Microsystem Technology

W. Menz, J. Mohr, 0. Paul

Microsystem Technology

Further Titles of Interest

M. Kohler Etching in Microsystem Technology ISBN 3-527-29561-5 S. A. Campbell, H. J. Lewerenz (Ed.) SemiconductorMicromachining ISBN 3-471-98084-6 S. Sinzinger, J. Sahns Microoptics ISBN 3-527-29428-7

W. Menz, J. Mohr, 0. Paul

Microsvstern Technology

@WILEY-VCH Weinheim - New York - Chichester Brisbane - Singapore * Toronto

Prof. Dr. Wolfgang Menz IMTEK (Institut fur Mikrosy stemtechnik) Albert-Ludwim-Universitat Georges-Kohl&-Allee 103 D-79110 Freiburg

Dr. Jurgen Mohr Institut fur Mikrostrukturtechnik Forschungszentrum Karlsruhe Postfach 3640 D-76201 Karlsruhe

Prof. Dr. Oliver Paul IMTEK (Institut fur Mikrosystemtechnik) Albert-Ludwigs-Universitat Georges-Kohikr-Allee 103 D-79110 Freiburg

This book was carefully produced. Nevertheless, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Illustration on front page: Array of CMOS-compatible silicon bulk micromachined thermoelectric infrared detector pixels (courtesy of the Physical Electronics Laboratory, ETH Zurich)

Library of Congress Card No. : applied for A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek - CIP Cataloguing-in-Publication-Data A catalogue record for this publication is available from Die Deutsche Bibliothek ISBN 3-527-29634-4

0 WILEY-VCH Verlag GmhH, D-69469 Weinheim (Federal Republic of Germany). 2001 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this hook, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Hagedorn Kommunikation, D-68519 Viernheim. Printing: Strauss Offsetdruck GmbH, D-69503 Morlenbach. Bookbinding: Wilhelm Osswald & Co., D-67433 Neustadt. Printed in the Federal Republic of Germany.

To Margrit, Sonja and Corinne

Preface Writing a book about microsystem technology is no simple task, since technology evolves at such high speed and new variants are reported all the time. If as an author one had to keep one’s manuscript up-to-date, one would have to continuously rewrite it, making a publication impossible. There is a further reason hindering the publication of such a book: the rapid expansion of microsystem technology into new fields of application. Not so long ago, applications in metrology, e. g., in domotics or in the automobile sector, dominated. Today, solutions in minimally invasive surgery, health care, or biochemistry are claiming a rapidly growing share of the market. When we set out to write this book it was not our intention to report the most recent research and development results. This should remain the privilege of the proceedings of relevant technical meetings. However we do believe that the microsystems engineer currently benefits from a large body of experience that has accumulated since the pioneering days of microsystem technology. With this book, we therefore wish to provide the student and the interested engineer with a source of information describing the necessary fundamentals and the basic techniques of the field. In particular is was our wish to show how many of its techniques have evolved out of microelectronics by transcending the limits of electronics, and how they have conquered new areas in physics, chemistry, and biology. On this basis, systems with mechanical, optical, fluidic, chemical and biochemical content can be built. Some day these will perhaps parallel the economic success of microelectronics. The material in this book has been taught and optimized in several courses at the University of Karlsruhe, at the Swiss Federal Institute of Technology ETH Zurich, and finally at the new Faculty for Applied Sciences of the University of Freiburg. The German predecessor of the book, i. e., ,,Mikrosystemtechnik fur Ingenieure” of VCH-Weinheim, in its second edition, was elaborated on the basis of these course notes. In view of the fact that microsystem technology is undissociable from the idea of international exchange and communication, we now present

VI

Prefuce

an English edition. The material contained in the German edition was thoroughly revised and Chapter 6 on silicon-based microsystems was considerably expanded. With these improvements, we hope that the book will add another bright piece to the colorful mosaic of the international endeavor in our fascinating field. Finally it is our pleasure to thank the numerous colleagues and collaborators who have contributed to this project with their contributions, proposals, and constructive criticism. An eminent role was played by Dr. Eric Kay, Mendocino CA, who reworked the manuscript with respect to language and technical content, so that hopefully the book can successfully compete on the market of technical literature in English. Freiburg, July 2000

Wolfgang Menz Jurgen Mohr Oliver Paul

Content

1 1.1 1.2

General Introduction to Microstructure Technology . . . . . . . What is Microstructure Technology? . . . . . . . . . . . . . . From Microstructure Technology to Microsystems Technology .

2 2.1 2.1.1 2.1.2 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.3 2.3.1 2.3.2 2.4

The Parallels to Microelectronics . . . . . . . . . . . . . . . The Production of Single Crystal Wafers . . . . . . . . . . . Production of Silicon-Single Crystals . . . . . . . . . . . . . Production of GaAs Single Crystals . . . . . . . . . . . . . . Basic Technical Processes . . . . . . . . . . . . . . . . . . . . Film Deposition . . . . . . . . . . . . . . . . . . . . . . . . . Lithography (Film Patterning) . . . . . . . . . . . . . . . . . Surface Modification . . . . . . . . . . . . . . . . . . . . . . Etching (Film Removal) . . . . . . . . . . . . . . . . . . . . . Packaging Technology . . . . . . . . . . . . . . . . . . . . . Requirements for Packaging Technology . . . . . . . . . . . Hybrid Technology . . . . . . . . . . . . . . . . . . . . . . . Clean Room Techniques . . . . . . . . . . . . . . . . . . . .

3 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.4

The Physical and Chemical Basics in Microtechnology . . . . . . Crystals and Crystallography . . . . . . . . . . . . . . . . . . Lattice and Types of Lattices . . . . . . . . . . . . . . . . . . Stereographic Projection . . . . . . . . . . . . . . . . . . . . The Silicon Single Crystal . . . . . . . . . . . . . . . . . . . Reciprocal Lattice and the Analysis of the Crystal Structure . . . Methods to Determine the Crystalline Structure . . . . . . . . . X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . Electron Beam Diffraction . . . . . . . . . . . . . . . . . . . Basic Concepts of Electroplating . . . . . . . . . . . . . . . . The Electrode-Electrolyte Interface . . . . . . . . . . . . . . . Polarization and Overpotential . . . . . . . . . . . . . . . . . Mechanisms of Cathodic Metal Deposition . . . . . . . . . . . Materials of Microsystems Technology . . . . . . . . . . . . .

. . . . .

.

1 1

9 15 15 17 24 26 27 30 31 35 36 36 37 39 45 45 46 48 53 55 62 62 64 66 69 73 74 81

VIII

Content

4

Basic Technologies in MEMS . . . . . . . . . . . . . . . . . . Basic Principles of Vacuum Technology . . . . . . . . . . . . . The Mean Free Path . . . . . . . . . . . . . . . . . . . . . . The Monolayer Time . . . . . . . . . . . . . . . . . . . . . . Velocity of Atoms and Molecules . . . . . . . . . . . . . . . . Gas Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . The Classification of Technical Vacuums . . . . . . . . . . . . Vacuum Production . . . . . . . . . . . . . . . . . . . . . . . Pumps for Rough- and Fine Vacuums . . . . . . . . . . . . . . High Vacuum- and Ultrahigh Vacuum Pumps . . . . . . . . . . Vacuum Measurement . . . . . . . . . . . . . . . . . . . . . . Pressure Transducer . . . . . . . . . . . . . . . . . . . . . . . Thermal Conductivity Vacuum Gauge . . . . . . . . . . . . . . Friction Type Vacuum Gauge . . . . . . . . . . . . . . . . . . Thermionic Ionization Vacuum Gauge . . . . . . . . . . . . . . Cold Cathode Ionization Gauge (Penning Principle) . . . . . . . Leakage and Leak Detection . . . . . . . . . . . . . . . . . . Properties of Thin Films . . . . . . . . . . . . . . . . . . . . Structure Zone Model . . . . . . . . . . . . . . . . . . . . . . Adhesive Strength of the Layer . . . . . . . . . . . . . . . . . Physical and Chemical Coating Techniques . . . . . . . . . . . Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ion Plating or Plasma Assisted Deposition . . . . . . . . . . . Ion Cluster Beam Technology . . . . . . . . . . . . . . . . . . CVD Processes . . . . . . . . . . . . . . . . . . . . . . . . . Epitaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plasma Polymerization . . . . . . . . . . . . . . . . . . . . . Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structuring of Thin Films with Dry Etch Processes . . . . . . . Physical Etch Technologies . . . . . . . . . . . . . . . . . . . Combined Physical and Chemical Etch Technologies . . . . . . Chemical Etching Technologies . . . . . . . . . . . . . . . . . Analysis of Thin Films and Surfaces . . . . . . . . . . . . . . Electron Probe Microanalysis (EPM) . . . . . . . . . . . . . . Auger Electron Spectroscopy (AES) . . . . . . . . . . . . . . . X-Ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . Secondary Ion Mass Spectroscopy (SIMS) . . . . . . . . . . . Secondary Neutral Particle Mass Spectroscopy (SNMS) . . . . . Ion Scattering Spectroscopy (ISS) . . . . . . . . . . . . . . . . Rutherford Back Scattering Spectroscopy (RBS) . . . . . . . . . Scanning Tunneling Microscope . . . . . . . . . . . . . . . . .

4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.4 4.4.1 4.4.2 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5 4.5.6 4.5.7 4.5.8 4.6 4.6.1 4.6.2 4.6.3 4.7 4.7.1 4.7.2 4.7.3 4.7.4 4.7.5 4.7.6 4.7.7 4.7.8

109 109 110 112

114 115 116

117 118 119 125 125

126 126 127 127 128

129 129 132 133

133 135

139 140 143 146 148 148 151 155

158 162 164 165

166 168 168 169 169 169 170

Content

IX

5 5.1 5.2 5.3 5.4 5.4.1 5.4.2 5.4.3 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.5 5.7 5.8 5.8.1 5.8.2 5.8.3 5.8.4

Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . Overview and History . . . . . . . . . . . . . . . . . . . . . . Resists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process of Lithography . . . . . . . . . . . . . . . . . . . . . Computer Aided Design (CAD) . . . . . . . . . . . . . . . . . CAD-Layout . . . . . . . . . . . . . . . . . . . . . . . . . . Alignment Patterns and Test Structures . . . . . . . . . . . . . Organization of the Design (Hierarchy. Layers) . . . . . . . . . . Electron Beam Lithography . . . . . . . . . . . . . . . . . . . Gaussian Beams . . . . . . . . . . . . . . . . . . . . . . . . Write Strategy with Gaussian Beams . . . . . . . . . . . . . . Shaped Beams . . . . . . . . . . . . . . . . . . . . . . . . . Post Processor . . . . . . . . . . . . . . . . . . . . . . . . . Proximity Effect . . . . . . . . . . . . . . . . . . . . . . . . Optical Lithography . . . . . . . . . . . . . . . . . . . . . . . Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shadow Projection . . . . . . . . . . . . . . . . . . . . . . . Imaging Projection . . . . . . . . . . . . . . . . . . . . . . . Further Developments . . . . . . . . . . . . . . . . . . . . . . Optical Lithography for Micromechanics . . . . . . . . . . . . Ion Beam Lithography . . . . . . . . . . . . . . . . . . . . . X-Ray Lithography . . . . . . . . . . . . . . . . . . . . . . . Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-Ray Sources . . . . . . . . . . . . . . . . . . . . . . . . . Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . Application of X-ray Lithography . . . . . . . . . . . . . . . .

171 171 171

6 6.1 6.1.1 6.1.2 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.2.6 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.4 6.4.1 6.4.2

Silicon Microsystem Technology . . . . . . . . . . . . . . . . . Silicon Technology . . . . . . . . . . . . . . . . . . . . . . . IC Processes and Substrates . . . . . . . . . . . . . . . . . . . Foundry Technologies . . . . . . . . . . . . . . . . . . . . . . Silicon Micromachining . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Wet Etching . . . . . . . . . . . . . . . . . . . . . . . . . . Basic Etch Shapes . . . . . . . . . . . . . . . . . . . . . . . Etching Control . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of Anisotropic Wet Etchants . . . . . . . . . . Dry Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Micromachining . . . . . . . . . . . . . . . . . . . . Polysilicon Micromachining . . . . . . . . . . . . . . . . . . . Sacrificial Aluminum Micromachining . . . . . . . . . . . . . Sacrificial Polymer micromachining . . . . . . . . . . . . . . . Stiction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micro Transducers and Systems Based on Silicon Technology . . Mechanical Devices and Systems . . . . . . . . . . . . . . . . Thermal Micro Devices and Systems . . . . . . . . . . . . . .

209

175

176 177 178 181 182 182 185 187 189 190 192 193 194 196 198 200 202 202 203 204 204 208

210

210 214 215

215 220 227 234 239 241 248 250 253 254 255 257 257 263

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Content

6.4.3 6.4.4 6.4.5 6.4.6 6.5

Devices and Systems for Radiant Signals . . . . . . . . . . . . Magnetic Devices and Systems . . . . . . . . . . . . . . . . . Chemical Microsensors . . . . . . . . . . . . . . . . . . . . . Micromachined Devices for Electrical Signal Processing . . . . Summary and Outlook . . . . . . . . . . . . . . . . . . . . .

273 275 280 285 287

7 7.1 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.3 7.3.1 7.3.2 7.3.3 1.3.4 7.4 7.4.1

The LIGA Process . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mask Production . . . . . . . . . . . . . . . . . . . . . . . . The Principle Construction of a Mask . . . . . . . . . . . . . . Production of the Carrier Foil . . . . . . . . . . . . . . . . . . Structuring of the Resist for X-ray Intermediate Masks . . . . . Electroplating with Gold for X-ray Masks . . . . . . . . . . . . Production of Process Masks . . . . . . . . . . . . . . . . . . Window for Alignment in X-ray Process Masks . . . . . . . . . X-ray Lithography . . . . . . . . . . . . . . . . . . . . . . . Production of Thick Resist Layers . . . . . . . . . . . . . . . Beam Induced Reactions and Development of Resists . . . . . . Requirements on the Absorbed Radiation Dosage . . . . . . . . Influences on the Quality of the Structure . . . . . . . . . . . . Galvanic Deposition . . . . . . . . . . . . . . . . . . . . . . Galvanic Deposition of Nickel for the Production of Microstructures . . . . . . . . . . . . . . . . . . . . . . . Mold Insert Fabrication . . . . . . . . . . . . . . . . . . . . . Electrodeposition of Further Metals and Alloys . . . . . . . . . Plastic Molding in the LIGA Process . . . . . . . . . . . . . . Production of Microstructures by Reaction Injection Molding . . Fabrication of Microstructures by Injection Molding . . . . . . . Fabrication of Microstructures by Hot Embossing . . . . . . . . Production of Metallic Microstructures from Molded Plastic Structures (Second Electroplating) . . . . . . . . . . . . . . . . Variatioiis and Additional Steps of the LIGA Technology . . . . Sacrificial Layer Technology . . . . . . . . . . . . . . . . . . 3D-Structuring . . . . . . . . . . . . . . . . . . . . . . . . . Production of Light-Conducting Structures by Molding . . . . . Examples of Applications . . . . . . . . . . . . . . . . . . . . Rigid Metallic Microstructures . . . . . . . . . . . . . . . . . Moving Microstructures. Microsensors and Microactuators . . . Fluidic Microstructures . . . . . . . . . . . . . . . . . . . . . LIGA-Structures for Optical Uses . . . . . . . . . . . . . . . .

289 289 291 291 294 295 298 299 301 301 302 303 306 310 316

7.4.2 7.4.3 7.5 7.5.1 7.5.2 7.5.3 7.5.4 7.6 7.6.1 7.6.2 7.6.3 7.7 7.7.1 7.7.2 7.7.3 7.7.4

316 321 322 324 324 327 333 337 341 341 344 347 349 349 353 366 368

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8 8.1 8.1.1 8.1.2 8.2 8.2.1 8.2.2 8.3

Alternative Processes of Microstructuring . Mechanical Micromanufacturing . . . . . Production Process and Primary Structures Examples of Applications . . . . . . . . . . Electro-Discharge Machining . . . . . . . The Basics of EDM . . . . . . . . . . . . . Applications of EDM for Microsystems . . Laser Micromachining . . . . . . . . . . .

9 9.1 9.1.1 9.1.2 9.1.3 9.1.4 9.2 9.2.1

Packaging and Interconnecting Techniques (PIT) . . . . . . . Hybrid Technology . . . . . . . . . . . . . . . . . . . . . . . Substrates and Pastes . . . . . . . . . . . . . . . . . . . . . . Layer Production . . . . . . . . . . . . . . . . . . . . . . . . Placement and Soldering of the Circuit Components . . . . . . Mounting and Contacting of Silicon Dies . . . . . . . . . . . Wire-Bonding Techniques . . . . . . . . . . . . . . . . . . . . Thermocompression Wire-bonding (Hot-Pressure Welding Bonding) . . . . . . . . . . . . . . . Ultrasonic Wire-bonding (Ultrasonic Bonding) . . . . . . . . Thermosonic Wire-bonding (Ultrasonic Hot-pressure Welding) . . . . . . . . . . . . . . . Ball-Wedge Bonding . . . . . . . . . . . . . . . . . . . . . . Wedge-Wedge Bonding . . . . . . . . . . . . . . . . . . . . . Advantages and Disadvantages of the Wire-bond Processes . . Test Processes and Alternatives . . . . . . . . . . . . . . . . New Contacting Technologies . . . . . . . . . . . . . . . . . The TAB Technology . . . . . . . . . . . . . . . . . . . . . . The Flip-Chip Technologies . . . . . . . . . . . . . . . . . . Adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotropic Adhesion . . . . . . . . . . . . . . . . . . . . . . . Anisotropic Adhesion . . . . . . . . . . . . . . . . . . . . . . Anodic Bonding . . . . . . . . . . . . . . . . . . . . . . . . Wafer-to-Glass Bonding . . . . . . . . . . . . . . . . . . . . . Wafer-to-Wafer Bonding . . . . . . . . . . . . . . . . . . . . New Packaging Technologies . . . . . . . . . . . . . . . . . Low Temperature Cofired Ceramics (LTCC) . . . . . . . . . .

9.2.2 9.2.3 9.2.4 9.2.5 9.2.6 9.2.7 9.3 9.3.1 9.3.2 9.4 9.4.1 9.4.2 9.5 9.5.1 9.5.2 9.6 9.6.1

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381 381 382 386 394 394 397 399

. 403 404 404 407 . 408 . 412 413

. 413 . 414 . 414

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415 416 417 417 418 418 420 422 422 424 425 425 427 428 428

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10 10.1 10.2 10.3 10.4 10.4.1 10.4.2 10.5 10.5.1 10.5.2 10.6 10.7

System Technology . . . . . . . . . . . . . . . . . . . . . . . Definition of a Microsystem . . . . . . . . . . . . . . . . . . Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . Signal Processing . . . . . . . . . . . . . . . . . . . . . . . . Signal Processing for Sensors in Microsystems . . . . . . . . . Neural Data Processing for Sensor Arrays . . . . . . . . . . . . Interfaces of Microsystems . . . . . . . . . . . . . . . . . . . The IE-Transfer . . . . . . . . . . . . . . . . . . . . . . . . . The S-Transfer . . . . . . . . . . . . . . . . . . . . . . . . . The Module Concept of Microsystem Technology . . . . . . . . Design. Simulation. Integration. and Test of Microsystems . . .

Literatur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

431 431

433 438 439 440 444

449 452 455 456

461 465

1 General Introduction to Microstructure Technology

1.1 What is Microstructure Technology? Microsystems technology leads the engineer away from a comprehensible seize regime to an area which is beyond the natural realms of perception. He must learn to work with these new opportunities as well as use his own experience, but not arbitrarily impose it onto the new technology. This mental transformation already started with microelectronics, the problem though was not as much apparent, since the electrical engineer is used to deal with “abstract” matter such as electricity. The real conflict with personal experience began initially with the merging of microelectronics with mechanical structures. Nowadays microsystems technology is mentioned everywhere. Unfortunately this does not contribute to clarity, but on the contrary the concepts seem frequently unclear and misunderstandings remain. First, however, the basic difference between microstructure technology and microsystems technology has to be clarified, although the choice of words should make it quite unequivocal. Microstructure technology is the tool, with which a particular geometrical structure of a body can be produced, whose dimensions lie in the micrometer region. In some cases however, only one of the body’s dimensions lies in the micrometer range, whilst the other two dimensions remain in the millimeter range. In other cases they are already in the sub-micrometer region. However, the actual dimensions are less important than the technology, which derives from microelectronics, and its potential to enter the micrometer region. Nanotechnology is even more difficult to define than microtechnology. Of course, it would be wrong to talk of nanotechnology if a structure is described, whose dimensions are only a fraction of a micrometer. Here too one must refer to the technology which enables the production and measurement of nanostructures. Both technologies have very different origins, therefore it would be very wrong to presume that one technology continually evolves into the other. On many occasions the terms microstructure technology and microsystems technology are mixed up or at least not well defined. By means of microstructure technology, it is possible to manufacture microbodies or microcomponents,

2

1 General Introduction to Microstructure Technology

whereas microsystems technology integrates these microcomponents to arrays, adds signal processing and controlling, and provides interfaces to the outside macroscopic world. An example from microelectronics should clarify this point: The smart connection of a hundred or thousand “dumb” transistors leads to an “intelligent” microsystem, the microprocessor, which constitutes the initial efficient capability of microelectronics. This book deals firstly with the basics of microstructure technology, followed by a discussion of microsystems and the technological requirements which lead to them. A basic question about the microsystem will be put to the reader first:

What is the motivation to pursue microstructure technology? To answer this question it is necessary to understand the development of microelectronics over the last 40 years. What has happened during this time? Before microelectronics, there existed conventional electronics and electronic components like resistors, capacitors and electron tubes. These components were assembled to form an electronic circuit, tested and adjusted by trimming the parameters of the components, until the circuit met the required specifications. Therefore each circuit was in its own way unique. The packing density and likewise the function density of an electronic circuit was limited by the size of the individual elements. By introducing microelectronics a dramatic change took place in electronics. Components were no longer produced and assembled mechanically but were optically transferred and multiplied using photolithography onto the workpiece i. e. the silicon wafer. It is noteworthy, that by optical imaging only two-dimensional structures could be transferred. At first this appeared to be a considerable disadvantage for the technology as a common concept was to design and manufacture in three dimensions. This disadvantage of optical imaging is, however, outweighed by its technological advantages because (i) it is possible to transfer structures whose dimensions are only limited by the wavelength of light, (ii) the imaging of the patterns is free of deterioration and thus highly reproducible and (iii)the optical transmission is parallel enabling an extremely high data flow (in state-ofthe-art-lithography 4.108 pixels are transferred with one exposure). Over the course of the last decades the development of microelectronics led to dimensions of the components being reduced by many orders of magnitude. These days critical dimensions far into the sub-micrometer region (0.3 pm) can be achieved. As the manufacturing process occurs “batchwise” i. e. with a group of wafers onto each of which millions of transistors are placed, many integrated circuits can be manufactured in parallel. It is also possible to reduce the cost of production by many orders of magnitude because of the increase in packing density. An important sign of quality of a circuit, which is necessary for a computer, is the switching speed. This can be greatly decreased by shortening the length of internal conductance paths and thereby improving the overall quality of an integrated circuit. Microelectronics rules our everyday lives. Microelectronics has infiltrated into all technical areas and has set the criteria for the development of the information

I . 1 What is Microstructure Technology? age. These influences are difficult to quantify, but taking the improvement of quality over a timespan of three decades, multiplied by the decrease in fabrication costs one receives a value of 10000 000. For comparison, take any other technology like manufacture of steel or construction of vehicles, and the huge differences against such developments become obvious. With microelectronics being successful to such a degree, the question arose whether similar technological advancements in other non-electronic areas are possible. Can these development concepts, processes, and materials not be transferred to mechanical, optical, fluidic or chemical and biological areas ? The answer to this question finally lead to microsystems technology. It can be stated :

Microsystems technology is the inevitable, logical development of microelectronics into non-electronic areas. Therefore microsystems technology profits from the enormous technological and theoretical base of knowledge of microelectronics, which has been developed over several decades. Many technologies that we take for granted today, were developed from the economically booming microelectronics industry involving high financial and personal expenditure. So for example, photolithography remains an essential ingredient. This notion will be emphasized throughout this book. Thin film technology, surface analysis and simulation are also areas, which have gained critical momentum from microelectronics. In order to be able to build on these experiences of a new technology it is first necessary to explore the underlying “philosophy” of microelectronics. In other words to define the recipe of success of microelectronics in order to be able to modify it so that its significance to other technologies can be ascertained. From numerous procedures and thought processes three topics can be highlighted, which will now be dealt with in turn. The design of an integrated circuit is done almost exclusively on the computer. The design of a circuit in the traditional trial and error approach, to find an optimum solution, is not feasible in microelectronics for economical reasons. The development steps must already be performed on the computer using expensive design- and simulation-methods. For an established process, a circuit (at least a digital circuit) already fills the requirements at the first production run. Only in a few cases a second production run is required to optimize the product. The simulation programs have been developed at great expense using many thousands of man years. It is both noteworthy and new, that knowledge of theoretical physics such as quantum mechanics, leads directly into the product configuration. Nowhere else does scientific basic research and product configuration come into such close contact as within microelectronics. A concept which illustrates this state is the phrase “band gap engineering” in which the knowledge of theoretical solid state physics and production know-how overlap with each other. The design, simulation and optimization of the product by means of the computer can be considered as objective one.

4

1 General Introduction to Microstructure Technology

The second objective is the transfer of the computer generated structure onto the work piece. The transference of geometrical data is done by optical means. The advantage of this method of transference is that it is free of “wear and tear” and therefore there is no attrition. On imaging, the structure can be reduced to such an extent which is limited only by the wavelength of the light used and by the inaccuracies of the optical system. Optical transference or “photolithography” has the largest technological influence on microelectronics, aside from the production of the basic material, the silicon single crystal. The word “photolithography” is taken from an old printing technology where a flat ground stone (hi000 = stone, ypaQe~v= to write) is appropriately etched, so that is takes on printing ink only at particular parts but prevents it from sticking to others. By reducing the structures to the sub-micrometer region, the packing density per square unit of the components brought onto the work piece is higher by many orders of magnitude than conventional technologies of transmission would allow. Therefore, despite increasing processing costs, the costs for the single element can be greatly reduced. By reducing the linear dimensions by a factor of two it is possible to produce four times as many structures on the substrate using a parallel process. Even if the expense for photolithography increases by a factor of three, one has still achieved an overall profit for the production. Besides the cost advantage, miniaturization brings with it a considerable quality advantage. Integrated circuits are generally measured by their function density and their speed of switching. By miniaturization, the electrical connections within the circuit are correspondingly shortened, which has a direct effect on the speed of signal processing. A simple calculation already shows that the vacuum speed of light amounts to 0,3 mm per picosecond. The pathways per picosecond of a signal on a circuit, which is affected by capacitances and inductances, are accordingly smaller and fall within the geometrical dimensions of the circuit. Optical imaging means a parallel transference of information. With a high quality objective, as is used in lithography, structures with minimum dimensions of 0.5 pm can be transferred onto a panel of 1 cm2. A parallel flow of 4.108pixels is thus achieved. At the same time the process of transference is independent of the pattern transferred. A complex structure requires no higher expenditure than a simple one, as long as the minimum dimensions are not surpassed. If by skilful design, structures are interlaced, it is possible to achieve high packing densities without the need for additional technological advances. At the moment one restriction is the two dimensionality resulting from optical transmission. As the image shows a limited focal depth, all transferred patterns are two dimensional. A microelectronic circuit tends to expand laterally by a few millimeters or even centimeters i. e. in the x- and y- direction. In the z-direction i. e. the depth, it rarely stretches to above 10 pm. Therefore it is claimed that the whole of microelectronics is quasi two dimensional. Of course it is possible with other processes to produce a circuit which is made up of more than one level stacked on top of each other. This does not affect the fact that the structures are transferred two dimensionally. It is actually surprising that one neglects one third of the possibilities of geometrical design, in comparison to conventional

1.1 What is Microstructure Technology?

5

electronics. However the optical transmission of the structure holds many more advantages, which greatly outweigh the disadvantages. The importance of microelectronics is proof enough. The second objective of microelectronics is, without a doubt, the application of photolithography. Due to the higher packing density of the building elements on the wafer, millions of structure elements experience exactly the same process on subsequent processing. In turn the process scattering is very small. Processes which, in the course of development, became more expensive, are compensated for by the minimum costs of producing thousands or millions of building elements on one single wafer. Due to lower process scattering and higher yield the processes in turn can be described more precisely and simulated. Therefore the assertions, which can be made with the software-tools to design the circuit, are more accurate and more realistic which results in a positive feedback in the design routine i. e. a closed loop. Batch fabrication, is considered to be the third objective in microelectronics. What has now changed in the development philosophy of electronics parallel to the manufacturing process towards microelectronics? Multiple individual building elements have been replaced by few, standardized sets of components within close tolerances. However by focusing the research and development onto the comparably less standard types, the performance could be increased by a large order of magnitude in contrast to conventional basic circuits. The basic building blocks, taken from libraries, can be combined and connected to optimal complex circuits using appropriate computer aided design tools. By improving the design-tools, just as by the constant improvement of quality of the building elements, nowadays functions can be obtained, which were technically not feasible a few years ago. An example would be the development of the personal computer, which has already surpassed the performance of the mainframe computers of the 80’s. Therefore, the use of standard components and the application of strictly standardized manufacturing processes are the fourth objective. The same basic concepts of microelectronics also apply to microstructure technology. For the latter, it is of great advantage to take advantage of the enormous technological know-how made available by microelectronics. Even if some of the processes have to be newly developed, essentially it is possible to build on the theoretical and technological basics, which have evolved with microelectronics. In conclusion, microstructure technology demands that: Microstructure technology must follow in general the path of microelectronics in order to be as successful. Within microstructure technology the development philosophy must include: 0

Enabling the supply of sufficiently powerful and efficient software development tools for microcomponents; development, simulation and optimization of structures on the computer, the avoidance of unnecessary process runs.

6

1 General Introduction to ~ ~ c r o ~ ~ r Technology u~tur~ 0

Transference of the structure, designed on the computer, onto the substrate by means of photolithography, using the advantages of a higher packing density and a decrease in the size of the structure.

0

Batch manufacturing with narrow manufacturing tolerances by precise control and monitoring of the process.

0

Development of only a few, but carefully designed basic structures which, by having a higher packing density and miniaturization, can be cost effectively multiplied onto the same substrate and can be integrated by a suitable connection technology to an “intelligent” system. This therefore leads (just as it did in electronics) to a complete change of ideas in the sensor and actuator technology and in precision engineering and finally mechanical engineering.

Although the processes of microstructure technology were closely related to those of microelectronics, the means to develop a microstructure into the third dimension had to be developed, since it is not supplied by microelectronics. Over the course of years this has lead to many variants, and each would fill a whole chapter of a book. Two of the most important are briefly described here and will be discussed in more detail in Chapter 6 and 7 in this book. Silicon micromechanics rely heavily in every aspect on microelectronics. Not only has it taken over a similar manufacturing process, but also the silicon single crystal is again the basic material for the microstructure. At the beginning of the 80’s and as the leading authority of this process K. E. Petersen, at that time a coworker of IBM, had already written a fundamental publication [Pete82]. The third dimension was developed from an anisotropic etching process, with which the single crystal can be subtractively levelled to a desired shape. Special etching solutions strip the material anisotropically from a single crystal, corresponding to the crystal morphology. Using silicon oxide or silicon nitride masks, parts of the silicon surface are exposed to the etching solution, which etches different crystal planes at different rates. Artificial layers can be build into the crystal, which serve as additional etch stop layers. Using suitable etching masks, etch stop layers and the application of isotropic and anisotropic etching solutions, three dimensional structures can be produced from silicon wafers, which in turn form the basic elements for sensors or any other components (Fig. 1.1-1). The specific advantage of silicon micromechanics lies in the ability to install on the same substrate both the microstructure body (e. g. sensor element) as well as the suitable electronic evaluation circuits. A second important process is the so-called LIGA technology which was developed at the beginning of the 80’s at the Nuclear Research Center, Karlsruhe (today Research Center, Karlsruhe) under the guidance of Erwin Becker, then head of the Institute for Nuclear Process Technology (today the Institute for Microstructure Technology), and Wolfgang Ehrfeld, as leading scientist. With this technology it was possible to manufacture components for isotope separation of uranium-hexafluoride, UF6 [Beck86].

1.1 What is Microstructure Technology?

7

Fig. 1.1-1 Array of front micromachined infrared detector pixels fabricated using commercial CMOS technology followed by compatible anisotropic silicon etching [Schn98] (Courtesy of the Physical Electronics Laboratory, ETH Zurich).

Fig. 1.1-2 Typical microstructure fabricated in LIGA technology. For comparison a human hair is put across the structure. The inner width amounts to 80 pm, and the wall thickness is 8 pm.

Figure 1.1-2 shows an array of identical geometric shapes, which are used as mechanical filter. This microstructure was produced using the LIGA process, a structure building process which employs X-ray lithography, electroplating and molding. This process is described in Chapter 7. A structure design which is generated on the computer is transferred by means of an electron beam writer onto a mask. The structure of this mask is imaged onto the workpiece using par-

8

1 General Introduction to Microstructure Technology

allel X-ray light (synchrotron radiation) as a shadow projection onto a radiation sensitive polymer layer. Because of the low absorption of the X-ray radiation in this polymer layer, the radiation penetrates without noticeable scattering deep into the layer. A layer thickness of more than one hundred micrometers can be irradiated without distortion of the structure. In microelectronics, light in the visible or near ultra violet region can be used to irradiate the photosensitive layer, since these so-called photoresists are only one micrometer or less thick. The parallel beam of the X-ray light and the extreme layer thickness permits the manufacture of structures with an aspect ratio (i. e. the ratio of structure height to the smallest possible lateral structure) of over 100. In microelectronics aspect ratios of only about 1 are normal. In this first step of the LIGA process a structured microbody is obtained with the lateral structure of the mask and a structure height, which is determined by the layer thickness of the resist. In further processing steps the irradiated and later removed polymer is replaced by electroplated metal. After the non-irradiated polymer is subsequently also removed, the negative form of the structure now remains as metal. This metal structure serves as a mold for further copying of the microstructure using processes such as injection molding or hot embossing. Although the manufacture of the original structure is relatively cost intensive, the process can be used for mass production of a variety of materials and therefore microstructure bodies can be manufactured highly economically. In addition to these two fundamental processes there are numerous variants which use some processes of the above described production technologies and which, for special applications, have their particular advantages. These alternative processes are fully described in a special chapter. At this point an example of mechanical microproduction (Fig. 1.1-3) should be mentioned [Bier88]. A microprofile is milled in a flat metal surface with an appropriately formed diamond. By

Fig. 1.1-3 Typical microstructure manufactured in mechanical microproduction. The structure is fabricated by milling a brass substrate with a diamond tool. The pitch size is about 100 pm (courtesy of Research Center Karlsruhe).

1.2 From Microstructure Technology to Microsystems Technology

9

manufacturing regular structures and by suitable stacking of microformed foils on top of each other, relative inexpensive three-dimensional microstructure bodies can be produced [BieBO]. The production method is different compared with both previously mentioned processes. However, partial steps from the other processes can be used such as, for example, the electroplating of the structure to produce a molding tool for the mass production of microbodies by injection molding or hot embossing.

1.2 From Microstructure Technology to Microsystems Technology The examples of microstructure technology given above would only be of moderate technological importance would it not have the potential to integrate components to a microsystem. Only then can microsystems technology develop to its full potential. This is again exemplified by microelectronics with the development of transistors which filled the basic requirements for economic success and which acted as a trigger on the development of microprocessors. Also within microsystems technology, the production of microcomponents is the basis, from which system technology derives. If however, technology were to idle at this stage, then the microsystems technology would be reduced to only replacing conventional components with microcomponents. If this were the case a technological revolution would not be under discussion. Only the integration of many sensors to an array, the connection with actuators and the control of all components by efficient signal processing on site makes an “intelligent” system out of “dumb” components. Microbodies which are manufactured using microstructure technology must be connected to each other on a common substrate. At first only the pure mechanical mounting of a structure onto a suitable support should be considered. This is not trivial if one thinks about the optical communication technology. At the moment a suitable method, which is stable over a long period and is inexpensive, is still being sought to precisely orientate a mono-mode glass fibre to an optoelectronics component within a fraction of a micrometer. Another problem illustrates the connection of several components with different thermal expansion coefficients. A particular problem exists in sensor technology, where on the one hand the delicate microstructure must be protected from damage and corrosion and on the other hand, be exposed as widely as possible to the environment to obtain a true and undistorted value of the physical and chemical properties of the environment. The connection and packaging technology plays a key sole, which can be seen later in many examples of the microsystems technology. A microsystem consists not only of a purely mechanical construction but also of a large amount of interfaces between the single components as well as between the macroscopic external environment and the microsystem and vice versa. These in-

10

1 General Introduction to Microstructure Technology

terfaces are of different types. The electrical interface, which predominates in microelectronics, is only one of many. For these interfacial processes microelectronics offers only techniques such as soldering, wire-bonding, the TAB (Tape Automated Bonding) technology or the flip chip process. Because the microsystem includes more than electronics, such interfaces as optical, mechanical, fluidic or acoustic interfaces must also be considered. The technologies for these are for a large part not yet developed. They differ greatly from electrical interfaces, so that it is necessary to find a new name which replaces the concept “interface” from microelectronics with its electrical joining processes. A suggestion would be to introduce the concept “coupling site”. An important method to protect sensitive microstructures mechanically is covering them with a glass plate. A suitable process for this is anodic bonding. The surfaces of an assembly (preferably silicon and glass) are brought into contact with one another. By heating to about 400°C and with the aid of an electrical field, ions in the dielectric are irreversibly displaced. The resulting electrostatic energy is large enough to bind both surfaces continually together leading finally to the formation of a chemical bond. Using the described processes, the technological requirements are met to integrate micromechanics, microoptics, microfluidics etc., and microelectronics within monolithic or hybrid solutions to complex systems. This would open a channel for new fundamental concepts of sensor technology, measuring and control technology, communication technology, environment and medical technology and other applications, which perhaps have not yet been thought of. The previous discussion dealt with the technological requirements which must be fulfilled for the production of microsystems. The following sections will discuss the role which information and software development plays in microsystems technology. The most important property of a microsystem is the possibility of having a whole array of sensors with high packaging density at low manufacturing cost, rather than one individual sensor. During a conventional construction, the analogue value of the one sensor is amplified and “delivered” at the output of the amplifier for further treatment. On the other hand, the intelligent system has the ability to pick up the signal using many sensors in parallel and process it on site. Every sensor displays cross sensitivity also for parameters, which are not supposed to be measured. A pressure sensor generally has a temperature dependence and is in addition dependent on the medium to be measured. If we consider a sensor array, then the true value without distortion by other parameters can be calculated, if the parameter function of each individual sensor element is known. The problem is then described as finding the solution of a system of n equations with m unknowns, with n as the number of independent sensor elements and rn the number of parameters to be measured. In other words, for every measurement with a sensor array, several parameters can be calculated in parallel. This can be a demanding task for an on-board microprocessor with mostly very limited capability. In many cases however, the quality of a measurement already increases significantly, if calculation of one or two of the most influential parameters is achieved.

I .2 From Microstructure Technology to Microsystenzs Technology

11

With the same technique it is possible to improve the selectivity of a sensor system. If for example a gas sensor had the task of detecting a complex gaseous composition in low concentrations, a single sensor e. g. CHEMFET (chemical field effect transistor) could not achieve this task. A whole array of CHEMFET’s each with a different parameter function, could solve such a task by using suitable algorithms for sample recognition. The special feature of this sensor system would be that with unchanged hardware configuration many different gas compositions could reliably be detected with a high degree of selectivity. The combination of sensor elements with analogue-digital converters, a microprocessor and an interface to the exterior are demanding requirements for a microsystem. Other components like multiplexers, ROM and RAM, complement the systems capabilities (Fig. 1.2-1). By storing a parameter mapping for a sensor array, the cost intensive laser trimming could be eliminated in the first operation and on replacement of a sensor array. Ageing processes could be identified by the recorded thermal history of sensors and can be compensated for. By averaging of several sensors the quality of measurement could be improved. Sensors with different ranges of sensibility could be connected and thus the working range of the sensor system could be increased considerably. Using microactuators, which again effect the sensor, feedback and motion compensated physical measurement systems can be released. Even though the list of possibilities is endless we should limit ourselves in this introduction. However at this point the ability to adapt such an intelligent system

Fig. 1.2-1 Scheme of a complete microsystem. The device consists o f arrays o f sen-

sors and actuators, as well as on-board signal processing and interfaces to the outside (macroscopic) world.

12

I General Introduction to Microstructure Technology

to a certain task will be highlighted once more. Microsystems could be used to explore unknown, dangerous, or usually inaccessible areas. These properties would be of an enormous advantage for applications such as environmental monitoring, exploration tasks, space missions and medical implants. In many cases a microsystem must be able to communicate with other systems. The microsystem must be able to transmit and receive data. These data must be transferred free of errors in sometimes highly disturbed surroundings. The compatibility with other systems or a central computer, with which the system communicates, is also of importance. Using simulation methods the system’s performance is to be checked whether it runs in a controlled mode or changes into a chaotic state. What are the tasks in microsystems technology for information technology? An important requirement for the concept of a microsystem is the system specification and the simulation of its properties on the computer. From this, one begins to establish the system specification and as a consequence describes the components and in particular, the desired properties of the sensor. If these properties are not realizable the system concepts must be varied until the optimum one has been discovered. As well as the system specification, the appropriate signal processing algorithms are of great importance. Here concepts must be developed in order to be able to solve complex tasks of measurement and control with the greatest computing efficiency for microprocessors in real time. Another field is the enabling of test routines for the self-test of microsystems. In some applications the requirements in reliability are crucial to such a degree, that the system must be tested at regular intervals or even continually. In case of malfunction the system will self correct or switch itself off in a defined way. In this way the bus system connecting a set of other systems or subsystems remains in operation and in emergencies other participating systems on this same bus system could take over the tasks of the switched off system. It has already been mentioned that an important property of microsystems must be the ability to communicate. However, these communications take place partly in a highly disturbed surrounding e.g. on a welding robot or next to the spark plugs in an automobile. Therefore for transfer free of distortion, methods of channel coding must be developed or present methods modified within microsystems technology. Production processes and performance of a microstructure are closely related. Here too the close parallels to microelectronics can be seen. The modern semiconductor plant of the late 90’s operates on strict computer integrated manufacturing, in which the production line is kept in the ideal 100%-yield state by a constant input-output comparison between the simulation program and the measured production parameters. For this, expert systems must be developed not only for the product development but also for the process control. The microsystems technology must go in the direction of these expensive methods, if it is to fulfill the technological demands. The opportunities of microsystems technology are so diverse, that only a small segment at this point can be mentioned. In which direction microsystems techno-

1.2 From Microstructure Technology to Microsystems Technology

13

logy will move is anybody’s guess. Likewise at the beginning of the development of microelectronics no one could predict that these developments would lead to personal computers and thereby revolutionize the whole of data processing. The future will undoubtedly bring still more surprises. For studying, it is important to acquire a wide knowledge and for the expert it is important to be interactive with other disciplines and be constantly prepared to move in new directions.

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2 The Parallels to Microelectronics

As already mentioned in the introduction, microstructure technology is based on an immense background of technology, which has been optimized and finally brought to almost perfection over the past decades. Studying the processes of microstructure and microsystems technology it is essential to get to grips with the methods of microelectronics. In the following paragraphs both the concepts of production of microelectronics and directions for future development with respect to production technology and the product, that is the integrated circuit, will be discussed. The packaging technology as well as clean room techniques have come about, as a result of these developments. They are integral components of circuit production and therefore should be dealt with in the introduction. The techniques, which operate microsystems technology, will be focussed on again in later chapters in more detail.

2.1 The Production of Single Crystal Wafers Silicon is the basic material for microelectronics as well as for the majority of areas within microsystems technology. Firstly the electronic properties of silicon single crystals are of prime importance and secondly their mechanical and chemical properties are significant. Although silicon is one of the most thoroughly researched materials (there are myriads of publications on the subject), its excellent mechanical parameters had almost been forgotten until the well known publication of “Silicon as a Mechanical Material” in 1982 by K. E. Petersen [Pete82]. This paper revealed to a wide audience the potential of silicon single crystals and how they could be used within silicon micromechanics for industrial production. In the following table a comparison between some of the physical properties of silicon and other materials is highlighted (Table 2.1-1). Note the tensile strength of silicon which overtakes that of stainless steel manifold.

16

2 The Parallels to Microelectronics

Table 2.1-1

Physical properties of silicon in comparison with other materials

Material

Quartz Silicon

Stainless Steel

Density [g/cm31

Hardness WaI

Young's Modulus [GPal

Tensile Strength [GPal

2.20 2.32

8.2 8.5-11

0.5-0.7 2.8-6.8

7.9-8.2

5.5-9

87 (100) 129.5 (110) 168.0 (111) 186.5 206-235

0.5-1.5

In general, semiconductors are solids whose electrical conductivity lies between that of metals ( 2 lo-' S .cm-') and that of dielectric materials ( 5 S .cm-'). By appropriate doping, the electrical conductivity of the semiconductor can be adjusted by many powers of ten. The electrical conductivity is also strongly dependent on temperature. The mechanism is fundamentally different from that of metals. In the case of a metal many electrons can be found in the conduction band and therefore a large transport of current can be achieved. In contrast, the electrons in a semiconductor must be raised from the valence band to the conduction band by thermal collisions. In the case of metallic conductors thermal collisions affect electrons within the lattice and with each other and lead to a reduction of electrical conductivity, whilst for semiconductors these thermal collisions make conduction possible, albeit to a much lesser degree than in metals. If one heats a semiconductor, the specific resistance decreases whereas if one heats up a metal it increases. Although there are many materials that can be classified as semiconductors, from a technical point of view most are of little importance. Semiconductors are categorized into elemental and compound semiconductors. Elemental semiconductors are, for example B, C (diamond), Si, Ge, S, Se and Te. Examples of compound semiconductors are binary compounds such as GaAs, InP and CdS. There are also a number of ternary semiconductors such as Hg,-, Cd,Te or Ga,_,Al,As as well as quarternary systems such as Gal-, In,As,-, P,. Even though the above mentioned semiconductors could be put to special use, silicon is now and also in the foreseeable future, the most promising semiconductor material for microelectronics. No other semiconductor combines such excellent physical, mechanical and chemical properties. The fact that silicon forms an oxide, that possesses special physical and chemical properties, has contributed to its prominent position within semiconductor technology. Compound semiconductors are available in a number of combinations of materials, which at best come into use for special applications such as the utilization of specific physical effects. To these material combinations belong the photoelectric sensors for wavelengths to which silicon is not sensitive. Of some technical significance are InSb, PbS, PSe, PbTe, CdS, CdSe and CdTe.

2.1 The Production of Single Cjystal Wafers

17

Most compound semiconductors are limited to special applications because of their particular properties, but also because of their limited availability, their high costs and expensive technology. However, GaAs is an exception because of its special band structure. It is a so-called “direct” semiconductor with the ability to emit light, in contrast to silicon. It has found a wide range of applications in optoelectronics, for microwave circuits and special sensory applications. The properties will be dealt with later in Chapter 3 . The technical difficulties in the production of GaAs single crystals leaves silicon’s leading role unchallenged.

2.1.1 Production of Silicon Single Crystals Silicon is not only widely used in microelectronics but it is also a basic material within microsystems technology. The next section will focus on the preparation of the purest material and the growth of an almost perfect single crystal [Sze88]. Over the last decade an enormous amount of labor and money has been invested in silicon technology. The result is a single crystal which far exceeds any naturally occurring material in its chemical and crystallographic purity. The requirements for purity of these crystals is almost unimaginable. In wafers for the production of VLSI-devices less than 1 heavy-metal atom is allowed among 1 trillion silicon atoms! The merit of success lies not only in the fact that one can produce such a highly pure material, but that large amounts are available on the market at low price. The production process which leads to a single crystal silicon disc or “wafer”, can be separated into the following steps: 0 0 0 0

Preparation and cleaning of the raw material, production of highly pure, polycrystalline silicon, growth of single crystals, mechanical processing of single crystal silicon bodes.

Raw silicon is obtained by reduction of the raw starting material, quartz sand. This reduction step is the most energy intensive within the whole chain of the production process, as 7 kWh per kg of silicon must be invested. The extraction takes place in an electric-arc furnace with carbon electrodes according to the following equation: SiO2

+ 2 C + Si + 2 CO at 2100 K

(2.1)

The reaction enthalpy AH,,,, is +695 kJ. The silicon obtained in this way has a purity of about 98 % and is known as “metallurgical grade silicon”. Further purification of the silicon is best carried out by fractional distillation. For that the material must be converted to the liquid form which occurs via a reaction with HCI. Trichlorosilane (SiHCl,) is formed which has a boiling point of 318 “C. The reaction goes as follows:

18

2 The Parallels to Microelectronics Si

+ 3 HC1+

SiHC13

+ HZ at 600 K

with a reaction enthalpy of AH600= -218 kJ. By successive fractional distillation the impurities, which would impair the electrical properties, are reduced down to a ratio with silicon of 10-9:l. After this purification step the reaction is essentially reversed, in order to obtain solid, elementary silicon. Therefore the process temperature must be adjusted in order for the reaction equilibrium to shift in the opposite direction.

4 SiHC13

+ 2 H2 -+3 Si + Sic14 + 8 HC1 at 1400 K

(2.3)

with AH,,,o = f964 kJ. At low pressure the trichlorosilane vaporizes and dissociates under the influence of thermal energy in a “chemical vapor deposition” process (see Section 4.5.5). In this step, very pure polycrystalline silicon is precipitated from the gas phase onto heated rod-like silicon substrates. So in a process which extends over a few days rods are formed, which have a diameter of over 200 mm and a length of several meters. The rods form the raw material, the so-called “electronic grade silicon” for the production of single crystal silicon. The fourth step of the production chain involves the growing or “pulling” of single crystals. Two processes have been developed on an industrial scale: 0 0

the Czochralski method, the Float Zone method.

The Czochralski Method In this process (abbreviated CZ) polycrystalline (electronic grade) silicon is melted in a quartz crucible by an induction or resistance heater in an inert gas atmosphere above 1415 “C, the melting point of silicon (Fig. 2.1-1). The inner melting crucible is made of quartz. As quartz at this temperature is already soft, the inner crucible is protected by an external casing made of graphite. Next, a nucleating crystal (seed crystal) of single crystal silicon with the desired crystallographic orientation, is brought into contact with the surface of the melt. By slowly rotating and retracting the seed crystal, and rotating the crucible in the opposite direction, a larger single crystal forms with a definite orientation and a constant diameter (apart from the initial “neck region”). During growth process the crystal can be “doped”, i. e. a defined amount of impurities can be introduced, which change the electrical properties in a pre-determined way. The doping elements that can be used are either trivalent e. g. boron, or pentavalent e.g. phosphorous, arsenic or antimony. Thereby it is possible to obtain a p- or n-doped raw material. The maximum permissible pulling speed depends on the material properties such as melt enthalpy and heat conducting coefficients as well as on technical parameters such as maintaining a flat isothermal surface in the melt and the crystal. Common pulling speeds are between 1 and 3 m d m i n .

2.2 The Production of Single Crystul Wufers

19

& ,--~

Axis of rotation Seed crystal

Vacuum,

Single crystal Melt Quartz crucible Heater

Radiation shield

Support (Graphite)

a

Fig. 2.1-1 Czochralski method for silicon crystal pulling. a) Scheme of the mechan-

ical set-up, b) view into the crucible with an ingot forming (Courtesy of Wacker Siltronic, Burghausen).

These pulled single crystals can weigh up to 60 kg and be up to 3 m long (Fig. 2.1-2). They are cut at either end. Crucible drawn material is available commercially with a diameter of up to 300 mm. Wafers with an even larger diameter (400 mm) are being introduced into pilot production in the near future.

20

2 The Purullels to Microelectronics

Fig. 2.1-2 A finished ingot being removed froin the pulling gear. Note the thin neck at the upper end carrying the full load of the ingot with a weight of up to 60 kg (Courtesy of Wacker Siltronic, Burghausen).

Float Zone Method

A further important process for the production of large silicon single crystals is the float-zone (abbreviated FZ) method. A rod of polycrystalline silicon, which already shows the external dimensions of the later single crystal, is clamped in a mounting so that the underside is in contact with a seed crystal. Using an induction coil, which melts only a small area of the polycrystalline rod and whose axis is movable, the melt- and solidification zone is moved over the length of the silicon rod to the upper end. On solidifying, the single crystal material grows in the same orientation as the seed crystal. At the trailing edge of the moving melt zone the polycrystalline material is transformed into a single crystal which exhibits the same orientation as the seed crystal (Fig. 2.1-3). This process must be carried out under high vacuum or in an inert gas atmosphere (usually argon). Using the float-zone process, crystals are obtained containing only minute amounts of chemical impurities. One advantage of this process is the absence of a crucible, which could be a possible source of contamination. A further advantage is the utilization of the segregation effect, which describes the difference in the equilibrium solubilities of the impurities of different materials, or different phases of the same material. The segregation coefficient ko = C, /C1 is defined as the relationship between the atomic concentration C, of an impurity content in the solid state (s = solidus) to that in the melt C1 (1 = liquidus).

2.1 The Production of Single Crystal Wafers Inert or

21

I Polycrystalline material Melting zone Movable induction coil

Monocrystalline material Seeding crystal

.p

to Pump

Fig. 2.1-3 a) Principle of float zone process, b) view of the melting zone and the movable coil (Courtesy of Wacker Siltronic, Burghausen).

The purification effect by zone-melting or segregation is then obtained when ko is smaller than 1, since then the contamination can accumulate at the rod’s ends (see Table 2.1-2) [SzeSS]. This purification process is, of course, also effective

22

2 The Parallels to Microelectronics

in the CZ crystal pulling method but in contrast to the CZ process the float zone method allows a multiple repetition of the melting and resolidification process, in order to obtain extremely pure crystals.

Table 2.1-2 Segregation coefficients ko of some elements in silicon Element

k0

Element

ko

A1 As Au B

2.10-~ 3.10-l 2.5.10-5 %lo-’ 4.1OP6

Fe Ga

8.10-6 %lop3

cu

Mg Na 0

1.6.10-3 1.25

As the melt-zone has no mechanical rigidity, both ends of the rod, above and below the melt-zone must be precisely fixed. The design of the induction coil and the corresponding field play an important role in the quality of the single crystal. The length of the melt zone should not be too extended, as the surface tension of the melt and the induced eddy currents must prevent the melt zone from collapsing and subsequently molten material running down the crystal. Phosphine (PH,) or diborane (B2H6) are typical doping materials, which are added to the ambient inert-gas atmosphere. FZ-silicon is available up to a diameter of 150 mm. A typical doping level for microelectronic application is 30 ppbw (parts per billion by weight) boron, which amounts to 1.5 x 1015 boron atoms per cubic centimeter of silicon or only 3 boron atoms to 100 million silicon atoms. The single crystal rods (also called ingots) must be processed further to discs or “wafers”. The blocks are accurately ground to cylinders and the crystallographic orientation is ascertained by means of an X-ray diffractometer. The orientation as well as the type of doping will be marked by means of so-called “flats” onto the cylinder surface. The coding is illustrated in Fig. 2.1-4. Then the ingots are cut using an inner diameter saw. Using this kind of saw the cutting edge can be put under tension and therefore kept in a stable position. The width of the cut has, of course, direct economical impact since the goal is to loose only as small an amount as possible of the costly single crystal. The very big diameter (300 mm and up) can not be cut in this way anymore, instead wire cutting saws are in use (Fig. 2.1-5). After cutting, further steps are taken to improve the wafer surface e. g. mechanical lapping and etching. This improves the surface crystal layers, which are damaged by the previous mechanical treatment. Finally a mechanical-chemical polishing step finishes the wafer surface. The contouring of the edges of the wafers is also important. This means a “trimming” of the edges, in order to avoid tiny silicon splinters in the manufacturing process arising from the edges of the wafer.

2.1 The Production of Single Crystal Wafers

23

Fig. 2.1-4 The coding marks used on a silicon wafer to distinguish doping type and crystallographic orientation.

Diamond coated cutting edge

a

b

Fig. 2.1-5 The slicing of the ingot into wafers. For wafers up to about 8 inches, this is done with a so-called inner diameter saw (a). Due to the circumferential tension the saw blade is stabilized and enables better cuts with thinner blades. For larger diameters the multiple wire saw is used (b).

Using the CZ method a relatively high fraction of oxygen is present in the crystal from the crucible material. The oxygen occupies interstitial sites and leads to deterioration of the advantageous electronic properties of the semiconductor. Therefore, frequently the backside of the wafer is either mechanically or electrochemically roughened. During a subsequent tempering process these oxygen atoms migrate from within the crystal and become anchored to the mechanical “traps” on the backside where they can cause no further damage. In the past the diameter of wafers increased by one inch every four years in order to meet the rapidly increasing demand for larger silicon surfaces.

2 The Parallels to Microelectronics

24

Alongside a growth in productivity due to ever increasing wafer size, there was also the miniaturization of the structures, which allowed a large increase in packing density. However, the delicacy of the structures demanded an ever improving flatness of silicon wafers. Ten years ago a flatness of 1 pm per 200 mm was required, today 0.3 pm per 300 mm has to be met.

2.1.2 Production of GaAs Single Crystals The production of GaAs single crystals is much more complicated than that of silicon. This is mainly due to the fact that the components Ga and As have differing vapor pressures which complicates a stoichiometric precipitation over a large crystal volume. These days three production methods are dominating and will now be briefly introduced: 0 0 0

the horizontal Bridgman method, the gradient freeze-method, and the modified crucible-crystal pulling method with sealing fluid according to Czochralski (LEC = liquid encapsulated Czochralski).

Bridgman and Gradient Freeze Methods Because of the different vapor pressure of both components Ga and As in the melt, the reactants are molten in a quartz ampoule, together with a separate reservoir of arsenic to compensate for the higher partial pressure. This additional amount of arsenic prevents thus the melt to deviate from the stoichiometric balance of Ga and As. A seed crystal must be introduced at an appropriate position, from which the crystal growth spreads out. The mixture in the ampoule is now melted in a furnace, without loosing the seed crystal. Then the ampoule is slowly moved out of the furnace in such a way that the solidification front of the seed crystal moves to the opposite end of the melt (Fig. 2.1-6a). Crystals which have been produced via this process do not exhibit a circular diameter but are D-shaped, rectangular or square. Another variation of this is to move vertically an ampoule hanging by a wire through the oven with an analogous temperature gradient. The advantage of this process is the ability to produce round wafers. As a consequence this method is called the “vertical” Bridgman method. A variation of this is the gradient freeze method, by which the oven and the melt are not moved relative to one another, but the solidification front moves through the melt by controlling the heater sections of the oven (Fig. 2.1-6b). The advantage of this process is that the agitation of the melt by movement of the ampoule is avoided, so less dislocation lines appear and therefore a higher quality of crystal is realized.

2. I The Production of Single Crystal Wafers

25

Fig. 2.1-6 Bridgman and gradient freeze method for GaAs. With the Bridgman method (a) the ampoule is pushed physically out of the furnace. The liquidus-solidus plane runs from the left (location of the seeding crystal) to the right. (b) The gradient freeze method avoids any mechanical movement by switching off the furnace section by section, resulting in a solidification front moving from left to right.

Fig. 2.1-7 The LEC (liquid encapsulated crystal) method is very similar to the Czochralski method sketched in Fig. 2.1-2. To prevent the evaporation of the components, the melt and the ingot is covered by a barrier layer.

26

2 The Parallels to Microelectronics

LEC Method The LEC method resembles the Czochralski method for silicon (Fig. 2.1-7). However, to maintain the stoichiometric relationship of the components, vaporization of the more volatile component (in this case arsenic) is prevented by covering up the melt and the hot single crystals with a gas-tight barrier layer. This barrier layer could be a liquid layer of B,O, which is chemically inert and in addition acts as a getter. Otherwise the process of crystal-pulling is identical to that of silicon single crystal production. The film of boron oxide remains on the outside of the ingot and must be ground off before cutting the wafers. The quality of the crystal, which has been manufactured by the LEC method is lower than that of the previously described processes. This is due to a steeper temperature gradient present in the LEC method, and therefore the density of the formed dislocation lines is larger than in Czochralski-silicon. Although crystals can be produced up to 100 mm in diameter with this method, common diameters that are produced on an industrial scale are between 50 mm and 75 mm only [Ober88].

2.2 Basic Technical Processes The production process of an integrated circuit leads back to relatively few basic technological steps, which repeat themselves a number of times in the whole process of fabrication of integrated circuits either individually or in combination. Individual steps are as follows: 0 0 0 0

film deposition, lithography (film patterning), surface modification (oxidation, doping), etching (film removal).

After manufacture of an integrated circuit on the wafer level, the wafer is cut into chips. All the subsequent processes can be defined by the term packaging technology with the subtitles: 0

0 0

dicing (cutting the wafer into chips), connecting (electrical contacting of the chips), housing (integrating the components to systems, encasing).

The earlier mentioned processes are outlined in the following. Processes with importance to microsystem technology will be treated in detail in later chapters.

2.2 Basic Technical Processes

27

2.2.1 Film Deposition The most important processes of film deposition, especially with respect to silicon technology, will be introduced briefly in the next section. Their impact on microstructure technology will be dealt with in more detail in Chapter 4.

Spin Coating Usually the first coating process consists of covering the wafer with a photosensitive layer of polymer, which is exposed by means of a mask and then developed. Upon exposure the solubility of the polymer changes, so that the exposed areas can subsequently be removed selectively by certain developers. With this the structural information of a mask can be transferred into the polymer film. After development and thermal treatment the polymer layer acts as a kind of stencil, adherent to the surface of the wafer and resistant to subsequent processes such as etching, doping, or other treatment. Therefore this polymer layer is also called resist or photoresist.

Chemical Vapor Deposition (CVD) The most versatile and definitely the most important layer deposition technology is the chemical deposition from the gas phase (Fig. 2.2-1). Variations of this are low pressure (LPCVD) and plasma enhanced (PECVD) deposition technology. The basic process of the CVD method is the condensation of a material from a thermally unstable, gaseous compound onto the substrate. The threshold energy of dissociation of the reactant molecule is supplied either thermally, by heating the

Exhaust

A

Heater Wafer boat

Fig. 2.2-1 A typical reactor for CVD processes. The tray of silicon wafers can be heated up to 1200°C.

2 The Parallels to Microelectronics

28

substrate, or by electrical or optical energy in a gas discharge. For the process to work it is important that the reaction products (except for the desired deposited material) are gaseous so that they can be removed (pumped off). In the last few years CVD has virtually replaced the evaporation technology within semiconductor technology. Using the technology of CVD it is possible to produce all the layers necessary for an integrated circuit. Single crystal growth can be accoinplished in CVD using epitaxial approaches and will be dealt with in the next paragraph. Single crystal layers can be manufactured from polycrystalline silicon (as produced by the standard CVD technology), if an "inner" epitaxy is present, by heating the film in the presence of seed crystals. Epitaxy Epitaxy from the gas phase, carried out at normal pressure as well as low pressure, is the standard method for the growth of thin single crystal silicon films. Figure 2.2-2 shows two standard reactor types. In the case of gas phase epitaxy accurate temperature control plays a crucial role in the quality of the layers. Therefore it is not only a matter of maintaining and controlling the absolute values of temperature of the wafer from 1050 "C to 1150 "C but also of minimizing the temperature variations across the wafer. In the case of low pressure processes, on heating the wafer, convection is of little influence, whereas heat conductivity (by direct contact of the whole surface) and radiation play a major role. Uniform heating of a substrate in vacuum is problematic since the wafer can not be fixed by a vacuum chuck nor can it be clamped down mechanically to the heater without destroying the delicate surface of the wafer. The largest possible surface area for thermal contact is Reactants

Si-wafers Radiation heater

Exhaust Reactants

Exhaust

b

a Fig. 2.2-2 Reactor types for epitaxy, with

a) an induction heater ("pancake"-reactor), and b) an reactor with radiation heating.

2.2 Basic Technical Processes

29

required. At high temperatures frozen-in stresses in the wafer are released and may lead to additional warping and consequently to non-uniform heating of the surface. In the case of a so-called “pancake” reactor a graphite support heats up the wafer. As described above, this leads to temperature gradients on the wafer, which in turn results in epitactic layers with a high number of crystalline faults. With cylindrical reactors, and heating the wafer without mechanical contact (thermal conduction) and using infra-red radiators, a uniform thermal distribution is obtained thereby yielding better results. The reaction gas consists of compounds such as SiH,, SiC1, or other gaseous silicon compounds (e. g. SiHC13) which are diluted with an inert carrier gas e. g. H2. On collision with a heated surface of a silicon wafer the molecule dissociates into its components some of which condense (in this case silicon atoms) onto the substrate. At the same time the temperature must be very precisely controlled, so that the deposition conditions remain constant over the whole wafer. At this process temperature, the silicon atom retains so much kinetic energy that it can migrate on the surface and occupy the most energetically favorable site on the surface. As a single crystal represents an energy minimum in the solid state, the deposited layer grows as a single crystal. If an appropriate mixture is added to the reaction gas, the deposited layers are correspondingly doped. Thereby, doping of greater than 10’’ atoms/cm3 can be achieved.

Physical Deposition Process (PVD) Almost without exception sputtering is the only remaining PVD processes for manufacture of semiconductors. Argon ions are accelerated onto the target by an externally applied voltage. Due to the transfer of energy from the ions to the surface layers of the target neutral particles are expelled from the target and subsequently condensed on a substrate. In sputtering the particles which arrive at the substrate have an energy 10 to 100 times larger than those in (thermal) evaporation. Consequently the adhesion to the substrate is much better with sputtered films. There are several variations of the sputtering process: 0 0 0

DC (direct current) sputtering, RF (radio frequency) sputtering, reactive sputtering.

The special features of these particular processes are emphasized in Chapter 4. The final breakthrough of the sputter technology resulted from the development of the so-called magnetron sputtering. Compared with conventional sputtering it is possible to achieve a higher growth rate ( e . g. 1 to 5 pm per minute with aluminum) which is comparable to a standard coating rate using evaporation.

30

2 The Parallels to Microelectronics

2.2.2 Lithography (Film Patterning) With photolithographic processes, the lateral structures of the electronic elements, designed and optimized on the computer, are transferred onto the wafer surface (a full description is given in Chapter 5). The semiconductor wafer is covered with a photosensitive coating (photoresist), onto which the lateral information of a photomask is imaged and fixed by development. The patterned coating protects against processes which may otherwise influence or attack the surface. As the coating is resistant to these effects, it is called “resist” (protective coating). The resist acts as an etching mask which is fixed onto the substrate and therefore, only exposed areas of the wafer surface can be changed by the subsequent process step. Different regions of the electromagnetic spectrum for the optical structure transference are used depending on the minimum structure sizes, which are to be transferred. Light in the visible region or near ultraviolet (UV) can be used for structures from above 0.5 pm. Far ultraviolet light (193 nm wavelength) is used for structures down to about 0.15 pm. X-rays (synchrotron radiation with wavelength between 0.2 and 20 nm) are still in the research and development state. For structures with dimensions down to a few nanometers, electron or ion beam lithography is used. In electron beam lithography an electron beam is guided in a scanning mode (similar to that used in television receivers) to the substrate and at the same time the beam is rapidly switched on and off. In contrast to the projection methods, which are used for mass production, electron beam writing is a serial information transfer process. It is therefore a slow process but with high accuracy. The key domain for electron beam lithography is in the production of precise masks, by which the CAD data are sequentially written into the electron sensitive layer. The production output is of secondary importance compared with the high precision which is required for the mask patterning. With today demanding precision of structure transfer, the optical (reduced) image impinges on its physical limits. No lens can transfer a structure with submicrometer details in “full wafer” exposure onto a wafer of 200 mm in diameter. As a consequence of greater demands on structural verification, only sections of the wafer are exposed at a time. The wafer is then moved mechanically under the optical set-up to the next position where the process of the exposure is repeated. This process is called “step and repeat”. The above also applies to electron beam lithography. Here the maximum writing area of an electron beam writer is about 1x1 m2. After every writing operation, the stage with the mask is mechanically shifted to the next writing field. As many structures spread over more than one field, the positioning error of the mechanical displacements may amount to only a fraction of the smallest structure, so that no misalignment is noticeable within a structure. Therefore these mechanical stages with interferometer control are very costly components of the lithography machines. An important feature of the optical irradiation process is the scaling of the mask. One can distinguish between a shadow projection, whereby the mask has the same size on the wafer as the transferred structure and the imaging (reduction) process. Imaging processes generally have scaling down ratios of 1O:l to 4:l.

2.2 Basic Technical Processes

31

Photoresists used in semiconductor technology are modified upon exposure to light in such a way that they change their molecular structure and therefore their chemical properties e. g. solubility. The maximum sensitivity generally lies in the UV-region and decreases for longer wavelengths. The resist can be handled under yellow light, without unintentionally exposing it. Therefore rooms in which the coated wafers are processed, are illuminated with yellow light (“yellow rooms”). The photolithographic process embraces different processes: 0 0 0 0

substrate preparation (cleaning), application of coating (spin on), drying, exposure, development, locally modifying the silicon surface, resist removal (stripping).

2.2.3 Surface Modification Thermal Oxidation At higher temperatures and in an oxygen rich atmosphere silicon forms a gas-impervious, chemically resistant layer of silicon dioxide (Si02). This layer plays an important role in the dominant position silicon holds within microelectronics. Silicon dioxide layers are found in applications such as: 0 0 0 0

passivation layers, masking layers, insulating layers, dielectric layers, adhesion promoting layers.

During thermal oxidation, the silicon atoms in the surface react layer by layer with atmospheric oxygen. The oxidation layer does not grow linearly with time, but exponentially. The oxygen atoms must diffuse through the ever thickening layer of silicon dioxide in order to meet with reactive silicon atoms. The process temperature for silicon is between 800°C and 1200°C. In the absence of water vapor in the atmosphere this process is referred to as a “dry” process. Water-free oxides have excellent dielectric properties and are mostly free of defects. However, the process time is about 10 h for an oxide with a thickness of 0.1 pm. If the oxygen in the reaction chamber is enriched with water vapor the process is referred to as a “wet” oxidation. Compared to dry oxidation, “wet” oxide layers are more porous and thus easier to penetrate by subsequent water molecules for reaction with new silicon atoms. Thick oxides are therefore produced by wet oxidations. The wet oxidation process produces layers with less density and a lower breakdown voltage compared with those prepared by dry oxidation processes.

2 The Parallels to Microelectronics

32

Diffusion

Doping in microelectronics is most important in producing p-n junctions. These junctions form the central element of each component within the semiconductor technology. During crystal pulling the ingots are already doped to n- or p-type semiconductors. On manufacture of electrical elements, laterally structured doping gradients must be introduced. Dopant impurities, such as boron or phosphorous atoms, are incorporated in minute amounts. The impurity concentrations are in the range of to lop4. Three boron atoms among 100 million silicon atoms is a conventional doping level. Doping is performed either by means of diffusion or ion implantation. During diffusion, regular lattice atoms are replaced with doping atoms in a thermodynamic equilibrium process. This has the effect of changing the electric properties of semiconductors. The two most important methods for doping are: 0 0

the constant-surface-concentration diffusion, and the constant-total-dopant diffusion.

In the “constant-surface-concentration diffusion” the dopant is supplied from a “non-exhaustible” gas source, so that at the outer surface of the crystal the concentration of the dopant remains constant over the whole process of the diffusion. In the case of the “constant-total-dopant diffusion” the surface of the crystal is initially covered with a solid layer of the dopant. However, the dopant supply is limited and eventually “exhausted” as the diffusion process proceeds. In the “constant-surface-concentration” approach, where the doping atoms are supplied from gas phase a constant doping concentration is regulated on the crystal surface, which corresponds to a particular doping agent solubility in silicon at a given temperature (Fig. 2.2-3a). If a doping layer of thickness dx and surface area A is placed on the surface of a silicon wafer as the constant-total-dopant source and the number of components at a given unit of time that can be diffused into the solid is N = A.J, where J is the particle current per unit area, then by applying Fick’s second law of diffusion: (2.4) The doping profile can be determined in the solid state as a function of the depth x and the number of particles N . D is the diffusion coefficient. In order to be able to solve the differential equation, the following boundary conditions are defined; at a time t = 0, all the particles No are kept at x = 0 in a surface A. The solution of the differential equation is therefore;

N ={ N ~ / A T D ~ ) ~ ~ ~ } ~ - ~ ~ ~ ~ ~ ~

(2.5)

The concentration profile is represented graphically in Fig. 2.2-3b. As can be seen from the Fig. 2.2-3a and 3b there is no diffusion profile which has its con-

2.2 Basic Technical Processes

33

Fig. 2.2-3 Doping profiles in a solid state body by a) constant-surface-concentration diffusion, b) constant-total-dopant diffusion, and c) ion implantation.

centration maximum below the surface, as this would be a violation of the first law of thermodynamics.

