news is it can be prevented! Some sources of particulate deposition are gas by-products of a CVD process, by-products of an etch process, and ammonium compounds. Stay tuned for many examples of how to prevent this.

2.2.19 SILANE Silane is a pyrophoric gas primarily used in CVD processes. A pyrophoric gas at certain concentrations reacts (oxidizes) with air without an ignition source. Silane at pyrophoric conditions can explode or be a flame. What is interesting is that silane below its pyrophoric concentration actually “survives” for a time (there is a general thought that silane immediately reacts with air no matter what the concentration, which is not accurate). Low concentrations of silane have been measured at the stacks of centralized wet scrubbers, hundreds of feet from the process emitting silane and in air almost the whole distance. In addition, the silane that does react in the exhaust system creates silicon dioxide, which can deposit in duct, deposit in centralized scrubbers, or be emitted. This information has encouraged some fabs to install POU devices with thermal oxidation technology rather than use passive POU air addition systems.

2.2.20 TOXIC GASES Some of the gases used in the semiconductor industry are considered toxics (arsine, phosphine, etc.). The semiconductor industry has established significant technology to deal with toxics. Examples are life safety systems, secondary contained gas lines, properly ventilated gas cabinets, specialized POU devices, and emergency release abatement devices.

2.2.21 ULTRAPURE WATER

AND

OTHER WATER SUPPLIES

The quality of water flowing to POU devices and centralized wet scrubbers is critical to their operation. Many effects can occur, such as the following: 1. Makeup water that is city water with high calcium concentration is used in a POU wet scrubbing system with no chemical addition for ammonia abatement: scaling of packing can result.

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2. A centralized acid scrubber operating at a scrubbing solution pH of 9 to 10, using first-pass reverse osmosis (RO) reject water with high calcium concentration as makeup water: causes significant scaling of packing and demister. 3. Makeup water that is recycled industrial wastewater or ultrapure water (UPW) in a POU thermal oxidation-wet scrubbing system removing high exhaust concentration of fluorine: corrosion of certain metal components in the POU device in a short time period. 4. Makeup water that is reclaimed rinse water (from wet bench wafer rinse baths), in a centralized acid scrubber: visible stack emissions. Ammonia was in the reclaimed rinse water at high enough concentrations to transfer to the exhaust air in the centralized acid scrubber, contact acid compounds, and form ammonium compounds. Ammonium compounds are the visible emissions.

2.2.22 VERTICAL TUBE CLEANERS

AND

OTHER SPRAY PROCESSES

Many vertical tube cleaners have been designed with a slot exhaust in the back of the spray chamber (not generally designed well for reducing exhaust acid mist entrainment). When acid solution is sprayed on furnace tubes or EPI bell jars, the negative static pressure of the exhaust removes acid mists directly into the exhaust duct. These acid mists condense in the exhaust duct and can be transported in the exhaust air to the centralized wet scrubbers. In some cases visible emissions can be found coming from centralized acid scrubber stacks (submicron acid mists are visible at around 1 ppmv and above). An orange opacity may occur in the vertical tube cleaner (and possibly coming from the centralized wet scrubber stacks) if polysilicon tubes or EPI bell jars are being cleaned with an acid solution (using concentrated nitric acid solution as a component), causing an exothermic reaction generating nitrogen dioxide (the orange color). Reformulating the acid cleaning solution can prevent this situation. Some spray processes use a chamber to spray different acid solutions and/or ammonium hydroxide-hydrogen peroxide solution separately onto wafers. Chemical mists can be entrained in the chamber exhaust, which then condense in the exhaust duct. Generally the more negative the static pressure of the exhaust is, the more chemical mists are entrained in chamber exhaust. Ammonia generated from the use of ammonium hydroxide-hydrogen peroxide solution in spray processes needs to be managed so as not to combine with acid compounds to form ammonium compounds. Possible remedies are ammonia being removed in a POU device before entering the centralized acid exhaust system or exhausting ammonia to the centralized ammonia exhaust system.

2.2.23 WET BENCHES (HOODS) Several wet benches (hoods) are generally used in a wafer fab. These wet benches contain one or several baths that contain chemical solutions or clean water for numerous wafer-processing steps. The exhaust from these wet benches should be

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Semiconductor Industry: Wafer Fab Exhaust Management

reviewed for proper exhaust segregation and to ensure that liquid does not enter the exhaust ducts (sometimes exhaust is installed at the bottom of the wet bench near the wastewater drain or overflow, which allows for liquid to flow into the exhaust duct below the wet bench). Proper exhaust segregation is a challenge, since many of these wet benches are exhausted in the same area below the baths and in the same area above the baths. Each bath should be exhausted separately in the area below the bath, since the use of the wet bench can change over its lifetime. The area above the bath is more challenging, since it is open to allow access, may have a minienvironment, and in many cases, to accommodate robotics. Therefore, “cross talk” can occur between exhausts from different baths. Balancing of the exhaust area above the baths is crucial to reduce cross talk. Once the segregation is complete, then each exhaust can be connected to the separate centralized exhaust systems for acid, ammonia, and VOCs (or POU devices can be installed if certain centralized exhaust systems do not exist). Some examples of bath solutions that should be reviewed for impact on exhaust management are the following: 1. Ammonium hydroxide solution (usually mixed with hydrogen peroxide solution); can be called standard clean (SC1) 2. Aqua regia solution (a mixture of three parts concentrated hydrochloric acid solution with one part concentrated nitric acid solution). Heated aqua regia is the one to look out for! 3. Heated nitric acid solution (e.g., concentrated nitric acid solution at 70˚C) 4. Heated photoresist stripper

2.2.24 “X-CESS” NEGATIVE STATIC PRESSURE

OF

EXHAUST

Excess negative static pressure of the exhaust can impact processes, POU devices, and centralized abatement systems. Excess exhaust can affect wafer quality, especially for atmospheric processes. Too much exhaust on POU devices causes operation issues, and with some technologies (e.g., a wet scrubbing system) it can cause mists (with contaminants) to be entrained into the exhaust. These mists condense in exhaust lines, causing corrosion, particulate buildup from contaminants in the mists, and other exhaust challenges. Excess negative static pressure of exhaust in POU oxidation systems, POU dry technology systems, and POU wet scrubbing systems can cause reduced gas residence time and can cause nonuniform gas flow through the POU device. Many POU device suppliers provide ranges of exhaust static pressure for their POU devices.

2.2.25 “Y” HEAT EXHAUST LINES Heating of exhaust lines should be used to keep particulates from building up in the exhaust lines from the process chamber to the process pump and from the process pump to the POU device for certain processes. Heat trace and adding hot nitrogen to the exhaust line are two ways to accomplish heating of the exhaust line. If heating of the exhaust line is used to keep the exhaust line clean without the use of a POU

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device (i.e., heating the exhaust line from the process pump to the centralized exhaust system), then the particulates being kept from the exhaust line are ending up in the centralized exhaust system! Some examples of processes that should be highly considered for heating of exhaust lines are in the following list. (Note that some CVD chamber cleanings can help in keeping exhaust lines clean.) 1. 2. 3. 4. 5. 6.

Nitride CVD Oxynitride CVD Tungsten CVD Tungsten silicide CVD LPCVD nitride Aluminum metal etch

Some other processes to review for heating the exhaust line (depends on how often it plugs and whether maintenance, cost, and downtime are considered serious): 1. Tungsten etch 2. TEOS-based oxide deposition processes 3. Other silane-based processes (e.g., polysilicon CVD and oxide CVD) Different exhaust-line heating products are available and can be very expensive.

2.2.26 ZERO UNPLANNED DOWNTIME Zero unplanned downtime CAN be achieved if the wafer fab has a detailed exhaust management strategy, which includes designing it right the first time and fixing it when it is found to be a challenge. Include persons on your team who understand exhaust systems, POU devices, and centralized abatement equipment; who can troubleshoot existing challenges; and who can design it right the first time. These persons are hard to find.

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3

Point-Of-Use Devices and Exhaust-Line Requirements

3.1 POINT-OF-USE DEVICE INFORMATION POU devices are designed for treating emissions from the outlet of the semiconductor processes, so as to remove the contaminants of interest before they enter the centralized exhaust system. POU devices have been installed to increase production uptime, to protect fab assets, and for personnel health and safety reasons; in a few cases POU devices can be installed for environmental reasons. Some of the POU devices are interlocked with the process equipment (i.e., when the POU device fails, the process equipment is shut down). A delayed shutdown can be set up to allow the wafer run to finish, as long as it is safe. POU devices are generally considered a “headache” to production and can be maintenance intensive. POU devices are essential for some process equipment to keep production operating, for safety reasons, and for protecting centralized exhaust systems. Therefore, it is essential to understand why POU devices are needed, the best POU device technology for the process application, and good surrounding exhaust line practices. This chapter will discuss these important items. There are numerous examples in this chapter of POU device selections for certain processes, and some examples of things that can cause POU devices to operate less effectively.

3.1.1 MAIN REASONS

FOR THE

USE

OF

POINT-OF-USE DEVICES

Many times the following questions are asked (and should be asked): why is a POU device for this process being used, or why was this POU device technology chosen for this process equipment or gases? Sometimes the answers to these questions are, it is what we always used, or it is what the past person installed on similar process equipment. Sometimes the POU device can actually increase plugging of the POU device exhaust line if it was not there at all! Sometimes the makeup water source is assisting in corrosion or plugging of the POU device, causing higher maintenance cost and reduced process wafer throughput. A good way to start is to list the main reasons for POU device use. 1. Prevent exhaust restrictions (blocked exhaust line or deposition in the centralized exhaust system). Reactions between gases, solids from the

29

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process, or condensation of vapors produce material buildup in the exhaust line and centralized exhaust system. This buildup can cause production downtime to the clean exhaust line and can cause back pressure, potentially allowing for gases to leak into a subfab or fab area. Centralized exhaust duct collapse can occur if the exhaust duct becomes too heavy with solids. Some processes that can cause deposition in exhaust lines and a centralized exhaust system are: a. b. c. d. e. f. g.

Aluminum metal etch Tungsten chemical vapor deposition (CVD) Tungsten silicide CVD Nitride CVD Oxynitride CVD Low-pressure chemical vapor deposition (LPCVD) nitride Tetraethylorthosilicate (TEOS)-based oxide CVD (mainly resulting from TEOS condensation and later reaction with chamber cleaning gas by-products) h. Titanium nitride CVD (using titanium tetrachloride and ammonia)

Heating the exhaust line to minimize particulate deposition can be used. 2. Prevent exhaust fires and explosions. Flammable gases (e.g., hydrogen) and pyrophoric gases (e.g., silane) can be used in semiconductor process equipment. These gases at specific concentration ranges in air can cause a fire or explosion in the exhaust line or centralized exhaust system. Flammable gases need a source of ignition, but pyrophoric gases do not at specific concentrations in air. Facility damage, extensive fab downtime, and personnel injury could result. 3. Prevent exhaust corrosion. Etching gases (e.g., chlorine, hydrogen chloride, and hydrogen bromide) and process by-products (e.g., fluorine and hydrogen fluoride) can damage exhaust lines and centralized exhaust ducts, depending on materials of construction. Production downtime and possible personnel exposure to these gases could result. Moisture from processes such as wet benches and spray processes and POU devices (e.g., wet scrubbing systems) can enhance corrosion when acid compounds, chlorine, and fluorine are present in exhaust lines and centralized exhaust ducts. In some cases, proper materials of construction can be used instead of installing POU devices. 4. Prevent exposure to personnel. Deposition of certain compounds (e.g., arsenic compounds from arsine, arsenic compounds from gallium arsenide etch by-products) can occur in a centralized exhaust system. Possible personnel exposure could occur when maintenance is conducted inside centralized exhaust ducts. 5. Prevent ammonium compound formation. Ammonia will react with acid compounds (e.g., hydrogen chloride and hydrogen fluoride) to form ammonium compounds (e.g., ammonium chloride and ammonium fluoride). These ammonium compounds can deposit in exhaust lines and the

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centralized exhaust system, and possibly generate visible emissions of submicron particles (ammonium compounds generally at 1 ppmv or greater concentration) at the centralized wet scrubber stack outlet. These ammonium compounds are not efficiently removed by standard centralized wet scrubbers. 6. Perfluorocompound (PFC) reduction. Because of the World Semiconductor Council (WSC) voluntary PFC reduction agreement, POU devices may be utilized to reduce PFC emissions. 7. Meet air regulatory requirements. POU devices may be installed to meet air permit limits. Examples are: a. Process equipment is too far to exhaust to centralized volatile organic compound (VOC) control equipment; therefore, a POU device is installed to abate VOC emissions. b. There is no centralized VOC control equipment. Key VOC-emitting processes can utilize POU devices to keep below regulatory VOC emission limits. c. A hazardous air pollutant (HAP) can be more efficiently removed by a POU device than by the centralized VOC control equipment, to meet regulatory HAP emission limits or be a minor HAP emission source. An example of a HAP is methanol. d. No centralized wet scrubbers are installed. POU devices are used to reduce emissions instead of use of centralized wet scrubbers. In many air regulatory jurisdictions POU devices are considered part of the process equipment (e.g., CVD, etch, EPI), and not considered abatement equipment (i.e., not regulated as abatement equipment). In addition, POU devices are difficult to test for compliance, using standard regulatory testing methods, because of small-diameter inlet lines and exhaust lines, difficult locations to sample, low gas flow rate, and flammable, pyrophoric, and reactive gases used. 8. Meet company philosophy. Some companies install POU devices on some process equipment that generally would not have a POU device. An example is in the etch process that uses hydrogen bromide as one of the process etch gases. The low flows of hydrogen bromide and other gases typically used in the etch process could exhaust to the centralized exhaust system and centralized acid scrubber; however, a few facilities believe it is important to add a POU device for hydrogen bromide.

3.1.2 POINT-OF-USE DEVICE TECHNOLOGIES As mentioned previously, it is important to select the right POU device technology for each process and process emissions. Many of the technology types for POU devices are listed next. A POU device can utilize more than one of these technologies (e.g., thermal oxidation with wet scrubbing; wet scrubbing, then catalytic oxidation, then wet scrubbing; and wet scrubbing, then thermal oxidation, then wet scrubbing).

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3.1.2.1 Point-Of-Use Wet Scrubbing Systems Exhaust gas comes into contact with a liquid in which the contaminants transfer from the gas phase to the liquid phase and dissolve in the liquid. In some cases, the contaminant to be scrubbed will react with the chemical in the scrubbing liquid (e.g., chlorine will react with a sodium hydroxide solution). The typical scrubbing solution is water based, with chemicals added in some applications for removing certain compounds (chlorine, ammonia, etc.) more efficiently. Examples of scrubber types are packed bed, venturi, and spray towers. Some typical contaminants removed in a wet scrubbing system are ammonia, boron trichloride, chlorine, diborane, dichlorosilane, hydrogen bromide, hydrogen chloride, hydrogen fluoride, and trichlorosilane. Some contaminants that are not removed by wet scrubbing to any extent are nitrogen trifluoride, nitrous oxide, tetrafluoromethane, hexafluoroethane, trifluoromethane, and sulfur hexafluoride, as well as others. Arsine and phosphine can be wet scrubbed using chemical solutions, but are not removed to any extent by standard water scrubbing systems (with no chemical addition). Each wet scrubbing system should be reviewed for design type (e.g., countercurrent packed bed, nonpacked spray venturi), adequate liquid-to-gas ratio, total gas flow rate, residence time, chemical addition used [if necessary], makeup water rate, chemistry of makeup water [e.g., calcium concentration], and inlet gas stream contaminant concentrations) to determine the best design and operation for removing the contaminants of interest. 3.1.2.2 Point-Of-Use Oxidation Systems There are primarily five different types of POU oxidation systems: passive air addition, flame oxidation, hot chamber oxidation, nonflame oxidation, and catalytic oxidation. 3.1.2.2.1 Passive Air Addition Passive air addition systems are primarily designed for adding air to mix with process exhaust contaminants to provide safe reaction or dilution (also called conditioning). For example, if silane is above the pyrophoric concentration in air, it will self react with air to form silicon dioxide and water, with the silicon dioxide depositing in the POU device and downstream exhaust line. If silane is below the pyrophoric concentration in air, some reaction of the silane with air will occur in the POU device; therefore, it acts more like a dilution device. It would also act as a dilution device for certain gases (e.g., phosphine, arsine, acid gases) that are not air reactive at their inlet concentrations into the POU passive air addition system. Other air reaction gases are tungsten hexafluoride and boron trichloride. It is important to note that particulates formed in a POU passive air addition system will most likely deposit both in the POU device and the POU device exhaust line. 3.1.2.2.2 Flame Oxidation Flame oxidation is the oxidation of process exhaust gases by utilizing a flame (typically hydrogen or natural gas with air).

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3.1.2.2.3 Hot Chamber Oxidation Hot chamber oxidation is when the process exhaust gas flows through an electrically heated chamber, where air is added for oxidation. Hydrogen or natural gas can be used to enhance oxidation. Hydrogen, natural gas, or water can be added to react with a certain gas to yield another gas (e.g., fluorine to hydrogen fluoride). 3.1.2.2.4 Nonflame Oxidation A nonflame oxidation system uses an incandescent porous wall for oxidation without a flame. Fuel (generally natural gas), air, and process exhaust are passed into a porous wall oxidation chamber. 3.1.2.2.5 Catalytic Oxidation This POU device utilizes a catalyst in a container (bed) to catalytically convert specific contaminants to less hazardous compounds (e.g., tetrafluoromethane to carbon dioxide and hydrogen fluoride; and organic compounds to carbon dioxide and water). The catalyst can be poisoned relatively easily by some contaminants (e.g., silicon-based compounds like silicon tetrafluoride and silicon dioxide), so removal of these contaminants before reaching the catalyst is crucial. Catalytic oxidation takes place at elevated temperatures, but usually at much lower temperatures than thermal oxidation. Flame oxidation, hot chamber oxidation and nonflame oxidation discussed previously are considered thermal oxidation, found in this book for describing POU devices (for example, POU thermal oxidation wet scrubbing systems). Many compounds (e.g., arsine, phosphine, silane, tungsten hexafluoride) that are oxidized in a POU oxidation system form oxidation products (e.g., acid gases, particulates, arsenic trioxide, etc.) that may need further removal. Oxidation by-products such as carbon monoxide and oxides of nitrogen should be reviewed for POU oxidation systems. (Note that carbon monoxide and oxides of nitrogen should be minimal for a POU passive air addition system.) It is common to have a wet scrubbing system follow either a thermal oxidation system or a catalytic oxidation system in an integrated POU device. Another integrated POU device has a wet scrubbing system before either a thermal oxidation system or a catalytic oxidation system, followed by a wet scrubbing system. There are also POU thermal oxidation systems that are stand-alone units (i.e., no integrated wet scrubbing system). The fate of oxidation by-products should be considered for the stand-alone thermal oxidation POU devices. 3.1.2.3 Point-Of-Use Cold Bed Systems There are two primary types of POU cold bed systems (also referred to as dry technology): adsorption and chemisorption. 3.1.2.3.1 Adsorption Exhaust gas flows through a container (canister, drum, etc.) filled with adsorbent material (e.g., activated carbon). The contaminants of interest are physically adsorbed onto the adsorbent material. A reversible desorption can occur with certain

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contaminants (e.g., arsine). Some contaminants on the adsorbent material may require air oxidation to oxidize and stabilize the contaminants. Care should be taken as oxidation can cause exothermic reactions. 3.1.2.3.2 Chemisorption This POU device is a container usually composed of adsorbent material coated with a reactive chemical, reactive porous media, or a resin matrix coated with reactive chemical. The contaminants of interest are adsorbed onto the material (media) and react either with the material itself or with the coating. Some contaminants require air oxidation on the chemisorption material to react and stabilize the contaminants. Care should be taken as oxidation can cause exothermic reactions. 3.1.2.4 Point-Of-Use Hot Bed Reactors POU hot bed reactors (also referred to as dry technology) have a system wherein exhaust gas flows through a container (bed) filled with reactive material that is at an elevated temperature. The contaminants of interest react with the bed material and are converted into inorganic salts that remain in the bed. The containers can have one or more zones of different bed material. Another variation of this system is that the exhaust gas flows through a hot bed concurrently with another gas such as air. The contaminants of interest react with the other gas, and possibly the bed material, forming solid salts that stay in the bed. 3.1.2.5 Point-Of-Use Reactor Systems In POU reactor systems (e.g., those having plasma or microwaves), exhaust gases from the semiconductor process equipment are treated by allowing them to flow through a chamber using energy to enhance reactions. In prepump applications, a blower can be placed between semiconductor process chamber and prepump reactor system to ensure that the process is not affected. Reactor systems can be used for silane; silane will react to silicon and hydrogen. The silicon will drop out in a collection area or be deposited on the reactor system chamber walls. Hydrogen is a by-product of the reaction of silane and phosphine in a nonoxygen environment; hydrogen is a flammable gas. Oxygen can be added to oxidize hydrogen to water in the reactor system. (Note: Care should be taken with hydrogen and oxygen concentrations so as not to generate a flammable or explosive mixture.) If oxygen is added to the reactor system for safe reaction of hydrogen, silane will also react with oxygen and form silicon dioxide. For carbon-based PFC destruction, sources of hydrogen atoms and oxygen atoms (e.g., hydrogen and oxygen, water) are added to react with monatomic fluorine and carbon (by-products of cracking carbon-based PFCs in reactor system) so that additional PFCs (e.g., tetrafluoromethane) are not formed in the reactor system. 3.1.2.6 Point-Of-Use Particulate Removal Systems Particulate removal systems (traps, filters, cyclones, precipitators) are installed as needed to remove solid particles and condensable vapors.

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3.1.2.7 Point-Of-Use Recycle or Reclaim Systems POU recycle or reclaim systems are rare in the semiconductor industry. An example is a condensation system for isopropanol in process exhaust air.

3.1.3 PURCHASING POINT-OF-USE DEVICE The best POU device technology should be matched with the process, process exhaust contaminants, and the POU selection goals determined by facility personnel. It is recommended that the future POU device owner decide the best POU device technology solution. The POU device supplier can suggest POU device technology solutions to the future POU device owner for his or her selection. The future POU device owner should utilize POU device suppliers that have the best POU device technologies. Some suggested POU supplier expectations that the future POU device owner should expect from the POU device supplier are detailed in the following sections. 3.1.3.1 Understanding Semiconductor Fab Processes, Materials, and Environmental, Health, and Safety The POU device supplier needs to understand the specific wafer fab operations, process equipment, process materials, surrounding process pump and exhaust line requirements, and wafer fab environmental, health, and safety requirements. Not understanding this information could lead to poor recommendation of POU devices for a given fab, which could cause process equipment downtime, excessive maintenance, safety concerns, etc. 3.1.3.2 Understanding Point-Of-Use Device Technologies and Keeping the Customer No. 1 This may sound strange, but there are POU device sales and marketing personnel that do not understand their own POU device products and which POU device products are best for each process application. In some cases they only have one or two POU device technologies and may try to sell them for almost any process application. The POU device supplier should provide a form to fill out to get a POU device recommendation and quote. This form should ask for process equipment type, process material flows, duration of process material flows, pump information (including pump purge), etc. The POU device selection is based on this information and is essential to ensuring that the right POU device technology is selected. Do not ask for or accept a POU device recommendation or quote without providing this information to POU device supplier. The POU device owner should expect to get the right POU device technology at a reasonable cost. 3.1.3.3 Understanding Site-Specific Information The POU device supplier should understand the buyer’s site-specific conditions. Examples are types of water available and its quality; natural gas and hydrogen

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availability; chemicals available from bulk chemical distribution (BCD); location and size restrictions for the POU device; exhaust systems available; regulatory requirements; site environmental, health, and safety requirements; maintenance staff availability; and wastewater treatment available. This information is required as part of the POU device recommendation and selection (e.g., do not recommend a POU device using natural gas if natural gas does not exist in the subfab; do not recommend a POU device using potassium hydroxide solution or sodium hydroxide solution if not available). 3.1.3.4 Propose the Best Point-Of-Use Device Technology Considering All Factors The POU device supplier should provide the best POU device technology that takes into account all of the important selection criteria (including cost of ownership). It is suggested that questions are asked of the POU device supplier about their POU device recommendation if there are doubts that the POU device technology recommended is the best. Why is a POU thermal oxidation-wet scrubbing system recommended when the process exhaust contaminants can all be water scrubbed directly? Why is POU dry technology system recommended since it uses large number of canisters, when a POU thermal oxidation-wet scrubbing system works fine? Why is a POU thermal oxidation-wet scrubbing system that is used primarily for PFC destruction at high temperature recommended for a CVD process where PFC destruction is not required? The discussion above is not to say that POU device suppliers cannot be trusted. The point is that the POU device owner needs to be educated enough to agree with the POU device supplier recommendation and feel comfortable that the POU device selected is the best fit for their process operations and keeps costs as low as possible. 3.1.3.5 Point-Of-Use Device Inlets and Number of Process Exhausts A POU device generally has one to four inlets, depending on the technology type and POU device supplier. One decision that needs to be made is how many process exhausts will be connected to a POU device. This decision should drive the number of POU device inlets needed and should include the input from the POU device supplier. (Note: Recommend one inlet per process exhaust; there are potential safety issues if more than one process exhaust is manifolded in some way to one POU device inlet.) Some thoughts to consider: 1. When the POU device is not operating, how will that affect wafer production (affects one process chamber vs. a few process chambers)? This assumes a POU device bypass of process exhausts to the centralized exhaust system cannot occur while the process is operating. If the POU device bypass can occur while processes are operating, then this question does not apply. 2. If there are multiple process chambers in one piece of process equipment, does it make sense to exhaust all of these process chambers to one POU device?

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3. Additional costs will be incurred for more POU devices if it is decided not to use maximum inlet configuration on a specific POU device. 4. The number of POU device inlets that can be used are based on the maximum capacity of the POU device itself. Capacity could be maximum inert gas flow rate, maximum specific contaminant input (e.g., fluorine), etc. Some fabs install POU devices for redundancy. An example would be installing two POU thermal oxidation-wet scrubbing systems with four inlets each to use on four process chambers. In a normal operation, two process chambers exhaust to each of the POU thermal oxidation-wet scrubbing systems. When maintenance is performed, a manual switchover is conducted to exhaust all four process chambers to one POU device while the other POU device is in maintenance mode. The POU device technology may be the best available and operate efficiently; however, the POU device inlets can plug frequently (i.e., process exhaust contaminants condense in the POU device inlet and do not reach the POU device itself) and cause excessive downtime. The POU device inlets need to be designed to prevent process exhaust contaminants from condensation and particulate deposition. Corrosion of POU device inlets can also occur. The POU device supplier should provide information on the maintenance requirements (i.e., downtime frequency) for their POU inlets for the specific process exhausts. It is crucial to spend quality time on specifying and reviewing the POU device inlet design and operation with the POU device supplier before purchasing the POU device. 3.1.3.6 Emissions and Destruction and Removal Efficiency Data Emissions and destruction and removal efficiency (DRE) calculations are not as good as actual measurements and sometimes are not representative. Ask the POU device supplier for DRE and emissions information for the contaminants entering the POU device that has been recommended. The best data are actual measurements using proper analytical equipment from another similar process or POU device operation. The next best thing is testing the POU device in the laboratory using similar contaminants simulated from actual process emissions and conditions. It is important to note that the semiconductor industry has guidance for analytical measurements for specific contaminants in exhausts. 3.1.3.7 Standards, Guidelines, and Guides Specify to the POU device supplier if there are any standards, guidelines, and guides that need to be followed. These could include safety standards, fire codes, air quality guidelines, etc. As mentioned earlier, regulatory requirements and site requirements should be communicated to the POU device supplier. 3.1.3.8 Quote Specifications Quotes for the POU device should be very detailed from each POU device supplier and allow for the POU device owner to determine which quote is the best POU device technology at a reasonable cost. The quotes should include purchase price,

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shipping cost, uptime, operation cost of ownership, utilities usage, DRE, etc. Each quote should be similar so that a direct comparison can be made between POU device suppliers for the final POU device selection. 3.1.3.9 Warranty The POU device supplier should provide a warranty (e.g., 1 year) on its equipment. This warranty can include uptime, DRE, POU device parts, labor, etc. The POU device owner can also ask for free maintenance from the POU device supplier for a certain time period (e.g., 1 year), or the POU supplier may provide it as part of their standard quote. There is a monetary value on this warranty; therefore, decide what that monetary value is so as to compare each quote equally. 3.1.3.10 Identifying Competent Local Maintenance Support and Training The POU device will need preventive maintenance and most likely will fail unexpectedly sometime. The details of this preventive maintenance should be provided in an operations and maintenance manual provided by the POU device supplier. Training should be made available to the POU device owner on how the POU device works and what maintenance is required; this is crucial (even if the POU device supplier is doing the maintenance support in the fab). If the POU device supplier is providing maintenance support, ensure that it is 24 hours per day, 7 days a week. (Note: If a third party is doing the maintenance, they need to also be available all the time.) Nothing is more frustrating than having a POU device down with no idea how to fix it and having to wait for someone to fly in to repair it! One issue that has been realized in fabs is a POU device supplier providing maintenance for a competitor’s POU device. Training is sometimes hard to receive due to competitive information. Also, there may be an advantage if the competitor’s POU device is down often, as it may be a reason to replace it with the supplier’s POU device. 3.1.3.11 Spare Parts Locally available spare parts are a must. It is difficult to explain to management that the spare part (especially if it a POU-device supplier recommended spare part to have on hand) that is needed must be flown in and will not be available for 24 hours, keeping a crucial piece of production equipment down! Stock critical spare parts at the fab or nearby off-site. Available spare parts can be part of POU device purchase.

3.1.4 POINT-OF-USE DEVICE INSTALLATION The selection of a high-quality, knowledgeable contractor for the POU device installation is essential. The POU device supplier, the POU device owner’s personnel, or a third party contractor can do this installation. If the latter two conduct the installation (which is usually the case), it is suggested that the POU device supplier review and “sign off” on the installation. Many issues can occur if the POU device is not installed correctly.

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Many POU devices are installed with interlocks, which shut down the process equipment when the POU device malfunctions (usually for gases such as silane). In some cases when it is safe to do so, the interlocks allow for the process to finish processing the wafers before shutting down the process equipment (usually for gases such as chlorine and boron trichloride, which the centralized acid scrubbers can remove to some extent). Another type of interlock is an audible or light alarm, notifying personnel of malfunction but not shutting the process equipment off. A safety review of the process equipment, process feed materials, process exhaust contaminants, and POU device is recommended for determining the type of interlock needed and how the shutdown should occur. Process exhaust bypasses are installed in some fabs, and are used to bypass the exhaust around the POU device, when the POU device is not operational, to allow for process equipment to keep operating. The exhaust bypass can be manual or automatic. Extreme care needs to be used when an exhaust bypass is used and to ensure that maintenance personnel are protected when doing maintenance on a POU device during exhaust bypass. Safety review of the use of process exhaust bypasses is crucial. Exhaust directional valves can be used for switching different process steps (e.g., deposition step and chamber cleaning step) to different POU devices; switching different process steps to either a POU device or to a centralized exhaust system without using a POU device; or switching different process steps to two different centralized exhaust systems without any POU devices. Here are some examples. 1. A CVD process exhaust is switched to one POU device for the deposition step and switched to a different POU device during the chamber cleaning step. 2. A CVD process exhaust is switched to a POU device for the deposition step and is switched directly to the centralized acid exhaust system during the chamber cleaning step. 3. Exhaust switches to a centralized acid exhaust system when acid solution is sprayed and to a centralized ammonia exhaust system when ammonium hydroxide solution is sprayed. Exhaust directional valves need to have a safety review and to be used with caution for certain process exhaust contaminants. Exhaust directional valves can plug relatively easily, so process exhaust contaminants should be reviewed for particulate deposition.

3.2 SURROUNDING EXHAUST-LINE REQUIREMENTS The right POU device technology can be selected, but the process exhaust line leading to the POU device can be installed in a way that causes particulate deposition (i.e., minimizes the effectiveness of the POU device, causing process equipment downtime and additional maintenance). Wrong construction materials can be used for the exhaust line from a POU device that utilizes a wet scrubbing system; this POU device exhaust line corrodes, causing process equipment downtime and additional

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maintenance. These are two examples why the surrounding exhaust line material is very important. Process wet pumps (using oil) will not be discussed in the next sections; however, it is important to install oil mist filters if process wet pumps are used. Oil mists can cause process pump exhaust line condensation, fires, and POU device or centralized scrubber stack issues. Nitrogen is sometimes added to the exhaust line at the process pump exit to purge the exhaust line in case of a sudden pump shutdown. Processes using flammable gases, pyrophoric gases, or toxic gases may use this postpump nitrogen addition. This additional nitrogen flow (if used) needs to be accounted for as inert gas flow into the POU device (if used).