Ion Implantation During ion implantation ionized doping atoms are electrically accelerated at room temperature (i. e. in a thermodynamic non-equilibrium) with energies of some keV up to 1 MeV and are shot into the crystal (Fig. 2.2-4). Penetration depths of 10 nm to about I pm can be achieved. At certain orientations of the crystal with respect to the direction of the ion beam, the crystal is more transparent to incident ions than is described by the statistical scattering theory. The ions can penetrate deeply into the channels in the lattice of the crystal at these special crystal orientations. This is called the channelling effect. Common ions used in the manufacture of semiconductors are e. g. B’, P+ and As+. 0 ’ and Ff are known as “special ions”. In the manufacture of semiconductors usual ion energies are 200 keV, but energies of up to 1 MeV or multiple charged ions can be used. The apparatus used can be separated into low and middle current implanters (ion currents up to 2 mA), and high current implanters for currents up to 25 mA. For more precise and low doping of up to about IOl4 atoms/cm3 the technology of ion implantation is applied. For higher doping, deeper penetration, and comparatively “crude” patterns a diffusion process is more cost effective. As most of the implanted ions end on arbitrary interstitial sites of the crystal, every implantation is followed by an annealing process which places the dopants into predetermined lattice sites and activates them electrically. The annealing is

34

2 The Parallels to Microelectronics

+200 kV DC

+I5 0 kV DC

Analyzer magnet

Aperture

Electrostatic

Fig. 2.2-4 Principle of an ion implanter. Ions are generated by means of a plasma. Ions with different mass and charge are separated by a mass spectrometer and then accelerated toward the target (silicon wafer). Usually the ion beam is scanned across the surface of the wafer.

carried out with silicon usually at temperatures between 900 "C and 1000 "C. Standard annealing times are between 10 and 30 minutes. In Fig. 2.2-3c a typical doping profile manufactured by means of implantation is shown. Independent of solubility limits and thermodynamic equilibrium, it is possible by ion implantation to produce practically any desired doping profile (also buried profiles) with almost any element. Therefore ion implantation is a useful tool for research and development of novel products. However, ion implanters together with equipment for lithography represent the largest investment in a semiconductor production line. One disadvantage of ion implantation is the contamination of the wafer by extraneous sputter products. The accelerated ion beam has a considerable sputter effect on the aperture and other internal pieces of the apparatus. Consequently the undesired impurities precipitate onto the single crystal wafer. One solution is to redesign the beam path: all components that the ion beam "sees" must be made of highly pure silicon.

2.2 Basic Technical Processes

2.2.4

35

Etching (Film Removal)

Until the late 70’s etching techniques were carried out by a wet chemical process in a dipping bath down to a structure size of 3 pm. The wet chemical etching process of amorphous, polycrystalline, and single crystal materials is basically isotropic i e. the etch attack is uniform in all directions. The resulting etch profile through a small opening in the resist mask is a spherical cavity. This process requires a precise control of the etch parameters and time in order to maintain the optimum between over etching and under etching of the wafer surface which is partially covered with resist. With ever smaller structures it becomes increasingly more difficult to maintain this optimum. On the structural level under 2 pm, the wet chemical etching has been taken over by a directional dry etching technique. In a diluted gas atmosphere with a relatively large mean free path of the particles, it is possible to accelerate ions electrostatically in a preferred direction, which results in the anisotropic etching process on the surface of the wafer. For liquids it is not possible for such an oriented transport process because of the short mean free path of the particles. A special process which plays only a secondary role within microelectronics is wet chemical anisotropic etching in which the removal rate of Si by alkali etching agents depends on the crystallographic orientation of the crystal to be etched. This latter process forms the basis of silicon micromechanics and is explained in detail in Chapter 6. The above mentioned etching principles can be classified according to the degree of anisotropy and selectivity of materials. The main processes are: 0

Plasma etching and barrel etching. The etching effect stems mainly from the chemical reaction of reactive species i.e. radicals which are formed by plasma discharge. Due to the relatively high working pressures the etching process is highly isotropic. By choosing the appropriate radicals the process can be made highly selective.

0

Reactive ion etching (RIE). This is a combination of physical and chemical processes. In the diluted plasma, an oriented movement of the highly energetic reactive species is maintained at right angles to the etching surface by an accelerating voltage.

0

Sputter etching or ion milling. This can be considered as a purely physical process. Chemically inert ions e . g . rare gas ions, are produced in a plasma and are accelerated onto the substrate with the aid of an electric field and give rise to physical sputtering.

These processes cover the full range in etch characteristics; plasma etching is isotropic but can be made highly selective, sputter etching on the other hand is strongly anisotropic with reduced selectivity, whereas RIE is a process in between, with medium anisotropy and fairly good selectivity.

36

2 The Parallels to Microelectronics

2.3 Packaging Technology All technological steps to manufacture systems and system subgroups by integrating components of different technologies onto one substrate or one housing are summarized under the term packaging technology. Under this technology the die-, wire-, flip-chip- and TAB-bonding, the encasing technology, and the PCB (printed circuit board) technology are categorized.

2.3.1 Requirements for Packaging Technology Modern packaging techniques describe a key technology for the development of highly integrated systems. By reducing the size of the structure, an increase in functional density has come about and at the same time the speed of signal processing has increased enormously. The VLSI (very large scale integration) circuits are characterized by a high working frequency, a short pulse rise, but also high power loss and numerous connection wires to the exterior. Considering circuits in communications technology, as well as gate arrays, standard cells and microprocessors, there is an increasing number of input and output signal junctions to control. A modern gate array normally has a chip area of 128 mm’, an internal lagtime of less than 150 ps, 20 W of energy dissipation and up to 320 junctions. The relatively large chip area, the high density of contact areas on the chip, and the increase of power loss puts the highest demands on the packaging approaches. By integrating a large number of logical functions, an isolated integrated circuit may play a decisive role in the total function of a complicated piece of equipment. At the same time the requirements increase for the highest reliability of integrated circuits. Modern integrated circuits can contain more than one million transistors. The dissipation of energy from integrated circuits increases with the number of transistors and the working frequency. Although the power loss per transistor could be decreased by applying new technologies, it increased on a per chip basis. In the field of communications technology ICs with more than 200 W of energy dissipation are anticipated. The power dissipation in a chip brings about a rise in temperature which leads to an exponential increase of the failure rate. Therefore the temperature of the chips must be kept low by lowering the heat transfer coefficient to an adjacent heat sink. A further cause for failure is mechanical stress. The empirical relationship for failure fractures in soldered junctions is: the number of temperature cycles which fail 50 % of the investigated objects, is inversely proportional to the square of the stress of the electrical contact. This is an important condition for the packaging technology: the material used for connecting the parts should be able to remove any hot spots immediately and the thermal expansion coefficient of all contributing components should be compatible in order to keep the shear stress on the junction points as small as possible.

2.3 Packaging Technology

37

chip I

hpack IC

SMD-components

encapsulated module

Fig. 2.3-1 Process sequences of packaging techniques for electronic hybrid fabrication.

2.3.2 Hybrid Technology With hybrid circuit integration the components, which have been manufactured using different technologies, are assembled to the system using joining techniques. A scheme of hybrid technologies is shown in Fig. 2.3-1 which are necessary for the assembly of a piece of equipment.

Screen Printing (Thick Film Technology) An important component of hybrid technologies is screen printing. The layers are brought onto a ceramic support by means of a screen printing process and then fused in. The technology of screen printing is very old, especially for decorative purposes. However, the dimensions of the decorative screen printing are orders of magnitude larger than those required for microelectronic circuits. Typical materi-

2 The Parallels to Microelectronics

38

als used are A1,0, ceramic substrates and various screen printing pastes. Prints for electronic circuits must be manufactured reproducible to about 50 pm with layer thicknesses of 1 to 80 pm. During the screen printing process a viscous material (the paste) is pressed onto a patterned screen and worked through the meshes by means of a squeegee (doctor blade). The structure of the patterned screen is thus copied by the paste onto the substrate underneath the screen. One of the essential processing steps in the production of a thick film circuit is the firing, which enables the electronic properties of the layers to be determined. Continuous furnaces come mainly into use, so that the necessary firing conditions can be satisfied at high throughput. As screen printing also plays an important role in microsystems technology it will be dealt with in Chapter 9 in more detail.

Placement and Soldering of Circuits Within hybrid technologies surface mounted devices (SMD) are today’s state-ofthe-art. However, the automated machines are relatively elaborate and expensive for small batches. In principle it is possible to handle the chips with tweezers but the small dimensions of the former make this an extremely laborious and cost intensive task. Two basic processes are used to connect the SMD components to the circuit boards: 0

reflow soldering for general application,

and the more recently introduced process: 0

laser soldering for thermally sensitive components.

Not every joining process is equally well suited for hybrid technology. The average wiring and contact density increases considerably with SMD with the consequence of large numbers of connections and partly, very small contact areas. In the manufacturing process a change from traditional soldering processes to new technologies is required.

Mounting and Contacting of non-encapsulated Semiconductor Chips In contrast to encapsulated semiconductors, whose junctions are connected to the substrate both, electrically and mechanically at the same time, two process steps are necessary with the chip-and wire-technologies. The first step serves to mechanically fix the semiconductor to the substrate, the subsequent process establishes the electrical connections. Besides the mechanical strength, the chip-substrate-connection must also be thermally and electrically conducting, in order to be able to dissipate the energy and to keep the chip at a defined electrical potential. A particular problem of semiconductor mounting illustrates the adaptation of the different temperature expansion coefficients of the materials to be joined.

2.4 Clean Room Techniques

39

After the mechanical fixing of the semiconductor chip onto the substrate, the electrical connections between the semiconductor and the interconnects of the circuit are carried out. In wire bonding thin wires are welded with the connector pads either by: 0 0

thermosonic welding, ultrasonic welding or thermocompression welding.

As the chip-and-wire technology is an important process for microsystems technology, further details are discussed thoroughly in Chapter 9. The chip and wire technology is sequential. In parallel, mass production processes were developed, with which a component can be mounted and connected in one procedure. These simultaneous contact processes are:

0 0

flip-chip-bonding, tape-automated bonding (TAB), isotropic adhesion, anisotropic adhesion.

These processes will again be dealt with in Chapter 9.

2.4 Clean Room Techniques For manufacture of structures in micro- or even sub-micrometer-regions, leading to yields for individual processes of 99.99% or more, the surroundings of the manufacturing area must be scrupulously controlled. This includes the room temperature, the air humidity and above all the particle density in the air and in the media being used. If one contaminant particle of 0.5 pm settles on a critical part of an integrated circuit, it can already cause considerable damage if not lead to failure of the circuit. Semiconductor technology, as well as microstructure technology and clean room techniques are therefore concepts that are inseparable with each other. Figure 2.4-1 shows the general concept of a clean room. An atmosphere with a low number of particles is of utmost importance and hence there is a constant exchange of “contaminated” air with recovered air free of particles. High quality clean rooms are operated in laminar flow. Air turbulence increases the retention period of particles in the laboratory surroundings. The actual clean room is a shell-type construction surrounded by a second enclosure, the “grey room”, in which the airflow is freshly prepared with respect to temperature and air humidity. After the addition of fresh air, it is pushed through the filter ceiling into the clean room. The air which flows laminarly at right angles to floor of the clean room is forced through a perforated flooring and transported through air channels again to

40

2 The Parallels to Microelectronics ke

Filter

Elevated floor

Fig. 2.4-1 The general concept of a high quality clean room with laminar flow.

the processing room above the filter cover. The air effluents of certain chemical processes are drawn directly from the process stage and separately disposed of. The circulating air is mixed with fresh air, humidified, temperature controlled and fed back into the pressurized room above the filter ceiling. Depending on the pore size of the filter, the air turnover and other parameters of the manufacturing environment, an atmosphere is obtained which is categorized into quality classes based on the amount of suspended particles. Table 2.4-1 shows the clean room classes according to the US federal standard 209b. The classification is given by particle size and number of particles per cubic foot. Table 2.4-1 Clean room classification as of the US federal standard 209E (particle concentration: (Particles per cubic foot) Clean room Concentration limits (particles/ ft3) 2 0.2 pm 2 0.3 pm classification 2 0.1 pm

1 10

100

1000 10 000 100000

not not not not

35 350 defined defined defined defined

7.5 75

750 not defined not defined not defined

3.00 30.0 300 not defined not defined not defined

2.4 Clean Room Techniques

41

In the general tendency toward metrification the cleanroom classification is about to be changed too. A new release of I S 0 standards 14644-1 and -2 has taken place [Dono99]. The new standard is shown in Table 2.4-2.

Table 2.4-2 ISO/TC209 14644-1 Airborne particulate cleanliness classes -~

Clean room

Concentration limits (partic1es/m3)

24 237 2 370 23 700 237 000

10 102 1 020 10200 102 000

4 8 35 83 352 832 3 520 8 320 35 200 83 200 352 000 3 520 000 832 000 35 200 000 8 320 000

29 293 2 930 29 300 293 000

The reference particle in Standard 209E is 0.5 pm. The number of particles L 0.5 pm in one cubic foot determines the class. This is different in the new class definition of I S 0 14644. The reference particle in this case is 0.1 pm and the classification is given by the power of 10 of particles 2 0.1 pm per m3. How does the old standard correlate to the new one? Class 1 of the old standard allows 1 particle per cubic foot. This corresponds to about 35 particles (20.5 pm) per m3. Applying the power law size distribution of Eq. (2.6) yields to 1000 particles of size 0.1 pm and larger: particle diameter I1 particle concentration I particle concentration I1 = (particle diameter I

(2.6)

with particle diameter I > particle diameter 11. The corresponding 1000 particles of diameter 2 0.1 pm can be found in Table 2.4-2 at I S 0 Class 3 . The comparison between old classes (209E) and new classes (IS0 14644) are listed in Table 2.4-3.

2 The Parallels to Microelectronics

42

Table 2.4-3 Correlation of old standard 209E to new standard I S 0 14644

FS 209E Class Class Class Class Class Class Class

I S 0 14644

IS0 IS0 IS0 IS0 IS0 IS0 IS0

0.01 0.1 1 10 100 1.000 10.000

Class Class Class Class Class Class Class

1 2 3 4 5 6 7

In Fig. 2.4-2 the classifications are graphically represented. The shaded area corresponds to the standard ambient environment. Even with predetermined air flow and filter type, the clean room quality or the particle precipitation on a surface is not constant but changes enormously depending on the exterior circumstances in the working area. A large source of unwanted particles in a clean room is generated by the working person. It is therefore necessary to protect the environment of a clean room from contamination generated by people. This corresponds to a dust free clean room and a disciplined behavior pattern e. g. avoiding hasty movements and refraining from wearing cosmetics such

1o7 1o6 1o5

1o4 1o3 1o2

10' 10" I 0.1

I

I

I

I

I

I

I

I

I

1

Particle Diameter [pm]

Fig. 2.4-2 Clean room classes after IS0 14644-1.

1

I

I

I 1

10

1OD

2.4 Clean Room Techniques

43

as face powder etc. Despite clean room apparel according to regulations, undisciplined conduct can increase the particle density by an order of magnitude. Hasty movements and wearing everyday clothes are expected to lead to a deterioration of two orders of magnitude. Machines, which are necessary for the process, are, of course, sources of particles, too. These machines are integrated in the clean room in such a way that, the “dirty” parts of the machine (e.g. gears, motors etc.) are placed in the grey room which is separated from the clean room. At the same time these machines can be serviced from the grey room and if necessary replaced with minimum disruption of the clean room atmosphere. This grey room has a reduced “cleanliness” and serves as a gate or buffer between the clean room and the outer atmosphere. The air must be continually replaced in order to be able to carry the particles away quickly which have been generated in the work area. The retention period of a particle is lowest, if it is flushed via the shortest route from the room. This is the case when operated in laminar flow. The higher the classification of the clean room, i. e. the higher the quality, the more frequent the air must be exchanged. With laminar flow, an average flow speed of 0.45-0.5 d s e c is necessary. The necessary air regeneration and input of added fresh air, temperature control and regulation of the air humidity is considerable. Table 2.4-4 shows some guidelines for the airflow depending on the cleanroom class [Whyt99].

Table 2.4-4 Guidelines for volume flow Clean room classification FS (209E)

Volume flow [m3h.m2]

10 100 1 000 10 000 100 000

1600-1 800 1600-1 800 700-1 100 60-120 60

A clean room of class 100 of 100 m2 of working area requires a turnover of up to 180000 m3 per hour of temperature controlled, humidified and purified air. A clean room is thus not only a considerable investment but also involves very high running costs. In order to lower the costs, especially in laboratory establishments where an optimum yield is not the priority, as is the case for manufacturing processes, clean rooms of a lower quality are installed (about class 10000). Clean air is blown into the clean room via turbulent flow inlets. To improve on the air quality for critical processes in limited areas additional “clean rooms within clean rooms”, socalled “clean benches” are installed. A clean bench installed in a clean room of class 10000 usually yields to a working area of class 100.

44

2 The Parallels to Microelectronics

With increasing purity demands (in the sub-micron technology clean room classes of 1 or even 0.1 are referred to), requirements such as wearing protective clothing and the behavior of the personnel are so critical that in reality it is no longer reasonable to expect a person to work in such an area. Newer concepts therefore include a personnel free environment and an automated or externally controlled process.

3 The Physical and Chemical Basics in Microtechnology

The modern technologies have come from a broad theoretical basis, without which manufacturing would not have been possible. They differ considerably from the traditional processes, which were developed empirically over decades or even centuries. Microelectronics is not imaginable without a profound knowledge of theoretical physics and, in particular, quantum physics. Therefore, for the microelectronics or microsystems technology engineer it is not just an intellectual exercise to explain the manufacturing processes with principles of theoretical solid state physics and in particular with quantum physics, but a necessity in order to understand, control and finally to optimize the manufacturing processes. It would go beyond the realms of this book to go into the basics of quantum mechanics. It is strongly recommended though to study the behavior of electrons in the field free space, in an periodic potential, and the meaning of electronic bands and band gaps in order to understand the basic characteristics of semiconductors. The crystalline state plays a dominant role in semiconductor technology. Silicon crystals are the material basis for the integrated circuit industry due to their special electronic properties. Since the publication of the famous paper of K. E. Peterson "Silicon as a Mechanical Material" [Pete821 also the unique mechanical properties of silicon have been recognized and present the basis of micromechanics. In the following section an introduction of crystallography with the emphasis on cubic crystals and especially silicon crystals is given.

3.1 Crystals and Crystallography A crystal is a solid body and consists of periodic repeats of a structural unit in three dimensional space. This structural unit can exist of a single atom or also of the most complex macromolecules each containing thousands of atoms. To understand the basic rules of crystallography some simplifications will be introduced:

46

3 The Physical and Chemical Basics in Microtechnology 0

The crystal is infinitely extended so that the boundary effects do not need to be considered.

0

The atoms or molecules of the crystal are first of all reduced to one point. Thereby a mathematical abstraction of the crystal is obtained and referred to as “lattice”. The physical model of the crystal is easily obtained, in which one assigns the points of the lattice again as atoms or molecules, which are also referred to as “bases”.

One can also formulate: Lattice (mathematical abstraction) = crystal (physical realization).

+ bases (atoms, molecules)

3.1.1 Lattice and Types of Lattices For each lattice one can construct a coordinate system, whose axes run parallel to the lattice vectors a,, a2, a3. These vectors run from one lattice point, which is arbitrarily chosen as the origin (in reality there is no origin in the infinitely extended lattice) to the next neighboring lattice points in the three directions 1, 2, and 3. One refers to the parallelepiped, with the lattice vectors defining up the sides, as a unit cell of the lattice. In other words: The smallest possible parallelepiped which is formed by the lattice vectors is called a unit cell. The unit cells by continuous repetition fill the infinite space of the ideal crystal completely. The unit cell which contains only one atom is called a primitive unit cell. The primitive unit cell of a given lattice can also be found by following the specification: from one lattice point connecting lines are drawn to the neighboring points. Now one establishes planes which are half way and perpendicular to the connecting lines. The smallest volume that can be surrounded by the planes is called the Wigner-Seitz cell and is the primitive unit cell. In Fig. 3.1-1 this specification for the two dimensional case is graphically shown. Primitive unit cells are not always found, namely if the crystal has a complex “inner structure“. Later in this section we will learn, that for example a body centered cubic crystal consists of 2 lattice points per unit cell. This is then referred to a non-primitive unit cell. Starting from any arbitrary unit cell within the crystal any other unit cell can be reached by a translation vector r with:

where ui must be whole numbers.

3.1 Crystals and Crystallography

a Fig. 3.1-1

47

b a) Construction of the unit cell by Wigner-Seitz. b) The unit cells coves entirely the whole surface (or the whole space respectively.

The number of possible lattices is limitless as there is no boundary with regard to the length of the vectors al, a,, a3 and the angle, which they form to each other. However, if one divides the angle- and length ratio into groups, which are invariant with regard to the possible symmetry operations, then according to Bravais one obtains only 14 lattice types which are given in Table 3.1-1. Table 3.1-1 Lattice types according to Bravais Designation of system

Lattice vectors and angular ratio

Number of possible lattices

Triclinic

al # a2 # a3

1

U # B # Y

Monoclinic Orthorhombic Tetragonal

al # a2 # a3 u = y = 90" # p al # a2 # a3 a = 0 = y = 90" al = a2 # a3 = p = = 900

Cubic Rhombohedra1 Hexagonal

2

4

2 3

al = a2 = a3 u = = y # 90" al = a2 # a, cx = = 90" y = 120"

1 1

48

a

3 The Physical and Chemical Basics in Microtechnology

b

C

Fig. 3.1-2 The three types of the cubic lattices: a) the simple cubic (sc),

b) the face centered cubic (fcc), and c) the body centered cubic (bcc).

In the following the cubic lattice type will be dealt with exclusively because the relevant materials for microtechnology, such as silicon and gallium arsenide, crystallize in the diamond lattice and so are cubic. As one can see, the cubic lattice consists of 3 variations (Fig. 3.1-2): the simple cubic lattice (sc), and the face centered cubic (fcc) the body centered cubic (bcc). The simple cubic has a primitive unit cell. On each corner point of the cube an atom is found, which is claimed uniformly from 8 neighboring unit cells. The number of atoms per unit cell is 8 . 1/8 = 1. The body centered cubic has the same number of corner atoms and in addition the body centered atom, i. e. two atoms per unit cell. Here we have a non-primitive unit cell. For the face centered cubic lattice the number is: 8 . 1/8 corner atoms and the 6 face centered atoms are claimed by two neighboring cells (6 . 1/2). The total number per unit cell adds up to 4.

3.1.2 Stereographic Projection In order to work with a crystalline polyhedron it would be very inconvenient to describe the crystal verbally in such a form as: The face a is at an angle of x degrees to face b and at an angle of y degrees to face c and so on. Instead of this a simple tool was developed for characterizing the typical microscopic and macroscopic shape of a crystal: The stereographic projection. One imagines a sphere into whose center the crystal polyhedron to be determined is placed. First the surface normals of the polyhedron are established, which cut through the sphere at certain points. The intersection points are referred to as the poles of the surface. In this way the surfaces of the crystal are transferred to poles onto the sphere's surface. For easy handling of such a projection, it must be transformed a second time into a plane surface. There are different methods to project a sphere's surface into a plane as can be seen in any geographical atlas. With the stereographic projection, the equatorial plane of the sphere is chosen as the projection planes (Fig. 3.1-3). The "south po1e"S of the sphere will be assumed to be the central point. If the surface pole P is connected to the south pole S , the straight line cuts through the equatorial plane at pole P'. The projection onto the equatorial plane is

3.1 Crystals and Crystallography

49

tN

Fig. 3.1-3 The stereographic projection.

finally used to characterize the shape of the crystal in the center of the sphere. At the double transformation the angle @ is invariant with this projection, the pole distance p is changed to p’ = R . tan pI2, where R is the radius of the sphere. With the stereographic projection, the upper half of the pole sphere falls within the equatorial circle, the lower half falls outside of this circle. Therefore as a rule the “southern“ half sphere is projected onto the north pole. The circles on the sphere whose centers coincide with the center of the sphere are referred to as “great circles” and play a special role. The great circle is the geometric site of the poles of all surfaces, whose normals lie in one plane (namely the plane of the large circle). A certain set of these surfaces one calls a zone. The surfaces which belong to one particular zone are called tautozonal. A grid of meridians and parallels of latitude are used for practical work with the stereographic projection. The projection of this graduated grid is called a Wulff‘s grid (Fig. 3.1-4). With this, one can place each pole according to @ and p into the grid. The value of the stereographic projection can be easily recognized from the following example. In Fig. 3.1-5a an unknown crystal is represented with some of its surfaces. The question is, are the surfaces tautozonal and which angles do they form with each other? By applying the Wulff‘s grid and rotation around the center, one can try to bring the poles to coincide with a great circle (Fig. 3.1-5b). In the example shown, the poles P I , PI and P3 lie on a large circle and so are tautozonal. The separation between the angles of the tautozonal poles can be counted simply with the aid of the grid. The angles between two arbitrary surfaces can always be determined, if both poles are aligned with a large circle.

3 The Physical and Chemical Basics in Microtechnology

50

Fig. 3.1-4 The Wulff's grid, a convenient tool for the evaluation of the stereographic projection.

For many purposes, it is useful to characterize the planes in the crystal by indices. For that the following guidelines developed by Miller are useful: 0

Determine the intersection points of the plane with the lattice vectors al, a2 and a3 and express the result in units of the lattice constant al, a2 and a3.

0

Form the reciprocal of these numbers. In case a whole number is not obtained the numerical triad is multiplied with the smallest common denominator. The result is put in brackets: (hkl) and is called Miller index.

v2

For one plane with intersection points 3, 1, 2 become the reciprocal $, 1, and the Miller indices (26 3). If the intersection points lie in infinity, the index associated with it is zero. This is the case, when a plane is parallel to one or two of the crystallographic axes. In Fig. 3.1-6 the Miller's indices of some planes in the cubic crystal are shown. The indices (hkl)do not specify an individual plane, but denote a whole family of planes which are parallel to each other, and are by definition indistinguishable from each other. The normal direction of these planes is given by the Miller indices too, which correspond to the relation of the three cosine directions of the normal vector:

h:k:Z = cos a:cos p:cosy

(3.2)

In this case the Miller indices are put in square brackets: [hkl].To indicate that a family of planes extends in the negative direction of the crystallographic direction or to indicate that the normal vector should point in the reverse direction, the

3.1 Crystals and Crystallography

51

Fig. 3.1-5 a) The stereographic projection of an unknown crystal, b) the evaluation of the crystallographic planes using the Wulff's grid. It is obvious that the three planes marked with circles are tautozonal.

corresponding Miller indices are marked with a minus sign over the index: (hkl) (Fig. 3.1-7). With this convention it is possible to mark the six (physically identical) surfaces of a cubic crystal: ( 100) (010) (001) (100) (010) (001). Due to the nature of the cubic crystal all the above listed families of planes are identical, i.e. indistinguishable from a physical point of view. This fact can be indicated by curly brackets around the Millers indices; the group of cubic surfaces is therefore characterized accordingly by { loo}.

52

3 The Physical and Chemical Basics in Microtechnology

z

A

(200)

(iO0)

Fig, 3.1-6 The definition of the Miller indices: define the intersection points of the planes to the three coordinates and calculate the reciprocals. Multiply the three figures in such a way, that three integers are received.

Fig. 3.1-7 Miller indices never indicate a specific plane, but always a family of planes. Negative indices indicate, that the normal vector is pointing into the reverse direction.

3.1 Crystals and Crystallography

53

3.1.3 The Silicon Single Crystal The silicon single crystal plays a dominant role in microelectronics as well as in microsystems technology. Therefore, the silicon lattice, which is identical to the diamond lattice, will be considered next in some detail. The construction of the silicon lattice can be thought of in two different ways: Method 1 Two face centered cubic lattices are brought together completely to self-coincidence. The second lattice is then extended along the space diagonal by a quarter of the length of the diagonal. Thus the silicon lattice is obtained as illustrated in Fig. 3.1-8a.

a

b

C

Fig. 3.1-8 a) The design of a silicon crystal: Two face centered cubic crystals are put together, then both lattices are pulled apart along the diagonal to a distance of '/4 of the diagonal. b) The second method to design a silicon crystal: Take a face centered cubic crystal, and replace each lattice point by a basis of two atoms, /4. with the two positions O,O,O and %, c) Some tetrahedric elements are graphically emphasized in the crystallic structure.

v4,

54

3 The Physical and Chemical Basics in Microtechnology Method 2

One starts again with a face centered cubic lattice and replaces every starting point by bases of two atoms. By so doing the one atom takes up the position k,,'/4 relative of the original point, whilst the second atom has the coordinates to the first (Fig. 3.1-8b).Visualizing this structure in three dimensions can still be demanding to the observer despite its relatively simple geometric arrangement. Finally Fig. 3.1-8c illustrates the tetragonal structure which leads to the chemical stability of the silicon. In the illustration, some tetrahedra are graphically emphasized. One can easily find more tetrahedra in the crystal. These are physically as well as chemically particularly stable building units, which explain the exceptional hardness of diamonds. The figures suggest anisotropy of properties, depending on the direction of viewing. Also the anisotropy of the physical and chemical parameters is an important characteristic of the crystal. In Fig. 3.1-9a one looks at the crystal in approximately the [loo]-direction i. e. in the direction of the normal of a (100)-plane. The cubic structure can be clearly recognized. In Fig. 3.1-9b the observer looks in the direction [110] into the crystal. In this direction a hexagonal structure appears in the foreground. Channels exist in the crystal which can be penetrated very deeply by implanted ions. This

v4,

a

b

C

Fig. 3.1-9 The silicon crystal in a) (100) view, b) in (110) view, and c) in (111) view.

3.1 Crystals and Crystallography

a

55

b

Fig. 3.1-10 The stereographic projection of silicon a) in (100) view and b) in (110)

view. effect is known as "channeling". It is either used intentionally, in order to place ions deeper into the crystal or intentionally avoided, in order to achieve as shallow a doping profile as possible. Figure 3.1-9c displays the crystal in (111) view with a perfect hexagonal structure. In Fig. 3.1-10 the stereographic projection of a silicon single crystal in both orientations, (001)-normal in the direction of the north pole and (110)-normal in the direction of the north pole, is shown. In part a) we see first of all by means of the Wulff's grid, that all 8 cubic surfaces (100) lie at 90" to each other. The { 111) surfaces form an angle of 54.7" to the { O O l } direction, however an angle of 90" among one another. In part b) of the stereogram, we recognize the central (110) direction and also at 90" the { 111) surfaces, which however do not form 90" angles among each other. We will return to these details with anisotropic etching of silicon single crystal wafers. They play a decisive role in structuring microbodies in the context of silicon micromechanics.

3.1.4 Reciprocal Lattice and the Analysis of the Crystal Structure The diffraction of particles (electrons, protons, neutrons) and photons (X-ray quanta) is a method for the structural analysis of an unknown crystal. These particle beams (waves) are diffracted by the unknown structure and can interfere with one another. The particles in this case appear wave-like in nature and can therefore interfere with each other. If they are in phase, they amplify each other and cause an increased intensity. If they are out of phase, then on average they cancel each other out, and the intensity becomes zero. If two waves originate from the same source, then the phase ratio depends only on the path length of both waves.

56

3 The Physical and Chemical Basics in Microtechnology

In a lattice we consider the lattice planes, which are parallel to each other, as reflection planes, where every plane reflects a part of the incident light intensity with regard to the reflection law (angle of incidence = angle of reflection). On constructive interference (i. e. a bright reflection), all reflected particle beams must fulfill Bragg’s condition (see Fig. 3.1-1 1):

2d . sin @ = nA

(3.3) with n = 1, 2, 3... the order of the reflection and d the distance between two planes. As sin 0 is always i 1, then the maximum wavelength which still fulfills Bragg’s condition is: Amax = 2d. Since lattice spacings of crystals are of the order of a few A, it is not possible to carry out any structural analysis on the crystal using visible light, since it has wavelengths of some thousand A, but instead, X-ray radiation or particle radiation is required. The wavelength of electrons with energy of I keV amounts to just 0.39 A. Next we will try to develop the elemental Bragg’s condition in several different ways. A monochromatic plane wave falls on a crystal whose lattice is defined by the translation vector:

Next the diffraction image of a lattice will be investigated. The scattering is assumed to be elastic i.e. the energy and frequency of the incoining beam is assumed to be the same as that of the scattered. Incident

Reflected

Lattice planes

Fig. 3.1-11 The graphical representation of Bragg condition for constructive inter-

ference.

3.1 Crystals and Crystallography

57

Deflected sectioinal beams

Incident beam

k

Fig. 3.1-12 The Bragg condition represented in vector algebra.

The incident beam is described by the wave vector k and the angular frequency w. Likewise it is valid for the diffracted wave k’ and w“. The k vector is perpen-

dicular to the wave front of the beam, and its quantity is:

Also is w = u’, because elastic scattering is assumed. The beam is represented by: E = &sin (kx - ut) or:

For constructive interference the phase difference between the scattered sections of the beam must amount to a multiple of 2n, otherwise they cancel each other out. Let us consider an incoming beam with a wave vector k, which is scattered partially at lattice point P and partially at point P’ (Fig. 3.1-12). Both sections of the beam are scattered toward the screen in the direction k’. The distance between P and P’ is r (the translation vector). How much is the phase difference of the two partial beams in the direction of k’? The partial beam, which is scattered at P’ has to travel longer in the direction of k in order in k’ direction. to reach P’ by a distance of ~, but gains the distance of Ikl The total path difference between the two partial beams is then (considering Eq. 3.4):

58

3 The Physical and Chemical Basics in Microtechnology

(k - k')r - Akr __ -

Ikl

Ikl

For constructive interference on the screen, the path difference of the partial beams have to be multiples of the wavelength:

Ak . r

=n

.A

(3.9)

lkl

(3.10) Equation 3.10 is another formulation of the Bragg condition. If these conditions are not maintained, then the phases of the partial waves are distributed uniformly across the whole interval (0, 2n), i. e. the partial waves extinguish each other. In the following the Bragg condition will be represented in still another elegant way. For that the concept of reciprocal lattice must be introduced. The reciprocal lattice is a translation lattice with the lattice vectors bk,which are connected to the basic vector ai of the actual lattice in the following relation: (3.1 1) The lattice vectors of the reciprocal lattice are formed according to the following procedure:

bl = 2 n .

a2 a3 a1 . a2 X a3

and so forth.

(3.12)

Vector bl is orthogonal to a, and a3. The denominator is the so-called "parallelepiped" volume. It has the volume of the primitive unit cell of the crystal lattice and has thereby the unit [length3]. The cross product in the numerator represents a surface [length2], which is stretched by the vectors ai and ap The reciprocal lattice vector has thereby the dimension [length-']. The translation vector of the reciprocal lattice has the general form:

g=ck=, 3

lkbk with integer lk

What seems to be the motivation to introduce a somewhat difficult to perceive reciprocal lattice? Without it being proven here, the real lattice is transformed in the Fourier space with the formulation law (Eq. 3.10). Each translation vector g of the reciprocal lattice represents a possible constructive diffraction image of the real lattice. The transformation of the real lattice in the reciprocal lattice is therefore as logical as the transformation of a real image into the Fourier space. Here the reciprocal lattice should be seen as a mathematical supportive formulation, with which one can elegantly explain the diffraction phenomena of the crystal. As a product of a translation vector r of the real lattice with the translation vector g of the reciprocal lattice, and under consideration of Eq. 3.11 we obtain:

3.1 Crystals and Crystallography

59 (3.13)

with m = u,l,, u212, uJ3. If we now return to the Bragg’s condition in Eq. 3.10 and multiply both sides by the corresponding translation vector of the reciprocal lattice g, then we obtain:

Note that m and n are not constants but running numbers. For all values of n there is a matching m, and the condition n/m = 1 can be fulfilled. With this the shortest and most elegant form of the Bragg’s relation is obtained:

Ak=g

(3.15)

A graphical interpretation of the interference conditions is the Ewald-construction (see Fig. 3.1-13). One depicts the reciprocal lattice of the lattice in question and inserts the k vector such that its vertex points to a lattice point. Now a circle is drawn around the base point of the vector with the radius I k I. If this circle cuts any other lattice points of the reciprocal lattice, then constructive interference reflections are possible. The vector k’ would then be inserted into the graphic representation such that it has a common basis with k and its vertex pointing to this second intersection point. If now with the Ewald-construction, the difference between the end points of k and k’ corresponds to a reciprocal translation vector, then under these conditions, we obtain a diffraction reflection. It was established above, that each reciprocal

Fig. 3.1-13 The Ewald construction at the reciprocal lattice for constructive inter-

ference.

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3 The Physical and Chemical Basics in Microtechnology

Fig. 3.1-14 The Laue method. The beam for investigation consists of “white” X-rays,

meaning a continuum of wavelengths with different lengths of k-vectors, but all in the same direction. translation vector represents a possible diffraction reflection. We thereby fulfill the Bragg’s condition (Eq. 3.16). With the Ewald-circle, the probability of intersecting another lattice point, is generally quite low. What can be done therefore in order to raise this probability? For this there are basically three methods available.

The Laue-Method Instead of a monochromatic wave, one allows a whole beam of different waves k,.ki..k, to enter into the crystal. For each ki we can construct an Ewald-circle. The sum of all circles then forms a region as is shown in the shaded area in Fig. 3.1-14. The probability of interferences is thereby considerably increased. To achieved this, we have lost knowledge of the exact wavelength. However, this method is very convenient for a first rough structural analysis.

The Bragg-Method Another possibility to raise the chance of the Ewald circle hitting one or more lattice points, presents itself if one turns the crystal relative to the wave vector k, as is shown in Fig. 3.1-15. In the process of a 360” rotation, a large area of the reciprocal crystal is scanned with the circle and numerous “Ewald conditions” can be found.

The Debye-Scherrer Method Instead of turning the single crystal, it can be pulverized, so that a light beam, which is represented by k, interacts with many crystallites which are now randomly oriented, that is, ordered in all orientations. There are always some which lie in the “Bragg orientation” (Fig. 3.1-16). This method is dealt with in detail in the next section, as it represents a simple method and is excellent for a first rapid structural analysis.

3.1 Crystals and Crystallography

61

Fig. 3.1-15 The Bragg method. The light is monochromatic, and the crystal is rotated during investigation.

Fig. 3.1-16 The Debye-Scherrer method. Many randomly distributed crystallites form a multitude of “Bragg-conditions”.

The Bragg relation is used to determine a diffraction direction only and does not allow a calculation of the intensity. The intensity of a reflection depends on the nature of the base of the crystal (that is the “physical” part of the crystal). To calculate the intensity one requires knowledge about the following parameters: 0

the arrangement of the atoms in the basis of the crystal and the corresponding binding forces,

3 The Physical and Chemical Basics in Microtechnology

62 0

the density distribution of the electronic cloud between the nuclei of the atoms,

0

and finally the oscillation of the atoms under the influence of the thermal energy.

Let the base unit consist of p atoms at the points:

+

(3.16) yja2 + zja3 with j = 1 , 2 , 3...p b; = xjal and xj, y,, zj fractions of the lattice vector. The spherical scattered wave which is emitted from the jth atom has the relative amplitude Aj. A, is also called the atomic structure factor. Its contribution to the diffracted beam (wave vector k’) is given by the phase difference bjdk or by the phase factor: ezbjhk

(3.17)

Therefore the scatter amplitude of the base makes a contribution which is given by the basis structure factor:

As mentioned above the contribution by the electron shell of the jth atom of the base is given by the atomic scattering factor AJ. Incidentally this scattering factor is different, depending on whether the diffraction is carried out with photons or particles, because the interaction with the atom is different depending on the nature of the scattered particle.

3.2 Methods to Determine the Crystalline Structure 3.2.1 X-ray Diffraction The possible reflections with the Ewald-construction were already mentioned briefly in the previous section and one can thereby raise the prospects of intersecting points of reciprocal lattices with the Ewald-circle, so that one either constructs a family of Ewald-circles with different diameters i. e. different k-vectors, or rotates the reciprocal lattice relative to the Ewald-circle. Another variation was developed by Debye and Scherrer, whereby the medium to be analyzed is a polycrystalline powder, in which the tiny crystals are available in all possible orientations to the investigating X-ray beam. A subset of it is always located in a position which fulfils the Bragg’s condition. The powder method is suitable for the qualitative analysis of a sample and for a first approx-

3.2 Methods to Determine the Crystalline Structure

63

imation of the size and symmetry of a unit cell and the average size of the crystallites; it can of course not compete with Bragg’s method, in which quantitative information about the electron density distribution in a single crystal can be obtained using monochromatic X-ray light. With the powder sample some crystals will be oriented such that, for example, their {loo} planes with the distance d,,, lead to a diffraction intensity within an angular distance 2@ from the original beam. This reflection condition is valid now for all crystal planes, which form an angle @ t o the optical axis of the probing beam. They lie rotationally symmetrical around the optical axis and thereby form a diffraction cone with the half aperture angle 2@. For other crystal planes {hkl} other aperture angles occur, so that eventually a Debye-Scherrer diagram with a family of diffraction cones inserted into each other results for all lattice planes. The original Debye-Schemer method is shown in Fig. 3.2-1. The sample is placed in a small tube that is rotated so that all random orientations of the tiny crystals are guaranteed. The diffraction cones are photographically recorded as arcs of a circle. In order to determine the angle @, one measures the reflections from the photographically recorded diffraction pattern. If the values for the numbers (h,k,Z)are known, then one can calculate dhwaccording to Bragg’s relation. In a cubic lattice with a unit cell of side length a, the distance between the planes is given by:

a (3.19) dh2 k2 12 From this follow the angles, where the diffraction conditions comply with the (hkl) planes: dhkl

=

+

A

sin@hkl= -2/h2 2a photographic film

beam

+

+ k* +

(3.20)

12

,diffraction cone

\

sample

Fig. 3.2-1 A Debye-Scherrer camera for crystal analysis.

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3 The Physical and Chemical Basics in Microtechnology

The possible reflections can be calculated, whereby h, k and 1 are replaced with numbers. However, not all whole number values for h2+,@+12 are present, as one can see in Table 3.2-1.

Table 3.2-1 The possible reflections and the associated sums of the squares of the Miller’s indices Miller Index

h2 + k2 + l2

It is apparent that 7 is missing and that 9 is present twice. The diffraction pattern has thus empty spaces, which are characteristic for simple cubic structures. The Debye-Scherrer powder diagram and its systematic spacings are schematically represented in Fig. 3.2-2. One should note however, that here only the positions of the theoretically possible reflections are represented, but not their intensities. If a theoretically possible reflection exhibits zero intensity it will not be detected. On the other hand it is not possible that a reflection appears at a place which is not given by the Bragg’s reflection condition.

3.2.2 Electron Beam Diffraction Besides analysis by photons, one can also use electron beams to analyze structures. Electron beams have much shorter wavelengths than photons (electrons which are accelerated to an energy of 10 keV, have a wavelength of 0.12 A). The interaction with the electron shell, as well as the nucleus of the atom to be analyzed, is much more intense. However, this has the consequence that the depth of penetration is smaller than with X-ray radiation. Therefore the potential applications and the preparation of the sample are significantly different. Electron beams are used preferentially to analyze thin layers, surfaces and gases. However, here also the Debye-Scherrer method is commonly used. Polycrystalline thin films of the material whose structure is to be determined are placed onto very thin amorphous carbon or A1,0, “carrier” foils. The experiment must of course take place in a vacuum (see Fig. 3.2-3).

3.2 Methods to Determine the Crystalline Structure n

0 0

2

nn

N r N r

2%

a simple cubic

b cubic bodycentered (h+k+l odd missing)

c cubic facecentered (h, k, I all even or all odd) Fig. 3.2-2 The theoretically possible reflections for the three cubic crystal types: a) simple cubic, b) cubic body centered, and c) cubic face centered.

Fig. 3.2-3 Debye-Schemer diffraction set-up for electron beams.

65

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3 The Physical and Chemical Basics in Microtechnology

With electron beam diffraction one can determine the distance between the atoms in a molecule. Likewise the unit cell parameter of the crystal can be determined easily with only a few atoms in the unit cell. However for a macromolecule the structural analysis becomes extremely complicated.

3.3 Basic Concepts of Electroplating In general, any chemical substance which is dissociated into ions in the liquid or solid state phase is called an electrolyte. Under the influence of an electrical field between two electrodes, submerged in an electrolyte these ions can carry electrical charges from one electrode to the other and thus generate an electrical current. If an ionic crystal, such as NaC1, is submerged in water it is dissociated into ions, Na’ and C1-. Since this process needs a high amount of energy, this can not be supplied by the thermal energy of the solvent. The decisive role for the dissociation is played by the hydrate molecules of the water which accumulate around the ions and thus deliver the necessary solvation energy. The ions with positive charge move towards the cathode of the electrolytic tank and are called cations, the negative ions are called anions. These ions are accelerated in the electrical field by a force:

where z = charge number and e = unit charge. Since the ions are not moving in a vacuum but in a liquid, with increasing velocity they are subject to a friction force which is proportional to the velocity of the ion. The friction force can be calculated with the Stoke’s equation:

with r = ion radius and r = viscosity of the solvent. Therefore after a short starting period these ions move with constant speed: (3.23) The so-called electrical mobility of the ions is defined only by the material parameters of the ions and the solvent: (3.24)

3.3 Basic Concepts of Electroplating

67

In the following we want to calculate the current in an electrolytic tank when voltage is applied across the two electrodes. We consider a tank with two electrodes in the distance 1 (Fig. 3.3-lc), The amount of substance of the electrolyte is defined as c = n/V For simplicity reasons we chose an electrolyte with only

a

4-1

b

Double layer

3 R-

RE

R+

Fig. 3.3-1 a) The electrolytic tank with its two electrodes (cathode and anode) and b) the equivalent circuit diagram. C, and C& are the capacitances of the cathode and anode respectively. c) Sketch of the electrolytic tank for the calculation of j, and j,.

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3 The Physical and Chemical Basics in Microtechnology

one cation with zf = 1, and one anion with z- = 1 per molecule. The current consists of all cations traversing an area A in the time t towards the cathode and all anions towards the anode. The number of cations participating in the current is then: c.NA.A.vt.twith the charge z+.e, and the amount of anions respectively: c.NA.A.V.twith the charge z-.e, with NA = Avogadro’s constant (NA = 6.02214 . mol-’). The total current is then:

I = -Q= N A .e . A . c . ( z + . v f + z -

. v-)

t

With E = U/l and NA.e= F (Faraday constant F Eq. 3.24 we can transform Eq. 3.25:

(3.25) =

9.648456.104 C mol-I), and

(3.26) The resistivity p of a conducting material is defined as (3.27) In the electrochemistry the conductivity 1 r c = - = F . c (u’.z+

+

u-

IC

.z-)

is the more preferred parameter: (3.28)

P

The question arises now whether Ohm’s law is applicable to a system like this or not. Even if the electrolyte could be considered as an Ohmic’s resistor, the whole system cathode-electrolyte-anode does not follow Ohm’s law, as can be seen from the equivalent circuit in Fig. 3.3-lb. At the interface electrode-electrolyte complex electric double layers are formed which act as nonlinear resistors in series with the electrolyte. The total resistance of the electrolytic system therefore sums up to: (3.29)

In order to measure the electrolytic resistance alone, alternating voltage is applied to the electrodes. At an appropriate frequency (usually 50 kHz) the capacitive resistance is shorted out, the electrode resistance R- and R f is high compared to the electrolyte and thus a fairly accurate measurement of the resistance or conductivity of the electrolyte is feasible.

3.3 Basic Concepts of Electroplating

69

a

Water maleeule Fig. - 3.3-2 a) The Hrater molecule with

[tive and negative charges and b) the preferred build-up of solvated cations with a sheath of water molecules.

When a molecule such as NaCl is dissoved in water the cation collects water molecules as a kind of sheath or shell. This process is called hydratation or solvatation. The solvatation energy then delivers the necessary supply for the dissociation. Due to the special polar structure of the water molecules the charge density for the negative charge is larger than that of the positive charge as can be seen in Fig. 3.3-2 This is the reason that cations usually have a more robust hydrate shell than anions. For simplicity, for the following cations are considered as solvated ions with hydrate sheath and anions without sheath.

3.3.1 The Electrode-Electrolyte Interface Electrical and Electrochemical Potential The electrode-electrolyte interface as represented in Fig. 3.3 -1a is oversimplified to allow a detailed study of any of the electroplating parameters. Therefore in the following, some electrochemical definitions will be introduced, and successively a more realistic model of the electrode-electrolyte interface will be developed. At the border between two phases (metal-electrolyte) always an electrical double layer is generated. There are many causes for this. A surface layer may consist of uniformly aligned molecules with a resulting double layer of charges. Another

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3 The Physical and Chemical Basics in Microtechnology

reason is the preferred adsorption of only one type of ions (cations), whereas the rejected anions collect near the cations and form the double layer. The overall difference in electrical potential between phase CI (metal electrode) and phase (3 (electrolyte) is called the inner potential or Galvani-potential. The Galvani-potential p is the sum of the so-called outer potential y and the surface potential x: (TJ=W+X

(3.30)

The Fig. 3.3-3 gives a graphical representation of the different potentials at the phase border. We have stated that the cations in the electrolyte are covered by a sheath of polarized water molecules and thereby building a molecular cluster. These clusters together with the electrode surface form a capacitor-like double layer, one side of which is formed by the presence of the electrons of the metal, the other side

Fig. 3.3-3 Graphical representation of the different potentials. q E is the inner potential of the bulk of phase (3 (electrolyte), whereas qMe represents the inner potential of phase a, the bulk of the metal electrode.

3.3 Basic Concepts of Electroplating

71

by the adsorption of the cations on the surface of the cathode. According to this idea which goes back to Helmholtz, the boundary electrolyte is limited to a monolayer film on the cathode made up of positively charged ions, whilst in the bulk volume of the electrolyte cations and anions mix in such a way that any arbitrary volume of electrolyte contains the same number of positive and negative charges. The thickness of the double layer is do*, as seen in Fig. 3.3-4a. The plane of charged particles (or better, the plane of the centered charges of the hydrated ions) is called the “outer Helmholtz plane”. The double layer can be considered as a capacitor with parallel plates. The capacitance is calculated as: C=-

cr&o. A dOH

with e, the relative permissivity, eo the dielectric constant, and A the area of the electrode.

a

b Fig. 3.3-4 a) The structure of the Helmholtz double layer and

b) the related potential function p(0.

(3.3 1)

12

3 The Physical and Chemicul Basics in Microtechnology

The related potential function is then: (3.32) This is a straight line as can be seen in Fig. 3.3-4b. In this model the whole potential drop between electrode and electrolyte happens within the distance doH, between electrode surface and outer Helmholtz plane. Considering a potential of about lV, then the field between Helmholtz plane and electrode amounts to lo7 Vcm-’. This extremely high field influences the molecular structure of the clustered ions, for example stripping the hydrate sheath of the ions, when traversing this region. The model of a potential drop only between electrode and outer Helmholtz plane again does not render the true parameters and has to be refined. The rigid wall of the outer Helmholtz plane is physically unrealistic, since there is always a Brownian motion and a diffusion force opposing the rigid wall. Gouy and Chapman independently developed a model considering diffusion effects and Brownian motion and thus defined a “diffuse double layer”, a layer extended in the x-direction into the bulk of the electrolyte with a smooth slope of the potential. The potential function can be calculated using the Poisson equation: (3.33) Solving the differential Eq. (3.33) and putting in the boundary conditions for this model leads to the potential function: (3.34) with p a measure of the radius of the ionic cloud. Finally Stern combined both models to a system with the outer Helmholtz plane and a diffuse double layer. This model was refined further by Bockris, Devanathan, and Mueller by adding an interfacial region, which consists of water molecules, neutral atoms, molecules, cations without hydrate sheath, and even ions of the same charge as the electrode. This indicates the fact, that at the electrode-electrolyte interface there are other bonding forces involved besides electrostatic, namely chemical bonds and van der Waals bonds. The plane involving the centered charges of this adsorbed layer is called the “inner Helmholtz plane”. This Be-. final model is sketched in Fig. 3.3-5 including the electrostatic potential ~ ( 8 tween the electrode surface and the inner and outer Helmholtz planes the potential is straight, followed by a slope within the diffuse layer toward the bulk of the electrolyte. This slope varies with the ion concentration of the electrolyte, it is steeper with high concentration and shallower with diluted solutions.

3.3 Basic Concepts of Electroplating

73

a

I

b

iH

OH

Fig. 3.3-5 The complete picture of the electrode-electrolyte interface including the models and adaption of Helmholtz, Gouy and Chapman, Stern, Bockris, Devanathan, and Mueller. In section b the potential drop is depicted for high concentration electrolytes (solid line) and diluted electrolytes (dashed line).

3.3.2 Polarization and Overpotential All these models above are static models, meaning that after an initial displacement current the external current between anode and cathode is zero. In the electroplating process, ions have to be moved from the bulk of the electrolyte to the electrode, where they exchange charges (electrons) with the electrode, to become neutral atoms and to be incorporated into the crystallographic structure of the electrode. The thermodynamic equilibrium is represented by the equilibrium potential or reversal potential q ( O ) , at which theoretically an exchange of electrons between electrode and electrolyte takes place. At realistic conditions (at non-equilibrium conditions) an overpotential can be observed. This is the difference between the

3 The Physical and Chemical Basics in Microtechnology

74

reversal potential q ( 0 ) and the potential q(j) in the current state, and is called polarization or overpotential r.

(3.35) The total overpotential is a combination of several components which are listed in the following: 0

The electron transfer overpotential

rTdue to an inhibited

charge transfer.

The electron transfer from the solid electrode to the adsorbed ion, and the change from the ion in solution to the adatom at the surface of the electrode is the most decisive step in the process chain of events in electroplating. This is the transition from electronic current to ionic mass transport. 0

The diffusion overpotential qdiffcaused by inhibited mass transport. Differences in concentration of electrolytes on the anode and cathode occur compared to the average electrolyte composition. These differences in concentration are caused e. g. by hydration-, dehydration-, complex formationand decay reactions near to the electrode. Thereby an inhibition in diffusion occurs on the electrode, which again causes a concentration or diffusion This diffusion overpotential is treated in more detail in overpotential rdiff. Section 3.3.3.

vr caused by a preceding or subsequent reaction. overpotential radcaused by stripping off the hydration

0

The reaction overpotential

0

The adsorption sheath and adsorption to the electrode surface.

0

The crystallization overpotential qcristcaused by migration over the surface to find an energetically favorable place for deposition. In the case of electroplating of metals, an additional inhibition of the crystallization process occurs. The surface diffusion of the adatom, i. e. the atom which is adsorbed onto the surface, and the incorporation into the crystal lattice at a growth point manifests itself in the crystallization overpotential rcryst.

3.3.3 Mechanisms of Cathodic Metal Deposition How does the deposition of a metal from an electrolyte solution occur on a metal cathode? Possible ways for solvated metal ions from the inside of the electrolyte to the uptake into the lattice of the cathode metals will be pursued below. For that the following steps have to be distinguished: 0

0

Migration to the reaction zone. Entering into the boundary layers on the phase boundary, traversing of the diffusion double layer,

-

3.3 Basic Concepts of Electroplating -

15

incorporation into the outer Helmholtz plane, desolvation of the hydratation sheath.

0

Migration of Me+ within the outer Helmholtz plane up to a point, opposite to an active site at the electrode surface.

0

Traversing the inner Helmholtz plane and electron transfer (neutralization).

0

Incorporation into the metal lattice.

The deposition process sees the largest energy barrier on transfer through the outer and inner Helmholtz planes. The metal ion must be squeezed through the closely packed components of the Helmholtz planes. One refers to this as a transfer polarization. Therefore, also the main component of the potential drop is found in the inner Helmholtz-layer. If one connects the average Galvani-potential to the average thickness of the inner Helmholtz layer of about to lo-* cm, then in a very confined space a field strength of the order of about lo7 Voltkm is achieved. With this voltage drop the cation is assumed to traverse through the layer fully dehydrated. Now the metal ion does not enter at an arbitrary place into the lattice. The microscopic surface is made up of regions with different specific surface energy. As a rule only the active regions i. e. the regions with the highest specific free surface energy, form the gateway to the metal lattice. The number of such active points is normally relatively small, compared to the total number of available lattice sites, as seen in Fig. 3.3-6,

Fig. 3.3-6 Energetically favorable places for crystallographic growth. The most favorable position is an already existing nucleus, the second favorable position is at a corner of a step, the next is to start a new row of a step. The least favorable position is to nucleate on a plain surface.

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3 The Physical and Chemical Basics in Microtechnology

A single nucleus as a growth side usually is most attractive for subsequent atoms arriving at the surface. The next most attractive position is to decorate a step in the crystallographic structure of the substrate. The completion of a layer is energetically more favorable than to start a new layer. The least attractive position is to start a new nucleus. A crystal therefore is not built up by arbitrary uptake of atoms or molecules but quasi periodically, in which lattice planes are arranged in succession. In order to bond to a growth site, the metal ion uses a relatively small amount of energy. However, it must first be able to reach this point by surface migration. If the primary adsorbed metal ion does not reach any growth sites, because on diffusion it does not cross such a place, or because it can not be taken up by such a site (because this site is already occupied by another reactant), then occasionally a new nucleus is formed. However, forming a new nucleus requires considerably higher energy, than arranging the metal ion on periodic growth sites, which can be seen from the height of the crystallization polarization. Inhibitors influence the Galvani-potential and often cause strong polarization. All electrolyte components, namely organic or colloidal substances, as well as reaction products of electrode reactions, are capable of inhibiting. Likewise also electrolyte concentration, acidification or introduction of so-called conducting salts contribute to growth retardation. Inhibitors retard the growth and decrease the work of nucleation by temporarily occupying the energetically favorable sites on the crystal. A competing occupancy of energy-favorable sites exists with metal ions and inhibitors, whereby the latter, forming weaker bonds with the substrate than the metallic bond of a cations, are eventually displaced from their sites by the metal ions. The inhibitors are therefore not incorporated into the lattice. It is important to note that the temporal dependence of the construction of the lattice is controlled with inhibitors and thereby the crystal structure can be changed. As a result of the various processes the material transport mechanism in the electrolyte must be studied in more detail. In practice different mechanisms compete, and the slowest process determines the overall speed. The ions to be deposited can be transported in three ways to the electrode:

Migration Migration means the ionic transport under the influence of an electric field, As discussed earlier, the ions move with a velocity of: (3.23)

which is in the equilibrium between the driving force of the applied electrical field, and the friction force due to the viscosity of the solvent. No field though in the electrolyte can be formed if there is a high excess of conducting salt in the electrolyte or a high concentration of ions to be deposited because of the high conductivity. In this situation the influence of migration can be neglected.

3.3 Basic Concepts of Electroplating

77

Diffusion By consumption of ions on the electrode surface a concentration gradient of the ions which take part in the reaction is generated in the electrolyte layer near the vicinity of the electrode. Ions diffuse to the electrode because of this concentration gradient. The ion current is determined by Fick's first law: N=-D.-

dc

dx

(3.36)

where N is the number of ions which reach the electrode in a unit of time. dnldx represents the concentration gradient across the diffusion layer dx and D is the diffusion coefficient, which contains the ionic radii of the participating reactants. The which was mentioned in diffusion is the cause of the diffusion overpotential rdiff the preceding section on polarization and overpotential at the electrodes. A thorough treatment of this matter leads to the following formula for the diffusion overpotential: (3.37) with R = universal gas constant (8.31451 JK-'mol-') F = Faraday constant (9.648456 .lo4 C mol-') 6, = Nernst diffusion layer thickness j = current density c, = concentration of the ions in the bulk of the electrolyte. Convection The diffusion layer can be reduced by forced convection. The stronger the convection in the electrolyte, the smaller is the diffusion layer thickness and the steeper the concentration gradient, which therefore leads to higher ion currents. The maximum possible current density of metal deposition is achieved, if the concentration of ions on the electrode is co = 0. This assumes that the reaction kinetics at the electrode is not the rate determining step and therefore all ions going through the diffusion layer are immediately taken up on the electrode. In Fig. 3.3-7 the relation is graphically represented [Hama98]. Curve 1 represents the diffusion limited reaction, which shows the concentration co = 0 at the electrode. At the intersection of the gradient line with the horizontal line that represents the concentration of the electrolyte far from the electrode, marks the Nernst diffusion layer 6,. Curve 2 in Fig. 3.3-7 shows the shape, if reaction limited (mixed) circumstances occure. In this situation the diffusion layer is called the hydrodynamic boundary layer. In order to achieve defined flow conditions, rotating disc electrodes are used. A metal cylinder is embedded centrally in an insulating material and only the face is

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3 The Physical and Chemical Basics in Microtechnology

Fig. 3.3-7 The formation of the Nernst diffusion layer with diffusion limited reaction (curve l), and with reaction limited (or mixed) conditions (curve 2 ) . dN is

called the Nernst diffusion layer.

exposed to the electrolyte. During rotation, the electrolyte is drawn along the rotational axes and ejected radially. The diffusion layer thickness is then: (3.38)

where y is the kinematic viscosity of the electrolytes, w is the angular velocity of the disc electrode and D is the diffusion coefficient of the species participating in the deposition. Electrodes with lateral dimensions in the micrometer region are denoted as microelectrodes (Fig. 3.3-8). Whilst a linear diffusion field forms on planar macroscopic electrodes, microelectrodes display a spherical, non-linear diffusion field. The edge diffusion is larger than the linear diffusion, i. e. per unit of time more ions diffuse to the electrode surface than in the macroscopic case. Thereby higher limiting current densities are achieved. In this connection the diffusion limiting current density is given by: (3.39)

with a = diameter of the disk.

3.3 Basic Concepts of Electroplating

79

a

b Fig. 3.3-8 Difference between a) macroscopic planar electrodes with predominant linear diffusion coefficient, and b) microelectrodes with spherical diffusion coefficient.

In the case of an embedded microelectrode, the electrode is located at the base of a cavity in a non-conducting material [Leye95]. Within this cavity the ions migrate through a linear diffusion field, and at the open end in the case of no convection, a spherical diffusion field is present (Fig. 3.3 - 9). The diffusion limiting current density for the embedded microelectrode without external convection is therefore given by

Conductive substrat

* a

-l

Fig. 3.3-9 Filling a microcavity with a high aspect ratio involves linear diffusion as well as spherical diffusion.

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3 The Physical and Chemical Basics in Microtechnology

with a h

= =

diameter of the cylindrical hole, and depth of the hole.

With forced convection, one has to distinguish between two cases between depending on whether the flow penetrates the cavity appreciably or not. With larger lateral dimensions the electrolyte is exchanged by the penetrating flow and so the effective diffusion distance is reduced. The calculation of the limiting current density becomes therefore more difficult, because it depends on the individual shape of the cavity. With small lateral dimensions the electrolyte solution does not penetrate into the cavity. The linear diffusion distance is therefore determined by the cavity height h. Therefore the limiting current density can be determined:

(3.41) The pH-value increases on the electrode surface as well as in the diffusion layer since in metallic deposition, such as nickel from a nickel sulfamate electrolyte, hydrogen is also deposited. However, as hydrogen shows a higher diffusion coefficient than the participating metal ions, the diffusion gradient of the former is flatter than that for the metal ion. At too low a concentration of hydrogen a colloidal deposition of nickel cations as hydroxide results. This happens at a pH-value of between 5 and 6. These hydroxides are formed in the deposited layer and cause an increased hardness and brittleness of the layers. Material Transport Processes During Microelectroplating It is desirable that during electroplating of a microstructure the layer growth should take place uniformly over the whole substrate. The layer growth is greatly influenced by two components, firstly by the local current density distribution and secondly by the particular conditions of material transport. The different behaviors, which can occur on an arbitrary microstructure, were already considered in the previous section. During growth, another time dependent component has to be added. With a microstructure, whose material transport is composed of a linear and a spherical diffusion distance, the transport conditions are very quickly and drastically changed after the filling of the cavity (the linear diffusion distance). Therefore after filling the cavities under linear diffusion conditions the deposit expands to mushroom-like structure as seen in Fig. 3.3-10. A lot of empirical knowledge is available from practical electroplating. Electrolytes contain additives such as wetting agents, brighteners, reaction products and so forth. The theoretical background of the function of these components in many cases is still obscure. The LIGA technology, which is treated in detail in Chapter 7, relates to a great extend to electroplating. Therefore a major part of the research activities went into the theoretical and experimental understanding of micro electroplating. The electroplating of microstructures, such as micro- and macro-throwing power, internal stress of thick electroplated layers, and the build-up of electro-

3.4 Materials of Microsystems Technology

81

Fig. 3.3-10 Mushroom-like structures received by “overplating” a cylindrical hole. The stem is plated in linear diffusion mode, whereas the dome grew in the spherical diffusion mode.

plated microstructures on top of processed silicon wafers are discussed in more detail in Chapter 7. For further reading on electrochemistry the book of Bockris and Ready [Bock981 is recommended.

3.4 Materials of Microsystems Technology Materials play an important role in microsystems technology, as they combine several functions. First of all they maintain, of course, the traditional role of shaping the outside dimension. Secondly, the surface properties of the materials are of great importance to the performance of the microcomponents or even the full system, as the surface to volume ratio increases with decreasing size of the structures. This increase is only linear if one assumes an unlimited thin layer in a mathematical sense. However, the ratio increases non linearly with the scale of miniaturization, if one considers a “physical” surface, i. e. if one takes into account that surface properties like for instance hardness, resistance to corrosion or electric breakdown, extend to a certain depth into the bulk (Fig. 3.4-1). Frequently the surface properties of the bulk material are changed by a desired or undesired adsorbed layer. A microcantilever, which is covered with an oxide layer, most probably exhibits other elastic properties than one with a “clean” surface. In other words the mechanical properties of certain microstructures depend on their processing history. A cantilever, which was produced with an etchant

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3 The Physical and Chemical Basics in Microtechnology

Fig. 3.4-1 The influence of surface properties with decreasing size of a structure. a) Assuming a constant “depth” of the surface independent of the size of the microstructure, b) the surface to volume ratio reaches infinity, as can be seen in the diagram.

A, has possibly another Young’s modulus than one which came into contact with an alternative etchant B. In microsystem technology, the materials and the determination of their properties play a most important role. In the conventional engineering sciences, material properties are normally listed in tables, which can be recalled when required. In most cases it concerns the bulk properties of the material. In bulk samples the decisive dimensions of the microstructure of the material (in this context by “microstructure” is meant as the microscopic crystalline structure of the material) is smaller by orders of magnitude than the size of the object. As a consequence the material properties appear, with some justification, to be independent from the sample shape and size. This assumption is not valid in microtechnology. In a polycrystalline material an object the size of a crystal may have different properties than one which still contains thousands of crystals. In a macroscopic body a thin oxide layer on the surface will have no obvious influence on the elastic properties. In contrast, for a microstructure the “volume” of the surface layer is no longer negligible and must be taken into account on calculating the mechanical properties. Moreover, the properties change depending on whether the surface layer is an oxide layer or a nitride layer. This again depends on the technological history of the microbody. One can already see that the determination of the material properties of a microbody is a much more complicated task, compared with that of the macroscopic bodies. As already mentioned above, the effects of the surface properties of the materials change non linearly in microtechnology. However in many cases, this can be

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83

used to advantage in microstructures by obtaining unique properties not found in macroscopic devices. Consider for instance the unusual high electric breakdown behavior of thin oxide layers, or the extreme mechanical hardness of diamondlike thin films, or the high corrosion resistance and freedom of pin-holes in plasma-polymerized films. In addition to the physical and chemical properties of the materials, which are normally connected with the mechanical performance of microcomponents, additional material research is needed since the development of new sensor and actuator principles may depend on particular characteristics of a given material. Material properties in general cannot be dealt with here in any detail. The authors will therefore concentrate only on the properties and phenomena, which are related specifically to certain applications in microsystems technology. As much research is being carried out in this area, the reader is advised to refer to the relevant literature for the most recent developments. In the next sections, materials for microsystem applications are discussed under the following topics: 0

materials materials materials materials

for for for for

shaping microstructures, sensor applications, actuator applications, auxiliary applications.

Materials for Shaping Microstructures

Single Ckystals Microsystem technology evolved from microelectronics. Therefore the first microcomponents were fabricated from silicon, and even today the majority of microsystems still relies on silicon. The famous paper of Kurt Peterson in 1982 [Pete821 “Silicon as a mechanical material” is considered by many researchers in this field as the kick-off for MEMS technology. Silicon is probably the most investigated material in science and technology, and yet it’s remarkable mechanical properties were almost totally obscured by the high commercial interest in it’s electronic characteristics. Peterson pointed out, that silicon is a suitable material for the design of MEMS devices. Some of the parameters are listed in Table 2.1-1 in Chapter 2. For details concerning silicon, the reader is advised to study Chapter 2 and 6. Compared to silicon, other single crystal materials play a less important role. GaAs as a mechanical material is not being used except in applications where a different Young’s modulus is important (EGaAs= 8 5 3 GPa, and Esi = 180 GPa) . Quartz (SO2) crystallizes hexagonally and displays a piezoelectric effect, which is maximum when the single crystal is cut normally to the c-axis. Because of this, quartz is frequently used as an oscillator in the watch industry and whenever a frequency standard is needed.

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3 The Physical and Chemical Basics in Microtechnology

Ceramics Ceramics as a substrate for hybrid microelectronics and microsystems as well has already innumerable applications. The standard substrate is aluminum oxide (A203) ceramic. It is the basis for almost all hybrid devices in combination with microelectronic circuits, screen printing technology, thin film technology, bonding processes and adhesion techniques. The chemical inertness, the mechanical stability, the surface finish, as well as the heat conductivity and thermal expansion coefficient all play an important role. In microelectronics the matching of the thermal expansion coefficient to that of silicon is of great importance. If there exists a sizable mismatch, a bonded component can be destroyed by the appearance of high shear forces upon thermal loading. When designing power electronics with a high amount of heat dissipation, ceramics with a high heat conductivity are needed. In this respect beryllium oxide (BeO) is outstanding. It is, however, technologically unpopular because of the toxicity of beryllium oxide dust. Aluminum nitride could be a replacement which has a heat conductivity near to beryllium oxide. However, aluminum nitride (AlN) as a non-oxide ceramic, has the disadvantage, that for the screen printing process special printing pastes have to be applied for firing in a reducing atmosphere. In Table 3.4-1, the electric and thermal properties of standard ceramics are listed and compared with the properties of silicon.

Table 3.4-1 Some parameters of common ceramics compared to silicon Silicon

Dielectric constant Thermal coefficient of expansion [ 10-7/K] Thermal conductivity [W/mK]

11.9 23.3 157

A1203

9.5 75

20

Be0

7.0 85

230

AlN

10.0 34 150

Polymers The atomic building blocks of current plastics or polymers are limited to relatively few elements. The majority consists of the elements hydrogen and carbon. The next most often occurring element is oxygen followed to a much lesser degree by the elements chlorine, fluorine and others which are specific for each polymer. Plastics or polymers are materials which consist of macromolecules made up of organic groups and are obtained by chemical synthesis. Their molecular weight is between 8 000 and 6 000 000 g/mol [Spex92]. Polymeric macromolecules consist of many monomer units. The properties of the polymers show no detectable change, if one either adds or subtracts only a

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85

few monomers. It is different with so-called oligomers, which consist of a few (ohiyoo = a few) monomers (10-20). The change in the degree of polymerization by 1 has the consequence of a detectable change of the chemical and physical properties. A large polymer molecule is made up of many monomer-"building blocks", in which dangling bonds stick out of at least two sides in order to be able to form a continuous chain. Depending on the monomer and initiation mechanism, different polymerization processes can be carried out. The two most important processes are : Addition polymerization as the chain reaction. One understands this as the joining of many monomers to give macromolecules in a chain reaction without cleavage of low molecular weight fragments. 0

Condensation polymerization. Generally two different monomers are connected to each other by a cleavage of low molecular weight material.

The Physical Behavior of ~ a c r o ~ o l e c ~ l e s Polymers can exist as amorphous or semicrystalline materials (Fig. 3.4-2). Amorphous polymers are described by a coil model. The macromolecules are looped spaghetti-like into each other. Semicrystalline polymers are described by a twophase model containing amorphous and crystalline regions. The crystals can only exist in two basic forms, i. e. lamellar or needle-like crystals. The form depends on the conditions of preparation. The molecules which appear on the sur-

I

a

b

Fig. 3.4-2 Solid state phases of polymers,

a) amorphous phase, b) lamellar crystal,

c) needle crystal.

C

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3 The Physical and Chemical Basics in Microtechnology

face of the crystal form the amorphous phase. The inter-crystalline areas are characterized by chain ends, connecting molecules, regular or loose loops, as well as entwinements. Polymers exhibit different phases depending on the temperature. In general four zones can be distinguished: the the the the

0 0 0 0

glass region, softening or plastic range, viscous region (for amorphous polymers), melting region (for semi crystalline polymers).

The softening region is associated with the glass transition temperature T,, the melting region with the melting temperature T, (Fig. 3.4-3). In the glass region the molecules are frozen in. It consists of a solid with known short range order. Only local oscillations and rotations of molecular groups are possible. The glass transition temperature TG is one of the most important parameters to characterize polymers. Many physical and mechanical properties undergo specific changes at the glass transition temperature. This is due to the cooperative motion of molecular chains in the softening region. By determination of these parameters it is possible to make statements about a series of physical, chemical and morphological effects. For injection molding or hot embossing it is desirable to have a small transition region and a well defined glass transition temperature TG.For injection molding the polymer has to be heated well beyond TG to fill the molding tool completely. To eject the microstructure the polymer has to be cooled below TG. This temperature cycling around TG is the major rate determining parameter in the mass fabrication of molded microstructures, since the thermal mass of the molding tool is

Duroplast W

.u) : *

I I

Semi-crystalline thermoplast ..-..\..-.. AmorDhous themoplast .-..I..-

P

Temperature T

TG

TG

TG TM

Fig. 3.4-3 The thermal regions of polymers.

oi'the compcm n t s

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87

usually large. Therefore it is desirable to have a small “thermal swing” to speed up production.

Materials for Sensor Applications

Ceramics for Utilizing the Piezoelectric Effect The piezoelectric effect is commonly used for sensors and actuators alike. This effect appears with materials which are not isosymmetric, such that a mechanical stress results which causes a polarization due to a non-symmetrical charge distribution. In Fig. 3.4-4 a simple model of a quartz single crystal shows the piezoelectric effect, when a force is applied to the crystal body. With externally applied electrodes on a test body, an electric voltage can be detected, which is a measure of the mechanical stress in the body. Conversely an applied field causes a mechanical distortion. In the simple one-dimensional case the piezoelectrical equation is:

P=Zd

+ q , E X ; e = Z s + Ed

(3.42)

where P is the polarization, Z the mechanical stress, d the piezoelectric coefficient, E the electric field, X the dielectric susceptibility, e the elastic extension and s the elasticity modulus. These equations describe the development of an electric polarization on applying a mechanical stress and vice versa the development of an elastic extension by applying an elastic field. The general definition of the piezoelectric coefficient is : (3.43) where i = x, y, z and k

= xx, yy, zz, yz, a,xy.

++

Fig. 3.4-4 Model of a quartz crystal and the mechanism of the piezoelectric effect. Mechanical stress stimulates electrical charges at the outside of the crystal and vice versa.

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3 The Physical and Chemical Basics in Microtechnology

The piezoelectric coefficient constitutes a tensor of third order. For quartz d is cmlVolt and for barium titanate in the order of cdVolt. A similarly diversely used material is lead zirconate-lead titanate (PZT). The sensitivity of piezo raw materials for use as sensors or actuators is characterized by the electromechanical coupling factor k stored mechanical energy stored electricsl ernergy

Ceramics as Material for Gas Sensors The multitude of ceramic sensors does not allow an in depth consideration of this topic in this book, therefore only some particularly common applications are singled out and briefly discussed. For a more comprehensive review the reader should become familiar with the technical literature [Kitt88]. With semiconductor sensors, the change of the electrical resistance is evaluated as affected by the experimental parameters. With metal oxide sensors, the experimental parameter is the surrounding gas atmosphere. An example of this is the SnO, sensor, which changes its resistance depending on the concentration of a series of gases. The resistance changes with the number of conduction electrons on the surface, which in turn is dependent on the number of adsorbed foreign gas atoms. A frequently used material is barium titanate BaTiO,. The number of conduction electrons is varied on the surface by reversible oxidation or reduction. Sensors with solid ionic conduction are wide spread. All sensing probes for the control of combustion gases of vehicle motors (lambda-probes) depend on this. Doped zirconium oxide ZrOz is used as such a material. The sensor consists of finger-like sensor bodies, onto which inner and outer platinum electrodes are attached. If there is a different oxygen partial pressure between the inner space of the finger and the outer space, then a voltage will develop between the electrodes, which is strongly non-linear. With a ratio of air to combustible gas of about 14 (A = 1) the voltage jumps by about a factor of ten. This jump of the potential is a precise measurement of the mixture of combustibles and thereby can be utilized for the combustion with minimal harmful exhaust gases.

Liquid Crystals Liquid crystals can be considered as a kind of a material which can be utilized as sensor and partially even as actuators. As a display it is vital for communication with the outside world. Therefore a general introduction into liquid crystal technology is provided in the following section. In 1888 the Austrian botanist Friedrich Reinitzer discovered organic substances, which possess between the solid crystalline and the liquid isotropic aggregate state a liquid crystalline phase, a so-called mesophase.

3.4 Materials of Microsystems Technology

a

89

b

C

Fig. 3.4-5

Liquid crystals and their different crystallographic morphologies: a) nematic, b) smectic, c) smectic C, and d) cholesteric.

Liquid crystal phases are typically formed from long, rod-like organic molecules. In the nematic phase the long-range molecular axes are aligned parallel, whilst the center of gravity of the molecules are randomly distributed in space (Fig. 3.4-5a). On cooling down the so-called nematic phase or from the isotropic liquid phase directly, different so-called smectic phases appear, in which the molecules in addition are ordered in layers (Fig. 3.4-5b). Different smectic phases are characterized by the ordering of the molecular center of gravity within the layers and with respect to their correlation of layer to layer. Besides the so-called orthogonal smectic phases, in which the long molecular axes on average are ordered perpendicularly to the layers, there exists also inclined smectic phases, in which the long molecular axes are at a sloping angle to the layer. A technically important variant is the smectic C phase shown in Fig. 3.4-5c. Another technically interesting phase is the cholesteric state (Fig. 3.4-Sd). The various orientations and alignments of the molecules in the liquid crystalline phases lead to a more or less unique dependency of the physical properties on direction. The optical anisotropy leads to a refractive index of light. In the liquid crystal, incident light is split into two rays, whose light vectors oscillate at right angles to one another and in general propagate with different speeds in the liquid crystal. On emergence of light from the liquid crystal, both beams, which are now Out of phase, reunite.

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3 The Physical and Chemical Basics in Microtechnology

Because of the anisotropic dielectric properties, the liquid crystal molecules try to align parallel or perpendicular to an externally applied electric field, depending on whether the largest dielectric moment is parallel to the long molecular axes or perpendicular to it. This mechanism is used for the switching of conventional nematic liquid crystal cells. The appearance of liquid crystalline alignment is found relatively frequently with small molecular compounds and was discovered first with low molecular weight materials. It appears with substances which are made of stiff rod-like molecules, which line up with each other and align parallel in the liquid phase. Molecules which align to give mesomorphic structures are called mesogens.

Materials for Actuator Applications

The Piezoelectricity as Actuator Principle The utilization of the piezoelectricity as an actuator principle is widespread in microsystem technology. Some selected applications should be mentioned as exemplary. The scan mechanism in AFMs (atomic force microscopes) works exclusively on piezoelectric actuators, mostly in the so-called inchworm-motion, as seen in Fig. 3.4-6. In many applications the buckling of a membrane is utilized for pumping a fluid in micropumps. Since the actual motion of single piezoactuators is only in the range of some micrometers, this pumping principle is only suitable to pump non-compressible media, since pumping a gas would need a higher compression ratio, and therefore a larger motion of the actuator. With stacks of

“Louse”

Tunneling tip unit

[------L----l-

Dielectric insulation

Vibration absorbing base plates

Fig. 3.4-6 Schematic view of a scanning tunneling microscope including a “louse” and the scanning tunneling tip unit 011 a low-vibration base plate. The louse carries the samle and is used for the rough approach. It is set into motion by alternatingly clamping the three feet of the louse to the base plate by appropriate voltages V F IVF2, , and VF3against V,,,, and appropiate contractions/expansions of the piezoelectric plate.

3.4 Materials of Microsystems Technology

91

Fig. 3.4-7 An example of a piezoelectric actuator. Due to the fact, that the displacement is very small, mechanical amplifiers or pantographs are used.

Fig. 3.4-8 An ultrasonic micromotor operated with piezoelectric actuators.

piezoactuators larger displacements are possible, but this set-up is complicated and costly. With mechanical amplifiers (pantographs) larger displacements are achievable too. A general principle is sketched in Fig. 3.4-7. Many derivatives of this basic design are presented in the literature. A further principle is used in micromotors by generating mechanical transient waves in rotor discs in combination with frictional counter plates as seen in Fig. 3.4-8. Very simple motors with low rotational speed without a reduction gear and relatively high torque can be realized. The arrangement of piezoelectric actuators in arrays is seen in Fig. 3.4-9. Each column is a single piezoelectric actuator. If they are controlled individually, the array can be operated as a so-called phased array. Without any moving part, this array can act as a kind of ultrasonic scanner or radar.

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3 The Physical and Chemical Basics in Microtechnology

Fig. 3.4-9 Raster electron micrograph of an piezoelectric actuator array. The array is fabricated with the lost form arrangement. The slurry is cast into the negative form of the array and after drying is fired to the final array. The polymer cast is burned off during the firing process.

Magnetostrictive Metals

A ferromagnetic material in its normal state consists of a multitude of individual domains, in which each domain is magnetized spontaneously in an arbitrary direction. The ensemble of these domains in the bulk, also called Weiss regions, are oriented such that they occupy an energy minimum, or in other words, all magnetization directions of the various domains compensate each other so that the magnetization disappears with respect to the exterior (Fig. 3.4-10). If now an external magnetic field is applied, with increasing field strength the Weiss domains flip to the direction of the outer field vector so that the resulting magnetization of the solid body is aligned antiparallel to the external

Low external field

High external field

t

Fig. 3.4-10 Magnetization of a ferromagnetic body a) without an external field, b) with a weak external field, c) with a strong external field.

3.4 Materials of Microsystems Technology

93

field. On magnetization a dimensional change of the solid body occurs, which generally lies in the region of less than lop6. The relative change in length /1 = AL/L is called magnetostriction. In this connection there are different cases which must recognized. The change in length parallel to magnetization, is called the “Joule magnetostriction”. Besides that, there is also a transversal magnetostriction, which amounts to only a fraction of the Joule magnetostriction. The saturation magnetostriction decreases with increasing temperature, up to the Curie-temperature, where it completely disappears. The magnetostriction is an effect, which can be used for microactuators, because it generates large forces with small changes in length. For certain applications in underwater sonars, materials were developed with an extremely high coefficient of magnetostriction like, for instance, Terfenol-D, Tb,Dy,_xFe,, where x is between 0.27 and 0.30 and y is between 1.90 and 1.98, with a value which exceeds that of nickel by a factor of 500. In Fig. 3.4-11 the magnetostriction for the new rare-earth-magnets is compared with the values for Permalloy and nickel.

2000

1500

E E

\

7

e

1000

I

4

500

0-

0

250

500

750 1000 1250 1500

H [kA/m] Fig. 3.4-11 The magnetostriction coefficient for different materials as a function of an

applied field.

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3 The Physical and Chemical Basics in Microtechnology

The calculation of the extension /Imag depending on the applied magnetic field H , is very complex; it will therefore be avoided and the reader is referred to the relevant literature [Bozo68], [ClarSO]. To give only an idea of the calculation, the energy balance of the individual energy terms is established, whereby:

E=Eo

+ E, + Erne + Eel

(3.44)

The individual terms have the following meaning: 0 0

0 0

Eo is the magnetic energy density, independent of the magnetization direction. Ea is the anisotropy-energy density. For different crystallographic directions the anisotropy energy is different. There are magnetically preferred alignments, which can be “easily” magnetized and some which are “difficult” to magnetize, which require a high magnetic field strength in order to attain saturation and the associated expansion. The difference between the so-called “easy” and “hard” direction magnetization can be considerable. Therefore with the so-called pseudobinary compound of the form MFel,9 (e. g. Terfenol-D) different components are combined, in order to achieve a partial compensation of the different anisotropy constants (that is reflected in the stoichiometric formula (Ml)x(M2)l - x h .9>. The anisotropy energy can be described for cubic crystals by anisotropy coefficients KO,K1 and K2. Erneis the magnetoelastic energy density, and Eelis the elastic energy density.

To calculate the desired relation /Imag (H), the energy balance must be selected, whereby the magnetic excitation and the mechanical stress are regarded as independent quantities and the direction cosine aibetween the magnetic field direction and the measuring direction and the extensions /IG are considered as dependent variables. Applications of Magnetostriction

An interesting application of the magnetostrictive effect in microsystems technology can be observed in a linear motor, which was introduced by Kiesewetter and Huang [KiesSS]. This motor makes use of the so-called inch-worm principle, which has also gained already numerous applications in microsystem technology in other designs and using other actuator principles. One starts with the assumption that the volume of a magnetostrictive rod in the first approximation does not change under the influence of an external magnetic field when: (3.45)

3.4 Muterials of Microsystems Technology

95

Then :

AV Ad A1 -=2.-+-=0 1 V d

(3.46)

and for the relative change: Ad A1 - -_ -d 21

(3.47)

A Terfenol-rod is mounted into a metal shell in such a way, that in the nonexcited state it is clamped along its whole length in the shell. This shell is now inserted into an array of coils, as can be seen in Fig. 3.4-12. If these coils are supplied with current in a particular sequence, then the Terfenol-rod elongates because of the magnetostriction in sections and moves through the shell opposite to the direction of the magnetic field. With one process run, a rod has a covered distance of

LA1 AS=-

(3.48)

1

D
,T I

1><1 r>a

=+------Coil +T -ere fnol =+Tube

As

(off) rod

Stepwidth

Fig. 3.4-12 A magnetostrictive linear inch-worm-motor.

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3 The Physical and Chevnical Basics in Microtechnology

(where L is the axial extension of the length of each applied magnetic field section). Meanwhile this principle has been further developed in other driving gears [Akut89].

Shape Memory Metals There are certain metal alloys, which on heating possess the ability to transform back to an earlier form, which was imprinted in its earlier history [Stoc89], [Taut%]. The changes in form of the so-called “shape memory metals” depend on a certain temperature, above which mechanical and thermal changes occur in the crystal structure of these metals. Above this critical temperature an austenite lattice structure is in existence, consequently the metal is hard and has a high strength. Below the transformation temperature a martensite lattice structure is adopted and the metal is soft and ductile. In principle, one must distinguish between the so-called “one-way effect” and the “two-way effect”. With the “one-way effect” the alloy takes on its earlier form only on exceeding the critical temperature and retains this form even after falling below this critical temperature again. With the “two-way effect” the alloy, depending on whether the temperature lies above or below the critical value, can take on two different forms where this shape transformation can occur frequently, as a result, the possible shape changes with the “one-way effect” are always larger than with the “two-way effect”. Until now Ni-Ti, Cu-Zn-A1 and Cu-Al-Ni alloys are used for practical applications. Depending on the alloy system the critical temperature can lie between -150°C and +150°C, so that a wide spectrum of applications are available.

One-way Effect The form changes, which occur with memory metals, are schematically shown in Fig. 3.4-13: Above the critical temperature, i.e. in the austenitic state, the alloy takes up form A. On cooling below the critical temperature (TAs) the material changes to the martensitic crystal lattice (“zig-zag alignment” of the lattice atoms) without changing the outside shape. At this temperature the alloy can be easily converted and shaped for example into form B. The degree of deformation should however not be too large, so that only a reversible martensitic deformation occurs e. g. shifting of twin boundaries in the crystal lattice. A too large deformation would cause dislocations and therefore a “memory loss”. On heating no shape change occurs, as long as the temperature remains clearly below TAs. If the temperature is raised above the critical value TAs, then the martensite lattice transforms into the austenite lattice and the original form A appears again. The total change of form A to form B is carried out within a relatively small temperature region of about 10 to 20°C. On re-cooling to below the critical temperature T,, the shape does not change anymore, although the martensitic crystal structure reforms.

3.4 Materials of Microsystems Technology Austenite

a

E 0

Martensite

Austenite

Martensite

a

a

5

U.

E

U.

U

97

- --

T>A,

T
Cooling

TcA,

+ Deformation

T >A,

Annealing

TCA,

Cooling

Fig. 3.4-13 The one-way shape memory effect. At a temperature of A, the crystallographic phase transition from martensite to austenite is completed, as can be seen in diagram 3.4-15.

Two-wayEffect For the “two-way effect” in which the material “remembers” a high temperature - as well as a low temperature shape, special mechanical and thermal material processing is required. For example the “two-way effect” can result from a strong deformation in the martensitic phase, so that an irreversible component remains, as is schematically represented in Fig. 3.4-14. It is also possible “to train” the martensitic form by repeating a small deformation in the martensitic phase with subsequent heating. With Ti-Ni alloys with a nickel content of over 50 % a two-step transformation can be seen instead of the single step austenite-martensite transformation. On cooling the austenitic phase first forms a pre-martensitic phase, which is called the rhombohedra1 or R-phase. Only on further cooling, the martensitic phase appears. If the sample in the martensitic phase is shaped and in this phase subjected to aging, then lens-shaped Ti,Ni, deposits form, which show a preferred orientation in the region of the deformation. Furthermore the aging brings about a transformation from the single step martensite transformation to the two stepped transformation austenite-R-Phase-martensite. Thereby preferential variations of the R-phase or the martensitic phase are formed and the alloy “remembers” also a low temperature form. A typical temperature-path-characteristic line for an element with the “two-way effect” is schematically represented in Fig. 3.4-15. On heating, the form change takes place at the so-called A, temperature and is completed at the temperature TAf,whereby this temperature interval is relatively small, typically 10-20 “C. With a drop in temperature the form change starts first at a temperature TMs,

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3 The Physical and Chemical Basics in Microtechnology

Austenite

Martensite

Austenite

a

m

E

0 U

E

U

warm

cold

Martensite

warm

t------,

cold

Fig. 3.4-14 The two-way-effect.

Temperature

Fig. 3.4-15 The typical displacement-temperature characteristics of a two-way element.

which is lower than TAfi. e. the temperature-path-characteristic line shows a hysteresis, which amounts to approximately 10 "C to 30 "C depending on the alloy. The low temperature form is then completed at the temperature TMf.

Suppressed Shape Memory If a part deformed in the martensitic phase is prevented from taking on its original (austenitic) configuration, then one refers to this as suppressed shape memory. Such a component can develop a very large force. The stress-expansion-characteristic line is schematically represented for such a case in Fig. 3.4-16, where an example was chosen, which has found a wide field of technical applications. A ring made of Ti-Ni alloy in the austenitic phase is produced e. g. by a metal-cutting processing,

3.4 Materials of Microsystems Technology

99

Cooling + elongation

n Heating and shrink on

Elongation

Fig. 3.4-16 Stress-elongation characteristics with suppressed memory effect. This is used to shrink tubes onto rods as for very tight connections.

with an inner diameter smaller than the axle over which it is to be pushed later. After cooling below the temperature TMf,the ring now in the martensitic state is expanded until the inner diameter is larger than the diameter of the axle. In this cooled state the ring can easily be pushed over the axle. On heating, the ring passes into the austenitic state and endeavors to take on its earlier form again, i. e. it shrinks until it comes into contact with the axle. As another form change is suppressed, a relatively large stress is built up, which leads to a solid bond between the axle and the ring. Compared with the form changes which result from metals, due to the thermal expansion coefficient (the shrinkage of “hot” rings on an axle is well established), the form changes of shape memory metals are more than two orders of magnitude higher (see Table 3.4-2). Table 3.4-2 Properties of technically applicable shape memory metals

Electrical conductivity [ 106 S/m]. Max. As-temperature [” C] Max. one-way effect (extension [%I) Max. two-way effect (extension [%I) Breaking elongation [%I

Ti-Ni

Cu-Zn-A1

Cu-Al-Ni

1-1.5 120 8 5 40-50

8-13 120 4 1 10-15

7-9 170 5 1.2 5-6

3 The Physical and Chemical Basics in Microtechnology

100

Application as Actuators In principle shape memory alloys can be used as actuators i. e. as controlling elements. A Ni-Ti spring has a relatively steep force-displacement characteristic-line in the austenitic phase (high temperature) and is extended by the force KO to a length LM. In the martensitic phase the characteristic line is considerably flatter and the spring can be extended by the force KOto a length LM,which is considerably larger than LA. This is schematically represented in Fig. 3.4-17 using the force-path-characteristics line. On exceeding the critical temperature TAP,the spring pulls together and is therefore in the position to perform work, whilst on falling below the critical temperature TMf,the spring again extends. Because of the high electrical resistance of Ni-Ti alloy, direct heating can also be accomplished by the passage of an electrical current. Thereby it is possible to drive relatively simply constructed actuators with the aid o f electrical signals or currents.

Properties of Shape Memory Alloys Shape memory alloys can be produced by vacuum melting, powder metallurgical processes, fusion spin processes and for thin layers by PVD processes (physical vapor deposition, see Section 4.3) [Walk90]. This presents many opportunities to use shape memory metals as microactuators. Some essential properties of shape memory alloys are summarized in Table 3.4-2. One can deduce from this table that Ni-Ti alloys possess the largest one-way and two-way effect and that the two-way effect is roughly lower by a factor 2 to 4 than the one-way effect. The maximum temperature T,, lies between 120°C and 170°C and depends greatly on the composition of the alloy. For example a change of composition o f 0.1 % can already lead to a shift of TAs by 10°C.

,Austenite al

2 0

U

KO

y /

! LA

T< Mf

f

LM

b

4

Displacement

Fig. 3.4-17 Force-displacement characteristics of a spiral spring in the martensite and the austenite phase.

3.4 Materials of Microsystems Technology

101

Gels A class of polymers with special potential for applications in microactuator technologies are the gels. Actuator principles like magnetostriction, thermal expansion and the “shape memory” effect vary in dimensions, of the order of only a fraction of a percent. It is different with gels, which frequently occur in nature and form the basis of many biologically active systems (i.e. sea cucumber). Also in microsystems technology it should be possible to build up actuators with reversible swelling gels which show a relatively large degree of conversion into mechanical energy. A gel consists of at least two components, a liquid form and a network of long polymer molecules, which can be either constrained to or alternately released from the liquid state. The individual molecular chains exist as polymer coils. One can imagine the whole gel as a pile of spaghetti. If one were to pull one strand of spaghetti only, it would be loosened from the whole pile. However, if one were to pull several strands simultaneously and pull very hard, then the whole structure would move. It is understandable that the volume will increase, if the gel takes up solvent and will shrink when it gives the solvent up again. The solvent uptake and release can be accelerated and increased by many effects. If one brings a single polymer molecule into solution, it can either inflate or shrink. If a polymer chain has for instance hydrophilic links, then in aqueous solution it would try to bind to water molecules and at the same time to extend itself. The effect would be the opposite if the polymer was hydrophobic; in an aqueous solution the chain would coil in the solution i. e. the entire gel would shrink in the presence of water. The polymer chains under consideration can also contain electrically charged groups. In an insulating medium the polymer chain tries to extend itself as far as possible, in order to minimize the repulsion between the charged centers. The gels will extend themselves to the point where the opposing elastic forces of the polymer network just balance the repulsion forces. If one puts an electrolyte into the insulating solution, then the ions of the solution would neutralize the electrically charged centers of the polymer chain and the elastic force of the polymer would allow the gel to shrink again. Examples for reversible shrinkable gels and gels capable of swelling are: 0 0

0

polystyrole (with really low swell behavior), polyvinyl alcohol and its derivatives (good swell behavior), polyacrylate (very large volume change).

As an example for polyacrylates, the poly-N-isopropylacrylamide (NIPA) is important and is frequently cited in the literature, because it represents a thermoreversible gel with large volume changes. Furthermore the monomer can be easily synthesized. The activity of gels capable of swelling can be stimulated by 0

change of the pH value of the solutions,

102 0 0 0

3 The Physical and Chemical Basics in Microtechnology thermal effects, action of light and electrostatic interaction.

With thermal gels capable of swelling, the transformation temperature can be influenced by the introduction of dissociable groups (-COOH) in the polymer. An example of the degree of swelling of an ionized P-NIPA gel is shown in Fig. 3.4-18 [Hiro87]. The volume phase transformation of gels by the action of light is reported by [SUZU~O] and [Mama90]. On irradiation with ultraviolet light, parts of the polymer chain are ionized. This induces an osmotic pressure, which leads to a swelling of the gel. By switching off the light source, the gel neutralizes again and shrinks to its original size. With visible light the reversible swelling is initiated solely by a thermal effect. This system is quicker than the photoionic system and could for example find uses in the production of light sensitive switches and artificial muscles. The concanacalin A is interesting for sensoric tasks, whereby a lectin is immobilized in the NIPA gel [Koku91]. The volume-phase transformation of this gel occurs at 34°C. If the gel saccharide dextransulfate (DSS) is added, the gel responds with an increase in volume of five times near to the transformation temperature. However, if the DSS is replaced by saccharide-a-methyl-d-mannopyranoside, the gel volume can be brought back to its original value. Also other saccharides seem to show these stimulating effects on the gel. This shows the potential of using swelling gels not only as actuator materials but also as sensors.

50

A

I

40

2 a

CI

1I

Ea3 30

I?!

32 20 0.1

I

10 100 Degree of swelling WN*)

Fig. 3.4-18 The swelling effect of ionized P-NIPA.

1000

3.4 Materials of Microsystems Technology

103

Liquid Crystal Polymers Liquid crystal polymers (LCP) exist if mesogenic units are introduced in a suitable way into a polymer. These mesogens are, as with low molecular weight compounds, made up preferably of aromatic building blocks. There are two different construction principles, which differ greatly in their range of properties. In the socalled side chain LCPs, mesogenic units are attached by flexible spacers to a flexible polymer chain. In contrast to the main chain LCPs, the mesogenic units are building blocks of the polymer chain, which in itself becomes stiff and rod-like due to the addition of stiff, rod-like mesogens. The mechanical, thermal and rheological properties of side chain LCPs corresponds well to those of conventional polymers. The mesogenic side chains provide additional optical and electric properties, which are comparable to those of low molecular weight liquid crystals. By the attachment of the mesogen to the main chain, the movement of the mesogens is limited. Nevertheless, on cooling, information which was “registered” in the liquid state, can be frozen in. The side chain LCPs are therefore of interest for optical storage media, which are able to store enormous amounts of data in the tiniest space. From the knowledge of the molecular parameters such as bond angle and bonding force constants, the theoretical maximum attainable durability and stiffness of a polymer can be calculated. For polymers, whose chains are linearly extended and packed parallel and densely, values are obtained which lie much higher than those that are actually measured for the polymer sample. Therefore the calculated tensile strength of linearly extended polyethylene exceeds that of steel, also the stiffness is greater. In conventionally processed polyethylene, the molecules are however not continuously linearly extended, but coil in the amorphous regions, so that polyethylene possesses a stiffness and tensile strength that is far below the theoretical maximum. It was therefore reasonable to synthesize polymers which are stiff due to their molecular structure, that they do not form a coil or ball, but also exist in the liquid state as stiff rods. This was achieved with the main chain LCPs. They are assembled from stiff mesogenic units and thus become themselves mesogenic. Like matches in a box, they align parallel in the liquid state and form liquid crystalline regions. The advantages of thermotropic main chain LCPs compared with conventional polymers result from an unusual combination of physical properties, which lead to a particular class of high performance plastics: 0 0 0 0

very high very high very high very low ceramic.

tensile strength, Young’s modulus, impact strength, thermal expansion coefficient, comparable to that of steel and

As the main chain LCPs essentially consist of aromatic building blocks, they possess in addition: 0 0

high chemical resistance and inherent flame resistance.

Materials for Auxiliary Applications

Polymers for Lithography Another group of materials gains importance in microsystems technology due to their ability to be patterned by subjecting them to the effect of photons or electrically charged particles: the group known as resists. An important polymer for microsystems technology, especially the LIGA technology (see Chapter 7), is polymethylmethacrylate, abbreviated to PMMA, whose monomer structure is shown in Fig. 3.4-19. The monomer has a molecular weight of 100 g/mol, as can be easily calculated. The polymer can form chains of several 1000 units and reaches molecular weights of 100000 g/mol. The chemical and physical properties of the polymer, for instance the solubility in certain “developers”, depends greatly on the molecular weight. The bonds, which lead to a polymer chain can be broken by means of high energy radiation. PMMA is therefore sensitive to such radiation in the region of /1 = 1 nrn and shorter or also to electron irradiation of 20 keVor higher. Therefore, for these energy regions, PMMA and similar polymers can be used as a “resist”. The meaning of resist in microtechnology is described in Chapter 5 , therefore here a brief explanation should suffice: The surface of a production part is covered with the polymer. Subsequently the structural features of a

Fig. 3.4-19 The structure of polymethylmethacrylate.

3.4 Muterials of Microsystems Technology

105

mask are transferred into the polymer layer by means of irradiating a suitable mask with a parallel beam of photons. In the irradiated regions the molecular weight of the polymer is reduced by chain scission and consequently the solubility is raised. After treatment with a solvent a relief image exists on the surface, which corresponds to the absorber structure of the mask. Under the presumptions that the insoluble structure of the remaining polymer protects the underlying surface and so is “resistant” to a subsequently used etchant (hence the name resist), the surface of the production part can be etched in a pattern. However, the wide application of PMMA is based on another property. The polymer is also best suited for shaping via injection molding. So pre-formed PMMA layers can be exposed again, in order to form several structural layers. However, in doing this compromises must be made with respect to the optimal molecular weight, as PMMA which was optimized as a resist does not simultaneously supply the best results with injection molding. In contrast to the above described “X-ray resist” there are numerous “photoresists”, which are used in optical lithography with light wavelengths between 193 and 400 nm. The mechanism of illumination is basically different. The positive resist consists of a resin in which an inhibitor, mostly diazonaphthochinone, is incorporated. This inhibits the rate of dissolution in a developer. On illumination, the inhibitor is neutralized and the rate of dissolution is drastically increased. With positive resists, the illuminated areas are preferentially dissolved in the developer. In contrast, with negative resists, an additional duroplastic cross-linking takes place on illumination which considerably reduces the rate of dissolution. With negative resists the non-illuminated parts of the resist are dissolved out, whilst the illuminated areas remain.

Electrorheological Fluids With electrorheological fluids the influence of an electric field changes their viscosity. This change of the viscosity can occur up to solidification of the liquid. After switching the field, the liquid goes back into the low viscosity state. This effect was already discovered and patented by Willis M. Winslow in 1942. There are basically two types of electrorheological or ER fluids: 0

the disperse ER fluids

and the 0

homogeneous ER fluids.

In the case of the disperse fluids, colloidal suspensions of semi-conducting particles in a dielectric fluid are used. Mineral oils, chlorinated hydrocarbons and hydrocarbonates are used as liquid components whilst the solid components consist

106

3 The Physicul and Chemical Basics in Microtechnology

a

b

c

Fig. 3.4-20 The electro-rheological effect.

of the finest powders of aluminum oxide, iron oxide, gypsum, cellulose, gelatin and other materials. Although the properties of ER fluids provoke in particular technical applications, over the course of the last decades this class of materials could not maintain its hold on the market. This was mainly due to the lack of durability of ER fluids. The particles have a tendency on the one hand to coagulate and on the other to sediment. In both cases the rheological effect disappears. In order to avoid this coagulation, tensids are mixed into the fluid. Basically a new situation has arisen with the development of homogeneous ER fluids. All ER fluids, that are known today, belong to the group of liquid crystals. As was described in the previous section, liquid crystals are made of polar polymer chains, which can align in an electric field and depending on the type, take up a crystalline phase. There exists not only a structural anisotropy in the ordered phase as well as optical anisotropy, which is used for displays, but also a mechanical and fluidic anisotropy. Depending on the degree of order, the viscosity of the fluid can be changed. Under the condition that liquid crystals undergo no change during the process (are cracked), no aging will occur, in contrast to disperse ER fluids. The electrorheological effect as an example of a disperse ER fluid, is shown Fig. 3.4-20. One distinguishes between two modes, the shear mode and the flow-mode. On applying a voltage between the two plates, the viscosity of the fluid changes and shows an increased resistance against the shear movement of the electrode plates or against the flow of the liquid parallel to the plates. In Fig. 3.4-21 the results of the viscosity measurements are given, which were obtained in an oscillating disc viscosimeter [Mori95]. The increase differs depending on the oscillation frequency. The change of the viscosity starts for instance with a field of 100 V/cm and achieves a saturation at 1500 V/cm with an oscillation frequency of about 300 s-'. Homogenous ER fluids could offer a wide range of applications in microsystems technology. So valves, pumps, switches and other microactuators without

3.4 Materials of Microsystems Technology

107

0.08

0.06

0.04

0.02

0

0

500

1000

1500

2000

2500

Electrical field (Wmm)

Fig. 3.4-21 The measurement of the viscosity on electro-rheological fluids.

mechanically moveable parts could be developed. The development is still in the early stage. The future will show, whether electrorheological fluids could form an alternative for mechanical components. If one subjects an LCP melt to a sheer- or extensional flow, as is the case e. g. with injection molding or extrusion, the mesomorphic domains are oriented in the flow direction. Oriented fibers exist in one direction from parallel rod-like molecules. On cooling this orientation is frozen in and remains in the solid state. Because the polymer is then reinforced with fibers from the same material, the thermotropic main chain LCPs are also called self-reinforcing polymers.

This Page Intentionally Left Blank

4 Basic Technologies in MEMS

4.1 Basic Principles of Vacuum Technology All processes to produce and characterize thin films, as well as a large part of the structuring processes, are carried out in vacuum. Therefore, the concept of the vacuum will now be explained. The word “vacuum” originates from Latin and means the ideal case of a material free space, which does not exist in reality. In reality vacuum means a volume with a significantly reduced pressure compared to the standard conditions of air pressure at sea level. The pressure decreases with increasing elevation. The air which surrounds us, like everything on earth, is influenced by gravity. If one imagines a cylinder over a unit area at sea level and if the pressure p o at this level is known, then the pressure distribution can be determined in the cylinder dependent on the height h. The corresponding formula is known as barometric formula:

where

m = mass of the air at a height h = 0 in a volume Vo and at a pressure po, h = altitude above sea level, g = gravity of the earth. This formula can be derived from the equilibrium of the downward flow of the particles under the influence of the gravity: j,,,

= -lumgn(h)

with y the mobility of the particles and n(h) the particle density (dependent on the height), and the upward flow due to diffusion (Fick’s law):

(4.2)

110

4 Basic Technologies in MEMS

+

The solution of the differential equation j,,,, jdi,= 0 finally leads to the barometric equation (4.1). If the standard values are inserted, then the following approximation formula is obtained: h

p = p0e-X

(4.4)

in which the height h is measured in km. As can be easily calculated, the “half value” for the pressure corresponds to h,,, = 5.5 km. In Table 4.1-1 some values of prominent locations are listed. Since the barometric formula is only an approximation (g is considered as a constant), it is not valid for heights beyond the stratosphere. To produce a vacuum on the earth’s surface or in the laboratory, as it only exists at great heights or in space, the pressure of the gas or the density of the gas molecules in a container must be reduced. This is performed with vacuum pumps that either pump the gas molecules out of the container or alternately adsorb the gas molecules onto a surface.

Table 4.1-1 Some pressure values for characteristic locations or conditions

Air pressure at sea level Variations due to weather CN tower Toronto Summit of Mount Blanc Summit of Mount Everest Space shuttle Geo-stationary satellite

* these values are not described

Height [m]

Pressure [mbar]

0

1013 930-1070 933 560 350 1 . lo4* 1 . 10-’0*

553 4807 8848 250 . lo3 36,000 . lo3

anymore by the barometric formula

4.1.1 The Mean Free Path The gas atoms in a container move linearly in one direction because of thermal energy until they collide with either the container wall or another gas atom. The mean free path h is that distance which on average an atom can freely cover (i. e. without collision with another atom). Basically, the less gas particles there are, on average the further an atom can move linearly i. e. the larger is the mean free path. In a simplified model we consider the gas atom as hard “billiard

4.1 Basic Principles of Vacuum Technology

111

balls” which collide if they approach each other at a distance of Zr, where r is the radius of the ball and Zmis the impact parameter of the two balls touching each other. The suffix indicates the behavior of the balls at temperature T +a where the particles travel with very high speed in straight lines without considering the attraction of particles when approaching each other. At lower temperature the attraction comes into play and consequently the cross section of the particles increases. The conditions are represented in Fig. 4.1-1. The particle travelling at a distance A1 through a gas of density n will collide with any other parAl. The number of particles in this ticle, which is found in the cylinder AV = cylinder is AN = n .AV = n .Ern .A/.The mean free path would therefore be in this approximation: (4.5) However, using this formula we neglect the fact that all other particles are moving too. By considering this relative movement, the mean free path must be re4x2, duced by a factor 1 / 4 2 . As can be seen in Fig. 4.1-1 C , = xR2, : = 40, therefore we get for the mean free path:

a

b

C

Fig. 4.1-1 a) Cross section (J, of two atoms considered as hard spheres (at a temperature T -00). b) Two atoms travelling at T c) atoms travelling at lower temperature and considering attractive forces.

112

4 Basic Technologies in MEMS

If one also considers that the atoms in reality do not move independent of each other, but are attracted over large distances and therefore describe non-linear pathways, then another correction must be introduced. The pathway depends now on the speed of the particles and therefore on the temperature i? If R, is the collision radius at a temperature T + and RTthe collision radius at T, then the so-called Southerland correction is valid: R;

= R$

(1

+ Td/T).

(4.7)

Tdis the “doubling temperature”, i. e. at T = Td, gT= 20,. The corrected formula for the mean free path is therefore:

Some values for the average free path are given in Table 4.1-2. Table 4.1-2 Mean free path of molecules at different pressures ~~

Pressure

Air

Hydrogen

100 mbar (lo5 Pa) 1 nibar (10’ Pa) mbar (lo-’ Pa) mbar ( Pa) loA9mbar Pa)

6.10-6 cm

2 . W cm 2.10-~cm 20 cm 200 m

6.10-3 cm 6 cm 60 m 60 km

200 km

4.1.2 The Monolayer Time If gas atoms collide on a solid wall, then they remain there with a particular probability on the surface, i. e. they are adsorbed. In competition with that is the process of liberation of atoms off the surface, i. e. desorption. The undesired desorption at the walls of a vacuum container decreases the quality of the vacuum and increases the pumping time, which is necessary to evacuate a container. If the surface plane is covered as densely as possible with adsorbed gas atoms adjacent to one another then one refers to it as a monomolecular layer or a monolayer. At lower coverages one refers to a coverage 6:

6=-- ii

nmono

with ti = actual particle density at the surface, = particle density of a monolayer. and ,,it

(4.9)

4.1 Basic Principles of Vacuum Technology

113

We can also define a monotime, which is the time necessary in order to cover an initially free surface with a monolayer. The calculation will not be carried out here, but it can be derived for gases from the equation of state and the Maxwellian distribution of velocities: (4.10) with ma = atomic mass, NA = Avagadro’s constant, R = universal gas constant, Mmolar= molar mass. For nitrogen molecules with a radius a monolayer is then: fi,,

M

Y =

1.6. lop’’ m, the surface density of (4.11)

1Oi9m-2 = 10~pn1-~.

With this value for firno,,the monotime can be calculated:

-

nmono

. -,/-

tmono= 3,8 .

(4.12)

P with M, the relative molecular mass (generally air with M , in mp2, p in mbar and T in K.

=

29), t,,,

in s,

fiilnollo

If one inserts the values for air, then the approximation formula is as follows: 3,6 . tmono =

(4.13)

P

Some values for the monotime for air are listed in Table 4.1-3.

Table 4.1-3 Values for tmonoversus pressure (for air) Pressure (mbar)

1

10”

10-7

10-l1

mono (s)

3.6.

3.6.10-~

36

-100 h

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4 Basic Technologies in MEMS

4.1.3 Velocity of Atoms and Molecules An important parameter for the fabrication of thin films is the velocity of the particles to be deposited. The velocity of an individual particle at a certain time to is technically of little interest since by collision and energy exchange the particles changes the velocity continually. Only the distribution of velocity of a large number of particles is of relevance. As can be seen in Fig. 4.1-2 the so-called Maxwellian distribution is quantified by the temperature. The maximum of the distribution curve defines the average velocity of the particles. With higher temperatures the distribution curve widens out and the average velocity increases. The average kinetic energy of a particle can be calculated in a first approximation by means of the relationship:

1 3 -mv2 = -kT 2 2

(4.14)

(4.15) Here the speed is averaged quadratically. With the more refined model of the Maxwellian distribution one gets: (4.16) m is the relative atomic mass, and M is the relative molecular mass.

Fig. 4.1-2 Maxwellian velocity distribution in dependence on temperature.

4.1 Basic Principles of Vacuum Technology

115

Inserting numerical values (T in K) leads to: (4.17) In Table 4.1-4 some values of average speed at room temperature are given.

Table 4.1-4 Thermal velocities of molecules at 20 "C (293 K) ~~

Type of gas

Rel. mole-cular mass

Average speed [ms-'1

H2 He H20

2 4 18 28 29 40 44

1755 1241 585 469 46 1 392 374

NZ Air Ar COZ

4.1.4 Gas Dynamics The state of a gas, i. e. pressure p , density p, temperature T, and velocity v, can be calculated by means of the gas dynamics. The theory looks at the gas as a compressible continuum. The limits of the application of this theory are given by the Knudsen number which is defined as:

A

Kn = D

(4.18)

with , I= the means free path of the molecules, and D = the relevant dimensions of the system under consideration.

D can be the diameter of a tube, the dimension of a vacuum pump or the typical dimension of a container. To apply the gas dynamical continuum theory, the Knudsen number should be Kn < 0.01, meaning the mean free path is considerably smaller than the dimension of the volume in question, or with other words, there are so many molecules that the behavior of a single molecule is statistically averaged out and the whole of the gas can be considered as a fluid. On the other hand for Kn > 0.5 the mean free path of the molecules is equal or larger than the dimensions of the container. The interaction with the container walls are predominant over the interaction of the molecules with each other. In this case the gas dynamics theory is replaced by the molecular flow theory. We

116

4 Basic Technologies in MEMS

will see the influence of the molecular flow when discussing the high vacuum pumps. To clarify the situation once more, a gas at a pressure of mbar may be treated as a molecular flow when enclosed in a container of SO 1 volume, but is a continuum under meteorologic conditions in the stratosphere. The region 0.5 > Kn > 0.01 is not well defined theoretically and is usually treated by extrapolation from either one of the defined regions.

4.1.5

The Classification of Technical Vacua

The technical vacuum is separated into different pressure regions. The distinct characteristics are the mean free path of gas molecules h and the monolayer time or monotime tmonoof the container walls. This classification is listed in Table 4.1-5.

Table 4.1-5 The classification of the technical vacuum region (b = container diameter) Pressure Region

Pressure [mbar]

Mean free path

Monotime [sl

Rough vacuum Fine vacuum High vacuum Ultrahigh vacuum

1013-1 1-10-~ 10-3-10-7

h <
T

h=b

T

h >>b h >>b

T > l

<

<
T < l

Knudsen number

Kn Kn Kn Kn

<< 1 = 1

>1 >> 1

In the first region - a rough vacuum -, the mean free path is still small compared to the container’s dimensions, which in the laboratory is usually under 1 m. The Knudsen number indicates a gas continuum. For the case of a fine vacuum, the mean free path is of the order of magnitude of the container dimensions. A gas molecule can already move some decimeters until it collides with another gas particle within the medium. In both cases the monolayer time is still considerably smaller than one second. At high vacuum the mean free path is already very large compared with the container dimensions. The Knudsen number is larger than 0.5 and the gas follows the molecular flow theory. Here the monolayer time, which is still under one second, is a characteristic feature. Depending on the type of molecule, the mean free path h is in the range of 60 m to 200 m at a pressure of mbar. With ultrahigh vacuum the mean free paths are in the order of kilometers and the monolayer time is longer than one second. For surface analysis and for maintaining very pure layers in microelectronics, an ultrahigh vacuum is needed, to avoid the formation of a contamination layer of gas molecules after a very short time. The number of molecules per mole is described by a physical constant, Avagadro’s constant, which is 6.0252. loz3particles per mole. The molar volume of any gas under normal conditions is 22413.6 cm3. If Avagadro’s constant is

4.2 VacuumProduction

117

divided by the molar volume, the particle number per cubic centimeter under normal conditions is obtained as no = 2.688. lo1’ particles per cm3: n=--

’’

1013

with the pressure p in mbar.

(4.19)

From this one can deduce that under ultrahigh vacuum of lo-’ mbar, still 2.6.107 particles per cm3 are present in the vacuum container. There is still 1 molecule per cm3 even in interstellar space.

4.2 Vacuum Production In the following sections the different technical methods and devices with which a vacuum can be generated will be described. Vacuum pumps are characterized depending on the kind of operation and of the transport mechanism of particles (Fig. 4.2-1). In the first section the commonly used gas transfer pumps will be characterized. These pumps are subdivided into displacement pumps for low vacuum regions and kinetic pumps for high vacuum regions. With sorption pumps discussed in later sections the gas molecules are trapped on the inner surface by sorption or condensation. These pumps work discontinually, that is after a certain working time they must be heated up in order to

Pumps

Displacement

t

Pumps Getter Pumps Cryosorption Pumps

Kinetic

Piston Pumps Rotary Vane Pumps

t

Oil Diffusion Pumps Turbomolecular Pumps

Fig. 4.2-1 Classification of vacuum pumps.

118

4 Busic Technologies in MEMS

release the adsorbed gas molecules. In the following section the operation and performance data of the most important types of pumps are described.

4.2.1 Pumps for Rough- and Fine Vacuums A displacement pump sucks in, compresses and discharges the gas by means of rotors, pistons or vanes. Displacement pumps can pump against atmospheric pressure, and they are therefore essential as back-up e. g. for diffusion-, turbomolecular- and cryo-pumps. The vane-type rotary pump is the most frequently used vacuum pump of all the displacement pumps. For nearly all high vacuum pumps they are used for pre-evacuation (roughing) of the container. There are numerous types with a wide range of dimensions with respect to the pumping speed and attainable ultimate pressure. The basic construction of a vane-type rotary pump is shown in Fig. 4.2-2. The aperture of the outlet valve is covered with a layer of oil (oil-immersed), in order to avoid back flow. The design also ensures that a sealing film of oil is always present between the pump casing and the rotor. Therefore, a good seal between the compression and inlet chamber can be attained. If water vapor or other condensable gases are transported with the gases, then condensation may occur in the compression phase. The condensed liquid penetrates the seal gap between the inlet chamber and the compression chamber and reduces the ultimately attainable pressure of the pump. Furthermore, the water can break down the oil

oil immersion

a

b

C

Fig. 4.2-2 The working principle of the vane-type rotary pump

4.2 Vacuum Production

119

film and at the same time lead to corrosion and finally to the destruction of the pump. To prevent the condensation a gas ballast inlet is integrated in the compression chamber near the exhaust valve. By means of this gas ballast valve air from the outside is added to the compressed gas and immediately expelled through the exhaust valve thus preventing the condensation of the pumped gas from the vacuum chamber. In addition another adverse effect is prevented. Without the gas ballast there is no gas dampening between the oil supply which is moved around by the rotating vanes and the sealing oil at the exhaust valve. With every rotation these oil portions are subjected to a heavy impact which can be noticed by a strong knocking noise of the pump (oil beating). By adding the gas ballast this beating is reduced and the pump runs smoothly. The typical pumping speeds for a rotary vane pump are between 4 to 65 m3h-l and the achievable ultimate pressure is for the single stage pump mbar mbar (with gas (with gas ballast lo-' mbar) and for a double stage pump ballast 5.10-2 mbar).

-

4.2.2 High Vacuum- and Ultrahigh Vacuum Pumps In the last two decades vacuum techniques have obtained considerable advances with technical improvements, especially in the ultrahigh vacuum region. With the turbomolecular, cryo- and ion getter and titanium sublimation pumps it has been possible, to achieve vacua in the super ultrahigh vacuum region (down to mbar). The most important representative of the mechanical kinetic vacuum pumps is the turbomolecular pump. It works in the region of molecular flow. The atoms which are to be pumped must have mean free paths in at least the region of the rotor dimensions of the pump, therefore the pump requires a starting pressure of about 10-1 to lop2 mbar. For this reason, the turbomolecular pump works always in conjunction with a roughing pump for instance a vane-type rotary Pump. The main components of the turbomolecular pump are the rotor and the stator. The blades of the rotor and the corresponding blades of the stator have an arrangement similar to that of a gas turbine (Fig. 4.2-3). The principle method of use of a turbomolecular pump is illustrated with the aid of Fig. 4.2-4. The side view of the blades of the rotor is shown. For simplification it is assumed that all the gas particles have an isotropic average speed v and that the rotor blade speed u = IvI. From Table 4.1-4 the speed amounts to about 500 m/ s for air. The simplified speed distribution of the atoms within the chamber is shown in Fig. 4.2-4 section a. In section b of Fig. 4.2-4, the speed distribution is shown as an observer would see it when travelling with the rotor blade. In the sketch it is assumed that the high vacuum part is on the left hand side and the low vacuum on the right hand side of the rotor blades. Studying the diagram it is obvious that all particles, which start from dA in the direction of the low vacuum within an angle p, can fly through the rotor area without collision with the blades. Particles, which stick to the rotor walls will again be desorbed isotropi-

120

4 Basic Technologies in MEMS

backing pressure

Fig. 4.2-3 Schematic view of a turbomolecular pump.

Low vacuum

Fig. 4.2-4 Working principle of a turbomolecular pump. a) Idealistic velocity distribution of the gas particles in the vacuum charnber with the uniform velocity C. b) Distribution as an observer would see it travelling with the velocity of the turbine blades 8,which is for simplicity reasons equal C. c) Even particles hitting the blades and being adsorbed in the mid-size position and, after some delay, being desorbed from the blades have a better chance (aperture angle) to travel from the high vacuum side to the low vacuum than in the reverse direction.

4.2 Vacuum Production

121

cally, after a period of time. Taking the mid-size element dC it is seen from the sketch in Fig. 4.2-4c that the aperture angle to the right is larger than that to the left. This again adds to the drift towards the left side (lower vacuum). The particles which fly directly through the rotor, as well as particles which have obtained an impulse transfer towards u during the time period of the rotor blades, show a drifting movement in direction u and thus can penetrate another neighboring stator (not sketched in the figure) more easily to the right than a particle which starts from right to left. The described operation of a turbomolecular pump is in fact very approximate, an exact analysis can however be carried out by bringing very sophisticated mathematics to bear on the problem. The suction speed of a turbomolecular pump is almost constant in the pressure region between and lo-'' mbar and differs little with different gases, with the exception of helium and hydrogen (which is 40 % and 25 % respectively less than other gases). Turbomolecular pumps have suction speeds of up to 600 l/s for nitrogen.The reason why there is hardly any back streaming and the vacuum is produced is free of hydrocarbons is due to the large compression ratio of turbomolecular pumps for gases with a high molecular weight (for nitrogen about lo8, for H2 about 104). Ejector Vacuum Pump In the field of the ejector vacuum pumps only the diffusion pump is of interest for high and ultrahigh vacuum techniques. With this type of pump a vapor pressure of 1-10 mbar is produced by heating oil in a boiling tank. The vapor expands inside the body of the pump and is deflected downwards diagonally through an annular jet. At this point, due to expansion, the vapor jet attains supersonic velocity (Mach 3-8). The gas particles from the high vacuum side diffuse into the vapor jet and by imparting a momentum from the oil jet move towards the pump outlet (Fig. 4.2-5). The oil jet on reaching the cooled walls condenses and flows back into the boiling tank. The pumped gas molecules migrate into the roughing pump and are finally discharged from the system. The ultimate pressure depends on the vapor pressure of the pumping oil and reaches the ultrahigh vacuum region (lo-'' mbar for NJ. A big disadvantage of diffusion pumps is the low compression rate. When the roughing pressure drops below lo-' mbar, oil particles can penetrate the vacuum chamber above the diffusion pump and spoil the high vacuum. Even with complex vapor barriers such as cold traps and cooled baffles the back-streaming cannot be suppressed totally. Diffusion pumps offer a wide range of pumping speeds between 10 and 50000 Us. Sorption Pumps The sorption pumps [Wutz89] belong to the group of gas-bonding pumps. There are basically three types of sorption pumps:

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4 Basic Technologies in MEMS

high vacuum

Fig. 4.2-5 Cross section of an oildiffusion pump. The oil vapor guided through the chimney is deflected towards the low vacuum side and condensed at the cold walls of the casing. The gas particles to be pumped from the vacuum chamber diffuse into the oil vapor jet and experience a drag toward the low pressure side.

Adsorption pumps The adsorption pump consists of a chilled surface to condense the atoms to be pumped. This type of pump is very simple in design due to the absence of any moving part. The disadvantage lies in the fact that it is very ineffective in pumping nobel gases. In general, with adsorption pumps activated carbon or zeolite is used. These materials possess an extremely high specific surface area of about lo6 m2/kg due to their “sieve structures”. As mentioned in Section 4.1.2, the surface binds about 1019atoms per m2 when fully covered. This corresponds to a surface absorbed materials amount: (4.20) To initialize the absorption pump it is heated to between 200 and 300”C, in order to remove the impurities of the absorbent from the previous pump cycles. Then the zeolite bed is cooled down with vacuum valve closed. To reduce the cooling time, the absorbent has a cooling coil immersed within it (Fig. 4.2-6). After reaching the working temperature the valve of the vacuum chamber is opened and the gas diffuses into the pump. All gases except hydrogen are pumped,

4.2 Vacuum Production

123

Vacuum flange

I Zeolith Coollant

package

Cooling Pipe

- Mesh

Coolant Fig. 4.2-6 Principle design of a sorption pump.

and the noble gases He and Ne to a lower degree. If an ultimate pressure of lop7 mbar should be attained, the content of noble gases in the standard atmosphere is preventing this. The partial pressure of neon is at about 2.10-' mbar and that determines the attainable ultimate pressure. If however the container is pre-evacuated with another type pump, which is efficient in pumping nobel to lop7 mbar can be obtained with an absorption gases, then a pressure of pump. Another possibility to reach a good ultimate pressure is flushing the container with clean N, prior to pumping. The advantage of an adsorption pump is seen in its simple construction and in the fact that a vacuum can be produced that is absolutely free of hydrocarbons. The disadvantages consist of the non-continuous mode of operation, and having to fill with activated charcoal, and the risk of explosion with inadvertent leakage of air. Another type of adsorption pump is the cryo-pump. In the case of cryo-pumps the gases which are in the chamber condense or absorb onto the adsorption agents (activated charcoal) which are held at very low temperatures; below -100 "C. Cryo-pumps supply an extremely clean, dry and oil free ultrahigh vacuum of up to lo-'' mbar. A roughing pump is required to initiate the cryo-pump and can subsequently be switched off as soon as continuous operation is reached. The initial roughing pressure should not exceed 100 mbar. Since the cryo-pump is an adsorption pump, it must be regenerated at regular intervals. To accomplish this the pump is heated to room temperature. The condensed and adsorbed gases are released and must be evacuated by pumping with the roughing pump. This recharging interval depends on the amount of condensed gas and varies strongly with the type of use.

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4 Basic Technologies in MEMS

The cryo-pump has the highest specific pumping speed of all high vacuum pumps. Some typical pumping speeds for pumps with 500 mm flange diameter are [Barf89]: Water 28000 Us, nitrogen 10000 l/s, hydrogen 9000 I/s.

Absorption pumps An absorption pump again collects the molecules or atoms to be pumped onto a surface. The gases, which are to be pumped, are bonded to an absorbing agent on a large porous, high surface area, which is kept at low temperatures using liquid nitrogen. The condensed atoms penetrate into the bulk of the absorbent by thermal diffusion and chemical bonding. In contrast to the adsorbing pump H2 is pumped very easily, since H-atoms have a high diffusion coefficient and disappear from the surface quite rapidly. The getter pump is also of technical significance. In this case, the atoms to be pumped are trapped by a material which is evaporated onto a shield. Due to the evaporation of the getter material (usually titanium) the gettering surface is continuously replenished and thus keeps up the pumping action.

Cathode (Titanium)

I

I

&

IT

Permanent magnet

‘ I

Titanium atom Argon ion

7

Recipient Wall

Fig. 4.2-7 Principle of an ion getter pump. By means of an electrical field ions are generated which are accelerated toward the titanium target. The titanium is sputtered and subsequently condenses at the walls of the pump gettering free gas atoms.

4.3 Vacuum Measurement

125

Ion getter pumps

A sophisticated type of getter pump is represented by the ion getter pump. This pump consists of a device which generates high energy electrons which in turn ionize the gas atoms or molecules to be pumped. These ion are accelerated toward a solid electrode made from titanium. By impact of the heavy ions the titanium is sputtered onto a shield and there absorbs the gas molecules by physical trapping or by chemical reaction with the titanium atoms (Fig. 4.2-7). With an ion getter pump even nobel gases and other chemically inert gases can be pumped.

4.3 Vacuum Measurement An integral part of vacuum generation is the measurement of the pressure. It is necessary to control the operation of the vacuum plant and/or to control the process within the vacuum plant (i. e. thin film deposition, etching, surface analysis). The technically available measurement range for pressure extends over about 15 orders of magnitude. It is immediately apparent that this large spread of pressure cannot be analyzed with only one technique or set-up. Essentially there are five techniques which are of interest for technical vacuum facilities. They are: 0 0 0 0 0

pressure transducers, thermal conductivity vacuum gauges, friction type vacuum gauges, thermionic ionization gauges, and finally cold cathode ionization gauges (Penning principle).

In the following sections the five principles will be briefly discussed, the advantages and disadvantages described and the measuring regions will be indicated.

4.3.1 Pressure Transducer The oldest technique to measure continuously pressures from about loi3 mbar to 1 mbar, is the pressure transducer, a closed container, of which one wall consists of a corrugated membrane. Depending on the differential pressure, this membrane curves either into the box or out of the box. This curvature can be monitored using a suitable lever and transferred to a pointer. The non-linearity error of such set-ups are lowered to a few percent by suitable electronic evaluation. Alternately a bent tube which is suspended in an evacuated container can also be used instead of a membrane box. The tube tries to straighten out depending on the pressure which is applied. This movement is transformed to a lever and a pointer.

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4 Basic Technologies in MEMS

4.3.2 Thermal Conductivity Vacuum Gauge A thermal conducting vacuum gauge consists of a tube connected to the vacuum to be measured, and contains a concentric spiral wire, which is heated electrically. The dissipated thermal energy is dependent on three components, firstly the thermal conductivity of the surrounding gas, secondly the thermal conductivity of the electrical contacts at the ends of the wire and thirdly the radiation energy of the hot wire. The heat conductivity is the only component that is pressure dependent (but also very dependent on the nature of the gas) and is therefore used for the actual measurement. The other two components can be kept constant if the temperature of the wire is kept constant. The measuring range for a TCVG is in the order of lop3 mbar up to some hundred mbar depending on the construction.

4.3.3 Friction Type Vacuum Gauge FTVG represents a new type of technique and one which is accurate over a large measuring range [Beam46, Frem821. Here a rotating steel ball is suspended magnetically in a sensing chamber (Fig. 4.3-1). The rotation, once the driving power is switched off, slows down due to the friction with the surrounding gas. This speed reduction is measured and used to determine the pressure. A re-calibration is not necessary because the instrument constants of such a device can be calculated precisely. Using this method, the absolute pressure in a container can be measured. The measuring range covers the range between mbar to about 2 . mbar and therefore serves the technically interesting areas of evaporation and sputtering.

ignet Fig. 4.3-1 A friction type vacuum gauge. A rotating ball is suspended magnetically. Due to the remaining gas particles, by friction, the rotating velocity is slowed down.

4.3 Vacuum Measurement

127

4.3.4 Thermionic Ionization Vacuum Gauge The arrangement of this vacuum gauge is seen in Fig. 4.3-2. Electrons are emitted from a heated filament and accelerated towards an anode. On the path to the anode the electrons collide with neutral gas atoms which are then ionized. The ions traverse through the anode grid and are caught by an ion collector. The ion current is proportional to the number of atoms per unit volume and consequently to the pressure in the measuring chamber. The proportionality is limited at the high pressure end by space charge effects, ionization by secondary electrons, and excitation of charged particles by electrons hitting the anode. The low pressure end is characterized by a residual current of secondary electrons, which are released from the ion collector by soft x-rays, which are in turn generated by electron bombardment of the grid. Electrons leaving the collector are indistinguishable from positive ions arriving at the collector. Therefore, in the so-called Bayard-Alpert gauge, the ion collector is shielded against photons, to keep the latter effect as small as possible. The measuring range for this gauge covers the pressure region of about lo-'' to 1 mbar.

4.3.5

Cold Cathode Ionization Gauge (Penning Principle)

With the Penning ionization vacuum gauge a gas discharge is struck between two electrodes. The gas discharge current is dependent on pressure and serves as a measurable parameter. In order to increase the probability of electrons ionizing neutral atoms on the path to the anode, a magnetic field is applied using a permanent magnet. Due to this field the electrons move in spiral paths and therefore

Pig. 4.3-2 a) Thermionic ionization gauge, b) Bayard-Alpert gauge.

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4 Basic Technologies in MEMS

have a larger ionic yield. The measuring range reaches from about to mbar. Although the Penning gauge does not work particularly accurately, it is very widely used because of its simple construction.

4.3.6 Leakage and Leak Detection There is no vacuum plant without leakage. Leaks are referred to as “real” leaks and “virtual” leaks. With a real leak, gas passes from the outside atmosphere into the vacuum chamber. The source of the leak is therefore inexhaustible. In the case of the virtual leak, the gas originates either from the chamber’s walls by desorption or from materials inside the chamber which absorb and give off gases easily, like many polymers, and finally from gases trapped in cavities within the container. In the latter case improper welding seams, screw and pocket hole combinations will serve as exhaustible sources for leakage. A leak is usually measured in a vacuum vessel, with the high vacuum valve closed, by an increase of pressure at a certain time interval. The leak rate Q, thus is expressed in the form: &I

(4.21)

= V . (dp/dt)

with V the volume of the vessel. The unit of the leak rate is [mbar 1 s-’]. In Fig. 4.3-3 the pressure versus time diagram shows a real leak and a virtual leak. Usually it is not an easy task to find the exact location of a leak, especially a virtual leak. Talking of a real leak though, an experienced operator sometimes knows the weak points of his equipment. All kinds of seals are latent sources of leakage. A leak at a particular location can be detected by exposing that location to a confined flow of Helium. Due to the high diffusion coefficient of He, the gas penetrates easily through the leak into the chamber. The He is detected very

t

a

t

b

t C

Fig. 4.3-3 The pressure-time diagram of a real leak (a), a virtual leak (b), and the combination of real and virtual leak [Cham98].

4.4 Properties of Thin Films

129

rapidly by a mass spectrometer which is connected to the exhaust line of the vacuum plant. With this approach the leak can be localized precisely. Sometimes it needs a lot of patience and experience to find and eliminate leaks.

4.4 Properties of Thin Films Thin films are the essential basis for the fabrication of integrated circuits as well as microsystems components in both the mass fabrication of products or in single samples for research applications. Thin films are either the carrier of the desired functions (sensors, actuators, electrical parameters), or serve an auxiliary purpose (e. g. isolation and protection layers, seeding layers for electrodeposition, removable sacrificial layers). Depending on the area of application the thickness of these films ranges from a few nanometers to some micrometers. The requirements which are placed on the films vary considerably, however the quality of films with respect to purity, absence of foreign matters, homogeneity of the internal structure, and adhesion to the substrate are always desirable properties. Thin films for microelectronics or MEMS applications are fabricated in most cases under vacuum conditions. Various methods of thin film production such as evaporation, sputtering, ion plating, plasma polymerization and chemical vapor deposition are discussed in the following Section 4.5. As the micro structure and properties of the growing films depend particularly on temperature during the deposition phase, this temperature influence is described by exemplary experiments performed by Movchan and Demchishin. Additionally the binding energy between the substrate and the thin films will be discussed, because good adhesion is a critical parameter of any thin film application.

4.4.1 Structure Zone Model The quality of a thin film which grows on a substrate via a vacuum process, is determined mainly by three parameters: 0 0

0

the physical state of the substrate surface, such as the roughness, the activation energy of the surface and the kinetic energy of the atoms to be deposited and finally, the binding energy between the adsorbed atoms and the substrate surface.

The roughness of the substrate can lead to shadowing effects for incident atoms which are incident obliquely at the surface and consequently leads to discontinuous coverage of the substrate. The result is a porous layer with low adhesion to the surface. At higher temperatures of the surface diffusion can compensate part of the shadowing effect. The temperatures necessary to accomplish this are proportional to their absolute melting temperature T,. The hypothesis therefore emerges that

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4 Basic Technologies in MEMS

one of the three effects, shadowing, surface diffusion, and volume diffusion, dominate in a certain region Tflm(i. e. the substrate temperature is normalized to T,,), and within this region the microstructure of the film has a characteristic appearance. This is the theoretical basis for the following structural model. Movchan and Demchishin [Movc69] investigated the structure of relatively thick (up to 2 mm) layers of Ti, Ni, W, Zr02 and Al,O, which were evaporated under high vacuum (10-3-10-7) as a function of TRm.The results are described by the three zone model shown in Fig. 4.4-la. This MD-model was extended by Thornton based on experiments using a hollow cathode sputter tube at a pressure of 10-400 mbar (Fig. 4.4-lb). Thornton thereby introduced the partial pressure of argon as a further parameter, in order to describe the influence of a gas atmosphere (however without ionic bombardment) on the film structure. He also introduced an additional state of film structure: a transition zone T between zone 1 and 2. This transition zone is not prominent among metallic and singlephased alloy layers, but probably with layers of refractory compounds and multiphase alloys, which are produced under high vacuum by either evaporation or, in the presence of inert or reactive gases, by sputtering or ion plating. In other respects, zones 1, 2 and 3 of both models are identical. Zone 1 includes the structures formed at low Tflm,Here the surface diffusion is not sufficient to cover the shadowed region. Needle-like crystals grow in height on a few nucleation sites, which become wider with increasing height by trapping other incoming atoms. The film is porous, displays low adhesion to the substrate, exhibits a rough, “cobblestone-like’’ surface, and has a high dislocation density and high internal stress. Due to the ramified appearance of the structure it is characterized as “dendritic.” In zone T the adsorbed atoms partially compensate the effect of shadowing due to an increased surface diffusion. In this zone the layer structure is fibrous and thicker compared to the previous structure. Zone 2 is defined by a temperature region, in which surface diffusion of the adsorbed atoms increases sharply and dictates the growth mode. The film has a columnar structure, in which the diameter of the columns increase with increasing substrate temperature. The films are denser and the adhesion is improved. The surface appears smoother than that of the dendritic structure. The structure of films from this zone is referred to as “columnar”. Zone 3 finally includes the Tflm region, in which the growth is determined by the volume diffusion of the evaporated atoms. It forms a re-crystallized structure of high density. The films display a polycrystalline structure, the adhesion is at its optimum. Under very clean and controlled conditions single crystal films can be grown in this region. The structure of films from this zone is characterized as “polycrystalline”. According to the extended model of Thornton [Thor74], the transformation temperatures TI and T2increase with increasing inert gas pressure P. The thermally activated atom on collision with the atoms of the inert gas loose part of their kinetic energy to the surrounding gas. Therefore, less energy is available for the surface migration, which is analogous to the case with lower substrate temperature without additional inert gas.

4.4 Properties of Thin Films

13 1

a

b Fig. 4.4-1

I a) The Movchan and Demshichin model, and b) the extension by Thornton showing the influence of an inert gas pressure on the microstructure of the film.

The situation is quite different when ions of the nobel gas are accelerated toward the substrate. The bombardment of the ions produce defects at the surface and thus increases the density of nucleation sites. By collisions of the thermal atoms with the high energy ions, energy is transferred to these atoms and their mobility is increased. This in turn results in more densely packed films, polycrystal-

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4 Basic Technologies in MEMS

line structure and higher adhesion to the substrate. In short: Evaporation combined with accelerated argon ions result usually in films with higher quality and improved adhesion to the substrate. However, an important difference exists between the constitution of evaporated thin films at high substrate temperature and those deposited at low temperature with additional ion bombardment: The structure formed by evaporation at high temperatures is the result of re-crystallization and crystalline growth by volume diffusion. With ion supported film growth the volume diffusion plays a less important role because of the lower substrate temperature, whereas with intensive ion bombardment new nuclei are continually formed so that a fine-grained but dense constitution exists. Furthermore, the ion bombardment cleans the substrate of foreign matter prior to deposition, and thus the successive adatoms encounter improved surface conditions and more intimate bonding to the surface.

4.4.2 Adhesive Strength of the Layer The adhesion of a thin film to the substrate depends on the state of the surface and the type of bonding, which the adatom with the surface is capable of forming. There are chemical, electrostatic and van der Waals bonding or a combination of these. The chemical bonding is the strongest of all bonding types which has a bond energy of 0.5 to about 10 eV. In the case of covalent-, ionic- and metallic bonds the bond strengths depend on the degree of electron transfer. In the first two cases compounds form, which are in general fragile and brittle, and in the latter are mostly ductile alloys. The van der Waals bonds are due to a polarization interaction, which requires no particularly intimate contact of partner atoms, but is weaker (0.1-0.4 eV) than the chemical bond and rapidly decreases in strength with increasing distance. The electrostatic bonds require formation of an dielectric double layer between the film and the substrate. Metal-insulator zones are preferred for this. Such a double layer, which results in a comparable bond energy to a van der Waals bond, can be established in layers of Al, Ag and Ag on polymer substrates. Theoretically it can be shown that a typical chemical bond (4eV) can counterbalance a mechanical stress of 1 .lo4 NmmP2 and a typical van der Waals bond (0.2 eV) can withstand a maximum stress of 5 . l o 2 NmmP2. Measurement of the adhesion strength often results in lower values than theoretically predicted. Often the inner stresses of a film are underestimated, which can exceed the external stress by far. This can lead to defects in the microstructure of the film which in turn decreases the adhesion strength well below the theoretically calculated value. As with ion bombardment, the nucleation density and at the same time the contact surface at the substrate can be raised, and in general films fabricated by sputtering rather than evaporation display adhesion strengths with superior values. These films though are surpassed in this respect by films produced by CVD-process, as in this case chemical bonds are formed. The measurement of adhesion strength of thin layers to their substrate is of importance, but usually not very satisfying, especially for achieving absolute values.

4.5 Physical and Chemical Coating Techniques

133

For a quick qualitative examination the “Scotchtape” film-test is suitable. An area of the film to be examined is cut crosswise with a scalpel into small squares. Over this a strip of Scotchfilm is stuck and stripped off at right angles. The number of squares, which stick onto the substrate and are not pulled off with the Scotch film, is a measurement for the strength of adhesion. A somewhat more precise method is obtained with a punch which is glued to the surface of the film to be tested. The punch is pulled off at right angles, shearing forces have to be avoided not to falsify the results. Therefore the punch is suspended on gimbals and the experiment is performed by means of a tensile testing machine. Frequently the scratch test is used with hard material layers, whereby a diamond tip is pulled across the sample with increasing load. At a certain load the film cracks like a thin layer of ice on a soft base. The scratch traces can be optically measured under a microscope. In another set-up the cracking of the layer is measured acoustically.

4.5 Physical and Chemical Coating Techniques Thin film deposition can be separated basically into physical coating techniques (physical vapor deposition = PVD) and chemical coating techniques (chemical vapor deposition = CVD). However with these categories, the possibility of thin film techniques are not yet exhausted. Besides a variety of combinations of the above, which are of special interest for microsystem techniques, LangmuirBlodgett techniques play an important role for the deposition of mono-layers especially in biosensor techniques. Also laser supported processes show potential application in this field.

4.5.1 Evaporation In evaporation, the material, which is to be deposited onto a substrate, is heated in a crucible above melting temperature (Fig. 4.5-1). Due to the thermal energy and the low binding force in a liquid some of the atoms can leave the melt by evaporation and travel to the substrate, where they condense and form a film. Unwanted deposits on the substrate surface can be removed by heating (desorption of the adatoms) prior to deposition. This process takes place in a vacuum chamber mbar. In this range the mean free path of the parunder a vacuum of to ticles exceeds that of the inside dimensions of the chamber and the probability of unwanted collisions with other gas particles decreases to almost zero. The process of deposition is thus running under well controlled conditions. The evaporation process is both theoretically as well as technically the best understood of all vacuum processes.

134

4 Basic Technologies in MEMS Substrate

Crucible

Fig. 4.5-1 The evaporation process. The crucible is heated, and the atoms move without collision (high vacuum!) to the substrate and condense forming a film. The average energy of the atoms is in the order of 0.2 eV.