3.2.1 PROCESSES

TO

LOOK OUT

FOR

Due to particulate deposition of process pump exhaust lines, the following processes are likely candidates for heating the exhaust line from process pump to POU device. Note that there may be some cases where heating the pump foreline is needed. A cold trap is very helpful for LPCVD nitride to remove ammonium chloride close to the process pump. 1. 2. 3. 4. 5. 6. 7.

Aluminum metal etch Tungsten CVD Tungsten silicide CVD Nitride CVD Oxynitride CVD LPCVD nitride TEOS oxide CVD (mainly due to TEOS condensation and later reaction with chamber cleaning gas by-products). 8. Titanium nitride CVD (using titanium tetrachloride and ammonia) Frequency of maintenance drives the installation of the heating system for the exhaust line from the process chamber to the POU device (or to the centralized exhaust system if a POU device is not used). For example, tungsten etch has some particulate buildup in the process pump exhaust line. The exhaust line may only need cleaning once a year without heating, which may be acceptable. Installing the exhaust line heating with a maximum rating of at least 140°C is suggested. Heating of exhaust lines at lower temperatures may be sufficient; the higher heating capability provides flexibility in case the exhaust line does not perform at lower temperature.

3.2.2 OUTLET CONNECTION

OF

PROCESS PUMP

One connection that may have positive exhaust static pressure is located at the outlet of the process pump (positive exhaust static pressure usually occurs at the process pump exhaust outlet before negative exhaust static pressure provided by the centralized exhaust system takes over). If the O-ring leaks, process exhaust can escape into the worker area. The right O-ring material needs to be installed for the contaminants

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and temperature experienced (do not forget temperature of the heat trace if used around the outside of the connection). High concentrations of fluorine from nitrogen trifluoride predevice CVD chamber cleanings can affect many O-ring materials.

3.2.3 KEEP IT STRAIGHT, KEEP IT CLOSE,

AND

KEEP IT

THE

SAME DIAMETER

The exhaust line from the process chamber to the process pump and from the process pump to the POU device should have a minimal number of bends (keep it straight), be as close as possible (short distance), and have the same diameter of exhaust line the entire way. Any exhaust line bends (e.g., 45º, 90º) can allow for condensation and particulate buildup. Condensation and particulate buildup can also occur where the exhaust line increases or decreases in diameter. The short distance will allow for the least amount of cooling in the exhaust line and the least amount of exhaust line heating if used (good heat tracing or a heated nitrogen injection device is very expensive!). It would be ideal to have a straight exhaust line with the same diameter from the process pump to the POU device (with the process pump right next to the POU device) with two exhaust line connections: one on the process pump outlet and one on the POU device inlet (see next section). Exhaust lines having the same diameter from process chamber to process pump would also be beneficial.

3.2.4 MINIMIZE CONNECTIONS Air leaks are a huge contributor to particulate formation and deposition. In addition, process exhausts containing pyrophoric or flammable gases can mix with oxygen from an air leak and possibly cause a fire or explosion. The basic rule of thumb is no air leaks allowed! Every connection installed is a possible source for an air leak. Connections also are difficult to heat trace. O-rings and gaskets can be affected by process exhaust contaminants (e.g., fluorine) and/or temperature (e.g., heated gases out of the process pump). It is best to minimize exhaust line connections and use the right construction materials for O-rings and gaskets.

3.2.5 INSTALL RIGHT EXHAUST-LINE CONSTRUCTION MATERIALS Review the contaminants flowing through the prepump, postpump, and POU device exhaust lines to ensure the materials of construction are appropriate for contaminants contained. Some POU devices will add moisture, which should be considered for condensation after the POU device (e.g., hydrofluoric acid can condense in the exhaust line from a POU thermal oxidation-wet scrubbing system; therefore, materials of construction should be compatible with hydrofluoric acid).

3.2.6 PITCH EXHAUST LINE COMING FROM POINT-OF-USE DEVICE THAT USES WET SCRUBBING SECTION Pitch the POU device exhaust line toward the POU device for POU devices that use a wet scrubbing section before the exhaust exits the POU device, to allow for any condensation to flow back to the POU device. This is essential even if the wet scrubbing section has a demister.

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3.2.7 KEEP STATIC PRESSURE

IN

EXHAUST NEGATIVE

Ensure the exhaust static pressure is negative from the centralized exhaust system back to the process (as mentioned earlier, the process pump exhaust is usually positive exhaust static pressure for a short distance right after the process pump). This will assist in keeping contaminants in the exhaust line (i.e., not leaking out and potentially exposing personnel). Measuring the actual exhaust static pressures at the POU device inlets and the outlet of the POU device are recommended (this will also provide a pressure drop across the POU device, which is useful). Many process pumps sense exhaust static pressure in case of high positive exhaust static pressure at the process pump. For an atmospheric process (no process pump), proper negative exhaust static pressure from the centralized exhaust system is essential as it directly affects the process.

3.3 EXAMPLES OF POINT-OF-USE DEVICES FOR SELECTED PROCESSES This section provides numerous POU device examples for some of the typical semiconductor processes for the reader’s use in semiconductor fabs. This is not all of the semiconductor processes and situations the reader will experience. It is provided for reference and the thought process when selecting POU device technologies and other items to be considered. The key is first determining the goals for why the POU device is going to be installed to begin with. Once it is known that a POU device is needed, determine the best POU device technology for the process and process exhaust contaminants. Each subsection is categorized into similar processes. Note that some examples have exactly the same process description but have different goals, which drives the POU device technology decision. It is important to state again that a nitrogen trifluoride predevice CVD chamber cleaning is a separate device (used prior to the process chamber) used to efficiently dissociate nitrogen trifluoride into monatomic fluorine and diatomic fluorine before they enter the process chamber for the purposes of cleaning the process chamber (this can lead to high concentrations of fluorine leaving the process pump during chamber cleaning, which requires special attention when specifying the POU device technology required).

3.3.1 OXIDE

AND

POLYSILICON (POLY) CHEMICAL VAPOR DEPOSITIONS

These examples are for oxide CVD using TEOS or silane-nitrous oxide with various chamber cleaning gases and for poly-CVD using silane with various chamber cleaning gases. EXAMPLE 3-1: OXIDE CVD WITH SILANE, NITROUS OXIDE, AND IN SITU PFC CHAMBER CLEANING. Also applies to poly-CVD with silane and in situ PFC chamber cleaning. Goals determined by site personnel: Silane must be below 25% of the lower flammable limit (LFL) and accepted that unreacted silane will enter the centralized acid exhaust

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Point-Of-Use Devices and Exhaust-Line Requirements system. The removal of by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: Passive air addition (conditioning) system. Other considerations: There will be some particulate deposition in the POU device exhaust line, and there will be unreacted silane entering the centralized acid exhaust system and creating some particulate deposition. Some unreacted silane will reach the centralized acid scrubber stacks and be emitted (silane is not water-scrubbed to any extent). Significant use of subfab clean air will most likely occur.

EXAMPLE 3-2: OXIDE CVD WITH SILANE, NITROUS OXIDE, AND IN SITU PFC CHAMBER CLEANING. Also applies to poly-CVD with silane and in situ PFC chamber cleaning. Goals determined by site personnel: Reduce silane to site-specific personnel exposure requirements, which is much lower than the LFL. The removal of by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: Thermal oxidation system. Other considerations: There will be some particulate deposition in the POU device exhaust line and centralized acid exhaust system. Significant use of subfab clean air will most likely occur.

EXAMPLE 3-3: OXIDE CVD WITH SILANE, NITROUS OXIDE, AND IN SITU CARBON-BASED PFC CHAMBER CLEANING. Also applies to polyCVD with silane and in situ carbon-based PFC chamber cleaning. Goals determined by site personnel: Carbon-based PFC to be abated by 95% or greater. Silane must be below 25% of the LFL. POU device technology selected: Thermal oxidation-wet scrubbing system. The thermal oxidation section needs to utilize higher temperature oxidation and a fluoridescavenging hydrogen atom source to destroy carbon-based PFC. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow the condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid resistant-material for the POU device exhaust line. POU device wastewater discharge needs to be evaluated for fluoride concentrations.

EXAMPLE 3-4: OXIDE CVD WITH SILANE, NITROUS OXIDE, AND NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Also applies to poly-CVD with silane and nitrogen trifluoride predevice CVD chamber cleaning.

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Semiconductor Industry: Wafer Fab Exhaust Management Goals determined by site personnel: High concentrations of fluorine from nitrogen trifluoride pre-device CVD chamber cleaning need to be abated by 99% or greater before entering the centralized acid exhaust system. Silane must be abated below 25% of the LFL. POU device technology selected: Thermal oxidation-wet scrubbing system, using natural gas, hydrogen, or water (hydrogen atom source) in the thermal oxidation section to convert fluorine to hydrogen fluoride before the wet scrubbing section. Internal materials of construction of the POU device need to be fluorine- and hydrogen fluorideresistant (dry and wet portions). Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow the condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. POU device wastewater discharge needs to be evaluated for fluoride concentrations.

EXAMPLE 3-5: OXIDE CVD WITH TEOS AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: After environmental, health, and safety review of TEOS emissions from this process, it was decided TEOS did not need abatement at POU. The removal of by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: None. Other considerations: TEOS can condense in the process exhaust line and process pump exhaust line, and can react with chamber cleaning by-product gases, causing exhaust line plugging. Therefore, the exhaust line from the process chamber to the centralized acid exhaust system will require heating if this occurs.

EXAMPLE 3-6: OXIDE CVD WITH TEOS AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: The company has an internal VOC emission limit per wafer; to meet this requirement, TEOS needs to be abated by 90% or greater. The TEOS emissions from the process chamber could be water scrubbed directly; however, there is a concern about foaming in a POU wet scrubbing system and putting organic compounds in wastewater. The removal of by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: Thermal oxidation system. Other considerations: The POU device supplier had to provide actual TEOS destruction data in the POU thermal oxidizing system to ensure removal efficiency requirements for TEOS were met. Thermal oxidation of TEOS and by-product silicon tetrafluoride will generate some particulates in the POU device exhaust line and centralized acid exhaust system. TEOS can condense in the process exhaust line and process pump exhaust line, and can react with chamber cleaning by-product gases, causing plugging. Therefore, the exhaust line from the process chamber to the POU device will require heating if this occurs. Significant use of the subfab will most likely occur.

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EXAMPLE 3-7: OXIDE CVD WITH TEOS AND IN SITU CARBON-BASED PFC CHAMBER CLEANING. Goal determined by site personnel: Carbon-based PFC to be abated by 95% or greater. POU device technology selected: Thermal oxidation-wet scrubbing system. The thermal oxidation section needs to utilize higher temperature oxidation and a fluoridescavenging hydrogen atom source to destroy carbon-based PFCs. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. The POU device wastewater discharge needs to be evaluated for fluoride concentrations. TEOS can condense in the process exhaust line and process pump exhaust line, and react with chamber cleaning by-product gases, causing plugging. Therefore, the exhaust line from the process chamber to the POU device will require heating if this occurs.

EXAMPLE 3-8: OXIDE CVD WITH TEOS AND NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Goals determined by site personnel: High concentrations of fluorine from nitrogen trifluoride predevice CVD chamber cleaning need to be abated by 99% or greater before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system, using natural gas, hydrogen, or water (hydrogen atom source) in the thermal oxidation section to convert fluorine to hydrogen fluoride before the wet scrubbing section. The internal materials of construction of the POU device need to be fluorine- and hydrogen fluorideresistant (dry and wet portions). Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. The POU device wastewater discharge needs to be evaluated for fluoride concentrations. TEOS can condense in process exhaust line and process pump exhaust line, and can react with chamber cleaning by-product gases, causing plugging. Therefore, the exhaust line from the process chamber to the POU device will require heating if this occurs.

3.3.2 DOPED OXIDE

AND

POLY-CHEMICAL VAPOR DEPOSITIONS

These examples are for doped oxide CVDs using TEOS and organic dopant(s) or using silane-nitrous oxide with inorganic dopants (e.g., phosphine and diborane) with various chamber cleaning gases, and for doped poly CVD using silane and inorganic dopants with various chamber cleaning gases. Note that if arsine is used as a dopant, the impact of arsenic compounds needs to be evaluated (not an example in this section).

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EXAMPLE 3-9: DOPED OXIDE CVD WITH SILANE, NITROUS OXIDE, INORGANIC DOPANTS (E.G., PHOSPHINE AND DIBORANE), AND IN SITU PFC CHAMBER CLEANING. Also applies to doped poly-CVD with silane, inorganic dopants, and in situ PFC chamber cleaning. Note that if arsine is used as a dopant, the impact of arsenic compounds needs to be evaluated. Goals determined by site personnel: Silane must be below 25% of the LFL, and unreacted silane will enter centralized acid exhaust system. Dilution (i.e., no removal) of inorganic dopants below site-specific personnel exposure requirements is acceptable. The removal of by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: Passive air addition (conditioning) system. Other considerations: There will be some particulate deposition in the POU device exhaust line, and there will be unreacted silane entering the centralized acid exhaust system and creating some particulate deposition. Some unreacted silane will reach the centralized acid scrubber stacks and be emitted (silane is not water scrubbed to any extent). Unreacted inorganic dopants will most likely reach centralized acid scrubber stacks and be emitted (phosphine is not water-scrubbed to any extent; diborane will be removed to some extent). Significant use of subfab clean air will most likely occur.

EXAMPLE 3-10: DOPED OXIDE CVD WITH SILANE, NITROUS OXIDE, INORGANIC DOPANTS (E.G., PHOSPHINE AND DIBORANE), AND IN SITU PFC CHAMBER CLEANING. Also applies to doped poly-CVD with silane, inorganic dopants, and in situ PFC chamber cleaning. Note that if arsine is used as a dopant, the impact of arsenic compounds needs to be evaluated. Goals determined by site personnel: Reduce inorganic dopants and silane to site-specific personnel exposure requirements. The removal of by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: Thermal oxidation system. Other considerations: There will be some particulate deposition in the POU device exhaust line and centralized acid exhaust system. Significant use of subfab clean air will most likely occur.

EXAMPLE 3-11: DOPED OXIDE CVD WITH SILANE, NITROUS OXIDE, INORGANIC DOPANTS (E.G., PHOSPHINE AND DIBORANE), AND IN SITU PFC CHAMBER CLEANING. Also applies to doped poly-CVD with silane, inorganic dopants, and in situ PFC chamber cleaning. Note that if arsine is used as a dopant, the impact of arsenic compounds needs to be evaluated. Goals determined by site personnel: Inorganic dopants to be abated by 95% or greater and abated to below site-specific personnel exposure requirements. Silane must be

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Point-Of-Use Devices and Exhaust-Line Requirements below 25% of the LFL. By-products hydrogen fluoride and silicon tetrafluoride abatement of 95% or greater is required at point-of-use. POU device technology selected: Thermal oxidation-wet scrubbing system. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet-scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. The POU device wastewater discharge needs to be evaluated for fluoride concentrations.

EXAMPLE 3-12: DOPED OXIDE CVD WITH SILANE, NITROUS OXIDE, INORGANIC DOPANTS (E.G., PHOSPHINE AND DIBORANE), AND IN SITU CARBON-BASED PFC CHAMBER CLEANING. Also applies to doped poly-CVD with silane, inorganic dopants, and in situ carbon-based PFC chamber cleaning. Note that if arsine is used as a dopant, the impact of arsenic compounds needs to be evaluated. Goals determined by site personnel: Carbon-based PFC to be abated by 95% or greater. Silane must be below 25% of the LFL. Inorganic dopants need to be abated by 95% or greater and abated to below site-specific personnel exposure requirements. POU device technology selected: Thermal oxidation-wet scrubbing system. The thermal oxidation section needs to utilize higher temperature oxidation and a fluoridescavenging hydrogen atom source to destroy carbon-based PFCs. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet-scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. The POU device wastewater discharge needs to be evaluated for fluoride concentrations.

EXAMPLE 3-13: DOPED OXIDE CVD WITH SILANE, NITROUS OXIDE, INORGANIC DOPANTS (E.G., PHOSPHINE AND DIBORANE), AND NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Also applies to doped poly-CVD with silane, inorganic dopants, and nitrogen trifluoride predevice CVD chamber cleaning. Note that if arsine is used as a dopant, the impact of arsenic compounds needs to be evaluated. Goals determined by site personnel: High concentrations of fluorine from nitrogen trifluoride predevice CVD chamber cleaning need to be abated by 99% or greater before entering the centralized acid exhaust system. Silane should be abated below 25% of the LFL. Inorganic dopants should be removed at 95% or greater. POU device technology selected: Thermal oxidation-wet scrubbing system, using natural gas, hydrogen, or water (hydrogen atom source) in the thermal oxidation section to convert fluorine to hydrogen fluoride before the wet-scrubbing section. The internal construction materials need to be fluorine- and hydrogen fluoride-resistant (dry and wet portions).

47

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Semiconductor Industry: Wafer Fab Exhaust Management Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet-scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. The POU device wastewater discharge needs to be evaluated for fluoride concentrations.

EXAMPLE 3-14: DOPED OXIDE CVD WITH TEOS, ORGANIC DOPANTS, AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: After an environmental, health, and safety review of TEOS and organic dopants emissions from this process, it was decided TEOS and organic dopants did not need abatement at POU. The removal of the by-products hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: None Other considerations: TEOS (and possibly other organic dopants) can condense in the process exhaust line and process pump exhaust line, and can react with chamber cleaning gases, causing plugging. Therefore, the exhaust line from the process chamber to the centralized acid exhaust system will require heating if this occurs.

EXAMPLE 3-15: DOPED OXIDE CVD WITH TEOS, ORGANIC DOPANTS, AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: The company has an internal VOC emissions limit per wafer; to meet this requirement, TEOS and organic dopants need to be abated by 90% or greater. The TEOS and organic dopant emissions from the process chamber could be water-scrubbed directly; however, there is a concern about foaming in a POU wet-scrubbing system and about organic compounds in wastewater. The removal of hydrogen fluoride and silicon tetrafluoride can be done by centralized acid scrubbers, and it is believed that the centralized acid exhaust ducts will not be damaged by these compounds. POU device technology selected: Thermal oxidation system. Other considerations: The POU device supplier has to provide actual destruction data on TEOS and organic dopants to ensure that removal efficiency requirements for organic compounds were met. There will be some particulate deposition in the POU device exhaust line and centralized acid exhaust system. TEOS (and possibly organic dopants) can condense in the process exhaust line and process pump exhaust line and react with chamber cleaning gases, causing plugging. The exhaust line from the process chamber to the POU device will require heating if this occurs.

EXAMPLE 3-16: DOPED OXIDE CVD WITH TEOS, ORGANIC DOPANTS, AND IN SITU CARBON-BASED PFC CLEANING. Goal determined by site personnel: Carbon-based PFCs to be abated by 95% or greater.

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POU device technology selected: Thermal oxidation-wet scrubbing system. The thermal oxidation unit needs to utilize higher temperature oxidation and a fluoride-scavenging hydrogen atom source to destroy carbon-based PFCs. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. POU device wastewater discharge needs to be evaluated for fluoride concentrations. TEOS (and possibly organic dopants) can condense in the process exhaust line and process pump exhaust line, and can react with chamber cleaning gases, causing plugging. The exhaust line from the process chamber to the POU device will require heating if this occurs.

EXAMPLE 3-17: DOPED OXIDE CVD WITH TEOS, ORGANIC DOPANTS, AND NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Goals determined by site personnel: High concentrations of fluorine from chamber clean need to be abated by 99% or greater and abated to below site-specific personnel exposure requirements before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system, using natural gas, hydrogen, or water (hydrogen atom source) in the thermal oxidation section to convert fluorine to hydrogen fluoride before the wet scrubbing section. The internal materials of construction need to be fluorine- and hydrogen fluoride-resistant (dry and wet portions). Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. POU device wastewater discharge needs to be evaluated for fluoride concentrations. TEOS (and possibly organic dopants) can condense in the process exhaust line and process pump exhaust line, and can react with chamber cleaning gases, causing plugging. The exhaust line from the process chamber to the POU device will require heating if this occurs.

3.3.3 NITRIDE

AND

OXYNITRIDE CHEMICAL VAPOR DEPOSITIONS

These examples are for nitride CVDs using ammonia and silane with various chamber clean gases and for oxynitride CVD using ammonia, silane and nitrous oxide with various chamber clean gases. EXAMPLE 3-18: NITRIDE CVDS WITH AMMONIA, SILANE, AND IN SITU PFC CHAMBER CLEANING. Also applies to oxynitride CVDs with ammonia, silane, nitrous oxide, and in situ PFC chamber cleaning. Goals determined by site personnel: Silane must be below 25% of the LFL and accepted that unreacted silane (below pyrophoric levels) will enter the centralized acid exhaust

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Semiconductor Industry: Wafer Fab Exhaust Management system. Ammonia must be abated to below 1 ppmv before entering the centralized acid exhaust system. POU device technology selected: Passive air addition (conditioning)-wet scrubbing system. Other considerations: There may be some particulate deposition (from unreacted silane) in the POU device exhaust line, and there will be unreacted silane entering the centralized acid exhaust system that may create some particulate deposition. Some unreacted silane may reach the centralized acid scrubber stacks and be emitted (silane is not water-scrubbed to any extent). Ammonia will be in the wastewater discharge of the POU device. Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating of the exhaust line from the process pump to the POU device may be needed.

EXAMPLE 3-19: NITRIDE CVDS WITH AMMONIA, SILANE, AND IN SITU PFC CHAMBER CLEANING. Also applies to oxynitride CVDs with ammonia, silane, nitrous oxide, and in situ PFC chamber cleaning. Goals determined by site personnel: Silane must be below site-specific personnel exposure requirements for silane, and ammonia must be abated to below 1 ppmv before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system. Other considerations: Ammonia will be in the wastewater discharge of the POU device. Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. The wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating of the exhaust line from the process pump to the POU device may be needed.

EXAMPLE 3-20: NITRIDE CVDS WITH AMMONIA, SILANE, AND IN SITU CARBONBASED PFC CHAMBER CLEANING. Also applies to oxynitride CVD with ammonia, silane, nitrous oxide, and in situ carbon-based PFC chamber cleaning. Goals determined by site personnel: Carbon-based PFCx abated by 95% or greater. Silane must be below 25% of the LFL, and ammonia must be abated to below 1 ppmv before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system. The thermal oxidation section needs to utilize higher temperature oxidation and a fluoridescavenging hydrogen atom source to destroy carbon-based PFCs. Other considerations: Ammonia will be in the wastewater discharge of the POU device. Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for

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condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating of the exhaust line from the process pump to the POU device may be needed.

EXAMPLE 3-21: NITRIDE CVDS WITH AMMONIA, SILANE, AND NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Also applies to oxynitride CVDs with ammonia, silane, nitrous oxide, and nitrogen trifluoride predevice CVD chamber cleaning. Goals determined by site personnel: High concentrations of fluorine from nitrogen trifluoride predevice CVD chamber cleaning need to be abated by 99% or greater before entering the centralized acid exhaust system. Silane must be below 25% of the LFL, and ammonia must be abated to below 1 ppmv before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system, using natural gas, hydrogen, or water (hydrogen atom source) in the thermal oxidation section to convert fluorine to hydrogen fluoride before the wet scrubbing section. The internal construction materials need to be fluorine- and hydrogen fluoride-resistant (dry and wet portions). Other considerations: Ammonia will be in the wastewater discharge of the POU device. Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating the exhaust line from the process pump to the POU device may not be needed as the high concentrations of fluorine from the nitrogen trifluoride predevice CVD chamber cleaning may keep the exhaust line relatively clean.

3.3.4 TUNGSTEN CHEMICAL VAPOR DEPOSITIONS These examples are for tungsten CVDs using tungsten hexafluoride, silane, and hydrogen with various chamber cleaning gases. Review previous examples if carbonbased PFC abatement is required. EXAMPLE 3-22: TUNGSTEN CVDS WITH TUNGSTEN HEXAFLUORIDE, SILANE, HYDROGEN, AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: Removal of tungsten hexafluoride at 98% or greater. Silane should already be below 25% of the LFL from the process pump due to maximum process flow for silane being low; therefore no silane abatement is required. Reduce hydrogen below 25% of the LFL. Minimize particulate deposition in the POU device exhaust line. POU device technology selected: Thermal oxidation-wet scrubbing system. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU

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Semiconductor Industry: Wafer Fab Exhaust Management device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating the exhaust line from the process pump to the POU device may be needed. Tungsten will also be in the wastewater discharge from the POU device.

EXAMPLE 3-23: TUNGSTEN CVDS WITH TUNGSTEN HEXAFLUORIDE, SILANE, HYDROGEN, AND NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Goals determined by site personnel: High concentrations of fluorine from nitrogen trifluoride pre-device CVD chamber clean need to be abated by 99% or greater and be abated to below site-specific personnel exposure requirements before entering centralized acid exhaust system. Removal of tungsten hexafluoride by 98% or greater is required. Silane should be abated to below site-specific personnel exposure requirements before entering centralized acid exhaust system. Reduce hydrogen below 25% of LFL. Minimize particulate deposition in POU device exhaust line. POU device technology selected: Thermal oxidation-wet scrubbing system, using natural gas, hydrogen, or water (hydrogen atom source) in the thermal oxidation section to convert fluorine to hydrogen fluoride before the wet scrubbing section. The internal construction materials need to be fluorine- and hydrogen fluoride-resistant (dry and wet portions). Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating the exhaust line from the process pump to the POU device may be needed, depending if fluorine from the nitrogen trifluoride predevice CVD chamber cleaning keeps the exhaust line relatively clean. Tungsten will also be in the wastewater discharge of the POU device.

EXAMPLE 3-24: TUNGSTEN CVD WITH TUNGSTEN HEXAFLUORIDE, SILANE, HYDROGEN, AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: Removal of tungsten hexafluoride at 98% or greater. Silane is at a low process flow, so dilution from nitrogen pump purge is determined sufficient for silane safety. Use centralized acid exhaust system airflow rate to dilute hydrogen below 25% of the LFL (where the process pump exhaust line connects to the centralized acid exhaust system). POU device technology selected: Wet scrubbing system, with safety metal cage. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow for condensate to drain back into wet scrubbing system. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Heating the exhaust line from the process pump to the POU device may be needed. Tungsten will also be in the wastewater discharge of the POU device.

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3.3.5 TUNGSTEN SILICIDE CVD EXAMPLE 3-25: TUNGSTEN SILICIDE CVD WITH TUNGSTEN HEXAFLUORIDE, DICHLOROSILANE, AND IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: Removal of tungsten hexafluoride and dichlorosilane by 98% or greater. POU device technology selected: Wet scrubbing system. Other considerations: Low concentrations of hydrofluoric acid and hydrochloric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install hydrofluoric acid- and hydrochloric acid-resistant material for the POU device exhaust line. Wastewater discharge from the POU device needs to be evaluated for fluoride concentrations. Heating the exhaust line from the process pump to the POU device may be needed. Tungsten will also be in the wastewater discharge of the POU device.

3.3.6 LPCVD NITRIDE EXAMPLE 3-26: LPCVD NITRIDE WITH AMMONIA AND DICHLOROSILANE. FURNACE TUBES CAN BE CLEANED OUTSIDE OF THE PROCESS EQUIPMENT IN A WET BENCH OR VERTICAL TUBE CLEANER. Goals determined by site personnel: Remove ammonia and dichlorosilane by 98% or greater before it enters the centralized acid exhaust system. Remove ammonium chloride (by-product from LPCVD process) so that ammonium chloride does not deposit to any extent in the process pump exhaust line. POU device technologies selected: Cold trap system near process pump, followed by a wet scrubbing system. Other considerations: Ammonium chloride will not be removed to any extent by the POU wet scrubbing system, so ammonium chloride removal by cold trap is important (as well as reducing ammonium chloride deposition in the process pump exhaust line). There may be more than one cold trap installed (e.g., one cold trap in the exhaust line close to the furnace tube and one cold trap in the exhaust line close to the process pump). Heating of the exhaust line between the furnace tube and process pump cold trap may be needed. Ammonia will be in the wastewater discharge of the POU device. Low concentrations of hydrochloric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install hydrochloric acidresistant material for the POU device exhaust line.

3.3.7 LOW-PRESSURE CHEMICAL VAPOR DEPOSITION (UNDOPED OR DOPED) USING SILANE

OF

POLY-

AND

OXIDE

LPCVD undoped poly and oxide deposition can use silane. LPCVD doped poly and doped oxide can use silane and one or more dopant gases. The furnace tubes can

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be cleaned out of the process equipment in a vertical tube cleaner or in a wet bench. Note that there are other LPCVD undoped poly and doped poly processes and LPCVD undoped oxide and doped oxide processes (e.g., TEOS-based processes) that will not be presented in this section. EXAMPLE 3-27: LPCVD UNDOPED POLY AND OXIDE USING SILANE AND FURNACE TUBE CLEANING CONDUCTED OUTSIDE OF PROCESS EQUIPMENT USING VERTICAL TUBE CLEANER. Goal determined by site personnel: Reduce silane to below 25% of the LFL. POU device technology selected: Passive air addition (conditioning) system. Other considerations: Unreacted silane will be exhausted to the centralized acid exhaust system and centralized acid scrubber. Some particulate matter (depending on how much silane reacts with air and forms silicon dioxide) may deposit in the POU device exhaust line and centralized acid exhaust system. A centralized acid scrubber will not remove unreacted silane to any extent.

EXAMPLE 3-28: LPCVD UNDOPED POLY AND OXIDE USING SILANE AND FURNACE TUBE CLEANING CONDUCTED OUTSIDE OF PROCESS EQUIPMENT USING VERTICAL TUBE CLEANER. Goal determined by site personnel: Reduce silane to levels below site-specific personnel exposure requirements before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation system. Other considerations: There may be some particulate deposition in the POU device exhaust line and centralized acid exhaust system.

EXAMPLE 3-29: LPCVD DOPED POLY AND OXIDE USING SILANE AND INORGANIC DOPANTS AND FURNACE TUBE CLEANING CONDUCTED OUTSIDE OF PROCESS EQUIPMENT USING VERTICAL TUBE CLEANER. Goals determined by site personnel: Reduce all hydride gases (including silane) by 95% or greater and to levels below site-specific personnel exposure requirements before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation system. Other considerations: There may be some particulate deposition in the POU device exhaust line and centralized acid exhaust system.

3.3.8 EPITAXIAL DEPOSITION Epitaxial (EPI) deposition processes can use dichlorosilane, trichlorosilane, hydrogen, inorganic dopants (e.g., arsine, diborane, and phosphine), and hydrogen chloride.

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EXAMPLE 3-30: EPI PROCESS USING DICHLOROSILANE, HYDROGEN, INORGANIC DOPANTS, AND HYDROGEN CHLORIDE. Goals determined by site personnel: Remove 99% or greater of dichlorosilane and hydrogen chloride, and try to prevent particulate deposition in the exhaust line to the roof. The EPI process exhaust line will be installed to above roof level for venting above LFL concentrations of hydrogen outside (i.e., no hydrogen destruction needed). There are no requirements to abate very low concentrations of inorganic dopants (hydride gases). POU device technology selected: Wet scrubbing system, with safety metal cage. Other considerations: Procedure to clean the exhaust line after wet scrubbing system should be established (will probably have to be cleaned regularly at some time interval). Hydrogen vented to outside may flare at vent outlet to atmosphere if ignition occurs; develop procedure in case that happens. Low concentrations of hydrochloric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install hydrochloric acid-resistant material for the POU device exhaust line.

EXAMPLE 3-31: EPI PROCESS USING DICHLOROSILANE, HYDROGEN, INORGANIC DOPANTS, AND HYDROGEN CHLORIDE. Goals determined by site personnel: Remove 99% or greater of dichlorosilane and hydrogen chloride, and prevent particulate deposition in the centralized acid exhaust system. Hydrogen will need to be oxidized to below 25% of the LFL before it enters the centralized acid exhaust system. Arsenic compounds (by-products of arsine being oxidized) are not wanted in wastewater; determined that small amount of arsenic compounds in centralized acid exhaust system is acceptable. POU device technology selected: Wet scrubbing system (with safety metal cage) followed by thermal oxidation system. Other considerations: Arsine is not removed to any extent in the wet scrubbing system. Oxidation of inorganic dopants (hydride gases) occurs, creating particulates including arsenic compounds. Cleaning of particulates (some containing arsenic) after thermal oxidation system will likely need to be done at some time interval. A procedure to clean exhaust line after wet scrubbing system and before thermal oxidation system should be established (will probably have to be cleaned periodically). Low concentrations of hydrochloric acid may be present in the exhaust line from the wet scrubbing system. Pitch the POU device exhaust line toward the wet scrubbing system to allow condensate to drain back into this POU device. Install hydrochloric acid-resistant material for the wet scrubbing system exhaust line.