The right choice of the evaporation source influences the quality of thin films. Essentially, there are three types of sources (Fig. 4.5 -2): 0

The directly heated resistance source (Fig. 4.5-2a). This consists usually of a metal sheet boat made of heat resistant material (tungsten or tantalum), in which the evaporant is placed. The passage of current heats the boat and the evaporant, which is melted and finally evaporated. Using this method only a small amount can be vaporized. If an alloy is used, where the components have different vapor pressures, the composition of the vaporized layer changes over the layer thickness. A frequently observed effect is the interdiffusion of the reactive vaporizing material with the boat and the subsequent corrosion of the set-up.

0

The inductively heated source (Fig. 4.5 -2b). Here an electrically conducting crucible (e.g. graphite) is heated using high frequency energy (usually 13.56 MHz) from an induction coil. If the evaporant is itself electrically conducting, then a crucible made of insulating material, such as quartz or boron nitride is used. The disadvantage of this method is the relatively expensive high frequency generator and the necessity of careful protection of the process environment against electromagnetic interference.

0

The electron beam vaporization (Fig. 4.5-2c). This process is thought to be the most frequently used. An electron beam is directed to the crucible ma-

4.5 Physical and Chemical Coating Techniques

135

Induction coit

Evaporation coil

crucible

b

a

C

Fig. 4.5-2 Different types of crucibles and sources for the evaporation.

terial and vaporizes it. In order to protect the hot cathode from bombardment by ionized source atoms, the cathode is placed sideways of or underneath the source, and the electron beam is bend and guided into the crucible by a permanent magnetic field. By additional electrostatic deflection of the electron beam in a predetermined phase ratio one can hit and heat up several crucibles at the same time. A wide range of alloys with exactly predetermined components (mix ratios) can be produced in this way. With all these evaporation sources the atoms emitted from the source have a mean energy E, of: E

-

“2

3 -kT, = 1,29 . T, [eV]

(4.22)

with k = 8,62.10-5 in eV K-’, T, = temperature of the source in K. At a source temperature of 1500 K the energy of the emerging atoms is then about 0.2 eV.

4.5.2 Sputtering The nature of plasmas A target (the source) and a substrate are placed opposite to each other in a chamber, which is filled with argon gas of to lo-’ mbar. The target and substrate are separated at a distance of a few centimeters (Fig. 4.5-3). When an electric field

4 Basic Technologies in MEMS

136

-

Substrate

reactive Gas

Argon _I_I,

Plasma

0.

*------.

- Target:

Fig. 4.5-3 The sputtering process. Neutral noble gas atoms (usually Argon) are ionized by collision with energetic electrons in the plasma. These ions are accelerated toward the target. By impact with the target neutral atoms are released from the surface. These neutral atoms condense on the substrate forming a film. The average energy of the atoms approaching the substrate is in the order of 10 to 100 eV. By adding a reactive gas to the chamber compound films such as A1,0, or TiN can be fabricated.

is applied between the target and substrate electrode, then the ever present few charged particles in the neutral gas are accelerated towards the cathode and anode respectively. The charged particles are due to cosmic radiation and natural radioactivity of the surrounding material. The accelerated electrons, when hitting neutral Ar-atoms on their way to the anode, can generate new ions and electrons. These electrons in turn can again generate new pairs of ions and electrons and so on. The ions on the other hand will generate secondary electrons when hitting the cathode. This avalanche of electrons maintains the plasma. Collision of electrons with neutral particles can cause one of the three effects: 0 0 0

Ionization, dissociation, excitation.

Dissociation is used in certain processes to generate radicals, which are neutral particles with free bonds. These radicals are chemically very reactive and are utilized for composite layers (A1,03) and for etching.

Atoms which arc being hit by electrons may get into an excited state. Thc absorbed energy is released again by emitting a photon. De-exitation and production of photons of characteristic wavelength is the reason for the glow of a particular color of a plasma. One can distinguish between a self-supported and ;i non-sclf-supportcd discharge. In the first case the ions arc generated by accelerafed electrons within the electric field and due to the secondary electron emission at the cathode. In the latter case electrons are supplied from an additional electron source such as a hot cathode. Self-maintained discharges are dependent on the electric field and the gas pressure (Paschen rule). In high vacuum the mean free path is very large. The possibility l'or an accelerated electron to collide with a neutral atom is therefor very low. At normal pressure (1000 mbar) the mean free path on the other hand is extremely small (some ten nanomctcrs). The electron can not be accelerated to the necessary ionization energy (in the order of 10 eV) on this short distance between two collisions.

The sputtering process Even if the evaporation is a well controlled process and is used to produce very pure films, it has the disadvantage of a relatively low adhesion due to the low energy of the condensing atoms. Sputtering gives better results fbr applications which require a strong adhesion to the substrate. The argon ions which are generated in the discharge are accelerated onto the target and because of their high kinetic energy kick out neutral atoms or molecular fragments from the target surface, which fly towards the substrate at high velocity and bit the surface. In contrast to thermal processes, the sputtered atoms have 10-100 tiines the amount of kinetic energy (Fig. 4 . 5 4 ) . The adhesion of the

3;6 eV

0.2,eV

Cu sputtered with ~

0

2

4

6

velocity

a

10

I 1o5cm/s I

Fig. 4.5-4 Comparison o f the energy o f sputtered and evaporated particle5

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4 Basic Technologies in MEMS

Fig. 4.5-5 Magnetron sputter targets.

film which forms on the substrate is also correspondingly high in comparison to evaporated films. Another advantage of the sputtering process is the fact that the target material does not have to be heated. Consequently, refractory materials such as tantalum or tungsten or ceramics, can also be sputtered. These obvious advantages are counteracted by the disadvantage of a lower deposition rate. In fact the rate could be raised by a factor of 10 by raising the ion yield in the plasma using a magnetic field in the so-called magnetron sputtering mode (Fig. 4.5-5). In this approach a magnetic field is superimposed onto the electric field. The electrons, instead of running straight along the field lines to the anode move on spiral paths and thus increase the possibility of finding a collision partner for ionization. The ion density and therefore the deposition rate is increased by at least a factor of ten compared to sputtering in the absence of a magnetic field. The magnets are usually placed underneath the target. A competing process to the deposition of material onto the substrate is the removing of matter from a surface. Depending on the operating parameters either one of these effects can dominate. Therefore one speaks of sputter-deposition and sputter-etching. Both processes can be applied to a substrate in a sequential order, meaning that prior to deposition a surface can be thoroughly cleaned first. The sputter etching is discussed in a later section (Section 4.6) together with other etching processes. High frequency sputtering is used for dielectric materials, as shown in Fig. 4.5-6. Instead of a DC-field a RF-field of usually 13.56 MHz is applied to the plasma. As the ions in the plasma have a much greater mass compared to the electrons, they can not reach the electrodes during a half phase of the high frequency voltage cycle, but oscillate between both electrodes. The electrons on the other hand have a higher mobility, and during a half period of the high frequency field can reach the electrodes. Due to the large capacitor at the RF-generator, the target electrode is charged negatively. The consequence is a superimposed DC-field to the RF-field. This is called a self-bias process. Due to the self bias the ions are accelerated toward the cathode until the negative charge is neutralized. In the next halfcycle the negative charge is build up again.

4.5 Physical and Chemical Coating Techniques

Fig. 4.5-6

139

High frequency sputtering with self bias effect. A RF field is applied to the electrodes. Due to the higher mobility of the electrons compared to ions, electrons can reach the electrode during one half phase of the high frequency voltage cycle whereas ions are to slow to reach the opposite electrode. Therefore electrons collect at the target forming a negative bias and accelerating the ions toward the target.

4.5.3 Ion Plating or Plasma Assisted Deposition With ion plating different processes are combined, such as thermal evaporation, sputtering, non-self-maintaining discharge, and ion bombardment. The thermally evaporated or sputtered atoms of a metal, e.g. Ti, pass through a plasma in which some of the atoms are ionized and some are impacted by argon ions in the direction of the substrate. By adding reactive gases, radicals are generated within the plasma discharge which react with the deposited atoms to form chemical compounds (Fig. 4.5-7). Due to the high energy ions impacting the substrate, layers of exceptional adhesion and hardness can be fabricated. High deposition rates can be achieved because of the thermal deposition source. Very good layers have a Vickers-hardness (diamond penetrator hardness) of about 5000 i. e. a value somewhere between hardened steel and diamond. Besides high hardness, these layer also show a reduced coefficient of friction, which has a positive effect on the wear of the coated working part. High performance tools such as drills and cutters are usually coated with TiN or TiAlNi by ion plating. The utilization and control of this process for uniform results is difficult due to the complex interdependencies of many parameters.

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4 Basic Technologies in MEMS

-

Argon

Electron source

Fig. 4.5-7 An arrangement of plasma assisted deposition. Energetic electrons emanated from a heated cathode and accelerated across the vapor cloud of a evaporation source ionize some of the neutral atoms to be deposited onto a substrate. These ions together with ionized argon atoms are accelerated toward the substrate.

4.5.4 Ion Cluster Beam Technology The ion cluster beam technology utilizes beams of ionized atom agglomerates (ICBD-ion cluster beam deposition) for coating purposes. The ICBD technology works with agglomerates (clusters) of some hundreds to thousands of atoms [Beck%]. These agglomerates normally carry only a single elementary charge (with much more than one elementary charge they tend to disintegrate). Therefore, in an electric field a cluster attains the same kinetic energy as a single ion. The popular theory that the cluster explodes into single atoms on hitting the substrate, in which the tangential components of energy enables the atoms to take up energetically favorable positions on the substrate, cannot be rigorously explained by physical theories. Much more likely is that the cluster drops are re-shaped on hitting the substrate surface and fuse with it (Fig. 4.5-8) [Gspa91]. Epitaxy should be possible at a relatively low temperature (about 250 "C) with suitable parameters. Large particle currents i. e. growth

4.5 Physical and Chemical Coating Techniques

141

x

Fig. 4.5-8 The two models of the ion cluster deposition.

a) The cluster disintegrates when hitting the substrate, leaving to the atoms a high migration energy. b) The cluster is reshaped when impacting the substrate and fuses with the surface. rates, can be relatively easily realized due to the large number of atoms in the cluster. Atom clusters are produced with a special evaporation source (Fig. 4.5-9). It consists of a closed vessel with a small nozzle opening. If the source substance is heated in the vessel, then the interior has a positive pressure (10 to lo3 mbar) compared to the high vacuum on the outside (lop5 to As the vapor of the source material flows out of the nozzle, it experiences a sudden cooling by adiabatic expansion. Subsequently it forms neutral atom clusters of some 100 to some 1000 atoms. These neutral atom clusters can now be bombarded with electrons from a filament cathode. A certain percentage (5-50%) of singly ionized clusters results, which are then accelerated in an electric field with energies of a few keV. In contrast to an atomic ion beam with similarly large mass current, the cluster ion beam does not disintegrate by coulomb repulsion (as the ratio of charge to mass e/m is too small). A large fraction of neutral clusters with low energy (0.1 eV per cluster atom) and a smaller fraction of ionized clusters with high energy (some eV per cluster atom) simultaneously impact the substrate surface. These are the appropriate en-

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4 Basic Technologies in MEMS

cathode

Radiation shield Heater

’

Evaporant’

0,l ...2 kvH

‘F

Fig. 4.5-9 Sketch of an ion cluster source.

ergies not only for the activation of the surface migration but also for the desorption of contamination layers on the solid surface. These characteristics lead to exceptionally favorable conditions for film formation. Important advantages of the ICBD technology are: 0 0 0 0 0

formation of layers at lower temperatures, layers with low defect density, layers of high purity, good adhesion, no electric charge with insulating substrates.

One of the aims of this technique is to make epitaxial single crystal films at low substrate temperatures. Other work concentrates on the surface layer removal and on surface structuring by means of CO, cluster beams. This cluster with about 1000 C 0 2 molecules shows a high sputter coefficient of more than 600 copper atoms per cluster ion. Experiments to drill holes into tungsten sheet and to polish nickel surfaces are carried out using cluster ion beams [Henk89, Henk9 11. If cluster projectiles with energy of 100 KeV hit a surface, they create a crater, which is temporarily about 10 times larger in diameter than the projectile (1 nm). C 0 2 clusters of 100 molecules and 100 keV of energy reach a velocity of about 20 k d s . As these clusters impact on the surface, a shock wave propa-

4.5 Physical and Chemical Coating Techniques

143

Fig. 4.5-10 Cluster erosion of a pyrex glass with C02 clusters. The cluster consisted of about 1000 C02 molecules with an energy of 100 keV.

gates not only within the cluster but also into the material, which results in a large compression and heating. The cluster evaporates and the material being bombarded becomes fluid. A part of the liquefied material is pushed out of the crater with a strong lateral component. This can also explain the strong polishing or leveling effect, which is observed with cluster ions of rough surfaces [Gspa95a]. The removal rate can be precisely controlled since under constant process conditions all clusters have the same size and energy. In this way one can process “sink holes” in a substrate, which are characterized by an uniform flatness of the bottom plane and uniform depth over the region of the process area. Figure 4.5 -10 shows hexagonal holes, which were produced by bombardment of clusters, each consisting of 1000 C 0 2 molecules, in pyrex glass. Nickel is used as the mask material. One can see in this electron micrograph that the hexagonal holes have a diameter of about 50 pm and a depth of 10 pm,and that the bottom planes are very flat. Experiments with silicon and diamond show similarly good results [Gspa95b].

4.5.5

CVD Processes

With chemical vapor deposition (CVD) molecules are dissociated at the surface of the substrate to be covered. The fragments consist of a solid component, which is deposited, and one or more gaseous components, which are removed together with a neutral carrier gas. Contrary to the processes discussed in the previous sections under the collective name of PVD, the CVD process is operated under normal at-

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4 Basic Technologies in MEMS

mospheric pressure or at reduced pressures down to some mbar. The dissociation energy is delivered by the heated substrate or by a plasma discharge (then the process is called PECVD = plasma enhanced chemical vapor deposition). A typical CVD process was discussed already in Chapter 2 in the context of the refining process of silicon: SiHCl3

+ H2 -+

Si

+ 3HCI

(4.23)

In this specific process the substrate temperature is of the order of 1100 "C. Many substrate materials or delicate structures on the substrate do not withstand these high temperatures. In such cases the PECVD process is applied. The dissociation energy in this process is supplied by the high energy electrons within the plasma or the photons emitted by excited atoms or recombined ion-electron pairs. In this case the process temperature can be lowered to about 250°C. The layer thicknesses are up to 10 pm, and under optimum reaction control the surface quality corresponds to that of the surface of the base material. Also components with complex shapes can be covered uniformly (conformal step covering) by working in the low pressure region. For reactions which occur on the substrate surface, not only the chemical properties of the reactants are important, but also the rate of material transport to the place of reaction (namely the substrate surface). In considering gas phase reaction for the deposition of solid components, the combined effect of the following three steps should be noted:

1. transportation of the source material to the reaction zone, 2. reaction, 3. removal of the unwanted reaction product. The flow rate of the gaseous reactants drops to zero on the substrate surface within the flow boundary layer, which forms as a consequence of friction. In order to be able to reach the substrate surface the reactants must therefore diffuse into the boundary layer. At atmospheric pressure (APCVD = atmospheric pressure chemical vapor deposition) the reaction process is usually mass transport limited. The deposited film in this case is dependent on the surface topography and position of the substrate with regard to the flow. Closely stacked substrates to be processed in a mass production environment would have different layer thicknesses depending on the position within the stack. However the diffusivity of the reactants increases by lowering the pressure (LPCVD = low pressure chemical vapor deposition). The gas molecules reach the surface at a faster rate than they are consumed by the chemical reaction. The process is then surface reaction limited. Under these circumstances the geometry of the set-up and the topography of the substrate is less important for controlling the coating thickness. However, since the reaction time is dependent on the temperature, heating the surface uniformly is a critical issue.

4.5 Physical and Chemical Coating Techniques

145

The actual flow behavior of the reactant near to the substrate surface can only be described approximately. It depends on the surface temperature of the substrate, the partial pressure and the single reactants on the substrate surface and in the flow boundary layer. A simple relation is given by the 1. Fick's law: (4.24)

where N is the current density of reacting molecules, n, the reactant density in the free flow, no the reactant density at the substrate surface, and 6 the thickness of the flow boundary layer. D is the diffusion coefficient with D T". T is the temperature in K and m is a coefficient between 1.75 and 2. Atoms or molecules, which impact a solid body surface, usually do not stay in their initial point of impact, as this location is probably not the most energetically favorable place; therefore these atoms or molecules sit in a labile equilibrium state on the surface. After gathering additional energy from the heated substrate or by collision with ions hitting the substrate, these atoms can migrate on the surface, until they have found a potential trough and there form a stable bond with an atom of the solid body. However, these adatoms during the migration phase can be kicked off the surface (desorption) by other competing adatoms or by ion sputtering. In the equilibrium state (saturation pressure pm over the substrate) adsorptionand desorption processes reach an equilibrium. In order to grow a thin film, the number of adsorption events must exceed those of desorption. One achieves this by raising the vapor pressure p,. above the saturation pressure pm. A number of the adsorbed atoms will coalesce to form a nucleus on the substrate and if this nucleus exceeds a certain size (critical nucleus) the growth of a film by further agglomeration will take place. With the supply of absorbed particles, three processes are initiated which generally occur simultaneously:

-

0 0 0

desorption, nucleus formation and nucleus growth and migration of dislocation steps.

An important property of a film is its ability to cover edges or troughs, in order to passivate or isolate an underlying surface against successive layers or the atmosphere. As shown in Fig. 4.5-11 one can distinguish between three cases. Figure 4.5-lla shows the ideal case: A continuous, uniform thick layer over the whole trough. This is achieved if the reactants have sufficient energy for surface migration after the first contact with the solid surface. If this surface migration disappears, then the reactants are left at the point of first contact with the surface. The thickness of the coverage is then a function of the possible arrival angle for the incoming particles as seen in Fig. 4.5-llb and 4.5-llc.

4 Basic Technologies in MEMS

146

a

b

C

Fig. 4.5-11 Edge and trough coverage in dependence on mean free path (deposition aperture) and surface mobility of the particles after the first contact with the surface. a) ideal situation with high surface migration, b) situation with large mean free path and reduced surface migration, c) situation as in b) but with smaller mean free path (higher deposition aperture).

4.5.6

Epitaxy

Epitaxy is a process whereby the single crystal structure of a substance is promoted and maintained by the coating processes. Mostly these are CVD processes, which run at higher temperatures, in order to give the atoms the necessary energy for surface migration, allowing them to search out energetically favorable sites. In epitaxy one distinguishes between homo-epitaxy, i. e. the growth o f layers of the same material as the substrate (ignoring different doping) and hetero-epitaxy in which the single crystal layer material differs from the substrate. However, also in hetero-epitaxy it must be ensured that the lattice constant of both materials is roughly the same, as otherwise a single crystal film is not achieved due to the high intrinsic stress.

Homo-epitaxy Homo-epitaxy will be discussed next using silicon as an example. Predominantly silane (SiH4) and chlorosilanes (SiCl,, SiHCl,, SiH,Cl,) are used as reaction gases together with a carrier gas of H,. The process is carried out above 1000°C. The reaction gas decomposes at such temperatures to silicon, which is deposited onto the substrate surface, and gaseous Cl, or HC1, depending on the composition,

4.5 Physical and Chemical Coating Techniques

147

are evolved. The reaction gas is diluted with an inert carrier gas, in order to avoid autoreaction i. e. to avoid a breakdown of the molecular species prior to the deposition. Furthermore, small concentrations of phosphine PH, or diborane B2H, are added, in order to obtain appropriate doping of the epitaxially deposited layers. The dopant concentration is about to lo2' atoms per cm3. Higher concentrations cannot be achieved with dopants, because they would exceed the limit of solubility in silicon. In epitaxy the deposition of the atoms takes place initially on the nucleation sites on the surface. These are generally corners and edges of incomplete crystal planes. Therefore, such planes are preferentially completed by deposition, before a new crystal layer is begun. Therefore, uniform growth of a single crystal epitaxial layer is ensured. The crucial process parameters in epitaxy are temperature, concentration of the reaction gas and gas flow control, as well as crystal orientation of the host crystal. Of course the state of the substrate surface at the start of epitaxy is important. The single crystal wafer must be thoroughly cleaned of all contaminating layers, in order to propagate an undisturbed growth of the substrate. The wafer is prepared therefore, by gas phase etching which proceeds the epitaxy process. Typical growth rates for (110)-wafers are between 0.5 pm and several micrometers per minute.

Hetero-epitaxy With stant have tions 0 0 0

hetero-epitaxy, crystals of a different material but with the same lattice conare built up on the host crystal. Single crystal layers of silicon on sapphire acquired special importance. For the specific technologies special abbreviaare used:

SOS technology: silicon on sapphire, ESFI technology: epitaxial silicon films on insulators, SO1 technology: silicon on insulators.

Due to the high bandgap of sapphire, SOS technology allows the production of silicon components on a sapphire chip, which even at elevated temperatures are safely insulated from each other. The deposition of epitaxial silicon layers is conducted predominantly in reactors with induction or radiation heating (see Fig. 2.2-1 and Fig. 2.2-2)

GaAs-Epitaxy Besides the silicon on sapphire technology, the deposition of GaAs on silicon is also of special technical interest. Unfortunately the lattice constants of silicon and GaAs are so different (0.5431 nm for Si and 0.5653 nm for GaAs), that these difficulties can only be overcome by very elaborate layer by layer accommodation to the different lattice constant. The current methods of epitaxy of gallium arsenide are:

148 0

0 0 0

4 Basic Technologies in MEMS CVD LPE (liquid phase epitaxy) MOCVD (metal organic chemical vapor deposition) MBE (molecular beam epitaxy)

Binary and ternary crystal layers are frequently obtained from liquid phase epitaxy, because the stoichiometric ratio can be best controlled. For GaAs generally the CVD process is used, even if the toxic reaction products in the production pose a problem. The MBE process gives very clean, well defined epitaxial layers, in which each atomic layer can be controlled precisely. However, the rate of growth is very low (about 1 y d h ) compared with the CVD process. For these reasons the MBE process is reserved mostly for small quantities needed in research and development. Epitaxy is used also in microsystem technology, because “buried layers” with a high concentration of dopants (especially boron) can be produced. Highly doped epitaxial layers play an important role as etch stop layers in the production of micromechanical structures of silicon, as discussed in Chapter 6.

4.5.7 Plasma Polymerization In plasma polymerization a small amount of a gaseous monomer is let into an evacuated chamber through a control valve. Such a monomer can be e. g. hexamethyl disiloxane. This monomer, which is a liquid at room temperature and standard pressure evaporates on admission into the vacuum chamber and spreads uniformly within the container. Inside the apparatus two electrodes are located, across which a high frequency voltage is applied (Fig. 4.5-12a). As a result, a glow discharge is excited, in which some of the monomer molecules are either fragmented into the radicals and/or are ionized. These fragments condense on the substrate and under the influence of energetic electrons, ions, and photons from the plasma form a highly cross-linked polymer layer (Fig. 4.5-12b). Therefore, a coating forms layer by layer with a degree of polymerization which would not be achieved by conventional polymer chemistry technology. Thin films fabricated by plasma polymerization display exceptional properties in corrosion resistance, freedom of pinholes, hardness and antifriction.

4.5.8 Oxidation Oxigen has a high affinity to silicon. Therefore silicon surfaces are usually covered with a native layer of S i 0 2 (about 20 A). S O 2 has exceptional properties with respect to chemical inertness, electrical break-through resistance, and as a diffusion barrier for the commonly used doping impurities such as As, B, and P. The properties of SiO, are a considerable asset of silicon as a material for microelectronics and MEMS as well.

4.5 Physical and Chemical Coating Techniques

149

Fig. 4.5-12 Principle of plasma polymerization. A monomer is dissociated in a plasma discharge. The fraction of the monomer diffuse onto the substrate and are polymerized due to the impact of energetic electrons and photons out of the plasma. a) the experimental set-up, b) the appearance of the “duroplastic” polymer.

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4 Basic Technologies in MEMS

SiO, layers thicker than the native layers are either applied by thermal diffusion or by CVD processes. For thermal diffusion there are two alternatives, the "wet process": Si

+ 2H20 -+ Si02 + H2,

(4.25)

and the "dry process": Si

+ 0 2 -+SiO2.

(4.26)

The dry process is considerably slower in the growth rate but results in a film of higher density with less pinholes and higher dielectric strength. The thermal diffusion process can be divided into three steps: 0

0 0

oxigen gas transport to the surface, diffusion through the existing oxide, reaction with the silicon and forming S O 2 [Mado97].

It is easy to understand that the growth rate slows down with increasing oxide thickness since the oxide atoms have to diffuse through the diffusion barrier of the increasing existing oxide layer. The oxide thickness can be calculated by a differential equation as a function of reaction temperature, crystal orientation and doping level, oxidant diffusivity, and partial pressure of the oxidant. At constant temperature the thickness of the oxide layer is proportional with the diffusion time at thin layers and changes to a parabolic dependency with time at thicker layers. Wet oxidation results in somewhat more porous layers. Diffusion of oxigen is facilitated and the growth rate of the oxide layers is increased by about one order of magnitude with otherwise similar parameters. A further increase in oxide growth is achieved by increasing the partial pressure of the water vapor. In Fig 4.5-13 the dependence of oxide thickness on vapor pressure after 1 hour process time is shown for two different temperatures on wafers in (100) orientation. Instead of thermally grown SiO, layers CVD processes can be applied for layers of reduced properties with respect to dielectric strength [Sze85]. For low temperature deposition (300 to 500" C) silane is used as reacting material:

Si&

+ 0 2 + Si02 + 2H2.

(4.27)

At intermediate temperatures (500 to 800" C) the reacting material is tetraethylorthosilicate (Si(OC,H5)4. The compound is abbreviated TEOS and decomposes in the following way: Si(OCzH5),

+ Si02

+ reactants.

(4.28)

4.6 Structuring of Thin Films with Dry Etch Processes

151

1

0,Ol 1

10 Water vapor pressure (atm)

Fig. 4.5-13 Oxide thickness versus vapor pressure at two temperatures [Sze85].

Finally at high temperatures (900" C) silicon dioxides is formed by reacting dichlorosilane with nitrous oxide: SiC12H2

+ 2N20 -+ Si02 + 2Nz + 2HC1.

(4.29)

In MEMS the oxide layers on silicon are used as etch mask, diffusion mask, passivation layers, and as sacrificial layer in silicon surface micromachining, since the Si02 layer is dissolved with hydrofluoric acid. Other applications besides electrical insulation are optical guidelines for integrated optics.

4.6 Structuring of Thin Films with Dry Etch Processes In general in etch processes material can be removed selectively from a surface, either by physical attack, such as ion milling, or by chemical means, such as dissolving the reactants, or by a combination of both. Depending on the process the material is removed isotropically meaning that is removed in all directions or anisotropically, where the removal process mirrors the direction of the incident etchant species (e. g. accelerated ions). Except for the relatively rare process of structuring with focussed light- or particle rays (see Chapter 5 ) , the structuring is carried out in so-called batch processes in which the whole surface (often several substrates) is subjected to an etching liquid or irradiated over a large area with etching particles. In order to subject certain areas of the substrate to these process

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4 Basic Technologies in MEMS

underetching

v

etchant

/

particle beam

,mask

\

substrate

Ih

Fig. 4.6-1 Principle of etching.

a) isotropic chemical wet etching, b) anisotropic physical etching with accelerated particles

steps, the parts of the surface which are not to be processed are covered with a resist layer, which was structured beforehand by lithography (see Fig. 4.6-1). The directionality is one important parameter of an etch process, the selectivity is another. Directionality or anisotropy means removing material in such a way, so that etch pits with steep and vertical walls are produced. The opposite is the isotropic etching without any preferential etch direction. In many processes the task is to remove a particular layer by etching without attacking adjacent structures. High selectivity means choosing the right chemistry, such that one type of material is removed completely without attacking other structures made from different materials. Usually directionality and selectivity are mutually exclusive parameters, meaning that processes with a high selectivity usually are isotropic, whereas processes with high directionality (anisotropy) usually have less selectivity. In Fig. 4.6-2 this fact is demonstrated schematically and will be discussed in more detail in the following. For many years chemical wet etching has been the method of choice in manufacturing, due to the high selectivity and etch rate of this process. With the exception of anisotropic etching of silicon single crystals, described in detail in Chapter 6, wet chemical processes are isotropic i. e. the etching speed is independent of the etch direction. Consequently, not only the material directly under the mask opening but also some of the material covered by the resist mask is removed so that the etched pattern no longer coincides with the resist pattern (lateral underetching). Wet chemical, isotropic etching can only be applied to structures, whose aspect ratio L (ratio of the structure height to its lateral dimensions) is smaller than 1 and with minimum lateral structure sizes of about 2 pm. The process of isotropic etching is applied whenever a layer has to be removed in total, such as resist layers, or so-called sacrificial layers which have to be removed underneath already existing structures.

4.6 Structuring of Thin Films with Dry Etch Processes Object 1

153

Object 2

Fig. 4.6-2 Selectivity and directionality of etching processes. Two objects 1 and 2 are

exposed. a) to isotropic etching with high selectivity, and b) to anisotropic etching with high directionality but low selectivity. The anisotropic etching is of increasing importance especially in microstructure technologies since it is a means of shaping three-dimensional microbodies in different materials with high aspect ratios. The technology has improved over the years to such a degree, that it is considered a serious competitor to the LIGA technique, which is famous for attaining high aspect ratios (Chapter 7). With the so-called dry etching processes the etching medium is gaseous. In most cases the etching gas is partially ionized, and the ions are accelerated toward the substrate to be etched. The impact of the ions with the substrate causes neutral atoms of the solid body to escape from the surface. The pure directional physical effect is often reinforced by chemical effects due to gaseous radicals, which are added to the process gas. As in pure chemical etching, radicals are also produced in a plasma, but at pressures of 10-1 to 10' mbar. In this pressure regime the mean free paths of the radicals are very short and therefore their directionality is low. Pure chemical etching in general is therefore highly selective but strongly isotropic. Within the frame of microelectronics many different dry etch processes have been developed, which differ according to the type of apparatus used (i. e. the production and transport of gaseous etch particles) and according to the type of active etch particles. In principle the process can be categorized as follows: 0 0 0 0

production of active etch gas particles, transport of these particles to the substrate, etching of the substrate surface, transportation of the etch products.

The production of active etch particles (i. e. ions) takes place in a plasma i. e. in a gas discharge. The necessary energy to ionize the gas particles is supplied either

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4 Basic Technologies in MEMS

by an electrical field (DC or RF) between two flat electrodes, or by inductive coupled high frequency fields, or by microwave energy. The transport of the active etch particles can occur either diffusely or directional and has a considerable influence on the properties of the etch process (directionality, etch rate and selectivity). One differentiates between the type of active etch particles: 0 0 0 0

inert ions (e. g. Ar') reactive ions (e.g. O', ClF,') reactive neutral gases (e. g. XeF2) reactive radicals ( e . g. F*, CF3*, O*).

In the following some of the most important dry etch processes are briefly listed: IE = Ion Etching Physical etching with inert ions, which are produced in a gas discharge and are accelerated onto the substrate, held within the plasma. IBE/IBM = Ion Beam Etching/Ion Beam Milling Physical etching with inert ions, which are produced in a plasma within an ion gun and are extracted and accelerated toward the substrate, i. e. the substrate is located outside the plasma in an etching chamber kept at high vacuum. The etch profile is very anisotropic, the selectivity very small. RIBE = Reactive Ion Beam Etching Directional, strong ion assisted etching with reactive ions, which are produced in the plasma and accelerated onto the substrate located outside the plasma like with IBE. The etch profile is isotropic to anisotropic depending on the pressure (and thus the mean free path of the ions), the selectivity is adequate to good. CAIBE = Chemical Assisted Ion Beam Etching The process is very similar to that of the RIBE. In this case the reactive gas is released into the reactor at a short distance from the substrate to be etched by means of a ring-shaped outlet. RIE = Reactive Ion Etching Directional, strong ion assisted etching with reactive ions, which are produced in a gas discharge, in which the substrate comes into contact with the plasma. The etch profile is isotropic to anisotropic, the selectivity is adequate to good. PE = Plasma Etching Chemical etching with free radicals and little assistance by ions. The etching profile is isotropic to anisotropic, the selectivity is good. BE = Barrel Etching The chemical etching takes place practically exclusively with free radicals. Therefore, the selectivity is good, the etch profile is however strongly isotropic.

4.6 Structuring of Thin Films with Dry Etch Processes

155

4.6.1 Physical Etch Technologies Zon Etching (ZE), Sputter Etching With ion or sputter etching the reactor consists of a vacuum chamber and two flat electrodes with differently sized surfaces, which are a few centimeters apart (parallel plate reactor, Fig. 4.6-3). The wafer to be etched is placed onto the bottom electrode so that it is in direct contact with the gas discharge. A value of 5.10-3 to lo-' mbar (0.5 to 10 Pa) is chosen for the pressure of the inert gas which is usually argon. The large electrode is grounded, whilst the other is connected via a coupling condenser to a high frequency voltage source with a voltage of about 0.1-1 kV. Under these conditions a gas discharge is generated between the plates, which exhibits a luminous zone limited to the middle area between the plates. A dark space is adjacent to each electrode. With dissimilar sizes of the electrodes, the voltage drop across the dark space of the small electrode becomes large compared to the voltage drop on the dark space of the large electrode, because the voltage drop is inversely proportional to the 4th exponent of the surface area. In such a high frequency discharge, the electrodes take up a negative potential with respect to the plasma potential. The reason for this can be seen in the larger mobility of the electrons, compared to the ions, so that the electrons can easily reach the electrodes during a half cycle of the RF (13,56 MHz). This negative potential leads to an acceleration of ions towards both electrodes and therefore -to sputtering of the substrate which is mounted onto the electrode.

Substrate small Electrode

Fig. 4.6-3 The parallel plate reactor for etching.

4 Basic Technologies in MEMS

156

The ion bombardment on the smaller electrode is considerably larger than (at the other half cycle) on the large electrode. Nevertheless there is also a minor sputtering effect on the larger electrode, which can lead to unwanted contamination. At low gas pressure the mean free path of ions is large compared to the dimensions of the structures to be etched, and the electrical field lines which are significant for the acceleration, are perpendicular to the substrate surface. The thermal energy of the ions, which lead to an arbitrary Braunian movement, is small compared to the energy due to the acceleration voltage, therefore sputter etching is highly anisotropic. At high gas pressures the ions are scattered in the dark space between the plasma and substrate, so that the transverse component of the velocity vector increases. If the surface structure dimensions approaches that of the dark space distance, then the electric field lines are additionally distorted. This effect could lead to a reduction in the anisotropy with sputter etching.

Zon Beam Etching (ZBE)/ Zon Beam Milling (ZBM) With ion beam etching the substrates to be etched and the plasma producing the ions are physically separated from each other (Fig. 4.6-4). The substrates are located in high vacuum mbar), and a gas discharge is maintained at relatively low pressures (about lop3inbar) in an adjacent chamber. In this chamber electrons emitted from a hot cathode are bent into spiral paths by an applied magsubstrate chuck (tiltingh-otating) \

neutralizing filament

acceleration grid

(0,l-lkV) discharge chamber (p = lobambar) gas input hot cathode

electron trajectory anode

0

ion

substrate

magnet

Fig. 4.6-4 Ion beam etching. Neutral atoms are ionized by collision with energelic electrons from a hot cathode, and accelerated by a charged grid toward the sample. By means of a neutralizing filament the particles are de-ionized by adding electrons. With this also insulating substrates can be etched.

4.6 Structuring of Thin Films with Dry Etch Processes

157

netic field, thereby enhancing collisions with neutral atoms leading to increased ionization. The ions produced in the plasma are extracted by an acceleration grid and guided onto the substrates to be etched. In order to avoid a spreading of the ion beam, the ions are neutralized with electrons after passing the acceleration grid. The ion energy can be varied between 0.1 and 1 keV. The substrate mount is rotatable and tiltable, so that the incident angle of the ions can be varied. In contrast to plasma sputter etching, with ion beam etching the ionic current density and the ion energy can be controlled independently of each other, and the incident angle of the ions on the substrate can be freely chosen. Also because of the separation of the plasma source from the substrate chamber, the contamination of the substrate is very much smaller because better control of unwanted sputtering is possible. The facilities associated with the IBE-process are however much more expensive than simple sputter etching.

Characteristics of Pure Physical Etching Processes With pure physical etching (IE and IBE, which are also generally denoted as "ion milling"), the removal rate depends not only on the energy and mass of the incident ions but also on their angle of incidence. The removal rate with inclined incidence (between 30 and 60") of the ions runs through a maximum (Fig. 4.6-5a) [Chap80, Wehn70, Wint831. This is due to the fact that removing an atom from the substrate must take place by momentum transfer between the incident ion and the substrate. With perpendicular incidence the impulse vector and associated collision cascade must be rotated by 180", whilst with smaller incident angles a smaller directional change of the impulse is necessary (Fig. 4.6-5b). As an opposing ef-

3

1

argon-ions

argon-ions

1 7 a

I

I

30'

60

I b

I

90'

angle of incidence Q Fig. 4.6-5 Characteristics of pure physical etching. a) dependence of the etch rate on the angle of incidence, b) the probability of momentum reversal is smaller at vertical incidence than on incidence at rp > 0".

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4 Basic Technologies in MEMS

Fig. 4.6-6 Artifacts of physical etching a) ideal situation b) realistic results

fect, the current density of incident ions per unit area decreases with decreasing incident angle. For this reason, surfaces which are oriented at an optimum incident angle will be etched faster. The pure physical etching can be characterized by the following propesties [Some761: 0

No absolutely perpendicular walls can be produced. The etching tends to form troughs, because the ion current density at the foot of a structure is raised by reflection from the inclined structure edges with inclined ion incidence (Fig. 4.6- 6b).

6

The etch rates are very low (some 10 n d m i n ) .

0

The selectivity is generally low.

0

Sputtered material can accumulate on the walls (especially in the deeper troughs) (Fig. 4.6-6b).

4.6.2 Combined Physical and Chemical Etch Technologies Besides pure physical and chemical etching processes, combined processes are predominantly used, since often the advantages of chemical and physical etching, i. e. high selectivity and anisotropy, can be combined [Bo1184]. Properties can be achieved, which cannot be achieved with pure chemical or physical processes alone.

4.6 Structuring of Thin Films with Dry Etch Processes

159

If the active etch gas itself can react with the substrate and form gaseous products, then additional energetic particle bombardment can accelerate these etch processes, so that etch retarding surfaces are broken down, atomic bonds are broken or the surface is locally heated. If the active etch gas alone cannot attack the substrate by formation of gaseous products, then local particle bombardment supplies the necessary activation energy, which leads to a gaseous product. It is also possible that products are in fact formed by the etch gas, but remain loosely bound to the surface and are removed only by the energetic particle bombardment. Reactive Ion Etching (RIE) In reactive ion etching (RIE) a parallel plate reactor is used. In contrast to plasma etching, with RIE the substrate to be etched is placed on the smaller electrode and the gas pressure is chosen to be as low as to lop2mbar. This takes advantage of the greater ion acceleration voltage at the smaller electrode in an RF capacitively driven plasma. It is an important characteristic of RIE that a physical bombarding component is necessary in order to initiate the directional etch process. The gas discharge with RIE produces not only reactive neutral radicals, reactive ions but after addition of inert gas into the container, also inert ions. Also neutral particles are always present. Therefore, the etching in an RIE system can result from a combination of several active etch species. Reactive Ion Beam Etching (RIBE) and Chemically Assisted Ion Beam Etching (CAIBE) With reactive ion beam etching, RIBE, the same arrangement as with ion beam etching (Fig. 4.6-4) can be used, in which the ion source is simply operated with a reactive gas instead of an inert gas. In the etch chamber where the substrates are located a high vacuum is maintained. In this case, only bombardment with reactive ions occurs because neutral particles are not accelerated through the high voltage grid onto the substrate, but reach the substrate only by diffusion. Another way in which reactive neutral species can be introduced, is by letting these gases directly into the etch chamber through a ring-shaped gas outlet near the substrate. This process is called Chemical Assisted Ion Beam Etching (CAIBE). Cryo-etch Technology The physical or chemical-physical dry etching with a high aspect ratio plays a particularly important role in microsystem technology. We have seen that the pure physical etching with accelerated ions does not bring the desired result, since as the etch rate with perpendicular incidence is smaller than with oblique incidence. In order to be able to produce perpendicular walls, special measures must be employed in order to prevent lateral removal. Cryo-etching seems promising for that. By cooling the substrate, the chemical reactions are changed or slowed down compared to etching at standard temperatures.

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4 Basic Technologies in MEMS I

Sm-Co

Fig. 4.6-7 Cry0 etching set-up. The substrate is cooled with liquid nitrogen. The magnetic field is to increase the ion yield in the plasma.

The main process for etching is reactive ion etching (RIE). HBr or S2F2are used as etch gases. In the first case SiBr, (x = 1.4) is formed during etching, which condenses on the cooled side walls and forms a passivation layer, whilst the floor layer is constantly being removed by the physical etch process. Using SzF2,sulphur compounds form, which similarly condense as passivation layers on the sidewalls. If one uses SF, as the etch gas, then another effect comes into play. [Tach91]. By lowering the temperature, the rate of reaction of the radicals with silicon is much reduced on the perpendicular walls and the ion enhanced etch process occurs only on the floor of the structure. A disadvantage of cry0 etching is however, that the re-deposition increases due to condensation of the etched material in very narrow cracks. In Fig. 4.6-7 a cryo-etch apparatus used by M. Esashi and co-workers is shown, [Esas95]. The silicon wafers to be etched are positioned on a cathode plate, which is cooled from the back side with liquid nitrogen. The cathode is isolated from the grounded cooling apparatus by an A1N ceramic plate. The plasma density is increased by rotating Sm-Co magnets. There are numerous other suggestions for the production of the plasmas in the literature. Figure 4.6-8 displays the experimental results on silicon [Esas95]. SF, is used as the process gas. The diagram shows the etch rate and the anisotropy dependence on the high frequency power. The substrate temperature was -I 20 “C. The anisotropy or “normalized lateral etching” is defined as the quotient of structure width to the etched depth.

The Advanced Silicon Etch Process (ASE-Process) Very recently a new process was introduced, which was developed by scientists at Robert Bosch GmbH and STS [Laer96, Hopk981. Here two processes are running in sequence. In a first step the silicon surface is etched in a RIE process with SF,, which dissociates into in SF,’ and F-. This reacts with Si to form SiF4. Since SiF,

4.6 Structuring of Thin Films with Dry Etch Processes

Nz Y

161

1.6

E a 1.2

Y

Q)

.w

E

c ,o w

rn 0.2

0.8

0.1

0.4

-.-3m E

0 0 1 2 3 4 RF power density [MI.cm2]

5 -2

Fig. 4.6-8 Results of the cryo-etch process of Fig. 4.6-7

is volatile under the etching conditions. It can be removed easily from the silicon surface. The parameters of the process are chosen in such a way that high directionality of the etching is achieved. In a subsequent process the reactive gas in the chamber is changed to CF4, which under the influence of the plasma undergoes a plasma polymerization process. Since this process is highly isotropic also the sidewalls of the silicon structure are covered by a polymeric sidewall protective film. After this, the process is switched again to RIE etching with SF,. All areas normal to the ion incidence are preferentially removed. This process is called Advanced Silicon Etch process (ASE). Typical results are given in Table 4.6-1 and an example of microstructures fabricated by means of the ASE process is shown in Fig. 4.6-9.

Fig. 4.6-9 Microstructures fabricated in silicon by means of the ASE process (by courtesy of A. Menz, IMSAS, Bremen).

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4 Basic Technologies in MEMS

Table 4.6-1 Typical results of the STS ASE process Etch rate Selectivity to SiO, Uniformity across wafer Aspect ratio Selectivity to resist Sidewall profile Feature size Etch depth capability

1.5 to 3.0 p d m i n 120 to 200: 1 k2.2 to 5.0% up to 30 50 to 1OO:l 9Ook2" 1 to >500 pm 10 to 800 pm

c

Characteristics of Reactive Ion- and Ion Beam Etching With RIE and RIBE the etching in general is the product of different gas species and ion bombardment of the substrate at the same time. Therefore, it is often difficult to separate the respective contributions. In contrast to pure physical sputtering (ion etching) the etch rate in the forward direction is considerably increased and has a maximum with perpendicular impact (4 = 90°), as here the impulse reversal is of minor importance for removing the substrate particle. Since in RIE and RIBE the particle current density decreases with decreasing angle (as with IE and IBE), the etch rate drops significantly at oblique incidence. This dependency on the rate of etching of the incident angle leads to a strong anisotropic etch profile, so that with RIE and RIBE perpendicular structure walls are possible. The etch rates are in general really high (RIE: 20 to 200 nm/min, RIBE: 50 to 500 n d m i n ) . Also by suitable choice of gases a high chemical selectivity can be achieved. In particular polymers can be etched selectively with oxygen as the etch gas, compared to metals, with a selectivity of over 50.

4.6.3 Chemical Etching Technologies Barrel Etching (BE) Barrel etching can be considered as a pure chemical etching process. For this process mostly a cylindrical-shaped reactor is used, which is schematically represented in Fig. 4.6-10a. A gas discharge is maintained by two outer electrodes and a high frequency voltage at a pressure between 0.1 and 1 mbar. The substrates to be etched isotropically are located in the middle of the container and are screened from the discharge by a cylindrical metal grid (etching tunnel). Neutral radicals which are formed in the plasma diffuse through the grid cylinder into the etching tunnel. Due to the choice of the reaction gas the process can be made highly selective, and due to the diffusion it is completely isotropic. It has to be assured though, that the life time of the radicals is long enough to survive the time of drifting to the substrate to be etched. Due to the construction of the barrel, ions are excluded from the etch process, the substrates therefore are not subject to energetic particle bombardment. To achieve

4.6 Structuring of Thin Films with Dry Etch Processes

163

homogeneous etching over a large surface or over several wafers at the same time, it is essential that the concentration gradient of the reactive particles (radicals e. g. O*, CF3*) in the reaction channel is negligible. To get uniform results the etch reaction should be reaction controlled rather than diffusion controlled. Pure chemical etching in a barrel reactor can be described by the following characteristics: 0 0 0

0 0

The selectivity is very high, the etch rate is average to high (20-100 nm/min), the attainable etch profile is strongly isotropic, the etch uniformity is, in general, small, the substrate to be etched is often contaminated with impurities.

The barrel reactor can be applied especially well if the etch process exhibits a very high selectivity and isotropic etch profiles are acceptable, or a complete removal of a layer, such as the stripping of photoresist, is required.

Downstream Etching A downstream etcher is in general of the same design as a barrel etcher. Here the radicals are generated in a parallel plate reactor, or by inductive coupling with an RF field. The gas flow including the radicals is directed over the surface to be etched outside the plasma. Ions, free electrons, or high energy radicals are excluded from the process (see Fig. 4.6-lob).

w e r tc etched

a

b

Fig. 4.6-10 Plasma etcher.

a) Barrel etcher. In the cylindrical plasma chamber radicals are generated which diffuse into the inner (field-free) chamber to attack the object to be etched. b) Down stream etcher. The radicals are generated in a plasma outside the etch chamber and guided toward the object.

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4 Basic Technologies in MEMS

4.7 Analysis of Thin Films and Surfaces The requirements of microanalysis of surfaces and thin films with regard to surface resolution and in depth understanding have lead to numerous new analytical methods [HaefS7]. The methods are based on the investigation of exchange processes between photons, electrons, ions or other particles with the surface or film to be analyzed. Elastically or inelastically scattered particles or secondary particles are emitted depending on the kind of excitation and the substance to be analyzed. The analysis of the particles takes place through an appropriate detector system. Many of these processes analyze only the top layers which may be largely made up of unwanted deposits from the environment. In order to be able to investigate the “true” surface and to have sufficient measuring time available, analysis is performed in ultrahigh vacuum. In this case the monolayer time, as described in section 4.1.2, amounts to several minutes or even hours, leaving enough time to investigate freshly cleaned or generated surfaces. Some of the most important methods are summarized in Table 4.7-1 to 4.7-3, arranged according to the analysis problem as well as exciting particles and particles to be analyzed. Overview of analytical surface processes Table 4.7-1 Chemical Composition AcronYm

Method

Exciting/Detectable Particles

Attainable Information

ISS

Ion Scattering Spectroscopy Auger Electron Spectroscopy X-Ray Photoelectron Spectroscopy Electron Probe Microanalysis

IondIons

at the surface

Electrons/Electrons

some nm in depth

Photons/Electrons

some nm in depth

ElectronslPhotons

up to 1 pm in depth

AES XPS EPM

4.7 Analysis of Thin Films and Surfaces

165

Table 4.7-2 Depth profile of the atomic concentration AcronYm AES ISS SIMS SNMS

RBS

Method

Secondary Ion Mass Spectrum Secondary Neutral Particle Mass Spectroscopy Rutherford Backscattering

ExcitingDetectable Particles

IonsIIons

Attainable Information by successive sputtering by successive sputtering some nm in depth

Ions/Atoms, Molecules

some nm in depth

IonsIIons

by variation of ion energy

Table 4.7-3 Surface topology AcronYm

Method

ExcitingDetectable Particles

SEM

Scanning Electron Microscopy Scanning Tunneling Microscope

Electrons/ Secondary Electrons Tunneling electrons

STM

Some of the above listed methods will be discussed in the following in more detail.

4.7.1 Electron Probe Microanalysis (EPM) Electron probe microanalysis (EPM), which is also called X-ray microanalysis, is the oldest of these processes. It is used for penetration depths of about 1 pm. On electron bombardment of the materials to be investigated, electrons from the inner shells of the atoms are knocked out. These shells are filled again by electrons from higher shells and therefore, X-ray radiation which is characteristic for each type of atoms is released (Fig. 4.7-la). This experimental method is frequently used in combination with scanning electron microscopy. In this case also a laterally resolved analysis of a surface can be realized. The lateral resolution is about 1 pm. Since one can “lock” the analyzer to a particular element specific wavelength, one obtains a lateral distribution of that particular element on the surface. To analyze these rays, either a wavelength dispersive crystal spectrometer or an energy dispersive Si(Li)-semiconductor spectrometer is used. The microprobe fails for low atomic number elements, with particle concentrations of less then 1 %, and with layers, which are thinner than the penetra-

166

4 Basic Technologies in MEMS

tion depth of the electron beam of approximately 1 pm. Therefore, in the last few years methods have been developed for the atomic analysis on the surface and in the region near to the surface which, in addition, also allow measurement of the change of composition with depth. The most important of these methods are: the Auger electron spectroscopy AES, electron spectroscopy for chemical analysis ESCA, secondary ion mass spectroscopy SIMS, the secondary neutral particle mass spectroscopy SNMS and Rutherford backscattering RBS.

4.7.2 Auger Electron Spectroscopy (AES) Auger spectroscopy is based on the energy analysis of emitted Auger electrons which appear on the replacing of inner shell electrons in competition with radiating transitions. As can be seen in Fig. 4.7-lb, the Auger electron is emitted in a sequence of processes, which begins with the ejection of an electron from the inner shell. On replacing the deficit electron, an electron can jump from an outer shell into the inner shell by emission of a photon. The photon can be immediately absorbed again by another electron from another outer shell. Thereby, the electron gains the necessary energy to leave the atom. The energy of the Auger electron is made up of four parts:

A uaer-electron v

n

n+l

a

b

Fig. 4.7-1 Principle of surface analysis.

a) EPM (electron probe microanalysis). An incident electron knocks off an electron from the inner shell which is replaced by an electron from an outer shell. The photon carries off the surplus energy. b) AES (Auger electron spectroscopy). The mechanism is similar to that of EPM except that the surplus energy is carried off by an Auger electron.

4.7 Analysis of Thin Films and Surjaces

167

where is the energy difference between the shell n and n-I, En is the ionization energy of the electrons in the nth shell and @A is the electronic work function of the spectrometer. In a crystal, the Auger spectrum changes significantly, depending on the nature of the neighboring atoms.This spectral change is referred to as a chemical shift. The interpretation of the chemical bonding is however difficult. Other processes (e. g. ESCA) are more suitable for chemical bonding evaluation. Auger spectroscopy is therefore preferentially used for the analysis of elements. It can detect all elements except H and He. The analysis depth lies between 0.5 and 5 nm, as electrons from a greater depths give up their Auger energy due to inelastic collisions. By successive sputtering of surface layers and subsequent Auger analysis of the remaining surface an exact image of the elementary composition as a function of depth can be built up. However, this method is not free of artifacts. The advantage compared to electron beam microanalysis can be seen in the ability to detect light atoms and in the fact that only the uppermost layers of a sample are detected by the analysis. Therefore Auger spectroscopy is considered to be a suitable instrument for the analysis, for example, of corrosion problems, which occur mainly on the surface. With the ability to scan the primary electron beam (energy region between 1 and 10 keV), laterally resolved images of a surface can be produced just as they can be in electron beam microanalysis. The energy analysis of Auger electrons are carried out with special cylindrical analyzers which are built axially symmetric with respect to the sample (Fig. 4.7-2) [Fuch90]. For analysis, the number of Auger electrons emitted per second is measured as a function of their kinetic energy. The maxima of the AES spectrum

Auger electrons Scanning of primary beam

-

Aperture

Detektor

/

Primary beam

Electrastatic energy anatyser

Fig. 4.7-2 Experimental set-up of an Auger spectrometer.

168

4 Basic Technologies in MEMS

give information about the type of sample atoms and their concentrations. To suppress and differentiate the background of the AES spectrum arising from scattered electrons, electronic means are used. The detection limit of particle concentration is about 0.1 %.

4.7.3 X-Ray Photoelectron Spectroscopy (XPS) Monochromatic X-ray radiation is used to excite the electrons of the surface to be analyzed. With XPS one uses usually the aluminum-K-radiation with the photon energy hv, = 1486 eV. Similar to AES analysis, the number of electrons emitted per second is measured as a function of energy and from this the concentration of various atoms is determined. The kinetic energy of the photoelectrons is calculated by the following relationship:

with EB = the binding energy of the emitted electron, and @A = the workfunction of the analyzer. Besides photoelectrons also Auger electrons are analyzed, whose peaks are recognized by the fact, that their energy is independent of the energy of excitement hv,. The atomic number of detectable elements is 2 > 3, the depth range is 0.5 ... 10 nm, and the detection limit of the particle concentration is at 0.1 %. An advantage of this method is that, apart from the analytical information, a statement about the bonding state of the atom can be obtained because of a chemical shift which depends on the kind of bonding.

4.7.4 Secondary Ion Mass Spectroscopy (SIMS) The probe is irradiated with ions, mostly Ar', with energies of up to 10 keV. Due to elastic impact cascades caused by the incident ions, some of the target atoms or molecular fragment gain enough energy to leave the surface by sputtering. A certain proportion of the emitted particles is charged positively or negatively, and can be analyzed in a quadruple mass spectrometer. The secondary ion mass spectroscopy obtained in such a way is characteristic for the distribution of the elements on the sample surface and also (because of the molecular fragments) for their bonding state. The average emission depth amounts to some nanometers. As the SIMS spectrum is in principle free of any background, the process has a very low detection limit (particle concentrations of 10-4 %). All elements (also hydrogen) are detectable, except isotopes. Whilst the processes AES and ESCA do not alter the probe (unless the sample is subjected to an additional sputter process), SIMS does change the probe irreversibly. However, the advantage is the depth profile information of the atom concentration.

4.7 Analysis of Thin Films and Surfaces

169

4.7.5 Secondary Neutral Particle Mass Spectroscopy (SNMS) In SIMS the process of coupled surface ionization and sputtering is influenced by the nearest neighbors of the particles to be analyzed (matrix effect). Therefore, the intensity of the single lines of the mass spectrum are not representative of the true composition of the investigated surface. In order to achieve a quantitative analysis, a standardization with samples of known composition is required. In the interest of accuracy, especially on analyzing sandwich structures with diffusion- and implantation profiles, it is preferable to carry out the emission process independently of the subsequent ionization process. This happens with SNMS, in which the neutral particles produced by ion bombardment, which constitute the main part of the sputter emission, are ionized only after they leave the surface and are then analyzed in a quadruple mass spectroscopy. The atom concentration of a given mass is then a function of the respective sputter yield and the probability of ionization. The electron impact in a RF low pressure Ar plasma has proven effective for ionization of sputtered particles. At the same time the Ar' ions of this plasma can be used as bombarding particles to produce the neutral particles which usually exceed secondary ions (direct bombardment mode, DBM). The process has the following advantages, in addition to avoiding the matrix effect: 0

Because of the low ionic energy (some eV), practically no change in the composition of the probe (atomic mixing) occurs with ion bombardment,

0

~ )the plasma, a higher ion curbecause of the high ion density (10" ~ m - in rent density (some mAcmp2) and therefore a higher sputter etch rate is achieved.

4.7.6 Ion Scattering Spectroscopy (ISS) This process enables an analysis of the uppermost atomic layer of the sample, whose surface is exposed to ions of a certain energy E, (some 100... lo3 eV) and mass mo. The primary incident ions are elastically scattered by the surface atoms of mass m and are analyzed in a detector according to their energy and their scatter angles. The ISS spectrum obtained by this means displays maxima with values E/Eo, which correspond to the mass ratio m/moaccording to the theory of elastic scattering.

4.7.7 Rutherford Back Scattering Spectroscopy (RBS) Rutherford back scattering spectroscopy is a high energy version of the ISS method. It is carried out with incident ions of hydrogen, helium or other light elements in the energy region of 0.1.. .5 MeV. The elastic scattering of primary ions occurs at a depth which is dependent on the experimental parameters. The energy

170

4 Basic Technologies in MEMS

depth profile of the atoms of mass rn > m, can be calculated from the energy loss of the ions (mo)at a penetration depth d, from the energy loss on impact with a mass m, and from the energy loss on returning to the sample surface. From the energy variation of the primary ions, and the energy distribution of the back scattered ions, depth profiles of the atomic concentration for 2 > 1 can be determined in the region 0.1.. .10 pm from significant damage to the probe material. A definite advantage of the RBS method, which displays a detection limit of particle concenis that the analytical method delivers absolute values, whilst the tration of previous methods must be calibrated appropriately by standard samples.

4.7.8 Scanning Tunneling Microscope The tunneling current, excited by a sharp-tipped metal electrode close to the surface to be analyzed, controls the scanning tunneling microscope. This powerful instrument is able to resolve structures smaller than an atom. A very small, pointed electrode is made to approach the surface to be investigated, until a tunneling current starts to flow due to the applied voltage between them. This current depends exponentially on the tip-surface distance, as can be seen in the following formula:

(4.32) The height of the local barrier Q = F(x,y,d,u> is a function of the location x,y on the probe, the distance d between tip and probe and the applied voltage. The very precise positioning of the tip and the scanning of the tip across the surface is achieved by piezoelectric actuators. Due to this, and the characteristic of the tunneling current, distances of down to 10-” m (0.01 nm) can be controlled. By means of the STM, crystallographic orientations and even single atoms can be visualized and measured.

5 Lithography

5.1 Overview and History The lithography process is probably the most important step in microelectronics as well as in microsystem technology, regardless of whether silicon micromechanics, reactive ion etching or the LIGA process is considered [Wolf86, Elli86, More88, Beck861. Only with the aid of lithography, using light or corpuscular rays instead of mechanical tools was it possible to produce structures, whose critical dimensions lie within the sub-micrometer regions. Another characteristic feature of lithography is that by its use mass production becomes economical: many structures can be produced in parallel on one wafer, and many copies of a pre-designed structure can be transferred to the wafers without wear and tear. The word “lithography” is described in a dictionary (Collins English) as “a method of printing from a metal or stone surface on which the printing areas are not raised but made ink-receptive as opposed to ink-repellent. The characters are transferred onto the stone by crayon or ink. On filling in with fatty printing color only the drawing takes up the color”. The lithography as it is used in microelectronics and microsystem technology though has very little in common with this definition. The meaning here is the transfer of patterns developed on the computer onto the workpiece (wafer) by optical means. The basic processes involved with lithography are outlined in Fig. 5.1-1.

5.2 Resists The wafer is first covered with a thin layer of a photosensitive polymer. This is done by so-called spin coating. For this the wafer is first mounted onto a rotatable chuck and fixed by vacuum. Next to the center of the substrate to be coated (generally a round silicon wafer) with polymer, a drop of liquid coating is applied and the wafer is brought into rotation with a high peripheral velocity. Due to centri-