EXAMPLE 3-32: EPI PROCESS USING DICHLOROSILANE, HYDROGEN, INORGANIC DOPANTS, AND HYDROGEN CHLORIDE. Goals determined by site personnel: Remove 99% or greater of dichlorosilane and hydrogen chloride, and try to prevent particulate deposition in the centralized acid exhaust system. Hydrogen will need to be oxidized to below 25% of the LFL before

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Semiconductor Industry: Wafer Fab Exhaust Management entering the centralized acid exhaust system. Abatement of inorganic dopants (hydride gases) is needed. Small amounts of arsenic compounds in wastewater are allowed. POU device technology selected: Thermal oxidation-wet scrubbing system. Other considerations: Small amounts of arsenic in wastewater. Some EPI processes have a hydride gas diluent bypass, which is before the hydride feed gas (in hydrogen) reaches the EPI process. It is important to determine to where this hydride gas diluent bypass flows, as it typically contains a majority of hydride gases being emitted. If this hydride gas diluent bypass is used, see next example. Low concentrations of hydrochloric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrochloric acid-resistant material for the POU device exhaust line.

EXAMPLE 3-33: EPI USING DICHLOROSILANE, HYDROGEN, INORGANIC DOPANTS, AND HYDROGEN CHLORIDE. Goals determined by site personnel: Remove 99% or more of dichlorosilane and hydrogen chloride, and try to prevent particulate deposition in the exhaust line to the roof. The EPI process exhaust line will be installed to above roof level for venting LFL concentrations of hydrogen outside (i.e., no hydrogen destruction needed). Inorganic dopants (hydride gases) need to be abated by 90%. After reviewing the flow of hydride gases from gas cylinders to the EPI process equipment, there will be a hydride gas diluent bypass to the exhaust line after the POU device. The hydride gas diluent bypass is estimated to be 95% of the total hydride gas emissions from the EPI operation. Therefore, abate the hydride gas diluent bypass with one POU device (best technology for hydride gases) and the EPI process with another POU device (best technology for hydrogen chloride and dichlorosilane). POU device technologies selected: Wet scrubbing system (with safety metal cage) for the EPI process exhaust line; dry technology system (canister) on hydride gas diluent bypass line. Other considerations: Procedure to clean the exhaust line after wet scrubbing system should be established (it will have to be cleaned periodically). Hydrogen vented to the outside may flare at the vent outlet to the atmosphere if ignition occurs; develop a procedure in case that happens. Low concentrations of hydrochloric acid may be present in the exhaust line from the wet scrubbing system. Pitch the POU device exhaust line toward the wet scrubbing system to allow condensate to drain back into this POU device. Install hydrochloric acid-resistant material for the wet scrubbing system exhaust line. Hydride gas sensing should be utilized in the dry technology system exhaust line to determine when the dry canister needs changing. The dry canister may require disposal and may be classified as hazardous waste.

3.3.9 OTHER EXAMPLES

OF

CHEMICAL VAPOR DEPOSITION PROCESSES

3.3.9.1 Titanium Nitride Chemical Vapor Deposition Titanium nitride CVD can use titanium tetrachloride and ammonia for deposition, and chamber cleaning gas (e.g., chlorine trifluoride).

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EXAMPLE 3-34: TITANIUM NITRIDE CVD USING TITANIUM TETRACHLORIDE, AMMONIA, AND CHLORINE TRIFLUORIDE (AS CHAMBER CLEANING GAS). Goals determined by site personnel: Abate titanium tetrachloride at 95% or greater; 99% or greater removal of chlorine trifluoride; and abate ammonia to below 1 ppmv before entering the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system. The thermal oxidation section may require a hydrogen atom source (e.g., hydrogen, natural gas, or water) to convert chlorine trifluoride to hydrogen fluoride and hydrogen chloride, so as to minimize effects on the materials of construction of the POU device and to assist removal efficiency in the wet scrubbing section. Other considerations: Low concentrations of hydrochloric acid and hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrochloric acid- and hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Ammonia and titanium will also be in the wastewater discharge of the POU device. Heating the exhaust line from the process pump to the POU device may be needed.

EXAMPLE 3-35: TITANIUM NITRIDE CVD USING TITANIUM TETRACHLORIDE, AMMONIA, AND CHLORINE TRIFLUORIDE (AS CHAMBER CLEANING GAS). The pump exhaust line has an exhaust directional valve that splits the exhaust to two separate exhaust lines, with one exhaust line for deposition exhaust and the other exhaust line for chamber cleaning exhaust. Goals determined by site personnel: Abate chlorine trifluoride at 99% or greater; remove 95% of titanium tetrachloride; and abate ammonia to below 1 ppmv before it enters the centralized acid exhaust system. POU device technologies selected: Two POU devices are going to used, one for deposition exhaust and the other for chamber cleaning exhaust; an exhaust directional valve in the process pump exhaust line will be used to switch to different POU devices. A wet scrubbing system is used for deposition exhaust and dry technology system is used for chamber cleaning exhaust (canister usage is estimated to be low). Other considerations: A safety review should be conducted for the directional valve of the process pump exhaust line. Low concentrations of hydrochloric acid may be present in the exhaust line from the wet scrubbing system. Pitch the POU device exhaust line toward the wet scrubbing system to allow condensate to drain back into this POU device. Install hydrochloric acid-resistant material for the POU device exhaust line. Ammonia and titanium will be in the wastewater discharge of the POU device.

3.3.9.2 Low-K Chemical Vapor Deposition Low-k CVD can use silicon-containing organic compounds for deposition and PFC gas for chamber cleaning.

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EXAMPLE 3-36: LOW-K CVD USING SILICON-CONTAINING ORGANIC COMPOUND WITH IN SITU PFC CHAMBER CLEANING. Goals determined by site personnel: Fab personnel have set a VOC requirement on emissions per wafer; therefore, abatement of 90% or greater is required for siliconcontaining organic compounds. Future PFC reduction of 95% or greater may be required, so this capability is specified for the POU device (note that the POU device may not be operated in PFC abatement mode until it is necessary). POU device technology selected: Thermal oxidation-wet scrubbing system, with PFC abatement capability. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations.

EXAMPLE 3-37: LOW-K CVD USING SILICON-CONTAINING ORGANIC COMPOUND WITH IN SITU PFC CLEANING. Goal determined by site personnel: No known reason for POU device determined. POU device technology selected: None. Other considerations: Particulate deposition in the centralized acid exhaust system needs to be determined. If maintenance becomes a concern, installation of the POU device can be done.

3.3.9.3 Furnace Process Using Ammonia A furnace process that uses ammonia only (i.e., other than inert gases) can be exhausted to the centralized ammonia exhaust system. If there is not a separate centralized ammonia exhaust system, then a POU device is needed. EXAMPLE 3-38: FURNACE PROCESS USING AMMONIA. Goal determined by site personnel: There is no separate centralized ammonia exhaust system. Ammonia should be less than 1 ppmv before entering the centralized acid exhaust system. POU device technology selected: Wet scrubbing system. Other considerations: If ammonia flow is high enough, a scrubbing solution of low pH may be needed to get less than 1 ppmv ammonia at the POU device outlet. This would require acid solution addition (e.g., sulfuric acid solution) to the scrubbing solution. Ammonia will be in the wastewater discharge of the POU device.

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3.3.10 ALUMINUM ETCH EXAMPLE 3-39: ALUMINUM ETCH PROCESS USING BORON TRICHLORIDE AND CHLORINE. Goals determined by site personnel: There should be 98% abatement of boron trichloride, chlorine, and aluminum trichloride (etch by-product). POU device technology selected: Wet scrubbing system. Other considerations: Heating the exhaust line from the process pump to the POU device may be needed. Low concentrations of hydrochloric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install hydrochloric acid-resistant material for the POU device exhaust line. Aluminum will also be in the wastewater discharge of the POU device.

3.3.11 OXIDE, POLY, NITRIDE ETCH PROCESSES An oxide etch process and nitride etch process generally use PFC gases. Poly etch process can use hydrogen bromide, chlorine, and PFC gases. EXAMPLE 3-40: OXIDE, POLY, NITRIDE ETCH PROCESSES. Goals determined by site personnel: Determined that centralized acid scrubber can abate silicon tetrafluoride (etch by-product), hydrogen fluoride (etch by-product), chlorine and hydrogen bromide. No hydrogen bromide damage of centralized acid exhaust system is expected. No abatement of PFC gases is required. POU device technology selected: None. Other considerations: None.

EXAMPLE 3-41: POLY ETCH PROCESS. Goals determined by site personnel: Hydrogen bromide damage of the centralized acid exhaust system is expected to be a concern. No abatement of PFC gases is required. POU device technology selected: Wet scrubbing system. Other considerations: Low concentrations of hydrobromic acid, hydrochloric acid, and hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install hydrobromic acid-, hydrochloric acid-, and hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations.

EXAMPLE 3-42: PFC ABATEMENT FOR OXIDE AND NITRIDE ETCH PROCESSES. Goals determined by site personnel: Review of site PFC emissions in relation to site PFC abatement goals was done. Abate PFC emissions by 95% or greater for oxide

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Semiconductor Industry: Wafer Fab Exhaust Management etch and nitride etch processes. Prefer installing one POU device for four etch chambers. Natural gas is available in the sub-fab. POU device technology selected: Thermal oxidation-wet scrubbing system, with higher oxidation temperature capability for destruction of tetrafluoromethane. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations.

EXAMPLE 3-43: PFC ABATEMENT FOR OXIDE AND NITRIDE ETCH PROCESSES. Goals determined by site personnel: Review of site PFC emissions in relation to site PFC abatement goals was done. Abate PFC emissions by 95% or greater for oxide etch and nitride etch processes. Prefer installing one POU device for four etch chambers. Natural gas is not available in the subfab. POU device technology selected: Wet scrubbing-catalytic oxidation-wet scrubbing system. Oxidation temperature for destruction of tetrafluoromethane is much lower for catalytic oxidation than thermal oxidation. Other considerations: Catalyst used must be protected from exposure to silicon compounds (e.g., silicon tetrafluoride, which is a nitride etch and oxide etch by-product). The first wet scrubbing section is used to remove silicon tetrafluoride to very low concentrations before bringing in the catalyst. Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations.

EXAMPLE 3-44: PFC ABATEMENT FOR OXIDE AND NITRIDE ETCH PROCESSES. Goals determined by site personnel: Review of site PFC emissions in relation to site PFC abatement goals was done. The results were to abate PFC emissions by 95% or greater for oxide etch and nitride etch processes. Natural gas is not available in the sub-fab. Space limitations exist in sub-fab. Prefer POU device technology that uses almost no floor space. POU device technology selected: Prepump reactor system, with hydrogen atom and oxygen atom compound(s) fed into the POU device. Use one POU device for each etch chamber. Other considerations: The by-products of the etch process and of the pre-pump reactor system will be exhausted to the centralized acid exhaust system and then to the centralized acid scrubbers. Some of these by-products are hydrogen fluoride and silicon tetrafluoride, which should be removed to some extent by centralized acid scrubbers.

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3.3.12 TUNGSTEN ETCH EXAMPLE 3-45: TUNGSTEN ETCH UTILIZING PFC GAS. Goals determined by site personnel: Abate tungsten etch by-products (e.g., tungsten hexafluoride) at 95% or greater. POU device technology selected: Wet scrubbing system. Other considerations: Low concentrations of hydrofluoric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing section of the POU device. Install hydrofluoric acid-resistant material for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations. Tungsten will also be in the wastewater.

3.3.13 ION IMPLANT EXAMPLE 3-46: ION IMPLANT USING ARSINE, PHOSPHINE, AND BORON TRIFLUORIDE. Goals determined by site personnel: Reduce arsine and phosphine concentrations below site-specific personnel exposure requirements before arsine and phosphine enter the centralized acid exhaust system. Prevent any arsenic compound deposition in the centralized acid exhaust system. POU device technology selected: Dry technology system (using canister). Other considerations: Canister will require replacement. Arsenic-contaminated canister may be hazardous waste.

3.3.14 WET BENCHES Wet benches can have separate baths containing different acid solutions, ammonium hydroxide-hydrogen peroxide solution or clean water. Ammonia and acid gases should not mix in the wet bench exhaust duct(s), to prevent formation of ammonium compounds. Ozone addition can be made to acid solution bath. EXAMPLE 3-47: WET BENCH WITH ONE AMMONIUM HYDROXIDE-HYDROGEN PEROXIDE SOLUTION BATH AND TWO ACID SOLUTION BATHS. Goal determined by site personnel: There is no centralized ammonia exhaust system. Ammonia entering the centralized acid exhaust system should be below 1 ppmv. POU device technology selected: Wet scrubbing system with low pH scrubbing solution (pH between 3 and 4). Concentrated sulfuric acid solution is added to the scrubbing solution for pH control. Other considerations: Segregation of exhaust inside the wet bench is crucial, so that ammonia and acid compounds do not mix in the gas phase (exhaust). The exhaust duct containing only ammonia will be exhausted to the POU wet scrubbing system; the

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Semiconductor Industry: Wafer Fab Exhaust Management POU device exhaust will be connected to the centralized acid exhaust system. The exhaust ducts containing only acid compounds will be exhausted to the centralized acid exhaust system. Low concentrations of sulfuric acid (used for pH control in the scrubbing solution) may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install proper materials of construction for the POU device exhaust line. Ammonia will be in the wastewater discharge of the POU device.

EXAMPLE 3-48: WET BENCH WITH TWO AMMONIUM HYDROXIDE–HYDROGEN PEROXIDE SOLUTION BATHS AND TWO ACID SOLUTION BATHS FOR WHICH IT IS DIFFICULT TO SEGREGATE EXHAUST. Goals determined by site personnel: The wet bench exhaust ducts are vertical downward from the bottom of the wet bench. The wet bench has two ammonium hydroxidehydrogen peroxide solution baths, each near one or more acid solution baths. Ammonium compounds are depositing in exhaust ducts from the wet bench, and ammonia is exhausted to the centralized acid exhaust system, forming additional ammonium compounds that are depositing in the centralized acid exhaust system and causing visible stack emissions. There is a centralized ammonia exhaust system available if wet bench exhaust can be segregated. This existing wet bench may be permanently damaged if physical exhaust segregation work is performed, the total cost of this work would be very high, and there would be significant production downtime for physical exhaust segregation work. POU abatement is necessary for the entire wet bench exhaust. Ammonia entering the centralized acid exhaust system should be below 1 ppmv. POU device technology selected: Wet scrubbing system with low pH scrubbing solution (pH between 3 and 4). Concentrated sulfuric acid solution is added to the scrubbing solution for pH control. Other considerations: Ammonium compounds will not be removed to any extent in the wet scrubbing system. There will still be particulate deposition (ammonium compounds) in the vertical exhaust ducts between wet bench and the POU wet scrubbing system (this is not an ideal solution). An exhaust duct washing system (low water flow) should be installed to periodically wash water-soluble ammonium compounds into the POU wet scrubbing system (the two exhaust ducts are installed to allow water to flow vertically into the POU wet scrubbing system, with no liquid pooling in exhaust ducts). Low concentrations of acid compounds may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install proper materials of construction for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations, if fluoride-containing compounds (e.g., hydrogen fluoride) are removed by the wet scrubbing system. Ammonia will be in the wastewater discharge of the POU device.

EXAMPLE 3-49: WET BENCH WITH BATH CONTAINING SULFURIC ACID AND OZONE ADDED. ALSO HAS OTHER ACID SOLUTION BATHS. Goal determined by site personnel: A wet bench bath was converted from sulfuric acidhydrogen peroxide solution to sulfuric acid solution-ozone. During the first few weeks

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of operation there were ozone odors reported; in addition, there was concern about ozone and the exhaust duct material. Ozone should be abated by 99%. POU device technology selected: Ozone destruct system Other considerations: None.

3.3.15 SPRAY PROCESSES USING ACID SOLUTIONS AND AMMONIUM HYDROXIDE-HYDROGEN PEROXIDE SOLUTION The spray chamber utilizes acid solutions, clean water, and ammonium hydroxidehydrogen peroxide solution during separate spray operations on wafers. EXAMPLE 3-50: SPRAY PROCESS USING ACID SOLUTIONS, AMMONIUM HYDROXIDE–HYDROGEN PEROXIDE SOLUTION, AND CLEAN WATER WITH NO SEPARATE CENTRALIZED AMMONIA EXHAUST SYSTEM. Goals determined by site personnel: Ammonia in exhaust entering the centralized acid exhaust system should be below 1 ppmv. Minimize mist carryover into the exhaust duct coming from the spray chamber. POU device technology selected: Wet scrubbing system using low pH scrubbing solution (pH between 3 and 4). Concentrated sulfuric acid solution is added to the scrubbing solution for pH control. Other considerations: The vent for the ammonium hydroxide solution storage tank should be connected to the inlet exhaust duct to the POU wet scrubbing system. Exhaust static pressure on the spray chamber should be between –0.5 and –0.75 inches water, to minimize mist carryover into the exhaust (the more negative the exhaust static pressure, the more mist will be removed from the spray chamber). Impact on wafers and safety of personnel will be evaluated. Low concentrations of acid compounds may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install proper materials of construction for the POU device exhaust line. Wastewater discharge of the POU device needs to be evaluated for fluoride concentrations, if fluoride-containing compounds (e.g., hydrogen fluoride) are removed by the wet scrubbing system. Ammonia will be in the wastewater discharge of the POU device.

EXAMPLE 3-51: SPRAY PROCESS USING ACID SOLUTIONS, CLEAN WATER, AND AMMONIUM HYDROXIDE-HYDROGEN PEROXIDE SOLUTION WITH SEPARATE CENTRALIZED ACID EXHAUST SYSTEM AND CENTRALIZED AMMONIA EXHAUST SYSTEM. Goals determined by site personnel: Ammonia in exhaust during spraying of ammonium hydroxide-hydrogen peroxide solution should be exhausted to a centralized ammonia exhaust system. Minimize mist carryover into the exhaust duct coming from the spray chamber.

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Semiconductor Industry: Wafer Fab Exhaust Management POU device technology selected: None. An exhaust directional valve is installed, with two exhaust ducts. One exhaust duct will be connected to the centralized ammonia exhaust system and the other exhaust duct to the centralized acid exhaust system. Proper switching of exhaust needs to be established between chemical process steps (one chemical step is spraying ammonium hydroxide-hydrogen peroxide solution and the other chemical steps are spraying acid solutions), so that ammonia is exhausted to the centralized ammonia exhaust system and acid compounds are exhausted to the centralized acid exhaust system. Other considerations: The vent of the ammonium hydroxide solution storage tank should be connected to the centralized ammonia exhaust system. Exhaust static pressure on the spray chamber should be between –0.5 and –0.75 inches of water, to minimize mist carryover into the exhaust (the more negative the exhaust static pressure, the more mist will be removed from the spray chamber). Impact on wafers and safety of personnel should be evaluated.

EXAMPLE 3-52: SPRAY PROCESS WITH HEATED AQUA REGIA SOLUTION. Aqua regia solution is mix of hydrochloric acid solution and nitric acid solution. Goals determined by site personnel: Odors have been detected on roof near centralized acid scrubber stacks and have been determined to be from aqua regia spray process. Odors need to be eliminated. A secondary concern is damage to the centralized acid exhaust system due to condensation of acid mists. POU device technology selected: Wet scrubbing system with high pH scrubbing solution. Sodium hydroxide solution is added to the scrubbing solution for pH control. Other considerations: Exhaust static pressure on the spray chamber should be between –0.5 and –0.75 inches of water to minimize mist carryover into the exhaust (the more negative the exhaust static pressure, the more mist will be removed from the spray chamber). Impact on wafers and safety of personnel should be evaluated. Low concentrations of hydrochloric acid and nitric acid may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install proper materials of construction for the POU device exhaust line. The rise of the centralized acid scrubber stack exhaust should be maximized by ensuring proper stack height and stack diameter, allowing for 3000 to 3800 feet per minute exit velocity.

3.3.16 CYLINDER AND OTHER STORAGE CONTAINER GAS-LINE (OR VENT GAS) PURGES Purging of gas lines from gas storage containers (e.g., cylinders) occur when gas storage containers are changed. Nitrogen is usually used for purging lines. EXAMPLE 3-53: LINE PURGES OF SEVERAL HYDRIDE CYLINDERS. Goal determined by site personnel: Install one POU device for 12 cylinder line purges for silane, hydrogen, and phosphine, which would require 12 POU device inlets. Hydride gases need to be abated to below site-specific personnel exposure requirements. POU device technology selected: Thermal oxidizing system.

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Other considerations: The small amount of particulate created in the thermal oxidizing system should have minimal impact on the centralized acid exhaust system.

EXAMPLE 3-54: LINE PURGES FOR ONE CYLINDER CONTAINING ARSINE IN HYDROGEN AND ONE CYLINDER CONTAINING PHOSPHINE IN HYDROGEN. Goals determined by site personnel: No arsenic particulate should contaminate centralized acid exhaust system. Arsine and phosphine need to be abated below site-specific personnel exposure requirements. High concentrations of hydrogen in these two cylinder line purges should be safe to vent unabated. Fuel is not available in gas cylinder storage room. POU device technology selected: Dry technology system (canister). Other considerations: The dry technology canister will need to be changed eventually. The canister may be a hazardous waste.

EXAMPLE 3-55: LINE PURGES FOR AMMONIA, CHLORINE, DICHLOROSILANE, AND HYDROGEN CHLORIDE. Goals determined by site personnel: Small amounts of ammonia, chlorine, dichlorosilane, and hydrogen chloride will be vented during gas cylinder changing. There is no centralized ammonia exhaust system. These gas cylinder line purges could be vented without a POU device to the centralized acid exhaust system. POU device technology selected: None. Other considerations: None.

EXAMPLE 3-56: LINE PURGES FOR AMMONIA, CHLORINE, DICHLOROSILANE, AND HYDROGEN CHLORIDE. Goals determined by site personnel: Small amounts of ammonia, chlorine, dichlorosilane, and hydrogen chloride will be vented during cylinder changing. There is a centralized ammonia exhaust system. Chlorine, dichlorosilane, and hydrogen chloride gas cylinder line purges can be vented without a POU device to the centralized acid exhaust system, and an ammonia gas cylinder line purge can be vented to the centralized ammonia exhaust system. POU device technology selected: None. Other considerations: None.

3.3.17 METAL ORGANIC CHEMICAL VAPOR DEPOSITION Gallium arsenide MOCVD can use arsine and gallium-containing organic compound as deposition chemicals, and indium phosphide MOCVD can use phosphine and indium-containing organic compound as deposition chemicals. Hydrogen is used as a carrier gas. Gallium arsenide MOCVD will be used as an example.

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EXAMPLE 3-57: GALLIUM ARSENIDE MOCVD WITH NO ARSENIC WASTEWATER TREATMENT PLANT AND EXHAUSTED TO ABOVE THE ROOF (OUTSIDE). Goals determined by site personnel: Arsine needs to be abated to below site-specific personnel exposure requirements, and arsine removal efficiency needs to be 99% or greater. No liquid arsenic wastewater should be generated (there is no mechanism to treat arsenic-contaminated wastewater on-site, wastewater handling is potentially “messy” and expensive to ship offsite for disposal). Hydrogen can be safely vented to above roof (outside), so hydrogen does not need to be oxidized. POU device technology selected: Dry technology system (canisters). Other considerations: Dry canisters will require replacement and may need waste disposal. Arsenic-contaminated canisters may be considered hazardous waste.

EXAMPLE 3-58: GALLIUM ARSENIDE MOCVD WITH ARSENIC WASTEWATER TREATMENT PLANT AND EXHAUSTED TO ABOVE THE ROOF (OUTSIDE). Goals determined by site personnel: Arsine needs to be abated to below site-specific personnel exposure requirements, and arsine removal efficiency needs to be 99% or greater. Liquid arsenic wastewater generated can be treated on site. Hydrogen can be safely vented to above the roof (outside), so hydrogen does not need to be oxidized. POU device technology selected: Chemical-dosed wet scrubbing system with metal safety cage. Other considerations: Low concentrations of certain chemicals may be present in the exhaust line from this POU device. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install proper materials of construction for the POU device exhaust line.

EXAMPLE 3-59: GALLIUM ARSENIDE MOCVD WITH ARSENIC WASTEWATER TREATMENT PLANT AND EXHAUSTED TO THE CENTRALIZED ACID EXHAUST SYSTEM. Goals determined by site personnel: Arsine needs to be abated to below site-specific personnel exposure requirements, and arsine removal efficiency needs to be 99% or greater. Liquid arsenic wastewater generated can be treated on site. Hydrogen needs to be oxidized to below 25% of the LFL before being exhausted to the centralized acid exhaust system. POU device technology selected: Thermal oxidation-wet scrubbing system. Other considerations: Generation of submicron arsenic particles in the thermal oxidation section will need to be reviewed to determine if these particles will go through the wet scrubbing section and deposit in the POU device exhaust line and centralized acid exhaust system. Pitch the POU device exhaust line toward the POU device to allow condensate to drain back into the wet scrubbing system. Install proper materials of construction for the POU device exhaust line.

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EXAMPLE 3-60: GALLIUM ARSENIDE MOCVD WITH NO ARSENIC WASTEWATER TREATMENT PLANT AND EXHAUSTED TO THE CENTRALIZED ACID EXHAUST SYSTEM. Goals determined by site personnel: Arsine needs to be abated to below site-specific personnel exposure requirements, and arsine removal efficiency needs to be 99% or greater. No liquid arsenic wastewater should be generated (there is no mechanism to treat arsenic-contaminated wastewater on-site, wastewater handling is potentially “messy” and expensive to ship off site for disposal). Hydrogen needs to be oxidized to below 25% of the LFL before entering the centralized acid exhaust system. POU device technology selected: Dry technology system (canister) followed by thermal oxidizing system. Other considerations: Canisters will require replacement and may need waste disposal. Arsenic-contaminated canisters may be considered hazardous waste.

3.3.18 GALLIUM ARSENIDE ETCH EXAMPLE 3-61: GALLIUM ARSENIDE ETCH. Goals determined by site personnel: Arsenic compounds are deposited in the centralized acid exhaust system and create hazardous cleanup conditions. Prevent arsenic compounds from depositing by preventing gallium arsenide etch by-products from reaching the centralized acid exhaust system. Arsenic compounds need to be abated by 98% or greater. POU device technology selected: Dry technology system (canister) that can remove gallium arsenide etch by-products. Other considerations: Canisters will require replacement and may need waste disposal. Arsenic-contaminated canisters may be considered hazardous waste. Heating of the exhaust line between the process pump and the POU device may be helpful in keeping gallium arsenide etch by-products from depositing.

3.4 FAB EXAMPLES OF POINT-OF-USE DEVICES Examples below are based on situations that may be found in the semiconductor industry. These situations are provided for the reader’s information and may be useful while working on projects. EXAMPLE 3-62: A NEW INSTALLATION OF TEOS-BASED OXIDE CVD PROCESS EQUIPMENT USING NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Tetraethylorthosilicate (TEOS)-based oxide CVD process equipment is sometimes installed without a POU device, as TEOS does not always need to be abated (this would also apply to doped oxide utilizing TEOS and organic dopants). The actual need for a POU device for this process equipment is for the chamber cleaning utilizing a predevice to fracture nitrogen trifluoride into monatomic fluorine and diatomic fluorine

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Semiconductor Industry: Wafer Fab Exhaust Management before entering the process chamber. A high concentration of fluorine comes out of the process chamber to the process pump, and then goes to the centralized acid exhaust system. Fluorine can impact duct material by reacting with liquid water in the duct to form hydrofluoric acid or directly from the gas-phase reaction of fluorine with duct material. In addition, any fluorine that reaches the centralized acid scrubber can react in the scrubber solution (depending on conditions) to form oxygen difluoride and be emitted from the stack. A POU thermal oxidation-wet scrubbing system that uses natural gas, hydrogen, or water in the thermal oxidation section can be used.

EXAMPLE 3-63: RETROFIT OF EXISTING CVD PROCESS EQUIPMENT (WITH EXISTING POU DEVICE) WITH NITROGEN TRIFLUORIDE PREDEVICE CVD CHAMBER CLEANING. Existing CVD process equipment using in situ carbon-based PFC chamber cleaning was retrofitted with nitrogen trifluoride predevice CVD chamber cleaning to reduce PFC emissions and to improve wafer throughput. An existing POU device on the CVD process equipment was not reviewed for compatible materials of construction or adequate fluorine removal efficiency for the high concentrations of fluorine from this chamber cleaning. High maintenance (downtime) of the existing POU device and fluorine effects on the centralized acid duct material occurred. Retrofitting or replacing the POU device was needed to efficiently remove fluorine and to minimize downtime of the POU device. A possible POU device replacement is a POU thermal oxidationwet scrubbing system that uses natural gas, hydrogen, or water in the thermal oxidation section.

EXAMPLE 3-64: PROPER INSTALLATION OF WET BENCH EXHAUST SEGREGATION AND POU WET SCRUBBING SYSTEM FOR AQUA REGIA BATH. Odors on the roof were traced back to a heated aqua regia solution bath (a mixture of concentrated hydrochloric acid solution and concentrated nitric acid solution), while the aqua regia solution was being heated and processing wafers. The wet bench had the heated aqua regia solution bath next to another acid solution bath. The wet bench exhaust was connected to the centralized acid exhaust system and a few centralized acid scrubbers. Wet bench exhaust segregation and POU device installation were selected as the solution for this odor concern. The POU wet scrubbing system that was installed is a wet scrubber with high pH scrubbing solution (with sodium hydroxide solution addition) followed by a submicron acid mist filter. It is important to note that the POU wet scrubbing system that was installed did not have the air flow-rate capacity to have the entire wet bench exhausted to it. The exhaust segregation work consisted of splitting the exhaust duct on the back of the wet bench, with one half closest to the aqua regia solution bath exhausting to the new POU wet scrubbing system and the other half exhausting directly to the centralized acid exhaust system. However, the exhaust internals of the wet bench were not changed. Odors continued on roof. Further inspection determined that the aqua regia solution bath emissions were mixing in the internal exhaust section of the wet bench and flowing to both exhaust ducts, with one exhaust not being abated by the POU wet scrubbing system (as mentioned previously). Further work was done to segregate the wet bench internal exhaust section to ensure

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Point-Of-Use Devices and Exhaust-Line Requirements aqua regia bath emissions were only exhausting to the POU wet scrubbing system. For future projects similar to this project, it was recommended that the entire wet bench that contains only acid solutions should be exhausted to a POU device and not perform wet bench exhaust segregation.

EXAMPLE 3-65: PROPER INSTALLATION OF WET BENCH EXHAUST SEGREGATION FOR AMMONIUM HYDROXIDE-HYDROGEN PEROXIDE SOLUTION BATH FROM OTHER ACID SOLUTION BATHS, AND INSTALLATION OF A POU WET SCRUBBING SYSTEM. A wet bench, containing one ammonium hydroxide-hydrogen peroxide solution bath, three acid solution baths, and four clean water rinses (one clean water rinse bath following each chemical bath), was causing particulate deposition in the centralized acid exhaust system. It was determined to be ammonium compounds formed in the wet bench exhaust. The common wet bench exhaust plenum area below the deck surface and the area above the baths were mixing ammonia and acid compounds to form ammonium compounds (e.g., ammonium chloride and ammonium fluoride). There was no separate centralized ammonia exhaust system; therefore, a POU device was required to remove ammonia before exhausting to the centralized acid exhaust system. The wet bench plenum area was segregated so that the ammonium hydroxide-hydrogen peroxide solution bath and corresponding clean water rinse bath had its own exhaust duct, so this exhaust duct could be connected to a POU wet scrubbing system with low pH scrubbing solution. Sulfuric acid solution was piped to the POU wet scrubbing system for pH control. The remaining acid solution baths were exhausted directly to the centralized acid exhaust system. The airflow in the area above the baths was set so that ammonia was below 1 ppmv in the acid solution bath exhaust ducts and that acid compounds were below 1 ppmv in the exhaust duct to the POU wet scrubbing system (note that it is very difficult to get no ammonia and acid compounds mixing in a wet bench containing acid solutions and ammonium hydroxide-hydrogen peroxide solutions if there is an open area above the baths and a robot moving wafers disturbing airflow).

EXAMPLE 3-66: POU DEVICE INLET PLUGGING ON TUNGSTEN CVD PROCESS. A tungsten CVD process generally uses tungsten hexafluoride and silane as deposition gases, with a PFC chamber cleaning gas. If tungsten hexafluoride reacts with air or water to form tungsten compounds, a POU device inlet can quickly plug. It is important to ensure that no air or moisture mixes with tungsten hexafluoride before it is successfully removed by a POU device.

EXAMPLE 3-67: GALLIUM ARSENIDE MOCVD CHEMICAL-DOSED POU WET SCRUBBING SYSTEM A gallium arsenide MOCVD process has a POU chemical-dosed wet scrubbing system that generates arsenic-contaminated wastewater that needs to be shipped offsite for

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Semiconductor Industry: Wafer Fab Exhaust Management hazardous waste disposal. The company has been having trouble finding acceptable hazardous waste disposal sites, and the disposal cost per gallon has risen significantly. In addition, the POU chemical-dosed wet scrubbing system is difficult to maintain because of liquid wastes generated and use of chemicals. The company is investigating a POU dry technology system to remove these concerns, or to install an onsite arsenic wastewater treatment plant to address the hazardous waste disposal concern.

EXAMPLE 3-68: SPRAY PROCESS USING ACID SOLUTIONS AND AMMONIUM HYDROXIDE–HYDROGEN PEROXIDE SOLUTION. A spray process using acid solutions and ammonium hydroxide-hydrogen peroxide solution had condensation in exhaust ducts, causing liquid pooling and damage. In addition, particulate matter was depositing in exhaust ducts. The exhaust static pressure on the spray chamber (bowl) ranged between –1.0 and –1.3 inches of water. High concentrations of ammonia emitted during short ammonium hydroxide-hydrogen peroxide solution spray process was reaching the centralized acid exhaust system and reacting with acid compounds, forming ammonium compounds. Ammonium compounds were depositing in the spray process exhaust duct and centralized acid exhaust system, and were being emitted from the centralized acid scrubber stacks as visible emissions. Also, the nearby chemical storage tank for ammonium hydroxide solution emitted high concentrations of ammonia when nitrogen pressure was released (the ammonium hydroxide solution storage tank was pressurized by nitrogen so as to feed it to the spray process chamber when needed). The fab did not have a separate centralized ammonia exhaust system. Therefore, a POU wet scrubbing system was installed for the spray chamber exhaust and small exhaust line for ammonium hydroxide solution storage tank venting. This POU wet scrubbing system has a sulfuric acid solution feed to keep the scrubbing solution pH between 3 and 4. The exhaust static pressure on the spray chamber was changed to –0.5 to –0.75 inches of water to reduce mists from being carried out of the spray chamber, causing condensation and ammonium compound particulate buildup in the exhaust duct (note that this change is required as condensation and particulate buildup can still occur between the spray chamber and the POU wet scrubbing system).

EXAMPLE 3-69: EPI PROCESS POU WET SCRUBBING SYSTEM INCIDENT. Maintenance had not occurred for some time on an EPI process POU wet scrubbing system. An exhaust line connection near the inlet of the POU wet scrubbing system began to leak air in (due to O-ring failure), mixing air with hydrogen during a deposition step. This air contacted pyrophoric solids near the POU device inlet and sparked the hydrogen-air mixture. The ignition disconnected the exhaust line at a connection and a flare resulted. The plastic wet scrubber melted (no fire occurred). Nearby maintenance personnel noticed the flare and notified manufacturing personnel, who shut down the EPI reactor, eliminating the flare.

EXAMPLE 3-70: USE OF POU THERMAL OXIDIZING SYSTEM ON NITRIDEOXYNITRIDE CVD PROCESSES.

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Point-Of-Use Devices and Exhaust-Line Requirements A POU thermal oxidizing system was installed on two nitride-oxynitride CVD processes (silane and ammonia as deposition gases for nitride CVD, and silane, ammonia, and nitrous oxide as deposition gases for oxynitride CVD; both processes use PFC chamber cleaning gas). The POU thermal oxidizing system was used to react silane before entering the centralized acid exhaust system. Most of the ammonia from these processes is not oxidized at the temperature of the POU thermal oxidizing system. Ammonia from one process during the deposition step mixes and reacts with byproducts silicon tetrafluoride and hydrogen fluoride from the other process chamber cleaning step, forming ammonium compounds; ammonium compounds deposited in the POU thermal oxidizing system and the exhaust duct. Ammonia that has not reacted flows to the centralized acid exhaust system, reacting with acid compounds, creating additional ammonium compounds which deposit in the centralized acid exhaust system and may be seen as visible emissions from the centralized acid scrubber stack. A POU thermal oxidation-wet scrubbing system to oxidize silane and scrub ammonia was installed to replace the POU thermal oxidizing system,. This POU technology also removed a majority of the chamber cleaning by-products (silicon tetrafluoride and hydrogen fluoride).

EXAMPLE 3-71: USE OF POU PASSIVE AIR ADDITION SYSTEMS FOR TWO PROCESSES. Two POU passive air addition systems (using air to condition process exhaust gases) were installed on two processes: LPCVD nitride using dichlorosilane and ammonia, and doped oxide CVD using silane, nitrous oxide, phosphine, and a PFC chamber cleaning gas. Testing was conducted on the exhaust duct from both POU passive air addition systems. It was determined that dichlorosilane, ammonia, nitrous oxide, and phosphine were diluted, not oxidized to any extent. Unreacted silane (which was below pyrophoric concentration as it entered the POU passive air addition system from the process pump) also was emitted from the POU device. Ammonia from the LPCVD process was found to be reacting with acid compounds in the centralized acid exhaust system and forming ammonium compounds, which were depositing in the centralized acid exhaust system. Phosphine required abatement, not dilution. The POU passive air addition system on LPCVD nitride was replaced by a POU wet scrubbing system to remove ammonia and dichlorosilane. The POU passive air addition system on doped oxide CVD was replaced with a POU thermal oxidizing system to oxidize silane and phosphine; it was determined that the by-product emissions from the chamber cleaning step did not need POU abatement and could be exhausted to the centralized acid exhaust system and centralized acid scrubbers.

EXAMPLE 3-72: USE OF POU THERMAL OXIDIZING SYSTEMS ON TUNGSTEN CVD. A POU thermal oxidizing system was installed on a tungsten CVD process to oxidize tungsten hexafluoride, silane, and hydrogen before entering a centralized acid exhaust system. Large amounts of particulate were depositing in the exhaust duct from the POU thermal oxidizing system. A POU thermal oxidation-wet scrubbing system was installed to replace the POU thermal oxidizing system, and abated hydrogen, silane, and tungsten hexafluoride. This POU technology also removed a majority of the chamber cleaning by-products.

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EXAMPLE 3-73: USE OF POU THERMAL OXIDATION–WET SCRUBBING SYSTEM ON NITRIDE–OXYNITRIDE CVD PROCESSES. A POU thermal oxidation-wet scrubbing system was installed on the exhaust from two nitride-oxynitride CVD process chambers (silane and ammonia as deposition gases for nitride CVD, and silane, ammonia, and nitrous oxide as deposition gases for oxynitride CVD). Ammonia was measured well above 1 ppmv in the POU device exhaust line. The POU device supplier was contacted to determine why the ammonia concentration guaranteed in the POU device exhaust line was not met. The POU device supplier stated that the thermal oxidation section temperature was not high enough to oxidize ammonia efficiently; therefore, ammonia needed to be scrubbed in the wet scrubbing section. The water makeup was increased until ammonia being emitted from this POU device was below 1 ppmv.

EXAMPLE 3-74: EXHAUSTING SILANE-BASED CVD PROCESSES (POLY, OXIDE, AND NITRIDE) TO A CENTRALIZED PYROPHORIC EXHAUST SYSTEM. A centralized pyrophoric exhaust system was installed to collect exhaust lines from several poly, oxide, and nitride CVD processes that did not have POU devices. The centralized pyrophoric exhaust system was plugging with particulate frequently, and the centralized wet scrubber on the centralized pyrophoric exhaust system was having sporadic visible emissions (determined to be ammonium compounds). There were periodic “pops” in the centralized pyrophoric exhaust system, which was probably from silane. To reduce maintenance, improve safety, and eliminate visible emissions, POU thermal oxidation-wet scrubbing systems were installed on all of the processes on the centralized pyrophoric exhaust system, eliminating the need for a centralized pyrophoric exhaust system.

EXAMPLE 3-75: NO POU DEVICE ON ALUMINUM METAL ETCH. An equipment engineer heat traced the exhaust lines from metal etch processes to the centralized acid exhaust system that was causing significant particulate buildup in the centralized acid exhaust system (if the exhaust line is heated to keep it clean, the particulate deposition just moves to another place!). POU wet scrubbing systems were installed on all metal etch processes. Heat trace on the exhaust line continued to be used to keep the exhaust lines as clean as possible from process pumps to POU devices.

EXAMPLE 3-76: POU DRY TECHNOLOGY SYSTEMS ON TUNGSTEN CVD PROCESSES. POU dry technology systems using canisters were installed on a few tungsten CVD processes. The hydrogen was allowed to pass through the POU devices to the centralized acid exhaust system, even though it was above the lower flammable limit after the process pump. A large number of canisters were used, with associated high costs and waste disposal required. POU thermal oxidation-wet scrubbing systems were

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installed to replace the POU dry technology systems. These POU devices also abated hydrogen, reduced operating costs, and eliminated canister waste disposal.

EXAMPLE 3-77: USE OF ACID WASTEWATER NEUTRALIZATION TREATMENT PLANT DISCHARGE AS MAKEUP WATER FOR POU THERMAL OXIDATION-WET SCRUBBING SYSTEM. Fab personnel decided to recycle the discharge water from the acid wastewater neutralization treatment plant to several POU thermal oxidation-wet scrubbing systems as well as other equipment. The metal parts in the POU thermal oxidation-wet scrubbing systems began to fail quickly due to corrosion. It was determined that the fluorine, hydrogen fluoride, and silicon tetrafluoride from chamber cleaning were dropping scrubbing solution pH and forming a strong acid solution with other acid compounds contained in discharge water from the acid wastewater neutralization treatment plant. Fab personnel went to a cleaner source of water for makeup water to POU devices.

3.5 DEVELOPING A POINT-OF-USE DEVICE AND SURROUNDING EXHAUST-LINE STRATEGY It is essential to develop a fab-specific or company-specific POU device and surrounding exhaust line strategy. This strategy will allow for consistent decisions, which should translate to reduced costs, more process uptime, less maintenance, etc. As mentioned earlier, the initial discussions should determine the reasons why POU devices are installed and the criteria for each chemical (e.g., remove ammonia to less than 1 ppmv before it enters the centralized acid exhaust system; reduce pyrophoric and flammable gases below 25% of the LFL).

3.5.1 DEVELOP OVERALL STRATEGY An overall strategy is needed so as to select the best process exhaust management and POU devices for a fab. An example below is provided so as to begin developing an overall process exhaust management and POU device strategy. 1. Silane should be below 25% of the LFL before it enters the centralized acid exhaust system. 2. Hydrogen should be below 25% of the LFL before it enters the centralized acid exhaust system. 3. Ammonia should be less than 1 ppmv into a centralized acid exhaust system from any process (including wet benches and spray processes). 4. Install a POU device on any process that will significantly impact the centralized exhaust systems and centralized scrubbers with particulate deposition and damage, or create odors, potential lead to personnel exposure, or produce visible emissions from the centralized scrubber stacks. An example is fluorine: the POU device requirement can be stated using DRE and/or POU device outlet concentration.

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5. Specific gases below one-half the IDLH concentration before they enter the centralized acid exhaust system. An example of specific gases are gases that have poor warning properties at one-half the IDLH concentration. 6. Total PFC emissions (based on global warming calculation method) are 5% or less of the total PFC usage for all new fabs. This requirement would drive PFC reductions being reviewed for each process in a new fab, so that the total PFC emission goal could be met. 7. Prevent any environment, health, and safety or production concerns not mentioned above. An example is reducing process emissions for air emission regulatory requirements. 8. All POU devices that discharge wastewater need to be reviewed (before installation) for impact on fluoride concentrations in site wastewater discharge and its impact on fluoride wastewater treatment system (if applicable). 9. Hazardous waste determination is necessary for dry canisters and any other wastes generated, including best waste handling and disposal practices. 10. Proper process pump exhaust line connections need to be installed for compatibility with contaminants and temperature inside the exhaust line (example: by-product fluorine from nitrogen fluoride predevice CVD chamber cleaning). This is due to possible positive exhaust static pressure for a short distance after the process pump so as to prevent leaks into the worker area, and to eliminate any leaks of air in at connections with negative exhaust static pressure that could cause unwanted reactions.

3.5.2 EXAMPLE OF PROCESS EXHAUST MANAGEMENT AND POINT-OF-USE DEVICE STRATEGY (THIS FAB DOES HAVE BOTH CENTRALIZED ACID EXHAUST SYSTEM AND CENTRALIZED AMMONIA EXHAUST SYSTEM) 1. Wet benches (using ammonium hydroxide solution): segregation of ammonia and acid compounds in the wet bench. Exhaust acid compounds to the centralized acid exhaust system and ammonia to the centralized ammonia exhaust system. Measure ammonia concentrations in the wet bench exhaust duct(s) connected to the centralized acid exhaust system to ensure ammonia is below 1 ppmv. If particulates are depositing in the wet bench exhaust duct connected to a centralized ammonia exhaust system, then acid compounds are getting into this exhaust duct; find the source and stop acid compounds from entering the wet bench duct connected to the centralized ammonia exhaust system. 2. Spray processes (using ammonium hydroxide solution): connect the spray chamber exhaust duct to an exhaust directional valve with two exhaust ducts. When ammonium hydroxide-hydrogen peroxide solution is used in the spray chamber, exhaust to the centralized ammonia exhaust system. When acid solutions are used in the spray chamber, exhaust to the centralized acid exhaust system. The switching to different centralized

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3.

4.

5. 6.

7.

8. 9.

10.

exhaust systems (using the exhaust directional valve) should be done when clean water rinses are used in the spray chamber (i.e., after chemicals are removed from the spray chamber during clean water rinse and before the next chemical spray). If the exhaust directional valve is not wanted by the process engineer, install a POU wet scrubbing system with a low pH scrubbing solution on the spray chamber exhaust duct to remove ammonia below 1 ppmv, before exhausting to the centralized acid exhaust system. The vent of the ammonium hydroxide solution storage tank should be connected to the centralized ammonia exhaust system or inlet exhaust duct to the POU wet scrubbing system. The exhaust static pressure on the spray chamber should be set between –0.5 and –0.75 inches of water to allow good operation of the spray chamber (to be verified by the process engineer and checked by safety personnel) and to minimize chemical mists from being carried into the exhaust duct. Install a POU device if needed for acid mists. Spray processes (with no ammonium hydroxide solution), including vertical tube cleaners: the exhaust static pressure on the spray chamber should be set between –0.5 and –0.75 inches of water to allow good operation of the spray chamber (to be verified by the process engineer and checked by safety personnel) and to minimize chemical mists from being carried into the exhaust duct. Install a POU device if needed for acid mists. Bulk chemical dispensing (BCD) for ammonium hydroxide solution (if ammonia concentration in the exhaust duct is above 1 ppmv): connect to the centralized ammonia exhaust system. Silane-based poly and doped poly CVD processes and silane-based oxide and doped oxide CVD processes: use a POU thermal oxidizing system. Nitride-oxynitride CVD processes: use a POU passive air addition-wet scrubbing system with heating of the exhaust line from the process pump to the POU device. TEOS-based oxide and doped oxide CVD processes: a POU device is only needed if particulate deposition in the centralized acid exhaust system is a concern. Heating of the exhaust line from the process chamber to the POU device (if used) or to the centralized acid exhaust system (if a POU device is not used) can be used if plugging of the exhaust line is a concern. Tungsten CVD processes: use a POU thermal oxidation-wet scrubbing system with heating of exhaust line from the process pump to the POU device. Tungsten silicide CVD processes (using dichlorosilane): use a POU wet scrubbing system with heating of the exhaust line from the process pump to the POU device. Nitrogen trifluoride predevice CVD chamber cleaning (due to fluorine emissions). This may occur with CVD processes already mentioned previously; the following recommendation takes precedence over other recommendations mentioned previously: a POU thermal oxidation-wet scrubbing system with use of natural gas, hydrogen, or water addition in thermal oxidation section and with fluorine-, hydrogen fluoride-, and hydrofluoric acid-resistant materials of construction in the POU device.

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11. LPCVD nitride processes: use a POU cold trap after the process pump, followed by a POU wet scrubbing system. A cold trap may also be installed in the exhaust line at the process exit. 12. EPI processes: use a POU wet scrubbing system. The first choice is to exhaust the POU wet scrubbing system directly outside of the fab (for safe venting of hydrogen). The second choice is to exhaust the POU wet scrubbing system to a POU thermal oxidizer system to oxidize hydrogen before it enters the centralized acid exhaust system. 13. Dry aluminum etch processes: use a POU wet scrubbing system with heating of the exhaust line from the process pump to the POU device. 14. Dry tungsten etch processes: use a POU wet scrubbing system. Heating of the exhaust line is only needed if the exhaust line between the process pump and the POU device plugs in less than 6 months. 15. Ion implanter process exhausts containing hydride gases: use a POU dry technology system (using canisters). Each canister will be considered hazardous waste. 16. Aqua regia bath (hydrochloric acid solution-nitric acid solution): use a POU wet scrubbing system with a high pH scrubbing solution followed by a submicron acid mist filter. Sodium hydroxide solution will be piped to the POU wet scrubbing system for pH adjustment of the scrubbing solution. Oxide etch processes, nitride etch processes, poly etch processes, sputterers, and evaporators do not need POU devices. Processes using organic compounds that are exhausted to a centralized VOC exhaust system do not require POU devices. Wet benches using acid solutions and clean water only will be exhausted to a centralized acid exhaust system.

3.6 “DEVELOPMENT OPPORTUNITIES” FOR POINT-OF-USE DEVICES Some thoughts below are provided for POU device suppliers, process equipment suppliers, and semiconductor fab personnel to consider.

3.6.1 MORE PROCESS

TO

POINT-OF-USE DEVICE INTERFACE

It would be beneficial if the process equipment could “talk” with POU devices. A current example of this is sending a signal to the POU device when CVD chamber cleaning is occurring, so higher temperatures can be used in the thermal oxidation section to more efficiently destruct the PFC of interest. Some other beneficial interfaces could be used as follows. 3.6.1.1 Water Usage Increase water makeup to the POU device when higher concentrations of fluorine, acid compounds, chlorine, and other contaminants are being emitted from the process chamber. A possible scenario would be setting makeup water at 0.2 gpm during the

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deposition step using silane and phosphine and then increasing to 0.5 gpm water makeup when the chamber cleaning is about to start. 3.6.1.2 Fluoride Wastewater Segregation Some wafer fabs have low fluoride concentration wastewater discharge requirements; therefore, segregate and treat high-concentration fluoride wastewater. The flow capacity of the existing fluoride wastewater treatment plant can be a limiting factor for wastewater from the POU devices with high fluoride concentrations. It would be beneficial to be able to send only the high-concentration fluoride wastewater (e.g., during CVD chamber cleaning) to the fluoride wastewater treatment plant, and send low-concentration fluoride wastewater (e.g., during idle time and deposition step) to the acid wastewater neutralization plant. A mechanical device could be used to switch back and forth from the acid wastewater neutralization plant to the fluoride wastewater treatment plant, depending on the process operation or actual fluoride wastewater concentrations (i.e., the switch could be based on the measured fluoride wastewater concentration being discharged from the POU device).

3.6.2 UNDERSTAND COST

OF

OWNERSHIP

The cost of ownership of POU devices needs to be better understood. Operating costs can be provided by the POU device supplier; however the estimated costs for utilities, exhaust, floor space, etc. provided by POU suppliers differ so much that it is difficult to compare them. Each fab can determine their own costs (e.g., electricity, water, natural gas) and supply these costs to each of their POU suppliers, so the cost of ownership can be compared. Generally, the largest POU device issue (cost to the fab) is lost wafer throughput that is caused directly from a POU device not operating. This is a big concern for fabs that cannot or do not bypass exhaust around a POU device when the POU device is not operating. For fabs that use exhaust bypass, this may only be an issue if the POU device goes down unexpectedly and has to be manually bypassed. The cost of lost wafer throughput should be factored somehow into the cost of ownership of the POU device. Truly understanding the POU device uptime and maintenance requirements is sometimes difficult for those fabs that can bypass exhaust. Many companies have developed data-gathering methodologies by process type, POU technology, and POU supplier to document POU device uptime. These data drive the decision-making process for future POU devices. Getting these data may be difficult if the POU device supplier also does the maintenance and can bypass exhaust when work is performed, thus not affecting process equipment (i.e., fab personnel do not know POU device uptime information). The loss of production is not a factor because exhaust bypass usually does not affect process equipment; however, the centralized acid exhaust system and possibly stack emissions will be impacted.

3.6.3 BETTER-PERFORMING INLETS

FOR

POINT-OF-USE DEVICES

If the process exhaust contaminants do not make it into the POU device, then the usefulness of the POU device is not fully realized. One of the main concerns is

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plugging POU device inlets, possibly requiring frequent maintenance and leading to process downtime (depends on exhaust configuration). Continued development of better-performing POU device inlets is suggested.

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4

Centralized Scrubbers

4.1 INTRODUCTION Centralized scrubbers (also called centralized wet scrubbers) are generally used at the end of a centralized acid exhaust system and a centralized ammonia exhaust system. There are other locations where centralized scrubbers are used, which will be discussed in this chapter. Three types of centralized scrubbers are generally found in the semiconductor industry: 1. Horizontal cross-flow packed-bed scrubbers: the exhaust air flows horizontally through packing while the recirculation scrubbing solution flows downward through the packing. This scrubber type is most prevalent for centralized scrubbers in the semiconductor industry. An advantage of this design is lower height. Typical design is rectangular shape. 2. Vertical countercurrent packed-bed scrubbers: the exhaust air flows upward through packing as recirculation scrubbing solution flows downward. This scrubber type is the second-most used for centralized scrubbers in the semiconductor industry. An advantage of this design is theoretically the best gas-to-liquid mass transfer. The best design and operation occurs with a cylindrical shape (note: a rectangular design is not an efficient design and creates operation concerns). 3. Vertical cocurrent packed-bed scrubbers: the exhaust air flows downward along with the recirculation scrubbing solution. This type is a very rare design in the semiconductor industry for centralized scrubbers. The best design and operation occurs with a cylindrical shape (note: a rectangular design is not an efficient design and creates operation concerns). Since vertical cocurrent packed-bed scrubbers are not used very much in the semiconductor industry as centralized scrubbers, the rest of this chapter will be dedicated to horizontal cross-flow packed-bed scrubbers and vertical countercurrent packed-bed scrubbers (this chapter only discusses packed-bed design, so will refer to these centralized scrubbers also as horizontal cross-flow scrubbers and vertical countercurrent scrubbers). Centralized acid exhaust systems commonly have the following contaminants in the airstream going into the centralized acid scrubber: hydrogen fluoride, hydrogen chloride, chlorine, fluorine, silicon tetrafluoride, perfluorocompounds (PFCs), hydride gases (arsine, phosphine, etc.), nitric acid, sulfuric acid, volatile organic compounds (VOCs), ammonia, etc. (note that ammonia and VOCs should be minimized or eliminated in the centralized acid exhaust system). Inlet concentrations for these contaminants are generally very low for centralized acid scrubbers, and inlet concentrations vary depending on what processes are emitting to the centralized acid 79

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exhaust system. Hydrogen fluoride and hydrogen chloride (which are usually in the highest inlet concentration) are generally below 5 parts per million by volume (ppmv); inlet concentrations can be commonly found at 1 ppmv or less for hydrogen fluoride and hydrogen chloride. Centralized acid scrubbers are generally designed for removal of hydrogen fluoride and hydrogen chloride. This can generally be accomplished at a scrubbing solution pH range of 7 to 8 (effects at lower and higher pH are discussed later). It is important to note that removal efficiencies in centralized acid scrubbers are near zero for PFC gases, silane, arsine, and phosphine. Low removal efficiencies can be expected for nitric acid and sulfuric acid in standard centralized acid scrubbers in the semiconductor industry. Inlet concentrations of VOCs and ammonia into centralized acid scrubbers depend on proper segregation of processes emitting VOCs into the centralized VOC exhaust system and processes emitting ammonia into the centralized ammonia exhaust system. They also depend on the use of point-of-use (POU) devices for processes emitting ammonia or processes emitting VOCs that exhaust to the centralized acid exhaust system. A small amount of VOCs is common in the centralized acid exhaust system due to general cleaning in the fab (e.g., isopropanol-water mixture used for cleaning surfaces that evaporates into fab air and is pulled into a wet bench exhaust). There is usually some removal efficiency for ammonia (“free” ammonia, not ammonium compounds) and organic compounds in the centralized acid scrubbers. Further discussion of ammonia and VOCs in centralized acid scrubbers is provided later in this chapter. Centralized ammonia scrubbers commonly have inlet concentrations of ammonia of 50 to 200 ppmv; inlet concentrations vary depending on what processes are emitting to the centralized ammonia exhaust system. Ammonia should be the primary compound, with near zero amounts of any other contaminants (such as acid compounds). High removal efficiencies of ammonia can be obtained if the pH of the scrubbing solution is maintained at 5 and below. Addition of sulfuric acid solution to control the pH of the scrubbing solution is common for centralized ammonia scrubbers. Some fabs use reclaimed sulfuric acid solution from fab processes. Acid scrubbers for large storage tanks of concentrated acid solution (e.g., concentrated hydrochloric acid solution used for clean water production) have similar design considerations. These acid scrubbers are commonly regulated by air regulatory agencies similarly to centralized acid scrubbers. A description of these acid scrubbers is included in this chapter. Wet scrubbing systems for higher inlet concentrations of acid compounds, chlorine, and acid mists will also be discussed in this chapter, as the acid scrubber is similarly designed, with the addition of a submicron acid mist filter after the acid scrubber. Also discussed in this chapter are centralized oxides of nitrogen scrubbing systems, which are primarily used in wafer manufacturing facilities.

4.2 GENERAL DESIGN AND OPERATION Many of the same design and operation information are similar for horizontal crossflow scrubbers and vertical countercurrent scrubbers. The first point to make is that scrubbers are mass-transfer units, not cooling towers! Many scrubbers are operated like a cooling tower: wastewater discharge (blowdown) of scrubbing solution occurs

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when conductivity increases to a set level. Scrubbers are installed to transfer gases (in air) to the liquid phase (scrubbing solution). Scrubber solution chemistry (i.e., fluoride concentrations, chloride concentrations, calcium concentrations, etc.) is essential to high removal efficiency, especially when inlet airstream concentrations are low (a few ppmv and less for acid compounds), which is the case in the semiconductor industry.

4.2.1 GENERAL DESIGN INFORMATION The following design parameters are suggested for designing centralized scrubbers (including fans and stacks). It is important to note that the suppliers of centralized scrubber may not use these criteria, so it is prudent for the future owner of the centralized scrubber to request that the supplier use specific parameters of scrubber design. Inlet concentrations of the centralized scrubber (from the specific fab in the centralized exhaust system) and required removal efficiencies for each contaminant should also be specified. 1. Recirculation rate of scrubbing solution: 6 to 10 gallons per minute (gpm)/ft2 of wetted packing surface and 10 to 30 gpm/1000 ft3/min air flowrate. Install proper sized pumps to get the recirculation rate required. Install strainers on the recirculation system of the scrubbing solution to prevent plugging of spray headers or settling in the weir design. For horizontal cross-flow scrubbers, install a spray header of scrubbing solution for the inlet air side of the packing section to ensure it is wetted properly. 2. Makeup water rate should be continuous. If no chemical addition is used, the recirculation rate of scrubbing solution should be a minimum of 3%. If chemical addition is used, a minimum recirculation rate of 1% of scrubbing solution should be used. Water chemistry of makeup water is important and will be discussed in a later section. 3. The pH of the scrubber solution can be maintained within a range of 7 to 8 for centralized acid scrubbers and a range of 3 to 4 for centralized ammonia scrubbers. Regulatory air permit limits need to be reviewed to ensure these pH ranges can be used, especially for centralized acid scrubbers. If not, consider the possibility of changing the pH range to 7 to 8 in the air permit limits. 4. Air velocity through packing: 300 to 500 ft/min (note that 500 ft/min and short packing depth [e.g., 4 feet] cause very short residence time for mass transfer). 5. Stack exit air velocity of 3000 to 3800 ft/min. Stack heights should be as high as possible to allow air to leave the building wake and not impact roof and air intakes. 6. Minimize organic compounds in the centralized acid exhaust system to reduce biogrowth. 7. Ammonia in the centralized acid exhaust system and acid compounds in the centralized ammonia exhaust system should be undetectable. 8. Install an efficient demister section between the packing section and fans.

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EXAMPLE 4-1: DESIGN OF VERTICAL COUNTERCURRENT CENTRALIZED ACID SCRUBBER. With the suggested design parameters listed previously, the following vertical countercurrent packed-bed scrubber was designed for 30,000 ft3/min. Packed-bed height: 8 feet. Cross-section diameter: 10 feet (equates to wetted packing area of 78.5 ft2 and 382 ft/min air velocity). Scrubbing solution recirculation rate: 470 gpm (equates to 6 gpm/ft2 and about 15.7 gpm per 1000 ft3/min). The makeup water rate is continuous at 5 gpm as sodium hydroxide solution is available for pH adjustment of the scrubbing solution. The pH range of the scrubbing solution is maintained between 7 and 8 by using sodium hydroxide solution (rarely added, since makeup water is city water at a pH of around 8). No measurable ammonia and organic compounds should be in the centralized acid exhaust system.

EXAMPLE 4-2: DESIGN OF HORIZONTAL CROSSFLOW CENTRALIZED ACID SCRUBBER. With the suggested design parameters listed previously, the following horizontal crossflow packed-bed scrubber was designed for 30,000 ft3/min. Packed-bed length: 6 feet. Height is 10 feet, and width is 8 feet. Height by width area: 80 ft2 (equates to an air velocity of 375 ft/min). Packing wetted area: 48 ft2 (6 feet times 8 feet). Recirculation rate for scrubbing solution: 300 gpm (equates to 6.2 gpm/ft2 and 10 gpm per 1000 ft3/min). The makeup water rate is continuous at 9 gpm using water only (no chemical addition). The pH range of the scrubbing solution is maintained between 7 and 8 by using city water only (pH of about 8) as makeup water. No measurable ammonia and organic compounds should be in the centralized acid exhaust system.

When specifying a centralized wet scrubber, it is essential to provide the supplier with the inlet concentrations (maximum and average) and required removal efficiencies for the contaminants needing abatement. The maximum airflow rate and the operational range are also important. Many suppliers of scrubbers design and test their scrubbers at inlet contaminant concentration of 100 to 500 ppmv. This generally works for centralized ammonia scrubbers. However, the very low inlet concentrations of acid compounds (typically

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<1 to 5 ppmv) in the centralized acid scrubbers make it very difficult to get high removal efficiencies (due to mass transfer difficulties and effects of scrubbing solution chemistry). Efficiency testing of removal should be conducted at the supplier’s facility using actual contaminant concentrations at the inlet of the centralized acid scrubber and conditions of the scrubbing solution chemistry to properly estimate the efficiencies of contaminant removal for centralized acid scrubbers in semiconductor fabs. EXAMPLE 4-3: TESTING OF CENTRALIZED ACID SCRUBBER IN A SEMICONDUCTOR FAB. A semiconductor silicon wafer fab used design parameters for a centralized scrubber that were suggested for new centralized acid scrubbers. It is important to note that there is no measurable ammonia in a centralized acid exhaust system. The first scrubber was installed and tested for removal efficiencies for hydrogen fluoride and hydrogen chloride. The average inlet concentration for hydrogen fluoride (measured as total soluble fluorides) was 2 ppmv with an average outlet concentration of 0.04 ppmv (removal efficiency of 98%); and average inlet concentration of hydrogen chloride (measured as total soluble chlorides) was 0.5 ppmv with an average outlet concentration of 0.1 ppmv (removal efficiency of 80%). As additional similar centralized acid scrubbers were installed, removal efficiency testing was performed. It was determined from this testing that common outlet concentrations of around 0.1 ppmv hydrogen chloride (measured as total soluble chlorides) and around 0.04 ppmv hydrogen fluoride (measured as total soluble fluorides) were found from inlet concentrations ranging from 0.1 to 10 ppmv for each acid compound. This was determined to be the mass transfer limits for these acid compounds for these similarly designed acid scrubbers and the makeup water used at this fab.