172

5 Lithography

Etching 1

f

J 2 Wafer

Exposure

~~~~~~~e~ resist

~ r ~ ~ e s s e ~ wafer

Fig. 5.1-1 The basic process of photolithography and subsequent fabrication steps.

fugal force the coating is spread very evenly across the wafer surface, and a very homogeneous layer can be produced. By increasing the speed of rotation the layer thickness decreases [Elli86]. This process of spin coating is used in the application of resist layers on the substrate for optical lithography and electron beam lithography as well, although the resist composition is different in both cases. In microelectronics, the final thickness after drying and pre-baking is only fractions of a micrometer, whereas in MEMS depending on the application, the thickness may amount of up to 100 pm. After the above processing the desired pattern is imaged onto the resist layer. During the lithographic process the resist changes its chemical properties, so that irradiated and non-irradiated regions have different solubility in a particular solvent or developer. In the subsequent development, the exposed areas are dissolved, whereas the non-exposed areas remain untouched. The pattern of the mask is now transferred into a chemical resistant stencil on the surface of the wafer. After the subsequent processes, such as etching, evaporation, or modification processes the resist is removed, leaving either an additive or subtractive pattern, or a pattern of a modified surface (i.e. oxidation or doping) on the wafer (Fig. 5.1-1). Since this polymer is resistant to subsequent processing steps, such as etching or oxidizing, it is therefore called “resist” or “photoresist”. Depending upon whether the irradiated or non-irradiated region is dissolved, it is possible to differentiate between positive- or negative resists. With a positive resist the polymer is changed such that the irradiated regions can be dissolved, whilst the non-irradiated regions remain unchanged. With a negative resist, the polymer becomes insoluble in the developer after radiation (Fig. 5.2-1). Positive resists are classified into single- and multi-component resists, depending on their reaction to the incident light or particle beam. Polymethyl methacry-

5.2 Resists

173

radiation

ddevelopmentb positiv-resis t

negativ-resist

Fig. 5.2-1 Positive and negative photoresist. By using positive resist the exposed areas are removed in the subsequent development process, whereas with negative resist the non-exposed areas are removed.

late (PMMA) is the typical single-component resist and is mainly used in X-ray or electron beam lithography. An example of a two-component resist is DQN (diazonaphtoquinone in a Novolak matrix), which is amongst the most frequently used photoresists in optical lithography. On irradiation of a mono-component resist, chain scission processes are induced in the long polymer chains so that the molecular weight of single-component resists decreases greatly. This can be a main chain scission process or can be initiated by a side chain cleavage [Ranb75]. An exact description of this process for PMMA in X-ray lithography can be found in Section 7.3. Two-component resists consist of a photoactive component and a basic polymer. In the case of DQN the polymer matrix is a Novolak resin (N), which can be dissolved in a basic solution. The photoactive component, which prevents the solvation of Novalak (solvent inhibitor) in its unmodified form, is made of diazonaphtoquinone (DQ). In the presence of light, DQ is converted by a so-called Wolff conversion, first into a carbene (by separating nitrogen), which is then transformed into a ketene (Fig. 5.2-2). On absorption of water the ketene is converted into an acid and so the resist is extremely hydrophilic and absorbs developer easily. On conversion of DQ into an acid the solvation of Novalak is no longer inhibited by a basic solvent [More88]. For most negative resists the decrease in solubility follows from a photoinduced polymerization or a crosslinking reaction. For some resists a change in polarity of the functional groups or the change in the degree of oxidation can give rise to the drop in solubility. In order to induce polymerization or crosslinking, a photosensitive component is attached to a polymer backbone, which on absorption of radiation changes into an excited state. This excited state is transferred to the polymer

174

5 Lithography Carben

Diazoquinon 0

0

II

" l o II

pap C-OH

R

R

lndencarbonacid

Keten

Fig. 5.2-2 The Wolff conversion of a DQN photoresist (diazonaphtoquinone in Novolak). By irradiation of the photoresist the diazonaphtoquinone is converted into a carbene, which is then transformed into a ketene, which is finally, under water absorption converted into an acid. This acid is water soluble.

so that the actual crosslinking reaction results from a reaction with two excited polymer chains. The photo sensitive component can also act as a kind of anchor by which both polymer chains are connected to each other. With lithography and resist technologies it is possible to expose precisely specified, and in some cases, down to nanometer sized regions of the surface. The subsequent processing step can be applied over the entire surface of the substrate; only those areas are changed which are not protected by the resist (Fig. 5.2-3). The material under the exposed surface can therefore be modified (by oxidation or doping with impurities) or removed by chemical or physical etching. Clearly new material build-up can subsequently be achieved by electro deposition or chemical as well as physical vapor deposition. resist

oxidation

wet etching

partial beam

resist

dry etching

electroforming

Fig. 5.2-3 The different subsequent process steps after exposure and development of the photoresist.

5.3 Process of Lithography

175

5.3 Process of Lithography In semiconductor technology lithography is the most frequently repeated process in the production of integrated circuits. Therefore it is easy to understand that in this area a great effort in research worldwide will continue to be taken. As a result of the demands of IC-production to manufacture the smallest structures possible, different processes have been developed and utilized. At the same time various types of radiation e. g. visible to deep UV-light, X-rays, and particle rays for patterning the resist have been employed. There are in general two separate processes, a serial process to fabricate masks, which are then used in a parallel process to image the mask pattern onto the substrate (Fig. 5.3-1). In the serial process, a precisely focussed beam (light-, electron- or ion-beam) is guided across the substrate and the desired pattern is “written” into the resist. Using this process, the computer pattern is freshly generated with every repetition of the exposure, and this patterning process is inherently relatively time consuming. The greatest importance of serial processes, and especially of electron beam pattern writing, is in mask production, although direct serial wafer exposure is

I

CAD Data of Microstructure

generator

I

I

I -_I

Reticle I O A , 5:1, 1:l Mask stepper

I

Mask 1 : l

I

i Fig. 5.3-1 The general procedure of the pattern transfer from the CAD file onto the silicon wafer.

176

5 Lithography

also being used for the manufacture of integrated circuits and other microstructures both in research and development. Due to time restrictions, it is not effective to produce the masks during the prototype phase of development of a new circuit, in which the geometrical layout is subjected to frequent design changes. Standard circuits however are produced entirely by a parallel mask based process. Not much can be expected to change even in the future since the increase of speed of serial patterning processes is more than compensated by the increase of structural details and a decrease in minimum feature size. In the case of the parallel process, the resist is irradiated over a large area by means of a mask, which constitutes the absorbing pattern. With visible or UV light, the mask may consist of a glass plate covered with a thin layer of metal (chromium) with a particular pattern. With X-ray lithography, the mask consists of a membrane from a low atomic number material and an absorber structure of a material with a higher atomic number. With electron- or ion-beams, stencil-like masks of metal sheets are used. Obviously for the manufacturing of masks, a serial-writing process is used, where the layout data from the CAD-system can be transformed to geometrical patterns. A distinctive feature of the parallel process is the scale of the mask. Masks with a 1:1 scale are used for shadow or contact printing. In this case mask and wafer are brought into close contact and uniformly illuminated through the mask. The size of the transferred structure is the same as on the mask. Contact printing is used for a minimum feature size of more than one micrometer. In any other case (especially for feature sizes of fractions of micrometers) imaging processes are in use, with image reductions of 1:4 until 1:lO. These masks are also called “reticles”. Using mask based processes, it is desirable to image a large area with one exposure, and if possible, the entire wafer. With increasing wafer diameter however, so-called “full-wafer” exposures are no longer possible, so that only sections of a silicon wafer can be exposed at a time, after that the wafer is moved mechanically to an adjacent position, and the next exposure takes place. This lithographic process is called “step and repeat”.

5.4 Computer Aided Design (CAD) In order to produce a mask, the information has to be exposed pixel by pixel into the resist layer of the mask. This is done with a pattern generator, which is usually an electron beam writer, as discussed in detail in Section 5.5. The pattern generator requires information about which sections of the mask should be exposed. In order to generate this information, a so-called CAD-system is used, with which the design can be carried out in an interactive mode with the designer at the computer terminal. After the pattern is designed geometrically, the data have to undergo several transformations until the electron beam writer can go to work. The design process and the necessary requirements will be described in the following.

5.4 Computer Aided Design (CAD)

177

5.4.1 CAD-Layout The aim of the CAD-layout is to specify light transmitting and absorbing areas of a mask. For handling reasons it is sensible to build up the entire mask area from smaller area units. These area units are specified by their border. In order to facilitate the conversion of these area units by the post processor program into machine-usable information, some general conventions are stipulated; 0 0

0

The border must be closed. The area contained within the border is the area to be filled (exposed). This stipulates, that a border is not allowed to enclose any another border. So-called “doughnut” structures (annulis) must therefore be produced in such a way, that two semicircular structures are fitted together to avoid an enclosed border. “Separated” areas, i. e. surfaces, which consist of several sections, though only connected with an infinitely thin connecting line are not permissible. The connecting line would not be written by the beam writer.

From these constraints a series of basic elements develop logically, which are present in a similar form with nearly all CAD-systems for microelectronics: 0

the polygon or square. This is the most general form of a surface element. It is framed by a closed border, without any other restriction. Almost any shape is allowed except the crossing over of the border lines, since this would violate the above conventions. In practice the number of corner points is generally restricted to some hundred i. e. complicated structures are composed of several polygons.

Some special cases of polygons are generally conveyed as an independent element like: 0 0

0

the rectangle (box), the connectors (wire), this is a shape with constant width, which can generally be used for the design of electric functions, circular elements like circles, annuli (doughnuts), arcs. However, most CAD-systems in microelectronics facilities do not supply these elements. Optical pattern generators and most electron beam writers are not designed to produce round structures. Instead, these pattern have to be approximated by polygons.

Finally there is also a series of auxiliary elements like 0

Text (labels). These auxiliary elements do not represent any area elements, which will later be present on the mask; they serve simply as an aid to the design (e.g. captions) and will be ignored by the postprocessor.

178

5 Lithography

Usually the patterns are designed interactively on a graphic screen of the CADsystem. At the same time certain sections can be zoomed, new elements added, and existing elements changed or deleted. The design data are stored in a datafile of a database. It is possible, to transfer the geometrical data of a pattern as a mathematical function or as a data matrix into the data file. This is especially advantageous, if the geometry compared to a complex design, can be more easily represented mathematically (e. g. the logarithmic spiral), or if the geometrical data are the result of another calculation program e. g. the result of extensive optimization calculations. This however assumes a suitable data interface. CAD-layout systems for masks differ generally from those which are designed for mechanical engineering as pointed out in the following list: 0

Mask design systems work with pixel oriented programs, whilst two dimensional CAD-systems for mechanical engineering are vector oriented.

0

Mask design systems possess a distinct hierarchical organization. With mechanical CAD-systems sub-structures are also used, but rarely hierarchically interlaced. In mask design systems, those capabilities necessary for the generation of diagrams are usually not, or only in a few cases, present ( e . g . automatic dimensioning).

CAD-systems developed for mask design in microelectronics can be adopted to design requirements in the microsystems field. In microsystem technology however, round shapes and more diversified structures are much more frequently designed than in microelectronics, the latter being generally based on rectangular structures (so-called Manhattan structures). But since such general structures do sometimes appear also in microelectronics, some modern CAD-systems have capabilities in this respect. Over the last few years a market for more flexible CADsystems has developed, since integrated optical applications require round patterns and structures with very large curvatures.

5.4.2 Alignment Patterns and Test Structures Sequential lithography pattern have to be precisely adjusted to each other. But even the first lithography pattern in the process sequence has to be centered to the virgin wafer and aligned with respect to the crystallographic orientation (i.e. with respect to the flats at the wafer circumference). Certain spaces in standardized mask design are provided for this kind of adjustment pattern. In Fig. 5.4-1 such an adjustment pattern is shown which facilitates the adjustment of the first mask to the geometry of the wafer. More complex structures are being build up with the subsequent lithographic steps in order to test the quality of the process steps, performed in between any two subsequent lithography steps. With suitable patterns below and above an insulating layer, the parameters such as dielectric constant, thickness and presence

Fig. 5.4-1 Adjustment pattern for alignment of the first lithographic masks with respect to the geometry (position of the flat) and crystallographic orientation of the silicon wafer.

Fig. 5.4-2 Test pattern for checking the degree of overetching at an etch process during the IC manufacture. a) The pattern on the mask. This pattern is transferred into the photoresist on top of the wafer surface. b) The appearance of the test pattern under ideal etch conditions (i. e. etch pits with vertical walls). c) The appearance of the test pattern with overetching. The alignment of the etch borders can be checked with the vernier-like test pattern and the degree of underetching can be easily estimated.

180

5 Lithography

of pinholes could be tested during the fabrication. An etched pattern could be checked with respect to overetching or underetching. For this a test pattern is transferred onto the etch mask of a wafer. The vernier-type arrangement of the two rows of etch pits facilitates the evaluation of etch quality (Fig. 5.4-2). There are numerous other patterns developed with high sophistication. These patterns are usually not published in the open literature, since they influence considerably the yield of a product, and therefore are kept confidential. The adjustment pattern are primarily designed to guaranty the precise alignment of subsequent lithographic patterns. These patterns therefore have to remain visible for the whole process sequence. A typical set of alignment patterns is shown in Fig 5.4-3 [Reye92].

a

h k Z

Nlask 3

Mask1+2

Nlask 1+2+3

b

Mask1

Mask 2

Mask 1+2

Fig. 5.4-3 A sequence of alignment patterns for three subsequent lithography masks. a) First the alignment pattern of mask 1 is transferred into the resist. By means of the second mask a second test pattern is exposed on top of the first pattern. With the third mask finally a third test pattern is superimposed to the alignment structure on the processed wafer after 3 lithography steps. b) By adding a vernier to the test pattern the degree of misalignment of two structures can be recognized and corrected.

5.4 Computer Aided Design (CAD)

18 1

The individual test pattern are designed to match into each other for subsequential alignment of the masks with respect to the already existing structures on the wafer. Another application for adjustment pattern are markers between the chips for the wafer scribing or sawing.

5.4.3 Organization of the Design (Hierarchy, Layers). With the large number of single elements, which usually appear in one mask design, a skilful organization of the design is most important. This is true, to the same degree, for both microelectronics and micromechanics. Mask design is therefore generally built up hierarchically. This means that the group of design elements are comprised of substructures. These substructures can be stored as “macros”, and can be added as a whole to a new design. This technique is very similar to the subprogram technique in general programming. For the implementation of the corresponding operation it is sufficient to call upon these subprograms. As substructures are allowed to contain references of other substructures, stacking (mostly to about 16 layers deep) is possible. Different transistor structures for example can be defined as substructures. Logical function can be designed conveniently from these substructures. The advantage of this hierarchical organization is a speed-up in design, and design failures are greatly reduced. A further aid for the organization of the design are the logical layers. To give an example: a pattern to be written by an electron beam writer can be subdivided in areas with high resolution i. e. sharp corners and other border contours), and areas of reduced requirements in resolution (i. e. the inner part of a square, which has to be filled). Therefore the exposure time can be clearly reduced, as the “non-exact” areas use a larger beam diameter and “filling” can be carried out much quicker than the exact areas, which must be written with small beam diameters. These areas will be assigned to different layers. Therefore, with the CAD-system there is the possibility to lay down different structures onto different logical planes or layers. In the graphical representation on a screen this is generally highlighted using different colors of the structure. After completion of the design, the information contained in the data base, i. e. the amount of elementary shapes, from which the design is made, inclusive of their coordination and their hierarchical organization, must be conveyed to a post processor program (see Section 5.5.4). As there are many different CAD-systems for data design, as well as for pattern generation, there is a requirement for suitable standardized interfaces. In the area of microelectronics, the so-called "Calms GDS I1 format” is accepted as a quasi standard format and is compatible with all pattern generators (or rather their processor programs).

182

5 Lithogrzlphy

5.5 Electron Beam Lithography Although electron beam lithography has not yet as much significance as optical lithography in mass fabrication, it is most important for the production of optical masks. On an industrial scale no other technology allows the production of arbitrary patterns with nanometer dimensions. An electron beam, which can be deflected by an electromagnetic or electrostatic field, is used in electron beam lithography, in order to write a specified pattern in an electron sensitive resist. One can distinguish between screen- or vector scan mode with round (Gaussian) or shaped beam [Brewso, Webe79, Pfei791. All these systems write a pattern in overlapping spot sizes and are therefore comparably slow. Besides electron beam writers so-called electron projectors have been developed, which image the mask as a whole onto the substrate, as it is done in optical lithography [Scot78]. Masks with free suspended stencil like absorber pattern have to be used, since there is no usable carrier transparent to electrons. The mask is illuminated with a widened electron beam. The electrons which pass through the mask are imaged by an electromagnetic lens onto the substrate. The disadvantage of electron projection systems are the special masks, which for example do not allow doughnut-patterns. For a complete pattern several supplementary masks have to be used for one exposure.

5.5.1

Gaussian Beams

In an electron beam writer with a round shaped beam, the intensity distribution across the beam corresponds to a Gaussian distribution. Therefore, one refers to a Gaussian beam. To clarify the principle construction and function of such an electron beam writer a simple illustration is shown in Fig. 5.5-1. The main components are: 0 0 0

0 0

the the the the the

electron source (cathode) with the anode aperture, electro-optical imaging system, blanking unit, deflecting unit and precision stage with laser interferometric position control.

Electrons are emitted from an electron source and accelerated under high voltage (between 10 and 100 kV) onto the target e . g . the mask. Tungsten “hair pin” cathodes or LaB,-crystals with very pointed emission areas are normally used as the electron source. For writers with high resolution (i. e. with a narrow beam diameter), also field emission cathodes are used. For the thermal cathode, it is desirable to have a low electronic work function of the cathode material (this is the reason why LaB, is used, regardless of some disadvantages in handling), since already at moderate cathode temperatures one obtains

5.5 Electron Beam Lithogruphy

accewation voltage

183

cathode

10 to lOOkV electron lens

~

condensor

for blanking deflection coils ,

electron detector

spotsize 25nm to lvm

,dser interferometer for positioning Fig. 5.5-1

Basic concept of an Gaussian beam electron beam writer. The electrons emerge from a cathode, are accelerated by an anode and focussed by an electron lens into a blanking unit. When a voltage is applied to the deflection plates the intensity of the beam down at the stage is decreased to zero. A second electron lens (final lens) focusses the electron beam at the stage to a spot of 25 nm to 1 pm. With the arrangement of deflection coils the electron beam is scanned across the focal plane at the stage.

a high electron current. The current density j, is described by the Richardson equation jR =

C T exp ~

(-$)

with Q, the workfunction of the cathode material and C a constant Thermal cathodes usually have a current density of under 1 A/cm2. For field emission-cathodes the current increases exponentially with increasing field strength. In this case the Fowler-Norheim equation describes the current density jFN:

with E = the applied electrical field and B and C constants.

184

5 Lithography

j,, usually is in the order of lo4 and 10' A/cm2 [Brod82]. The beam, emerging from the cathode and accelerated by the anode, is focussed by a sophisticated electromagnetic lens system. Depending on type of apparatus and the operating parameters, the beam diameter d is between a few nanometers and 1 pm, and can be estimated from the focal lengths of the single lensesx, the distances of the lenses to the aperture Liand from the size of the anode aperture do. For a system of three perfect lenses d is calculated as

The beam diameter on the substrate can be varied over a wide range (by a factor of about 20-50) by the lens system, similar to an optical zoom lens. The beam is moved meanderlike over the field to be exposed. During the scanmotion the intensity is switched on and off to generate the desired pattern. This very fast switching is achieved by running the beam through the electrical field between two capacitor plates. In off-mode (i. e. with full electrical field) the beam is deflected, and the intensity at the substrate drops to zero. The scanning of the beam in x- and y-direction is achieved by electromagnetic fields applied orthogonal to the beam. The deflection of the beam in the x- and y-direction is effected by magnetic coils on utilization of the Lorenz power:

FL = - e . v X B

(5.4)

where v is the speed of electrons and B the magnetic field. Note that v and B are vectors. To keep the scanning beam as close as possible to the center of the final lens (to avoid excessive lens distortions), a double deflection mode in both directions is applied, as sketched in Fig. 5.5-2. In order to keep the imaging errors to a minimum, the maximum deflection amplitude of the electron beam at the substrate is usually in the order of one millimeter. With large deflections, the electro-optical errors (beam diameter, astigmatism, linearity of the deflection etc.) become too large. Within the deflections of above mentioned dimensions the beam astigmatism can be compensated for by a stigmator, a magnetic octopole inside the final lens. For large interconnected patterns which exceeds the maximum scan amplitude, the substrate has to be moved mechanically to the adjacent writing field. Therefore the substrate is mounted onto an interferometer controlled x-y-stage. In order to achieve the highest possible accuracy, a so-called two frequency laser interferometer is applied. The line of a He-Ne-laser is split by an external magnetic field (Zeeman effect) and the beat frequency between these two lines is measured. The reference beat frequency is guided by a beam separator directly into the register, whereas the measuring frequency is shifted by the movement (Doppler effect) of a reflector, which is mounted to the stage. With this arrangement mechanical displacements can be detected, which are about 100 times smaller than the wavelength of light i. e. a distance of more than 100 mm can be measured with an error less than f 1 0 nm.

5.5 Electron Beam Lithography

185

! I

;

Electron beam

a

-2J

Mask

Fig. 5.5-2 The double deflection mode in electron beam writers. In order to feed the scanning electron beam through an aperture at the optical axis the beam is double deflected by two sets of coils in x- and y-direction. (In this arrangement the beam is actually deflected perpendicularly to the paper plane!).

For control and calibration of the stage- and beam-positioning, as well as for the determination of the beam diameter, defined markers are positioned on the x-y-stage. Electrons which are backscattered from these markers are detected and processed by means of scintillation detectors or photomultipliers. The control of the deflecting coils, the magnetic lenses, the blanking unit, the x-y-stage and the evaluation of the signal of the electron detectors are all computer controlled.

5.5.2

Write Strategy with Gaussian Beams

To write on a mask or a wafer with the Gaussian beam there are two basic strategies: 0 0

the screen scan process (Fig. 5.5-3a) and the vector scan process (Fig. 5.5-3b).

With screen scan processes the electron beam is guided meander-like over the entire area (scan-field), which can be scanned by the beam deflection system. The

c Screen scan versus vector scan. The thin lines indicated the path of the electron beam with zero intensity (blanked beam). For certain patterns with small areas to be exposed the vector scan is more economical than the screen scan. intensity is switched on only when the beam scan moves into areas to be exposed, whilst on other parts it is blanked. With only small areas to be exposed, the beam is scanned mostly in the blanked mode, which is a waist of time and money. A variation of this strategy is the “writing on the moving stage” (“writing on the fly”). In this case the electron beam is deflected periodically only in one direction, whilst at the same time the stage under the beam is continuously moved in the perpendicular direction over the entire length of the mask. Since the table is to be moved synchronously with the beam scan, the limiting parameter is the slow mechanical displacement. When only small parts of the writing field are being exposed, again the electron beam writer is “idling” most of the time. An attempt to minimize the non-effective writing time is achieved by using the vector scan principle. Instead of moving the electron beam over the whole of the available scan field, the beam jumps to the areas to be exposed and scans only the designated areas. With both processes the patterns are built up from elementary trapezoids, which are exposed successively. The trapezoids or rectangles in turn are filled by the electron beam meander-like line for line. The line again is exposed by a sequence of overlapping dots. Therefore the beam is not moved continuously over the area to be exposed, but stepwise spot to spot. The distance between two spots is called the “beam step size” and is considered the geometrical resolution of the electron beam writer (Fig. 5.5-4). This procedure is chosen to keep changes in the deflection coil current as small as possible to avoid excessive eddy currents and thus nonlinear deflection errors. The deflection voltage for the electron beam therefore follows a step function. In practice however due to the ever present parasitic in-

5.5 Electron Beam Lithography

time I

187

A

ii' time

beam-step-size t -

Fig. 5.5-4 Writing mode of a Gaussian beam electron beam writer. By applying a staircase-shaped current to the double deflection coils the beam is moved stepwise from position to position. The beam-step-size is the resolution limit of the system.

ductances and capacitances of the coils, the current is smoothed out and the beam is scanned more or less continuously. Due to this writing strategy the beam deflection is mostly divided into two sequences. The position of the trapezoid to be exposed in the main working field is approached with a relatively slow scan motion. For the subsequent trapezoid deflection it is necessary, that software as well as hardware are as fast as possible, since this part of the exposure determines essentially the working speed of the electron beam writer.

5.5.3

Shaped Beams

To expose a pattern onto a mask, many single overlapping exposure steps must be carried out with the Gaussian electron beam. Even if the frequency at which the beam is moved from step to step, is high (with particularly fast writers it can be 160 MHz), the total exposure time can sum up to several hours for one mask. To speed up this process special electron beam writers were developed with a rectangular shaped beam. At the expense of resolution, large elementary areas can be exposed in one step. The size of the rectangle can be varied by imaging lenses. The principal set-up for shaping the beam is relatively simple (Fig. 5.5-5): A square aperture in the upper section of the writer is imaged into the plane of a second aperture. Without deflection the image of the first aperture just corresponds with the second aperture and this shape is reproduced on the sub-

188

5 Litlzogruphy

-1

cathode 1. square

aperture

condensor lens

1. image of

deflection for image shaping

cathode

‘M-----:i:i:/j ...

Fig. 5.5-5

full size beam

/

Shaped beam electron beam writer. By shifting the image of the first square aperture with respect to the second square aperture, rectangular shaped images can be formed in the the final focal plane.

strate. If the image of the first aperture is shifted with respect to the second aperture, only the overlapping part of the two apertures is illuminated, and a small rectangle is exposed onto the mask. Each arbitrary combination of x- and y-deflection can be used, in order to produce the desired rectangular shape. The substrate is then exposed with these individual elementary patterns, using the corresponding deflection- and blanking-systems. The electron beam writer with shaped beam was developed, in order to reduce the writing time. However, a decrease in writing time can only be attained, if the writing geometry can be allotted within a simple rectangle. This is generally the case in microelectronics (Manhattan structures), but less so in micromechanics as often really complicated geometrical structures must be produced.

5.5 Electron Beam Lithography

189

5.5.4 Post Processor Now that the principle operation of an electron beam writer has been described, the task of a post processor program should be briefly introduced, whose function is to convert the CAD-data into the control data for the writer (Fig. 5.5-6). This field is still in development, and there are up to now very few common standards. The post processor program has to organize the beam movements for the large deflections within the main working field (with the blanked beam) and the stepwise motion within the elementary trapezoids (with full intensity). In addition it has to control the accompanying stage movements. The parameters for writing a pattern are stored in a control data file, for the control of the following tasks :

a which of the CAD-output files is in use, a which layer should be written, a in which format the input data are stored, a which corrections should be carried out on the data,

a which machine parameters on writing are to be used (the beam diameter, the beam step size and the size of the writing area). According to these instructions the post processor program controls the input file (output file of the CAD-system) and generates the positional information for the electron beam in the form of an address file. Each path of the electron

Fig. 5.5-6 Data flow for a pattern to be processed by an electron beam writer system.

190

5 Lithography

beam to be followed, is treated as a series of address data, which are supplied by the post processor. The address data is then converted by a digital-analogue converter (DAC) into a voltage, which causes the deflection of the electron beam. A compromise must be made with respect to the number of binary addresses for the storage of positional information. With the increasing positioning resolution, the number of addresses, the necessary storage space and the conversion time for the transformation in the DAC is also increasing. Usually about fifteen bits per positioning coordinate are chosen. With this the final resolution of the writer is predetermined. All positions are multiples of the smallest unit length, the intercept length of an electron beam (beam step size). The data generated by the post processor are placed in an output file, the binary file. In order to carry out the handling and exposure of the pattern, another auxiliary file (the jobfile) is required. This jobfile determines the operation parameters, i. e. which substrate has to be loaded from the magazine in time, which calibration operations are to be carried out, which part of the pattern should be placed where on the substrate, and so on. In the actual writing process the writer then executes the write- and movement-operations, which are contained in the binary file.

5.5.5

Proximity Effect

For electron beam writers to obtain the smallest possible pattern geometry, it is not only a matter of the smallest possible beam diameter, but depends also greatly on the electron-resist system and the layers underneath the resist. When electrons penetrate the resist layers, they are scattered both elastically and inelastically. The electrons therefore experience a deviation from their original direction. This deviation depends on both the energy of the incident electrons as well as on the atomic mass of the resist molecules and is known as forward scattering. As the scatter angle increases quadratically with decreasing energy and the electrons in the resist give up their energy quasi-continuously, a strong broadening of the electron beam occurs in particular at the end of the electron pathway. The electrons interchange energy with their environment until they come to a rest. If the resist thickness exceeds the penetration depth of the electrons, the deposited electrons fill a lobe-shaped body within the resist. Naturally the size of this lobe is dependent on the original energy of the electrons. Depending on the energy too is the quasi-vertical section of the lobe at the surface of the resist. Thick resist layers have to be exposed by high energy electrons in order to obtain good vertical walls (Fig 5.5 -7). Unfortunately there are other unfavorable effects which increase with the electron energy as we will learn in the next section. In the case of writing a mask, the energy of the penetrating electron is in general so high, that nearly all the electrons pass through the thin resist layer entirely and enter the substrate underneath the resist. Due to the large atomic mass of the substrate material, the electrons are scattered by a large angle, which can exceed 90".

5.5 Electron Beam Lithography

191

electron beam

20kV

60kV

Ill/

/Ill

Fig. 5.5-7 Electron scattering in thick resist for electrons with different energy.

resist

substrate Fig. 5.5-8 Proximity effect of particle beams in resist.

Therefore some of the electrons appear again at the boundary of substrate and resist. The resist is now exposed from underneath in areas which are not supposed to be irradiated. This effect is called back scattering (Fig. 5.5-8). The scattering of the electrons in the resist as well as the backscattering from the substrate cause the exposure of areas which do not correspond to the desired pattern. Small structures next to large areas are overexposed due to this "proximity effect'"Chan751. In serious cases the small detail is not distinguishable anymore from the large area. The proximity effect is one of the limiting factors for resolution of electron beam exposures.

192

5 Lithography

The proximity effect is influenced by: The energy of the electrons. It decreases with increasing energy. Therefore it is necessary to use the highest acceleration voltage possible for the structuring of thicker resist layers with parallel side-walls. The substrate material. The effect is smaller for materials with lower atomic weight. Consequently materials with low atomic number, (e. g. beryllium, atomic number = 4) are especially suitable in micromechanics as substrates in the mask production. The resist material and its thickness. The smaller the average atomic number of the resist and the thinner the resist, the less the effect is noticeable. The contrast (y-function) and the development conditions of the resist. The influence of the effect decreases with the higher contrast. To correct the proximity effect it is possible e. g. to divide the structure which is to be written, into several areas. These areas are then exposed with respect to the background dose with adapted parameters, such as different electron current density or different exposure time. These different local irradiation doses are calculated with Monte-Carlo-methods.

5.6 Optical Lithography Optical lithography is most important in the production of microelectronic circuits and also in microtechnology. It is being used to fabricate the coating masks for the successive etch- and diffusion processes. For many years optical lithography was considered to come to an end at pattern resolution of under 0.5 pm. Nevertheless due to intensive development this limit could be reduced to below 0.2 pm. Significant contributions were made by continuous achievements in many areas, which determine the structure, such as: 0 0 0

wavelength of light (diffration), focal length and numerical aperture of the lens system, contrast and resolution of the resist, reflection from the substrate (standing waves in the resist).

In photolithography the pattern is produced by imaging a mask into the photosensitive resist. Mercury vapor lamps are conventionally used as illumination sources, which have strong emission lines at 435 nm (G-line), 405 nm (H-line) and 365 (I-line). Recently eximer-lasers have been used, which work at wavelengths of 248 nm (gas: krypton fluoride) or 193 nm (argon fluoride).

5.6 Optical Lithography

193

Low resolution systems but fairly simple and inexpensive in design are 1:1 shadow projection systems, which either form a direct contact of the mask to the resist (contact printing), or leave a small gap (20-50 pm) between mask and resist, to prevent the scratching of the mask due to trapped dust particles (proximity printing). High resolution systems work with lenses by generating a reduced image of the mask artwork onto the resist (projection printing).

5.6.1 Masks The masks which are used in optical lithography consist of glass- or quartz plates with a thickness of about 1.5 to 3 mm. A sputtered chromium layer serves as the absorber and has a thickness of usually 0.1 ym, which is sufficient for complete opacity. A resist layer of about 0.5 to 1 pm thickness is applied onto a chromium layer using a spin coater. After drying and baking, the resist layer is exposed by an optical pattern generator or by an electron beam writer. The subsequent step is the development of the exposed resist by either a simple dip-development or by a more elaborated spray development, in which the rotating mask is sprayed with developer, which is followed by an extensive washing cycle and finally a drying process. With the help of the resist mask the chromium layer can be etched to the desired pattern. This is carried out primarily, as a wet chemical process, where the etch liquid is sprayed onto the rotating mask. However, dry etch processes are used on masks with high resolution requirements. In the final step the remaining resist is removed in an oxygen plasma (see Section 4.6.3).

Mask Repair With increasing complexity and size of the pattern field, the mask inspection and repair become ever more important. With very large designs and minimum structure width under 1 pm, it is a fact, that absolutely error free masks can no longer be produced. The mask check is carried out with the aid of an expensive computer program, with high performance optics and with a high precision stage. Usually for mass fabrication the same pattern is repeated many times on one mask. Therefore a direct comparison of two written patterns (die-to-die) can be carried out. At great expense a comparison of the exposed pattern with the CAD- data (die-to-database) is performed, and in this case also recurring errors are found. With modern mask inspection systems all errors with a diameter of 0.35 pm and larger can be detected with a probability of 95 %. In principle two classes of errors can be defined: “opaque defects”, where absorbing spots are left behind on sites, which should be transparent, and “clear defects” where pieces of absorber structures are missing. To eliminate the opaque defects the chromium must be removed, which can be carried out by irradiation with laser light or with a focussed ion beam (see Section 5.7). The elimination

194

5 Lithography

of clear defects is in general more expensive because the non-transparent layer must be deposited onto the mask. This can be done in a kind of local CVD-process. The gaseous metallic compound is guided over the area to be repaired, and a laser or particle beam is then aimed to the position of the defect, to supply the necessary threshold energy for inducing the decomposition of the compound and the deposition of the non transparent layer.

5.6.2 Shadow Projection The simplest form of lithography, the optical 1:l shadow projection, which used to be the standard process of semiconductor technology over many years and is still in operation in many applications with less critical components, is schematically represented in its two variants, the contact- and proximity printing in Fig. 5.6-1. With contact printing, masks and wafers are stacked close to each other with a small separation gap. After precisely adjusting each to the other by utilizing special alignment patterns, they are brought into close contact for exposure. In principle this allows a good pattern resolution down to the submicrometer region. However when dust particles are entrapped between mask and wafer, this may result in defects on the mask. A defect produced in such a way on the mask would appear with every successive exposure. A further problem in contact printing is the slight unevenness of the resist layers, which prevents ideal contact

Fig. 5.6-1 a) contact printing, b) proximity printing.

5.6 Optical Lithography

195

over the entire wafer surface. Therefore the pattern resolution on the wafer is varying depending on the contact situation. This problem even increases with subsequent process steps as well as the existence of structures already present on the wafer. With proximity printing a small defined gap is left between the mask and wafer, typically 10-50 pm. This prevents the mask from being damaged by small entrapped particles However due to the wave characteristics of light, in this case shadow projection does not result in an ideal intensity function, which is precisely congruent to the mask pattern, but the intensity distribution is ruled by Fresnel diffraction and results in a loss of resolution. Figure 5.6-2 demonstrates the difference between contact printing and proximity printing with otherwise similar parameters. The exact value of the attainable resolution depends on the resist-developer system. As an approximation, the minimum attainable dimensions b,, can be specified by: bmin=

dh.d,,,

(5.5)

with /1 = wavelength of the applied light, dprox = the proximity distance. For a wavelength of 436 nm (G-line of a mercury vapor lamp) and a proximity distance of 20 pm, a minimum resolution of about 3 pm can be expected. With contact printing and using a 1 pm thick resist layer, the resolution would be 0.7 pm. Due to the low resolution and the above mentioned problems, neither method is considered for modern semiconductor lines. However, in microtechnology, where the requirements on minimum structure widths are generally less, these method are still of importance.

Fig. 5.6-2 Comparison of the experimental results of a structure gained by a) contact printing and b) proximity printing. The openings are 24 x 40 pm2, the resist thickness is 30 pm (Courtesy of Fraunhofer Institute for Reliability and Microintegration (IZM), Berlin).

196

5 Lithography

5.6.3 Imaging Projection Improved pattern resolutions are attained with imaging systems. In this case the resolution amounts to:

where NA = numerical aperture of the imaging system. From this it is apparent, that with decreasing wavelength the resolution increases. Also an increase of the numerical aperture leads to a higher resolution. However, the numerical aperture also limits the focal depth Af, which can be estimated by the following formula:

For example, a wavelength of 436 nm and a numerical aperture of 0.35 results in a resolution of 0.6 pm and a focal depth of 1.8 pm. An unevenness of the wafer, its topography, a resist thickness variation and additional equipment errors can easily lead to an out-of-focus situation. To optimize the conditions for optical lithography, a compromise between the higher resolution and a greater depth of focus has to be made. If a large depth of focus is required, as well as a high resolution, then the only way to achieve this is to reduce the wavelength further. The image quality of a projection device is specified by the modulation transfer function (MTF) [King77]. This gives the intensity modulation as a function of the spatial frequency of a line design:

with Zmin and I,, = minimum and maximum intensity. and Imin/Im, = contrast in the pattern. The MTF for a diffraction limited lens system is calculated by [Di1175]:

with v0 = 2.NA I /1 (optical cut-off frequency). The maximum attainable spatial frequency depends on whether the exposure is carried out with coherent or incoherent light. For coherent light v,,, = NA/L For incoherent light this value doubles. Therefore in order to attain a higher resolution, it is advisable to use incoherent light.

5.6 Optical Lithography

197

light beam mask displacement mask mirror

wafer

Fig. 5.6-3 The Perkin-Elmer mirror lens system (MICRALIGN) for full wafer expo-

sure.

Full Wafer Exposure There is no lens capable of exposing the full wafer with the required resolution. Therefore Perkin-Elmer Corp. developed a compromise solution by using a mirror lens system. This is represented in principle in Fig. 5.6-3. A mirror lens does not exhibit a chromatic error like a diffraction lens. But the spherical correction of the mirror lens is not possible over the entire surface area but only in a sickleshaped surface area. Therefore the mirror lens is moved synchronously with regard to the mask and wafer, using a precise scan-mechanism. With this the entire mask is imaged in one scan-motion onto the mask on a scale 1:l. The MICRALIGN-series which have been evolved from this idea by Perkin Elmer formed the “working horses” of the production of the 2 pm structure resolution area for many years. Step and Repeat-Processes Modern high resolution lithography is performed with the so-called optical wafer stepper. The essential element consists of a high resolution, extremely highly corrected lens, which however has the required performance only within an image field of about 1 cm2 and a focal depth of 1 pm or even less. During the lithographic process the entire wafer is exposed step by step and field by field. This process is therefore called “step and repeat”. The exposure steps have to be followed by precise stage movements. The position of the table is controlled by laser interferometer. To overcome the little depth of focus, every field has to be focussed individually.

198

5 Lithography

5.6.4 Further Developments Over the last years much effort has been invested, to reduce the limits of optical lithography. The greatest success in this respect is represented by the use of socalled phase shifting masks, which have meanwhile found their way into the production process [Leve92]. In addition to the usual absorber design, the phase masks possess a layer on every second transparent structure, which rotates the phase of the transferred light wave (phase shifter). The thickness of the transparent layer is given by:

(5.10)

t=h.(N-l),

with N = the refractive index of the layer. The optical beam path is manipulated in this way between two adjacent elements, such that both partial beams have a phase difference of 180". As a result the diffracted light of the partial beams interfere destructively in the overlapping area at the resist level (which by definition should be a non-irradiated area). With this the image contrast and the resolution in the optical projection illumination is improved considerably. Structures with resolutions down to 0.2 pm can be achieved. Meanwhile there are many variations of phase masks, a simple form is represented in Fig. 5.6-4.

phase mask

conventionel mask

Fig. 5.6-4 Example of a phase shifting mask. The additional optical path of the phase shifter cause a 180" phase shift of adjacent light beams. The amplitude immediately below the mask is shown in a). At the focal plane the square shaped amplitude is modified by Fresnel diffraction. Due to the destructive interference in the overlapping parts the amplitude (and therefore the intensity too) is zero. In comparison with conventional masks the resolution limit is pushed towards smaller dimensions.

5.6 Optical Lithography

199

In order to be able to work with a very small focal depth, multi-layer resists were developed, in which only the uppermost thin resist layer is optically structured. The developed pattern of this thin layer is then transferred, by etching processes, to the underlying thicker layers, which also serves to even out the topography of an already partially processed wafer. An example of this technique is the so-called tri-level process (Fig. 5.6-5). The lowest resist layer which forms the mask of subsequent processes, is relatively thick (1-3 pm). This polymer layer is coated by a sputter process with a very thin (20-100 nm) auxiliary layer of a material which is resistant to oxygen plasma. Mostly silicon nitride layers are used, but also thin metal layers (e. g. titanium) are useful. A resist layer, which can be lithographically patterned, is finally applied on top. Using this tri-layer process the upper resist layer can be optimized for sensitivity, resolution, reflectivity and so on, without the need for compromises with the layers underneath. The resist pattern is transferred into the secondary layer by sputter etching with argon ions, and this pattern in turn is transferred into the thick polymer layer (polyimide) by ion etching with oxygen ions. Using the so-called DESIRE-process (Dry Etching of Silylated Image Resist) silicon atoms are incorporated into the uppermost area of the resist layer by irradiation (silylation), whereby these areas will become resistant to oxygen ions

IIIillilllIIllJI

optical mask

SiaN, or Ti 0,02-0,lym polyimid , 1-3ym substrate -

I -

I I 1 I 1 I I i 1 1 1 1 1 I Ar+ sputter etching with argon ions

reactive ion etching with oxygen ions Fig. 5.6-5

The process sequence of the tri-level process. By using three layers of masking materials high resolution patterns can he transferred onto substrates with uneven topology.

200

5 Lithography

[Reuh91] [Kato92]. An etch mask is thus formed directly into the resist. In this process, the resist layer serves as a lithographically active layer and as an ion etching layer at the same time. Oxygen ions are used to transfer the pattern, which in the first instant is only existent at the uppermost layer of the resist, into the bulk of the entire resist layer. Due to the high directionality of the oxygen ions, etch masks with steep edges can be produced.

5.6.5 Optical Lithography for Micromechanics Until a few years ago optical lithography was used exclusively for the manufacture of microelectronic circuits. However, in the meantime, it has become ever more important in the microstructure technology. It is used to facilitate dry- and wet etch processes, lift-off processes for very delicate metal structures, and for shaping polymer tools for electroplating. The latter process has become possible with the development of optically sensitive coatings, which can be put down in layers of over 100 ,um and can be exposed and developed with a fairly high aspect ratio (the relation between minimum lateral structure to structural height). In the present case aspect ratios of 1:10 are possible. Applications include highly viscous positive photoresists based on Novolak (e. g. AZ 400 series by Hoechst) as well as negative polyimide resists. The thick resist layers are usually structured using UV-light by shadow illumination. The physical limits due to scattering and diffraction described above also hold good for the thick film resist which is the reason why the aspect ratio of the microstructure is limited to between 8 and 10. The shape of the walls is not only influenced by the exposure, as can be seen in (Fig. 5.6-6). The concave shape is a

Fig. 5.6-6 A columnar structure influenced by photon scattering during exposure and by the effect of progressive development.

5.6 Optical Lithography

20 1

result of the variation of solvent concentration in the thick resist layer and due to the fact that with the progressive development different parts of the structure are subjected to the solvent for different lengths of time [Schu96]. The spreading out of the structure on top is a result of the pre-bake process (prior to exposure), as this region is very dense and the solvent concentration correspondingly lower. Consequently the contrast of the resist is much higher. The widening in the area of the foot is a consequence of the short contact time with the developer. These distortions can be minimized, if the structure as a whole is exposed to the developer as short as possible. The structures produced by optical lithography with thick resists are normally used as molds for a successive electroplating step, designed to manufacture micromechanical metallic structures. Since this process is similar to the more elaborate LIGA-process (Chapter 7), with some concessions with regard to aspect ratio and resolution, this process is also called “UV-LIGA” or “Poor Man’s LIGA”. Of interest for the manufacture of structures with arbitrary shaped walls i s the so-called grey-tone lithography (Fig. 5.6-7). The actual pattern comprises a kind of raster, i. e. small squares or circles with progressively different dimensions. A similar principle is used when printing grey tones in newspapers. Due to the inevitable diffraction on exposure, these structures have a lateral gradient in dosage, resulting in prism- or sinusoidal shaped surfaces.

Fig. 5.6-7 Grey-tone lithography. A pixel pattern with dimensions below the resolution limits of the development process yields to “semi-exposed” areas on

the substrate.

202

5 Lithography

5.7 Ion Beam Lithography In addition to electron beam machines, exposure systems working with ion beams have been developed in which a resist is directly patterned with accelerated and focused ions. The main advantage of ion beam lithography is less scattering of the impinging ions in the resist when compared to electron beam lithography. This arises from the larger mass of the ions which also prevents backscattering of the ions to a very large extent. In effect this means that the proximity effect can be almost completely neglected in this case. Furthermore the energy deposited over a unit distance is much higher than in electron beam lithography resulting in a much higher sensitivity. On the other hand, because the penetration depth of heavy ions with energies below 1 MeV is only in the range of 30 to 500 nm and a fixed maximum depth to which the ions can penetrate exist, only very thin layers can be patterned. Therefore, for practical purposes a tri level technique seems to be indispensable. With the exception of the ion source the principal construction of both, electron and ion beam systems, is identical. However while the generation of free electrons is relatively easy (e.g. by thermal emission), for the production of ions a much greater effort is necessary. The non-availability of reliable ion sources is the major hurdle for a more widespread use of this technology. In principal two different types of ion sources have been developed thus far: The ions (e. g. Hz+) can be extracted either from a gaseous atmosphere or from a liquid. In contrast to the simpler gas sources, with liquid metal sources a large variety of ions can be utilized. The advantage of micro-plasma ion sources lies in their high efficiency [Frey92]. Because the heavy ions can not be deflected as efficient as electrons, ion beam exposure tools will probably not be able to solve the throughput problem connected with serial writing systems despite the higher sensitivity of the resist. The advantages of ion beam writing systems lie more (i) in the achievable resolution where feature sizes of less than 10 nm can be obtained, (ii) in the repair of optical masks (removing excess chromium on a mask) and (iii) in direct (maskless) ion implantation.

5.8 X-Ray Lithography It can be deduced from Eq. (5.5) that the minimum attainable structure width decreases with the root of the wavelength of the applied light. By moving from the visible region to the short wavelength UV region, not even a factor of 1.5 can be reached. A much larger effect is possible when changing to X-rays with wavelengths of 0.2 to 2 nm. As there are no optical diffraction components for imaging in the X-ray region, X-ray lithography is carried out as a simple 1:l shadow projection with a proximity gap between masks and substrate. Figure 5.8-1 shows the principle construc-

5.8 X-Ray Lithography

203

normal pressure

or He (some 100 mbar) / /

proximity (40W

, I

vacuum seal ,,(capton or beryllium) ,beam line

tructure

u

I

synchrotron radiation

Fig. 5.8-1 Exposure station for X-ray lithography.

tion of an X-ray exposure station. To avoid the adsorption of X-rays in the air either the path between the X-ray window and the mask must be kept very short, 0s the mask- and sample-holder must be mounted in a chamber flooded with helium. This makes the construction expensive. A further problem of X-ray beam lithography concerns the mask and the related alignment issues as well as in the availability of the X-ray source.

5.8.1 Masks The X-ray masks consist of a very thin carrier film, which minimizes loss of the X-rays penetrating the film, and of an absorber pattern, which absorbs the X-rays entirely as far as possible. For safe handling of the masks, the thin membrane film is suspended over a stable frame. The membrane film is made from material with a low atomic number for example beryllium, silicon, silicon nitride, boron nitride, silicon carbide, and titanium. Plastic film has not yet proven to be a good membrane film because of the low shape stability and X-ray durability. At the moment diamond layers are in their development stage. Electroplated gold is usually used as absorber material, but also tungsten and tantalum are suitable. In order to keep distortions within the masks to a minimum during irradiation, the X-ray masks usually do not exceed a size of 50x50 mm2. Consequently a wafer cannot be exposed in a single step but the already discussed step and repeat mode is applied. This, as well as the lack of a sufficiently bright radiation source, limits the output to some ten wafers per hour with diameters of 200 mm. In general, the output is

204

5 Lithography

actually still lower, since the beam intensity can not be increased beyond certain lirnits due to the thermal heating of the mask and associated geometric distortion of the pattern.

5.8.2 X-Ray Sources The required wavelength region suitable for available masks and resists, ranges between 0.2 and 2 nm. The limits at the short end of the wavelength region is caused by the fact that the resist layer is increasingly transparent for shorter wavelength X-rays and therefore only little energy conversion occurs in the resist. The absorbing structures on the other hand have to be relatively thick which presents a technological problem. The long wavelength region is limited by the decreasing transparency of the carrier membrane. Further requirements of X-ray sources for lithography include high brightness, a small source size and a parallel beam, in order to apply proximity printing without a scaling error. Available X-ray sources are: 0 0 0

high efficiency X-ray tubes, plasma sources, and synchrotrons.

To generate a beam with X-ray tubes, an electron beam with high energy is accelerated onto a target. On deceleration of the electrons, so-called Bremsstrahlung is generated. The maximum X-ray energy corresponds to the energy of the incident electron. As long as this magnitude is greater than the characteristic absorption band of the target material, the radiation of this characteristic energy is emitted predominantly. The efficiency for the production of X-rays is very low (lop4lop5).A high electron current is necessary for a sufficiently large irradiation efficiency. Consequently, extensive cooling must be applied to the target. With plasma sources a high energy laser pulse is aimed at the target and an electrical discharge is generated, so that the target material vaporizes and forms an extremely hot plasma [Nage75]. The ions recombine by emission of X-rays. The efficiency is at least one order of magnitude better with plasma sources than with X-rays tubes. At present, the attainable performance and the efficiency of the laser are not yet sufficient for economical usage in X-ray photolithography. Besides a low efficiency, X-ray tubes and plasma sources also have the disadvantage of non-parallel beams and extended sources in common. For a parallel beam small apertures have to cut out of the major part of the source and thus the usable intensity of the source is very small.

5.8 X-Ray Lithography

205

5.8.3 Synchrotron Radiation Synchrotron radiation is produced from relativistic electrons, which are guided in a polygon-shaped vacuum tube by applying a centripetal acceleration [Kunz79]. This centripetal acceleration is caused by strong magnets arranged on the corners of a polygon. The electrons are emitted from a cathode and accelerated linearly outside the ring up to several MeV. With a special arrangement (kicker magnet) packages of electrons are forced into the ring. With RF sources these packages are accelerated mostly to some GeV and nearly the velocity of light. At this energy the electrons emit so-called synchrotron radiation tangential to their trajectory. Synchrotron radiation encompasses a continuous spectral region from infrared (with a photon energy of a few meV) to hard X-ray radiation with a photon energy of up to 100 keV. This light source is especially attractive for research as it shows the following properties: 0 0

Continuous spectral distribution, extremely directional and therefore highly parallel, high brightness, well defined time resolution in the picosecond region, polarized, very high long-term stability, exactly calculable.

It ideallly fulfils the required boundary conditions of lithography regarding brightness and parallelism. Synchrotron radiation was observed for the first time in an accelerator by General Electric Corp. in America in 1947 [Elde47]. In the early years it was considered as the unwanted side product in the process of high energy particle acceleration for nuclear physics, because it greatly limits the maximum attainable energy of the electron. In the mid 70’s worldwide many radiation sources were built based on electron- and positron-storage rings (e. g. BESSY in Berlin, Germany, Photon Factory in Tskuba, Japan, or the NSLS in Brookhaven, USA), which are exclusively used for the production of synchrotron radiation and, among others, for experiments of ultra high resolution lithography. The principle of the production of synchrotron radiation is schematically represented in Fig. 5.8-2. Electrons, which travel circular with nearly the speed of light (i. e. with a steady rotational frequency), emit electromagnetic energy tangential to their orbit. The radiation is strongly collimated by an aperture angle y , which is determined by the energy E of the electron:

m.c (5.11) E As the radiation occurs over the entire circumference of the synchrotron, the collimation is manifested only in the vertical direction. Whilst the radiation power in the vertical direction shows a good approximation of a Gaussian distribution, in the horizontal direction it is band-shaped. The illumination pattern q l -

206

5 Lithography

t

inwntial

point

cone

scan

Fig. 5.8-2 Principle of an electron synchrotron. Electrons are forced along a circular

“race track”. Due to the centripetal acceleration of the relativistic electrons synchrotron radiation is emitted tangential to the curvature. The light is guided to the exposure station. Due to the band-shaped irradiation the mask and substrate to be exposed have to be scanned up and down in order to expose a rectangular field.

generated from a synchrotron can be compared with the beam emitted from a light tower with rotating reflector. The width of the illuminated band surface is about lcm at a distance from the tangential point of about 10 m, which corresponds to an aperture angle of about 1 mrad. The total irradiated output P and the spectral distribution, which is specified by a characteristic wavelength ,Ic, depends on the energy of the electron and the radius of the trajectory. The characteristic wavelength ,I, is defined such that the integral radiated output above and below this wavelength is of the same magnitude. With increasing energy, the available output of the radiation increases to the power of 4 and the spectrum shifts to shorter wavelengths with the energy to the power of 3.

P=88,5.-

.I R

~4

R /E,=5,99.E3

(5.12)

(5.13)

5.8 X-Ray Lithography

207

l10o -2 E n

U

2 E

E

lo1 loo

U

3

10-1

L

i 10-l

10 (I

10

wavelength Inm] Fig. 5.8-3 Spectral output of synchrotron radiation versus electron energy.

with A, = characteristics wavelength in nm, P = total radiated energy in kW, E = energy of the electron in GeV, I = electric current in A, R = curvature radius of the electron trajectory in m. The curvature radius of the electronic trajectory in the magnetic field of the bending magnets is given by:

E R = 3,335. B

(5.14)

The dependency of the spectral output for 5 different electronic energies on the wavelength is illustrated in Figure 5.8-3; the electric current (I = 0.1 A) and the curvature radius of the electronic trajectory ( R = 10m) were kept constant. In the figure the characteristic wavelength A, is shown, which differs from the wavelength with maximum spectral output of the radiation by a factor of 0.65. Figure 5.8-4 shows a typical synchrotron which is used for LIGA. This machine, which is located in Baton Rouge, Louisiana, USA, was not yet finished when the photograph was taken. The circular vacuum tube, containing the accelerated electrons is still missing. One can distinguish between the bending magnets and in between the focussing magnets. The guidance of the X-ray radiation, from the tangential point (Fig. 5.8-2) in the storage ring to the place of the experiment, is carried out in an evacuated steel tube, the so-called beam lead. As the irradiation experiment usually is carried out under a standard atmosphere or under helium gas, the vacuum region of the beam lead (and the storage ring) must be separated from atmospheric pressure

208

5 Lithography

Fig. 5.8-4 A synchrotron in the construction state (Courtesy Prof. V. Saile, Research

Center Karlsruhe). by an X-ray transparent window. This window is generally made of beryllium which is both light in weight and transparent to X-ray radiation. But even so the radiation spectrum is attenuated, especially in the region of long wavelengths (beyond 1 nm). Further attenuation of the soft radiation components is caused by the carrier membrane material of the X-ray mask (Si, Be, or Ti).

5.8.4 Application of X-ray Lithography Applications of X-ray lithography in the semiconductor manufacture has been extensively discussed for many years. At the moment the main technical problem is seen in the mask technology where very stringent requirements exist with regard to (thermal and radiation) stability of the mask carrier membrane film and on the precision of alignment. The introduction of X-ray lithography as a technology has been delayed because over recent years significant improvements were achieved in the area of optical lithography, whereby the limits of the minimum manufacturable structures were constantly pushed to smaller values. It is questionable today whether X-ray lithography will ever be utilized on a large scale in the manufacture of semiconductors. In contrast to microelectronics, the use of synchrotron radiation in X-ray lithography due to its unique spectral properties has gained importance in producing microoptical and micromechanical structures. With this in mind the LIGA process is fully discussed in Chapter 7.

6 Silicon Microsystem Technology

The preceding chapters have laid out the foundation for the theoretical and technological tools and methods useful for the fabrication of microtransducers and microsystems. A considerable number of devices and systems are based on silicon technology enhanced by a range of micromachining techniques [Pete82, Midd94, Sze94, Trim97, Proc981. Originally micromachining was synonymous with the selective wet chemical etching of silicon or thin films. Already in its infancy, micromachining enabled the fabrication of numerous microcomponents including cantilevers, beams, and thin membranes. Many of these structures are still being implemented today in, e. g., pressure sensors and accelerometers. The initial micromachining developments were motivated by the success story of silicon based microelectronics, by adding non-electronic functionality to IC-like substrates. To this day, silicon related microsystems have expanded at a breathtaking pace in number and design, due to the rapid diversification of micromachining and integration techniques. Wet etching has been complemented by a host of dry etching methods. The combination of anisotropic and isotropic etching methods enables the fabrication of a fascinating variety of three-dimensional microstructures. Micromachining methods fully compatible with standard integrated circuit (IC) processing have been developed. Materials other than silicon, in particular thin films deposited on top of silicon substrates, have become the focus of microfabrication. Moreover, the addition of materials by chemical or physical deposition, spin-on processes, electrodeposition, and hybrid integration on top of IC-like substrates has broadened the range of structures and addressable applications. After a brief review in Section 6.1 of IC process integration, Sections 6.2 and 6.3 provide overviews of bulk and surface micromachining techniques, respectively. Section 6.4 illustrates these methods with selected classical and more recent examples of microtransducers and microsystems. Among the more spectacular devices and systems fabricated using silicon micromachining are neural microprobes, integrated accelerometers, digital micromirror arrays, microgears, microoptoelectromechanical systems, and thermal and mechanical microsystems based on commercial CMOS application specific integrated circuit (ASIC) technologies.

2 10

6 Silicon Microsystem Technology

6.1 Silicon Technology Silicon micromachining starts with the preparation of a suitable substrate. The simplest substrates are silicon wafers covered by a photolithographically structured dielectric layer. This stack allows the fabrication of only the most basic structures. Electrical and thermal functions are not available to complement the mechanical transduction in such devices. At the other end of the complexity scale, the substrates may be chips or wafers fabricated using integrated circuit technology. In this second case, they consist of silicon with various diffusions and several dielectric thin films with conductive levels integrated in-between. With these materials, digital, analog, and mixed signal circuitry can be integrated on-chip. Micromachining then represents a major challenge, since the circuitry has to be preserved, while selected constituent materials are locally removed. To give a better feeling for the various integrated circuit (IC) materials, processes, and challenges in the specific context of micromachining, this section summarizes the main steps used to build IC structures. Individual process step such as CVD, metallization, and dry etching were discussed in Chapters 4 and 5 . The following section focuses mainly on the integration of these steps into process sequences. The main representatives of IC technologies are complementary metaloxidesemiconductor (CMOS), bipolar, and BiCMOS (bipolar CMOS) processes. Charge-coupled device (CCD) and memory technologies are economically important, but have not been extensively adapted to microstructure fabrication yet. Special processes build on unconventional substrates such as SIMOX (silicon separated by implantation of oxygen) and SO1 (silicon-on-insulator) wafers. Micromachining has taken advantage of the unique properties of these advanced materials. The reader acquainted with IC technology is invited to jump straight to Section 6. on micromachining, since the description of the basic integration techniques is being restricted to a minimum here.

6.1.1 IC Processes and Substrates Micromachining techniques are often qualified as CMOS-compatible as soon as they preserve a few thin films deposited onto some silicon substrate. However, to give an impression of how much more may be involved in CMOS-compatible sensor fabrication, this chapter summarizes the basics of standard silicon processing, in particular CMOS technology. CMOS technologies vary considerably when considered in detail. They are the result of the impressive skills of process engineers in fine-tuning delicate individual process steps into robust routines and integrating them into stable process sequences. Nevertheless, since the operating principle of the basic electronic building block, the field effect transistor (FET), is their common denominator, some rules and procedures common to many CMOS technologies can be noted.

6.1 Silicon Technology Source

Gate

Silicon substrate

211

Drain

Gate oxide

Fig. 6.1-1 Schematic cross-section of a FET with a single interconnect level

As schematically shown in Fig. 6.1-1 the heart of every FET, one finds a conducting gate separated from the semiconducting substrate by an extremely thin dielectric, the gate oxide. This is the metal-oxide-semiconductor sandwich which gives the technology the three last letters of its name. Upon application of an appropriate voltage to the gate, the effective polarity of the semiconductor is locally inverted and a conducting channel is formed. To either side of the gate, highly doped regions in the substrate, of polarity opposite to that of the substrate, define source and drain of the FET. They are contacted to metal lines providing the connection to other FETs or to the outside world. Upon formation of the channel, the source and drain are electrically connected, i. e., short-circuited. FETs in p-doped silicon with n-polarity of source, channel, and drain (NMOS devices) and other, complementary devices with p-channel, source, and drain in an n-substrate (PMOS devices) are cointegrated in CMOS technology. The art of circuit design then consists of appropriately dimensioning these basic elements and of their sophisticated interconnection to realize anything from simple switches to stateof-the-art systems on a chip with currently more than lo7 transistors. Technologically, these structures are constructed by the local doping of the silicon substrate with electrically active impurities and the successive growth or deposition and patterning of conductive and insulating thin films. Extensive accounts of these techniques are found in the literature [Wolf87, Sze88, Runy90, Chan961. A considerably simplified CMOS technology might be based on the process sequence summarized in Fig. 6.1-2, starting with a (100) silicon substrate. In most cases this is heavily p-doped material with a lightly doped epitaxial layer with controlled impurity concentrations, or a lightly p-doped substrate with oxygen concentration around 5 X 1017~ 1 1 1In ~~ the . latter case, a thermal gettering treatment is performed to cause the homogenous precipitation of the oxygen over the entire wafer except close to the surfaces. The resulting denuded zones are depleted of oxygen and other undesired impurities. Common wafer diameters are 3 inch, 100 mm (“4 inch”), 125 mm (“5 inch”), and 150 mm (“6 inch”) with respective thicknesses of up to 675 pm. Major IC companies however fabricate their products on 200 mm wafers and are currently moving on to 300 mm and beyond. Active areas designated to contain the field effect transistors are defined next. This is done by a process referred to as local oxidation of silicon (LOCOS). First,

6 Silicon Microsystem Technology

2 12

Fig. 6.1-2 Simplified CMOS process. Silicon substrate before process (a), after LOCOS nitride patterning (b), end of LOCOS process (c), n-well diffusion (d), gate oxide growth and gate definition (e), source and drain implants (f), contact oxide and first metal deposition and structuring (g), intermetal and second metal deposition and patterning (h), and passivation deposition and pad opening (i).

a thin oxide (ca. 30 nm), the so-called pad oxide, is thermally grown, and an LPCVD silicon nitride (ca. 100 nm) is deposited (cf. Section 4). The nitride is patterned using photolithography and dry etching. Areas where it is left untouched are the future active regions. The wafer next undergoes a wet oxidation, which forms a dense and electrically reliable silicon oxide, i. e., the field oxide, with a thickness of roughly 0.5 pm to 1 pm in the areas free from LOCOS nitride. The silicon nitride and pad oxide are finally removed. This leaves the silicon wafer protected by thermal oxide with well defined openings. Next, n-wells for p-channel FETs are formed using photolithography, implantation, and drive-in. The surface of the wafer is wet etched and cleaned before the thin gate oxide is thermally grown. A roughly 0.3 pm thin polysilicon layer is then deposited by CVD, using the pyrolysis of silane (SiH,). It is heavily n-doped (No 1020cm-3)and thus is turned into a reasonable conductor (“metal”). Finally it is patterned by photolithography and dry etching. The resulting structures define the logic gates.

-

6.1 Silicon Technology

213

Sources and drains of one polarity are then implanted. Those of the complementary devices follow next. During this doping process, the respective gates and field oxide surrounding the active regions serve as doping masks and thus define the corresponding localized source and drain regions. The fabrication of the so-called front-end is then completed. What follows is the back-end process concerned with the construction of the device interconnects. A first silicon oxide dielectric is deposited by CVD and opened locally to the source and drain regions and the gates. Next follows the deposition and patterning of a first metalization layer contacting sources, drains, and gates. A second CVD dielectric is then deposited and opened locally onto the first metal layer. A second structured metal layer finally allows the realization of more complex interconnection topologies. The process is completed by the deposition of a silicon (oxy)nitride passivation thin film. Openings over extended metal structures form electrical contact pads and enable the interconnection of the chip to the external world. For more detailed information and the description of other important techniques such as channel stop implants, threshold adjustment, sidewall formation, selfaligned processes, silicidation, and such trendy topics as via plugs, multilevel and copper metalizations, low-k dielectrics, etc., the reader is referred to the literature on IC-technology [Chan96]. Bipolar ICs use somewhat simpler technology. The main electronic activity occurs in a hierarchy of oppositely doped regions, i. e., the emitter, base, and collector regions. The art of bipolar technologies consists of carrying out the required doping with sufficient accuracy in depth and concentration. Above the silicon surface one usually finds a stack of dielectric layers with metal interconnects, topped by a passivation with pad openings. The fabrication of silicon based microstructures may follow two approaches. In the first, use is made of these materials as dictated by the technology, with the respective geometrical limitations and physical properties. In the second, these basic ingredients are adapted to more specific needs by varying the materials and the processes. The ability to cointegrate circuitry next to the micromachined device is then often sacrificed. Finally, SIMOX (separation by implantation of oxygen) wafers have found specific applications in micromachining. A schematic cross-section of such a wafer is shown in Fig. 6.1-3. The main body of silicon is separated from thin silicon surface layer of a few micrometer by silicon oxide. This is produced by heavy im-

Silicon substrate

Fig. 6.1-3 Schematic cross-section of a SIMOX wafer.

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6 Silicon Microsystem Technology

plantation with oxygen. An anneal causes these impurities to concentrate and react with the silicon atoms, thus forming a dense buried silicon oxide layer. The silicon surface region is then cleared of oxygen and keeps its crystalline order and is available for the fabrication of circuit components isolated from the main substrate. The main silicon body retains only its thermal and mechanical functions. Advantage has been taken of the buried oxide as an etch stop layer allowing the fabrication of thin monocrystalline silicon structures EDiem93, Mii1195al. A disadvantage of these substrates is the prohibitively long times required to implant roughly 10" oxygen atoms per cm2 into the wafer. An alternative approach to produce such substrates proceeds by thermal bonding of a polished wafer onto a second wafer covered by silicon oxide. A solid chemical bond forms and reliably connects the two parts (Section 9.5). The top wafer is then lapped and polished down until only a thin silicon layer remains on the oxide, providing a silicon-on-insulator (SOI) wafer. Like the SIMOX process, the fabrication of SO1 substrates is time-consuming.

6.1.2 Foundry Technologies An attractive approach to microstructure fabrication uses commercial IC foundries for the preparation of the silicodthin-film substrates. This approach has several advantages : The silicon process is performed by trained operators and kept up-to-date by dedicated technologists. More time is left to solve microtransducerspecific problems. Process conditions are stable. Indeed, a considerable effort is invested by IC fabs to guarantee the stability of the relevant geometrical or electronic process parameters. The processes are optimized for mass production. Large scale production of microtransducers is possible. Circuitry can be integrated without neck-braking technological feats. Equipment maintenance and upgrade are taken care of by the foundry. CMOS technologies are available to the microsystems community through several organizations and offered directly by silicon foundries dedicated to application specific integrated circuit (ASIC) fabrication. The latter have traditionally dealt with a broad spectrum of clients and after clarifying discussions may even accept minor adaptations of their technologies for micromachining needs. Organizations representing .microsystems-oriented IC technologies include MOSIS in the USA and Europractice and TIMA-CMP in Europe. Cronos Integrated Microsystems North Carolina offers bulk and surface micromachining and LIGA-like options (multi user MEMS processes, MUMPS) in addition to the basic CMOS process. Multilevel polysilicon micromachining is commercialized by Sandia National La-

6.2 Silicon Micromachining

2 15

boratories, Albuquerque. In Europe, ASIC foundries active in microsystem developments include Austria Mikro Systeme (Unterpremstatten, Austria) and EM Microelectronic-Marin SA (Marin, Switzerland). Also more special silicon based sensor processes have been made publicly available by, e. g., Robert Bosch AG (Germany) and SensoNor asa (Norway) through the Normic fabrication cluster. However, when opting for the IC approach to microsystem technogy, one should also be aware of a few limitations: 0

Only a few materials are available: silicon, dielectric thin films, polysilicon, interconnect metals. These materials offer a limited number of physical effects to be exploited in microtransducers. Strict design rules allow the construction of relatively few basic constellations of materials and thus of only a few basic microstructures.

Whether these restrictions outweigh the advantages has to be evaluated in each particular case. Fascinating CMOS devices and systems have been demonstrated, as several examples discussed in Section 6.4 will how. However, before functional devices are described, the second technological ingredient required for their fabrication, namely micromachining, has to be discussed. This is done in the following two sections.

6.2 Silicon Micromachining 6.2.1 Introduction A first approach to enhance the functionality of IC-based substrates and material combinations proceeds by micromachining the silicon substrate. Silicon can be wet or dry etched by various techniques. Some wet etchants, such as nitric acid/ hydrofluoric acid based mixtures have isotropic etching properties. In contrast, alkaline solutions etch anisotropically, i. e., preferentially remove certain crystal planes, while preserving others. Various dry etching methods in the gas phase or based on plasma processes are available. A rapidly growing collection of shapes can thus be realized in materials with functionality considerably enhanced in comparison with their original electrical purpose. The entire field of silicon micromachining is described in depth in [Elwe98, Mado971. Wet and dry etching processes have in common a three-step sequence consisting of 0 0 0

the transport of one or several reactants to the silicon surface, the chemical reaction of the reactant on and with the surface, and finally the transport of the reaction products away from the surface.

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6 Silicon Microsystem Technology

In the case of dry etching, reactive radicals, ionized molecules and energetic particles diffuse and drift through the gas or plasma to the surface, driven by density gradients, electrostatic fields, and electrodynamic forces. Whether diffusion or drift dominates depends strongly on process conditions such as gas pressure, composition, temperature, frequency and power of the plasma excitation, and bias voltages. In the optimal case, the volatile products desorb from the surface and are finally eliminated from the dry etching chamber by diffusion and vacuum pumps. In the wet case, the reactants are provided in solution. Concentration gradients drive the dominantly diffusive transport of the agent to the silicon surface. In the usual configuration, shown in Fig. 6.2-1, the size of the concentration gradient dc/dx=(c,-c,)/d is essentially limited by two factors. The first is the difference in the concentrations cs and co of the reactant, respectively, at the surface and in the homogeneous volume of the etching solution, where the reactant is at its nominal concentration c,. The concentration cs is determined by the efficiency at which the reactant is depleted at the surface, i.e., by the etch rate. The second factor is the thickness d of the concentration boundary layer that separates the surface from the homogeneous solution. Agitation of the solution reduces d and thus increases the etching efficiency. Dissipation of ultrasound enhances the transport steps, but damages fragile structures. When choosing an etching technique in order to reach a particular goal, one may base one’s choice on criteria such as etch rate and anisotropy, selectivity, microstructure shape, process compatibility, ease of use, safety, and cost, among others, as described in the following.

-&C

Fig. 6.2-1 Reactant concentration c in wet silicon etchant close to etched surface. The reactant is depleted below the nominal concentration co in a boundary layer of thickness d.

6.2 Silicon Micromachining

2 17

Etch Rates and Anisotropy The anisotropy of an etching process results from the fact that some crystal planes are etched more rapidly than others. Anisotropy may be expected, since etching as a thermodynamic phenomenon is related to crystal growth, which is well-known to yield faceted single crystals. Examples of anisotropic etchants include a list of alkali hydroxide solutions and solutions of amine-based organic solutions with properties described below. A common property of these anisotropic etchants is that they etch (111) crystal planes much more slowly than (100) and (110) planes. In contrast, isotropic etchants remove material in all directions with identical rate. Etch cavities with cross-sections shown in Fig. 6.2-2 (a) and (b) are thus obtained using isotropic and anisotropic etchants, respectively. The stability of (111) planes can be exploited to accurately define the final shape of a microstructure. The geometry of a patterned mask layer on a silicon surface is thus transferred into the silicon substrate. The accuracy of this transfer has its limit in a small but non-negligible etch rate of (111) planes, which leads to mask underetching as shown in Fig. 6.2-2 (c). If required, this can to some extent be compensated by appropriate mask design. In dry etching processes, anisotropy is achieved by the acceleration of the reactive species by suitable substrate bias, and by surface passivation, as explained in Section 6.2.6. Wet and dry etchant formulations without direction-sensitivity are also available. A commonly used wet system is based on solutions of nitric and hydrofluoric acid (Section 6.2.2). Whether an etchant is isotropic or anisotropic depends on the predominance of transport processes (isotropic diffusion) or surface reaction rates (orientation dependent). Diffusion limited etching tends to be isotropic; with etch rate limited processes, the converse is true and anisotropy may result. Silicon etch rates vary from almost zero for the (111) crystal planes in anisotropic etchants to several hundred micrometer per minute in isotropic etching solutions. In anisotropic wet etchants, etch rates of the fast etching planes can be several tens of micrometers per hour at temperatures between 50 "C and 100 "C.

Fig. 6.2-2 Cross-sections of trenches etched by isotropic (a) and

anisotropic (b, c) wet silicon etchants. Result of underetching is shown in (c).

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6 Silicon Microsystem Technology

Selectivity Like in IC technology, the selectivity of an etchant designates the ratio of the etch rate of different materials. In the technologist’s jargon, an etchant for a material A is selective against another material B when it etches A more rapidly than B. Quantitatively, the selectivity SA.B is the ratio R A : R B of the respective etch rates R A and RB. While the goal of silicon micromachining is to remove silicon, frequently other materials have to be preserved in the process. These may be a mask layer, such as a structured silicon nitride, silicon oxide, metal or polymer mask defining the initial etch shape. For the reliable etching of a distance detchinto the silicon (etch depth or underetching distance), the thickness dmaskof the mask layer should be sufficiently large. Explicitly, one must have dmask> detch. Frequently, more complicated material systems than a silicon/mask combination are to be structured. An extreme example is the microstructuring of wafers produced using a complete CMOS process [Leng94, Miinc971. The exposed metal of contact pads on such wafers is removed by alkaline solutions within minutes. The lack of an etchant formulation with sufficiently high selectivity against aluminum based alloys is one of the reasons why silicon micromachining of CMOS wafers is not yet a wide-spread commercially used batchwise process.

Process Compatibility Contamination, temperature budget, and the deposited energy density are additional issues to be considered. Contamination primarily concerns the possible diffusion of alkaline ions from anisotropic etchants into the dielectric layers of electrically active components. In the worst case, alkaline contaminants diffuse to the gate oxide and lead to uncontrollable shifts of the threshold voltages. When circuitry is integrated on-chip with microtransducers, wet alkaline micromachining may be safely carried out as a preprocess or postprocess, both followed by thorough cleaning. The reader be reminded that current chemical-mechanical polishing (CMP) processes use stabilizers such as KOH and oxidizers such as KIO, in their solutions. Such processes are applied at all levels of the back-end of stateof-the-art IC processes. Contamination seems to be well under control. Thermal budget could be a second concern. Wet etching temperatures rarely exceed 120°C. This is far below the temperature limit of 450°C not to be exceeded after the first metalization has been deposited. It is even below the upper temperature for stable operation of integrated circuits (ca. 150 “C). Doping profiles thus remain stable and reliable operation of circuitry is expected. In contrast with wet etching, dry etching methods deposit considerable power densities on the wafer surface. Substrate temperatures are usually at or slightly above room temperature. Nevertheless, in view of the possible thermal isolation of microstructures, they may heat up. In addition to the preprocessing and post processing methods mentioned above, other process variants best termed “in-between processes” have been developed. As an example, the processes of bipolar and CMOS fab DIMES associated with

6.2 Silicon Micromachining

2 19

the Delft University of Technology have a modular structure [Sarr92]. Some of these modules are dedicated to microsensor fabrication. They are fully compatible with the preceding and following process sequences and enable surface and bulk micromachined devices to be integrated with IC structures. Ease of Use and Safety

Ease of use is an issue closely related with safety. Besides physical barriers, probably the best protection against perilous incidents are clear thinking and slow, controlled movements. Thorough training of operators is a top priority. Alkali hydroxide solutions have a low hazard potential if reasonable safety rules are observed. The contact with the etchants and with their fumes and etch products should be avoided by using impermeable gloves and providing appropriate ventilation. The same comments apply to the use of HF solutions. Safety measures such as those typically used in IC wet processing (chemically resistant gloves, apron, face protection, ventilation) are imperative. In the case of solutions based on ethylene-diamine, pyrazine, and pyrocatechol, additional security barriers should be provided. A closed vented cabinet is mandatory. These organic compounds are highly toxic. Their combination has repeatedly been mentioned as potentially carcinogenic. Admittedly, manipulating tweezers and chips using clumsy gloves and observating a microsensor chip across the window pane of a chemical cabinet are challenging. However, momentary loss of a sample is preferable over potentially long-time health problems. Dry etching is inherently easier to use, especially if the manipulation is performed at the batch level by a cassette handling system. As in IC foundries, adequate protection of operators against corrosive fluorine or chlorine components must be ensured by proper installation, vented gas cabinets, gas sensors, and alarm signals. cost

Wet etching equipment is usually less expensive than state-of-the-art dry etching tools. The cost of the second category of equipment easily exceeds a quarter of a million dollars stand-alone, or considerably more if combined in a cluster tool with other dry processes. In addition to the initial investment, overall cost is however determined as well by running costs, average down-time, chemical waste treatment and disposal, width of process windows, achievable specifications, and yield. In practice, criteria such as availability of equipment or previous experience with a method are often equally important in determining the decision whether or not to use a micromachining technique.

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6 Silicon Microsystem Technology

6.2.2 Wet Etching Wet etching of silicon has been successfully used over the past twenty years to produce an amazing variety of microstructures: membranes, bridges and cantilevers made of silicon or dielectric materials supported on silicon, etch grooves for optical or fluidic application, spirals, sieves, etc. A variety of etching formulations exist with widely differing properties. We now describe some of the more widely used recipes in more detail. A setup for wet silicon etching is schematically shown in Fig. 6.2-3.

Reflux condenser

Etch solution __

Heating fluid

at controlled

temperature

Fig. 6.2-3 Schematic cross-section of an experimental setup for wet silicon etching.

HNA Etchants Silicon is isotropically etched by mixtures of HF, HNO,, and CH3COOH, i.e., hydrofluoric acid, nitric acid, and acetic acid, hence the designation NHA, and water. In a second class of isotropic silicon etchants, the acetic acid is entirely replaced by water. The hydrofluoric acid is usually provided as HF(49.2%), i.e., with a weight-percentage of 49.2% HF in water. Similarly, HNO, is frequently provided as the standard aqueous solution HN03(69.51 %). Etch rates and properties of the two families of mixtures have been reported in IRobb59, Robb60, Schw61, Schw761 and are shown in Fig. 6.2-4. Etch rates up to 940 y d m i n for solutions with 20 to 46 % HN0,(69.5 1 %) complemented by HF(49.2 %) have been achieved at room temperature. The quality of the etched silicon surfaces depends on the etchant composition. Smoother surfaces are achieved in solutions with higher nitric acid and lower acetic acid concentration.

6.2 Silicon Micromachining

100

50 CH3COOH Wt.-%

0

100

50 CH3COOH W.-%

22 1

0

Fig. 6.2-4 Silicon etch rate of hydrofluoriclnitriclacetic acid mixtures in p d m i n (a)

and resulting surface quality (b). In (b), dark areas correspond to smooth surfaces and round edges, while white areas are characterized by rough surfaces and sharp edges [Robb60, Schw761. The etching by HNA involves two steps. First, the silicon surface is oxidized by the HNO,, a powerful oxidant. Simultaneously, the resulting oxide is removed by the diluted hydrofluoric acid. Generally, etchants without acetic acid show lower etch rates. The discrepancy increases with the acetic acid content of the solution. It has been suggested that this is due to the lower polarity of the acetic acid molecule in comparison with the highly polar H,O molecule. The following overall reaction [Will961 has been suggested: 18HF

+ 4HN0, + 3 Si -+

3H,SiF,

+ 4NO(g) + 8H,O.

(6.1)

The reason why HNA solutions etch isotropically is that the etching process is diffusion-limited. A disadvantage of HNA etchants is that their selectivities against silicon dioxide are low. Etch rates of S i 0 2 between 30 and 70 n d m i n have been reported [Pete82a]. Silicon nitride and noble metals are mask materials with better resistance. Cured negative photoresist has also been used as a simple yet not too effective mask.

Alkali Hydroxide Etchants Aqueous solutions of the alkali hydroxides KOH, NaOH, LiOH, CsOH, and RbOH are anisotropic silicon etchants. Among them, KOH is most widely used. All etch the (100) planes faster than the (110) planes and much faster than the (111) planes. Early explanations of the anisotropy were based on the different numbers of bonds with which the surface silicon atoms are bonded to the bulk crystal. In these simple models atoms on (100) surfaces presented two dangling bonds to the solution. In contrast, (111) atoms leave only one bond dangling, as schematically shown in Fig. 6.2-5. Thus the etching solution has to disrupt two

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6 Silicon Microsystem Technology (1 11) Surface

(1 00) Surface

Fig. 6.2-5 Coordination of the outermost atom layer on the (111) and (100)-oriented

silicon surfaces.

and three crystal bonds, respectively, in the two cases, hence the slower etch rate of (1 11) surfaces. However, this simple picture is unable to explain the difference in etch rate between (111) and (110) planes, to predict etch rates close to the (111) planes [Elwe971 or to account for the spontaneous appearance of hillocks. It also conflicts with results obtained using scanning probe microscopies or with the tendency of (11 1) planes to reconstruct into more complex topographies where individual atoms no longer show the expected number of dangling bonds. A recent theory based on concepts of physical chemistry, the surface tension of various crystal faces and the free energy of atomic steps on etched surfaces appears to be successful in accounting for some of the more detailed observations [Elwe97, Elwe981. More refined versions of microscopic anisotropic silicon etching theories are still under discussion. It is apparent that alkaline silicon etching is reaction rate limited. Stirring the solution helps more by cleaning the etched surface from obstructing gas bubbles than by narrowing the diffusion boundary layer. Further, activation energies of 12-16 kcal/mole of the etching process are typical of chemical reactions rather than of diffusion-controlled processes. The reaction of KOH solutions with the silicon surface was studied in detail in [Seid87, Seid901. These authors concluded that the presence of hydroxyl ions, OH-, is responsible for the etching. They proposed the overall scheme

Si

+ 20H- + 2H20 + Si02(OH);- + 2H2.

Si

+ 20H- + Si(OH),’- + 4eC.

(6.2) The reaction starts by the oxidation of the silicon by hydroxyl ions, according to

(6.3)

Four electrons per oxidized Si-atom are injected into the conduction band of the silicon crystal. Attracted by the negatively charged complexed silicon, the electrons remain near the Si surface. In combination with the Si(0H);- ions, they build up an electrolytical dipole layer. Finally, the electrons react with water molecules according to 4e-

+ 4 H 2 0 -+ 40H- + 2H,

(6.4)

6.2 Silicon Micromachining

223

This prevents the silicon crystal from being negatively charged up further and restitutes the hydroxyl ions to the solution. Both effects contribute to keep the reaction (6.2) running. The silicon complex finally reacts with four further OH- ions to produce the soluble complex Si02(OH)22-and two water molecules. Concentration of useful KOH solutions typically range from 20 wt% to 50 wt%. Common are concentrations around 30 wt% or 6 molar (M). Concentrations below 20 wt% tend to produce rough surfaces. Etching temperatures between 50 and 95 "C have been reported in the literature. Generally the quality of etched structures increases with temperature. However this is achieved at the cost of a reduced anisotropy ratio R~loo):R~lll). At 72"C, a solution with 15 wt% KOH has a (100) etch rate of roughly 55 pm/h [Seid87]. At 95OC, a solution with 6 M KOH provides an etch rate of 150 pm/h on commercial CMOS substrates [Jaeg96]. The addition of isopropyl alcohol (IPA) to KOH solutions enhances the anisol l l ~ . up to 400:l have been reported, enabling the fabritropy ratio R ~ l o o ~ : R ~Ratios cation of structures essentially defined by the resolution of the mask [Pete82a]. The most reliable etch mask against KOH available in standard IC technologies is LPCVD silicon nitride, as used, e.g., in the LOCOS process. Its etch rate in KOH at all relevant concentrations and temperatures is negligible. Its adhesion to the silicon substrate is excellent and its pinhole density is low. Both properties make it an excellent choice as a mask layer. PECVD silicon nitride also has low etch rate usually below 1 nm/min. However, depending on process conditions, larger defect densities result from the higher pinhole densities of the PECVD layers. Also, underetch rates tend to be higher due to the lower adhesion of the layers to silicon. Similarly to high temperature (LPCVD) silicon nitride, thermal silicon oxide provides practically defect-free and uniform etch protection. The selectivity of KOH against thermal Si02 is 300: 1 for 30 wt% KOH at 60 "C. It decreases with increasing temperature and KOH concentration. PECVD silicon oxide has much higher KOH etch rates and can be used, if necessary, for short etching experiments. Exposed aluminum such as that of contact pads is attacked violently with etch rates larger than 1 pm/min. at 95 "C. Even aluminum structures protected by 2-pmthick PECVD silicon nitride and oxynitride passivation sandwiches usually do not survive extended KOH etching without being locally destroyed through pinholes. Negative photoresist can be cured to become resistant against KOH and in principle can be used as a protection layer for the delicate active wafer side. Unfortunately, in solution the photoresist is lifted off starting from the edges and finally floats off, leaving the wafer unprotected. A breakthrough in KOH etching consists of micromachining standard CMOS wafers using a 6 M aqueous KOH solution. "Standard CMOS" means here that prior to micromachining the wafers underwent a complete CMOS process, from the initial gettering treatment to the deposition of the final passivation. The process has been transferred to the ASIC CMOS foundry EM MicroelectronicMarin SA [Munc97]. Micromachining is carried out as a post-process. Its goal is the fabrication of membranes composed of the entire sandwich of dielectric CMOS layers with integrated polysilicon and metal structures. These membranes are used for thermal microsensors such as gas flow sensors [Maye97], infrared detectors [Paul98], and chemical sensors [Kol199]. The development of the process

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6 Silicon Microsystem Technology

involved several challenges. First, on-chip circuitry cointegrated with the microsensors have to be preserved from KOH. Second, the rear surface of the CMOS wafers routinely shows a root mean square roughness of the order of 1 pm, reducing mask adhesion. Finally, the bulk silicon material contains oxygen precipitates which increase the (111) etch rate. These challenges were solved by the process sequence shown in Fig. 6.2-6. Micromachining starts with the deposition of a front silicon nitride passivation with tensile prestress. In the final membranes this stress compensates the average compressive stress of the CMOS dielectric sandwich and improves the membrane yield. In order to protect the active wafer side including the sensitive aluminum pads, a PECVD silicon oxide based on tetraethoxysilane (TEOS) is then deposited on the front, conformally covering contact pads and passivation. About 50 pm of silicon are then removed from the rear surface of the wafers by commercial chemical polishing. A PECVD silicon nitride layer is next deposited onto the wafer back and structured. The wafers are then mounted in an etch box made of stainless steel, with O-rings and teflon spacers. After roughly four hours, the 600-pm-thick 6-inch wafers are completely etched through and the KOH etching has stopped at the field oxide. Finally, the wafer is rinsed and diced. Overall membrane yield after dicing higher than 98 % was reported [Leng94]. (a)

CMOS thin films CMOS silicon

(b)

Sensor passivation Rough surface

(4

Protection layer Polished surface Mask Etch box

(9) Membrane

Fig. 6.2-6 CMOS-compatible KOH etching of membranes consisting of the CMOS dielectric layers.

6.2 Silicon Micromachining

225

Ammonium Hydroxide Etchants The above reaction scheme shows that a base is needed to etch silicon. The alkali ions in solution do not contribute directly to the etching. They participate indirectly through maintaining a pH value favorable to the formation of a soluble silicon complex. The search for alternative silicon etchants was thus naturally guided to the investigation of nonalkaline bases. Examples are solutions of the inorganic compounds ammonium hydroxide (NH,OH), tetraethylammonium hydroxide, and tetramethylammonium hydroxide ((CH,),NOH), abbreviated TEAH and TMAH, respectively. Ammonium hydroxide at concentrations between 1 and 18 wt% at 75 "C etches silicon anisotropically with etch rates up to 30 p d h . The resulting silicon surfaces, however, are rough and the etch rate is thus unpredictable over longer etch times. Ammonium hydroxide is a gas under ambient conditions. It evaporates from the solution, leading to unstable etchants [Schn90]. Besides KOH, TMAH is the most popular silicon anisotropic etchant. Since it is used in aqueous solution it is sometimes referred to as TMAHW or TMAW (= TMAH with Water). The main advantage of TMAH is that it can be made selective against aluminum by appropriate additives. Solutions compatible with the standard CMOS metallization thus appear feasible. [Taba95] deals with this issue. The TMAH etch rate is maximum at 2 wt% TMAH and decreases with concentration (1.5 p d m i n at 10 wt% and 0.5 pm/min at 40 wt%, both at 90 "C), i. e., with increasing pH value. Simultaneously, the quality of the etched surfaces is improved. Above 20 wt%, smooth etch walls and bottoms are obtained. Below 15 %, the formation of hillocks leads to irreproducible results. Anisotropy ratio R~loo):R~lll) decreases from 35 to 10 between 5 and 40 wt%. Ristic reports (100) etch rates of approximately 4 p d h , 33 p m h , and 80 pm/h for 20 wt% solutions at 50 "C, 80 "C, and 95 "C, respectively [Rist94]. Surface roughness is due to hillock formation which was suggested to be due to persistent H2 bubbles locally screening the silicon surface from the etchant. Oxidizers including ammonium peroxydisulfate ((NHJ2S208) have been added to TMAH solutions to bind hydrogen ions, with various degrees of success [Klaa96a]. Unfortunately the aluminum etch rate increases with increasing TMAH concentration, jeopardizing the CMOS-compatibility of TMAH. The reason is that the natural passivation layer Al(OH,) of aluminum is dissolved by strong bases or acids. By lowering the pH value of the TMAH solution, the selectivity to aluminum can thus be improved. Tabata et al. showed that by lowering the pH value by 1 unit by adding the acids (NH4)2C02and (NH,),HP04, the aluminum etch rate was reduced by a factor of roughly 10, [Taba95]. An alternative scheme consists of adding silicon to the TMAH solutions ("silicon doping" of the solution). Orthosilicic acid, i. e. Si(OH4) (c-? Si02(OH),2- 2H+), is formed and the pH value of the solution is lowered. As a welcome side-effect, the silicon complexes react with the aluminum to form a less soluble aluminum silicate. Adding silicon hy dissolving wafer fragments or silicon powder is time-consuming. An alternative approach consists of directly adding orthosilicic acid instead of silicon.

+

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6 Silicon Microsystem Technology

A clear advantage of TMAH solutions is their high selectivities against the various available dielectrics. [Rist94, Schn9 11 have reported selectivity values of different etch formulations. In every case the selectivity against LPCVD silicon nitride and thermal silicon oxide was found to be larger than 2X104 and 5X103, respectively. PECVD layers show selectivities larger than 103. In silicon-doped solutions, selectivities are even larger. Consequently, a single thin dielectric layer reliably protects silicon against TMAH solutions, even if the etching goes across entire wafers. Ethylene-diamine Pyrocatechol Etchants Historically, solutions based on hydrazine (N,H,) and pyrocatechol (C,H,(OH),) [Cris62] were the first with which anisotropic silicon etching was demonstrated. Later, the toxic hydrazine was replaced by ethylene-diamine (NH,(CH,),NH,) by Finne et al. [Finn67]. The resulting solution is frequently referred to as EDP or EDW (W for Water). Water was shown to be an essential ingredient of the reaction, through the ionization of ethylene-diamine according to NH,(CH2),NH2

+ H 2 0 -+ NH,(CH,),NH,+ + OH-.

(6.5) The surface silicon atoms react with hydroxyl ions to produce Si(OH),,+. In a second step the reaction with water produces the complex Si(OH);-. This is then further complexed by the pyrocatechol through the reaction Si(0H);-

+ 3C&,(OH),

-+ [Si(C6H40,)3]2-

+ 6H20.

(6.6) Customary EDP solutions have the following compositions. Solution of type S (= Slow) consists of 1 liter ethylenediamine, 160 g pyrocatechol, 6 g pyrazine and 133 ml water. This formulation has a relatively low (100) etch rate in comparison with alkaline etchants. At 70"C, 8O"C, and 90"C, its etch rate is 14 pm/h, 20 p d h , and 30 p d h , respectively. Its anisotropy ratio R~,oo,:R~,,,, is approximately 35 [Pete82a]. A second EDP formulation (type F, Fast) contains 320 ml instead of 133 ml water. Its etch rate is larger, however it has lower aluminum selectivity and etch quality. EDP is much appreciated for its CMOS compatibility. Its selectivity against aluminum is roughly 300: 1 at 90 "C. Consequently, during the three hours required to produce say a 90 pm deep etch pit, the top 0.3 pm of contact pads are removed. With typical CMOS metallization thicknesses between 0.6 pm and 1 pm, sufficient material remains on-chip for acceptable wire bonding. Bond reliability may be increased by designing contact pads with two or more (if available) superposed CMOS metals. Typical selectivities of EDP type S against silicon oxides are larger than 2000: 1 and even larger against silicon nitrides [Mose93]. If CMOS chips are to be etched in EDP, their silicon substrate must be locally exposed. This is achieved by cutting across all dielectrics, either during the CMOS process by superposing active, contact, via, and pad masks [Mose93], or after the CMOS process by a dry etch across the dielectric layers. The high selectivity against oxide makes an HF dip just before EDP etching necessary. This removes the native oxide protecting the silicon areas to be niicro-

6.2 Silicon Micromachining

227

structured. Even at a thickness of only 30 A, the native oxide would cause irreproducible and uncontrolled results. After completion of the EDP etching, cavities are often found to be plugged up with insoluble white deposits, likely to be solid Si(OH)4. Also, bonding pads have been found to bccome unbondable, possibly due to their passivation with Al(OH),. A cure against both problems consists of thoroughly rinsing in deionized water, dipping in 5 % ascorbic acid and cleaning by a second rinse in water [Leng94]. EDP solutions are toxic and corrosive. If this micromachining approach is adopted, extreme care should be applied in handling this solution. EDP oxidizes in contact with air. With time, its color changes from opaque dark-red to transparent brownish and unstable etching characteristics are obtained. Optimally the chamber volume in the reflux condenser above the solution is continuously rinsed with dry nitrogen. Even then, after roughly two weeks at room temperature, the solution usually has to be replaced by a fresh mixture.

6.2.3 Basic Etch Shapes The anisotropy of etchants provides a straightforward method to fabricate a variety of micromechanical structures. Their geometry is defined by the fact that (111) crystal planes etch slowly, while (100) surfaces and others are rapidly etched. The relatively high etch rate of intermediate planes such as (122) or (133) allows convex structures to be undercut. Using these basic characteristics, grooves, membranes, mesas, cantilevers, bridges and more complicated structures are readily obtained. Etch Grooves Usually the substrate wafer to be etched has (100) orientation and is covered by a masking layer, i.e., SiO, or Si,N4. This is patterned to locally expose the silicon below. We now let an anisotropic wet etchant act on the silicon surface. One atom layer after the other of the (100) surface is removed. As shown in the schematic cross-section in Fig. 6.2-7, the progression of the (100) plane is laterally constrained by (111) planes. Once a portion of (111) crystal plane has been exposed by the etchant, it is etched at the much lower rate R(lll). Thus, apart from slow underetching the lateral (111) planes appear stable. Their slope angle with respect to the vertical direction is a(1lll = arctg(J2) = 54.7'. After a sufficiently long time, the (100) etch bottom dies out at the intersection of the lateral (111) walls, as shown in Fig. 6.2-7. The etching then virtually comes to a stop. The result is a v-groove of width W and depth D, related by W = J2D. Viewed from above, the three-dimensional structures obtained using rectangular mask openings with edges oriented along [ 1101 directions, as shown in Fig. 6.2-7, are rectangular grooves delimited by four (1 11) planes originating at the edges of the openings. With a square opening, the resulting etch pit is an inverted pyramid. The angle between (111) walls is 70.5'.

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6 Silicon Microsystem Technology

Fig. 6.2-7 Rectangular mask openings aligned with [Oil] and [OlT] crystal directions on (100) wafers result in anisotropic etch grooves delimited by (1, + 1, 1)

+

and (100) planes. Ultimately the (100) plane vanishes.

A mask opening of arbitrary geometry on a (100) substrate will ultimately lead to a v-groove. At edges with orientation other than <110>, crystal planes with underetch rate higher than R(lll)will be exposed, thus enabling the edge to be underetched. Ultimately the corresponding etch front is stopped at the outermost (111) planes defined by the mask opening. These observations are summarized in Fig. 6.2-8 where two mask openings and the resulting v-groove are shown. To obtain the etch pit geometry for a given opening, construct the smallest rectangle with sides oriented along the [ O l l ] and [OlT] directions, which completely contains the opening. If the circumscribed openings of two mask openings overlap, construct the enveloping rectangle of the overlapping rectangles; iterate to convergence. This defines the ultimate v-groove as illustrated for example in Fig. 6.2-8. For some applications, (1 10) substrate material is preferred over (100) silicon. This choice is justified by the fact that (110) wafers contain (111) planes perpendicular to the wafer surface. The etching then proceeds as shown in Fig. 6.2-9 (a) and (b). The surface (110) plane is efficiently etched and the progression of the etch front exposes vertical (1 1I) walls. Viewed from above, rhomboidal structures as shown in Fig. 6.2-9 (c) are obtained. In view of the verticality of the four side walls, the etching proceeds until the wafer is etched across. Two further (111) planes intersect the (110) surface at an angle of 35.3 degrees and may be exploited for other structures, using appropriately designed etch masks.

6.2 Silicon Micromachining

229

Fig. 6.2-8 Irregularly shaped mask openings lead to anisotropic etch grooves defined by the smallest enveloping rectangle aligned with <110> directions. Openings with overlapping grooves ultimately result in large etch grooves.

Fig. 6.2-9 Rhomboidal mask openings on (1 10)-oriented silicon with edges aligned along [T12] and [1i2] directions may result in etch grooves with vertical sidewalls.

6 Silicon Microsystem Technology

230

Membranes With large openings, the (100) etch front on (100) material reaches the opposite side of the wafer before the lateral (111) walls intersect. If the etching is stopped shortly before this occurs, a silicon membrane laterally clamped to the silicon substrate is obtained. Such membranes now belong to the most successful commercial silicon microstructures. In numerous silicon pressure sensors, the deflection of the membrane under differential pressure gives access to various kinds of pressure dependent signals (see Section 6.4). For reproducible fabrication and reliable operation, the final membrane thickness has to be accurately controlled. Several methods to ensure this are described in Section 6.2.4.

Mesas and Tips It is a greater challenge to etch protrusions than cavities. Mesas, as protrusions are often also referred to, are obtained by convex mask structures surrounded by exposed silicon. As shown in Fig. 6.2-10, etching then produces a truncated pyramid which progressively emerges from the receding silicon. The difficulty of the task results from the underetching of the convex mask corners by planes with etch rates comparable to R(loo).A square mask thus does not lead to a mesa with square top face. The resulting pyramid is defined by (111)-oriented and higher index planes A solution to this problem consists of adding compensation structures to the corners of the mask, as shown in Fig. 6.2-11. The underetching then starts from the corners of the compensation structures. If the compensation structures are judiciously designed, the various underetching fronts converge to the desired top geometry of the mesa just when the desired mesa height is reached. Understandably, this requires reliable underetch rates, i. e., a well controlled and stable etch solution. Note that the dimensions of compensation structures scale linearly with the height of the mesa and thus dictate the minimal distance between mesas. Since

(a)

Mask

Silicon

(b)

Mesa

Fig. 6.2-10 Convex mask structures (a) result in mesas with corners rounded by rapidly etching higher index planes. In (b) mask has been omitted for clarity.

6.2 Silicon Micromachining

(a)

Mask \

(001)

Silicon I

(111)

(b)

23 1

Mesa

Higher index planes

Fig. 6.2-11 Simulation of the effect of corner compensation structures on anisotropic silicon etching. The higher index etch fronts converge to four corners and result in principle in a well defined silicon mesa.

no intrinsic etch stop is reached and processes variations are common, mesas often have irregular corners. If one lets the underetching planes of a square mask converge, the mask material ultimately floats off. Left behind is a prismatic cone with sharp tip, as shown in Fig. 6.2-12. Such sharp structures have been used in a number of applications including scanning tunneling and force microscopes and biomedical applications [Henr98].

Cantilevers Beams with one-sided support, also referred to as cantilevers, can be produced as shown in Fig. 6.2-13 on (100) and (110) wafers. The mask opening in Fig. 6.2-13 has two convex corners which are underetched during the micromachining process. While the etch bottom recedes and the peripheral (1 11) planes remain stable, higher index etch fronts progressively release the cantilever until the rear (1 11)

Fig. 6.2-12 Due to underetching, small convex etch masks ultimately result in sharp silicon tips or ridges after mask lift-off.

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6 Silicon Microsystem Technology

Fig. 6.2-13 Cantilevers are produced by underetching of convex corners. Correspond-

ing cross-sections between arrows are shown on the right hand side. plane is reached. It should be noted that the receding underetch fronts intersect at a sharp corner. Local thin film stress at this corner sometimes leads to the fracture of the microcantilever. Cantilevers serve a variety of purposes in resonators, accelerometers, and infrared, gas flow, and chemical microsensors, and test structures. The useless but illustrative example of a miniaturized piano for micropianists is shown in Fig. 6.2-14.

Fig. 6.2-14 SEM micrograph of an array of dielectric cantilevers fabricated using anisotropic silicon etching (courtesy of DASA, Munich).

6.2 Silicon Micromachining

233

Bridges Bridges, i. e., beams clamped at both ends are feasible with restrictions. Fig. 6.2-15 shows a design with etch openings intended to produce bridges. While the left design produces disconnected parallel etch grooves, the second layout enables the the bridge to be formed. A bridge will result if the enveloping rectangles of the separate mask windows overlap. The underetch time required to release the structure depends on the relevant underetch rates and should be evaluated already during the design phase. Especially bridge orientations too close to (110) should be avoided in view of their slow underetching.

Fig. 6.2-15 Design rule for successful microbridge fabrication using wet anisotropic silicon etching. The left design with bridge parallel to <110> directions fails to produce a bridge.

Fig. 6.2-16 CMOS microstructure fabricated using EDP front etching of CMOS processed silicon. The micro test structure is used to measure the heat capacity of CMOS thin film sandwiches [Arx98al.

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6 Silicon Microsystem Technology

As an example, a more complex microbridge etched in EDP type S at 95 “C is shown in Fig. 6.2-16. It was fabricated using a complete CMOS process and is composed of all CMOS dielectric layers, a meandering polysilicon line and a rectangular metal cover sandwiched in-between. The structure has been used to determine the heat capacity of CMOS thin films [Arx98a].

6.2.4 Etching Control Reproducible micromachining requires an accurately controlled etching process. This means controlling the progression of etch fronts in the various relevant directions and stopping the process once a desired geometry is obtained. The simplest way of achieving this is by timing; the etching is interrupted by rinsing the sample after a predetermined time. For this method to be successful, however, one has to know the relevant etch rates accurately enough, have a sufficiently stable solution, and control the etch temperature with sufficient accuracy. With an activation energy of 0.4 eV, at 90°C for example, an etch rate varies by 3.6%/K. This shows that a considerable inaccuracy in etched distance and microstructure geometry may be expected. More reliable control methods which exploit intrinsic mechanisms of the etching process rather than an arbitrary time scale are described in the following. Etch Stop Mechanisms A straightforward method is provided by the selectivity of etchants against materials other than silicon. In the context of IC technology, dielectric layers such as silicon nitride and silicon oxide provide “natural” etch stops. An example of this method was provided with the CMOS compatible KOH membrane etching in Fig. 6.2-6. Once the KOH has crossed the approximately 600-ym-thick wafers, it is stopped by the field oxide of the CMOS sandwich. Ultimately, the field oxide is also etched, finally exposing integrated polysilicon structures to the etchant. With a thermal oxide etch rate of about 200 h m i n . of 6 M KOH at 95 ‘C, one is given approximately 20 minutes before half the field oxide is removed. A sufficient time is therefore left to ensure that the process is completed over the entire wafer. With EDP and TMAH, the situation is even more favorable and dielectric structures are reliably and reproducibly released. The dopant concentration in silicon profoundly influences etch rates. The general observation is that the etch rate is decreased significantly in p-doped silicon at boron concentrations higher than 2 X 10’9cm-3. Figure 6.2-17 shows the dependence of the (100) silicon etch rate of two anisotropic etchants. In the case of EDP, as shown in curve (a), a reduction by a factor lo3 in comparison with weakly doped silicon is observed at a doping level of 1.7 X 1020cm-3.This value is close to the saturation concentration of boron in silicon. The reduction is rather independent of surface orientation. Similar findings for KOH are summarized in curve (b) of Fig. 6.2-17. Again, doping close to saturation leads to a significantly reduced etch rate.