4.2.1.1 Distribution of Airflow Horizontal cross-flow scrubbers and vertical countercurrent scrubbers do have difficulty in providing uniform air distribution throughout the packing. A horizontal cross-flow design has a very short distance from exhaust duct to packing inlet. Baffles can be installed, but there may still be nonuniform air distribution. A vertical countercurrent design generally has the exhaust duct entering horizontally with a sharp turn upward into the packing. Again, baffles can be used, but uniform air distribution is even more difficult in this situation. Bypassing of air through or around the packing with little or no contact with the scrubbing solution can significantly impact efficiency of removal. Some examples of this are the following: 1. Bypassing air above the packing section of a horizontal cross-flow scrubber can occur if there is an opening at the inlet of the top side of the packing section or if the packing level drops due to settling. An installation of a full-length horizontal piece can be done to block any air from going above the packing section. It is recommended to check for settling of the packing soon after filling the packing section and periodically afterward. Sometimes the packing is filled a little higher than needed to anticipate

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settling. This can be an issue if the packing level blocks the spray patterns of the spray nozzles, causing incomplete wetting of the packing and possibly causing the air not to contact the scrubbing solution in the packing section itself. 2. Air not contacting the scrubbing solution in the packing section can happen if the distribution system of the scrubbing solution for either horizontal cross-flow or vertical countercurrent design does not fully wet the packing section. Examples of this are nonuniform spray patterns from spray nozzles of scrubbing solution or nonuniform wetting of packing using a weir distribution system of scrubbing solution. 3. Air can bypass in corners of rectangular vertical countercurrent scrubbers. It is recommended never to design a vertical countercurrent scrubber in a rectangular shape (i.e., a cylindrical shape should be used). 4. In horizontal cross-flow scrubbers, air bypassing under the packing section is possible if the sump level of scrubbing solution is not high enough (i.e., does not extend above the bottom of the packing section). This can occur by poor design of the scrubber or when a sump system of the scrubbing solution is retrofitted without ensuring the level is high enough. Plugging of packing can cause poor distribution of air and scrubbing solution. Many times this plugging cannot be seen from the sides or top of the packing (i.e., the plugging is near the center of the packing section). Another concern with plugging of the packing is the increase of air velocity through the packing (i.e., reduces the residence time for scrubbing). This can affect removal efficiency. 4.2.1.2 Distribution of Scrubbing Solution It is essential that all packing be wetted. This assists in the best mass transfer possible (i.e., prevents air passing through nonwetted sections of packing) for the type of packing selected. In addition, chemical treatment for prevention and removal of biogrowth can be most efficiently done by entire wetting of the packing (i.e., getting chemical to the biogrowth). The two most-used distribution systems of scrubbing solution for centralized scrubbers in the semiconductor industry are spray nozzles and the weir distribution system. Spray nozzles can be used for horizontal cross-flow and vertical countercurrent scrubbers. Spray nozzles allow for uniform distribution of scrubbing solution across the packing. Spray nozzles can plug and require more maintenance than a weir distribution system. A weir distribution system can be used for vertical countercurrent scrubbers but is not recommended for horizontal cross-flow scrubbers (due to the distance required for distribution of the scrubbing solution). A weir distribution system for scrubbing solution has low maintenance requirements and is more dependable. The concern about a weir distribution system is that it takes some distance for the scrubbing solution to spread out and wet all the packing (i.e., it could lose a few feet of packing at the top for complete scrubbing use). If the weir distribution system does not wet

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all the packing area horizontally (as the scrubbing solution travels downward), some of the air may pass through the scrubber without contacting the scrubbing solution (i.e., partial scrubbing of the entire airstream occurs). Horizontal cross-flow scrubbers can have a packing level on top that can drop, which allows air to bypass through the top of the packing section. Also, the sump level of scrubbing solution should be above the bottom of the packing section while the horizontal cross-flow scrubber is operational (note: idle sump solution level of the scrubber could be higher). A sheet can be installed to block air if air is bypassing on the top or bottom. Install sheet on the inlet and outlet air sides of the packing section, vertically past the packing about 3 to 4 inches. A spray header should be installed on the inlet air side of the packing section for horizontal cross-flow scrubbers. This spray header wets the front of the packing that is usually not well wetted due to incoming air forcing scrubbing solution sprayed from the top away from the front of the packing section. 4.2.1.3 Designing Centralized Scrubbers and Fans for Installation Two common installation types for centralized acid scrubbers and centralized ammonia scrubbers are seen in the semiconductor industry: 1. Several centralized scrubbers are attached to one main header, which collects the exhausts from the main ducts from the fab. It is common to see a set of fans that pull air through a bank of centralized scrubbers. For backup centralized scrubber capacity, one extra centralized scrubber is installed, which is idle until one of the other centralized scrubbers needs to be idled for maintenance. 2. One centralized scrubber is attached to a main duct from the fab. There can be several centralized scrubbers, one for each main duct from the fab. Generally, each centralized scrubber will have two fans and two recirculation pumps for scrubbing solution, one operating and the other as a backup when maintenance is required. A few regulatory agencies allow for bypassing centralized scrubbers for maintenance (i.e., uncontrolled emission to the atmosphere for a short time for maintenance). In these fabs backup centralized scrubbers or mechanical equipment may not be installed.

4.3 GENERAL OPERATION INFORMATION Many factors should be reviewed because the centralized scrubber is being designed and installed to ensure high gas-to-liquid mass transfer, maximum uptime, prevention of visible emissions, and no odor reports. Most of this section concerns centralized acid scrubbers, as this is where most of the “development opportunities” are (note: centralized ammonia scrubbers are generally installed and operated for an exhaust airstream which has ammonia as its major contaminant).

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4.3.1 AMMONIA – CENTRALIZED ACID SCRUBBER ENEMY NO. 1 Ammonia can cause issues for centralized acid scrubbers, as well as the centralized acid exhaust system. Most new fabs segregate ammonia away from the centralized acid exhaust system and send ammonia-containing air to a separate centralized ammonia exhaust system (and usually centralized ammonia scrubbers). Why? Ammonia will react immediately with acid compounds (e.g., hydrogen chloride, hydrogen fluoride, nitric acid, and sulfuric acid) to form ammonium compounds (ammonium chloride, ammonium fluoride, etc.). These ammonium compounds settle as particulates in the ducts, causing plugging. Ammonium compounds will not mass transfer to any extent in standard centralized acid scrubbers. The ammonium compounds exiting the centralized acid scrubber will be in the form of submicron particles (formed with water vapor in the scrubber air). These particles are generally visible coming from a centralized scrubber stack at around 1 ppmv and higher. Many regulatory agencies have opacity requirements for stack exits (also called visible emissions requirements). Note that the visible emissions can be sporadic, as they need both acid compounds and ammonia to form; concentrations of ammonium compound vary in the centralized acid exhaust system depending on what the emissions of ammonia and acid compounds result from fab processes (fab process emissions are very variable). Another concern is the very low efficiency of removal of ammonium compounds in centralized acid scrubbers as it relates to meeting air regulatory limits for efficiency of scrubber removal of acid compounds and acid compound emissions (e.g., ammonium chloride can be measured as hydrogen chloride by some sampling methods; since very little ammonium chloride is removed in centralized acid scrubbers, measured hydrogen chloride removal efficiency would be very low). The measurement technique (sampling method) used is also important. It is crucial to really understand what is being sampled and how best to measure it. Do not assume that sampling contractors understand the different aspects of sampling centralized acid scrubbers (inlet and outlet). Spend time educating yourself, or ask someone who is educated to properly review all sampling protocols and data to be generated before performing testing. Many times, “bad” data have been generated that were not useful to the fab personnel. Three types of ammonia and acid compound scenarios can be found (with each of its characteristics listed). 1. Ammonia rich: the inlet airstream of the centralized acid scrubber has a higher concentration of ammonia than total acid compounds. Uncombined ammonia can be scrubbed to some extent in the centralized acid scrubber, which causes the pH of the scrubbing solution to rise (which could lead to scaling of packing). Negative removal efficiencies of acid compounds can be measured, as ammonia can react primarily with chloride and fluoride ions (for example) in the scrubbing solution to form ammonium compounds; this especially occurs if ammonia concentration is much higher than the concentrations of total acid compound in the centralized acid scrubber inlet airstream.

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2. Acid-ammonia neutral: concentrations of ammonia and total acid compounds are the same in the centralized acid scrubber inlet airstream. Removal efficiencies for both ammonia and acid compounds are near zero. 3. Acid rich: concentrations of total acid compounds are higher than the ammonia concentration in the centralized acid scrubber inlet airstream. Acid compounds that do not form ammonium compounds should be removed by the centralized acid scrubber to some extent. EXAMPLE 4-4: AMMONIA RICH. An existing silicon wafer fab had no ammonia exhaust segregation. Wet method sampling trains (using glass sampling vessels containing liquid for scrubbing contaminants that are soluble in solutions used) were used on the inlet and outlet of the centralized acid scrubber. Average test results were reported to fab personnel. The inlet ammonia concentration was 8 ppmv, and the outlet ammonia concentration was 4 ppmv. The inlet hydrogen fluoride concentration was 1.0 ppmv, and the outlet hydrogen fluoride concentration was 1.1 ppmv. The inlet concentration of hydrogen chloride was 0.40 ppmv, and the outlet concentration of hydrogen chloride was 0.50 ppmv. Removal efficiencies of hydrogen fluoride and hydrogen chloride were reported as –10% and –25%, respectively. The visible emissions seen at the stack outlet were determined to be ammonium compounds (total of 1.6 ppmv) instead of water vapor from the centralized acid scrubber.

EXAMPLE 4-5: ACID-AMMONIA NEUTRAL. An existing silicon wafer fab had no ammonia exhaust segregation. Wet method sampling trains (using glass sampling vessels containing liquid for scrubbing contaminants that are soluble in solutions used) were used on the inlet and outlet of the centralized acid scrubber. This testing methodology sampled for total soluble fluorides, total soluble chlorides, etc., reporting the sampling results as hydrogen fluoride, hydrogen chloride, etc. Average test results were reported to fab personnel. The inlet ammonia concentration was 0.5 ppmv, and the outlet ammonia concentration was 0.5 ppmv. The inlet hydrogen fluoride concentration was 0.4 ppmv, and the outlet hydrogen fluoride concentration was 0.4 ppmv. The inlet concentration of hydrogen chloride was 0.10 ppmv, and the outlet concentration of hydrogen chloride was 0.10 ppmv. Removal efficiencies for both hydrogen chloride and hydrogen fluoride were reported as zero. There were no visible emissions seen at the scrubber stack. This was attributed to ammonium compounds being 0.5 ppmv, below 1 ppmv which is typically the stack outlet concentration when visible emissions first occur.

EXAMPLE 4-6: ACID RICH. An existing silicon wafer fab had some ammonia exhaust segregation. Wet method sampling trains (using glass sampling vessels containing liquid for scrubbing contaminants that are soluble in solutions used) were used on the inlet and outlet of the centralized acid scrubber. This testing methodology sampled for total soluble fluorides, total soluble chlorides, etc., reporting the sampling results as hydrogen fluoride, hydrogen

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Semiconductor Industry: Wafer Fab Exhaust Management chloride, etc. Average test results were reported to fab personnel. The inlet ammonia concentration was 0.5 ppmv, and the outlet ammonia concentration was 0.5 ppmv (a reported removal efficiency of zero). The inlet hydrogen fluoride concentration was 1.0 ppmv, and the outlet hydrogen fluoride concentration was 0.4 ppmv. The inlet concentration of hydrogen chloride was 0.4 ppmv, and the outlet concentration of hydrogen chloride was 0.3 ppmv. Removal efficiencies of hydrogen fluoride and hydrogen chloride were reported as 60% and 25%, respectively. The centralized acid scrubber did not remove a total of 0.7 ppmv of hydrogen fluoride and hydrogen chloride; 0.5 ppmv was most likely ammonium compounds. There were no visible emissions seen at the scrubber stack. This was attributed to ammonium compounds being 0.5 ppmv, below 1 ppmv which is typically the stack outlet concentration when visible emissions first occur.

It has been found in the semiconductor industry that ammonia can also assist in biogrowth. Significant biogrowth has occurred in a centralized acid scrubber with no detection of VOCs in the centralized acid exhaust system, and with ammonia detected in the centralized acid exhaust system and being partially scrubbed by the centralized acid scrubber. When ammonia was significantly reduced in the centralized acid exhaust system (i.e., not going into the centralized acid scrubber), the biogrowth was reduced. It is important to mention that acid compounds should not be exhausted to centralized ammonia scrubbers, as ammonium compounds will form. Ammonium compounds will have very low removal efficiency in centralized ammonia scrubbers, and stack visible emissions will occur at around 1 ppmv ammonium compounds and above.

4.3.2 ACID MISTS Acid mists can be formed in the centralized acid scrubber generally due to nitric acid and hydrochloric acid (sulfuric acid can also form acid mists, but sulfuric acid is not usually at high enough inlet concentrations to centralized acid scrubbers to cause stack visible emissions). Acid mists at around 1 ppmv and above are usually visible at the stack exhaust exit. It is important to note that acid mists with no ammonia present will have visible emissions. Standard centralized acid scrubbers are marginally efficient for nitric acid (especially at low inlet concentrations such as a few ppmv) and can be “overwhelmed” by higher concentrations of hydrogen chloride. Examples of sources for acid mists are solution baths and spray processes of heated aqua regia, hot nitric acid baths, and venting of large hydrogen chloride cylinders and tube trailers. Odors can result from these acid compounds and byproducts of heated aqua regia solution (e.g., chlorine and nitrosyl chloride). The use of a centralized acid scrubbing system for higher inlet concentrations of acid compounds, chlorine, and acid mists is discussed later in this chapter. EXAMPLE 4-7: HEATED BATH OF AQUA REGIA SOLUTION. A fab had a separate centralized ammonia exhaust system and undetectable ammonia in the centralized acid exhaust system. A common mixture of aqua regia solution (one part concentrated nitric acid solution and three parts concentrated hydrochloric acid

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Centralized Scrubbers solution) was used in a bath in a wet bench. This bath of aqua regia solution was heated to 70°C. The exhaust from this bath went to the centralized acid exhaust system and on to centralized acid scrubbers. When the bath was heating and then processing wafers, visible emissions occurred at the centralized acid scrubber stacks for about 2 hours, with varying intensities. Odors were noticed downwind of the centralized acid scrubber stacks, and metal equipment and accessories on the roof were rusting. Fab personnel designed and installed a POU wet scrubbing system operated at a pH range of 10 to 11 (with an addition system of sodium hydroxide solution) followed by a submicron acid mist filter. Visible emissions and odors did not occur after this POU device was installed.

EXAMPLE 4-8: HEATED BATH OF NITRIC ACID SOLUTION AND HEATED BATH OF NITRIC ACID–SULFURIC ACID SOLUTION. This fab had a separate centralized ammonia exhaust system and undetectable ammonia in the centralized acid exhaust system. The fab had a bath of concentrated nitric acid solution at 70°C and a bath with a 50/50 mixture of concentrated nitric acid solution and concentrated sulfuric acid solution at 80°C in a wet bench. The exhaust from this wet bench containing these baths was connected to the centralized acid exhaust system and on to the centralized acid scrubbers. Visible emissions from the centralized acid scrubber stacks varied during the day. The most visible emissions occurred after both baths were replenished with new acid solutions, heated to temperature, and wafers were introduced to the baths. Fab personnel designed and installed a POU wet scrubbing system operated at a pH range of 10 to 11 (with an addition system of sodium hydroxide solution), followed by a submicron acid mist filter. Wet bench exhaust was connected to this POU device. The visible emissions from the centralized acid scrubber stacks were eliminated.

EXAMPLE 4-9: VERTICAL TUBE CLEANERS. A fab had a separate centralized ammonia exhaust system and undetectable ammonia in the centralized acid exhaust system. Sporadic visible emissions were seen at the centralized acid scrubber stacks. Review of fab processes and times of visible emissions correlated with the use of two vertical tube cleaners (using hydrofluoric acid solution and nitric acid solution for furnace tube cleaning). The vertical tube cleaners sprayed acid solutions on furnace tubes in an enclosed chamber, with exhaust slots in the back of the enclosed chamber drawing acid mists into the centralized acid exhaust system; the amount of acid mists being entrained increased as the exhaust static pressure became more negative. The exhaust static pressure was set at negative 1.2 inches of water on both vertical tube cleaners. Acid mists were being significantly entrained in the exhaust. An additional concern was found to be the amount of acid mist condensation in the exhaust ducts near the vertical tube cleaners. The static pressure of the exhaust was reduced to a negative 0.60 inches of water. Safety personnel reviewed the use of the vertical tube cleaners at this static pressure and found the vertical tube cleaners to be safe for operation (i.e., adequate protection of workers). The operation of the two vertical cleaners at a static pressure of the exhaust of negative 0.60 inches of water produced no visible emissions at the centralized acid scrubber stacks and reduced acid mist condensation in the exhaust ducts.

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EXAMPLE 4-10: SPRAY PROCESSES FOR ACID SOLUTION. A fab had a separate centralized ammonia exhaust system and undetectable ammonia in the centralized acid exhaust system. Sporadic visible emissions were seen at the centralized acid scrubber stacks. A review of fab processes and times of visible emissions correlated with the use of four spray processes of acid solution (using different acid solutions at varying temperatures), usually when more than one acid spray process was in use. These spray processes were in an enclosed chamber with exhaust on one side, pulling directly on the chamber entraining acid mists in the exhaust; the amount of acid mists being entrained increased as the exhaust static pressure became more negative. The exhaust static pressure ranged from negative 1.0 to 1.3 inches of water for these four spray processes for acid solution. Acid mists were being significantly entrained in the exhaust. An additional concern was found to be the amount of acid mist condensation in the exhaust ducts near the spray processes for acid solution. The static pressure of the exhaust was reduced to negative 0.70 inches of water. Safety personnel reviewed the use of the spray processes for acid solution at this static pressure and found the spray processes to be safe for operation (i.e., adequate protection of workers). No wafer quality issues were found. Visible emissions from the centralized scrubber stacks were eliminated, and acid mist condensation was reduced by the static pressure changes of the exhaust made on the spray processes for acid solution.

EXAMPLE 4-11: LINE VENTING OF HYDROGEN CHLORIDE CYLINDER. Hydrogen chloride was vented through a line during a cylinder-changing operation. The hydrogen chloride (from this venting) flowed through an exhaust duct to the centralized acid exhaust system (airflow rate of 60,000 ft3/min) and to centralized acid scrubbers. A visible emission occurred at the centralized acid scrubber stacks even though the hydrogen chloride concentration at the inlet into the centralized acid scrubbers was estimated to be only 50 ppmv, which called into question the removal efficiency of hydrogen chloride for the centralized acid scrubbers. There was no measurable ammonia present in the centralized acid exhaust system, so ammonium chloride was not causing the visible emissions. The gas supplier and centralized scrubber supplier claimed it was due to hydrogen chloride mist being generated at the time of hydrogen chloride venting, even though the relative humidity in the centralized acid exhaust system was well below 100%. Fab personnel decided to investigate this claim further. Inspection with a flashlight of the inlet duct to one of the centralized acid scrubbers showed no visible acid mists. The installation of a submicron acid mist filter (with no inlet water scrubbing section) did not remove hydrogen chloride (if it was hydrogen chloride submicron mist, it would have been removed by this filter); visible emissions at the centralized acid scrubber stacks still occurred. Therefore, it was determined that hydrogen chloride as a gas was forming the submicron acid mist in the centralized acid scrubbers. A water scrubbing section was installed before the existing submicron acid mist filter in the exhaust duct near the hydrogen chloride cylinder. The water-scrubbing section allowed the hydrogen chloride acid mist to form and then be removed in the submicron acid mist filter. The visible emissions were eliminated from the centralized acid scrubber stacks when the hydrogen chloride cylinder line was vented.

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4.3.3 FLUORINE Fluorine can be exhausted to a centralized acid exhaust system from many processes (e.g., chemical vapor deposition [CVD] process chamber cleanings), especially if there is no POU device utilized for fluorine removal. The chamber cleaning that generally has the most fluorine emitted is the nitrogen trifluoride predevice CVD chamber cleaning (note: this chamber cleaning is when nitrogen trifluoride is fractured into monatomic fluorine and diatomic fluorine in a device prior to flowing to the CVD chamber). Fluorine will react to some extent with water in the air in the centralized acid exhaust system to form hydrogen fluoride, and with liquids in ducts to form acid solutions; these acid solutions can do significant damage to ducts, depending on the construction materials exposed in the exhaust duct. However, unreacted fluorine can reach the centralized acid scrubbers. Fluorine is not removed efficiently by typical centralized acid scrubbers. In addition, many centralized acid scrubbers are operated at pH levels above 8, with the addition of sodium hydroxide solution. Under these conditions, oxygen difluoride can form. Both fluorine and oxygen difluoride odors may be detected outside the fab; health effects from these gases should also be evaluated. EXAMPLE 4-12: ODORS OUTSIDE FAB. A fab had been experiencing odors (similar in characteristic to fluorine and oxygen difluoride) in areas surrounding the fab building. The odors were traced back to the centralized acid scrubbers. This fab did not have a centralized ammonia exhaust system, so there were periodic visible emissions (below local air regulatory agency opacity requirements). Fab personnel decided to review the design and operation of the centralized acid scrubber first and leave fab process review until later if odor issues still existed. The design parameters of the centralized acid scrubbers were reviewed and were determined to be marginal at best. The following changes were implemented: 1. The recirculation rate of the scrubbing solution was changed to meet good scrubber design parameters of 6 to 10 gpm/ft2 of wetted packing surface and 10 to 30 gpm/1000 ft3/min airflow rate. 2. The pH of the scrubbing solution was reduced to a range of 7 to 8 from a pH that was above 9. This was done to reduce the possible formation of oxygen difluoride from fluorine. Sodium hydroxide solution was still added to the centralized acid scrubbers to be able to keep the pH range between 7 and 8 while using a minimum amount of makeup water. 3. The makeup water rate was changed to be continuous (previously makeup water was periodically added based on conductivity of the scrubbing solution) and at 1% of the recirculation rate of scrubbing solution. 4. The air velocity through the centralized acid scrubbers was reduced to 500 ft/min. The odors and sporadic visible emissions continued. The fab processes and effectiveness of POU devices were reviewed to determine likely sources of ammonia and odor. Several ammonia sources were found. The odor source was determined to be oxide CVD processes using nitrogen trifluoride predevice chamber cleaning with no POU

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Semiconductor Industry: Wafer Fab Exhaust Management devices. Fab personnel decided to remove ammonia from the centralized acid exhaust system first for three reasons: 1. Ammonium compounds were determined to be contributing to the odor found outside the fab. The centralized acid scrubbers could not remove these ammonium compounds. 2. The fab wanted to eliminate all visible emissions from centralized acid scrubber stacks. Even if fluorine emissions were significantly reduced to the centralized acid scrubbers, visible emissions coming from centralized acid scrubber stacks would still occur. 3. Ammonium compounds were depositing in the centralized acid exhaust system. The ammonia segregation and abatement project was implemented. There were no more visible emissions! However, odors were still being reported outside the fab, but less often. In addition, review of the centralized acid exhaust system showed damage to ducts. Fab personnel finally decided to install POU devices (with the ability to efficiently remove high concentrations of fluorine) on the oxide CVD processes with nitrogen trifluoride predevice chamber cleaning. The odor reports stopped after this POU device project was implemented.

EXAMPLE 4-13: DUCTWORK DAMAGE. Fab personnel did a camera scoping and integrity evaluation of the centralized acid exhaust system. In some areas there were ducts with significant damage. Testing was done to find out the cause. Acid damage was the cause (primarily from hydrofluoric acid). A process review was done for fab equipment that was exhausted upstream of the damaged ducts. It was determined that there were several CVD processes using nitrogen trifluoride predevice chamber cleanings without POU devices. POU devices that can efficiently remove fluorine were installed on these CVD processes.

4.3.4 VOLATILE ORGANIC COMPOUNDS Organic compounds should be eliminated from the centralized acid exhaust system and centralized ammonia exhaust system as much as possible (exemptions to be discussed later). Centralized ammonia scrubbers are generally operated at low pH which can eliminate biogrowth; therefore, organic compounds in a centralized ammonia exhaust system can be more forgiving as far as biogrowth is concerned. Organic compounds are a great food source for biogrowth. Organic compounds will also get into wastewater to some extent, depending on its mass transfer in air to scrubbing solution and solubility in the scrubbing solution. These organic compounds can be emitted from wastewater in the wastewater treatment plant and be discharged in the wastewater outside the facility. Many fab sites have emission requirements for VOCs; therefore have a separate centralized VOC exhaust system with centralized VOC control equipment. These fab sites usually limit VOC emissions from other centralized exhaust systems. Fab sites that do not have VOC emissions requirements may have a separate centralized VOC exhaust system with no centralized VOC control equipment (for the few fabs

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that do not have a separate centralized VOC exhaust system, they may want to consider it!). Review of all processes and exhaust connections is important to ensure that VOC-emitting processes are going to the centralized VOC exhaust system. There are some activities (e.g., general cleaning using a mixture of isopropanol and water) that happen in the fab area, in which organic compounds are evaporated in fab air; this fab air flows through process exhausts (e.g., wet benches) into centralized exhaust systems. VOC concentrations from general cleaning are typically very low in the fab air and usually have minimal impact on biogrowth in centralized acid scrubbers and centralized ammonia scrubbers. However, VOC emissions should be estimated for general cleaning. An example of an organic compound that may be commonly exhausted to a centralized acid exhaust system or centralized ammonia exhaust system is hexamethyldisilazane (HMDS). HMDS (usually used in vapor prime on a wafer before photoresist coating) can cause issues with some centralized VOC control equipment by being oxidized and forming silicon dioxide. Silicon dioxide can plug the thermal oxidizer and associated heat exchangers (if any); silicon dioxide can also poison the catalyst in a catalytic oxidizer and plug associated heat exchangers (if any). To prevent this from occurring, vapor prime exhaust containing HMDS can be connected to a centralized acid exhaust system or centralized ammonia exhaust system. A centralized ammonia exhaust system would be the better of the two centralized exhaust systems, as the HMDS may form ammonia when it reacts with water. It is important to note that permitted VOC emissions should be reviewed to determine if HMDS can be exhausted to non-VOC centralized exhaust systems. EXAMPLE 4-14: VOCS IN CENTRALIZED ACID EXHAUST SYSTEM EXAMPLE NO. 1. A centralized acid scrubber was having biogrowth issues. VOC sampling of the inlet airstream to the centralized acid scrubber using a hand-held monitor showed 2 ppmv VOC measured as isobutylene. Calculations of possible VOC emissions assuming no VOC removal efficiency of the centralized acid scrubber showed projected VOC emissions close to permitted VOC emissions. Fab personnel decided to review process and exhaust connections to determine VOC-emitting sources. An isopropanol wafer dryer in a wet bench was found to be exhausted to the centralized acid exhaust system; this process exhaust was disconnected from the centralized acid exhaust system and connected to a centralized VOC exhaust system. The inlet airstream of the centralized acid scrubber was resampled using a hand-held VOC monitor. VOC concentration was below detection.

EXAMPLE 4-15: VOCS IN CENTRALIZED ACID EXHAUST SYSTEM EXAMPLE NO. 2. Periodic hand-held VOC sampling of inlet airstream to a centralized acid scrubber had shown varying VOC concentrations. The effect of these VOC concentrations appeared to be biogrowth in the centralized acid scrubber and projected VOC emissions close to permitted VOC emissions. A team was established to review all processes for VOC emissions, including a fab-wide review of process exhaust connections. The results of

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Semiconductor Industry: Wafer Fab Exhaust Management this study showed that the CVD processes of tetraethylorthosilicate (TEOS)-based oxide without POU devices and HMDS from vapor prime operations were the VOCemitting sources. It was important to keep operation exhausts of HMDS vapor prime on the centralized acid exhaust system, since HMDS was causing issues with centralized VOC control equipment, there was no centralized ammonia exhaust system, and HMDS emissions should not be exhausted to centralized general or heat exhaust system. The team decided to install POU devices on CVD processes of TEOS-based oxide.

EXAMPLE 4-16: VOCS IN CENTRALIZED AMMONIA EXHAUST SYSTEM. A fab site conducted a yearly VOC monitoring program on all centralized exhaust systems, to determine if there were any VOC exhaust connections that were made to centralized exhaust systems other than the centralized VOC exhaust system. A measurement of 2 ppmv measured as isobutylene was found on the inlet to the centralized ammonia scrubber. No biogrowth was found in the centralized ammonia scrubber (scrubbing solution was kept at a pH range of 3 to 4). Assuming no VOC removal efficiency in the centralized ammonia scrubber, the estimated VOC emissions were within permitted VOC emissions for the fab site. Fab personnel still wanted to pursue where the VOC emissions were coming from. The VOC sources were found to be HMDS vapor prime operations. The vapor prime operations were recently exhausted to a centralized ammonia exhaust system due to HMDS issues with the centralized VOC control equipment.

4.3.5 SOURCE

OF

MAKEUP WATER

Make-up water quality (source) is very important for centralized acid scrubbers. Centralized ammonia scrubbers usually operate at low pH (e.g., in the range of 3 to 4), so are less sensitive to water quality. However, water quality still should be reviewed before its use in centralized ammonia scrubbers. The following discussion will concentrate on centralized acid scrubbers. The makeup water chemistry (quality) of scrubbing solution is very important to ensure good mass transfer from gas phase to liquid phase for acid compounds and for scrubber uptime. An example is water with high calcium concentration (e.g., city water or reverse osmosis reject) at a pH above 8 can lead to increased scaling of packing and mist eliminator. Another example is use of water with high concentrations of fluorides (e.g., fab reclaimed water or discharge water from an acid wastewater neutralization plant) can limit hydrogen fluoride mass transfer from the gas phase to the liquid phase. Use of water (e.g., reclaimed water) with high concentrations of ammonia can cause ammonia to move from the liquid phase to the gas phase, resulting in the formation of ammonium compounds (i.e., visible emissions). EXAMPLE 4-17: USE OF RECLAIMED WATER AS MAKEUP WATER FOR CENTRALIZED SCRUBBERS. A fab-wide water reclamation or recycling program was initiated and completed. Water was reclaimed from wafer rinse-water processes and used for centralized acid scrubbers and centralized ammonia scrubber makeup water. Soon after the reclaimed water was

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Centralized Scrubbers used as scrubber makeup water, there were reports of visible emissions from centralized acid scrubber stacks (it is important to note that the fab had no measurable ammonia in the centralized acid exhaust system, as the ammonia was properly segregated to a centralized ammonia exhaust system). Fab personnel conducted ammonia sampling on inlet airstreams to the centralized acid scrubbers, and ammonia was undetectable (below 1 ppmv ammonia). They next reviewed processes to determine if visible emissions were acid mist; no source was found. After further discussion, it was decided that maybe the outlets of the centralized acid scrubbers should be sampled. Results from this sampling showed ammonium compounds were the source of visible emissions. There was no ammonia coming into the centralized acid scrubbers, but ammonia was coming out of the centralized acid scrubbers! It was thought that the source of ammonia might be the reclaimed water. The reclaimed water was tested, and ammonia was found in the reclaimed water. Calculations were done using the concentration of ammonia in the reclaimed water and makeup water rate. A similar mass rate of ammonia was found for the makeup water input to the centralized acid scrubbers as compared to the ammonia emissions of the centralized acid scrubbers. The reclaimed water was the ammonia source. Further investigation determined that the rinse water from wafers that had just contacted ammonium hydroxide-hydrogen peroxide solution was the source of ammonia in the reclaimed water. This rinse water with ammonia was removed from the water reclamation system, and the visible emissions were eliminated.

EXAMPLE 4-18: REVERSE OSMOSIS REJECTED WATER USED AS CENTRALIZED SCRUBBER MAKEUP WATER. Facilities personnel decided to use first-pass reverse osmosis (RO) rejected water as makeup water for the centralized scrubbers as part of the water conservation program. The centralized acid scrubbers were designated for the use of the first-pass RO reject water. The centralized acid scrubbers were operated at a pH range of 9 to 10 for the scrubbing solution. No scrubber review of the impact on the use of first-pass RO reject water was done. Scaling of packing occurred quickly causing a shutdown of the centralized acid scrubbers. The site’s air quality permit required the scrubbing solution to operate at a pH above 8. Fab personnel requested a trial test using RO reject water as makeup water at a scrubbing solution pH range of 6 to 7, to determine if this makeup water source could be used without significant impact on scrubber packing. The trial test was approved by the local air regulatory agency with the stipulation that efficiency testing of hydrogen chloride and hydrogen fluoride removal would be conducted during the first 2 weeks of the trial test, to ensure the removal efficiency of 90% in the air permit would be met at a scrubbing solution pH range of 6 to 7. The testing was conducted, with hydrogen chloride and hydrogen fluoride removal efficiencies both above 90%. Scaling of centralized acid scrubber packing was greatly reduced at the pH range of 6 to 7.

EXAMPLE 4-19: NEUTRALIZATION PLANT DISCHARGE OF ACID WASTEWATER USED AS MAKEUP WATER. A fab wanted to use some of the discharge water from the site’s acid wastewater neutralization plant, which was discharged from the facility and flowed to the city’s wastewater treatment plant. The first trial use of this discharge water was as makeup

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Semiconductor Industry: Wafer Fab Exhaust Management water to the centralized acid scrubbers. This makeup water source was utilized with no additional chemicals to the scrubbing solution (i.e., no sodium hydroxide solution use). There was no measurable ammonia in the centralized acid exhaust system. The operation of the centralized acid scrubbers went well, except for the makeup water’s aggressive impact on the recirculation pumps. The yearly efficiency testing of acid compound removal was conducted for the local air regulatory agency. The results showed that there was similar removal efficiency for hydrogen chloride from the previous year’s testing; however there was a large drop in removal efficiency for hydrogen fluoride. It was determined that the higher fluoride concentrations in the makeup water (from the discharge of the acid wastewater neutralization plant) were affecting removal efficiency of hydrogen fluoride.

The use of first-pass RO reject water, discharge water from the acid wastewater neutralization plant, or reclaimed water may be satisfactory for operation of centralized ammonia scrubbers (scrubbing solution usually at a low pH) and for high removal efficiencies of ammonia. Additional equipment maintenance (e.g., scrubbing solution recirculation pumps) may be required.