6.2 Silicon Micromachining I

1017

235

I

1018

1019

1020

Boron concentation (ems) Fig. 6.2-17 Dependence of the (100) silicon etch rate in EDP type S at 100 "C (a) and 24 wt% KOH at 60°C (b), (after [Rist94] and [Heub91]). The dependences are almost independent of temperature.

This so-called boron etch stop is simple to implement by heavy doping. In practice, however, several disadvantages are revealed. At the high doping levels required, more than 1000 ppm of boron atoms are present in the silicon crystal. Due to the smaller size of the substitutional boron impurities, the released microstructures shrink or, if suspended on opposite sides, are subjected to considerable tensile stress. The second disadvantage is technological. In view of the high surface density of boron atoms, prohibitively long and thus costly doping predepositions or implantations are necessary to load the crystal with the desired number of impurities. In addition, the degenerate doping prevents circuitry from being implemented in these areas. The boron etch stop mechanism is understood in terms of the fundamental silicon etching mechanism. Electrons injected into the silicon in the first reaction step, Eq. (6.3), are minority carriers in the p-doped sample. In degenerately doped samples, their recombination life time is short and they recombine before the dissociation of water according to Eq. (6.4) is able to occur. Examples of microstructures fabricated using the boron etch stop are shown in Fig. 6.2-18. Such microneedles are used by neurophysiologists as a minimally invasive tool to monitor neural signals in-vivo. The individual needles are fabricated using a two-step boron diffusion with different depths, the deeper of which ensure the stability of the beam over most of its length, while the shallow diffusion allows extremely fine tips to be realized. CVD dielectrics cover the top surface of the structures ; integrated metallizations with exposed contact areas at various locations along the needle make it possible to monitor neural activity. Needles with widths down to 20 pm in the main beam area, typical thicknesses of 12-15 pm, and lengths up to 20 mm have been demonstrated. More than six parallel needles have been integrated on single devices. Recently, even microchannels were integrated in similar structures, for controlled drug delivery.

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6 Silicon Microsystem Technology

Fig. 6.2-18 SEM and optical micrographs of neural probes fabricated using the boron etch stop technique. (a) Perspective view of a probe tip showing the deep and shallow boron diffusions to define the shank. (b) shows probes with several different two-dimensional geometries (courtesy of K. D. Wise, University of Michigan, Ann Arbor, USA, [Wise98]).

A second example is schematically shown in Fig. 6.2-19. Similar to the above KOH membrane etching, dielectric membranes are fabricated by etching across entire wafers. Inaccuracies in the in-plane dimension of the membrane due to uncontrolled underetching are limited by a circumferential boron diffusion at which the etchant stops. At the same time, boron diffusion provides a method to produce large dielectric membranes supported by a grid of degenerately p-doped silicon lines [Yoon921.

Silicon

Mask’

Fig. 6.2-19 Micromachined membrane accurately defined by heavily boron doped edge regions (after [Yoon92]).

6.2 Silicon Micromachining

237

Electrochemical Etching Since wet etching of silicon is an electrochemical process, one may expect the etch rate to be influenced by an electrical potential Vapplied between etch solution and silicon sample. The I-Vcharacteristics in Fig. 6.2-20 show that this is the case for both, n and p-doped silicon in a 40 % KOH solution at 60 "C. Similar behavior is observed or expected in the other silicon etchants. Depending on its direction, the current I results from a dominance of either chemical oxidation or reduction of the silicon surface. In any case I has to be returned to the solution by complementary reactions at a counter-electrode. At a voltage of approximately -1.55 V, no current flows into or out of the sample. At this so-called open-circuit potential, the reduction/oxidation reactions at the silicon surface are charge-balanced and the silicon etches as though its potential were floating. A second important feature in Fig. 6.2-20 is the sudden drop of the current for both n and p-doped samples to close to zero around -0.9 V. This corresponds to an electrochemical regime where the silicon surface is oxidized (in the chemical sense) without ensuing dissolution of the oxidation products. The surface is said to be passivated. At sample voltages more anodic (positive) than -0.9 V, the silicon etch rate is negligible. This fact now routinely serves in the fabrication of silicon membranes with well-controlled thickness using variants of the three-electrode setup schematically shown in Fig. 6.2-21. A Pt reference electrode defines the reference potential, i. e., 0 V. The silicon sample to be etched consists of a p-doped substrate with n-doped epilayer containing circuit blocks and sensor elements. Its epi thickness corresponds to the desired membrane thickness. To protect the wafer front, the

h

OS8 0.6

I -

0.4

-

0.2

-

B

2

4

E

U

.-

Ic.

u)

s P

0 :

4-

c L

3

7

i

- o * 2 ~ -0.4 -0.6

1 .o

-1.8

-1.4

-1

-0.6

-0.2

Potential (V)

Fig. 6.2-20 I-V characteristics of the electrochemical etching of n and p-doped silicon in 40 wt% KOH at 60°C (after [Rist94]).

23 8

6 Silicon Micmsystrin Tt)chnology

Epi contact

\

Mask

Potentiostat

I I=O

Referenceelectrode

1

Counterelectrode

Fig. 6.2-21 Three-electrode sctup for the electrochcmical etching of silicon ineiiibrancs with accurately defined Lhickness. ‘The etching stops at the

built-in pn-junction [Kloe89].

wafer is usually inserted into a protection box or covered by an etch resistant wax. Only the rear face with its structured mask layer, the p-silicon is locally exposed to the etchant. An anodic potential larger than -0.9 V on the epi ensures its immediate passivation when it enters into contact with the etchant. In contrast, the potential of the p-substrate automatically adjusts itself to a value close to the open circuit potential. The pn-junction is then reverse biased and except for a small leakage current, no current flows through p-substrate nor pn-juntion. The sample is therefore etched at its unbiased speed, with etched structures defined by the rear mask, until the etch front reaches the pn-junction. At this point, the n-silicon surface is passivated and the etching stops. Membranes with controlled thickness are thus obtained. To ensure the proper potential definition, a counterelectrode is used in addition to the sample and reference electrodes. By returning the sample current to the solution, it enables the reference electrode current to remain at zero volts. This is achieved by using a so-called potentiostat, which simply is a feedback circuit maintaining Ire,= 0 by appropriate adjustment of the counterelectrode potential. A result of such processing is shown in Fig. 6.2-22. A lightly n-doped membrane is suspended on a p-doped substrate. In contrast with the boron etch stop, this technique permits the integration of additional active or passive elements into the micromachined structures, such as p-doped diffusions, serving as piezoresistors or heating resistors, or even of CMOS circuitry [Reay95]. Dielectric CMOS membranes with a suspended silicon island have been fabricated using electrochemical KOH etching, allowing the thermal decoupling of circuitry components from the rest of the chip [Mii1198]. Similarly, electrochemical front side etching in TMAH has enabled the production of silicon islands suspended on dielectric cantilevers. Such structures are again thermally well isolated

6.2 Silicon Micromachining n-Epilayer

Membrane

239

Circuitry

Piezoresistor

Fig. 6.2-22 Silicon membrane with integrated piezoresistors fabricated by the electro-

chemical etch-stop. The structure is used for pressure sensing or ultrasound generation and detection [Bran97].

from the rest of the chip. Micromachined thermal converters have been fabricated using this method IKlaa96bl. Recently, a different electrochemical method to etch silicon has been developed. When silicon is biased to a sufficiently anodic potential with respect to diluted HF solutions, pores with diameters between a few nanometers and tens of micrometers and pore pitch to diameter ratios between 1.1 and 10 are etched into the silicon. Pore parameters depend on bias voltage, HF concentration, and temperature [Lehm9 11. Nanoporous silicon with irregular nm-size pores has been used to turn extended regions of exposed silicon into a sponge-like material which is easily removed using diluted alkaline etchants. In contrast, ,urn-size mask openings combined with proper bias and HF concentration yield perfectly regular vertical holes useful, e. g., for high-area integrated capacitors, high aspect ratio microstructures, and microoptical components [Lehm96].

6.2.5 Characterization of Anisotropic Wet Etchants Several reasons make it necessary to characterize the direction dependent etch rates of anisotropic etchants : the evaluation of underetch times required to produce microstructures, the accurate prediction of etch shapes, the design of optimized etch masks. The space of etch parameters to be varied is large. In addition to their direction dependence, etch rates show strong variations with etchant composition, temperature, bias potential and illumination level, among others. The temperature dependence is frequently handled by determining etch rates R(T) at several temperatures and extracting the activation energy EA of the process, by fittting Arrhenius’ law R(T) = Roexp (-EA/kT) to the data. An elegant method to efficiently acquire such data has recently been proposed by Sato et al. [Sato97]. It is schematically shown in Fig. 6.2-23. A polished hemispherical silicon sample with a diameter of 22 mm is used. Such a hemisphere exposes all existing crystal planes. Consequently the etch rates of all crystal faces can in principle be determined. The sample is mounted on a goniometer and its

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6 Silicon Microsystem Technology

Initial shape

Final shape

Fig. 6.2-23 Schematic cross-section of initial and final shape of anisotropically etched silicon sphere.

shape is accurately determined. It is then etched for a certain time in the solution to be characterized, and rinsed. Its final shape is the result of the progression of all etch fronts originating from the hemisphere. From the difference of the two topographies, a map of the etch rates is extracted. The etch duration has to be chosen judiciously. If it is too short, the etched distances are small and inaccurate rates are extracted. If it is too long, some crystal planes between fast-etching directions are reduced to edges between fast etching fronts. Nevertheless the hemisphere method gives straightforward access to complete sets of orientation dependent etch rates. A disadvantage is the cost of the samples. A method for more modest budgets is based on the “wagon wheel”, denoting a planar etch mask which consists of a large number N of radial spokes, as shown in Fig. 6.2-24 (a) [Csep83]. Mask spokes and separating transparent areas alternate with an angular period of 2rrlN. Photolithography is used to transfer the pattern into a mask layer on a polished (100) silicon wafer. Spoke-shaped areas of exposed silicon then separate spoke-shaped mask structures. Through the mask openings, the etchant rapidly progresses in the (100) direction normal to the wafer surface. Simultaneously, the different mask edges are underetched at individual rates depending on their respective orientations. Thus, after an etch duration tetcha series of etch grooves is produced, delimited by a (100) floor and side walls composed of one or several crystal planes parallel to the corresponding inask edge. The overall result is a pattern similar to that shown in Fig. 6.2-24 (b). The clover-shaped clear area shows where the mask spokes were completely underetched due to the merging of neighboring side walls. The underetch rate R,(0) is thus obtained as a function of edge direction 8. It is given by R,(8) = r ( 8 ) d 2NtetCh, where the radius 4 0 ) denotes the radial extension of the clover-shape. In addition a quantitative analysis of etch rates requires the inclination angle (Miller indices) of smooth sidewalls with respect to the wafer normal to be determined, e. g., optically. With this input, the unteretch rates can be translated into etch rates R(0) of crystal planes by R = R,sinp. The wagon wheel method yields a discrete set of etch rates.

6.2 Silicon Micromachining

241

Resist mask

1001

Fig. 6.2-24 (a) Wagon wheel mask for the determination of direction-dependent un-

deretch rates and etch rates; (b) micrograph of etched wagon wheel pattern.

6.2.6 Dry Etching The general terminology and basic effects involved in dry etching were introduced in Section 4.7. Here, further information is presented concerning the dry etching of silicon and IC thin films. In recent years, in fact, the dry etching for microsystem fabrication has developed into an art of its own. Several exciting techniques have emerged and spread rapidly. These include isotropic silicon etching using xenon difluoride (XeF,) and a variety of techniques allowing the fabrication of high aspect ratio structures, i. e., features with small width at comparatively large height.

XeF, Etching XeF, is an isotropic silicon etchant with high etch rate [Chan95]. It is one among a few rare gas compounds of the inert noble gas Xe. It may be purchased in bottles in the solid form from which it sublimates under ambient conditions. On a silicon surface it decomposes readily into the volatile Xe, and silicon tetrafluoride (SiFJ. Silicon etch rates up to tens of ymlmin have been reported. However, etching appears to rapidly enter the diffusion-limited regime, when microstructures are being underetched. The etch process is then progressively slowed down. In this case, pulsed operation with repeated etching and pump-down cycles has been found to be beneficial to the etch rate. Relatively simple equipment is sufficient for XeFz etching: a bell jar with the XeF, bottle with a valve and a pump with pressure gauge and throttle valve appear to be sufficient. No external energy input such as plasma nor heating are required. However, safety precautions have to be taken in view of the aggressive fluorine. Typical etching pressures at ambient temperature are a few torr.

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6 Silicon Microsystem Technology

XeF, attacks neither IC dielectrics nor aluminum metallizations. It is therefore fully CMOS compatible. Microstructures have been produced by this method on CMOS chips fabricated through the MOSIS service [Hoff95]. Similarly to EDP front micromachining, the stack of dielectric layers has to be opened at well-defined locations through appropriate design of the field, contact, via, and pad masks. This locally exposes the substrate silicon to the etchant. A possible disadvantage is the resulting rough silicon surface which makes the method less reproducible than selective wet silicon etching. However, since XeF, does not attack ceramic or plastic materials, it is even suitable for micromachining of packaged CMOS microsystems. XeF, micromachining can thus be the very last step of the entire microsystem fabrication process. Further variations of halogednoble gas etching were developed at the Forschungszentrum Karlsruhe, Germany [Koh196]. These processes are based on fluorine and bromine compounds carried by a noble gas. Both Ar and Xe were shown to be well-suited for this purpose. By adding Xe to F,, etch rates up to 1.1 ymlmin were achieved. Determinant for the etching are the XeF, formed in the plasma and the sputtering action of the heavy Xe ions. However, under certain process conditions these ions cause rough silicon surfaces. Optimal surface qualities are achieved using a mixture of fluorine, bromine, and Xe as the carrier gas. The reactions Br, F, + 2BrF, BrF F, + BrF,, and BrF, F, BrF, occur in the plasma. BrF and BrF, are strongly fluorinating compounds and react rapidly with silicon to form silicontetrafluoride (SiF,). At a Br:F mixing ratio of 1:3, BrF, is preferentially formed. In the reaction

+

4BrF,

+

+ 3Si + 2Br2 + 3SiF4,

+

(6.7)

with silicon, Br, is thus regenerated and continues to feed the reaction chain until all available F, is depleted. Experimentally, the surface quality is controlled and optimized by varying the ratio of partial pressures between the Br/F compounds and the carrier gas Xe. All reaction products are volatile. Hemisperical etch cavities have been produced in silicon using this etching method and a mask layer with circular holes (Fig. 6.2-25). Reaction products escape through the small openings in the etch mask. In view of their excellent surface quality, the resulting silicon wafers, after stripping of the etch mask, can be used as a moulding tool for the fabrication of microlenses in PMMA or other thermoplastic materials. Figure 6.2-26 shows an SEM micrograph of an array of such lenses. These are to be inserted and used in medical catheters.

High Aspect Ratio Silicon Micromachining Besides these isotropic etchants, several highly anisotropic dry etching methods for silicon have recently emerged. They carry names such as DRIE (deep reactive ion etching), HARM (high aspect-ratio micromachining), ICP-RIE (inductively coupled plasma RIE), BSM (black silicon method) or just ASE (anisotropic silicon etching). Improvements in several areas of plasma process technology have

6.2 Silicon Micromachining

243

Fig. 6.2-25 Fabrication of moulding tool for hemispherical microlenses using dry silicon etching by Xe halogenides.

Fig. 6.2-26 SEM micrograph of an array of PMMA microlenses produced using the moulding tool in Fig. 6.2-25.

enabled this progress. The first is the use of higher plasma densities. This was made possible by novel plasma excitation methods. One method implemented in several commercial high-aspect ratio etchers is inductive coupling using a coil surrounding the plasma chamber. The magnetic field emanating from an alternating current at a frequency of 13.56 MHz causes more efficient ionization of the etching gas than in conventional plasma etchers. Biasing of the wafer substrate accelerates the ions into trajectories perpendicular to the wafer surface. Other meth-

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6 Silicon Microsystem Technology

Fig. 6.2-27 Sidewall passivation as used in CMOS technology for fabrication of narrow metal lines.

ods of creating denser plasmas use a static magnetic field, e.g., in MIE (magnetron ion etching) or the combination of a static magnetic field and dynamic electric fields through electron cyclotron resonance. In all these processes, intense vertical ion bombardment of the silicon surface becomes a central component of the etching process. SF, is the reactant of choice. The second improvement consists of passivating the resulting silicon sidewalls during the process. Sidewall passivation has been a standard process in dry aluminum etching in IC technologies for many years. There, chlorine based etching chemistries lead to the rapid attack of horizontal surfaces and, simultaneously, to the deposition of a polymer film on the resulting vertical sidewalls. The polymer layer acts as passivation against further etching. Anisotropy is thus achieved. This process enables much better dimensional control of aluminum features than isotropic wet etchants. The process is schematically shown in Fig. 6.2-27. Similar concepts are implemented in various high-aspect ratio silicon etching processes. This is achieved by adding O2 and/or fluorocarbons such as CHF, or C4F, to the plasma. These produce polymerized fluorocarbon coatings. While vertical walls are covered by such a polymerized layer, horizontal surfaces, e. g., the bottom of the trenches are cleaned from the protecting layer by the on-going ion bombardment. Appropriate concentrations of the etching SF6 and the passivating species allows the fabrication of trenches with walls at 9 0 k 2 " to the silicon surface and aspect ratios (trench height to width ratios) of 30:l and more. A process pioneered and patented by Bosch [Laer96] has been implemented in several commercial etchers. It temporally splits the etching step from the passivation step. A brief period of etching with SF6 based plasma chemistry is followed by fluorocarbon based passivation, which is followed by etching, and so on. A resulting trench cross-section is schematically shown in Fig. 6.2-28. A second sidewall passivation process uses cryogenic cooling. The wafers are mounted on a special chuck cooling them to temperatures as low as liquid nitrogen temperature (77 K). This is achieved by cooled He gas flowing over their rear face. At such low temperatures, sidewalls are protected by condensed gas, while condensation at-the etch bottoms is prevented by the ion bombardment.

6.2 Silicon Micromachining

245

Fig. 6.2-28 Anisotropic dry silicon etching using alternating isotropic etching and anisotropic passivation steps. The process results in deep high-aspect-ratio trenches. Applications of Dry Silicon Micromachining

Single Crystal silicon Reactive Etching and Metallization (SCREAM) is a process for the fabrication of released structures such as beams, bridges, and more complex structures from monocrystalline silicon. The process has been reported by the Nanofabrication Facility of Cornell University, USA [Shaw96]). As shown in Fig. 6.2-29, the process starts with a substrate covered by a patterned silicon dioxide. Anisotropic silicon etching produces trenches with depth up to 10 pm. A thin silicon oxide layer is then conformally deposited, that is, it covers sidewalls and horizontal surfaces with similar thickness. Anisotropic etching removes the oxide from horizontal surfaces, while leaving vertical surfaces protected. This is followed by isotropic silicon etching to undercut the material defining the trenches, which leads to the formation of underetched, suspended structures. By appropriate layout of the initial oxide mask, laterally suspended structures are produced. Finally the deposition of a metallization and its subsequent patterning turns the high aspect ratio beams into, e. g., capacitive elements. The method has been used to fabricate the scanning and tunneling units for scanning tunneling microscopes, linear resonators, accelerometers, and electrostatic lenses and quadrupoles. All individual process steps of SCREAM are performed at temperatures below

Fig. 6.2-29 Fabrication steps of SCREAM process.

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6 Silicon Microsystem Technology

Fig. 6.2-30 High force bidirectional comb-drive actuator, part of a multiple-level monolithic single-crystal silicon torsional resonator fabricated using SCREAM3D, a recent variant of SCREAM (courtesy of W. Hofmann, Nanofabrication Facility, Cornell University, USA [Hofm98]).

300°C, so that in principle the process is CMOS-compatible. A variant of SCREAM [Hofm98] has enabled the electrical isolation of beams from the substrate, and the fabrication of up to three independently mobile structural levels, see Fig. 6.2-30. This variant exploits the thermal oxidation of the entire cross-section of thin silicon beams and the different etching rates of trenches of different widths. A less involved, but also less versatile dry silicon micromachining process is Silicon Micromachining by PLasma Etching (SIMPLE), as shown in Fig. 6.2-31 [Fren96]. The process exploits the surprising modulation of the dry etch rate by different doping levels in silicon. Using C1-based plasmas, these authors showed that undoped or lightly doped silicon regions can be etched anisotropically, whereas degenerately n-doped buried layers can be etched isotropically and selectively against less doped regions. Cross-sections of a typical structure before and after such processing are shown in Fig. 6.2-3 1. For the etching process to become selective, very high doping concentrations close to lo2' cmp3 are required. Since such highly doped buried layers are unusual in standard IC technologies, this precludes the straightforward combination of SIMPLE with standard IC technologies. An elegant way of combining CMOS-compatible dry etching of dielectric thin films with isotropic etching of silicon has been demonstrated in [Fedd96]. The process is schematically shown in Fig. 6.2-32. It starts with a die fabricated using a triple metal 0.8 pm CMOS process of Hewlett-Packard accessible through MOSIS. Metal levels 1 and 2 are used as electrically active layers, while the third level is used as an etch mask for the subsequent micromachining. An anisotropic plasma etch with CHF3/0, chemistry is then applied. This removes the passivation over the entire chip. The dry etch stops on the third metal and the CMOS thin film

6.2 Silicon Micromachining

247

Fig. 6.2-32 CMOS-compatible dry etching for the fabrication of laterally suspended CMOS dielectric sandwich structures with integrated conducting lines.

sandwich below is thus preserved. In contrast, in regions where the third metal was opened, the entire CMOS stack is etched down to the silicon substrate. A second dry etching step with SF6/02 chemistry isotropically etches the silicon, thereby selectively underetching the dielectric structure. Narrow dielectric sandwiches with integrated conducting layers are thus released and again provide beams and bridges for, e. g., electromechanical microstructures such as comb-

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drives. Although [Fedd96] did not demonstrate the cointegration of circuitry with microstructures, this is in principle feasible in view of the CMOS-compatibility of the process.

6.3 Surface Micromachining The term suqace micromachining is somewhat misleading, since all micromachining processes remove material from a surface. Rather, the suface micromachining summarizes a number of techniques producing microstructures from thin films deposited onto the surface of a substrate. In contrast with bulk micromachining, surface micromachining leaves the substrate intact. The resulting microstructures are thus entirely built above the substrate surface. Surface micromachining uses the sacrificial layer method. The following process steps and materials are required, as illustrated in Fig. 6.3-1: 1. The base material, for instance an IC dielectric onto which the microstructure is built. The base layer may be structured (Fig. 6.3-1 (a)) to provide electrical contact paths between electrical components below and the microstructure above. 2. The so-called sacrificial layer is then deposited and patterned. Its sole purpose is to define the spacing between base layer and subsequent structural thin film (Fig. 6.3-1 (b)). 3. A structural thin film is then deposited and patterned. In the final device it performs the desired mechanical, thermal, and electrical functions. Its layout defines the geometry of the final device (Fig. 6.3-1 (c)). The microstructural layer is fixed to the base layer where the sacrificial layer was opened. Electrical contact is established in such areas. 4. Eiaally, a selective etchant removes the sacrificial material while preserving the structural material, the base layer and all subjacent materials (Fig. 6.3-1 (d)). The result of such processing is a large variety of possible microstructures. As examples, Fig. 6.3-2 shows a simply clamped beam, a microbridge, and a microchannel. Steps 2 and 3 may be iterated several times to build up structures composed of more than one structural layer. The more complex topologies of micromotors and microgears for instance require such cyclic processing. Up to five levels of polysilicon have been deposited to fabricate micromechanical gears with, e. g., wheels, bearings, and transmission shafts [Rodg98]. The basic mechanisms of sacrificial layer etching are identical to those of wet bulk silicon micromachining : transport of reactant from the solution to the etch front, chemical reaction at the surface, transport of reaction products away from the etch front into the solution. In bulk micromachining, the efficiency of the two transport steps can be kept high, e. g., by stirring. With the exception of the rather violent HNA reaction system, therefore, the efficiency of bulk silicon micromachining processes are reaction rate limited. Etch fronts progress linearly with time. In sacrificial layer etching the situation is often different. Take the example of the microchannel in Fig. 6.3-2 (c). Once the etching has cleared a suffi-

6.3 Sui$ace Microvnackining

249

Fig. 6.3-1 Process steps of surface micromachining.

cient portion of the channel, reactant molecules have to diffuse a considerable distance along the channel before reaching the etch front. Similarly, reaction products are eliminated froin the etch front and the channel by diffusion. Consequently, after some time, diffusion dominates the efficiency of the process [Monk94a, Monk94b, West961. Agitation of the microstructure is ineffective in this case. Viscous friction of the etchant in the narrow etched structures is too high to allow laminar, and even less, turbulent motion of the etchant. In the diffusion-limited case the etch length is roughly proportional to fi, where 1, D, and t denote the etched channel length, the diffusion constant of the reactant in the solution and Note however the etching time [Pau197]. The etch rate thus decreases as (D/t)-112. that this simple dependence applies only to linear channels with constant crosssection. Other geometries show different time dependences and required further analyses. However, they all have in common that the etch rate drops rapidly with increasing distance.

Fig. 6.3-2 Basic structures feasible by surface micro-

machining: cantilevers (a), beams and bridges (b), channels (c).

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6.3.1 Polysilicon Micromachining Since the late eighties, polysilicon has become the structural material of choice for surface micromachining. Its popularity has much to do with its readily controllable mechanical properties and the fact that it is compatible with high temperature processes, is easily doped, and can be structured with high accuracy. The method is usually referred to as polysilicon micromachining, in the sense that microstructures are built from polysilicon. Like gate or capacitor polysilicon layers in standard IC processes, micromechanical polysilicon is usually deposited at temperatures near 600"C, either by low pressure chemical vapor deposition (LPCVD), atmospheric pressure CVD (APCVD), or plasma-enhanced CVD (PECVD). The deposition process is based on the pyrolysis (thermally induced decomposition) of silane (SiH4) or chlorinated silanes (SiH,Cl, SiH,Cl,, SiHCI,, and SiCI,), where Si atoms are deposited onto the surfaces while H,, HC1, or Cl, are pumped out of the reaction chamber. The morphology of polysilicon layers strongly depends on processing conditions such as reactant pressure and substrate temperature. At lower deposition temperatures, amorphous films tend to be produced. At temperatures above 620 "C, mostly polycrystalline layers are obtained. The polycrystalline film results from columnar growth, with submicrometerwide columns perpendicularly to the substrate surface [Chan96]. Layers with thicknesses between 0.3 pm (typical CMOS gate thickness) and several micrometers have been produced. Most applications require electrically conductive polysilicon. For this purpose, it is doped using a variety of methods. Adding phosphine (PH,), arsine (ASH,), or diborane (B2H6) to the pyrolytic gas mixture incorporates the necessary impurities into the film. If the sacrificial layer is a phosphorous doped silicon oxide, it may serve as a phosphorous source during deposition and the subsequent anneal. Thin film stress is an important issue with polysilicon. The residual stress of polysilicon films depends strongly on the deposition parameters. Residual stress in as-deposited films is compressive with values down to -700 MPa for films produced under standard LPCVD conditions at 900 "C [Howe95]. For polysilicon microstructures, however, the stress and stress gradient of the constitutive film has to be as low as possible. This objective is reached by annealing the layers at temperatures above 1000°C. Significant reduction of the residual stress to values below 50 MPa is achieved by such a treatment. Phosphorous silicate glass (PSG) is frequently used as the sacrificial material. It is deposited using LPCVD or PECVD equipment with SiH4 or tetraethoxysilane (TEOS) and O2 or N 2 0 as silicon and oxygen sources, respectively. Phosphine (PH,) provides the phosphorous dopants. PSG films contain up to 14 % of phosphorous. Selective etching is done in HF-based solutions at rates of the order of 1 p d m i n . PSG films deposited at higher temperatures (LPCVD) have usually lower etch rates than those produced at lower temperatures (PECVD). Also, etch rate increases with phosphorous concentration. Unfortunately, HF etching of PSG is rapidly limited by diffusion. Prohibitively long times are required to un-

5.3 Sugace Micromachining

25 1

Fig. 6.3-3 Surface micromachining of an electrostatic micromotor.

deretch distances longer than a few micrometers. This is one of the reasons why extended polysilicon microstructures are usually structured with dense arrays of through-holes. The holes are also a necessity in view of the degradation of polysilicon under prolonged exposure to HF solutions. Electrostatic micromotors were among the earliest microstructures fabricated using this technology. A three level polysilicon process schematically shown in Fig. 6.3-3 was used for their fabrication. A finished device is shown in Fig. 6.3-4. Despite the initial excitement which they created, such structures have remained academic curiosities, mainly due to lubrication problems which severely limit their lifetime. Coupling torque out of the devices was another problem. A solution to this problem using five levels of polysilicon was recently proposed at Sandia National Laboratories [Rodg98]. These authors demonstrated the fabrication and operation of transmission gears and successfully translated the rotational motion of micro$omponents into linear displacements and vice versa.

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Fig. 6.3-4 SEM micrograph of an electrostatic micromotor fabricated using polysili-

con surface micromachining (courtesy of the Berkeley Sensor and Actuator Center, CA, USA).

Further applications include comb-drive actuators (see Section 6.4) and threedimensional structures. In the latter, polysilicon structures are flexibly mounted on micromachined polysilicon hinges. Appropriate combinations of such components can the be raised into three-dimensional assemblies usable, e. g., as optical elements: mirrors, Fresnel lenses, or supports for active optical components, as shown in Fig. 6.3-5. A field referred to as microoptoelectromechanical systems (MOEMS) is developing around this original approach [Yeh95]. In the above examples, polysilicon taylored specially to the needs of micromechanics was used. A group at Siemens avoided this restriction by using a standard n-doped gate polysilicon layer of a 0.8 pm CMOS process [Hier96]. The 600 nm thick field oxide was used as the sacrificial material. During the CMOS process, the various dielectric layers were opened above the polysilicon structure to be released. After completion of the CMOS process, micromachining using buffered HF released the polysilicon. An accelerometer microsystem with on-chip signal conditioning circuitry was demonstrated.

Clamp

Hinge

Fig. 6.3-5 Hinged “origami” structures fabricated using polysilicon micromachining.

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253

6.3.2 Sacrificial Aluminum Micromachining Fully CMOS compatible surface micromachining is possible if one of the CMOS metallizations is used as the sacrificial material. This process has been referred to as SALE (Sacrificial ALuminum Etching). In many ASIC CMOS processes, two such metal layers made from aluminum alloys are available. Removal of the first metal as a sacrificial layer allows structures to be fabricated which are composed of the intermetal dielectric, the second metal, and the passivation. The second metal is then sandwiched between two dielectrics. Appropriately structured, it may serve as a mirror, electrode, heating resistor or thermistor. Membranes with integrated heater/thermistors [Pau195a], microchannels [West97a,West97b], resonant plates, and clamped beams have been fabricated using this technique. As an example, the process shown in Fig. 6.3-6 was used to fabricate the thermal pressure microsensor and microsystem shown in Figs. 6.4-16 and 6.4-17, respectively. Etching aluminum alloys selectively against silicon based dielectrics is achieved with several etchants. A first mixture is based on Nitric acid, Phosphoric acid and Acetic acid (NPA), and water in concentrations of 2.29 wt%, 72.88 wt%, and 11.37 wt%, and 13.46 wt%, respectively. Nitric acid oxidizes the aluminum, likely producing aluminum hydroxide, which is then etched away by the phosphoric acid. As in the HNA etchants, the acetic acid slows down the decomposition of the nitric acid into less favorable compounds. Initial etch rates of 6.8 p d m i n , 68 pm/min, and 170 pndmin were measured at 30 "C, 50 "C, and 65 "C, respectively, in the initial etch-rate limited regime. However, the etching process rapidly crosses over to the diffusion limited regime. Although diffusion considerably slows down underetching, microstructures underetched by several hundred micrometers are nevertheless feasible in view of the high selectivity of the above etch mixture against the dielectric layers. Clearing a 150 pm long channel takes roughly 2.5 hours at 65°C. Krumm etch, composed of hydrogen peroxide (H,O,), phosphoric acid and acetic acid achieves similar results [West96, Pau1971. The resulting etch fronts are rougher and less reproducible. Hydrochloric acid (HC1) mixtures with water or with diluted hydrogen peroxide (H,O,) violently attack aluminum. A linear distance of 150 pm is etched within 30 min. For SALE to be compatible with the CMOS process, all exposed non-sacrificial metal structures, i. e., electrical contact pads have to be protected from the etchant. Satisfactory protection against the NPA solution and Krumm etch is ensured by photoresist cured at 140°C. The photoresist layer should be thicker than the largest topographical steps on the MEMS die for smooth step coverage and protection uniformity. Similarly, electroplated Au bumps covering the pads are reliable shields against the etchant [Pau197]. Aluminum has also been used in non-CMOS micromachining. Nickel structures used for the fabrication of vibrating gyroscopes (see Section 6.4.1) were electroplated on a sacrificial A1 base. Similarly, microstructures made of two polyimide layers with integrated TiW heating resistors were built on a 2 pm thick sacrificial aluminum layer [Suh95]. The released structures were used as thermally driven miniaturized ciliary actuators.

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Fig. 6.3-6 Sacrificial aluminum etching (SALE). Microstructure after CMOS process (a), photoresist protection (b), sacrifical layer etching (c), and photoresist

removal (d).

6.3.3 Sacrificial Polymer Micromachining Polymer layers have also been successfully used as a sacrificial material. In view of the limited temperature resistance of these organic compounds, they preclude high temperature processing. Evaporation, sputtering, and electrodeposition however are compatible processes to be used for the construction of structural levels. The particular attraction of organic materials is that first they provide excellent planarization and second they are easily removed by ashing in oxygen. Texas Instruments used two-level polymer sacrificial micromachining to fabricate impressive two-dimensional micromirror arrays, described in more detail in Section 6.4.3. The structures are all-metal devices with mechanical, electrical, and optical functionality. A novel application of sacrificial polymers has emerged recently [Cros98]. Micrometer-thin parylene coatings have been used for years as a protective coating of micromachined pressure sensors against potentially aggressive media. Parylene-C is deposited from the gas phase and converted into a dense layer by plasma polymerization. The reason for using Parylene is the highly conformal and pinhole-free coverage achievable. A coating is said to be “conformal” when it uniformly covers even complex topographies. In the novel sacrificial method, vertical metal structures intended as capacitor electrodes are electroplated on the substrate using thick photoresist techniques. The entire surface is then coated with a 5 -ym-thin parylene film onto which a second layer of metal is electroplated. After removal of the organic material, metal structures with narrow gaps usable as interdigital electrodes

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255

are obtained. The parts built directly on the substrate provide fixed electrodes, while those floating above the substrate are mobile. These flexible electrodes are attached to the substrate at selected locations where the parylene coating was opened before the second electroplating. The process is a candidate for the fabrication of inexpensive electrostatic sensors and actuators on a broad range of substrates including silicon and CMOS wafers.

6.3.4 Stiction Sticking or stiction, as it is also referred to, is a serious problem in micromachining, especially surface micromachining. In the last step of the micromachining process, the microstructures are rinsed and dried. Towards the end of the drying process, liquid has vanished from the chip surface, while it still fills the micromachined gaps. The surface of the residual liquid forms menisci with negative average (Gaussian) curvature. Radii of curvature are comparable in size with representative gap widths. Due to surface tension, liquid surfaces with negative curvature require an underpressure in the liquid with respect to the ambient pressure. This pressure difference pulls the microstructure towards the substrate. Two things may then happen. Either the structure breaks or it is pulled into contact with the substrate surface. The last liquid drop between structural material and substrate surface contains highly concentrated impurities which may ultimately glue the microstructure to the surface at undesired locations. As an example of typical forces consider water against air at 25 "C. With a surface energy of o, = 7.2X1OP2 J m-2, the pressure difference Ap resulting from a cylindrical meniscus with a radius of curvature of Y = 0.72 pm amounts to Ap = qJv = 1 atm. Repeated rinsing to clean away residues is no cure. Clean rinsing liquid dissolves the impurities or even introduces more of them. Upon concentration during the next drying process the microstructures are usually reattached. In some cases, the problem is relieved by using rinsing liquids with low surface tension such as heptane or hexane. A method to totally avoid the formation of a liquid meniscus uses the sublimation of frozen solvent. After rinsing in water, and possibly an intermediate solvent the microstructures are rinsed in the final solvent. This is then frozen and sublimated at reduced pressure [Take91, Koba92, Lin951. Another method to avoid the liquid/air interface is critical point drying [Mulh93], as schematically shown in Fig. 6.3-7. The rinsing liquid is changed into the gaseous state without the appearance of a meniscus separating the liquid and gaseous phases. This is achieved by circumventing the critical point in the p-Tdiagram at sufficiently high pressure and temperature. With its critical point at 31 "C and 72.8 atm, CO, is well suited for critical point drying. After the microstructures have been extensively rinsed in deionized water, they are transferred into methanol to replace the water by dilution. The structures in methanol are then placed in a pressure vessel, where the methanol is exchanged against liquid C 0 2 at roughly 25 "C and 80 atm. Heating up to 35 "C changes the liquid C 0 2 into the gaseous phase. Venting the

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pressure vessel completes the procedure and the fabrication of released microstructures. Appropriate design of microstructures is another way of alleviating stiction [Abe95]. Sharp protrusions added locally to the edges of the structural components reduce the effective contact area between substrate and microstructure. The elastic stiffness of the released microstructure pulls the two components more easily apart.

P

Liquid Initial state

i

?+-B

2

cT

I

T,

Fig. 6.3-7 Principle of critical point drying for stiction-free surface micromachining. Process path 2 surrounds the critical point (highest temperature and pressure at which liquid and vapor phases coexist), whereas process path 1 crosses the liquidhapor phase transistion line. Drying using path 2 avoids surface tension caused by liquid/vapor interface.

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6.4 Micro Transducers and Systems Based on Silicon Technology Complex micro structures and entire microsystems can be produced using silicon technologies and the micromachining techniques described in the previous sections of this chapter. Numerous research and development teams around the world work on such devices. Excellent detailed accounts of the entire field are available [Sze94, Trim97, Kova98, Proc98, Sens991. For this reason this section only describes a few representative structures and systems for the transduction of mechanical, thermal, radiant, magnetic, fluidic and electrical signals. The examples given below are by no means exhaustive and are intended to introduce and illustrate a selection of transduction effects. An early spectacular achievement of microsystem technology was the demonstration of micromotors smaller than the diameter of a human hair. However, these devices have quietly vanished from the stage, to be replaced by no less impressive other systems. Tribological effects are difficult to control in miniaturized, rapidly rotating microstructures. In particular, it is difficult to achieve a precise, wobble-free bearing of rotors on a central shaft, fabricated by surface micromachining techniques. Since the height of the resulting structures is small in comparison with their lateral dimensions, only a restricted support area is available. In comparison with conventional macroscopic gears, the relative precision of microparts is smaller by orders of magnitude. In a micromotor, the ratio between shaft diameter and rotorkhaft gap width is roughly 10’ (100 pm : 1 pm). For comparison, consider a typical macroscopic component such as the crankshaft in an automobile. Here the same ratio is roughly 5X104 (50 mm : 10 pm). Admittedly, the shrinking of motors to microscopic dimensions was a strong demonstration of the power of micromachining. The imitation of macroscopic devices at the microscopic level by exploiting the shrinking power of micromachining still dominates in many actual devices and systems. At the same time, micromotors have become a constant reminder that reduction alone does not guarantee success. As many other applications demonstrate, an important opportunity of silicon micromachining is the realization of unconventional devices, taking advantage of favorable scaling properties of selected physical effects, and the formidable potential of large-scale production based on the established IC fabrication techniques, enhanced by selected micromachining techniques.

6.4.1 Mechanical Devices and Systems Mechanical transducers such as pressure sensors and accelerometers are among the most successful microsystem products. Miniaturized pressure sensors have found applications in fields as diverse as process control, differential-pressure flow measurement, altimetry, barometry and medical pressure monitoring. The main customer of miniaturized accelerometers is the automotive sector using

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them mostly as crash sensors. In addition, much attention has recently been focused on miniaturized gyroscopes for inertial navigation.

Pressure sensors Many of today’s mature pressure microsensors are based on a silicon membrane supported by a rigid silicon die. Rudimentary bipolar technology followed by electrochemical anisotropic silicon etching suffices to fabricate such devices. Depending on the pressure range, membranes have thicknesses of a few micrometers and a few tens of micrometers. As schematically shown in Fig. 6.4-1, the membrane is supported on its sides by a silicon rim. Under a pressure difference AP, the membrane is deflected. Deformation is then translated into an electrical signal using the piezoresistive response of four diffused strain gauges. The strain gauge resistors are placed close to the membrane edges (or close to one edge) where the curvature of the deflected structure is maximum. Two of the resistors are parallel to the respective supporting edges, while the other two are perpendicular to them. and The individual resistances undergo changes according to ARIl/Rll= KIl&(AP) AR,IR1 = KL&(AP),respectively, where E, Klland KL denote the pressure-dependent local strain and the two relevant piezoresitive gauge factors, respectively. For p-doped silicon with NA = 1019cmp3,one has Kii = 29 and K1 = 7 [Ober86]. Connection of the four structures into a Wheatstone bridge produces a differential signal AU proportional to (K,-KII)~(AP),which grows with deflection and ideally vanishes in the undeformed state of the device. The arrangement is schematically shown in Fig. 6.4-2. At small deflections the response of such a device is linear. At larger pressures, it is dominated by geometrical nonlinearities in the device and piezoresistive nonlinearities in the resistors. The resulting overall nonlinearities and an unavoidable residual offset can be compensated by trimming resistors or on-chip circuitry, which also take care of the various temperature coef-

n-Epilayer

Pietoresistor

Membrane

Circuitry

Differential pressure

Fig. 6.4-1 Silicon membrane pressure sensor. The deflection of the membrane under differential pressure causes the contraction or expansion of the integrated piezoresistors. The pressure-dependent resistances are integrated into a Wheatstone bridge as shown in Fig. 6.4-2.

6.4 Micro Transducers and Systems Based on Silicon Technology

259

Fig. 6.4-2 Wheatstone bridge configuration of piezoresistor on silicon membrane

pressure sensor. Resistors parallel and perpendicular to the membrane edges experience opposite resistance changes.

ficients. An example of an industrial piezoresistive pressure sensor with integrated signal conditioning circuitry is shown in Fig. 6.4-3 [Kres94]. After IC processing and micromachining, the individual chips are anodically bonded to a borosilicate substrate and mounted on a metal header. The sensor fulfills the severe specifications required by the automotive industry: operation between -40 "C and 125 "C at pressures between 20 kPa and 115 kPa. A shipping device is shown in Fig. 6.4-4.

Fig. 6.4-3 Schematic cross-section of an industrial version of a piezoresistive pressure sensor. Signal processing circuitry is integrated on-chip (courtesy of Robert Bosch GmbH, Stuttgart, Germany).

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Fig. 6.4-4 The industrially fabricated and packaged pressure sensor schematically shown in Fig. 6.4-3 (courtesy of Robert Bosch GmbH, Stuttgart, Germany).

Accelerometers Acceleration is measured by its action on masses. In an accelerometer these masses are usually referred to as proof or seismic masses and are elastically hinged to a rigid substrate exposed to the acceleration. Two principles have been implemented in various configurations. In the first, the deformation of the hinges under the inertial force by the proof mass is measured. In the second, the inertial force on the seismic mass is balanced by applying a counterforce in a feedback system to keep the seismic mass in a stable position with respect to the substrate. Parameters describing accelerometers are the fundamental mechanical resonance frequency and the sensitivity, i. e., the signal output per unit acceleration. Construction of accelerometers often involves a trade-off among these parameters. In order to suppress spurious low frequency signals, the stiffness of the device is increased. However, at the same time this reduces sensitivity. A third important parameter is cross-sensitivity, i. e., the response of the device to accelerations perpendicular to its principal sensing direction. Usually, cross-sensitivity is expressed in percent. Miniaturized devices show cross-sensitivities down to a few percent. Finally, bandwidth is an important specification of accelerometers. In order to trigger the inflation of an airbag within milliseconds, crash sensors have to cover a bandwidth of several kHz. Fig. 6.4-5 shows a structure using the first sensing principle. It is realized by anisotropic etching of silicon wafers from both sides. The proof mass is suspended on a solid silicon frame by two silicon hinges with integrated piezoresistors. The seismic mass of an accelerometer with capacitive readout is shown in Fig. 6.4-6. Again, the structure is produced by double-sided anisotropic silicon etching. It is

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261

Fig. 6.4-5 SEM micrograph of a piezoresistive inertial sensor (courtesy of Robert Bosch GmbH, Stuttgart, Germany).

Fig. 6.4-6 SEM micrograph of the suspended seismic mass of a capacitive micro

accelerometer. supported by eight thin tensile ribbons at its corners which guarantee the parallelity of seismic mass and counterelectrodes. In principle the sensitivity of capacitive accelerometers can be increased by shrinking the gap width. This has the advantage of leaving the resonance frequency unchanged except for squeeze film effects between the capacitor plates. Fascinating examples of force-compensated accelerometers are produced and commercialized, e. g., by Analog Devices. These structures are fabricated using polysilicon micromachining and implement an electrostatic comb geometry. The. basic structure of the accelerometer is schematically shown in Fig. 6.4-7 [Chau95]. Its polysilicon proof mass is attached to the silicon substrate via flexural beams optimized to restrict its motion to linear displacements in essentially one direction. Other modes are highly constrained. The proof mass has equally spaced long electrode fingers on both sides, alternating with pairs of electrodes fixed on the substrate. The electrode arrangement on one side is used to sense the deflection of the proof mass out of its equilibrium position. Those at the

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Fig. 6.4-7 Schematic top view and cross-section of surface micromachined electromechanical accelerometer. The deflection of the central shuttle from its equilibrium position under the inertial force F, is detected as the unbalance of the capacitances between the shuttle and, respectively, the fixed electrode systems 1 and 2.

other side are at respective potentials of V, and Vddenabling to recenter the proof mass by applying an appropriate potential deviating from (V,,+Vdd)/2to the proof mass. This provides the acceleration signal. Both parts are connected by a feedback loop ensuring that the proof mass stays within k10 nm of its equilibrium position. The substrate die contains all the necessary circuitry, including squarewave oscillators, a demodulator, low-pass filter, differential amplifier, and preamplifier, all on a chip of 3x3 mm'. Systems with ranges of +5 g and 'r50 g, mainly for automotive applications, are available. In contrast to the bulk micromachined devices, the polysilicon micromachined structures offer the advantage of enabling a straightforward realization of two dimensional accelerometers. They measure two orthogonal components of the acceleration by combining two one-dimensional devices on a single chip.

Gyroscopes Micromachining techniques have enabled the fabrication of impressive gyroscopes, that is, devices for the measurement of yaw rate (angular rotation) or angular acceleration. Whereas the most sensitive macroscopic yaw sensors are optical devices, their micromachined counterparts exploit subtle mechanical effects.

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263

These basically originate in the Coriolis force F, = 2m (v X u) acting on any mass m moving with velocity v in a frame of reference rotating with angular rate w. In many devices, two orthogonal oscillators with identical fundamental frequency are weakly coupled to each other through the Coriolis force. One oscillator is harmonically excited. At a vanishing yaw rate, both oscillators remain independent. On the other hand, under rotation the motion of the driven oscillator is transferred to the other oscillator with a coupling strength proportional to the yaw rate. A recent realization of this principle is schematically shown in Fig. 6.4-8 [Putt94, Spar971. The device essentially consists of a wheel fixed on the substrate by its axis. Curved suspension arms give it the freedom to vibrate in the ring plane. It is surrounded by evenly spaced electrodes. They are used to excite selected resonance modes and detect their shapes. Excitation occurs electrostatically through the capacitance between the electrodes and the biased wheel. When the device is excited using two opposite electrodes, at vanishing yaw rate it responds by an elliptical breathing mode symmetrical with respect to the electrodes, as shown in Fig. 6.4-9. Under rotation the principal axes of the breathing mode are rotated with respect to the line connecting the excitation electrodes. The rotation angle @ is proportional to the yaw rate. In practice, neighboring electrodes are made to participate in the excitation through a feedback loop guaranteeing the breathing motion to remain unrotated. The size of the required driving voltage is then proportional to the yaw rate and provides a more reliable response to the external input. The device in Fig. 6.4-8 was fabricated on a silicon substrate with integrated circuitry. The mechanical structure consists of electroformed nickel (Ni) on a sacrificial A1 base layer. After removal of the aluminum, the wheel is released and held in place only by the central post. The high aspect ratio (19 pm high, 5 pm wide) Ni structure was fabricated using a LIGA-like process, with a polymer electroplating template. The wheel/electrode gaps are 7 pm wide. Yaw rate resolution of the device is 0.5”/s with a rate range of klOO”/s.

6.4.2 Thermal Micro Devices and Systems Most microstructures are sensitive to temperature. Temperature dependent material properties are often responsible for this often undesired effect. In devices not specifically designed as temperature sensors, the art consists of reducing the temperature cross-sensitivity to a minimum. Counter-measures range from the inclusion of reference structures and temperature-coefficient compensating analog circuitry, to the inclusion of on-chip temperature sensors and programming of look-up tables and interpolation schemes. On the other hand, numerous microstructures using silicon technology do enable the measurement of the absolute temperature T or at least temperature changes AT with respect to a reference temperature To. In addition to their stand-alone use, temperature sensors are applied in many “tandem” transducers where a non-thermal signal is first converted into a thermal signature which is finally transduced into an electrical signal. Initial parameters to be measured include

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Fig. 6.4-8 Schematic top view and cross-section of vibrating ring gyroscope. The ring is centrally attached to the substrate. Its in-plane vibration mode is excited and monitored using the peripheral fixed electrodes.

Drive electrode

w =0

Sense electrode

Vibration mode

wf 0

Fig. 6.4-9 Resonant mode shapes of vibrating ring gyroscope without (left hand side) and with (right hand side) yaw.

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gas flow, vacuum, and pressure, as described below. Further devices based on thermal principles include radiation detectors and miniaturized incandescent lamps. Temperature Sensing

Silicon devices designed to measure temperature include pn-junctions, bipolar transistors, thin film thermistors, and integrated thermocouples and thermopiles. A silicon diode shows a strong temperature dependence, as seen from the basic I-V characteristic I = I,(T)exp(qV/kT) of the forward-based diode, where Is, q, and k denote the device's temperature-dependent reverse saturation current, the elementary charge, and the Boltzmann constant, respectively. When the device in biased at constant current, the applied voltage decreases approximately linearly with increasing temperature, with a slope of approximately -2.2 mV/K. This slope shows variations of about k 0 . 2 mV/K depending on the device, technology, and bias current. Diodes are simple in operation and easily fabricated. They can be cointegrated beside more demanding microtransducers, to provide a reference substrate temperature at the cost of negligible silicon area. Appropriately calibrated, they provide temperature values with an accuracy down to 0.1 "C. Nonlinear contributions to the temperature-dependent diode response are effectively cancelled by structures referred to as PTAT (proportional to absolute temperature) circuits. The basic idea is the combination of two different diodes with different junction areas. If these device are forward biased with identical currents, I, = I,, the difference AV = V. - V, in the respective voltages is equal to AV = kT/q X ln(r), where denotes the ratio of junction areas. The only temperature dependence is in the prefactor. It is linear with a slope determined by universal constants and the design parameter r. In practice, the diodes are commonly replaced by bipolar transistors of different area, driven by identical currents from a current mirror. The example of a simple circuit realizing this elegant temperature sensing principle with output current proportional to T, is shown in Fig. 6.4-10.

Fig. 6.4-10 Transistor circuit providing an output current proportional to absolute temperature (PTAT) (after lout

[Midd94]).

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6 Silicon Microsystem Techtzology

A third method to measure temperature is provided by resistors. Indeed pure metals have large temperature coefficients of resistance at room temperature. By suitable alloying, the temperature coefficient can be changed over a wide range. CMOS aluminum metallization are usually alloyed with silicon and copper. They show temperature coefficients between 2900 p p d K and 4000 ppm/K at 300 K [Arx98b]. Resistors used for temperature measurements are often referred to as thermistors. Accurate local temperature measurements are best performed using the four-point configuration shown in Fig. 6.4-11. The resistor is contacted with four lines, two of which are used to provide the sensing current, while the other two serve to accurately measure the potential drop over the resistor. This configuration is recommended, because it results in a resistance value insensitive to the resistance of the sometimes long leads connecting the device. Due to design constraints, their resistance often cannot be neglected in comparison with the sensing part of the device and has to be effectively discarded from the measurement. Polysilicon is also a useful thermistor material. The degenerately doped gate polysilicon (No = 10” cmP3) of MOS-based IC processes shows a temperature coefficient between 650 ppm/K and 900 p p d K . Polysilicon and silicon samples with n-doping concentrations below 1019cm-3 and p-doped samples generally show negative temperature coefficients of resistance. A value of -500 p p d K was for instance measured on a p-doped diode polysilicon layer of a CMOS ASIC process [Arx98b]. A possible layout of a polysilicon temperature sensor is shown in Fig. 6.4-11. Diffused silicon resistors are also suitable for temperature measurements. The task is however made more difficult because of their biasing-dependent junction depth. Generally, resistive temperature measurements require calibration of the device, since even commercial IC technologies guarantee sheet resistances with at best a reproducibility of f 10 %. Even resistor matching is rarely better than 1 %. However, for the measurement of small temperature excursions of a few tens of degrees, resistors are a simple and versatile tool, if their temperature coefficient of resistance can be calibrated in-situ or is available from independent measurements. Finally, thermocouples enable the straightforward determination of temperature differences on thermal microstructures. They are particularly appealing since their

?-

J”

Fig. 6.4-11 Schematic view of integrated temperature dependent resistor (thermistor) and its use at constant bias current Zo. The voltage measurement is accurately performed in the four contact configuration independent of contact resistances in the current path. For clarity, surrounding dielectric layers are not shown.

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signal is self-generating [Midd94], i. e., they need no external driving or biasing. The basic structure of a thermocouple is a pair of different materials A and B contacted at one end, as shown in Fig. 6.4-12. The contact is exposed to the temperature To AT to be measured while the other two ends are at the reference temperature of To , e. g., an efficient heat sink such as the silicon substrate material. When the measurement and reference contacts are exposed to a temperature difference AT a voltage appears between the two reference ends. This is the thermoelectric potential, given for sufficiently small temperature differences by AU = aABAT, where aAB= ciB- aA denotes the relative Seebeck coefficient between the two materials. In silicon technology, thermocouples can be realized using pairs of the available conducting materials, i. e., doped monocrystalline silicon, n-doped or p-doped polysilicon and various metallizations. Largest thermoelectric coefficients have been obtained using weakly doped silicon legs against aluminum, with Seebeck coefficients up to 1 mV/K [Midd94]. Approximately, the Seebeck coefficient of polysilicon follows c, 0.79(kB/q)ln(no/n)and aP 0.86(kB/q) In(p,/p) as a function of carrier concentration n and p , with the coefficients no= 9.8 X lo2' cmP3and po = 1.28X1021 cmp3 [Arx98b]. Gate polysilicon layers (Rsq -25 a) against CMOS metallizations achieve values between -87 pV/K and -115 pV/K. Positively doped CMOS polysilicon layers (p 128 Pam) have shown thermopowers up to 268 pV/K at 300 K [Arx98b]. A simple way to boost the relatively small signals provided by individual thermocouples is to connect them in series into so-called thermopiles, as shown in Fig. 6.4-12. Thermal gas flow sensors, pressure sensors, infrared radiation detectors, chemical sensors, and ac-dc converters have been equipped with thermopiles for temperatures measurements. It should be noted that the use of thermopiles is most beneficial when temperature differences

+

-

-

-

- -

Material A. a,

Material B, as

T = To+ AT

Fig. 6.4-12 Schematic view of a thermocouple and a thermopile composed of N = 3 thermocouples. Under a temperature difference AT, thermoelectric voltages V, = uABATand Vip= NuA,AT are measured, respectively.

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6 Silicon Microsystem Technology

are to be measured. They do not give access to absolute temperature. If absolute temperature needs to be known, this is better achieved using a diode or a transistor-based circuit described above. One should also note that polysilicon/metal thermopiles are sandwiched in-between the stack of dielectric IC materials and the silicon below their measurement contact can be entirely etched away, providing optimal thermal isolation.

Flow Sensors Flow velocities of fluids, i. e., gases or liquids are conveniently measured using thermal methods. Generally, one of two principles is applied: (i) the heat transfer from a heated structure into the fluid or (ii) the heat transport from one area of a heated structure to another area, mediated by the fluid in motion. Case (i) is referred to as hot-wire anemometry, in view of macroscopic sensors based on a heated wire immersed in the flowing fluid. The heat power P transferred from wire to fluid is well approximated by P = ( c , c21/;)AT, where c, and v denote system-dependent constants and the fluid velocity, respectively, and AT is the teinperature increase of the hot wire above the reference temperature defined by the fluid. Principle (ii) has been referred to as hotfilm anemometry. In practice the sensor structure is heated centrally and temperatures are measured at upstream and downstream locations. An important advantage of hot film over hot wire anemometry is the fact that the strong ,,&nonlinearity does not appear in its response. An early micromachined flow sensor (now commercially available) implementing the hot film principle was reported by Honeywell [John87]. The device is schematically shown in Fig. 6.4-13. It consists of a pair of parallel, 500-pmlong diagonal silicon nitride bridges on a silicon substrate, fabricated by silicon front micromachining. As described in Section 6.2.3, the bridge is formed by the overlap of etch cavities resulting from the triangular openings in the silicon

+

Fig. 6.4-13 Schematic view of diagonal bridge based thermal flow sensor developed by Honeywell [John87]. Flow-induced temperature differences are measured thermoresistively.

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269

nitride to either side of the double-bridge and the slit between the bridges. The structure contains integrated thin-film metal resistors. The resistor winding around the central slit serves as the heater, while the other two are used as thermistors enabling the determination of the temperatures upstream and downstream. The output of the device is the flow-rate dependent difference of the thermistor resistances. Integrated into a Wheatstone bridge, the devices provide a flow-rate dependent voltage. In the commercial version, the device is encapsulated in a ceramic/plastic package with integrated flow channel and dust filter, if required. The thermal time constant of the device is 5 ms. Flow rates down to 1 cm3/min can be measured. Conversion of the strongly nonlinear response has to be performed externally using calibration data. Recently, CMOS technology and compatible micromachining were used to fabricate the highly integrated flow sensor microsystem shown in Fig. 6.4-14. The system is produced using a 2 pin double polysilicon double metal CMOS ASIC process of EM Microelectronic-Marin SA (EM). The chip comprises a membrane-based flow sensor, power management to heat the sensor, a two-stage amplifier to boost the output signal of the device into the mV range, and an AID converter [Maye97]. Electroplated Au structures for the packaging of the die are provided on-chip. The sensor consists of a silicon oxidehitride membrane fabricated by the CMOS-compatible micromachining method discussed in Section 6.2.2. It measures approximately 300 pm by 600 pm and contains a polysilicon heating resistor with 1.9 kL2 and two polysilicon/aluminum thermopiles. These provide the upstream and downstream temperature signals with respect to the common chip tem-

Fig. 6.4-14 Integrated CMOS flow sensor microsystem including thermoelectric membrane based flow sensor, power management and output amplification circuitry, and electroplated structures for the subsequent low-cost packaging (courtesy of the Physical Electronics Laboratory of ETH Zurich, Switzerland) [Maye97].

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6 Silicon Microsystem Technology

perature. Connected differentially, they provide an output signal proportional to the temperature difference of their respective hot contacts on the membrane. The membrane is composed of the entire sandwich of dielectric CMOS layers. The heater is operated at a constant voltage. The thermal output voltage is fed into the two amplifier stages and provided at the system output as a digital signal. Overall gains of 25 and 250, corresponding to resolution of 12 and 14 bits, can be selected externally. After CMOS fabrication and before micromachining, the chip undergoes the bumping process of EM. This normally serves to cover the aluminum bonding pad with ca. 25 -pm-thick electroplated Au bumps for tape-automated bonding (see Section 9.3.1). By using an unconventional layout of the bump mask, the sensor is surrounded after bumping by a rectangular Au frame, beside the usual contact bumps. This is used for the encapsulation of the system: the die is flip-chip mounted on a carrier, as schematically shown in Fig. 9.3-2. However, the carrier has a rectangular opening corresponding to the flow sensor. In this process the electroplated frame is solder-bonded to an identically shaped structure on the carrier. Simultaneously, the contact bumps are attached to the pads on the carrier. Since the sensor is now surrounded by a hermetic seal, the delicate circuitry is protected from potentially corrosive media flowing over the microsensor. An additional barrier is finally built in by filling the space between carrier and microsystem die using an epoxy underfill [Maye98]. A pen-sized prototype anemometer was fabricated based on this microsystem and state-of-the-art packaging materials: a flexible substrate with copper metallizations sandwiched in-between polyimide layers was used as the carrier. After the chip has been mounted on the flex and this has been soldered to a standard

Fig. 6.4-15 Flow sensor microsystem mounted on flexible substrate (left and middle)

and inserted into plastic housing with flow channel. Housing diameter is ca. 10 mm (courtesy of the Physical Electronics Laboratory of ETH Zurich, Switzerland) [Maye97].

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271

socket, the microsystem is introduced into a plastic housing with a flow-channel (Fig. 6.4-15). The sensor is tangential to the channel and measures the pressure difference between the inlet and outlet ports of the channel due to the external wind velocity. Wind speeds between 0.02 m s p J and 38 m s-’ have been measured by this “flow-pen”. Vacuum and Pressure Sensors Nonmechanical gas pressure sensors exploit the pressure-dependent thermal conductivity of gases. Traditionally known as Pirani gauges, such devices have recently been miniaturized (so-called micro-Pirani gauges) using silicon technology. An example is shown in Fig. 6.4-16 [Pau195a]. The sensor is fabricated using an industrial ASIC CMOS process followed by sacrificial aluminum etching (SALE), as shown in Fig. 6.3-6. It consists of a polygonal membrane clamped along its edges to the IC substrate. The membrane is composed of the intermetal isolation and passivation dielectrics with a meander made of the second CMOS metallization sandwiched in-between. The meander serves to heat the membrane. It has a resistance of ca. 50 The membrane is released by removing a polygonal sacrificial structure made of the lower CMOS metal. Access to the sacrificial layers is ensured by eight lateral openings in the dielectric sandwich. Such processing clears a gap with a width of roughly 0.65 pm, i.e., the thickness of the lower CMOS metal. During pressure measurements, the pressure of the gas filling the gap via the access holes is determined. When heated, the membrane experiences a temperature change AT given by AT = P/(G Ggas(p)),where P, G, and G,,(p) respectively denote the dissipated power, the thermal conductance of the materials between the heater and the substrate, and the pressure-dependent thermal conductance of the gas in the

+

Fig. 6.4-16 SEM micrograph of thermal CMOS pressure sensor fabricated using commercial CMOS technology and sacrificial aluminum etching (SALE)

[Pau195a].

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6 Silicon Microsystem Technology

micromachined gap. The dissipated power is easily determined by current and voltage measurements over the resistor, while the temperature change AT is accurately deduced using its temperature coefficient of resistance. In practice the heating resistor is compared to a gapless reference structure with resistor in direct contact with the substrate. This enables possible substrate temperature drifts to be compensated. Such pairs of structures were integrated into a microsystem including a bandgap reference, current sources for sensors and references, a differential amplifier, and two A/D converters for current and voltage measurements [Habe96]. The product of their output signals provides a measure of the pressure. The circuitry constitutes a feedback loop tracking the difference of sensor and reference resistors to be constant, and thus the temperature increase of the sensor above the die temperature to remain stable, by adjusting the pressure-dependent heating power. Whereas the membrane-based devices in Section 6.4.1 are absolute pressure sensors independent of gas composition, thermal pressure sensors measure the thermal conductivity of the gas within a gap. This parameter depends on the pressure, heat capacity, gap dimensions, and surface properties [Paul95b].

Fig. 6.4-17 CMOS pressure sensor microsystem containing four pressure sensors like that in Fig. 6.4- 16, four reference structures, two A/D converters, bandgap reference and current sources (courtesy of the Physical Electronics Laboratory of ETH Zurich, Switzerland) [Habe96].

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6.4.3 Devices and Systems for Radiant Signals Microtransducers for the detection, emission, or modulation of electromagnetic radiation are available in a large number of variants. They cover the energy spectrum from far infrared to X-rays. Low energy radiation is detected using cooled narrow-bandgap semiconductor materials. Detection of light using photoconductors and diodes is among the oldest applications of semiconductor materials. State-of-the-art high efficiency silicon solar cells make use of anisotropic silicon micromachining for improved light trapping. The ionizing power of high energy radiation is exploited in elementary particle detectors. Semiconductor light-einitting diodes and lasers have become ubiquitous. The following few examples illustrate the use of various micromachining techniques in such radiation handling devices. The examples address the three topics of uncooled infrared detectors, thermal scene simulators, and light modulators for image projection. Uncooled Infrared Detectors

A recent trend in micromachined detectors for infrared (IR) radiation is the fabrication of two-dimensional arrays. Such devices have applications, among others, in building control, smart intrusion and presence detection, spatially resolved radiation temperature monitoring, and fire detection. Radiation of thermal origin has wavelengths between 4 pm and 20 pm, i.e., energies between 60 meV and 300 meV. Since direct detection using narrow-bandgap semiconductor materials requires cryogenic cooling, a tandem detection principle is frequently implemented in uncooled microsystems: in a first step, the radiative energy is absorbed by a thermally isolated structure and causes the structure to heat up. In the second step the temperature change is quantified using thermoelectric conversion or thermoresistively. A recent thermoelectric array based on a commercial 1 pm CMOS process of EM Microelectronic-Marin SA serves as an example of this tandem approach [Pau198]. In view of its application in building control, cost considerations ruled out vacuum encapsulation and, consequently, surface micromachining in view of the narrow gaps and high thermal losses at ambient pressure associated with surface micromachining. The detector with its 10 by 10 pixels is entirely located on a single robust membrane. A schematic cross-section of the device is shown in Fig. 6.4-18. The membrane is composed of all dielectric CMOS layers from field oxide to passivation layer. It is fabricated using the CMOS-compatible 6-inch wafer KOH processing described in Section 6 [Munc97]. It is stiffened by a gold grid electroplated onto the CMOS-processed wafers using the standard bumping process of EM, similar to the packaging structures of the thermal gas flow sensors described above. The grid lines have a height and width of 25 pm and 80 pin, respectively. They are anchored to the bulk silicon supporting the sensor membrane. The gold lines subdivide the membrane into pixels with an effective radiation absorbing area of 250 pm square. Each pixel contains a thermopile composed of 12 polysilicon/aluminum thermocouples sandwiched

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6 Silicon Microsystem Technology

ElectroplatedAu

Pixel N

Pixel N+l

Fig. 6.4-18 Schematic cross-section of thermoelectric infrared detector array fabricated using commercial CMOS technology, Au electroplating, and CMOS-compatible KOH micromachining (courtesy of the Physical Electronics Laboratory of ETH Zurich, Switzerland) IMUnc971.

between the dielectric layers, with cold contacts below the gold lines and the hot contacts in the center of the pixel. Signal lines are integrated below the gold lines which simultaneously act as thermal separation lines between neighboring pixels. Output signals of the pixels are selected by a multiplexer beside the array and are forwarded to a highly sensitive low noise/low offset on-chip amplifier optimized for low frequency sub-microvolt sensor signals. The amplifier achieves a low frequency input noise power spectral density of 13 nV/1/Hz, 800 nV offset, gain of lo4 at a bandwidth of 600 Hz, and a common mode rejection ratio of 135 dB. The pixels have an average sensitivity of 4.1 V/W. Pixels and amplifier achieve a noise-equivalent temperature difference of 320 mK at a bandwidth of 10 Hz. Using a 12.7 mm diameter Fresnel lens with a focal length of 9.4 mm, and an average transmission of 53 %, the microsystem is able to thermally “see” its environment with a resolution of roughly 1 K at a rate of one frame per second and an angular resolution of 2”. Figure 6.4-19 shows the 6.2 mm by 5.3 mm large microsystem die. IR detector arrays with higher integration density are based on micro bolometers. These require temperature stabilization and vacuum encapsulation for reliable and effective operation. The basic element is shown in Fig. 6.4-20. It consist of an absorber plate with integrated thermistor suspended on two arms. In a microsystem reported by Honeywell, each pixel is fabricated above an SRAM cell on a CMOS substrate, the absorber material is a PECVD silicon nitride sandwich, and vanadium oxide serves as the temperature dependent resistor, with a temperature coefficient of -2 %K-’. The structures are fabricated using sacrificial layer micromachining, deposition and structuring of dielectric layers and finally sputtering and patterning of the thermistor material. Arrays with up to 240 by 336 pixels were demonstrated. They can be operated at a frame rate of 60 Hz [Cole98].

6.4 Micro Transducers and Systems Based on Silicon Technology

215

Fig. 6.4-19 CMOS infrared radiation imager with 10X 10 thermoelectric detector pixels, multiplexers, and on-chip amplifier system [Pau198].

Silicon substrate

Fig. 6.4-20 Schematic view of a micro bolometer for infrared radiation imaging (after [Cole981).

Thermal Scene Simulators Microlamps have potential use as single elements or linear arrays in miniaturized infrared spectrometers, and as two-dimensional arrays in infrared projection applications. The second application was successfully addressed by structures similar to the surface micromachined infrared detectors described in the previous subsection. The difference with the detectors is that the integrated resistor (a TIN meander) is used as a heating resistor rather than thermistor [Cole95]. In view of the excellent thermal isolation of lo7 K/W of the individual radiation pixels in vacuum, pixels have been operated at temperatures as high as 900 K. The array consists of 5 12 by 5 12 pixels.

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6 Silicon Microsystem Teclznology

Fully CMOS based CGait93, Swar931 thermal scene simulation devices functional without vacuum encapsulation were obtained by silicon front micromachining using EDP, as described in Section 6.2.2. By appropriate design layout of the field, contact, via, and pad masks and silicon bulk micromachining subsequent to completion of the CMOS process (through MOSIS), microbridges with integrated (gate) polysilicon resistors were fabricated. The pixels have a thermal resistance of 3.7X lo4 K/W and a thermal time constant of about 1 ms.

Light Valves Impressive arrays of light switches were developed by Texas Instruments (TI) and are being commercialized under the name digital micromirror device (DMD) as a component in professional projection systems and high performance displays [Kess98]. The arrays are composed of up to 1280 X 1024 torsional mirrors, each built over a CMOS SRAM cell fabricated using a 0.8 pm CMOS process of TI. As shown in Fig. 6.4-21, each mirror structure is composed of three levels. The bottom level consists of a pair of landing electrodes preventing electrostactic sticking between the deflected mirror and the substrate and a pair of actuation electrodes. The next level includes support posts for the torsional hinges and for address electrodes used for the electrostatic deflection of the device. A yoke bearing the third structural level is supported between the two hinges and includes two pairs of landing tips defining the maximum deflection angle of roughly f15" of the mirror. The third level consists of a vertical post and the mirror surface. Each mirror stack has horizontal dimensions of 16 pm by 16 pm and an estimated height of 6 pm in the undeflected state. During operation the mirrors are switched

Yoke and mirror ,4' address electrode

CMOS substrate with logic cell

-\

Landing electrode

Fig. 6.4-21 Exploded view of a surface micromachined micro mirror for digital projection applications. The device is integrated on top of an addressing logic cell. Under appropriate bias voltages it rotates around the torsional hinges into one of two stable tilted positions (off and on).

6.4 Micro Transducers and Systems Bused on Silicon Technology

211

appropriately between the two fully deflected positions corresponding to the on and off states. In the on-state, light from a localized source is reflected into the projection optics and further onto the screen, producing a light spot. In the offstate, the reflected light is projected into a light sink, so that the spot on the screen remains dark. The position of each mirror is updated fast enough to make it possible to display about 80 frames of three colors with an 8-bit brightness range per second. Deflection of a mirror is achieved by applying about 30 V between the mirror and the actuation electrodes. The mirror then rotates into either direction until it is stopped by the corresponding landing electrode. Which direction is chosen is defined by a small bias voltage applied to the appropriate address electrode. Once the mirror is deflected its position remains (bi)stable under the actuation voltage and preparation of the following on/off update of the biasing electrodes can proceed. Fabrication of the device is based on sacrificial polymer micromachining and aluminum-based metallizations. CMOS fabrication of the RAM array is followed by the deposition and patterning of the bottom electrode metal. Next a polymer layer is spun onto the wafer, planarizing the CMOS/electrode topography, and opened to define the first level of posts. Hinges, supporting posts and the yoke are then formed by a two-level metallization scheme resulting in 60 nm thick hinges and more rugged structural components. A second level of polymer is then spun on and structured, defining the upper support posts for the mirror. A final metallization with mirror quality and subsequent ashing of the polymer in oxygen plasma completes the process. For a description of testing and encapsulation, the reader is referred to the pertinent literature [Kess98]. Deformable grating light modulators are simpler structures also able to switch light on or off for projection applications [Bloo97]. The principle is shown in Fig. 6.4-22. An arrangement of parallel thin beams is the central component of the device. The beams are made of LPCVD silicon nitride coated with aluminum

Fig. 6.4-22 Surface micromachined reflective/diffractive digital micromirror for projection application.

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for optimum reflection. They are clamped at their two ends, released using surface micromachining, and can be electrostatically attracted onto the substrate surfaces. In their equilibrium position, the upper surface of the beams is at a distance of h/2 above the reflecting substrate surface. The entire arrangement then acts as a mirror, since partial waves from beams and interstitial areas interfere constructively. In the deflected state, the distance is reduced to h/4 so that the beams act as a phase grating, diffracting an incoming light wave. Appropriate choice of the grating period allows projection of different colors from a white incident source into the projection optics. Pixels with sizes down to 25 pm by 25 pm were demonstrated. A third approach - among several others - is pursued with the torsional micromirror system shown in Fig. 6.4-23 [Kran98]. They are elastically supported on torsional silicon bars and are individually deflected from their equilibrium position by electrostatic forces. In the present arrangement, the mirrors have a size of 3 X3 mm2 and a maximum deflection angle of & 18". The mirrors are fabricated in bulk silicon and coated with aluminum for improved reflection. They are deflected electrostatically. Considering the high creep resistance of monocrystalline silicon, long life-times can be expected of such devices.

Fig. 6.4-23 Photo of a torsional mirror array. Individual pixels are electrostatically deflected from their equilibrium position (courtesy of the Zentrum fur Microtechniken (ZfM)of the Technical University of Chemnitz-Zwickau,

Germany).

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6.4.4 Magnetic Devices and Systems Silicon-based magnetic sensors use a range of galvanomagnetic effects, the best known of which is the Hall effect, discovered in 1879. In a sample carrying a current, the Hall effect produces an electrical field perpendicular to both the applied magnetic induction and the current flow direction. The field can be directly measured, e. g., in an integrated Hall plate of classical geometry fabricated using bipolar or CMOS-like technologies, or indirectly, via various carrier deflection, concentration, or modulation effects in split-electrode devices, magnetic FETs, horizontal and vertical magnetotransistors, and current domain devices, among others. Excellent descriptions of these more advanced devices are found in [Popo91, Balt941. In view of its low carrier mobility, silicon is not the optimal material for magnetic sensors. Nevertheless, the availability of commercial fabrication technologies weighs heavily in its favor when less demanding applications are addressed. Magnetic semiconductor sensors show offsets and sensitivity variations due to temperature and stress (from packaging and thermomechanical effects). If silicon is used, such effects can at least partially be compensated by on-chip circuitry. Companies including Honeywell, Siemens, and Texas Instruments produce and commercialize Hall devices with on-chip amplification and stabilization circuitry. Depending on their implementation, Hall sensors are able to measure magnetic fields perpendicular to the chip surface or in-plane. A conventional Hall plate fabricated using bipolar technology is schematically shown in Fig. 6.4-24. It consists of a laterally isolated n-doped epilayer on a p-substrate, with four n+-contacts obtained by the emitter/collector contact diffusion of the IC process. These contacts are the two current contacts for the injection and the extraction of a current Z, respectively, into and out of the plate and two side contacts for the measurement of the Hall voltage proportional to the perpendicular magnetic induction. In the absence of an external magnetic field, the current flow in the plate is symmetric and equipotential surfaces are symmetric with respect to the longitudinal symmetry axis of the device. Due to the Lorentz force acting on the carriers, a magnetic induction B tilts the equipotential lines by the Hall angle 8,. Consequently a Hall

p+-lsoiation

Fig. 6.4-24 Conventional Hall plate in epilayer technology with p+ isolation well. The device measures the out-of-plane component of the magnetic induction.

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6 Silicon Microsystem Technology

Top Hall c o n t a c i

i”l

,,,,,!Bottom Hall contact

Fig. 6.4-25 Vertical Hall plate in CMOS technology combined with deep trench etching. An in-plane component of the magnetic induction is detected

[Stei99]. voltage VH= GtBr,/qnt appears, where G is a factor depending on the geometry of the plate (G < l), and r,, n, and t denote the Hall factor (empirically, r, = 1.15 in weakly n-doped silicon), the carrier density in the plate and the plate thickness, respectively [Balt94]. An interesting vertical Hall plate for the detection of an in-plane magnetic induction component has been reported recently [Stei99]. It uses state-of-the-art IC trench etching to define a vertical high aspect ratio silicon plate delimited by two parallel trenches. As schematically shown in Fig. 6.4-25, five contacts are defined on the chip surface, three of which are on the plate and two on its sides. Current is forced through the plate using the two outer plate contacts. The external contacts probe the voltage at the bottom of the plate, while the middle contact measures the voltage on the top face. At B = 0 T, the locations of middle and external contacts coincide with the same equipotential surface. In a non-vanishing magnetic induction perpendicular to the plate, equipotential surfaces are tilted and a Hall voltage V, appears between middle and external contacts.

6.4.5 Chemical Microsensors Detection of chemical species in gases or liquids has applications in process control in the chemical or food processing industry, in environmental control, and in clinical diagnostics, among others. Frequently, the first step of the detection process is the adsorption of the chemical compound on the surface of the device, or the absorption into its volume. Several sensor effects enable the quantification of the sorbed species. A resulting mass change can be measured as a change in the resonant frequency of the device. Reaction enthalpies may lead to a temperature change measurable directly on thermally well isolated microstructures or indirectly via thermomechanical effects, e. g., using the bimorph effect or resonant shifts in bilayered or multilayered cantilevers [Lang99]. A more direct transduction of a chemical concentration into an electrical signal is achieved using modified field effect transistors. These devices are summarized

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under the term CHEMFET (chemical FET) and belong to the earliest chemical microsensors. Depending on their specific application for the detection of ions in solution, or gaseous compounds, they also have names such as ISFET (ion sensitive FET), GASFET (gas sensitive FET), and ADFET (adsorption FET). The main use of ISFETs is pH determination [Berg90]. The usual gate conductor of the FET is replaced by the liquid containing the ions to be detected. As shown in Fig. 6.4-26, the device is thus reduced to a semiconductor/dielectric/ liquid sandwich. Silicon oxide covered by the chemically more robust silicon nitride and metal oxides have been used as gate dielectric material. When the device is exposed to a liquid, ions adsorb on the gate dielectric and form the Helmholtz double layer (see Chapter 3) with corresponding potential difference y E In alkaline solutions, dominantly OH- ions are adsorbed, while protons adsorb preferentially from acids. To define the potential of the liquid, the source or substrate of the ISFET is biased with respect to a reference electrode immersed in the solution with well-defined potential with respect to the solution. By varying the bias voltage, the channel of the ISFET can be turned on or off, like in a conventional FET. The main difference is the double layer potential. The bias potential required to yH(pH) VRef,where VRefdescribes switch on the channel is V h H ) = V,, the reference/liquid interface, and V , , is the thrdshold voltage in the absence of a double layer. The pH-dependent potential y H depends on pH through a relation of the form y H= (CkT/q) X (pH-pH,), where C characterizes the chemical activity of the chemical species on the gate surface and q and pH, denote the elementary charge and the p H leading to no double layer, respectively. For SO,, C = 2.303. In principle the threshold voltage of the FET therefore linearly follows pH. ISFETs still present several technological challenges. First, it is difficult to produce gate dielectrics that simultaneously are a reliable barrier to the species in solution and show the required surface activity without being dissolved by the solutions. Second, the reduction of macroscopic electrode concepts to microscopic dimensions and their desirable integration on an ISFET chip is still difficult. As

+

+

Reference electrode

Solution

t\

Silicon substrate

Source

Gate oxide

Fig. 6.4-26 Schematic cross-section of an ISFET.

Metal interconnect Drain

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6 Silicon Microsystem Technology

Source

Drain Silicon substrate

Gate oxide

Fig. 6.4-27 Schematic cross-setion of a GASFET.

a result of the imperfect solutions to these and other problems, ISFETs tend to drift and thus require periodic recalibration. The principal example of a GASFET is the Pd-gate H2 sensor [Lund75]. Its structure is more similar to the conventional FET than the ISFET and is schematically shown in Fig. 6.4-27. Instead of polysilicon, Pd is used as a gate material on silicon oxide. Palladium absorbs large quantities of hydrogen. When the device is exposed to an atmosphere containing H2, hydrogen atoms diffuse to the gate oxide/Pd interface and form a dipole layer again causing a threshold voltage shift AV,. Similar devices sensitive to CO [Dobo85] an CC14 [Lund81] have been demonstrated. Some of these sensors have gates with pores for improved access of the species to be detected to the gate/dielectric interface. A gate-less variant of the ISFET, with sufficiently thin gate dielectric ( 5 nm) is operated in air. Polar molecules absorbed on the gate surface, cause an electric field to penetrate into the channel region of the FET. This again results in a threshold voltage shift. Such devices are referred to as ADFETs. Sensitivity to NO, NO2, SO2, and HCl among others was demonstrated [Cox74]. Like ISFETs, GASFETs and ADFETs suffer from drift. Long-term reliability is also a concern. Other aspects to be taken into account are the cross-sensitivity of sorption-based chemical sensors to other species than that to be detected. A tradeoff in many devices is that specific sensitivity is often obtained only at the expense of a slow response.

Microfluidic Components and Systems A modem trend in microfluidic systems is to combine valves, pumps, flow sensors, fluid injectors, mixers, cells for optical, chemical, electrical, or electrochemical analysis, and other microcomponents into compact, possibly handheld instruments. The resulting systems have been referred to as micro total analysis systems (pTAS) and labs-on-a-chip. Examples are blood analysis systems, chemical screening in miniaturized immuno-assays, and automatized DNA sequencers, among others. Silicon micromachining has started to play a role in the development of some of their components. Considering the ambitious goal of pTAS, however, numerous other techniques in addition to silicon micromachining are needed to make these systems possible.

6.4 Micro Transducers and Systems Based on Silicon Technology Actuator membrane

Valve seat I

\

\\

Outlet

Fig. 6.4-28

283

Aluminum

Inlet

1

Silicon

Base plate

Schematic cross-section of a microvalve with bimorph actuation. By heating the bossed silicon actuator membrane, the boss is lifted from the valve seat, opening the valve (courtesy of of the Institut fur Mikro- und Informationstechnik HSG-IMIT, Villingen-Schwenningen, Germany).

Fig. 6.4 -28 illustrates the principle of a microvalve constructed using two structured silicon chips mounted on a base plate. The upper silicon level includes the actuator consisting of a bossed silicon membrane fabricated by anisotropic silicon etching. A metal film with thermal expansion coefficient differing from that of silicon is deposited on the actuator. The lower silicon level is anisotropically structured from both sides and acts as the valve seat. The valve is opened by heating the actuator membrane and thereby exloiting the bimorph effect in the silicon/

Fig. 6.4-29

Cross-section of the valve schematically shown in Fig. 6.4-28. All components including the A1 layer, bossed membrane, and valve seat are clearly visible (courtesy of the Institut fur Mikro- und Informationstechnik HSG-IMIT, Villingen-Schwenningen, Germany).

6 Silicon Microsystem Technology

284

Heating resistor

,/+

XI

\

Pyrex Silicon Pyrex

/

Actuation liquid

'

Diaphragm \

Valve seat

Fig. 6.4-30 Exploded schematic view of thermopneumatic valve fabricated from two Pyrex levels and one anisotropically etched silicon level. Upon heating, the actuation liquid expands, forcing the silicon membrane down onto the valve seat [Zdeb94).

metal sandwich. Fig. 6.4-29 shows a cross-section of the fabricated device. Gases and liquids are switched on and off reliably, at differential pressures up to 1 bar. A second valve actuation principle exploits the thermopneumatic effects provided by a phase change in a trapped liquid. The principle is shown in Fig. 6.4-30. Such devices have been realized [Zdeb94] in an anodically bonded Pyrex/silicon/Pyrex sandwich, with an anisotropically micromachined silicon level. A liquid is enclosed in the volume defined by the resulting cavity and the upper Pyrex level. Power dissipation by a heating resistor locally vaporizes the liquid. This increases the pressure in the cavity and leads to an expansion of the micromachined silicon membrane towards the valve seat in the lower Pyrex level. The fluidic connection between inlet and outlet is thus interrupted and the Electrostatic drive.

Pneumatic chamber

Inlet valve

Substrate

Fig. 6.4-31 Schematic cross-section of an electrostatically driven bidirectional micropump [Zeng95]. At lower drive frequencies, fluid is pumped from left to right, as shown; at high frequencies, fluid flow is reversed.

6.4 Micro Transducers and Systems Based on Silicon Technology

285

valve is closed. Redwood Microsystems Inc. in Menlo Park, CA, develops these devices under the name “fluistor”. A further important component of fluidic microsystems are micropumps. It is not surprising that the most successful micromachined silicon based devices are membrane pumps rather than friction-plagued rotary structures. An example is shown in Fig. 6.4-31. The device is electrostatically driven and is composed of four anisotropically micromachined silicon levels. The two lower levels form elastic valves for the suction of liquid into the pump chamber through one valve and its subsequent ejection through the second valve. Suction-ejection cycles are driven by a time-dependent voltage between levels three and four, which leads to the periodic contraction and expansion of the pump chamber. It is interesting to note that the pump is bidirectional [Zeng95]. At low driving frequencies it operates as expected, with liquid sucked in by the left valve. In contrast, at frequencies higher than about 3 kHz, the operation of the pump is reversed, due to a phase shift between the membrane and valve motions, caused by the inertia of fluid and micropump components [Ulri96].

6.4.6 Micromachined Devices for Electrical Signal Processing The fabrication of components with purely electronic function has also benefitted from micromachining. Three examples dealing respectively with the filtering of electrical signals, high-Q inductors for millimeter-wave circuits and the separation of cointegrated analog and digital circuit components by silicon bulk micromachining are briefly described in the following. A micromechanical electronic filter is shown in Fig. 6.4-32. Similar in structure to comb-drive accelerometers, it can be produced using, e. g., polysilicon surface micromachining or deep reactive ion etching of monocrystalline silicon [Nguy98]. If a signal is applied to the lower pair of interdigitated electrodes, the central part Flexural suspension

Resonator 0

Al

Fig. 6.4-32 Microelectromechanical electronic filter based on two electrostatic comb structures with a movable resonator mass (after [Nguy93]).

286

6 Silicon Microsystem Technology

including shuttle, folded arms and the mobile halves of the interdigitated electrodes is excited into resonance by signal components predominantly in the interval f R i fRIQ,where f ,and Q denote the fundamental frequency of the resonator and its quality factor, respectively. Finally, the upper pair of interdigitated electrodes translates the motion into an electrical ac-signal corresponding to the mechanically selected narrow interval of the input signal. Resonance frequencies between 20 kHz and 8.5 MHz and quality factors higher than 8 X lo4 in vacuum have been achieved. Higher-order filters have been demonstrated [Wang97]. Current efforts aim at extending the applicability of such devices into the GHz range. Using micromachining techniques it is also possible to reduce the loss in integrated inductors. Such passive components are highly desirable for low-cost GHz circuits. When these inductors are realized on-chip in standard CMOS technology as integrated coils made of the available metal layers, eddy currents in the silicon substrate materials degrade the Q factor of integrated inductor based resonators to unacceptably low values. A straightforward solution is to locally remove the silicon below the inductor by CMOS compatible silicon micromachining. The inductor then consists for instance of a coil-like structure made of the CMOS metal layers sandwiched between the CMOS dielectrics. The coil is laterally suspended over a micromachined cavity if silicon front micromachining is used. If etched from the rear, the inductor may be integrated into a laterally clamped CMOS dielectric membrane. Quality factor values of 20 have been achieved in vacuum, at oscillator frequencies in the GHz range [Mila97]. Finally, the undesirable cross-talk between digital circuitry and on-chip analog components through the common substrate can be eliminated by the approach shown in Fig. 6.4-33 [Base95]. By appropriate design layout, analogue components are gathered in the center of the die and are surrounded by digital circuitry. Using post-processing rear bulk silicon micromachining, a rectangular trench is removed from the silicon die. All analogue components then sit on an isolated silicon island, electrically connected to the surrounding digital mainland by signal and power lines. Signal coupling through the substrate silicon is thus suppressed CBase95, Mii11981.

Analog circuitry

Digital circuitry

linesa

Siiicon

‘Trench

Fig. 6.4-33 Use of bulk silicon micromachining for the suppression of digital crosstalk to analogue circuit components integrated on the central mesa struc-

ture.

6.5 Summary and Outlook

6.5

281

Summary and Outlook

Silicon based micromachining offers breathtaking opportunities. Nevertheless it has its limits. In particular the use of available, e. g., commercial IC technologies leads to severe restrictions. In this case, materials are predefined in composition, geometry and properties, pushing the microsystem designer to become creative in inventing novel, meaningful two-dimensional mask layouts. The absence of highly efficient actuation effects, of piezoelectric, pyroelectric and ferromagnetic effects, and of a decent optical activity requires the often costly integration of nonstandard thin films or nonsilicon components. In addition, the original driving force of micromachining, i. e., the benefit of continuing miniaturization has lost some of its intensity and miniaturization is no longer the primary goal of silicon-based microsystem technology. As the example of electrostatic micromotors has shown, the scaling behavior of certain effects with miniaturization can be disadvantageous to the scaled down system. Yet, the prospects of silicon based microsystems are bright. Indeed, they have already conquered a strong position in several transducer markets including pressure sensor, accelerometer, and inkjet printer applications, and more recently digital light projection. These examples address mass markets. Further stories of success can be expected in the future. The convenient cointegration in silicon technology of microstructures with dedicated signal amplifying and conditioning circuitry is a strong argument in favor of silicon based microsystem fabrication. Complex systems including large microtransducer arrays or large collections of sensors are likely to continue being realized in silicon technology. Sensors and actuators with spatial resolution and electronic noses, papillae, cochleae, retinae, and skin are possible candidates for such future highly integrated silicon based microsystems.

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The LIGA Process

7.1 Overview The LIGA process is related to some technologies used in microelectronics fabrication. However, it is technologically very different compared to silicon microtechnology, which will become clearer to the reader in the following description of the process. The developments of the LIGA process began at the end of the 70’s at the Nuclear Research Center, Karlsruhe (KfK), within the development of the so-called nozzle separation process of uranium isotopes [Beck82, Beck861 in order to be able to produce extremely small, and specially designed slotted separation nozzles inexpensively. The application of the process is not only limited to this use but the process is suitable for the production of other microstructures (see Section 7.7) for numerous applications, like measurement and regulation, communication- and/or automobile- and medicine-technology [Ehrf87]. The production of a microstructure by the LIGA process is schematically represented in Fig. 7.1-1. The essential process steps are X-ray lithography with synchrotron radiation, the electroplating of metals and the molding of plastics. This combination of process steps have been given the name LIGA; LI for X-ray lithography, G for galvanic or electroplating and A for Abformung (German word for molding). In the first step of X-ray lithography, a plastic layer several hundreds of micrometers thick is applied to a metallic base plate or an isolated plate with an electrically conductive cover layer, used as the substrate. The X-ray sensitive plastic is either polymerized in place directly on the base plate or glued to it. Up until now PMMA (polymethyl methacrylate) is used almost exclusively as the X-ray resist because of its high contrast known from electron beam lithography. However, because of its low sensitivity chemically strengthened X-ray sensitive negative resists are also used [Sche96]. To form the micro structure an absorber pattern of a mask is transferred into the thick plastic layer with the aid of extremely parallel and high intensive synchrotron radiation with a characteristic wavelength between 0.2 and 0.6 nm.

290

7 The LIGA Process

a lithography

"

' 1111111 1

~ ~X-Rays ~~~~~~~ Absorber (Au)

Irradiation

Maskmembrane Resist

\ Metallic

Development of the Resist

Substrate \ Microstructure Made of Plastic

b Electroformin Metal Deposition

Microstructure Made of Metal

Stripping of the Unirradiated Resist

C

Molding Process

Mold Insert (Metal) Injection Hole

Mold Filling

Molding Mass Gate Plate (Metal)

Demolding Process

d Second Electroformina

I

Gate Plate= Electrode Plastic Metal

Fig. 7.1-1 The basic process steps of the LIGA technology.

7.2 Mask Production

291

The X-ray radiation which passes through the mask is absorbed in the resist and leads to a chemical modification. In the case of PMMA the chemical resistance changes because of bond breaking in the long molecular chain, so that these regions can be dissolved with a suitable developer. Using micro-electroplating a complementary structure can be formed from the resultant resist structure after the development process. The metal e.g. copper, nickel or gold is deposited in the void spaces of the electrically non-conducting resists where the deposition of the metal starts on the electrically conducting base plate. Using these metal templates in injection molding, reaction resin casting or in hot embossing, almost any number of highly detailed plastic copies can be fabricated at relatively low costs [Noke92]. These plastic structures can again be filled by electrodeposition with metals or serve as ‘lost forms’ for the production of ceramic microstructures. In the following chapters the individual steps of the LIGA process will be described in detail. The first process step, X-ray lithography, which defines the structure quality for the following steps and thus, represents the most critical process step, puts especially high requirements on the necessary X-ray masks. So first the important steps of the production of suitable masks are discussed, before X-ray lithography, electroplating and molding techniques are dealt with.

7.2 Mask Production 7.2.1 The Principle Construction of a Mask A mask which can be used in the LIGA process consists of an absorber, the carrier foil and the frame of the mask [Bach91]. In contrast to masks for optical lithography as used in microelectronic fabrication, the required absorbing characteristics are much higher while the thickness of the carrier foil is much lower. This requires a different mask fabrication process. Absorber The information which is to be transferred into a thick resist, is given by the structure of the absorber, which is to shield certain parts of the resist from synchrotron radiation. Whilst in optical lithography using UV light, an approximately 0.1 pm thick chromium layer on the mask is already sufficient, the absorber in X-ray lithography must consist of a material with a high absorbency for X-ray radiation in the particular wavelength region of interest. Materials are considered which have a high atomic weight and therefore a high absorption coefficient, such as gold, tantalum or tungsten. Mostly Gold is used due to its ability to be deposited by electroplating. Tantalum or tungsten find reIatively few applications and are structured by reactive ion etch processes.

292

7 The LIGA Process

The retention capability for X-ray radiation does not only depend on the absorption coefficient uAu(l), but also on the thickness dAuof the gold layer. The transmission of a layer is given by:

TAu(d)= e - a a a ( A ) ' d A u

(7.1)

To achieve a low X-ray transmission, that is, to maximize the absorption of the appropriate synchrotron radiation necessary for structuring in the LIGA process, the gold absorber must have a thickness of more than 10 pm (see Section 7.3.4). The required thickness depends not only on the characteristic wavelength of the synchrotron radiation, but also on the the thickness of the resist to be irradiated. The absorber thickness increases with decreasing wavelength and increasing resist height. In the case of gold absorber the structure is built up by micro-electroplating. Thus, a resist layer is structured, whose thickness is somewhat larger than the absorber thickness and subsequently gold is electro-deposited in the developed voids. With the exception of structuring with synchrotron radiation itself, there is no process at the moment which guarantees either the required precision or the structuring of this resist layer with a height of more than 10 pm and a precision in the sub-micrometer region. Therefore, initially an X-ray mask with an absorber thickness of about 3 pm has to be produced, a so-called intermediate musk. There are several processes for precise structuring of the resist layer of this height which will be described in more detail in the following. Using synchrotron radiation, the pattern of the intermediate mask is transferred into a resist layer of approximately 20 pm thickness, which after development serves as the mold for gold electrodeposition. The mask produced in this way which has sufficiently large absorber thickness, is called a process musk. The use of synchrotron radiation does not display any noticeable structure deterioration in this copy step due to the highly parallel beam and the small wavelengths.

Carrier Foil The absorber structures are fabricated on a suitable carrier [Scho91]. In optical lithography approximately 2 mm thick polished glass- or quartz plates are used. This material cannot be used in X-ray lithography since at such thickness glass plates absorb almost entirely the incoming x-radiation. The carrier foil must have a low absorption coefficient and low thickness, in order to absorb less of the X-ray radiation (see Eq. 7.1). Therefore, materials with a low atomic weight like e. g. beryllium, carbon (diamond), silicon and its compounds, plastics or metals with a lower atomic number are chosen for membrane materials. On selecting materials an optimum must be found between mechanical rigidity, dimensional stability and transparency to the synchrotron radiation. Furthermore, the carrier material must be resistant to X-ray radiation. This limits the use of plastics. Previously in the LIGA process thin metal foils have been used as the carrier material. Only by using these materials was it possible to obtain low stress of free-standing membranes over a large area (e.g. 25 mm X 65 mm). Among the metals, beryllium shows an ideal transmission behaviour (Fig. 7.2-1). With comparably thick

7.2 Mask Production

293

100

80 CI

<

e 60

.-

.v)

0

40

E

U

20

0

f

Fig. 7.2-1 Transmission of different materials for mask carriers (synchrotron radiation A, = 0.556 nm).

carrier layers of several 100 pm, transparency is still high in the useful wavelength region. For reasons of high toxicity (dust from Be and its oxides can cause lung diseases) the handling of Be layers represent a problem in laboratories which are not especially equipped for such work. Nevertheless, in recent years Be gained in popularity because it allows a decrease in distortions generated by thermal load during X-ray exposure. Titanium is an alternative to beryllium as a carrier layer. Because of the high absorption coefficients of titanium compared to beryllium, the membrane thickness must be considerably smaller and should not exceed several micrometers. The thin carrier foils are stretched across a rigid frame to provide the necessary mechanical robustness for the electroplated absorbers. This allows easier handling of the mask during fabrication and alignment. The mask production in the LIGA process consists of the following process steps: 0 0 0 0

manufacture of the carrier foil, structuring of the resist layer of an intermediate mask, electroplating of gold for the absorber structure, copying the intermediate mask onto the process mask and electroplating once more with gold.

294

7 The LIGA Process

7.2.2 Production of the Carrier Foil The process steps for the production of an intermediate mask are schematically represented in Fig. 7.2-2. The carrier is a membrane which is freely suspended across a frame. In the case of metals it is typically produced by PVD processes onto a solid substrate. Matured processes from the semiconductor technology are used to fabricate the membrane from silicon and its compounds. After deposition of the carrier film a window of a desired size is etched into the substrate. In the case of a titanium mask, the frame is made from invar (alloy of 18 % Cu, 28 % Ni and 54% Fe) which best matches to the thermal requirements. In the case of silicon a Pyrex ring, onto which the wafer with its mask window is fastened. is used as a frame. sputtering of Titanium etching of substrate spincoating resist

11 11 1 I I

patterning by e-beam writer

development

electroforming of gold

removal of resist Fig. 7.2-2 Process steps for an intermediate mask with shallow absorbing structures (< 3 Pm>

7.2 Mask Production

295

A process has been especially developed for the production of X-ray intermediate masks with titanium membranes, which avoids the handling of solid invar carriers with cost and time intensive, mechanical handling steps (milling, lapping, polishing). These steps are otherwise necessary to produce surfaces with a roughness R,,, of less than 100 nm to avoid pinholes in the membrane. This alternative process uses a silicon wafer with its high surface quality as an intermediate support for the membrane. First a layer of carbon is applied to the silicon wafer, leaving the outer rim of the wafer uncovered. Substrate temperature and other coating parameters are adjusted such, that a low adhesive layer is achieved. A titanium layer of 2-3 pm thickness is sputtered onto this carbon layer. This layer exhibits good adhesion at the outer most periphery of the wafer, whilst on the carbon layer adhesion is low. Next a solid frame is glued to the inner part of the Ti-layer. After curing the frame together with the Ti-carrier foil can be lifted off the support by gently bending the Si wafer. By this technique a carrier foil is obtained having an excellent surface finish.

7.2.3 Structuring of the Resist for X-ray Intermediate Masks In the case of an intermediate mask a resist of about 3-4 pm thick is spun onto a free suspended membrane. The resist is structured e. g. with the aid of an electron beam writer or by optical lithography or by reactive ion etching. After developing, the voids are filled with gold by electroforming. Finally the non-irradiated resist is removed by a strong solvent or by applying an oxygen plasma. On copying the X-ray intermediate mask (gold absorber thickness < 3 pm) to the X-ray process mask (gold absorber thickness >10 pm) no quality deterioration is noticeable due to the advantageous properties of the synchrotron radiation (see Section 7.3.5). The quality of the microstructure is therefore largely determined by the quality of the intermediate mask. Therefore on the intermediate mask, the absorber structure should have the most acute edges possible, to produce immaculate working masks. Depending on the requirements which the microstructures must fulfil, different processes can be utilized for the production of intermediate masks. Optical Lithography For structures with reduced requirements with respect to precision and miniaturization, the X-ray intermediate mask can be produced by an optical copy of a conventional chromium mask in an approximately 3 pm thick photosensitive coating. The requirements for the perpendicular absorber walls can be attained by optimizing the baking process and the irradiating- and development conditions, so that structures with an angle of slope of about 88" are possible [Schu96]. The minimum achievable lateral dimensions of the structure in the photosensitive coating are about 2 pm because of the unavoidable effect of diffraction in optical lithography. In addition a certain rounding of sharp edges is inevitable. The structural loss

296

7 The LIGA Process

(decrease of structural width of about 0.5 pm), which appears with the optical copying, is compensated by an increased structure size on the chromium mask. Direct Electron Beam Lithography

An electron beam writer with high acceleration voltage (e. g. 100 keV) can be used for direct structuring of an approximately 3 pm thick resist layer [Hein92]. The 100 keV acceleration voltage keeps the necessary steepness of the edges because the electron lobe which results from scattered electrons in the material, takes place primarily in the substrate. PMMA is used as the resist especially for high precision requirements. PMMA shows a very high resolution in electron beam lithography, however one of its disadvantages is a low sensitivity which results in long writing times. The 3 pm thick PMMA layers which are required for the intermediate masks are produced by double coating with a spin coater with a very homogeneous thickness. A temperature annealing step is carried out after every coating step to reduce the susceptibility to stress cracking. In case of less demanding precision also negative resist materials based on the diazo systems are available, which represents a compromise between resolution and writing time. In order to attain precision in the submicrometer region, the structures are subdivided into a fine region, near the border periphery, and directly connected to it, a coarser inner region. The fine region, which has a width of 1 pm, is illuminated with a small beam diameter (e. g. 0.02-0.5 pm). For the coarse region a considerably larger beam diameter (up to 0.5 pm) is sufficient. Thus, the border can be structured with high precision and at the same time the total writing time is considerably reduced, as the writing time per surface unit for the coarse region is about a factor of 50 smaller than it is for the fine region. A graded distribution in regions allows a gradation of the surface dosage, so that a simple correction of the proximity effect (see Section 5.5.6) is possible, in which the edge region is written with a higher dosage than the inner region. The limits of electron beam lithography in a 3 pm thick resist result in the smallest line and space structures in the range of about l pm, because of the proximity effect. Nevertheless, the measurement limitations of the detailed structure are between 0.1 pm and 0.2 pm. Reactive Ion Etching X-ray intermediate masks can also be produced for the LIGA process in a so called tri-level process (see Section 5.6.4) by structuring a layer system from photoresist, titanium and polyimide, whose lateral dimensions also lie below 1 pni. The structuring of the three layers is carried out by optical lithography, sputter etching with an argon plasma and reactive ion etching with oxygen. First a 3-4 pm thick polyimide layer is brought onto a titanium foil, which after its structuring serves as the electroplating template for the gold absorber. The structuring of this polyimide layer is carried out by reactive ion etching in oxygen. A thin titanium layer is used as the etch mask, which is applied onto the polyimide by magnetron sputtering.

7.2 Mask Production

297

For the structuring of the polyimide layer, the operation parameters of the oxygen plasma must be chosen such, that a very high selectivity of the etch rate between titanium and polyimide is obtained. Using the right operation parameters titanium is removed about 300 times slower than polyimide, which allows the thickness of the titanium layer to be very thin, e.g. 10-15 nm. The titanium layer is structured by sputtering in an argon plasma, using a photosensitive layer as the mask. In general, using this structuring process, plastic layers are much faster removed than metal layers. However, under the optimum operation parameters, the photosensitive layer is removed only 2-3 times faster than titanium. As the titanium layer is chosen to be very thin, the thickness of the photosensitive layer is also in a range of 100 nm. In such layer structures, very small lateral dimensions can be precisely reproduced even by photolithography. If structures with very small lateral dimensions are to be produced, the structuring of this uppermost layer is carried out directly with an electron beam writer.

Comparison of Structuring Methods on the Production of Intermediate Masks Figure 7.2-3 shows the comparison of three resist structures of an intermediate mask, which were produced using the three above described processes. With the structure produced by optical lithography, rounding caused by the diffraction effect, can be clearly seen. The three production processes of the intermediate mask are compared with each other in Table 7.1.

Table 7.2-1 Comparison of different processes to produce X-ray intermediate masks Process

Smallest line width

[WI

Structure detail [am1

Energy expenditure