4.3.6

PH OF

SCRUBBING SOLUTION

The pH of the scrubbing solution in centralized scrubbers can affect removal efficiencies, for example, ammonia, hydrogen fluoride, fluorine, and chlorine. Removal efficiency of hydrogen chloride is less dependent on scrubbing solution pH. Since the inlet air concentrations of chlorine and fluorine are usually very low for centralized acid scrubbers, pH levels for centralized acid scrubber scrubbing solution are usually set for hydrogen fluoride. Since centralized ammonia scrubbers are for ammonia only, desired removal efficiency of ammonia sets the scrubbing solution pH levels. If excess ammonia (i.e., ammonia in higher concentrations than acid compounds) is in the inlet air of the centralized acid scrubber, then the pH of the scrubbing solution will most likely rise when ammonia is absorbed in scrubbing solution. Continuous pH monitoring is a good way to track what is happening with the scrubbing solution in centralized scrubbers. Another way is to track chemical usage as pH is monitored and controlled; one example is the use of sulfuric acid for pH control in centralized ammonia scrubbers. The pH of scrubbing solution changes as it absorbs acid compounds, ammonia, and other specific contaminants while traveling through the packing. The pH of the scrubbing solution in the packing will be lower than the actual measured pH levels in the scrubbing solution sump when acid compounds are scrubbed in centralized acid scrubbers. The pH of the scrubbing solution in the packing when ammonia is scrubbed will be higher than the actual measured pH levels in the scrubber solution sump in centralized ammonia scrubbers. For example: 1. Centralized acid scrubbers: a typical minimum scrubbing solution pH that is used for scrubbing hydrogen fluoride is 5. Due to dropping scrubbing solution pH in packing section as acid compounds are absorbed, a minimum scrubbing solution sump pH of 7 is used to ensure good removal efficiency

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for hydrogen fluoride. Sodium hydroxide solution addition to scrubbing solution sump can be used if water source for makeup water cannot keep pH above 7. 2. Centralized ammonia scrubbers: a typical maximum scrubbing solution pH that is used for scrubbing ammonia is 5. Since scrubbing solution pH increases in the scrubber packing as ammonia is scrubbed, the use of a scrubbing solution pH range of 3 to 4 in the scrubbing solution sump is common. Chemical addition (e.g., concentrated sulfuric acid solution) is typically used to control pH of the scrubbing solution EXAMPLE 4-20: HIGH PH SWINGS OF CENTRALIZED ACID SCRUBBER (ALONG WITH VISIBLE EMISSIONS). Two centralized acid scrubbers had been experiencing periodic higher swings in pH of scrubbing solution (e.g., pH baseline was 7.8 with pH spikes to 8.5). Some of these pH spikes occurred at the same time as visible stack emissions. Ammonia was the likely source of both occurrences. The source of the ammonia was determined to be spray processes using ammonium hydroxide-hydrogen peroxide solution. Why are the visible emissions from centralized acid scrubber stacks occurring only some of the time when scrubbing solution pH goes up? Ammonia needs to mix with acid compounds to form ammonium compounds, which form submicron particles in the scrubber that contribute to visible emissions. If ammonia concentration is higher than total acid compounds and ammonium compounds that are formed are below 1 ppmv, then the pH of the scrubbing solution will rise, and there will be no visible emissions at the centralized acid scrubber stacks. If ammonia and total acid compounds are same concentration, or if total acid compounds are at a higher concentration than ammonia, with ammonium compounds formed above 1 ppmv, then there could be visible emissions at the centralized acid scrubber stacks with no pH rise in the scrubbing solution (a scrubbing solution pH drop can occur when total acid compounds are at a higher concentration than ammonia). Based on these situations, one cannot assume that a visible emission and an increase in pH of the scrubber solution will occur at the same time when processes are emitting ammonia.

EXAMPLE 4-21: LOW PH SWINGS IN SCRUBBING SOLUTION OF CENTRALIZED ACID SCRUBBER. A centralized acid scrubber was operating at a scrubbing solution pH range of 7 to 8 with continuous flow rate of makeup water and no chemical addition. There were episodes where the pH dropped below 7 to as low as 4. The minimum allowable pH of the scrubbing solution on the air permit was 5, so corrective action was required. A feed system of sodium hydroxide solution was installed to add sodium hydroxide solution when the pH of the scrubbing solution dropped below 7 in the scrubber solution sump and stopped addition of sodium hydroxide solution at pH of 8 in the scrubbing solution.

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4.3.7 BIOGROWTH

AND

SCALING

Biogrowth and scaling are a challenge for centralized acid scrubbers. Usually centralized ammonia scrubbers do not have biogrowth and scaling issues due to low pH of the scrubbing solution operation. Packing and plugging of the mist eliminator on centralized acid scrubbers can occur due to scaling or biogrowth. It is common to see both biogrowth and scaling occurring together. Good distribution of scrubbing solution in the packing is crucial to minimize scaling and biogrowth. Poor wetted packing can be starting points for scale and biogrowth buildup. Examples of where and how slightly wet areas occur are the inlet face of a horizontal cross-flow scrubber without face sprays, a poor distribution system of scrubbing solution, and underdesigned recirculation rate of scrubbing solution. A common scale is calcium carbonate, which commonly occurs at higher pH levels of scrubbing solution (e.g., 9 and above) when makeup water of high calcium concentration is used. Operating at a pH range of 7 to 8 in scrubbing solution that uses makeup water with nonscaling concentrations of calcium at a proper continuous flow rate can be a solution (i.e., review calcium concentrations in the supply of makeup water to the centralized acid scrubber). First-pass RO rejected water is almost always an issue. Biogrowth is enhanced by a food source such as ammonia and organic compounds. Biocide is commonly used to control biogrowth. Good distribution of scrubbing solution (i.e., good wetting of all packing to get biocide to the biogrowth) is crucial to controlling biogrowth. EXAMPLE 4-22: BIOGROWTH AND SCALING OF CENTRALIZED ACID SCRUBBERS. Facility personnel were having difficulty providing proper negative static pressure to the exhaust (not enough) in the centralized acid exhaust system to the fab. The pressure drop (scrubber inlet to fan inlet) across each centralized horizontal cross-flow acid scrubber had been increasing, which was the cause of the exhaust issue to the fab. Visible inspection of the packing was conducted, which showed that the packing had biogrowth and scale. This was confirmed with analysis of the material on the packing. The centralized acid scrubbers were taken down one at a time to replace packing, which was partially plugged. The interesting development was that each scrubber’s packing was not plugged the same and that the pressure drop across the scrubber was not reduced much after the packing was replaced. Facility personnel decided to look more closely at the mist eliminator on each centralized acid scrubber, which is between the packing and the fan. The mist eliminator was almost completely plugged with scale and biogrowth; the mist eliminator had never been replaced or cleaned. Because of the poor conditions of the mist eliminators, they were all replaced. Proper negative static pressure of the exhaust in the centralized acid exhaust system returned to what the fab needed. How can this scale and biogrowth be prevented? Why was each packing condition for biogrowth and scale different? Biocide had been consistently used for years with varying results. The distribution system for the scrubbing solution was then reviewed for each centralized acid scrubber. The lower the recirculation flow rate of scrubbing

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solution, the more the packing was plugged. All the centralized acid scrubbers were operating at a recirculation flow rate below the minimum of good design for scrubbing solution. In addition, face sprays of packing at the scrubber inlet were used with very little flow rate of scrubbing solution. Recirculation flow rates of scrubbing solution were increased to the minimum flow rates of good design (note: the recirculation system for scrubbing solution was reviewed to make sure each centralized acid scrubber could operate at its higher recirculation flow rate for scrubbing solution). Biocide continued to be used. The scaling and biogrowth of the packing were greatly reduced. The preventive maintenance program was revised for each centralized acid scrubber, which included cleaning of the mist eliminators.

4.3.8 PREVENTIVE MAINTENANCE Preventive maintenance for centralized scrubbers is crucial. Good scrubber design and operation necessitates less maintenance. However maintenance is still required! Many fabs are constructed in a way to allow for one centralized scrubber to be down for preventive maintenance during normal fab operation. Some fabs have all of their centralized scrubbers operating but have redundant fans and recirculation pumps for scrubbing solution so as to conduct maintenance on one pump or one fan while the other is operating; this type of centralized scrubber setup usually requires yearly downtime for internal maintenance of the scrubber. Safety interlocks are important when doing maintenance on these backup mechanical systems.

4.3.9 WASTEWATER DISCHARGE

FROM

CENTRALIZED SCRUBBERS

It is important to review the constituents in the wastewater being discharged from centralized scrubbers. An example is fluoride concentrations in wastewater discharge of the centralized acid scrubber. This wastewater discharge may have to be treated for fluoride concentrations before discharging it outside the facility. If centralized acid scrubbers are made more efficient for hydrogen fluoride, then the fluoride concentrations in wastewater discharge would increase. Examples of making a centralized acid scrubber more efficient would be removing ammonia from the centralized acid exhaust system or making scrubber modifications that would increase removal efficiency of hydrogen fluoride. Another way that fluoride concentrations could increase in the wastewater discharge of the centralized acid scrubber is an increase in hydrogen fluoride, silicon tetrafluoride, and fluorine emissions from fab processes (this assumes there is no ammonia to react with these contaminants to form ammonium compounds).

4.4 ACID SCRUBBERS FOR LARGE STORAGE TANKS OF CONCENTRATED ACID SOLUTION Acid scrubbers can be used for large storage tanks containing solutions of concentrated hydrochloric acid for production of clean water supply to the fab, and for large storage tanks containing solutions of concentrated hydrofluoric acid or concentrated nitric acid used in the wafer manufacturing facility. The filling of these tanks is the primary emission event. High-efficiency scrubbers are generally used

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on the exhaust from these tanks to treat high concentrations of acid compound during filling of the tank. It is important to note that the exhaust duct from these tanks should not be “hard connected” (i.e., there is no way for outside air to be pulled in) to these storage tanks when a fan is used; if the exhaust duct is hard connected, the fan will cause excessive negative static pressure of the exhaust on the storage tank and increase emissions (evaporation) from the tank. Good design of the exhaust duct will reduce loss of acid solution (reducing emissions and costs). EXAMPLE 4-23: LARGE STORAGE TANK OF CONCENTRATED HYDROCHLORIC ACID SOLUTION. Odors were detected around the large solutions tank of concentrated hydrochloric acid when it was filled from a tanker by pressuring the tanker with air, forcing the solution of concentrated hydrochloric acid into the storage tank with a several thousand-gallon capacity. The tank vent (stack) allowed hydrogen chloride in air to be emitted upward just above the tank (there was no fan). A pumping method (instead of using pressurized air) for filling the storage tank with concentrated hydrochloric acid solution from the tanker was evaluated, and it was decided not to implement it. A high-efficiency countercurrent wet scrubber was installed with makeup water and no chemical addition; there was monitoring of the pH of the scrubbing solution. A fan was installed on the outlet of the wet scrubber with a stack that discharged 50 feet above ground level. The exhaust duct was hard connected from the storage tank to the wet scrubber inlet. The efficiency of hydrogen chloride removal was tested during filling and was greater than 99%; the scrubbing solution pH dropped to 2 during filling of the storage tank with the solution of concentrated hydrochloric acid. No odors were detected. The pH range of the scrubbing solution was found to be between 4 and 5 during nonfilling operation (almost all the time). Why was the scrubbing solution pH so low? It was determined that the hard connection of the exhaust duct from the storage tank to the scrubber inlet was causing excessive emissions of hydrogen chloride (evaporation of concentrated hydrochloric acid solution). The amount of solution of concentrated hydrochloric acid used per week was more than before the wet scrubber and fan system were installed, which was good confirmation of higher emissions from the storage tank of hydrogen chloride. An exhaust duct “T”, designed with proper capture during higher airflow and higher hydrogen chloride emissions during filling of the storage tank, was installed above the storage tank on the exhaust duct to allow air to flow in. After the exhaust duct T was installed, the pH of the scrubbing solution in the wet scrubber operated between 7 and 8 during nonfilling operations, and the amount used per week of concentrated hydrochloric acid solution dropped to a level similar to what was used before the wet scrubber was installed.

4.5 CENTRALIZED ABATEMENT SYSTEMS FOR HIGHER INLET CONCENTRATIONS OF ACID COMPOUNDS, CHLORINE, AND ACID MISTS As mentioned before, acid mists above 1 ppmv can be visible at stack discharge from centralized wet scrubbers (without any ammonia present). It may be beneficial to segregate and exhaust the processes that cause higher inlet concentrations of acid

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compounds (that create stack visible emissions from centralized acid scrubbers) and high concentrations of gases like chlorine in a separate centralized exhaust system, especially if there are enough of these processes to justify another centralized exhaust system in the fab. A centralized abatement system suggested for this situation is a vertical countercurrent packed-bed scrubber operated at high pH for the scrubbing solution (using sodium hydroxide solution addition), followed by a submicron filter for acid mist. The actual pH range for the scrubbing solution is determined by what is being removed (e.g., chlorine). Operation at high pH of the scrubbing solution could lead to scaling of packing, so quality of makeup water should be reviewed. EXAMPLE 4-24: INSTALLATION OF CENTRALIZED ABATEMENT SYSTEM FOR HIGHER INLET CONCENTRATIONS OF ACID COMPOUNDS, CHLORINE, AND ACID MISTS. A large fab building containing a few individual fabs had numerous centralized acid scrubbers, many of which had stack visible emissions. Odors were commonly detected on the building roof, and there were occasional fab evacuations (air intakes entrain air containing odors). Ammonia had been segregated from the centralized acid exhaust system and put into a separate centralized ammonia exhaust system with its own dedicated centralized ammonia scrubber (with pH control of the scrubbing solution). Some of the facility’s personnel had thought the stack visible emissions were from the water vapor from the centralized acid scrubbers; however, the visible stack emissions varied in intensity during the same meteorological conditions and time of day (i.e., if the visible stack emissions were from water vapor from the centralized acid scrubbers, the emissions would be the same under similar meteorological conditions and time of day). A task force was established to determine the processes contributing to the odors and visible stack emissions. The odor was described as chlorine-like. Sampling of the stacks of the centralized acid scrubber and technical information found determined that the visible stack emissions were most likely nitric acid mist and hydrochloric acid mist (above a total of 1 ppmv at the outlets of the centralized acid scrubber stack). The review of the chlorine-using processes showed that all chlorine-using processes had appropriate POU devices that abated chlorine efficiently. The source of chlorine and possibly similar contaminants were needed to be found; these could be a process byproduct. All of the individual fabs’ processes were then reviewed for chlorine as a by-product and acid compounds that could contribute to odors and visible acid mists. Estimates of emissions were calculated for processes found as possible sources, to determine how much each individual process contributed to the odors and visible emissions. Technical information reviewed showed that heated aqua regia solution can generate chlorine and nitrosyl chloride as process by-products. The following processes in the individual fabs were found to be contributing to the odors and visible emissions. 1. Four baths of aqua regia solution at either 60°C or 70°C. Two of these baths were in large wet benches which had other acid solution baths in them. It was decided to exhaust the entire wet bench to whatever abatement system was determined, as the age of the wet bench made it difficult to do physical exhaust separation (i.e., permanent damage would occur to the wet bench during physical exhaust segregation).

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2. There was one spray process of heated aqua regia solution. 3. Two baths of concentrated nitric acid solution were operated at 70°C. One of these baths was in a small wet bench, and the other bath was in a large wet bench with other acid solution baths. It was decided to exhaust this large wet bench to a new abatement system, due to the age of the wet bench and difficulty in physically separating exhaust. 4. There were two spray processes of acid solution that used heated concentrated nitric acid solution. The total airflow rate for the above processes was approximately 5000 ft3/min. A determination was made that future processes of these types could be used. In addition, a centralized exhaust system and one centralized abatement system would be used instead of numerous POU devices, so as to accommodate future processes, to use no fab or subfab floor space, and to better exhaust large wet benches. The centralized abatement system installed was a vertical countercurrent packed-bed scrubber operated with scrubbing solution at pH between 10 and 11 followed by a large submicron acid mist filter (almost the same size as the vertical countercurrent scrubber). The size of this centralized abatement system was 10,000 ft3/min, to accommodate future processes. The odor reports and visible emissions were eliminated by this installation.

4.6 CENTRALIZED SCRUBBING SYSTEM FOR OXIDES OF NITROGEN This centralized scrubbing system is designed to remove oxides of nitrogen, usually from wafer chemical thinning in a wafer manufacturing facility. The main reasons for abatement are that nitrogen dioxide is generally orange in color (above certain concentrations) when leaving the stack, and there may be air regulatory emission limitations for oxides of nitrogen. Nitric acid and hydrogen fluoride are usually also in the exhaust air from wafer chemical thinning processes. One important aspect of designing this scrubbing system is how much nitric oxide is in the airstream. Sampling of the exhaust stream from the wafer chemical thinning processes can determine the concentrations of nitric oxide and nitrogen dioxide. If there is a sufficient amount of nitric oxide, then another scrubbing stage is built in (usually the first scrubbing stage) to convert nitric oxide to nitrogen dioxide, so the nitrogen dioxide can be scrubbed in the next stages. One way to determine if nitric oxide needs abatement is if the overall removal efficiency for oxides of nitrogen would require removal of nitric oxide. Hydrogen sulfide can be generated in the nitrogen dioxide scrubbing stage, so another (final) stage can be installed to remove hydrogen sulfide. EXAMPLE 4-25: DETERMINING IF NITRIC OXIDE NEEDS ABATEMENT. Sampling of the three wafer chemical thinning processes was conducted using continuous monitoring that can distinguish between nitric oxide and nitrogen dioxide. Eight hours of continuous sampling provided the following average concentrations: 10 ppmv nitric oxide and 90 ppmv nitrogen dioxide. The minimum removal efficiency of oxides

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of nitrogen required by the local air regulatory agency was 95%. Therefore, nitric oxide needed abatement to reach a minimum of 95% removal efficiency of oxides of nitrogen. The centralized scrubbing system was installed with four stages: Stage 1 converts nitric oxide to nitrogen dioxide by strong chemical oxidation. Stages 2 and 3 use a scrubbing solution of high pH to scrub nitrogen dioxide. Stage 4 uses a scrubbing solution of high pH (with the addition of sodium hydroxide solution) to scrub hydrogen sulfide.

It is important to note that wastewater from certain oxides of nitrogen scrubbing systems (chemistry dependent) can release hydrogen sulfide when the pH is dropped close to neutral and below (e.g., if the pH of wastewater is adjusted downward from a high pH, hydrogen sulfide may be released). This situation should be considered when designing a scrubbing system for oxides of nitrogen.

4.7 SPECIFYING A CENTRALIZED WET SCRUBBER As mentioned earlier, it is important for the facility personnel to specify the best centralized wet scrubber for their specific application. It should not be assumed that the scrubber supplier will supply what is needed. Four examples of specifications are listed later for use in developing facility-specific centralized wet scrubbers or centralized wet scrubbing systems; additional requirements may be needed based on facility and company requirements. Note that the size (airflow rate) of the scrubbing system needs to be determined correctly (i.e., it is not good to exhaust 35,000 ft3/min through a scrubber designed for 30,000 ft3/min!). The maximum rate of airflow needed and the range of operational rate of airflow should be specified.

4.7.1 CENTRALIZED ACID SCRUBBERS FOR FAB AND ACID SCRUBBER FOR LARGE STORAGE TANK OF ACID SOLUTION These two types of acid scrubbers can generally have similar specifications. Installation of each acid scrubber most likely will be different. A horizontal cross-flow packed-bed scrubber or vertical countercurrent packed-bed scrubber can be used (use of a vertical countercurrent packed-bed scrubber for a large storage tank for acid solution (e.g., concentrated hydrochloric acid solution) is recommended, especially if pressurized air is used to fill the storage tank with acid solution). Some items are listed next to consider when specifying requirements of an acid scrubber to suppliers. 1. Maximum rate of airflow (e.g., 30,000 ft3/min) and range of operational rate of airflow (e.g., 18,000 to 30,000 ft3/min) for each centralized acid scrubber. Maximum rate of airflow (e.g., 3000 ft3/min) and range of operational rate of airflow (e.g., 1500 to 3000 ft3/min) for the acid scrubber for the large storage tank of concentrated hydrochloric acid solution. Ensure that it is constructed large enough to include future process exhausts (i.e., specify some additional airflow capacity if needed).

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2. Average and maximum inlet concentrations of acid compounds for which the facility wants to design the acid scrubber. An example for centralized acid scrubber is: inlet concentrations of hydrogen chloride and hydrogen fluoride will each be 1 ppmv average and 5 ppmv maximum. Specify removal efficiencies required with corresponding inlet concentrations (e.g., inlet concentrations of 95% at 1 ppmv hydrogen fluoride, and 95% at 1 ppmv hydrogen chloride). Average concentration is used instead of maximum concentration, as it is more difficult to remove 1 ppmv at 95% removal efficiency. An example of an acid scrubber for a large storage tank for concentrated hydrochloric acid solution would be 5 ppmv average concentration for hydrogen chloride and 1000 ppmv for maximum hydrogen chloride concentration (short duration during filling). In this case, facility personnel may require 99.5% removal efficiency at 1000 ppmv. It is important to note that the outlet concentration at these conditions (if they actually occurred) would be 5 ppmv, which as hydrochloric acid mist would be a visible stack emission. 3. Recirculation rate of scrubbing solution: 6 to 10 gpm/ft2 of wetted packing surface and 10 to 30 gpm/1000 ft3/min rate of airflow. Install proper-sized pumps to get the required recirculation rate of scrubbing solution (include a backup pump with automatic switching if needed). Install strainers on the recirculation system of scrubbing solution to prevent plugging of the spray headers or settling in weir design. For horizontal cross-flow scrubbers, install a spray header for scrubbing solution for inlet air side of the packing section to ensure it is wetted properly. The horizontal cross-flow scrubber should utilize spray nozzles. Specify a distribution system for scrubbing solution if a vertical countercurrent scrubber is used: nozzles or weirs. 4. Rate of makeup water should be continuous. If there is no chemical addition, the rate of makeup water should be a minimum of 3% of the recirculation rate of scrubbing solution. If chemical addition is used, a minimum of 1% recirculation rate of scrubbing solution can be used for the rate of makeup water. Provide the chemistry of makeup water to the scrubber suppliers and request that the scrubber suppliers explain how it will affect the packing, mist eliminator, etc. 5. The pH of the scrubber solution should be maintained between 7 and 8. If a chemical addition system is required, specify the solution of base to be used and how it will be supplied to the acid scrubber. Have scrubber suppliers provide estimated chemical usage. 6. Air velocity through packing: 300 to 500 ft/min. Ask what type of packing will be used. The packing depth for a horizontal cross-flow scrubber and packing height for a vertical countercurrent scrubber are crucial. Have scrubber suppliers provide this information. Suggest investigating more than 4 ft of packing, especially if the scrubber is designed to a maximum of 500 ft/min air velocity through packing.

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7. Exit velocity of stack exhaust of 3000 to 3800 ft/min. State what the stack heights will be (as high as possible to allow for air to leave the building wake and not impact fab air intakes). Request air regulatory stack ports for sampling to be installed, with proper straight-duct, no-disturbance distances before and after ports to ensure approval by local air regulatory agency if compliance testing is required. Provide safe access to ports (e.g., ladder, platform, and railings). 8. Install efficient demister section between packing section and fans. 9. Ductwork between scrubber and fans is required. 10. Request information on construction materials for everything. Hydrogen fluoride-resistant materials should be requested if hydrogen fluoride will be in the airstream into the scrubbing system. 11. References should be provided for similarly designed acid scrubbers with similar inlet concentrations and maximum rate of airflow. 12. A preventive maintenance schedule is requested. 13. Yearly operating cost should be provided. It would be beneficial for facility personnel to provide actual utility costs and other costs (e.g., costs for makeup water, electricity, wastewater treatment, labor, chemicals used if applicable). 14. Ask about installation cost if it is to be conducted by scrubber supplier. If not to be done by scrubber supplier, ask for scrubber supplier to provide a final inspection and startup assistance, including written signoff on installation before startup. 15. Total purchase cost including shipping should be provided. 16. What spare parts are needed? What are their costs? Which of these spare parts should be considered critical? 17. Warranties are needed, with any issues fixed by the scrubber supplier at their cost. Examples are removal efficiencies and equipment warranties. 18. Who will provide local maintenance support 24 hours a day, 7 days a week? 19. Operations and maintenance manuals are needed. 20. Training of onsite personnel is required. 21. What is the time from purchase order until the equipment reaches the facility for installation? 22. Removal efficiency (inlet and outlet) testing: how would the scrubber supplier like to be involved? It is important to know the scrubber supplier’s involvement, as the scrubber supplier will be asked to fix the acid scrubber if the guaranteed removal efficiencies are not met. This testing is usually coordinated by the facility personnel (usually done by their testing contractor), but it is good to have the scrubber supplier approve the test methods and contractor. 23. Specify direct drive fans or variable frequency (speed) drive fans to scrubber suppliers. State whether a backup fan is required, with automatic switching if the exhaust is lost.

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24. Specify instrumentation needed (e.g., pH monitoring, pressure drop, outputs to facilities management system) to scrubber suppliers. 25. Provide information to scrubber suppliers on available utilities, mechanical requirements, and electrical requirements. 26. Provide company requirements on environmental, health, and safety to scrubber suppliers.

4.7.2 CENTRALIZED AMMONIA SCRUBBER

FOR

FAB

A horizontal cross-flow packed-bed scrubber or vertical countercurrent packed-bed scrubber can be used. Some items to consider when specifying a centralized ammonia scrubber to suppliers are listed next. 1. Maximum rate of airflow (e.g., 20,000 ft3/min) and operational range of rate of airflow (e.g., 12,000 to 20,000 ft3/min). Ensure that it is constructed large enough to include future process exhausts (i.e., specify some additional capacity of airflow if needed). 2. Average and maximum inlet concentrations of ammonia for which the facility wants to design the centralized ammonia scrubber. An example for a centralized ammonia scrubber would be ammonia inlet concentrations of 50 ppmv average and 200 ppmv maximum. Specify removal efficiency required with corresponding inlet concentration (e.g., 95% at 200 ppmv ammonia inlet concentration). The maximum inlet concentration is used to ensure high removal efficiencies at the maximum inlet concentration of ammonia to prevent any ammonia odors. 3. Recirculation rate scrubbing solution: 6 to 10 gpm/ft2 of wetted packing surface and 10 to 30 gpm/1000 ft3/min airflow rate. Install proper-sized pumps to get the required recirculation rate of scrubbing solution (include a backup pump with automatic switching if needed). Install strainers on the recirculation system for the scrubbing solution to prevent plugging of spray headers or settling in the weir design. For horizontal cross-flow scrubbers, install spray header for scrubbing solution for the inlet air side of the packing section to ensure it is wetted properly. A horizontal crossflow scrubber should utilize spray nozzles. Specify a distribution system for the scrubbing solution if a vertical countercurrent scrubber is used: nozzles or weirs. 4. Rate of makeup water should be continuous. Since chemical addition will be used to control a low pH of the scrubbing solution, a minimum of 1% rate of recirculation of the scrubbing solution can be used for the rate of makeup water. Provide chemistry of makeup water to scrubber suppliers and request that the scrubber suppliers explain how it will affect the packing, mist eliminator, etc. (this is less crucial for operation at low pH of the scrubbing solution). 5. The pH of the scrubber solution should be maintained between 3 and 4. This will require a system of chemical addition using an acid solution (e.g., concentrated sulfuric acid solution) which has a very low vapor

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14.

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pressure (since any acid compound emitted will react with ammonia and form ammonium compounds). Specify the acid solution to be used and how it will be supplied to the centralized ammonia scrubber. Have scrubber suppliers provide estimated chemical usage. Air velocity through the packing: 300 to 500 ft/min. Ask what type of packing will be used. Packing depth for a horizontal cross-flow scrubber and packing height for a vertical countercurrent scrubber are crucial. Have scrubber suppliers provide this information. Suggest investigating more than 4 ft of packing, especially if the scrubber is designed to allow a maximum air velocity of 500 ft/min through the packing. Exit velocity of stack exhaust of 3000 to 3800 ft/min. State what the stack heights will be (as high as possible to allow for air to leave the building wake and not impact fab air intakes). Request that air regulatory stack ports be installed for sampling, with proper straight-duct, no-disturbance distances before and after ports to ensure approval of the local air regulatory agency if compliance testing is required. Provide safe access to ports (e.g., ladder, platform, and railings). Install an efficient demister section between the packing section and fans. Provide ductwork between scrubber and fans. Request information on construction materials for everything. References should be provided for similarly designed ammonia scrubbers with similar inlet concentrations and maximum rate of airflow. A preventive maintenance schedule is requested. Yearly operating cost should be provided. It would be beneficial for facility personnel to provide actual utility costs and other costs (e.g., costs for makeup water, electricity, acid solution used, wastewater treatment, and labor). Ask for the installation cost if the equipment is to be installed by the scrubber supplier. If it is not to be done by the scrubber supplier, ask the scrubber supplier to provide a final inspection and startup assistance, including written signoff on installation before startup. The total purchase cost including shipping should be provided. What spare parts are needed? What are their costs? Which of these spare parts should be considered critical? Warranties are needed, with any issues fixed by the scrubber supplier at their cost. Examples are removal efficiency and equipment warranties. Who will provide local maintenance support 24 hours a day, 7 days a week? Operations and maintenance manuals are requested. Training of on-site personnel is required. What will be the time from purchase order until the equipment reaches the facility for installation? Removal efficiency (inlet and outlet) testing: how would the scrubber supplier like to be involved? It is important to know the scrubber supplier’s involvement, as the scrubber supplier will be asked to fix the centralized ammonia scrubber if the guaranteed removal efficiencies are not met. This

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23.

24. 25. 26.

testing is usually coordinated by the facility personnel (usually done by their testing contractor), but it is good to have the scrubber supplier approve the test methods and contractor. Specify the direct drive or variable frequency (speed) drive fans to scrubber suppliers. State whether a backup fan is required, with automatic switching if exhaust is lost. Specify the instrumentation needed (e.g., pH monitoring, pressure drop, outputs to facility’s management system) to scrubber suppliers. Provide information to scrubber suppliers on available utilities, mechanical requirements, and electrical requirements. Provide company requirements on environmental, health, and safety to scrubber suppliers.

4.7.3 CENTRALIZED ACID SCRUBBING SYSTEM FOR HIGH INLET CONCENTRATIONS OF ACID COMPOUNDS, ACID MISTS, AND CHLORINE A vertical countercurrent packed-bed scrubber followed by a submicron filter for acid mist can be used for this application. Some items to consider when specifying this centralized acid scrubbing system to suppliers are listed next. 1. Maximum rate of airflow (e.g., 10,000 ft3/min) and operational range of rate of airflow (e.g., 6000 to 10,000 ft3/min). Ensure that it is constructed large enough to include future process exhausts (i.e., specify some additional airflow capacity if needed). 2. Average and maximum inlet concentrations of the primary acid compounds, acid mists, and chlorine for which the facility wants to design the scrubbing system. An example is: inlet concentrations of hydrogen chloride will be 100 ppmv average and 1000 ppmv maximum; inlet concentrations of nitric acid will be 20 ppmv average and 100 ppmv maximum; and inlet concentrations of chlorine will be 20 ppmv average and 100 ppmv maximum. Specify the removal efficiencies that are required with the corresponding inlet concentrations (e.g., removal efficiency of 99.5% at 1000 ppmv hydrogen chloride, 95% at 100 ppmv nitric acid, and 95% at 100 ppmv chlorine). There could be other contaminants needing abatement, such as hydrogen fluoride and nitrosyl chloride; if so, develop the average concentration, maximum concentration, and removal efficiency for each of these contaminants. Specify that there be no visible emissions from the stacks of the centralized scrubbing system. 3. Provide a submicron filter for acid mist after the vertical countercurrent packed-bed scrubber. No visible emissions should be allowed from the stacks of the centralized scrubbing system stacks. 4. Recirculation rate of scrubbing solution: 6 to 10 gpm/ft2 of wetted packing surface and 10 to 30 gpm/1000 ft3/min rate of airflow. Install proper-sized pumps to get the required recirculation rate of scrubbing solution (include

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6.

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9. 10. 11.