~~~

optical lithography direct electron beam lithography reactive ion etching

1.5 to 2.5 0.8 to 1

-1 0.1

0.3 to 0.4

0.1

small average (100 keV E-beam necessary) energy intense process

298

7 The LIGA Process

a

b

C

Fig. 7.2-3 Resist structures for intermediate masks, fabricated by a) optical contact printing, b) electron beam lithography, c) reactive ion etching. The resist height in all three cases is about 3 pm, the smallest lateral dimensions is (a) 2.5 pm, (b) 1.0 pm, and (c) 0.3 pm.

7.2.4 Electroplating with Gold for X-ray Masks The electroplating of microstructures will be dealt with in detail later in this chapter as an essential process step of the LIGA process (see Section 7.3). As for the fabrication of both intermediate- and process-masks, an electroplating process for microstructures is required [Mane88]. The essential requirements of electroplating with gold for the absorber will be summarized in this section:

7.2 Mask Production

299

The deposition should be carried out with the least amount of stress, so that no mask distortions are induced and to prevent the stripping of large absorber regions. The homogeneity of thickness of the deposited layer must, both in the microscopic as well as in the macroscopic region, show a high uniformity, resulting in a uniform and maximum X-ray contrast. Dimensions of the resist structure even in the submicrometer range must be replicated by electroplating. The absorber structure must adhere well to the carrier material, as the cross sectional area of the absorber structure often amounts to only a few square micrometers.

As titanium belongs to the group of materials which are difficult to cover by electrodeposition, either suitable intermediate layers or a pre-treatment is necessary. For example, with titanium foils, a wet chemical oxidation of the surface has proven to be a suitable method [Mohr88]. It does improve not only the adhesion of the resist, but also the adhesion of the absorber structure. In order to ensure a uniform initiation of the electroplating, a gold layer of about 10 nm thickness is sputtered onto the titanium oxide layer. For electroplating in the LIGA process, cyanide electrolytes are used for the deposition of gold absorber structures. However, the cyanide electrolytes do have some disadvantages; above all the toxicity, small attack of the resist and rough surfaces. By using suitable sulfite gold electrolytes, fine-grained depositions with smoother surfaces can be achieved.

7.2.5 Production of Process Masks The process mask is produced, by transferring the pattern of an intermediate mask into a resist using synchrotron radiation (Fig. 7.2-4). PMMA is used as the resist, which is applied to the mask carrier by direct polymerization with a thickness of about 20 pm (see Section 7.3.1). In contrast to the spin coated layers of similar thicknesses polymer resist structures can be produced which are not susceptible to stress cracking. In case of the process mask, beryllium instead of titanium can be used as mask membrane. Carrier foils of beryllium can be much thicker than titanium foils due to their lower absorption of X-ray radiation. They are produced by mechanical processing of metal sheets. At present this preparation process is limited to a thickness of 500 pm, so that the beryllium carrier can be used only for Xray masks. This metal sheet is coated on both sides by a silicon nitride layer, in order to prevent corrosion. A PVD deposited gold layer is used as the electroplating seeding layer. This may cause problems in resist adhesion which have to be overcome initially by adhesion promoters.

300

7 The LIGA Process sputtering of Titanium casting of PMMA

1 1 1 1 1 1 1

copying of intermediate mask with synchrotron radiation

development and electroforming of gold removal of PMMA

etching of substrate Fig. 7.2-4 Process steps for the final process mask on a titanium carrier with absorbing structures of up to 15 pm height.

As X-ray lithography involves shadow projection of the absorber structure an exact image of the absorber structure in the resist results with a sufficiently thick gold layer. Also lateral roughness of the absorber structure of the interrnediate mask, which is only of some tens of nanometers, is completely transferred onto the process mask. For this copy step the wavelength of the synchrotron radiation must be considerably larger than for the structuring of resists with a thickness of several 100 pm (see Section 7.3.3). With the small thickness of the gold absorber of the intermediate mask the necessary contrast between the illuminated and nonilluminated region can be achieved only by using soft X-ray radiation.

7.3 X-ray Lithography

301

7.2.6 Window for Alignment in X-ray Process Masks If microstructures are to be produced by X-ray lithography, which fit exactly on previously structured auxiliary layers, then the X-ray process mask must be aligned relative to the sample prior to exposure. Because titanium or beryllium mask membranes are not optically transparent, perforations are etched locally into the membrane, whereby perforation and alignment pattern on the substrate can be adjusted simultaneously by an optical microscope. To etch the hole into the membrane the entire mask is covered with a photosensitive coating, which is then removed only at those sites where the windows for alignment are to be etched. In the case of titanium, etch undercutting of the photosensitive coating plays a secondary role due to the thin membrane and hence the short etching time. In contrast, with thick beryllium masks the entire membrane cannot be etched through, as this would lead to a strong etch undercutting due to isotropic etching. For this reason windows must be machined into the beryllium prior to etching. The gold crosses, together with the absorber structure, are produced on the mask and are therefore very precisely positioned with regard to the other structures. These crosses are circumscribed by a gold edge, which subsequently defines the dimensions of the perforation in the titanium foil and guarantees the necessary stability. In the previously applied alignment process a gold cross, which is mounted over an opening in the titanium foil, is exactly aligned with an appropriate cross on the sample. The adjustment precision is limited to 1 pm by the separation distance of both planes (mask, substrate) which is pre-determined by the resist thickness.

7.3 X-ray Lithography By X-ray lithography, the pattern of the mask is transferred into a resist layer, which may have a thickness of up to some millimeters, using synchrotron radiation (see Section 5.8). The region which is exposed to X-ray radiation undergoes a chemical modification. The degree of modification depends on the X-ray sensitivity of the material and on the energy of the absorbed radiation in the resist. The quality of the structure which can be achieved depends on the divergence of radiation, the diffraction of the radiation at the absorber edges and the range of the photoelectrons which are produced in the resist layer. In addition secondary effects must be considered, like fluorescence electrons produced in the mask rnembrane and photoelectrons which are released in the substrate. Also mask distortions resulting from thermal load of the mask membrane have to be taken into account.

302

7 The LIGA Process

7.3.1 Production of Thick Resist Layers The resist layer thicknesses used in microelectronics usually do not exceed 1 pm and the resist consists normally of a polymer, which is soluble in a solvent. On applying this resist solution onto rotating substrates, the solution is spread out. After evaporation of the solvent a homogeneous layer of solid resist remains, whose thickness decreases as the speed of rotation increases. Using this spin coating method very uniform and homogenous resist layers can be produced with thicknesses of about 1 pm. However, this process is not suitable for thick resist layers, especially for those of the LIGA process which are several 100 pm thick. The spin coated layers show only a loose bonding of the polymer chains which, unfortunately, show strong intrinsic stress with increasing layer thickness. In the development stage, this leads to a non-uniform penetration of the developer into the resist layer and to cracking and stress corrosion. Therefore, in the LIGA process a resist layer is polymerized directly onto a base plate [Mohr88] or a polymerized plate is glued or welded onto the base plate. Polymethylmethacrylate (PMMA) is almost exclusively used as the resist material. In case of direct polymerization, the raw material is a viscous cast resin, which consists mainly of a low viscosity monomer, methylmethacrylate, and a solid component dissolved in it. After addition of a polymerization starter (e. g., peroxide), polymerization of MMA to PMMA results, either at a raised temperature or by addition of an initiator (e. g., anilin) at room temperature. The molecular structure of the monomer MMA and the polymer PMMA are represented in Fig. 7.3-1. The PMMA added to MMA remains unchanged on polymerization i. e. no secondary polymerization occurs. On polymerization the resulting molecular weight distribution can be strongly influenced by the different initiators and starters, their con-

Monomer MMA

CH 3

I

CH2=

c I

C = O I

O-CH,

Polymer PMMA

,

CH 3

-CH 2

--L

CH3 CH 3 I -CH2-C-CHz-C -CHz-

1

C=o I

O-CH,

I

C=o I O-CH,

I

C=o I

O-CH,

Fig. 7.3-1 Structure of the monomer methylmethacrylate (MMA), and the polymer

polyrnethylmethacrylate (PMMA).

7.3 X-ray Lithography

303

centration and by the reaction conditions. As the added solid fraction does not take part in the polymerization, normally a bimodal molecular weight distribution (see Fig. 7.3-5a in Section 7.3.3) is achieved. After polymerization the sample is subjected to an annealing process in order to reduce the inner stress. In case of using solid PMMA plates two approaches are pursued. In one of the processes the above described cast resin is applied as a thin adhesion layer (10 pm) onto the base plate and the PMMA plate is pressed onto this. The cast resin used as adhesive, hardens at room temperature by using the same material as with direct polymerization. Alternatively a thin monomer layer is spun onto the base plate, which usually contains adhesion conducive components. Using pressure and temperature the resist plate is welded with this adhesion layer resulting in a bond between substrate and the resist plate [Skro95]. On fabrication of resist structures it is particularly important that after the lithography process i. e. after irradiation and development, both very small and high structures still adhere to the base plate. As the voids are mostly filled with metal in a subsequent electroplating process (see Section 7.4), the adhesion layer on the substrate must guarantee a good and uniform initiation of electroplating on the base plate. In addition, the effective cross-section for the production of photoelectrons in the base plate should be small, which requires a material of low atomic number for the adhesion- and electroplating initiation layer. These requirements can be fulfilled by a compromise, i. e., a sputtered titanium layer, which is subsequently oxidized by a wet chemical process. On oxidation a microporous surface results in which a mechanical teething of the plastic layer is possible. In addition, the internal adhesives which are added to the cast resin, lead to a good adhesion of microstructures. These adhesives are typically siloxanes, which form an oxygen bridging bond with the oxidized surface [Boer81]. Not only titanium, but also copper, nickel and gold are used as adhesive- and electroplating seeding layers, however in all these cases the adhesion is smaller compared to titanium. In the case of gold, adhesion can be improved by addition of thiophenols. If only polymer structures are to be fabricated on a substrate, carbon is well suited as an adhesion layer [ElkhOO]. Although carbon gives better adhesion to polymer structures compared to the titanium adhesion layer, it unfortunately cannot be used as an electroplating seeding layer.

7.3.2 Beam Induced Reactions and Development of Resists The irradiation leads to a scission of the polymer chain of PMMA i. e. to a radiation induced reduction of the molecular weight [Schn78]. From Fig. 7.3-2 it can be assumed, as with PMMA, that by increasing the dosage of radiation the average molecular weight decreases from an initial value of 650 000 g/mol to a minimum limiting value between 2 500 g/mol and 3 000 g/mol at very high dosages of radiation. This is initiated by an electronic excitation of the molecular bonds. The X-ray photons which hit the PMMA with energy in the keV region, are absorbed by single atoms due to the photo effect and release high energy photo- and Auger-elec-

7 The L E A Process

304

e

0

5

10

15

20

dosis [kJ/cm3] Fig. 7.3-2 Influence of the radiation dosis D on the average niolecular weight M , of PMMA.

trons. Their energy is transferred to other molecules or molecular building blocks thereby generating secondary electrons, which are available for further excitation, until they are ultimately thermalized. Finally ionized and excited molecules as well as thermal electrons remain. In the case of PMMA a cleavage of the ester side chain results from the excitation (see Fig. 7.3-3), so that a radical ester group and a radical C-atom are available in the polymer main chain. The radical C-atom generates the scission of the chain resulting in a radical part and a saturated part via a double bond, which originally forms from the free C-atom [SchnSl, Schn831. Thus, a short stable chain and two radicals remain, the radical polymer chain and the radical ester chain, which are free to react with one another or with other radicals to form stable molecules. Other recombination processes after chain scission are energy- or electron transfer to other molecules. The chains can also persist in a metastable excited state. The so-called G-values are a quantitative measure for the chemical effect of incident rays. The G(s)-value gives the number of main chain scission per 100 eV of absorbed radiation energy. The G(s) value can be determined by the relationship

with

= molecular weight prior to irradiation in g/mol, M,t,D= molecular weight after irradiation in g/mol, D = dosage in eVJg which is deposited in the resist, NA = Avogadro’s constant.

7.3 X-ray Lithography

I.fraction of side chain

305

CH 3 -CH

-

C

0

-

CH *-

radicales

I

2. generation of a double bond -CH

-

C =CH

3. fraction of backbone chain Fig. 7.3-3 Mechanism of chain fracture of the PMMA molecule due to exposure by

high energy radiation. Cross-linking of PMMA in competition with chain scission is also observed with higher doses. In this case chemical bonds are formed between different main chains and so the molecular weight increases. Analogous to the G(s) value for chain scission, the G(x) value is defined by each 100 eV of absorbed energy for the cross-linking reaction. Chain scission and cross-linking are processes with opposite influences on the molecular weight. As long as G(s) is larger than G(x) the average molecular weight decreases with increasing dosage. If both G values are comparable in size, the molecular weight remains unchanged. This is the reason for the lower limiting value of the molecular weight, as can be seen in Fig. 7.3-2. For the production of microstructures, the regions of low molecular weight must be selectively soluble in a suitable developer. At the same time the developer should not dissolve the non-irradiated region i. e. the removal by the developer in non-irradiated regions must be negligible. This is especially important for the development of high structures produced in the LIGA process, because the upper regions of the structure are exposed for a much longer time to the developer than the lower regions. Also a swelling of the non-irradiated region cannot be tolerated, as this would lead to unacceptable stress induced crack formation. A very good developer suitable for PMMA in X-ray lithography is a mixture of ethylene glycolmonobutyl ether, monoethanol amine, tetrahydro-l,4-0xazine and water [Ehrf88]. The solubility of PMMA in this developer at different temperatures is shown in Fig. 7.3-4 dependent on molecular weight. For example, one can see, that at a developing temperature as low as 40 "C, PMMA with a molecular weight of over 60000 can no longer be dissolved in the developer. With increasing temperature the solubility increases, and the solubility curve becomes much flatter. Nevertheless, up to 50% of PMMA molecules with a molecular weight

7 The LIGA Process

306

100

80

20 0' 0

I Y

0,5*105

I

1,o -10

I

1,5 -10

mean molecularweight [g/mol] Fig. 7.3-4 Influence of mean molecular weight and temperature T on the solubility of

PMMA during exposure. of 130 000 are dissolved in the developer at 80 "C. For low temperatures the solubility curve is much steeper which results in a higher contrast between non irradiated and irradiated parts.

7.3.3 Requirements on the Absorbed Radiation Dosage PMMA excels in that it gives very good image reproducibility i. e. a high resolution [Gree75]. However, it has the disadvantage that it is relatively insensitive, so that a high radiation dosage is necessary to realize a significant reduction in the molecular weight. A typical molecular weight distribution of PMMA prior to irradiation is shown in Fig. 7.3-5a. The distribution is bimodal with an average molecular weight of 600 000 [Elkh93]. At a temperature of 38 "C the developer (described in the previous section) dissolves at least 50 % of the PMMA molecules with a molecular weight of up to about 20 000 (shaded area in Fig. 7.3 -5). As the fraction of such polymer chains in the non-irradiated PMMA is very small, the developer can in principle dissolve only a very small fraction of the non-irradiated resist. An impairment of the microstructure is not noticeable due to the presence of this small amount of low molecular weight components because it is integrated among the long chained molecules. Irradiation of a dosage of 4 kJ/cm3, results in a mono-modal distribution (Fig. 7.3-5b) with an average molecular weight of 5,700. This distribution lies almost completely in the area in which the developer dissolves more than 50% of the PMMA, so that during the developing process it can be removed. With a radiation dosage below 4 kJ/cm3 the molecular weight is not sufficiently reduced, i. e. the fraction of insoluble polymer chains would be too large, so the irradiated region could not be completely dissolved, and therefore PMMA residues would remain.

7.3 X-ray Lithography

307

Fig. 7.3-5 The distribution of the molecular weight of PMMA before and after exposure with 4 and 20 kJ/ cm3. The shaded area in the diagram indicates the solubility of PMMA in LIGA developer of 38 "C by more than 50 %.

Therefore, 4 kJ/cm3 represents a limiting value for the minimum dosage to be deposited. This limiting value depends on the temperature of the developer. It can be further decreased, if the developing process is supported by convective measures (e. g. stirring, ultrasonic). With a radiation dosage of 20 kJ/cm3 a distribution results with an average molecular weight of 2 800, as shown in Fig. 7.3-52. The whole of the PMMA is dissolved relatively quickly in the developer at such a high dosage. PMMA however, should not be irradiated with a higher dosage, as it can then lead to damage by formation of bubbles, which would prevent a defect free production of microstructures. Therefore, 20 kJ/cm3 corresponds to the upper limiting value, i. e. the maximum irradiation that PMMA can be subjected to. This value is dependent on the temperature of irradiation and decreases at higher temperatures in the resist. As a consequence there is a decrease in the value by 14 kJ/cm3, especially for tall samples (e. g. 1 mm), where the heat dissipation to the substrate is reduced due to the higher heat resistance of the thicker PMMA.

7 The LIGA Process

308

It is essential for X-ray lithography with PMMA, that the necessary radiation dosage required to fully remove the resist and to produce defect free microstructures, lies between the limiting values of 4 and 20 kJ/cm3 (or 14 kJ/cm3). Thus the dosage which is ‘deposited’ in the resist can be varied by a factor of 5 at the most. From this, an estimate of the radiation wavelength can be made. The depth at which the adsorbed dosage, that is to say the intensity, has dropped by a factor of 5 , is shown in Fig. 7.3-6 in dependence on the wavelength of monochromatic X-ray radiation. From this dependence it follows that to structure, e.g. 500 pm thick resist layers, the wavelength of monochromatic X-ray radiation should only be 0.25 nm at the most. One can also assume from Fig. 7.3-6, that the long wavelength radiation is almost fully absorbed in the upper layer of the resist and cannot penetrate into the deeper layers. When considering synchrotron radiation, the wide spectral distribution (see Fig. 5.8-3, Section 5.8.3) must be considered. If the long wavelength component of the radiation is not already absorbed by the X-ray window in the beam tube, it must be filtered out by a further pre-absorber, so that the dose ratio is not exceeded. Fig. 7.3-7 shows, to what degree and in which regions the synchrotron radiation is absorbed in a typical experiment used to produce microstructures. One can now calculate how much dose would be deposited in a 1 pm thick PMMA sample placed at a particular location in the X-ray beam [Bley91]. The curve is a typical representation for the electron-stretcher ring ELSA at Bonn university with a characteristic wavelength of 0.5 nm. The experimental conditions, especially the irradiation time and the pre-absorber, are chosen such that on the underside of a 500 pm thick resist layer, a value no less than 4 kJ/ cm3 (limiting dose D,) and on the upper side a value no more than 20 kJ/cm3 (surface dose 0,)is maintained. As the long wavelength beam is absorbed in relatively thin layers near the surface, it does not contribute to the attainment of the lower limit dosage deep in the sample. In order not to exceed the upper limit, the

-5.

103-

I

c, 102 8 U e

0

E 101 L

. L

aa e aa n

10010-1

100

10’

Wavelength [nm] Fig. 7.3-6 Influence of the wavelength of monochromatic X-ray radiation on the penetration depth in PMMA (decrease of the initial intensity to 1/5).