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a backup pump with automatic switching if needed). Install strainers on the recirculation system of the scrubbing solution to prevent plugging of the spray headers or settling in the weir design. Specify the distribution system of the scrubbing solution distribution system: nozzles or weirs. The rate of makeup water should be continuous. Since chemical addition is used, a minimum of 1% recirculation rate of scrubbing solution recirculation rate can be used for the rate of makeup water. Provide the scrubbing system suppliers with the chemistry of the makeup water and request that they explain how it will affect the packing, the mist eliminator, etc. (this is crucial due to the high pH of the scrubbing solution operation). The pH of the scrubber solution should be maintained between 10 and 11. This will require chemical addition. Specify the base solution (e.g., sodium hydroxide solution) and how it will be supplied to the centralized scrubbing system. Ask the scrubbing system suppliers to provide the estimated chemical usage. Air velocity through the packing: 300 to 500 ft/min. Ask what type of packing will be used. The packing height for a vertical countercurrent scrubber is crucial. Have the scrubbing system suppliers provide this information. Suggest investigating more than 8 ft of packing, especially if the scrubber is designed to a maximum of 500 ft/min air velocity through the packing. Velocity of exhaust at the stack exit should be 3000 to 3800 ft/min. State what the stack heights will be (as high as possible to allow air to leave the building wake and not impact fab air intakes). Request that air regulatory stack ports for sampling be installed, with proper straight-duct, nodisturbance distances before and after ports to ensure approval of local air regulatory agency if compliance testing is required. Provide safe access to ports (e.g., ladder, platform, and railings). Install an efficient demister section between the packing section and the submicron filter for acid mist. Provide ductwork between the scrubbing system and fans. Request information on construction materials for everything. Hydrogen fluoride-resistant materials should be requested if hydrogen fluoride will be in the airstream going into the scrubbing system. References should be provided for similar scrubbing systems with similar inlet concentrations and maximum rate of airflow. A preventive maintenance schedule is requested. Yearly operating cost should be provided. It would be beneficial for facility personnel to provide actual utility costs and other costs (e.g., costs for makeup water, electricity, wastewater treatment, chemicals used, labor). Ask for the installation cost if the equipment is to be installed by the supplier of the scrubbing system. If it is not to be done by the scrubbing system supplier, ask the scrubbing system supplier to provide a final inspection and startup assistance, including written signoff on the installation before startup. The total purchase cost including shipping should be provided.

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17. What spare parts are needed? What are their costs? Which of these spare parts should be considered critical? 18. Warranties are needed, with any issues fixed by the supplier of the scrubbing system at their cost. Examples are removal efficiencies and equipment warranties. 19. Who will provide local maintenance support 24 hours a day, 7 days a week? 20. Operations and maintenance manuals should be requested. 21. Training of onsite personnel is required. 22. What will be the time from purchase order until the equipment reaches the facility for installation? 23. Removal efficiency (inlet or outlet) testing: how would the supplier of the scrubbing system like to be involved? It is important to know the scrubbing system supplier’s involvement, as the scrubbing system supplier will be asked to fix the centralized scrubbing system if the guaranteed removal efficiencies are not met or if there are visible stack emissions. This testing is usually done by the facility personnel (usually by their testing contractor), but it is good to have a scrubbing system supplier approve the test methods and contractor. 24. Specify direct drive fans or variable frequency (speed) drive fans to the scrubbing system suppliers. State whether a backup fan is required, with automatic switching if exhaust is lost. 25. Provide information on available utilities, mechanical requirements, and electrical requirements to scrubbing system suppliers. 26. Provide company environmental, health, and safety requirements to scrubbing system suppliers. 27. Specify instrumentation needed (e.g., pH monitoring, pressure drop, outputs to facility’s management system) to scrubbing system suppliers.

4.7.4 CENTRALIZED SCRUBBING SYSTEM

FOR

OXIDES

OF

NITROGEN

A vertical countercurrent packed-bed scrubber design should be used. A few scrubber stages will probably be required, with each stage having a purpose. Some items are listed next to consider when specifying to suppliers a centralized scrubbing system for oxides of nitrogen. It is important to note that a submicron filter for acid mist may need to be added after the scrubber stages if acid mists (e.g., nitric acid mist) need to be abated due to visible stack emissions attributable to acid mists. 1. Maximum rate of airflow (e.g., 10,000 ft3/min) and operational range of rate of airflow (e.g., 6000 to 10,000 ft3/min). Ensure that it is constructed large enough to include future process exhausts (i.e., specify some additional airflow capacity if needed). 2. Average and maximum inlet concentrations of oxides of nitrogen (including the split between nitric oxide and nitrogen dioxide) for which the facility wants to design the scrubbing system. An example for a centralized scrubbing system for oxides of nitrogen is: inlet concentrations of nitric oxide will be 10 ppmv average and 50 ppmv maximum, and of nitrogen

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6.

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dioxide will be 90 ppmv average and 450 ppmv maximum. Specify required removal efficiencies with corresponding inlet concentrations (e.g., 99% removal efficiency of oxides of nitrogen for 500 ppmv maximum and 95% for 100 ppmv average). It is also important to specify inlet average and maximum concentrations of hydrogen fluoride and nitric acid, with the required removal efficiencies. Concentration of nitric acid mist above 1 ppmv at the stack outlet most likely will appear as visible emissions. No visible stack emissions are allowed, which could occur from nitrogen dioxide (orange) or nitric acid mist (white). The recirculation rate of scrubbing solution: 6 to 10 gpm/ft2 of wetted packing surface and 10 to 30 gpm/1000 ft3/min rate of airflow. Install proper-sized pumps to get the required recirculation rate of scrubbing solution (include a backup pump with automatic switching if needed). Install strainers on the recirculation system of scrubbing solution to prevent plugging of spray headers or settling in the weir design. Specify the distribution system for the scrubbing solution: nozzles or weirs. The rate of makeup water should be continuous. Since chemical addition is used, a minimum of 1% recirculation rate of scrubbing solution can be used for the rate of makeup water. Provide suppliers of the scrubbing system with the chemistry of the makeup water and request that they explain how it will affect the packing, mist eliminator, etc. Scrubbing system suppliers should recommend chemicals to be fed, how the chemicals will be fed, and from where, and should recommend the pH range of the scrubbing solution for each stage. Ask the scrubbing system suppliers to provide estimated chemical usage for each chemical used. Air velocity through the packing: 300 to 500 ft/min. Ask what type of packing will be used. The packing height for a vertical countercurrent scrubber stage is crucial. Have scrubbing system suppliers provide this information, with support information if a maximum of 500 ft/min air velocity through the packing is used. Exhaust velocity at the stack exit should be 3000 to 3800 ft/min. State what the stack heights will be (as high as possible to allow for air to leave the building wake and not impact building air intakes). Request that air regulatory stack ports for sampling be installed, with proper straight-duct, no-disturbance distances before and after ports to ensure approval of local air regulatory agency if compliance testing is required. Provide safe access to ports (e.g., ladder, platform, and railings). Install an efficient demister section between packing stages, and between the final stage and fan. Provide ductwork between the scrubbing system and fan. Request information on construction materials for everything. Hydrogen fluoride-resistant materials should be requested if hydrogen fluoride will be in the airstream into the scrubbing system. References should be provided for similar scrubbing systems with similar inlet concentrations and maximum rate of airflow.

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13. A preventive maintenance schedule is needed. 14. The yearly operating cost should be provided. It would be beneficial for facility personnel to provide actual utility costs and other costs (e.g., costs of makeup water, electricity, all chemicals used, wastewater treatment, and labor). 15. Ask for the installation cost if the equipment is to be installed by the supplier of the scrubbing system. If it is not to be done by the scrubbing system supplier, ask for the supplier to provide a final inspection and startup assistance, including written signoff on the installation before startup. 16. The total purchase cost including shipping should be provided. 17. What spare parts are needed? What are their costs? Which of these spare parts should be considered critical? 18. Warranties are needed, with any issues fixed by the scrubbing system supplier at their cost. Examples are removal efficiencies and equipment warranties. 19. Who will provide local maintenance support 24 hours a day, 7 days a week? 20. Operations and maintenance manuals are needed. 21. Training of onsite personnel is required. 22. What will be the time from purchase order until the equipment reaches the facility for installation? 23. Removal efficiency (inlet or outlet) testing: how would the scrubbing system supplier like to be involved? It is important to know the supplier’s involvement, as the supplier will be asked to fix the centralized scrubbing system if the guaranteed removal efficiencies are not met or if there are visible stack emissions. This testing is usually done by the facility personnel (usually by their testing contractor), but it is good to have the scrubbing system supplier approve test methods and contractor. 24. Determine if a submicron filter for acid mist after the scrubber stages is needed for abatement. 25. Specify direct drive fans or variable frequency (speed) drive fans to the scrubbing system suppliers. State whether a backup fan is required, with automatic switching if exhaust is lost. 26. Provide information to scrubbing system suppliers on available utilities, mechanical requirements, and electrical requirements. 27. Provide company environmental, health, and safety requirements to scrubbing system suppliers. 28. Specify what instrumentation is needed (e.g., pH monitoring, pressure drop, outputs to facilities management system) to scrubbing system suppliers.

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5

Centralized Equipment to Control Volatile Organic Compounds

5.1 INTRODUCTION Centralized equipment to control volatile organic compounds (VOCs) is generally installed at fabs because of local air regulatory requirements to reduce VOC emissions. Centralized equipment to control VOCs is usually preferred (rather than having numerous point-of-use [POU] devices) as the VOC emissions from processes can be safely exhausted and abated collectively. Fabs typically have one up to a few centralized equipment units to control VOCs when VOC emission control is needed. The rate of airflow of centralized equipment to control VOCs is generally between 5000 to 60,000 ft3/min. The VOC concentrations at the inlet can range between 20 and 600 parts per million by volume (ppmv) total VOCs measured as methane. The design of centralized equipment to control VOCs needs to account for a high rate of airflow and low VOC concentrations at the inlet, as well as the individual organic compounds (e.g., methanol, isopropanol, xylenes). Low VOC concentrations at the inlet have an effect on removal efficiency of VOCs for centralized equipment to control VOCs in the following ways. 1. Many equipment technologies for performance of VOC control (removal efficiency) depend on VOC concentration at the inlet. 2. The measurement of VOC concentrations at the outlet may be difficult if they are very low. If VOC concentration at the inlet is low, then it may be difficult to show high removal efficiency of VOCs. For example, if the average inlet concentration of VOCs is 50 ppmv total VOC measured as methane and the average outlet concentration can only be measured accurately at 5 ppmv total VOC measured as methane, then the best removal efficiency of VOCs that can be shown is 90%. Therefore, it is suggested that air-permit conditions reflect removal efficiency of VOCs or VOC concentration at the outlet for determination of compliance (e.g., centralized control equipment for VOCs should have 95% removal efficiency for VOCs or outlet concentration less than 20 ppmv total VOCs measured as methane). 3. Some equipment technologies to control VOCs perform differently with specific organic compounds (e.g., VOCs of low molecular weight). 4. The inlet concentration of each organic compound entering the centralized control equipment for VOCs typically changes. This variation tends to impact the performance of centralized control equipment. 113

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5. The continuous variations in total VOC concentrations at the inlet make it more difficult to establish average total removal efficiency of VOCs, especially when VOC concentrations at the inlet are low. Several different centralized equipment technologies to control VOCs have been or are being used in the semiconductor industry. The next section describes these technologies. Each equipment technology is presented with similar text and examples for the use by the reader (i.e., so the reader need only read the section for the technology in which they are interested). The selection of these equipment technologies to control VOCs can depend on the following: 1. Maximum rate of airflow and operational range of the rate of airflow, in cubic feet per minute 2. Average and maximum inlet concentrations for each organic compound and for total VOCs 3. Removal efficiencies required for individual organic compounds and for total VOCs 4. Possible by-products created (e.g., carbon monoxide, oxides of nitrogen, and silicon dioxide) 5. Available utilities (electricity, natural gas, etc.) 6. Safety considerations (e.g., below a specified percentage of lower flammable limit [LFL] in inlet exhaust air to the centralized control equipment for VOCs; preventing carbon fires) 7. Location of installation and size of VOC control equipment 8. Regulatory agency requirements, which could include air, water, and waste. Determine if a bypass to the atmosphere is allowed for the centralized equipment to control VOCs (design of VOC control equipment can change due to this). 9. Company preferences 10. Waste-disposal options and wastewater treatment availability 11. Available local maintenance support and spare parts 12. Purchase, shipping, installation, and operating costs There may be other selection criteria. Any one of these selection criteria could eliminate one possible equipment technology or supplier. Examples of selection criteria will be presented after discussion of centralized equipment technologies to control VOCs.

5.2 CENTRALIZED EQUIPMENT TECHNOLOGIES TO CONTROL VOLATILE ORGANIC COMPOUNDS Centralized equipment technologies to control VOCs can be broken into four major categories: oxidation systems; capture-oxidation systems; capture-recovery systems; and other systems. Some typical types with advantages and disadvantages of each category are provided next.

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5.2.1 CENTRALIZED OXIDATION SYSTEMS The primary oxidation systems used as centralized equipment to control VOCs in the semiconductor industry are regenerative thermal oxidation systems and recuperative catalytic oxidation systems. These use whole-airstream oxidation (i.e., the entire airstream is heated to a set temperature at a particular residence time). Both of these systems are used to retain a majority of the heat to preheat incoming airstream to the oxidation section; this is also called reuse of heat. One disadvantage is the VOC concentrations from many semiconductor fabs are very low, so these whole-airstream oxidation systems are not able to best utilize heat from oxidation of organic compounds so as to reduce use of fuel. An advantage of oxidation systems is that no liquid or solid waste is generated during normal operation. Whole-airstream oxidation without heat-recovery technology can be used but is very costly. This equipment technology to control VOCs is rare for centralized equipment in the semiconductor industry. Due to this, whole-airstream oxidation without heat-recovery technology is not discussed. 5.2.1.1 Regenerative Thermal Oxidation Systems This centralized equipment technology to control VOCs uses two or more sections with heat-adsorbing material (media) in it. The following example is for a twosection system of regenerative thermal oxidation for simplicity. One media section (call it the first media section) is used to preheat the incoming airstream while the second media section is being heated by the exiting airstream of the thermal oxidizer (thermal oxidation is occurring between the two sections). When the second media section is heated up to a set temperature, valves switch direction of the exhaust, allowing the second media section now to be the inlet air preheated section and the first media section then to be the heat-adsorbing section. Switching of the valves happens quickly (these valves can switch every few minutes). VOC outlet concentrations can spike during the valve switching. Another design for regenerative thermal oxidation uses a system of rotating exhaust diversion instead of valves to control airflow of the inlet between media sections (heat adsorption and heat desorption). Heat recovery is generally 90 to 95% for regenerative thermal oxidation systems. The thermal oxidation temperature is generally between 1350 and 1500°F. An advantage of thermal oxidation is that it is a known technology. Once the thermal oxidation temperature is set and oxidation is optimized, the removal efficiency of VOCs is relatively steady at similar inlet VOC concentrations. The thermal oxidation temperature of the regenerative thermal oxidation system can be used for compliance with the air-permit condition for VOC removal efficiency (after testing the removal efficiency of VOCs has been conducted at that thermal oxidation temperature to demonstrate the actual VOC removal efficiency). EXAMPLE 5-1: AIR-PERMIT REQUIREMENT FOR REMOVAL EFFICIENCY OF VOCS BY CENTRALIZED REGENERATIVE THERMAL OXIDATION SYSTEM

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A fab installed a regenerative thermal oxidation system for centralized equipment to control VOCs that was operated at 1400°F. An inlet-outlet compliance test was conducted with the local air regulatory agency. The removal efficiency for VOCs was 95% at the inlet concentration of 200 ppmv total VOCs measured as methane. Continuous monitoring of the thermal oxidation temperature is the only air-permit condition to show compliance with VOC removal efficiency of 95%.

Natural gas (or another fuel) is necessary for thermal oxidation (combustion) in the regenerative thermal oxidation systems. By-products of thermal oxidation (carbon monoxide, oxides of nitrogen, and silicon dioxide) need to be evaluated and most likely included in the air permit. Carbon monoxide emissions come from the incomplete oxidation of both the fuel and the incoming organic compounds. It is essential to get the accurate emissions for carbon monoxide from the regenerative thermal oxidation system supplier, as some equipment suppliers only report the carbon monoxide emissions from the thermal oxidation of the fuel. It is not unusual to have the actual carbon monoxide emissions three to five times higher than carbon monoxide emissions just from fuel oxidation itself (i.e., carbon monoxide emissions are higher for incomplete oxidation of incoming organic compounds than incomplete oxidation of fuel). As a general rule, carbon monoxide concentrations increase as thermal oxidation temperature decreases with everything else staying the same. Emissions of oxides of nitrogen come from oxidation of air and fuel at a certain temperature, and from oxidation of any nitrogen-containing organic compounds. Examples of nitrogen-containing organic compounds are hexamethyldisilazane (HMDS) and n-methyl pyrrolidone (NMP). Suppliers of VOC control equipment may not include emissions of oxides of nitrogen from nitrogen-containing organic compounds unless it is requested. Low oxides-of-nitrogen burners may be available from equipment suppliers; determine the impact of these burners on carbon monoxide emissions. EXAMPLE 5-2: EMISSIONS OF CARBON MONOXIDE AND OXIDES OF NITROGEN FROM REGENERATIVE THERMAL OXIDATION SYSTEM. A facility purchased a regenerative thermal oxidation system with a stated VOC removal efficiency, carbon monoxide emission rate, and emission rate for oxides of nitrogen from the supplier of VOC control equipment. An air permit was filed with and issued by the local air regulatory agency with VOC removal efficiency, VOC emission rate, carbon monoxide emission rate, and emission rate of oxides of nitrogen similar to what was provided by the supplier of the regenerative thermal oxidation system. After installation, an air regulatory compliance test was conducted for VOC removal efficiency, VOC emissions, carbon monoxide emissions, and oxides of nitrogen emissions. The VOC removal efficiency and VOC emissions passed the compliance test. Carbon monoxide emissions and oxides of nitrogen emissions were above the stated amounts in the air permit. A meeting between facility personnel and the equipment supplier determined that the carbon monoxide emission rate and oxides of nitrogen emission rate provided by the equipment supplier were only for the fuel oxidation. Carbon monoxide emissions from incomplete oxidation of organic compounds and emissions of oxides of nitrogen from oxidation of NMP were not included in the information

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from the supplier. The air permit was refiled with the local air regulatory agency with the higher emission rates for carbon monoxide and oxides of nitrogen. Fortunately, these higher emission rates did not make the entire site a major source for carbon monoxide or oxides of nitrogen.

Silicon dioxide is another possible by-product if silicon-containing organic compounds (e.g., HMDS from photocoater vapor prime operation) are emitted to the regenerative thermal oxidation system. Silicon dioxide can plug heat-adsorbing media of the regenerative thermal oxidation, causing an eventual shutdown of the unit. In addition, removal of this plugged media can be very difficult. It is suggested that processes emitting silicon-containing organic compounds be exhausted to another centralized exhaust system (either centralized ammonia exhaust system or centralized acid exhaust system), if the VOC emission limit on the air permit allows for it. Tetraethylorthosilicate (TEOS) is another silicon-containing organic compound that can be used in the fab; however, the processes using TEOS are usually exhausted to a centralized acid exhaust system. It would be beneficial to determine where processes emitting TEOS and other silicon-containing organic compounds are exhausting to, so as to ensure no effects on the regenerative thermal oxidation system. 5.2.1.2 Recuperative Catalytic Oxidation Systems This centralized equipment technology to control VOCs has a catalytic oxidation section (containing a burner followed by a catalyst) followed by a heat exchanger, that reuses some of the heat for preheating exhaust air containing organic compounds before the air enters the catalytic oxidation section. The inlet airstream entering the catalyst is usually heated to between 600 and 700°F; the catalyst allows for oxidation of the organic compounds at a much lower temperature. The oxidation of organic compounds in the catalyst causes the temperature to rise (exothermic reaction). Heat recovery in the recuperative catalytic oxidation system is generally 50 to 75%. Higher catalyst outlet temperature allows for the opportunity for more heat recovery (and possibly using less fuel). The inlet catalyst temperature can be measured continuously to show compliance with air-permit conditions for VOC removal efficiency (after a regulatory compliance test for VOC removal efficiency is performed successfully). Catalyst activity can degrade over time due to catalyst poisoning, causing VOC removal efficiency to also be reduced; it is important to avoid certain contaminants (e.g., silicon compounds) that can poison the catalyst. The temperature increase across the catalyst section can be measured to show that oxidation of organic compounds is occurring. This temperature difference would be difficult to use as a measure of catalyst performance, as the VOC concentrations into the centralized recuperative catalytic oxidation system usually vary greatly due to variable process VOC emissions (i.e., the temperature across the catalyst would vary with VOC concentrations at the inlet). Therefore, a modest decrease in catalyst performance may be difficult to determine by the temperature difference of the inlet-outlet catalyst. A periodic check of VOC removal efficiency or VOC outlet concentration is a possible way to watch catalyst performance; a portable VOC monitor can be used.

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EXAMPLE 5-3: AIR-PERMIT REQUIREMENT FOR REMOVAL EFFICIENCY OF VOCS BY CENTRALIZED RECUPERATIVE CATALYTIC OXIDATION SYSTEM. A fab installed a recuperative catalytic oxidation system for centralized equipment to control VOCs and was operated at 700°F at the inlet to the catalyst section. An inletoutlet compliance test was conducted with the local air regulatory agency. The site environmental staff took portable VOC measurements at the same time to compare with continuous VOC measurements conducted for the compliance test. This comparison was to be helpful for the future in-house use of a portable VOC monitor. The VOC removal efficiency was 95% at an inlet concentration of 200 ppmv total VOC measured as methane. Continuous monitoring of the inlet temperature to the catalyst section was the only air permit condition to show compliance with the VOC removal efficiency requirement of 95%. Environmental personnel continue to conduct a weekly inlet-outlet test using a portable VOC monitor to watch VOC removal efficiency.

Natural gas (or another fuel) is necessary for oxidation (combustion) in the catalytic oxidizer. By-products of oxidation (carbon monoxide, oxides of nitrogen, and silicon dioxide) need to be evaluated and most likely included on an air permit. Silicon dioxide generation in the burner before the catalyst section could seriously impact the catalytic activity, so this should be avoided. Carbon monoxide emissions are generated from the incomplete oxidation of both the fuel and the incoming organic compounds. It is essential to get the accurate emissions for carbon monoxide from the supplier of the recuperative catalytic oxidation system, as some suppliers of VOC control equipment only report the carbon monoxide emissions from the oxidation of the fuel. It is not unusual to have the actual carbon monoxide emissions much higher than the carbon monoxide emissions just from fuel oxidation itself (i.e., carbon monoxide emissions can be higher for incomplete oxidation of incoming organic compounds than incomplete oxidation of fuel). Emissions for oxides of nitrogen come from oxidation of air and fuel at a certain temperature and from oxidation of any nitrogen-containing organic compounds. Examples of nitrogen-containing organic compounds are hexamethyldisilazane (HMDS) and n-methyl pyrrolidone (NMP). Suppliers of VOC control equipment may not include data for emissions for oxides of nitrogen from nitrogen-containing organic compounds unless they are requested. EXAMPLE 5-4: EMISSIONS FROM CENTRALIZED RECUPERATIVE CATALYTIC OXIDATION SYSTEM FOR CARBON MONOXIDE AND OXIDES OF NITROGEN. A facility purchased a recuperative catalytic oxidation system for centralized equipment to control VOCs with a stated VOC removal efficiency, carbon monoxide emission rate, and emission rate for oxides of nitrogen from the supplier of the VOC control equipment. An air permit was filed with and issued by the local air regulatory agency with VOC removal efficiency, VOC emission rate, carbon monoxide emission rate, and emission rate of oxides of nitrogen similar to what was provided by the supplier of the recuperative catalytic oxidation system. After installation, an air-regulatory compliance

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test was conducted for VOC removal efficiency, VOC emissions, and emissions of carbon monoxide and oxides of nitrogen. The VOC removal efficiency and VOC emissions passed the compliance test. The emissions of carbon monoxide and oxides of nitrogen were above the stated amounts in the air permit. A meeting between facility personnel and the supplier of the VOC control equipment determined that the emission rates of carbon monoxide and oxides of nitrogen provided by the supplier were only for the fuel oxidation. Data for carbon monoxide emissions from incomplete oxidation of organic compounds and oxides of nitrogen emissions from oxidation of NMP were not included in the information from the supplier of the recuperative catalytic oxidation system. The air permit was refiled with the local air-regulatory agency with higher emission rates for carbon monoxide and oxides of nitrogen. Fortunately, these higher emission rates did not make the entire site a major source for carbon monoxide or oxides of nitrogen.

Silicon dioxide is another possible by-product if silicon-containing organic compounds (e.g., HMDS from a photo-coater vapor prime operation) are emitted to the recuperative catalytic oxidation system. Silicon dioxide can poison the catalyst, causing loss of removal efficiency of VOCs. The catalyst is usually very expensive to replace. Silicon dioxide can also plug the heat exchanger after the catalytic oxidizer, causing an eventual shutdown of the unit. Cleaning of the heat exchanger usually is very difficult; plugging could lead to replacement of the heat exchanger. It is suggested that processes emitting silicon-containing organic compounds be exhausted to another centralized exhaust system (either a centralized ammonia exhaust system or a centralized acid exhaust system), if the VOC emission limit on the air permit allows for it. TEOS is another silicon-containing organic compound that can be used in the fab; however, the processes using TEOS are usually exhausted to a centralized acid exhaust system. It would be beneficial to determine where processes emitting TEOS and other silicon-containing organic compounds are exhausting to so as to ensure no effects on the recuperative catalytic oxidation system.

5.2.2 CENTRALIZED CAPTURE-OXIDATION SYSTEMS This centralized equipment technology for control of VOCs uses adsorption media (e.g., zeolite, carbon, resin) to capture organic compounds and then desorbs the organic compounds in a much lower airflow rate to an oxidation system. Captureoxidation systems are ideal for many semiconductor fabs as these systems are designed to concentrate organic compounds from an exhaust system with a high airflow rate and low VOC concentrations at the inlet (e.g., centralized VOC exhaust system), and then oxidize with a much lower airflow rate with much higher VOC concentrations. The higher VOC concentrations at the inlet to the oxidation system will allow for better use of the heat generated when organic compounds are oxidized, reducing the use of fuel. Another advantage of capture-oxidation systems may be lower levels of emissions from carbon monoxide and oxides of nitrogen. Another advantage of capture-oxidation systems is no liquid or solid waste is generated during normal operation. A disadvantage is the capture portion of these systems usually has difficulty with low-molecular-weight organic compounds like methanol. Using special adsorbing media, a larger amount of adsorbing media, or putting adsorbing units in a series can improve the capture (removal efficiency) of low-molecularweight organic compounds.

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The following centralized capture-oxidation systems for VOCs will be discussed, which represents the majority of the centralized VOC capture-oxidation systems in the semiconductor industry. 1. Rotor-concentrator, recuperative thermal oxidation system 2. Rotor-concentrator, recuperative catalytic oxidation system 3. Fluidized-bed, thermal oxidation system Fluidized-bed, catalytic oxidation systems; fixed-bed, thermal oxidation systems; and fixed-bed, catalytic oxidation systems will not be discussed, as these centralized equipment technologies for control of VOCs are rare in the semiconductor industry. Similar technology and issues can be applied to these VOC control equipment from the three capture-oxidation systems that are actually discussed. 5.2.2.1 Rotor-Concentrator, Recuperative Thermal Oxidation Systems This centralized VOC capture-oxidation system has a rotor-concentrator (a wheel which rotates slowly) made of zeolite or other adsorbing media that captures the organic compounds from the inlet airstream. Most of the outlet airstream from the rotor-concentrator wheel goes to the atmosphere. A small portion of the outlet airstream from the rotor-concentrator wheel is used as desorbing air, which is heated by a heat exchanger and passes back through a small section of the rotor-concentrator wheel to desorb the organic compounds. This desorption air (containing desorbed organic compounds) enters the thermal oxidizer and is heated to generally between 1350 and 1500°F. The exit air from the thermal oxidizer passes through two heat exchangers before being emitted to the atmosphere. One heat exchanger is used to preheat desorption air containing organic compounds (i.e., after desorbing organic compounds from the rotor-concentrator wheel) before entering the thermal oxidizer. The other heat exchanger is used to heat the desorption air before it enters the rotorconcentrator wheel. The removal efficiency of VOCs for the rotor-concentrator wheel is very important. If the rotor-concentrator wheel is not adsorbing properly, then the unadsorbed organic compounds will be emitted to the atmosphere. The condition of the adsorption media on the rotor-concentrator wheel, VOC-laden air leakage around seals, improper rotating wheel speed (including stopped), and inefficient desorption of organic compounds from rotor-concentrator wheel are some of the situations that cause lower removal efficiency of VOCs for the rotor-concentrator wheel. Another way that the adsorption media of the rotor-concentrator wheel can lose VOC removal efficiency is the permanent condensation of high-molecular-weight organic compounds (e.g., heated photoresist stripper) on the rotor-concentrator wheel, decreasing its adsorbing ability. Installation of a stationary nonregenerated (passive) carbon bed in the airstream prior to the rotor-concentrator wheel can remove these high-molecular-weight organic compounds (i.e., prevent these highmolecular-weight organic compounds from reaching the rotor-concentrator wheel). The second advantage of the stationary nonregenerated carbon bed is that it can decrease the maximum concentrations of VOCs at the inlet and increase the minimum

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VOC concentrations at the inlet (i.e., adsorbs when inlet VOC concentrations are high and desorbs when inlet VOC concentrations are low). This is an advantage as the adsorbing material in the rotor-concentrator wheel will most likely perform better with less deviation in VOC concentration at the inlet have lower maximum and higher minimum concentrations of VOCs at the inlet. This carbon bed should be replaced when it becomes “loaded” with high-molecular-weight organic compounds and no longer performs. It is important to consider fire protection in the stationary nonregenerated carbon bed. For air-permitting consideration, monitoring of the rotor-concentrator wheel speed and temperature of desorption air should be considered parametric monitoring. Inhouse periodic monitoring of the VOC concentration in the air of the rotor-concentrator wheel outlet using a portable VOC monitor is worth considering, ensuring there is not degradation of the rotor-concentrator wheel resulting in decreased removal efficiency of VOCs over time. Visible inspection of the rotor-concentrator wheel and seals should be conducted during normal preventive maintenance. An advantage of the thermal oxidizer (oxidizes the concentrated organic compounds in the desorption airstream from the rotor-concentrator wheel) is that it is a known technology. Once the thermal oxidation temperature is set and oxidation is optimized, the VOC removal efficiency is relatively steady at similar VOC concentrations at the inlet. Continuous monitoring of thermal oxidation temperature can also be used to show compliance for removal efficiency of VOCs required in the air permit (after testing of removal efficiency of VOCs has been conducted at that thermal oxidation temperature to demonstrate the actual VOC removal efficiency). EXAMPLE 5-5: REQUIREMENT FOR REMOVAL EFFICIENCY OF VOCS BY CENTRALIZED ROTOR-CONCENTRATOR, RECUPERATIVE THERMAL OXIDATION SYSTEM. A fab installed a rotor-concentrator, recuperative thermal oxidation system for centralized equipment to control VOCs, and the thermal oxidizer was operated at 1400°F. An inlet-outlet compliance test was conducted with the local air regulatory agency. The site environmental staff took portable VOC measurements at the same time to compare with continuous VOC measurements conducted during the compliance test. This comparison was to be helpful in future in-house use of the portable VOC monitor. The VOC removal efficiency was 95% at the inlet concentration of 200 ppmv total VOC measured as methane. Continuous monitoring of the oxidation temperature in the thermal oxidizer was required on the air permit to show compliance with the requirement of 95% removal efficiency of VOCs. The air permit also required continuous monitoring of the rotor-concentrator wheel speed and desorbed air temperature. The environmental personnel continue to conduct an in-house weekly inlet-outlet test to determine VOC removal efficiency for the rotor-concentrator wheel.

Natural gas (or another fuel) is necessary for oxidation (combustion) in the thermal oxidizer of the rotor-concentrator, recuperative thermal oxidation system. By-products of oxidation (carbon monoxide, oxides of nitrogen, and silicon dioxide) need to be evaluated and most likely included in the air permit. Carbon monoxide emissions come from the incomplete oxidation of both the fuel and the incoming

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organic compounds. It is essential to get accurate emissions data from the control equipment supplier for carbon monoxide, as some suppliers only report the carbon monoxide emissions from the oxidation of the fuel. It is not unusual to have total carbon monoxide emissions three to five times higher than carbon monoxide emissions just from fuel oxidation itself (i.e., carbon monoxide emissions are higher for incomplete oxidation of incoming organic compounds than incomplete oxidation of fuel). As a general rule, carbon monoxide concentrations increase as oxidation temperature decreases with everything else staying the same. Emissions of oxides of nitrogen come from oxidation of air and fuel at a certain temperature, and from oxidation of any nitrogen-containing organic compounds. Examples of nitrogen-containing organic compounds are HMDS and NMP. Suppliers of VOC control equipment may not include emission data for oxides of nitrogen from nitrogen-containing organic compounds unless data are requested. Low oxidesof-nitrogen burners may be available from suppliers of VOC control equipment; determine the impact of these burners on carbon monoxide emissions. EXAMPLE 5-6: EMISSIONS OF CARBON MONOXIDE AND OXIDES OF NITROGEN FROM CENTRALIZED ROTOR-CONCENTRATOR, RECUPERATIVE THERMAL OXIDATION SYSTEM. A facility purchased a rotor-concentrator, recuperative thermal oxidation system for centralized equipment to control VOCs, with a stated VOC removal efficiency, carbon monoxide emission rate, and oxides of nitrogen emission rate from the equipment supplier. An air permit was filed with and issued by the local air regulatory agency with VOC removal efficiency, VOC emission rate, carbon monoxide emission rate, and oxides of nitrogen emission rate similar to what were provided by the VOC control equipment supplier. After installation, an air regulatory compliance test was conducted for VOC removal efficiency, VOC emissions, carbon monoxide emissions, and oxides of nitrogen emissions. The removal efficiency and emissions of VOCs passed the compliance test. Emissions of carbon monoxide and oxides of nitrogen were above the stated amounts in the air permit. A meeting between facility personnel and the equipment supplier determined that the emission rates for carbon monoxide and oxides of nitrogen that were provided by the equipment supplier were only for the fuel oxidation in the thermal oxidizer. Carbon monoxide emissions from incomplete oxidation of organic compounds and oxides of nitrogen emissions from oxidation of NMP were not included in the supplier’s information. The air permit was refiled with the local air regulatory agency with higher emission rates for carbon monoxide and oxides of nitrogen. Fortunately, these higher emission rates did not make the entire site a major source for carbon monoxide or oxides of nitrogen.

Silicon dioxide is another possible by-product if silicon-containing organic compounds (e.g., HMDS from a photo-coater vapor prime operation) are emitted to the rotor-concentrator, recuperative thermal oxidation system and desorbed to the thermal oxidizer. Silicon dioxide can plug the heat exchangers after the thermal oxidizer, causing an eventual shutdown of the unit. Cleaning of the heat exchangers usually is very difficult; plugging could lead to replacement of heat exchangers. Exhausting the processes that emit silicon-containing organic compounds to another centralized

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exhaust system (either centralized ammonia exhaust system or centralized acid exhaust system) is suggested, if the VOC emission limit on the air permit allows for it. TEOS is another silicon-containing organic compound that can be used in the fab; however, the processes using TEOS are usually exhausted to a centralized acid exhaust system. It would be beneficial to determine where processes emitting TEOS and other silicon-containing organic compounds are exhausting to so as to ensure no effects on the VOC control equipment. 5.2.2.2 Rotor-Concentrator Recuperative Catalytic Oxidation Systems This centralized VOC capture-oxidation system has a rotor-concentrator (a wheel which rotates slowly) made of zeolite or other adsorbing media that captures the organic compounds from the inlet airstream. Most of this outlet airstream from the rotor-concentrator wheel goes to the atmosphere. A small portion of the outlet airstream from the rotor-concentrator wheel is used as desorbing air, which is heated by a heat exchanger and passes back through a small section of the rotor-concentrator wheel to desorb the organic compounds. This desorption air (containing desorbed organic compounds) enters the catalytic oxidizer (burner followed by a catalyst) and is generally heated to between 600 and 700°F before it enters the catalyst. The exit air from the catalytic oxidizer passes through a heat exchanger, which is used to heat desorption air before it enters the rotor-concentrator wheel to remove organic compounds. A second heat exchanger may be used to preheat desorption air containing organic compounds (i.e., after desorbing organic compounds in the rotorconcentrator wheel) before entering the catalytic oxidizer; however, the use of this second heat exchanger depends on available heat from the catalytic oxidizer (i.e., is there enough heat left over after the desorption air heat exchanger) to make it worthwhile to use two heat exchangers. The removal efficiency for VOCs of the rotor-concentrator wheel is very important. If the rotor-concentrator wheel is not adsorbing properly, then the unadsorbed organic compounds will be emitted to the atmosphere. Condition of the adsorption medium on the rotor-concentrator wheel, VOC-laden air leaking around seals, improper rotating wheel speed (including stopped), and inefficient desorption of organic compounds from the rotor-concentrator wheel are some of the situations that cause lower removal efficiency of VOCs for the rotor-concentrator wheel. Another way that the rotor-concentrator wheel adsorption media can lose removal efficiency of VOCs is the permanent condensation of high-molecular-weight organic compounds (e.g., heated photoresist stripper) on the rotor-concentrator wheel, decreasing its adsorbing ability. Installation of a stationary nonregenerated (passive) carbon bed in the airstream prior to the rotor-concentrator wheel can remove high-molecular-weight organic compounds (i.e., prevent these organic compounds from reaching the rotor-concentrator wheel). The second advantage of the stationary nonregenerated carbon bed is that it can decrease the maximum and increase the minimum VOC concentrations at the inlet (i.e., adsorbs at the inlet when VOC concentrations are high and desorbs when VOC concentrations are low). This is an advantage as the adsorbing material in the rotor-concentrator wheel will most likely perform better with less deviation in inlet VOC concentration, lower maximum

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inlet concentration, and higher minimum inlet VOC concentration. This carbon bed should be replaced when it becomes “loaded” with high-molecular-weight organic compounds and no longer performs. It is important to consider fire protection in the stationary nonregenerated carbon bed. For air-permitting purposes, monitoring of the rotor-concentrator wheel speed and temperature of desorption air should be considered as parametric monitoring. In-house periodic monitoring of the VOC concentration in the rotor-concentrator wheel outlet air using a portable VOC monitor is worth considering, ensuring there is not degradation of the removal efficiency of VOCs in the rotor-concentrator wheel VOC over time. Visible inspection of the rotor-concentrator wheel and seals should be conducted during normal preventive maintenance. For the catalytic oxidation section, the inlet catalyst temperature can be continuously measured to also show compliance with VOC removal efficiency in the air permit conditions (after regulatory compliance testing of the removal efficiency of VOCs is performed successfully). Catalyst activity can degrade over time due to catalyst poisoning, causing removal efficiency of VOCs to be reduced; it is important to avoid contaminants (e.g., silicon compounds) that can poison the catalyst. The temperature increase across the catalyst section can be measured to show that oxidation of organic compounds is occurring. This temperature difference would be difficult to use as a measure of catalyst performance, as the VOC concentrations at the inlet usually vary to the catalyst section from the rotor-concentrator wheel, due to variable VOC emissions from fab processes (i.e., the temperature across the catalyst varies with variable VOC concentrations at the inlet). Therefore, a modest decrease in catalyst performance may be difficult to determine by the difference in catalyst temperature. A periodic check of the removal efficiency of VOCs or VOC concentration at the outlet by using a portable VOC monitor is a possible way to watch catalyst performance. EXAMPLE 5-7: AIR-PERMIT REQUIREMENT FOR REMOVAL EFFICIENCY OF VOCS BY CENTRALIZED ROTOR-CONCENTRATOR, RECUPERATIVE CATALYTIC OXIDATION SYSTEM. A fab installed a rotor-concentrator, recuperative catalytic oxidation system for centralized control equipment of VOCs and was operated at a 700°F inlet air temperature to the catalyst section. An inlet-outlet compliance test was conducted with the local air regulatory agency. The site environmental staff took portable VOC measurements at the same time to compare them with continuous VOC measurements conducted during the compliance test. This comparison would prove helpful in the future in-house use of portable VOC monitor. The removal efficiency of VOCs was 95% at the inlet concentration of 200 ppmv total VOC measured as methane. Continuous monitoring of the inlet temperature to the catalyst section was required in the air permit to show compliance with the requirement of removal efficiency of VOCs of 95%. The air permit also required continuous monitoring of the speed of the rotor-concentrator wheel and the temperature of desorption air. The environmental personnel conducted in-house weekly inlet-outlet testing to watch the removal efficiency for VOCs of the entire rotorconcentrator, recuperative catalytic oxidation system. If higher than expected VOC

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emissions were found, the rotor-concentrator airstream at the outlet and the catalytic oxidizer airstream at the outlet would be measured separately for VOC concentrations to determine the source of higher VOC emissions.

Natural gas (or another fuel) is necessary for oxidation (combustion) in the catalytic oxidizer. By-products of oxidation (carbon monoxide, oxides of nitrogen, and silicon dioxide) need to be evaluated and most likely included in an air permit. Generation of silicon dioxide in the burner before the catalyst section could seriously impact the catalyst activity, so this should be avoided. Carbon monoxide emissions are generated from the incomplete oxidation of both the fuel and the incoming organic compounds. It is essential to get accurate emission data for carbon monoxide from the control equipment supplier, as some suppliers only report the carbon monoxide emissions from the oxidation of the fuel. It is not unusual to have the actual emissions of carbon monoxide be much higher than those just from fuel oxidation itself (i.e., carbon monoxide emissions can be higher for incomplete oxidation of incoming organic compounds than incomplete oxidation of fuel). Emissions of oxides of nitrogen come from oxidation of air and fuel at a certain temperature and from oxidation of any nitrogen-containing organic compounds. Examples of nitrogen-containing organic compounds are HMDS and NMP. Suppliers of equipment to control VOCs may not include data for emissions of oxides of nitrogen from nitrogen-containing organic compounds unless data are requested. EXAMPLE 5-8: EMISSIONS OF CARBON MONOXIDE AND OXIDES OF NITROGEN FROM CENTRALIZED ROTOR-CONCENTRATOR, RECUPERATIVE CATALYTIC OXIDATION SYSTEM. A facility purchased a rotor-concentrator, recuperative catalytic oxidation system for centralized equipment to control VOCs with a stated VOC removal efficiency, carbon monoxide emission rate, and emission rate of oxides of nitrogen from the equipment supplier. An air permit was filed with and issued by the local air regulatory agency with VOC removal efficiency, VOC emission rate, carbon monoxide emission rate, and emission rate of oxides of nitrogen similar to what was provided by the equipment supplier. After installation, an air-regulatory compliance test was conducted for VOC removal efficiency, VOC emissions, carbon monoxide emissions, and emissions of oxides of nitrogen. The VOC removal efficiency and VOC emissions passed the compliance test. Emissions for carbon monoxide and oxides of nitrogen were above the stated amounts in the air permit. A meeting between facility personnel and the equipment supplier determined that the emission rates of carbon monoxide and oxides of nitrogen provided by the control equipment supplier were only for the fuel oxidation. Carbon monoxide emissions from incomplete oxidation of organic compounds and oxides of nitrogen emissions from oxidation of NMP were not included in the specifications for VOC control equipment. The air permit was refiled with the local air regulatory agency for higher emission rates of carbon monoxide and oxides of nitrogen. Fortunately, these higher emission rates did not make the entire site a major source for carbon monoxide or oxides of nitrogen.

Silicon dioxide is another possible by-product if silicon-containing organic compound (e.g., HMDS from a photo-coater vapor prime operation) are desorbed from

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the rotor-concentrator wheel and then emitted to the catalytic oxidizer. Silicon dioxide can poison the catalyst, causing loss of removal efficiency of VOCs. The catalyst is usually very expensive to replace. Silicon dioxide can also plug the heat exchangers after the catalytic oxidizer, causing an eventual shutdown of the unit. Cleaning of the heat exchangers usually is very difficult; plugging could lead to their replacement. It is suggested that processes emitting silicon-containing organic compounds be exhausted to another centralized exhaust system (either a centralized ammonia exhaust system or a centralized acid exhaust system), if the VOC emission limit on air permit allows for it. TEOS is another silicon-containing organic compound that can be used in the fab; however, the processes using TEOS are usually exhausted to a centralized acid exhaust system. It would be beneficial to determine where processes emitting TEOS and other silicon-containing organic compounds are exhausting to so as to ensure no effects on the catalytic oxidizer and heat exchangers. 5.2.2.3 Fluidized-Bed, Thermal Oxidation Systems This centralized VOC capture and oxidation system has a fluidized bed of adsorbing material that captures the organic compounds from the inlet airstream. The outlet airstream from the fluidized bed goes to the atmosphere. An example of one methodology to desorb the organic compounds from the adsorbing material is presented here. Heated carrier gas desorbs the organic compounds from the adsorbing material in a separate chamber. This desorption carrier gas (containing desorbed organic compounds) enters the thermal oxidizer and is generally heated to between 1350 and 1500°F. The adsorbing material can be made of combustible material; if it is, a safety review should be conducted for possible fires caused by certain organic compounds adsorbed on this combustible material from the centralized VOC exhaust system. Fire protection should be installed as necessary. The removal efficiency of VOCs from the fluidized bed is very important. If the adsorption material in the fluidized bed is not adsorbing properly, then the unadsorbed organic compounds will be emitted to the atmosphere. The condition of the adsorption material and inefficient desorption of organic compounds from adsorption material are some of the situations that could cause lower removal efficiency for VOCs. Another way that the adsorption material in the fluidized-bed can lose removal efficiency of VOCs is that the permanent adsorption of high-molecular-weight organic compounds (e.g., heated photoresist stripper) takes place on the adsorption material (i.e., cannot be effectively desorbed), decreasing its adsorbing ability. Installation of a stationary nonregenerated (passive) carbon bed in the airstream prior to the fluidized bed can remove high-molecular-weight organic compounds (i.e., prevent these organic compounds from reaching the fluidized bed). The second advantage of the stationary nonregenerated carbon bed is that it can decrease the maximum and increase the minimum of the VOC concentration at the inlet (i.e., adsorbs when inlet VOC concentrations are high and desorbs when inlet VOC concentrations are low). This is an advantage as the adsorbing material of the fluidized bed will most likely perform better with less deviation in VOC concentration at the inlet, and with lower maximum and higher minimum VOC concentration at the inlet. This carbon

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bed should be replaced when it becomes “loaded” with high-molecular-weight organic compounds and no longer performs. It is important to consider fire protection in the stationary nonregenerated carbon bed. For air-permitting purposes, monitoring of the fluidized bed conditions to ensure that adsorption material is contacting VOC-laden air and that temperature of desorption carrier gas in the desorption chamber should be considered as parametric monitoring. In-house periodic monitoring of the VOC concentration in the outlet air of the fluidized bed using a portable VOC monitor is worth considering, ensuring there is not degradation over time of the adsorption material in the fluidized bed or there are no issues with the fluidized bed. Visible inspection of the fluidized bed should be conducted during normal operation if possible. An advantage of a thermal oxidizer (which oxidizes the carrier gas stream containing organic compounds from the fluidized bed) is it is a known technology. Once the thermal oxidation temperature is set and oxidation is optimized, the removal efficiency of VOCs is relatively steady at similar VOC concentrations at the inlet. Continuous monitoring of the thermal oxidation temperature can also be used to show compliance with the efficiency requirement in the air permit for VOC removal (after efficiency testing has been conducted at that thermal oxidation temperature to demonstrate the actual removal efficiency of VOCs). EXAMPLE 5-9: REQUIREMENT OF REMOVAL EFFICIENCY OF VOCS FOR CENTRALIZED FLUIDIZED-BED, THERMAL OXIDATION SYSTEM. A fab installed a fluidized-bed, thermal oxidation system for centralized equipment to control VOCs; the thermal oxidizer was operated at 1400°F. An inlet-outlet compliance test was conducted with the local air regulatory agency. The site environmental staff took portable VOC measurements at the same time to compare with continuous VOC measurements conducted for the compliance test. This comparison would be helpful in the future in-house use of the portable VOC monitor. The VOC removal efficiency was 95% at an inlet concentration of 200 ppmv of total VOCs measured as methane. Continuous monitoring of the oxidation temperature of the thermal oxidizer was required on the air permit to show compliance with a removal efficiency requirement of 95% for VOCs. The air permit also required continuous measurement of the temperature of the desorption chamber carrier gas, flow of fluidized-bed adsorption material, and periodic visual inspections of the fluidized bed operation. The environmental personnel continue to conduct an in-house weekly inlet-outlet test to determine removal efficiency of VOCs by the fluidized bed.

Natural gas (or another fuel) is necessary for oxidation (combustion) in the thermal oxidizer of the fluidized-bed, thermal oxidation system. By-products of oxidation (carbon monoxide, oxides of nitrogen, and silicon dioxide) need to be evaluated and most likely included on an air permit. Carbon monoxide emissions come from the incomplete oxidation of both the fuel and the incoming organic compounds. It is essential to get accurate emissions data for carbon monoxide from the supplier of the equipment to control VOCs, as some suppliers only report the carbon monoxide emissions from the oxidation of the fuel. It is not unusual to have

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the actual carbon monoxide emissions three to five times higher than carbon monoxide emissions just from fuel oxidation itself (i.e., carbon monoxide emissions are higher for incomplete oxidation of incoming organic compounds than incomplete oxidation of fuel). As a general rule, carbon monoxide concentrations increase as oxidation temperature decreases, with everything else staying the same. Emissions of oxides of nitrogen come from oxidation of air and fuel at a certain temperature, and from oxidation of any nitrogen-containing organic compounds. Examples of nitrogen-containing organic compounds are HMDS and NMP. Suppliers of the equipment may not include emissions data on oxides of nitrogen from nitrogen-containing organic compounds unless data are requested. Low-oxides-of-nitrogen burners may be available from suppliers of VOC control equipment; determine the impact of these burners on carbon monoxide emissions. EXAMPLE 5-10: EMISSIONS OF CARBON MONOXIDE AND OXIDES OF NITROGEN FROM CENTRALIZED FLUIDIZED-BED, THERMAL OXIDATION SYSTEM. A facility purchased a fluidized-bed, thermal oxidation system with a stated VOC removal efficiency, carbon monoxide emission rate, and emission rate of oxides of nitrogen from the supplier of VOC control equipment. An air permit was filed with and issued by the local air regulatory agency with VOC removal efficiency, VOC emission rate, carbon monoxide emission rate, and emission rate of oxides of nitrogen similar to what was provided by the equipment supplier. After installation, an airregulatory compliance test was conducted for removal efficiency of VOCs, VOC emissions, carbon monoxide emissions, and emissions of oxides of nitrogen. Results for the VOC removal efficiency and VOC emissions passed the compliance test. Emissions of carbon monoxide and oxides of nitrogen were above the stated amounts in the air permit. A meeting between facility personnel and the supplier of the equipment to control VOCs determined that the emission rates of carbon monoxide and oxides of nitrogen provided by the supplier were only for the fuel oxidation in the thermal oxidizer. Data on the carbon monoxide emissions from incomplete oxidation of organic compounds and emissions of oxides of nitrogen from oxidation of NMP were not included in the information from the equipment supplier. The air permit was refiled with the local air regulatory agency with higher emission rates for carbon monoxide and oxides of nitrogen. Fortunately, these higher emission rates did not make the entire site a major source for carbon monoxide or oxides of nitrogen.

Silicon dioxide is another possible by-product if silicon-containing organic compounds (e.g., HMDS from a photo-coater vapor prime operation) are emitted to the fluidized-bed, thermal oxidation system and desorbed to the thermal oxidizer. Silicon dioxide may deposit in the thermal oxidizer, causing an eventual shutdown of the unit. It is suggested that the processes emitting silicon-containing organic compounds be exhausted to another centralized exhaust system (either centralized ammonia exhaust system or centralized acid exhaust system), if the VOC emission limit on the air permit allows for it. TEOS is another silicon-containing organic compound that can be used in the fab; however, the processes using TEOS are usually exhausted to the centralized acid exhaust system. It would be beneficial to determine where

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the processes emitting TEOS and other silicon-containing organic compounds are exhausting to so as to ensure no effects on the centralized equipment for VOC control.

5.2.3 CENTRALIZED CAPTURE

AND

RECOVERY SYSTEMS

Centralized capture and recovery systems utilize an adsorption material for capturing organic compounds in the inlet airstream and then desorb the adsorption material typically with a hot carrier gas or steam; the organic compounds are then condensed, collected, and can be disposed of or reused (note: the reuse of these liquid organic compounds for semiconductor manufacturing processes is not common; these liquid organic compounds can be sent off-site for incineration [heat recovery], for example). The capture portion of this VOC control equipment may have difficulty in removing low-molecular-weight organic compounds. One advantage of capture and recovery systems is the insignificant emissions of carbon monoxide and oxides of nitrogen. One potential disadvantage is the generation of liquid streams during the recovery of organic compounds that needs to be dealt with. This technology of VOC control equipment is rarely used for centralized control equipment for VOCs in the semiconductor industry. Some examples of centralized capture and recovery systems are briefly described next. 5.2.3.1 Systems of Fixed-Bed Carbon with Steam Regeneration This centralized equipment to control VOCs uses fixed-bed carbon vessels to adsorb the organic compounds from incoming airstream. The carbon beds are regenerated with steam, and the VOC-laden steam is condensed into a liquid. There is usually one carbon bed in the regeneration phase, while the other carbon beds are in adsorption phase. This liquid stream (consisting mostly of organic compounds and water) is usually sent to one or more distillation columns to separate into two different liquid streams: water with organic compound contamination and organic compounds with water contamination. The water with organic compound contamination may be sent to wastewater treatment of organic compounds or discharged without organic-compound treatment off-site (if this wastewater discharge is allowed to be sent off-site without treatment). The organic compounds with water contamination may be sent off-site for incineration (the water content will determine whether it can be used for heat recovery; it will probably be more expensive due to the water content). The organic compounds adsorbed in the carbon vessels should be reviewed for possible carbon fires. 5.2.3.2 Fixed-Bed Systems and Recovery of Liquid Organic Compounds This centralized equipment to control VOCs adsorbs organic compounds from the incoming airstream in a fixed bed filled with adsorbent material; the airstream can be desorbed by heated carrier gas (if steam is used, see the previous section). This VOC-laden carrier gas is condensed in a heat exchanger (condenser), and the liquid organic compounds can be sent off-site for incineration (heat recovery). If the adsorbent material is combustible, fire safety should be reviewed.

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5.2.3.3 Fluidized-Bed Systems and Recovery of Liquid Organic Compounds This centralized equipment to control VOCs adsorbs organic compounds from the incoming airstream in a fluidized bed using adsorbent material; the airstream can be desorbed by heated carrier gas or steam. The VOC-laden carrier gas or steam is condensed in a heat exchanger (condenser), and the liquid organic compounds can be sent off-site for incineration (heat recovery). Separation of water and organic compounds may be required if steam is used. If the adsorbent material is combustible, fire safety should be reviewed.

5.2.4 OTHER CENTRALIZED EQUIPMENT SYSTEMS OF VOLATILE ORGANIC COMPOUNDS

FOR

CONTROL

Fixed-bed carbon vessels with no regeneration systems can be used for low VOC emissions from the fab. The carbon will need to be replaced once the organic compounds break through the carbon (i.e., carbon becomes loaded with organic compounds). The VOC-laden carbon may be able to be regenerated and reused (e.g., carbon vessels sent to an off-site facility for carbon regeneration and subsequent reuse). Sometimes fixed-bed carbon vessels are used as a short-term backup when the primary centralized equipment to control VOCs is not operating due to equipment malfunction or lack of preventive maintenance. Each organic compound adsorbed into the carbon vessel needs to be reviewed for the possibility of generating a fire with the type of carbon used. Centralized equipment technology to control VOCs that reuses heat to generate electricity (turbine) or generate steam or hot water (boiler) has been discussed in the semiconductor industry. These types of VOC control equipment may be investigated more in the future as the semiconductor industry strives to reduce energy. A biofilter system is a type of centralized equipment technology that is rare in the semiconductor industry. This technology utilizes bio-organisms to remove and destroy organic compounds. This equipment to control VOCs can be used as a system using centralized whole-airstream equipment or in conjunction with a centralized capture-desorption system.

5.3 EXAMPLES OF SELECTION CRITERIA FOR CENTRALIZED EQUIPMENT TO CONTROL VOLATILE ORGANIC COMPOUNDS As presented earlier, there are criteria that can be used for selecting centralized equipment to control VOCs. Some examples with explanation are provided next. 1. Maximum rate of airflow and operational range of airflow rate. The rate of airflow is very important for the physical design of equipment to control VOCs. It can also impact operating cost (i.e., the more the air, the more the fuel to heat it). Setting the right capacity of the rate of airflow is crucial. It may be prudent to put in a 10 to 20% safety factor in addition

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to maximum rate of airflow determined from fab processes that emit organic compounds. It is important to note that the purchase price of equipment that has a larger rate of airflow is not linear (e.g., equipment to control VOCs at 30,000 ft3/min is not 50% more expensive than control equipment at 20,000 ft3/min for similar VOC concentrations at the inlet; maybe a purchase price of 10 to 20% more, for example). The operation of the equipment selected to control VOCs should be evaluated before it is purchased to make sure it can be operated at a lower or much lower rate of airflow. EXAMPLE 5-11: CENTRALIZED EQUIPMENT TO CONTROL VOCS IS UNDERDESIGNED FOR AIRFLOW RATE FOR A NEW FAB. A wafer fab was being built and the VOC-emitting processes had been initially determined by production personnel. The maximum anticipated airflow rate in the centralized exhaust system for VOC-emitting processes was 20,000 ft3/min. The centralized equipment selected to control VOCs was a regenerative thermal oxidation system. Fab personnel decided to purchase a regenerative thermal oxidation system with a rated capacity of 20,000 ft3/min (even though a 30,000-ft3/min unit was not much more). When the fab building was built, the VOC control equipment arrived and was installed. Process equipment selection and purchase were changing while this was occurring. It was now determined that the total airflow rate for VOC-emitting processes had grown to 30,000 ft3/min; also, it was determined that additional equipment for VOC-emitting processes could be installed in a year to further increase wafer production! A second order was placed for VOC control equipment at 20,000 ft3/min (for a total equipment capacity of 40,000 ft3/min) and would delay the startup of some of the VOC-emitting processes (i.e., a fab ramp to full capacity was delayed). Installation of this second regenerative thermal oxidation system was difficult and very costly.

EXAMPLE 5-12: CENTRALIZED EQUIPMENT TO CONTROL VOCS IS UNDERDESIGNED FOR AIRFLOW RATE FOR AN EXISTING FAB AND FAB EXPANSION. An existing fab building was required to install centralized equipment to control VOCs by the local air regulatory agency. The fab decided to order a rotor-concentrator, thermal oxidation system for 20,000 ft3/min for its current operations. This control equipment was installed and began operation at maximum capacity of airflow rate. Company management notified the fab personnel that it needed to increase its wafer throughput by 50% (note: this had been decided 6 months before as long-term company strategy but was not communicated to fab personnel; company management that approved the purchase of the newly installed centralized equipment were “not in the loop”). A new rotor-concentrator wheel was purchased and major equipment modifications had to be made on the newly installed rotor-concentrator, thermal oxidation system that totaled hundreds of thousands of U.S. dollars. Due to local air regulations, a fab shutdown was required to do these modifications and had to be done before the fab wafer project to increase throughput could be implemented. This loss of wafer production during fab shutdown was a much higher cost than equipment modifications.

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2. Average and maximum inlet concentrations for each organic compound and total VOC. Inlet concentrations for each organic compound need to be estimated or measured to select the centralized equipment technology required to control VOCs, and to design it properly. Average and maximum inlet concentrations for each organic compound and for total VOCs are needed. These inlet concentrations of VOCs can be used to estimate VOC removal efficiencies and determine if there will be a safety concern. EXAMPLE 5-13: SELECTION OF CENTRALIZED EQUIPMENT TO CONTROL VOCS BASED ONLY ON TOTAL VOC CONCENTRATIONS AT INLET. An existing fab needed centralized equipment to control VOCs for a 30,000 ft3/min airstream containing an average concentration of 200 ppmv total VOCs at the inlet measured as methane, a maximum inlet total concentration of 600 ppmv total VOCs measured as methane, and a total removal efficiency of 96% for VOCs. No individual organic compound concentrations were specified. A rotor-concentrator, thermal oxidation system was purchased and installed. VOC testing of this control equipment showed an average total VOC removal efficiency of 92%. Review of the individual organic compounds provided an inlet concentration range of 20 to 60 ppmv of methanol. The rotor-concentrator could only remove approximately 50% of the methanol. A regenerative thermal oxidation system (replacing rotor-concentrator, thermal oxidation system) was installed to be able to achieve 96% total VOC removal efficiency.

EXAMPLE 5-14: SPECIFY CENTRALIZED VOC CONTROL EQUIPMENT FOR NEW FAB. A new fab to be built needed centralized equipment to control VOCs for a 30,000 ft3/min airstream containing an average VOC concentration of 100 ppmv total VOCs at the inlet measured as methane, a maximum VOC concentration of 300 ppmv total VOCs at the inlet measured as methane, and a total VOC removal efficiency of 95%. Inlet concentrations (average and maximum) of individual organic compounds were specified based on projected VOC-emitting processes. A fluidized-bed, thermal oxidation system was purchased and installed. The fab process equipment was installed and operation began. After 2 months of fab operation, a fire started in the fluidized bed (which used combustible adsorbent material) and permanently damaged the VOC control equipment. An investigation determined that ketone compounds and similar organic compounds were being used (emitted) in the fab; these organic compounds were not listed as being used in the VOC-emitting processes when the centralized control equipment was specified. A rotor-concentrator, thermal oxidation system was installed to replace the fluidized-bed, thermal oxidation system.

3. VOC removal efficiency required. The total VOC removal efficiency should be specified. A specific VOC removal efficiency can be required by the local air regulatory agency so as to meet VOC emission limits or other regulatory requirements. A few of the common organic compounds used in the semiconductor industry

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may be regulated separately or as part of another category (e.g., hazardous air pollutants). It may be important to specify removal efficiencies of individual organic compounds to the equipment supplier. EXAMPLE 5-15: ONE ORGANIC COMPOUND NEEDING HIGHER REMOVAL EFFICIENCY. An existing fab was required to install centralized equipment to control VOCs due to local air regulations that had been recently changed. The VOC control equipment had to reduce total VOC emissions by 90% to meet the total emission limit for VOCs specified in the local air regulations. Air-dispersion modeling was done on the average and maximum emission rates of each individual organic compound to compare with local regulatory agency ambient air guidelines. It was determined that one of the organic compounds had to be abated to 95% to meet the local regulatory agency ambient air guideline for that organic compound. Fab personnel specified to equipment suppliers that a total VOC removal efficiency of 95% and an individual organic compound removal efficiency of 98% was necessary (for the organic compound to meet the ambient air guideline). The higher VOC removal efficiencies were specified to ensure that the local air regulation total VOC emission limit and the ambient air guideline for the specific organic compound were met. Centralized equipment technology for VOC control was selected and installed.

4. Possible by-products created (e.g., carbon monoxide, oxides of nitrogen, and silicon dioxide). Carbon monoxide emissions and oxides of nitrogen emissions are usually regulated by the local air regulatory agency to some extent. Silicon dioxide as a particulate can also be regulated. In addition, silicon dioxide created in certain centralized control equipment for VOCs can cause plugging and downtime; silicon dioxide can poison the catalyst of the VOC control equipment technology and can be determined by limitations on these byproduct emissions. Thermal oxidizers and catalytic oxidizers need to be reviewed for emissions of carbon monoxide, oxides of nitrogen, and silicon dioxide. Carbon monoxide is generated by incomplete oxidation of fuel and incomplete oxidation of incoming organic compounds. Oxides of nitrogen are generated by oxidation of fuel and oxidation of nitrogencontaining organic compounds. Silicon dioxide is generated by oxidation of silicon-containing organic compounds (note: generation of silicon dioxide can be minimized by minimizing silicon-bearing organic compounds from entering the centralized VOC control equipment). EXAMPLE 5-16: LARGE MANUFACTURING SITE WITH EXISTING HIGH EMISSIONS OF CARBON MONOXIDE AND OXIDES OF NITROGEN. A large manufacturing site was trying to stay below emission limits for a major source of carbon monoxide and oxides of nitrogen. The addition of any type of oxidation

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system for centralized VOC control equipment would cause emissions to exceed major thresholds source for carbon monoxide and oxides of nitrogen. Fab personnel selected a fluidized-bed system for recovery of liquid organic compounds for the centralized VOC control equipment. A fire safety system was installed to detect any fires and protect the VOC control equipment.

EXAMPLE 5-17: FAB SITE WITH LIMITATIONS ON EMISSIONS OF OXIDES OF NITROGEN. An existing fab had to expand the current capacity of its centralized VOC control equipment. The fab site needed to be careful not to increase emissions of oxides of nitrogen enough to exceed the emission limit for them as a major source. The emissions of carbon monoxide were not so much of a concern. The fab selected a rotor-concentrator, thermal oxidation system, with a low-oxides-of-nitrogen burner, for centralized equipment to control VOCs.

5. Available utilities (electricity, natural gas, etc.). It is important to ensure the proper utilities are available to operate the centralized equipment to control VOCs. Installation of a natural-gas-fired thermal oxidizer or catalytic oxidizer when natural gas is not readily available can be an issue! Design and operation of a thermal oxidizer or catalytic oxidizer with other fuel sources such as propane, No. 2 fuel oil, etc. need to be completely evaluated. Specifications of available electricity need to be communicated to the equipment supplier. If recovery of organic compounds is needed, then the following operations may need to be accomplished that require utilities: heating of carrier gas to desorb organic compounds from adsorbing material and cooling of hot carrier gas in a condenser; or generation of steam for regeneration of adsorbing material, subsequent