7.3 X-ray Lithography

309

surface dosis D O

/

minimum dosis D G

102

resist

-

10 0

100

2000 1 0

preabsorber thickness [pml

5

10 0 100

300

mask thickness

resist thickness

Iwl

Iwl

500

Fig. 7.3-7 Absorbed energy by a PMMA test piece of 1 pm thickness at exposure to the radiation of the electron stretcher machine ELSA at Bonn university (2.3 GeV, h, = 0.5 nm, electron current I = SO mA, l h exposure time to an area of 10 x 100 mm2>.

long wavelength component of the synchrotron radiation is filtered out through a 200 pm thick pre-absorber, which is placed in the beam path in front of the mask. Polyimide (Kapton) is used as the pre-absorber. In principle, beryllium or other low Z materials can also be used. For these pre-absorbers it is important that the absorption coefficient for soft X-ray radiation is as large as possible in contrast to that for hard X-ray radiation. The area of the resist which is shadowed by the absorber of the mask must not be attacked by the developer, therefore in this region the deposited dose must lie under 100 J/ cm3 (damage dose 0,).From this value the requirement on the contrast of the mask is deduced to be 200. In order to reach such a high value, the height of the gold absorber must amount to about 10 pm in this example. Other considerations (see Section 7.3.4) may call for an even greater gold absorber thickness. The optimization of the spectrum must be re-assessed each time, for every source and every resist thickness. However, the consequences for the radiation spectrum which hits the resist are similar. The spectrum must be modified such that it best fits with the wavelength, as derived from the approximation (see Fig. 7.3-7a).

310

7 The LIGA Process

7.3.4 Influences on the Quality of the Structure One of the outstanding properties of the LIGA-process is the ability to produce tall structures, whose walls show only a minimum deviation from the perpendicular and therefore maintain constant quality over the entire structure height. Fig. 7.3-8a shows a 400 pm high test structure produced by X-ray lithography, whose width was measured optically (Fig. 7.3-8b). The regressing line indicates a variation of width of 0.04 pm over 100 pm [Mohr88]. A deviation from a perpendicular sidewall could be caused by many reasons or processes: An inclination during the scan-movements through the beam, low selectivity of the developer and physical effects, which are brought about by the X-ray radiation [Munc84], as well as effects which result from the construction of masks and samples. The different sources of error will now be considered in more detail.

a

b 400

= a

300-

I

E

.P

2

200

-

aa L

B

s

;;;; 100 -

a

Q

-7,O

7,2

7,4

7,6

lateral dimension [pm]

Fig. 7.3-8 a) SEM picture of a LIGA test structure of 400 pm height, b) measured width of the structure in dependence on height.

Fresnel-Diffraction, Photoclcctrons In Fig. 7.3-9 the effect, which results from the Fresnel diffraction in an absorber edge, is shown, as well a s (he inlluence of photoclcclrons, which are produced in the rcsist. Furthermore the cllcct of the divergence of synchrotron radiation is shown . If thc incident light beam is shadowed by an edge, then any location in the plane of the edge has l o be considered as the starling point for new and circular diffusc light waves. Thcsc circular waves interfere with each other (Huygens principle) and lead to diffraction (local variation in the light intensity) whose position to the edge is dependent on (he wavelength and on optical path from the point of origin. These diffraction phenomena result in the fact that also in geometrically shadowed areas, the radiation is absorbed and in the lighter regions less radiation is deposited. 'The effect is especially pronounced with monochromatic radiation, as in this case the conditions for the extinction of the light wave (path difference (A/ 2) can be fulfilled exactly. With polychromatic synchrotron radiation the entire spectrum at the point of interest must always be considered. Since the condition for interference extinction is fulfilled only for one specific wavelength, other wavelengths at this point are not extinguished, the diffraction influence is small com-

fresnel diffraction

photoelectrons

beam divergence

Fig. 7.3-9 Resolution limiting effects at X-ray lithography with synchrotron radiation.

7 The LIGA Process

3 12

pared to monochromatic radiation. The calculation shows that the devitation Ax of the first minimum of the Fresnel diffraction from the ideal shadow line increases linearly with the wavelength. This deviation Ax or the blur of the edge is plotted in Fig. 7.3-10 against the characteristic wavelength for a resist depth of 500 pm. In the wavelength region of interest for X-ray lithography the blurred edge is smaller than 0.1 pm. The X-ray radiation releases photoelectrons as well as Auger-electrons in the resist, whose reaction with the resist material brings about the desired chemical changes. These electrons collide with resist molecules and have a fairly wide range due to their high energy. In traversing the resist the electrons gradually loose their energy. Thereby energy can also reach the shielded regions, which leads to a decrease in the edge definition. As the range of the released electrons increases approximately by a power of two with the energy, this effect increases with shorter wavelengths of the synchrotron radiation (Fig. 7.3-10). For the wavelengths region in X-ray lithography the range of interest lies in the region of 0.1 Pm. The total ei-ror caused by both effects, is represented in Fig. 7.3-10. It can be recognized that the blurred edge amounts to about 0.1 pm. The minimum error is obtained with a wavelength of about 0.3 nm.

x -4 c 0

=

0,l

e

0 U

0,Ol 0,Ol

091 190 characteristic wavelength [nm]

10,o

Fig. 7.3-10 Influence of characteristic wavelength on edge definition in 500 pm deep resist with regard to Fresnel diffraction and photo electrons.

7.3 X-ray Lithography

313

Divergence of Radiation The synchrotron radiation is not ideally parallel, it has a finite divergence for two reasons: The first reason is the natural divergence due to the finite electron energy, the second is the divergence due to the oscillations of the electron orbits in the accelerator around the ideal reference orbit, the so-called Betratron oscillations [Koch87]. With an ideal current filament (that is without consideration of the Betratron oscillation) every wavelength of the synchrotron radiation has a unique opening angle. The opening angle decreases with increasing electron energy. The divergence due to the Betratron oscillation, is strongly dependent on the type of machine used and also on the location of the tangential point of the electronic orbit. Typical values lie between 0.1 and 1 mrad in the horizontal and vertical direction. In order to analyze the influence of the beam divergence on the precision of structure manufacture, the divergence of the radiation at the point of origin in the beam line must be considered. During exposure, the mask and the substrate (the sample) is scanned up and down. On scanning every point of the sample is exposed under different angles of the beam. Therefore, on a vertical edge the full beam divergence has an effect on the structure quality. In the horizontal direction the divergence locally leads to an inclination of the structure on the base plate. At a distance of 10 m of the point of origin the walls of a 100 pm wide structure are filled against the vertical by about 0.01 mrad. The influence of the horizontal divergence on the structure quality can therefore be neglected. Inclination of the Absorber Walls to the Beam An inclination of the absorber walls to the beam can come about in two ways. First, the mask- and the sample plane can be adjusted only with limited tolerance vertical to the synchrotron beam. The other reason can be a slope in the absorber walls of the mask due to non-ideal lithographic processes. Especially in the production of masks by optical lithography the angle of the slope can lie in the region of 85". If one considers the dose deposition under an inclined absorber, then the isodose line (points of the same dose) has a larger increase by a factor 200 [Mohr88] for all machines, which may be used for X-ray lithography. This means a conicity of the structure of 0.8 mrad for an edge inclination of 5" on the mask, which for a structure height of 500 pm leads to a declination of the structure width over the height of 0.4 pm. Fluorescence Radiation from the Mask Membrane The absorption of all materials is defined by characteristic edges on which the absorption coefficient increases stepwise with increasing energy. This increase arises because the energy of the irradiated photon is sufficient to raise the electron from the inner shells (K, L) into the continuum. If this excited state recombines again, radiation will be released, which corresponds to this energy level (fluorescent ra-

3 14

7 The LIGA Process

diation). This radiation which is generated in the mask membrane is homogeneously radiated over the solid angle and therefore can also reach under the absorber. There, damage of the resist is caused, which is comparable to the directly irradiated areas and can lead to a dissolution under normal development conditions. Thus, a rounded surface edge will be the consequence. This effect will especially occur, when the increase of the absorber coefficient lies in the region of the synchrotron radiation spectrum used for the structuring. This applies to titanium which is used as the mask membrane and whose absorption edge lies at 0.497 nm. In the example represented in Fig. 7.3-11, a 1 mm high structure which is irradiated with a titanium mask, has a rounding radius of about 150 ,urn. The effect is clearly less using a beryllium membrane (rounding of only about 2 pm), as beryllium does not possess any absorption edges in the wavelength region of interest [Pant95].

Fig. 7.3-11 1 mm thick resist structure exposed by using a titanium mask. Due to fluorescence radiation from the titanium membrane the edge is rounded.

Production of Secondary Electrons from the Adhesionand Electroplating Seeding Layer As already mentioned in Section 7.3.1, the substrate surface onto which the resist is deposited, consists of metal e. g. titanium or gold. This layer has a considerably higher absorption coefficient than the resist material. Therefore, at the boundary of the resist-/adhesion layer, the effective cross-section for the production of secondary electrons increases greatly. These electrons lead to an increased destruction of the resist in the boundary layer. This effect is insignificant in the irradiated area, but can have a drastic effect on the adhesion structure in the region shielded by the absorber, if the absorber thickness is insufficient. Figure 7.3-12 shows how the adhesion decreases using similar thickness of gold absorber, but different synchrotron radiation spectra for structuring [Pant94]. The consequence of this is that the

7.3 X-ray Lithography

315

Fig. 7.3-12 Similar resist structures exposed with radiation of different spectra: a) exposure with a “soft” spectrum (preabsorber Kapton 25 pm), b) exposure with a “hard” spectrum (preabsorber Kapton 125 pm).

thickness of the absorber has to be clearly raised above the minimum value discussed in Section 7.3.3. Alternatively the fraction of highly energetic photons in the spectrum can be reduced by using X-ray mirrors which are irradiated under very shallow angles by the synchrotron radiation.

316

7 The LIGA Process

7.4 Galvanic Deposition The microstructures produced by X-ray lithography from plastic, mostly PMMA, can be the end product in some cases, e.g., microoptical components (see Section 7.7.4). In many cases however, metallic microstructures are manufactured in which the cavities of the plastic are filled with metal by electroforming. To produce microstructures in an economical replication technique (see Section 7.5), robust and shape retaining mold inserts from metal are necessary. The fabrication of these molds is carried out likewise by X-ray lithography and electroforming. In addition, micro-electroforming is applied to produce the gold absorber structure of the X-ray mask (see Section 7.2.4). Thus, micro-electroforming plays an important role in the different manufacturing steps of the LIGA process. To produce the absorber on the mask, gold plating is used, whereas nickel plating is predominantly used to produce microstructures. Up until now copper plating has been of less importance. Also alloy plating from a nickel-cobalt solution (for hardness) or from a nickel-ion alloy (for magnetic properties) is used for special applications.

7.4.1 Galvanic Deposition of Nickel for the Production of Microstructures A nickel sulfamate electrolyte is used for the galvanic production of nickel microstructures and likewise for molds, which are required for plastic molding (see Section 7.4.5). It contains 75-90 g/l of nickel in the form of nickel sulfamate, 40 g/l of boric acid as a buffer and approximately 4g/l of an anion active wetting agent. The pH value of the bath is between 3.6 and 4 and the bath is running at a temperature between 50 and 60°C. Electroplating is carried out on the substrate which has been structured by Xray lithography, which generally is covered by an oxidized titanium adhesion layer. This is a compromise between the requirements for good resist adhesion and ability to initiate electroplating. For several reasons gold or copper surfaces would be much more suitable, but in this case the resist adhesion is not sufficient. On oxidation of titanium under the influence of a hydrogen peroxide solution, an approximately 40 nm thick oxide layer is produced, which differs considerably from a natural oxide layer, which is only 3 nm thick. It was shown that the artificially produced oxide layer represents a very good initial seeding layer for the metal electrodeposition and provides adequate adhesion of the fabricated metal microstructures. This is because on oxidation, TiO, is formed where x is somewhat smaller than 2. In contrast to crystalline Ti02, which is an excellent insulator, the amorphous TiO, is electrically conducting and can therefore be used as a primary electroplating seeding layer. Wet chemical oxidation of the titanium surface leads to microscopically roughened surfaces containing microchannels, in which the re-

7.4 Galvanic Deposition

3 17

sist can be mechanically anchored. Using this method, good adhesion between plastic microstructures and metal can be achieved [Bach92]. Relatively high Ni concentrations (75-90 g/l) in the electrolytes are chosen, in order to attain the highest possible so-called microthrowing power, meaning, structures with small cross-sections are filled at a similar rate as those with larger cross-sections. This avoids the use of leveling agents which also yield to high uniformity of the microscopic metal deposits. Their application, however, leads to negative results if the concentration does not match the particular material transport situation with microstructures, which is different for different geometric sizes. The uniformity of the height of the structure with different cross-sectional dimensions is crucially dependent on the flow characteristics [Leye95]. Material transport differences are minimized, if the current density for electroplating is kept small in comparison to the diffusion limited current density. In this case, the material transport mechanism is dominated by diffusion. This requires a Reynold's number of less than 2. With large Reynold's numbers the material transport is determined by both diffusion and convection. Besides good uniformity on the microscale, the area over the entire surface, which could be several square centimeters, must be electroplated evenly i. e. no intolerable camber should appear at the perimeters. The cause for such macroscopic metal camber is a non-uniform field distribution, usually at the outer margin of the structure. It can also result from non-uniform coverage of resist material on the substrate. In order to attain an equalization of the field distribution and thereby a uniform layer thickness, suitable apertures of dielectric materials are placed in front of the substrate. The anion active wetting agent contained in the electrolyte facilitates the penetration of the electrolyte into the deep and narrow channels in the PMMA-resist, so that the electroplating starts evenly on the metallic base plate. Whereas the optimum wetting agent concentration value normally considered is one at which the surface tension no longer decreases on further addition of wetting agent, a higher wetting agent concentration is used with the LIGA process. The wetting ability was directly analyzed by measuring the contact angle between PMMA and electrolyte. For nickel sulfamate electrolytes at 50 "C, the optimum concentration of the wetting agent was shown to be about 0.5 %. At this dosage the surface tension drops to 25 mN/m compared to 75 mN/m without wetting agent. The contact angle between PMMA and electrolyte, which lies at 70" to 80" without wetting agent was measured to be 5" after 10 min of wetting time which is regarded a sufficiently good wetting. By evacuating the microstructure plate, the penetration of the electrolyte in the resist structure can be further improved. The deposition is carried out with current densities of 1-10 A/dm2, which leads to a growth rate of 12-120 p d h . The current density is the critical dimension for the intrinsic stress which results in the structure, and which should not be greater than 10 to 20 N/mm2. These low intrinsic stresses are necessary so that a large structure e.g. a large honeycomb or a mold, do not bend after being detached from the base plate or do not peel off the base plate during the electroplating. The intrinsic stress varies across the thickness of the deposited layer. Depending on the current density, compressive as well as tensile stress could occur

318

7 The LIGA Process

[Hars88]. Figure 7.4-1 shows a typical example of the influence of the layer thickness and the current density on the intrinsic stress. With increasing current density the intrinsic stress changes from compressive to tensile stress. With increasing thickness the intrinsic stress decreases rapidly and becomes approximately constant for thickness greater than 50 pm. In this example, layer thicknesses greater than 50 pm could be deposited with almost no intrinsic stress for a current density of 5 A/dm3. The intrinsic stress is additionally influenced by the type and concentration of added wetting agents. With nickel sulfamate electrolytes, the intrinsic stress can be controlled by the bath temperature, because the stress decreases with increasing deposition temperature. A large increase in the process temperature is however detrimental due to evaporation loss and a reduced exposure time of the electrolyte. The current density also influences the Vicker's hardness of the structure. As shown in Fig. 7.4-2, at low deposition current densities (1 A/dm2) 350 is the highest value measured, whereas with increasing current density, the hardness falls firstly rapidly and then more gradually to a value of 200. This is due to the hydrogen deposition, which increases with increasing current density. The hydrogen bubbles produced can adhere to the walls of the microstructure, so that at these locations no nickel will be deposited and pores in the nickel layer are formed. These pores only appear when impurities are present in the electrolyte, which serve as nucleation agents for effervescence and as an adhesive for

40

30 20 10

0

-40' 0

I I

30

I

60

90

I

I

I

I

120 150 180 210

thickness [vm] Fig. 7.4-1 Influence of thickness and current density on intrinsic stress of electroplated Ni layers (nickelsulfamate)

7.4 Galvanic Deposition

-

3 19

500

a

9 400 O

I[

structural height 400pm

u m

300 u)

0

W

Ecc

200

v)

5

% 100 .> 0,

2

4

6

8

1

Current density (A/dmz ] Fig. 7.4-2 Influence of deposition current density on Viker’s hardness of a 400 pm nickel layer (testing load 100 p).

hydrogen. Impurities can consist of solid particles such as dust from the air and anode deposits, nickel hydroxide or components of the wetting agent and its decomposition products. Therefore, solid impurities which are either introduced into the electrolytes or are produced on hydrolysis, must be removed by pumping the electrolyte continually through a filter with 0.2 pm pore size. Defects are also formed by an organic decomposition product of the wetting agent in the electrolyte. By exceeding a critical concentration, an increase in pore formation is observed. The organic impurities are eliminated by an “active carbon” purification. This must be carried out periodically depending on the bath loading. As Fig. 7.4-3 shows, no pores appear with a freshly prepared nickel sulfamate. With current densities between 1 and 10 A/dm2 no defects were observed over a period of about 14 days by electroplating of other samples. However, after that the defect rate increases drastically. After “active carbon” purification, in which also the unused wetting agent is removed, and on restoring the original wetting agent concentration, defect free microstructures can again be electroformed. With the above discussed operation parameters of the nickel bath, it is possible to fill narrow and deep channels in the resist exclusively with pure metal by electroforming. Details of the plastic form, which lie in the submicrometer region, are still formed with high precision. Figure 7.4-4 shows a nickel honeycomb structure as an example of an electroformed structure. The height is 180 pm, the wall thickness 8 pm. One can see small non-uniformities of the edges. These are already present in the mask in the form of ridges.

7 The L E A Process

320

200

I

I

I

5

10

15

I 1

VJ

%

150

=

100

r: L

al

g

=I z

50

D-

0

20

25

Runing time [days] Fig. 7.4-3 Number of pinholes on a substrate of 5 cm2 with electroplated microstructures in dependence on service life of a nickelsulfamate electrolyte, and after cleaning in active graphite (AK is a german acronym for active graphite cleaning).

Fig. 7.4-4 Metallic (Ni) honeycomb structure in LIGA technology. The structure is electroplated in nickelsulfamate. The wall thickness is 8 pm, the hole diameter 80 pm. A human hair (about 60 pm in diameter) serves as a comparison.

As can be seen from the picture the surface of the deposited nickel structure is rather smooth. This depends strongly on the roughness of the substrate and the layer thickness. In general, for layers with heights above 100 pin a roughness (R,) of somewhat under 1 pm results. The roughness can be reduced by mechanical finishing (lapping, polishing, milling) if required.

7.4 Galvanic Deposition

321

7.4.2 Mold Insert Fabrication The production of a LIGA mold insert is schematically represented in Fig. 7.4-5. In principle, it is processed similar to the production of metallic microstructures. In contrast however, the metallic deposition is not interrupted when the metal has reached the upper edge of the resist structure, but continues to “overgrow” the resist microstructure i. e. the structures are completely covered by the metal. The metal deposition over the top of the structure is continued, until an approximately 5 mm thick metal layer is formed. As the microstructure and the metal plate are produced in a continuous deposition process, an excellent bonding between the microstructure and the metal plate results. Special care must be taken to avoid intrinsic stresses during the electroforming of the stable metal layers, which could lead to a bending of the mold insert. Because of this risk the current density is limited.

Fig. 7.4-5 Principle of mold insert fabrication: a) fabrication of microstructures in resist by X-ray lithography, b) electroplating of the microstructure, c ) machining of the mold insert, d) removal of the substrate and final machining, e) reinoval of the remaining resist.

322

7 The LIGA Process

After surface treatment of the backside of the mold insert, the base plate is removed. In order to separate the two components by parallel stripping, the base plate is treated prior to electroforming, to ensure a low adhesion between the mold insert and the base plate results. However, it is also possible to deposit metallic interfacial layers onto the base plate by electroplating. If these interfacial layers are selectively etched away from the nickel mold, it can be separated from the base plate without mechanical stress. The same is possible by fully removing the substrate by etching. The surface quality of the front of the mold reflects the surface quality of the base plate. No surface finishing is needed when the surface quality is sufficient on the base plate. However, prior to the removal of the resist, it is possible to process the front of the assembly to the required surface topography i. e. by milling, lapping or polishing. The base of the mold cavity is produced by coating the plastic mold with the electrodeposited metal. As the roughness of the resist surface is generally very small, so is the base of the mold cavity. In the middle of wide resist structures where two plating fronts in the process of overgrowing come together a very narrow trench may remain. It can be shown, that electrically isolated resist areas with a width of less than 0.4 mm can be overgrown and the depth of the trench is in the range of 0.2 pm only. In the case where larger areas must be overgrown, the surface of the resist is made electrically conducting by additional coating. In this case, electrodeposition starts immediately on the resist surface after the overgrowing metal gets into contact with this layer, resulting in very small trenches only where the growing fronts come into contact. The LIGA mold inserts from nickel produced in such a way are used in reaction molding, injection molding and in hot embossing (see Section 7.6). They can easily withstand temperatures up to 150"C and pressures up to 10 MPa. Even after several thousand molding cycles the microstructures show no obvious wear or other degradation.

7.4.3 Electrodeposition of Further Metals and Alloys In principle microstructures can be produced by the LIGA process from any metal and alloy which can be electrodeposited. However, the requirements on microthrowing, wetting, intrinsic stress and minimal gas formation must be fulfilled by the electrolyte. This excludes the practical use of several metals. Besides absorber structures for X-ray masks, gold microstructures with heights of several 100 pm already have been produced. Although either cyanidic or sulfite gold electrolytes can be used. The sulfite gold electrolytes have several advantages which have already been discussed in the context of mask production (see Section 7.2.4). For the production of copper microstructures, sulfate as well as fluoroborate electrolytes are considered. Sulfamate electrolytes lead to ductile and almost stress free layers with good planarization and with 100 % current efficiency. However, organic bath additives are required. They are difficult to analyze which makes

7.4 Galvanic Deposition

323

the process altogether difficult to control. Also copper sulfamate baths are sensitive to impurities. Fluoroborate baths enable a 100 % current efficiency as well as layers which have low stress, no additives needed and little sensitivity towards impurities. The disadvantage of fluoroborate baths is the high corrosivity and the lower hardness of the deposited layers (about 120 Vicker’s hardness). In some cases, especially with reactive primary layers a pre-copper plating is necessary. Different LIGA-structures from copper can be produced successfully with sulfamate- as well as with fluoroborate electrolytes (see Section 7.7). LIGA structures can also be manufactured from metal alloys. The first alloy which was applied to manufacture microstructures in LIGA technique consisted of nickel and cobalt, which could be successfully deposited by a modified nickel sulfamate electrolyte [Eich92]. These Ni-Co alloys were much harder than nickel. Thus, they are of high interest for mold insert fabrication. Iron-nickel alloys, such as permalloy, with 80 9% by weight of nickel and 20 % iron are of particular interest. Permalloy is ferromagnetic and displays not only a high magnetic saturation but also a low coercitive field strength. These magnetic characteristics are important for actuator applications (see Section 7.7.2). The deposition of an alloy with homogeneous and specified constituents from appropriate electrolytes, is particularly difficult for microstructures. Besides the composition of the electrolyte and the deposition parameters (current strengths, temperature etc.), the current intensity is particularly important with respect to the composition of the microstructures. At high current densities the iron deposition is diffusion controlled, which means that insufficient iron will be supplied through the diffusion boundary layer which therefore results in an impoverishment of iron in the structure. At lower current densities, the composition of the alloy is not influenced by the material transport. At a certain current density the transport of metal is controlled by diffusion and depends on the thickness of the diffusion layer. With increasing diffusion layer thickness the diffusion controlled process starts at lower current densities. Due to the high aspect ratios, the diffusion layer thickness is in the range of the structure height. Thus, the current densities in case of alloy electroplating are clearly lower compared to conventional electroplating. They are in the region of less than several A/dm* [Thom95]. Since the diffusion layer thickness varies with the different aspect ratios of the microstructure, the highest aspect ratio on the substrate always defines the current density. Taking this effect into account, Ni/Fe microstructures with a constant nickel-iron content are produced over the whole substrate. The measurement of magnetic properties of such microstructures yields to values which are comparable to alloys made by fusing. The magnetic saturation lies at 1.1 Tesla, about 5 % below fused alloys. These small differences are accounted for by a mixed crystal structure of the microstructure. Whilst no property differences were measured in the 80:20 alloy either perpendicular or parallel to the growth direction, they are clearly present in pure nickel and 5050 alloy. They can be avoided by annealing the sample. For microstructured Ni/Fe-samples the magnetic saturation increases to about 1.4 Tesla with increasing iron content up to about 55 %. At iron content higher than 55 9% the magnetic saturation drops again. The reason for this is a change of crystal structure from face centered cubic Awaruit to body centered cubic Kamacite [Abe195].

324

7 The LIGA Process

7.5 Plastic Molding in the LIGA Process The process for the production of a primary structure made of PMMA as well as using galvanic deposition leading to a metal microstructure, were outlined in the previous chapter. The process steps are laborious and therefore expensive. For commercialization, the LIGA process is especially interesting because of the possibility of mass fabrication by injection molding, reaction injection molding or hot embossing. Therefore, the processes of molding will be described next. In the context of micromolding, these processes are characterized to a lower degree in the injection machine than in the tool retention fixture which is of special importance for microtechnology as well as in process control. Other particular constraints are imposed by the mold insert. The mold insert must be designed such that a non-destructive mold release of the molded structure is possible i. e. the surface roughness of the matrix and the adhesion of the plastic on the surface of the mold insert must be very small, likewise the microstructures on the mold insert should show no undercut. On mold release, a tilt of the matrix must be avoided, so that the microstructure is not damaged. That means that the mold release should be carried out exactly parallel to the walls of the microstructure. To be able to fill the smallest structure dimensions, which could be in the submicrometer range, the molded plastic must have high filling ability, which means low viscosity. Measures must be taken utilizing process control to avoid or counteract volume changes of the plastic on hardening. Then the formation of cavities or a shrinkage of the plastic on the metal structure can be prevented and the dimensional stability of the molded structure can be guaranteed.

7.5.1 Production of Microstructures by Reaction Injection Molding The materials used in reaction injection molding are low viscosity monomers, which are blended with a soluble initiator for polymerization in a mixing chamber shortly before injection into the mold. After injection into the mold, the molding compound hardens by polymerization. The classical material used for reaction injection molding is polyurethane. The scheme of a RIM machine is shown in Fig. 7.5-1 [MacoSS, POts951. Two or more of the liquid reactants are injected into the mixing chamber under high pressure, typically 100-200 bar. The dosage in the mixing chamber must be very precise, and the correct stoichiometric ratio should be maintained throughout the reaction. At higher pressures the low viscosity components attain a higher speed in the mixing chamber and are blended with each other. The mixture flows under a comparatively low pressure of 10 bar or less (in contrast to injection molding), and with low viscosity into the mold, because the polymerization reaction starts with a time lag. The low viscosity and the resulting low injection pressure are the reasons

7.5 Plastic Molding in the LIGA Process

r i g h pressure

325

1

High pressure Component B

Control valves

Mixing chamber

Fig. 7.5-1 Functional scheme o f a reaction injection molding (RIM) machine. Components A and B are blended in a mixing chamber and injected into the mold. After curing the castings is released from the mold.

for the growing application of the RIM-process. Molding parts of up to 50 kg can already be processed. Large molds for the reaction injection molding are relatively inexpensive to produce because of the low mechanical requirements due to the low pressure. In contrast to conventional reaction injection molding, with molding of microstructures it is necessary to evacuate the molds prior to filling, in order to be sure that no gas bubbles are trapped in the structures, which cannot escape due to the high surface tension of the molding material. It must be taken into account on polymerization that the molding material shows an unavoidable reaction shrinkage, which can amount to 20% of the volume. Whilst the associated contraction of the mold can have a positive effect on the mold release, it must be ensured that the reaction shrinkage does not lead to contration cavities or bubbles in the microstructure. During polymerization a constant high pressure is applied to the molding compound to balance this reac-

326

7 The LIGA Process

tion shrinkage. Additional molding compound which is still fluid, is pushed into the mold cavity by this high pressure as soon as a volume decrease takes place due to the reaction shrinkage. As this high pressure affects the structure of the mold insert from all sides, no damage is done to the mold insert. However, it must be ensured that the molding compound does not harden prematurely in the feeding channel prior to entering the mold. For this reason the feeding channel generally is kept at a lower temperature than the mold. A laboratory apparatus for the molding of microstructures using plastic in a vacuum-reaction injection molding process is schematically represented in Fig. 7.5-2. Essentially the apparatus consists of a vacuum chamber, in which a twopart tool (injector and lock side) is constructed together with a container, a holding pressure cylinder and a hydraulic motor, with which the tool and the vacuum chamber are opened and closed. In the figure the apparatus depicts a vacuum chamber already closed, but a tool that is still opened. In this stable state the tools and the feeding channel are evacuated. Prior to filling with the molding compound the evacuated tool is closed by the hydraulic motor with an adjustable mold clamping force (0 to 50 kN). Both tool parts i.e. injector and lock side tool body, can be operated by closing and opening the mold with control guide pins and bushes. The blended reaction mold compound is brought into the process container via a hopper, which is locked by the stopper rod to the vacuum chamber. After degassing the molding compound during evacuation of the process chamber, to remove the entrapped air, a gas pressure is applied in the working container above the molding compound. Pressures of up to 3 mPa can be realized from this applied pressure. Filling funnel Polymer resin Vacuum connector

Pressure connector Holding pressure cylinder

Resin container Feeding channel Thermal isolation

LIGA mold insert

Molding tool injector side lock side

Vacuum chamber

Hydraulic drive

Fig. 7.5-2 Scheme of a vacuum RIM machine for laboratory use.

7.5 Plastic M o l d i q in the LIGA Process

327

After filling the mold voids, in order to apply a large holding pressure on the molding compound, the working chamber can be exchanged by a hydraulic constant pressure cylinder. With this cylinder, a straight pin is pressed into the feeding channel, such that pressures of up to 30 MPa can be achieved. For release molding the vacuum chamber is maintained at a positive pressure of 0.1-0.3 MPa. As the vacuum chamber remains closed at some millimeters above the ejection path, the positive pressure in the vacuum chamber is maintained during the mold release process. On returning the closing unit against the pressure of the chamber the tool body can be opened smoothly. Usually the polymerization of the reaction injection molding resin is carried out at raised temperatures. For this reason the tool body of the apparatus can be heated and cooled by oil which is supplied by an integrated channel system. By means of a programmable control device specified temperature cycles can be maintained precisely. An insulating applicator decouples the temperature sensitive molding compound from these temperature changes in the process container. By annealing the polymers such as PMMA at temperatures above the glass transition temperature (about 110 "C) prior to mold release, several tasks can be accomplished. Firstly a degree of hardening is achieved in which the residual concentration of the monomer is reduced, and secondly, the majority of the intrinsic stress in the molded parts is canceled out. Therefore, after mold release both a distortion of the microstructure as well as a stress crack corrosion in the subsequent electroforming process can be largely avoided. As already mentioned, the reaction shrinkage during polymerization can positively affect the ejection force. The ejection force is lowered further by a mold release agent exuding out of the molding compound during polymerization and thereby forming a film between the mold insert and the structure. Despite these measures the mold release process is very critical with respect to the fragile microstructure. Both, mold release speed and mold release temperature, must be matched exactly to the polymer and the structure. The rule of thumb is that the mold release temperature should lie about 20 "C under the glass transition temperature. The mold release speed should be very small, at least at the beginning of the demolding process. Reaction injection molding shows advantages for the production of microstructures such as a lower process pressure and related lower loading of the microstructure on filling. However, a major disadvantage is that the polymerization reaction in the microstructured mold insert is difficult to control and often highly reactive. Sometimes even explosive reactants must be used.

7.5.2 Fabrication of Microstructures by Injection Molding In the injection molding process [Pots95, Joha941 polymerized plastics are processed from granulate, powder or as extruded profile material. The molding material is melted in the plasticizing unit of an injection molding machine. It is injected into the mold voids of an injection mold insert in this viscous state. The solidification of the molding compound is carried out by cooling down the plastic melt in

328

7 The LIGA Process

the injection mold insert. Typical materials for injection molding are polyvinyl chloride (PVC), polyacrylnitrile butadiene styrol (ABS) and also PMMA. Correspondingly the equipment of an injection molding machine can process thermoplastics, duroplastics and elastomers. Thermoplastics become ductile and can be processed several times. Amorphous and semi crystalline thermoplastics are to be distinguished by their structure. Duroplastics react under the influence of heat and subsequently cross-link. Unlike thermoplastics they can not be melted again by heating. The technically important duroplastics are: phenolformaldehyde, melamine formaldehyde, epoxy resin, silicon resin and polyurethane. Elastomers are plastics, whose plastic-elastic behavior is similar to natural rubber, i. e. elastomers run mainly under the collective term of synthetic rubber. The typical operation of an injection molding machine is represented in Fig. 7.5-3 [Joha94]. It can be divided in three main steps: 0

0

Plasticization, i.e. melting the raw material by heating the polymer in the region of the screw conveyer. Injection of the melt in the normally cold mold insert under high pressure. This task is carried out by the injection unit. Opening the tool and ejecting the hardened molded article.

The retention capability of the tool is brought about by two mounting plates, where at least one is movable, in order to be able to open and close the tool. On the movable mounting plate a device is attached to apply and maintain a mold force. This part of the injection molding machine is called a closing unit. The hydraulics consist of a pump and a coil system with valve, washer and throttle for the production of pressure to control the machine movements.

b)

c)

Fig. 7.5-3 The functional principle of an injection molding machine.

7.5 Plastic Molding in the LIGA Process

329

The granulate or powder molding compound is in a funnel, which is above the feed opening of the injection cylinders and can be opened or closed by a recorder. A screw moves in the axial direction inside the injection cylinder. By turning the screw, the molding compound reaches the feed opening and on further rotation reaches the screw tip. At the same time the molding compound is melted by these cylinders which are heated by electrical power or a temperature controlled oil bath. During initial feeding of the screw, the melt is compacted and injected under pressure through a die into a closed mold form. It solidifies in the desired shape and is ejected after cooling down. The parameters, temperature, time and pressure must be very carefully controlled so that the functions, which include the molding cycles, plasticization, injection and cooling, can be carried out reproducibly. The material parameters which are influenced by the individual function cycles are listed for the case of macroscopic injection molding in Fig. 7.5 -4 [Habe90]. Although the process of injection molding of microstructures is not very different to standard injection molding, two essential points are crucial. As with reaction injection molding, in micro-injection molding the mold insert must also be evacuated. Only by so doing, can typical aspect ratios for the LIGA process be realized. Whereas for conventional injection molding the temperature of the mold on filling and shaping is consistently low, with micro-injection molding it is necessary that during the entire filling time the mold form temperature is well above the glass transition temperature of the plastic that is being used, in order to prevent solidification of the melt [Eich92]. It is however, not necessary

injection

compression

dwell time

4 I

T

surface ,I I appearance ,I

I

I I I

sunk spots voids internal orientation strain crystallisation

-0 0

CI

.-C ?!

z

fn

fn

weight dimensions

I

I

2!

P

time

4

Fig. 7.5-4 Typical pressure-time sequence of an injection molding process and the influence on material parameters.

330

7 The L E A Process

to raise the mold form temperature to the melting temperature (about 240 "C); it is sufficient to remain about 70 "C below this value. Thermal analytical experiments lead to the conclusion that, on injection molding of microstructures at high mold insert temperatures, as mentioned above the molding compound is not decomposed. Longer fill times and therefore lower injection velocities are possible by choosing these high temperatures, so that the injection pressure can be greatly reduced. Due to the low injection pressures, the mechanical damage of the microstructure of the assembly can be completely avoided. However, this means, that after the filling process, the entire mold must be cooled down to solidify the molding compound. It is therefore obvious, that the cycle times with micro-injection molding are higher (some minutes) compared with those of the conventional injection molding (less than one minute). The injection molding machines for microtechnology do not differ much in their principle construction from the conventional molding machine. At the beginning of the injection phase, the mold form is closed, evacuated and the mold pressure is building up. Subsequently, the filling of the mold form cavity with plastic melt is carried out via the injection unit. The mold filling i. e. the mold fill process, is separated into at least two (and maximum six) steps. The switch over between the individual steps is carried out depending on the screw stroke (injection phase) or the time (holding pressure phase). The recording of the screw path with time gives, in general, information about the screw position at the end of the holding pressure phase (residual mass). The flow front speed adjusts proportionally to the screw initial feed speed according to the molding part geometry. In general, during the first injection phase the molding part will be volumetrically filled. The volume shrinkage on cooling of the molding parts is compensated for in the successive injection steps, which form the holding pressure phase. With a reduced pressure and with a very small injection speed, continuously molten polymer is pressed into the mold cavity up to the moment of sealing. The presence of a residual mass is a necessary requirement for the successful holding pressure phase. If the pressure during the holding pressure phase is raised, then in the vicinity of the feed head a positive residual pressure results in the mold cavity, which can lead to formation of stress cracking near to the feed head. A typical injection process is schematically shown in Fig. 7.5-5 with the screw path S, and the hydraulic pressure pH, both as a function of time t [Heus88]. In injection step 1 the screw moves from the end point sp of the plastification path within a time t, to the cross-over point sl. This results in a screw preliminary speed v1 = (s -s )he.The hydraulic pressure increases with increasing time and at4 1 tains its maximum value p I at the cross-over into injection step 2. Directly after cross-over the hydraulic pressure drops to the holding pressure level p z . In injection step 2 the screw moves from the cross-over point 1 to the cross-over point 2 within the time tnl. The initial feed speed of the screw is very small in this phase. In this example, yet another holding pressure phase is effective, the injection phase 3. Here the hydraulic pressure falls to p3. The initial screw feed speed is likewise very small. The middle of the screw position s3 and the front end position

7.5 Plastic Molding in the LIGA Process

331

Screw path SS

SO

. . . . . ..

i

Residual

s3 s2

S1

SP Hydraulic pressure PH

PI P2 P3

P St

Fig. 7.5-5 The screw path versus the hydraulic pressure as a function of time for an

injection cycle.

of the molding compound which is situated in the screw (so) represents the residual mass. Subsequently, in the last injection step the injection die is closed, and the plasticization of new mold compounds are carried out. The plasticization path s3 is put back in the time tp with a hydraulic pressure pst (stagnation pressure). Immediately after the first injection step the cooling phase is initiated. The mold forming parts of the injection mold insert, and therefore the molded parts, are cooled down at a defined temperature gradient. The demolding temperature must lie below the glass transition temperature of the molding compound. After the casting has been demolded with the help of the ejector unit, the warming up phase of the next injection cycle begins. Typical LIGA-structures with high aspect ratios and spaces of 10 pm width and a depth of more than 100 ,urn, can be filled up with PMMA by microinjection molding. A defect free mold release is attained, however only when starting with extremely smooth walls of the mold insert, as is attainable by the LIGA pro-

332

7 The LIGA Process

cess, whilst with a spark erosion processing even in the polishing mode it is not possible to have inold release of structures with similar dimension because o f the large surface roughness. An example of a microstructure made of PMMA, which was manufactured by the injection molding process, is shown in Fig. 7.5-6. The honeycomb structure is characterized by an especially large height of 700 pm, the diameter of the openings is 100 pm, the wall thickness 70 pm [Eich92]. As can be seen in Fig. 7.5-7, the microstructures are fixed on a base plate of the same material. The reason for this is that on closing the mold an interspace remains between the assembly structure and feed plate. Using this method, the filling of the connecting honeycomb structure is possible, which is carried out over a plate with a sprue bush. The feed plate is specially made, so that adhesion of the

Fig. 7.5-6 Example of a microstructure in PMMA, which was fabricated by injection molding (height 700 pni, openings o f the honeycoinb structure 100 pm, wall thickness 70 pm).

n Feed plate

Feeding hole

A

-

Polymer __ base plate Polymer __ microstructures

Mechanical interlocking Molding material Molding tool Mold insert

Fig. 7.5-7 Injection molding of microstructures fixed to a baseplate of polymer. The

picture shows the situation during demolding.

7.5 Plastic Molding in the LIGA Process

333

solidified mold material is ensured and stresses due to volume shrinkage are compensated. In order to produce a suspended microstructure the base plate must be separated from the microstructure in a subsequent processing step. This can be carried out by milling or by laser processing. In this case the whole structure is covered with a soluble plastic material which acts as a structural stabilizer. Afterwards the base plate is milled from the backside. After removing the whole base plate the soluble plastic material is removed resulting in a free standing microstructure. A further possibility to directly produce suspended structures using injection molding, is to use a feed plate with special feed channels. This feed plate itself represents a microstructured mold and can be extremely costly to produce. In this case, as with macroscopic injection molding, predetermined breaking points were generated in the feed, from which the microstructures could be broken off. To conclude this chapter, typical parameters for the previously described processes of reaction injection molding (RIM) and thermoplastic injection molding (TIM) are shown next to each other in the following Table 7.5 -1 for a better comparison.

Table 7.5-1 Comparison of typical parameters of reaction injection molding (RIM) and of thermoplastic injection molding (TIM)

Reaction temp. [ "C] Mold temp. [ "C] Viscosity [Pa s] Injection molding pressure [bar] Molding clamping force [Pa]

RIM

TIM

40 70 0.1-1 10-100 5 x lo5

200 2s 102-10s 100 3 x lo7

7.5.3 Fabrication of Microstructures by Hot Embossing In hot embossing an already polymerized plate of thermoplastic material is shaped by compression at high temperatures to form the microstructures. In the simplest case the plastic plate is put onto a solid base plate, which is made such that by hot embossing interlocking takes place with this base plate. Subsequently both plates are brought to a temperature, which lies above the glass transition temperature of the polymer, with PMMA about 160°C. At this temperature the molding material is in the viscoelastic state, and the mold insert can be pressed relatively easily into the molding material (Fig. 7.5-8). The mold release takes place after cooling below 80 "C. The pressure in hot embossing is about lo7 Pa. Also in this case, to avoid bubbles, pre-evacuation of the mold insert and the space between the mold insert and the plastic plate is necessary. Although the plastic is in a viscoelastic state, and the

334

7 The LIGA Process

Mold insert Polymer layer

Substrate preparation

Adhesion layer Substrate

Hot embossing

Separation

Reactive ion etching

Q.

... ...... ..... ..... ...

..... ...... .... ... ...

Individual structure Fig. 7.5-8

Process steps for the fabrication of isolated microstructures by hot embossing.

embossing pressure is high, the molding material between the base plate and the end of the mold is not squeezed out entirely. A thin layer of several tens of micrometers remains on the base plate, depending on the pressure and the embossing temperature. In order to isolate the microstructures which are connected to this residual layer on the substrate, the film is removed by reactive ion etching (RIE) in an oxygen-plasma (see Section 4.5.7). The advantage of this process to produce isolated microstructures is that a costly feed plate, which is required with injection- or reaction injection molding, is not necessary. Also volume shrinkage does not occur during hot embossing because a polymerized plate is used for the raw material. Therefore, the damage by distortions is definitely less and the accuracy of the microstructure is improved.

7.5 Plastic Molding in the LIGA Process

335

This process is suitable to produce microstructures on processed silicon wafers i. e. on top of microelectronic circuits. The process sequence is schematically represented in Fig. 7.5-9 [Mich92]. In the first step the molding compound is applied to the wafer, which is covered with protection- and metallization layers. This can be done by direct polymerization (a). After solidification the above de-

LIGA molding tool

plastic metallisation passivation layer processed Si-wafer

oxigen plasma microstructures in plastic

microstructures in metal

wet etching

Fig. 7.5-9 Process sequence for microstructures on integrated circuits by hot embossing a) deposition of a passivation layer, a metallic film, and the polymer layer onto the silicon, b) hot embossing of the microstructure by means of a LIGA tool, c) removal of the polymer at the foot of the structure by oxygen etching, d) electroplating of the metallic microstructure, e) insulating the metallic structures by chemical wet etching of the metallic film on top of the passivation layer.

336

7 The L E A Process

scribed embossing is carried out (a, b). In this case after mold release the thin residual layer is located directly above the metallic layer of the wafer. This is removed by reactive ion etching (RIE) in an oxygen plasma (see Section 4.5.7) (c). Therefore, the RIE process is carried out such that the ions impact the substrate as perpendicular to the wafer as possible and thereby hardly any erosion occurs on the side walls of the plastic microstructure. In this way the metallizing layer between the individual microstructures is exposed and can be used as the electrode in a successive electroforming process (d). After metal deposition, the plastic structures on the wafer are removed by solvents. To electrically isolate the metallic microstructures the metallizing layer is removed between the structures (e). A sputter etch process can be used with argon (see Section 4.5.1), but also wet chemical etch processes are possible, which are carried out in such a short period of time, that the metallizing layer under the microstructure is not attacked. In addition, a metallization layer may still exist on the wafer as a protection layer, which is only opened by photolithographic processes for bonding pads to the integrated circuits underneath. Figure 7.5-10a shows an example of a plastic structure, which has been produced by hot embossing. The cross like structures of PMMA are molded with a height of 180 pm, with a minimum width of only 4 pm and without defects. Figure 7.5-lob shows an example of metallic structures, a Ni-honeycomb mesh, which was produced on microelectronic circuits by hot embossing and subsequent electroforming .

Fig. 7.5-10 Samples which were produced by hot embossing (a) and subsequent electroplating (b). a) Detail of a microstructure produced on a plane metallic substrate (height of the structure 180 pm, width of the ribs 4 pm, b) nickel honeycomb mesh on a processed Si-wafer (height of the structure 50 pm, hole diameter 80 pm, wall thickness 9 pm). In the ground of the holes, details of the electronic circuit are visible.

7.5 Plastic Molding in the LIGA Process

337

7.5.4 Production of Metallic Microstructures from Molded Plastic Structures (Second Electroplating) In order to use the material variations to the full extent that the LIGA process offers, it is necessary to convert microstructures produced by molding techniques, into metal structures in a successive electroforming process. A basic requirement is therefore the ability to do electroforming at the molded plastic structures, which requires the presence of an electric conducting layer, on which the metal deposition can be initiated. Furthermore, for the production of metallic structures in which size and shape integrity is preserved, compatibility of the applied molding material with the electrolyte is necessary. In particular the molding material should not swell and no organic components should seep from the molding material, which could lead to impurities in the electrolytes (see Section 7.4.1). Therefore, separate methods were developed with respect to the different molding processes, which enable second electroplating. Second Electroplating of Hot Embossed Microstructures It is obvious that according to the already described process (Section 7.5.3) for the production of structures on processed silicon wafers, metallic microstructures can also be produced. The only difference in this case is that instead of a silicon wafer, a metallic plate is used as the base plate in the molding step. Therefore, in the process, the likelihood of cracking the silicon wafer does not have to be taken into account. Usually a selectively etchable layer (sacrificial layer), often titanium, is applied to the metallic base plate, which is etched off for releasing the microstructure. Second Electroplating with the Aid of a Metallic Feed Plate If the second electroforming is to be carried out with injection- or reaction injected microstructures, it is convenient -if possible - to use a metallic feed plate to fill the mold cavity of the mold insert with a molding compound. By applying pressure to the mold insert (Fig. 7.5-ll), the residual surface will not be covered with plastic and the metal surface remains clean. After hardening of the molding compound, the molded parts are released by separation of the mold insert and the feed plate. A form-fitting compound results from the feed plate and the plastic because of the undercutting of the sprue bush. Therefore, the microstructures remain fixed to the feed plate, even during mold release. These plastic structures represent a reproduction of the resist structure which was produced by lithography using synchrotron radiation. By using the feed plate as an electrode, the plastic molds are filled by electrodeposition with metal. Depending on the application, the feed plate used as electrode, remains fixed to the microstructure, or is detached from the metallic microstructure by a special selective etch process. The disadvantage with this process is the application of high pressure to the delicate microstructure and the possible damage of the fragile structure side walls. In addition only mold inserts can be used, which are completely fillable by means of

338

7 The LIGA Process Feeding hole Feed plate Molding material Mold insert

Molding material Micro structures

Electrode Metal

Fig. 7.5-11 Molding of microstructures by means of a sprue bushing and subsequent electroplating. a) molding of the cavity between sprue bushing and molding tool, b) demolding, c) electroplating of the complementary structure by utilizing the sprue bushing as an electrode.

relatively few sprue brushes, which should have diameters not smaller than 1 mm. Otherwise, the cost for the production of a feed plate with many sprue bushes becomes exceptionally high. This means that the microstructures must have a connection with injectable macroscopic structures. A matrix for a honeycomb-like metal grid, which consists of thousands of microscopic small chambers with diameters of a few micrometers, cannot be produced using this process.

Second Electroplating with the Aid of Electrically Conducting Plastics Another process for the production of metallic microstructures by the second electroplating, is based on the production of electrically conductive bases in the molding process, which serve as support plates for the insulating plastic structures [Harm90]. With this process optimally designed metallic microstructures can be produced, because the support plate serves as an electrode and onto which microstructures can be integrated made from insulating plastic. With vacuum-reaction-injection-molding in the first molding step, the mold cavity of the mold inserts being used is filled with electrically insulating reaction

7.5 Plustic Moldiizg in tlze LIGA Process

339

resin compound. Subsequently, in the second molding step the matrix is coated with an electrically conducting molding compound. By suitable process control the electrically conducting and non-conducting plastic are welded together, whilst the adhesion onto the metallic mold insert is low (possibly by addition of mold release agent). The matrix structure can then be released from the mold without destroying any of the sensitive microstructures. The metallic deposition starts then on the base plate made from electrically conducting plastic. In order to achieve a defined transition between the conducting and nonconducting molding materials, at least one of the molding compounds during thc second molding step with the overlaycrs sliould be in solid form. Based on thc characteristics of non-conducting and conducting reaction resin compounds and mo Idi ng compciunds, thrce production "I)p ro aches result lrom the mo Id i n g concept, which arc represented in Figure 7.5-12. In approach I, the mold insert in the first molding step is filled wilh t h e rcsiri compound. In the second molding s ~ c pa scparately manulacturetl support plate from conducting molding matcrials is presscd onto the filled mold insert, whcrcby 1.Molding step : filling of the mold insert non-conductive molding material

non-conductive molding material

+

I

Removing of surplus molding material

Hardening

2. Molding step : covering of the filled mold insert Conductive polymer

1

Removing of surplus molding material

Conductive polymer

Polymer welding

Conductive

Hardening

Fig. 7.5-12 Production variants of molding of micrnstructures with subsequent electi-oplating by tncans ol conductive polymers.

340

7 The LIGA Process

excess resin compound is expelled from the edges of the mold insert. Due to the diffusion of the liquid is into the solid support plates, a form-fit of both plastics results. The reaction resin compound in the mold cavity hardens to the mold material after filling. In the first molding step of approaches I1 and 111, the matrix is filled over the entire surface with reaction resin compound. The excess resin compound is then expelled from the mold insert by applying a kind of plunger. The resin compound in the mold cavity then hardens with the plunger remaining in position. In the second molding step of the production variation I1 after hardening of the molding material, a separately manufactured support plate made of electrically conducting molding material is welded onto the mold insert which is filled with molding material. The welding is done at elevated temperatures. In the second molding step of production approach I11 in the reaction injection molding, a viscous electrically conducting reaction resin compound is applied, which hardens and becomes the electrically conducting support plate and binds strongly to the previously hardened molding compound. PMMA filled with carbon black or silver is successfully used as an electrically conducting plastic. The optimum degree of filling is about 75 wt% silver and about 12 wt% of carbon black.

Fig. 7.5-13 SEM-micrographs of Ni-honeycomb structures at four different process

steps: a) structures in PMMA after X-ray exposure (primary structure), b) molding tool in nickel (secondary structure), c) molded polymer structures (ternary structure), d) honeycomb structure in nickel after 2. electroplating (quaternary structure).

7.6 Variations and Additional Steps ofthe LIGA Technology

341

A nickel honeycomb grid at four different process steps is shown in Fig. 7.5-13: No structural degradation is observed on plastic molding and electroplating of these structures (minimum dimension = wall thickness = prism distance = 8PI. Also with injection molding electroformable structures can be produced using granulates filled with conducting carbon black. At high flow speed of the melted granulates, a segregation of polymer and conducting carbon black is observed. This segregation is utilized when structures are filled at high injection speeds resulting in a gradient in the conductivity from the structure base to the structure surface. The base plate however, will be filled at a lower injection speed so that there is a homogeneous distribution of carbon black particles and thus the desired conductivity is attained. This process has rendered the production of arrays with an aspect ratio of 3-5, which could be electroformed without any defects.

Second Electroplating by Covering of the Plastic Structures A possibility to produce metallic microstructures by a second electroplating, which appears obvious at first glance, will be mentioned here for the sake of completeness. The structures are coated by a metallic conducting layer. This is most simply achieved by a PVD-process (physical vapor deposition, see Section 4.5). However, in using this process the side walls are also coated. Even if the layer is very thin, the metal deposition starts simultaneously at the structure floor and the side walls. For structures with high aspect ratios, the structure at the top of the cavity will tend to close up whereas the structure at the floor may not yet be filled leading to undesired cavities. Therefore, this process is only considered for those structures which have small aspect ratios.

7.6 Variations and Additional Steps of the LIGA Technology In order to cover the widest possible spectrum of uses, the standard LIGA process is extended by numerous process variations. They will be introduced and discussed in the following chapters.

7.6.1 Sacrificial Layer Technology If micromechanical sensors or actuators are to be produced with microfabrication methods, in many cases stationary microstructures as well as moveable microstructures must be designed. Often movable and stationary microstructures are integrated, so that hybrid assembly is not possible. Such hybrid assemblies are also often hindered by the low dimensional tolerances which are required. These con-

342

7 The L E A Process

straints apply to e. g. acceleration sensors, gyros, linear actuators, resonators and similar structures. Movable structures are produced in silicon micromechanics, in which e. g. a pit is made by anisotropic etching underneath a thin elastic cantilever. In surface micromechanics freely moving structures are produced, in which several thin, structured layers which are made of different materials are placed on top of each other. The so-called sacrificial layers are selectively etched off the layers placed above and below (see Fig. 6.3-4). It is also possible with the LIGA process to produce moveable microstructures by introducing sacrificial layers [MohBO]. Therefore, for movable sensors and actuators a large range of materials is available as well as the possibility of large structure height with no limitation in the lateral shaping. As an example, the process steps for the production of an acceleration sensor is shown in Fig. 7.6-1 for. In this example microstructures have electrical functions, like most sensors and actuators, so that the individual parts of the microstructure must be electrically isolated from each other. The process is therefore based on an electrically non-conducting substrate, e. g. a silicon wafer equipped with an insulation layer or a ceramic substrate. A metallization layer is applied onto this layer using a PVD process. Stringent requirements are made on this layer with respect to the adhesion to the substrate and also to the subsequently electroplated metal layer. These requirements cannot be readily fulfilled by a single layer. Therefore two different metal layers are used, one being the adhesion- and the other the primary electroplating initiating layer. The layer systems made of chromium and silver have proven to exhibit good adhesion where chromium possesses a good adhesion to the substrate and silver possesses a good adhesion to the electroplated layer. When necessary, passivation layers are introduced, in order to avoid problems with the different etch processes. In order to isolate the microstructures from each other, produced later by the LIGA process, the layer system is structured by

metallisation (h 1pm)

electro forming

insulating substrate (Si, ceramik)

metal (Ni, h > 100pm)

sacrificial layer (Titanium, h = 5pm)

c)

resist (PMYA, h > 100pm)

sacrificial layer

f)

Fig. 7.6-1 Process steps for movable microstructures (sacrificial layer technique).

7.6 Variations and Additional Steps of the LIGA Technology

343

optical lithography and wet etching processes. Similarly, in the metallization layer, conducting paths are structured to electrically connect isolated areas of the system. The sacrificial layers is now applied using the PVD process on the pre-treated substrates. The following requirements are placed on these layers: 0 0 0 0 0

good ability to patterning, good adhesion of the resist used in X-ray lithography, good initiation and good adhesion of the subsequent electroplating, good selective etchability compared with all materials which are used as substrate, metallization layer or sensor- and actuator materials, fast etching without residues even underneath structures with large areas.

In the LIGA process titanium has proven to be a good sacrificial layer, as it possesses both good adhesion to the resist and to the electroplating initiating layer and can also be etched with hydrofluoric acid, which does not attack standard materials used in the LIGA process (Cr, Ag, Ni, Cu). The thickness of the titanium layer should be large enough, so that the movable structures have clear openings to move freely. With too small a gap the microstructure may be blocked by contaminants. Also for etching of large areas under microstructures, it is favorable if the gaps which are formed are not too narrow. However, with increasing thickness of the sacrificial layers the precision, with which these layers can be structured by simple photolithography and wet etch processes, decreases. In addition, with larger thicknesses, the inner stresses of the applied layers is too large so that a good adhesion onto the substrate is no longer guaranteed. A titanium thickness of 5 pm is a good compromise between these opposing requirements. This titanium layer is structured, using optical lithography and etch processes, so that on subsequent structuring with X-ray radiation the movable parts of the microstructure are placed above the titanium layer, whilst the stationary parts are connected directly onto the electroplating initiating layer. The resist having a thickness of several hundred micrometers is applied to the substrate in the standard way over the structured metallization- and sacrificial layers. This resist layer is finally irradiated with synchrotron radiation through a mask. The X-ray mask is adjusted with respect to the previously structured layer by alignment marks on the mask and on the metallization layer (see Section 7.2.6). After exposure with synchrotron radiation the irradiated area is developed and the primaty structure is filled with metal by an electroplating process. The electroforming takes place on top of the metallization, as well as on the sacrificial layer. Finally the non-illuminated resist is removed. In the subsequent process sequence the titanium sacrificial layer is selectively etched. Hydrofluoric acid (0.5 %) has shown to be particularly suitable. Thus, the part of the microstructure which was placed on top of the sacrificial layer becomes freely movable, whilst the other parts of the metallic microstructure on the metallization layer are well anchored to the substrate. The electric contacts can be made between the individual parts of the microstructure via bond pads and conducting leads, which were produced in the metal-

344

7 The TJGA Process

lization layer. As a consequence electrical connection with separately manufactured integrated circuits is accomplished. The micromechanical and microelectrical components that are connected in such a way, are then packaged in a common housing. However, direct structuring on wafers with integrated circuits is not possible using X-ray radiation since, for example, the gate-oxide in an electronic circuit is damaged by X-ray radiation. Direct integration of electric circuits and microstructure can be carried out using hot embossing described in Section 7.5.3. In this case the bond connections are avoided and a very high integration density is attained. At the same time, of course, also moving parts can be produced. For this the substrate used in the hot embossing process is provided with a structured sacrificial layer and the molding, aligned with the pre-structured substrate, is carried out. Several examples for the movable microstructures are introduced in the following Section 7.7.

7.6.2 3D-Structuring In principle, the standard LIGA process allows only the production of structures with a constant structure height and perpendicular walls. However, many structures require a variation in geometry in the third dimension. This can be achieved by the structuring in different planes (stepped structures), by tilting of masks and substrates relative to the beam (oblique or conical structures), by additional processes (structures with spherical surfaces) or by effective use of secondary radiation (conical structures). Stepped Structures The production of stepped structures, of which some examples are shown in Fig. 7.6-2, can be fabricated by three variations in the process.

Fig. 7.6-2 Stepped structures fabricated by a twofold LIGA process.

7.6 Variations and Additional Steps of the LIGA Technology

345

In the first process, microstructures which were structured in the first lithography step, are irradiated through a second mask. This mask, which must be aligned with the structure of the first exposure, contains the structural details of the overlay. The radiation dose is chosen such that the lower limit dose is supplied (see Section 7.3.3) to the specific height within the structure. During the subsequent development step, the resist is therefore not fully developed all the way to the base. This results in a step in the structure. Disadvantageous of this relatively simple method is the undefined dose limit which is especially dependent on the development time. Therefore, the parameters for this process have to be optimized for every individual microstructure. Also the surface of the step is quite rough due to the development properties of the resist. In principle, a similar effect can also be achieved, if the absorber on the mask consists of two different materials. Whereas this leads to an expensive mask production, no second radiation is necessary during the production of the structure. In the second method, a structure is produced by X-ray lithography and electroplating in a first sequence. The desired step height of the finished structure is achieved by plan-milling the surface. In a subsequent step, a second structural layer is placed onto this substrate. Although by using this method an exact step height can be achieved, the problem remains that the position of the two structure parts with respect to each other depend on the alignment precision during the irradiation. Variations in the vertical position of more than 1 pm must be accepted. Furthermore, the adhesion of both structure parts is not optimal. In the third method, a base plate is pre-structured such that a stepped substrate is available [Miill96]. The structuring is carried out either with mechanical methods or via lithography and electroplating depending on the requirements of precision. The lateral precision of the steps can be adjusted to less than 1 pm by sputter etching processes. These stepped plates are used as the substrate in the LIGA process and are coated with resist. The structuring with X-ray radiation is carried out all the way to the substrate base. The vertical position is determined exclusively by the second structuring process and does not depend on alignment precision. This method is particularly suitable for microoptical applications because the lateral precision of the location of the individual structures lies in the range of 0.1 ym. Inclined Structures Structures, whose side surfaces have an angle other than 90" to the surface or to the substrate, can be produced by inclination of the mask and substrate to the appropriate angle with respect to the X-ray beam. The L E A process is carried out in the standard way. These structures are also of particular interest for optical applications such as the production of prisms (see Section 7.7.4). By irradiation with different inclination angles and by using negative resists, not only simple structures but also more complex structures can be realized [Feie95].

346

7 The LICA Pvocess

Conical Structures and Structures with Spherical Surfaces The effect described in Section 7.3.4, where secondary electrons are released from the mask membrane and isotropically emitted, enables the manufacture of conical structures on the upper side (see Section 7.3.11). The patterning is Iimited by choice of the spectrum and the membrane material in which the fluorescence radiation is generated. In order to produce structures with defined spherical curvature, an additional process was developed. Column structures produced by the LIGA process are exposed to a second irradiation of X-ray radiation (Fig. 7.6-3). The spectrum is adjusted such that the dose is deposited predominantly in the upper part of the structure. The glass transition temperature is thereby reduced by this dose deposition because of the change in the molecular weight of the plastic, i.e. the structure has a glass transition temperature that varies according to the height. The material in the upper part of the structure melts at lower temperatures compared with that in the lower region. When the structure es heated to a temperature between both Synchrotron radiation

Irradiation

Resist ...................... ///////////////////I//<

Substrate

Synchrotron radiation

++w+++++ X-ray filter

Fig. 7.6-3 Fabrication of microstructures with spherical surfaces by LIGA technique and subsequent flush exposure, melting, and resolidification.

7.6 Variations and Additional Steps of the LIGA Technology

347

Fig. 7.6-4 Examples of lens arrays fabricated as explained in Fig. 7.6-3 and in the

text.

glass transition temperatures, only the material in the upper part of the structure can melt. Because of the surface tension the melt contracts to a hemispherical shape. Columns with spherical caps originate in this way. This process is especially suitable to produce lenses or lens arrays (Fig. 7.6-4).

7.6.3 Production of Light-Conducting Structures by Molding In the area of optical information technology and optical sensors (see Section 7.7.4) components based on polymer wave guides are of increasing interest, since such components can be produced by molding processes in a large quantities and therefore at low costs. For the production of mono-modal structures with dimensions in the region of some micrometers, hot embossing is used to produce either trench- or so-called corrugated structures. Ideally the corrugated structures are directly connected with trench structures for fibers using the process described in Section 7.6.2. The trench structures are often filled with a monomer which has a high refractive index and hardens on e. g. UV-polymerization [Klei94]. Other methods use different processes in order to change the refractive index locally, such as ion bombardment [Fran96], monomer diffusion or UV irradiation [Fran94]. For the production of wave guiding structures for multi-mode applications, the hot embossing technology is combined with a welding process, so that a double layered microstructured polymer construction can be produced [Mi.i1195a]. The process is schematically represented in Fig. 7.6-5. In the first process step the core layer of the wave guide covered by a foil is compressed into the mold insert. This is carried out by using hot embossing above the glass transition temperature of the polymer. By using a foil of a material with higher glass transition temperature the material on the front side can be almost fully removed. In the second step the foil is removed and then the cladding layer is welded under pressure with the present core layer in the mold insert at a temperature of about the glass transition temperature. Therefore, a diffusion of both polymers is carried out in a layer of

348

-

7 The LIGA Process

separation stamp foil

preparation of moulding tool

core foil mould insert

hot embossing of core layer n

cladding foil

welding of cladding layer

core foil

demoulding

silver

sputtering of reflection layer (optional)

sputtering mask

finishing insert of multi mode fibre

+

--

UV hardened cladding layer

Fig. 7.6-5 Process steps for the fabrication of light-guiding structures including optical components and adjustment stops.

1 pm to 10 pm, which leads to a good adhesion of both polymer layers. Subsequently, the entire structure is mold released, additional optical elements (e. g. glass fibers) are introduced, reflecting layers are deposited by sputtering and the entire arrangement is covered in a UV resin polymer, in order to apply the second cladding. The advantage of this process is that the simple waveguide structures can be equipped with optical structures and that additional optical elements can be introduced in a hybrid fashion in the waveguide structure. This leads to an increase in the complexity of optical components.

7.7 Examples of Applications

349

7.7 Examples of Applications The free lateral shaping with the LIGA process, the large variety of materials, structural heights of several hundred micrometers, as well as the formation of mold details in the submicrometer range, enable the production of components for different areas, like micromechanics, microoptics, sensor and actuator technology and fluid technology. These components find uses in the automotive technology, process technology, general mechanical engineering, analytical techniques, communication technology and chemical, biological and medical technology and many other fields. In this section examples of some applications are given. The section is predominantly about prototypes produced at the Research Center, Karlsruhe. Further examples froin other groups can be taken from the literature.

7.7.1 Rigid Metallic Microstructures Simple structures, which are produced by the LIGA process, are either completely dissolved from the substrate or form one unit with the substrate. Examples are simple honeycomb structures in polymer or metal, which can be used as particle or wavelength filters. Other examples are microcoils or microgears. Filters for the Far Infrared Metal membranes with precisely defined, periodically arranged slit apertures over a large layer thickness, e. g., cross- or Y-structures, can used as band pass filters for the far infrared. They are called resonant gratings [Ulri68, Comp831. The spectral quality of the filter depends on the shape and dimensions of the slit apertures, but also on the relative thickness of the membrane. The wavelength region, in which these filters allow rays to pass through, lies in the same order of magnitude as the slit length. The width of the slits and the distance to the neighboring element, which represent the critical dimensions, are just about one order of magnitude smaller. Therefore, it follows that for a transmission wavelength of about 20 pm, the critical structures lie in the order of only a few micrometers, which must be controlled very accurately. In order to attain a high separation sharpness, the thickness of the filter should be large compared to the critical dimensions. Thus the LIGA process is best suited for the production of such filters [RuprBlI. As an example for a band pass filter, a copper membrane with a cross-shaped slit aperture, which has a length of 18.5 pm, a width of 3 pm and a minimal distance to the neighboring elements of 2 ym, is shown in Fig. 7.7-la. The thickness of the membrane is 20 pm. The membrane is produced by X-ray lithography followed by electroplating. From Fig. 7.7-lb, in which the measured transmission curve is shown, the filter is transparent to radiation wavelengths between about 27 pm and 35 pm, whilst the transmission drops abruptly toward the high- and low frequency spectral regions.

350

7 The LIGA Process Fig. 7.7-1 Bandpass filter for the far infrared. a) SEM micrograph of crosslike slits in a metal membrane of 20 pm thickness (dimensions of the structures: length 18.5 pm, width 3 pm, distance to the next structure 2 pni), b) measured transmission as a function of wavelength (in a pm).

100 1

b

Wavelength [ pm

1

Fig. 7.7-2 Highpass filter for the far infrared. Transmission with a honeycomb structure of 80 pm hole diameter, wall thickness of 8 pm, and height of 180 pm.

7.7 Examples of Applications

35 1

High-pass (or low-stop) filters for the far infrared are metal membranes with uniform hole apertures, whose thickness is more than twice the hole diameter. Therefore, honeycomb-shaped grids, as was described in Section 7.5.4 (see Fig. 7.5-13d), can be used as high-pass (or low-stop) filters. The transmission curve for the honeycomb grid, with a diameter of 80 pm, is shown in Fig. 7.7-2. One can deduce that these honeycomb grids are transparent to radiation with a wavelength of less than 120 pm, and for higher wavelengths (lower energy) exhibit a sharp cut-off (transition between the shut-off and transmission regions) and in the maximum wavelengths give a transmission of over 95 %. Microcoils Planar microcoils with large current carrying ability and arbitrary lateral form can be produced with the LIGA process because of the large structure height and the use of metals with low electric resistance, e. g., copper. In order to avoid short circuiting, the metallic coil torsion must be constructed on an insulating substrate. Therefore, a ceramic plate or a silicon wafer, onto which an insulation layer (e. g. Si,N4) is applied if necessary, is topped with a metallization layer. A sputtered titanium layer has proven to be effective for this purpose, as in many applications. After the structuring of the plastic (by X-ray lithography or molding), the electroplating and the removal of the plastic mold, the titanium layer between the coil spirals is removed. This is carried out either by sputter etching (see Section 4.5.1) or wet chemically etching, analogous to the sacrificial layer technique (see Section 7.6.1). However, on wet etching the etch times must be kept very precise so that the titanium layer under the coil turns is not also removed. The test for full removal of the metallization layer between the coil turns can be carried out by measuring the inductance of the coil. Figure 7.7-3a shows a plastic mold made of PMMA for a microcoil, which consists of two coils entangled with each other. Figure 7.7-3b shows a section of a copper coil on a ceramic substrate after re-

Fig. 7.7-3 Microcoils (width and distance of the windings 20 pm, height 100 pm, a) SEM micrograph of the complementary structure in PMMA (primary

structure), b) SEM micrograph of a coil made in copper on a ceramic substrate.

352

7 The LIGA Process

moval of the titanium layer between the coils. The width and distance between coils are each 20 prn and the height about 100 ,urn.

Microgears The advantage of producing microcogs for microgears using the LIGA process is that the teeth can be formed in these tiny dimensions €or an involute gear. Also the tiny gear rims can be produced with teeth structures lying inside, which exceeds conventional precision engineering capabilities. Therefore, various gears with

Fig. 7.7-4 Planetary gear a) gear components made from nickel and polymer, b) assembled planetary gear. The overall diameter of the gearbox is 3 mm (by courtesy of RMB, Biel, Switzerland).

7.7 Examples of Applications

353

small dimensions (e. g. harmonic drive gears) can be realized, which was not possible before. Fig. 7.7-4a shows the different cogs for a planetary gear. The diameter of the sun wheel is 0.8 mm, the teeth have a minimum width of 65 pm. The thickness of the structure lies between 300 pm and 400 pm. Figure 7.7-4b shows a mounted planetary gear in a casing.

7.7.2 Moving Microstructures, Microsensors and Microactuators The spectrum of producible sensors and actuators with the LIGA technology has increased substantially with the development of sacrificial layer technology (see Section 7.6-1). In addition to these actuators additional LIGA-structures are constructed resulting in more complex devices [Lehr96, Guck951.

Acceleration Sensors Figure 7.7-5a shows the principle construction of a capacitive acceleration sensor, which has been produced using the sacrificial layer technique [Burb91]. A seismic mass suspended on a leaf spring is located between two electrodes which are well attached to the substrate. In response to an acceleration, the distance between electrodes and seismic mass changes, and thus the capacitance change due to the distance change can be electronically detected. By exploiting the shaping capabilities of the LIGA process the seismic mass can be branch shaped, (Fig. 7.7-5b), so that disturbing changes in capacitance due to temperature variations of the structure are compensated. A further increase of the precision is achieved by suspending the seismic mass on two elastic cantilevers, which leads to a linear deflection and therefore to a linear signal. Furthermore, the seismic mass can be split into several fork structures, which consequently raises the basic capacitance. With the distribution of opposite electrodes in single blocks, the air damping can be reduced [Stro95]. Figure 7.7-5c shows a sensor element produced by X-ray lithography and electroplating and Fig. 7.7-5d shows a detailed picture of the nickel structure. The distance between the opposite electrode and the seismic mass is only 4 pm, the width of the elastic spring about 20 pm. Data are listed in Table 7.7-1 for sensors equipped with appropriate electronics. Table 7.7-1 Characteristic data of the LIGA acceleration sensors Measurement Range

1 2 g

Resolution Band width (3 dB) Sensitivity Temperature range Temperature sensitivity Non-linearity

1 mg/JHz DC ...700 Hz 2.5 V/g -20.. .loo "C 300 p p d K < 0.6%

354

7 TheUGA Process

7.7 Examples of Applications

355

Because it is possible to produce acceleration sensor elements via a common mask process, a two-dimensional arrangement of sensor elements is achieved with high precision. Both sensor elements are arranged orthogonally and are sensitive in the x- and y-directions, respectively. By combining them with a silicon sensor sensitive in the z-direction, a planar 3D sensor can be constructed at minimum cost of assembly (Fig. 7.7-6) [Mohr94].

Fig. 7.7-6 Acceleration sensor system for all three directions of the space. The system consists of two LIGA sensor elements (for x- and y-direction), and one silicon sensor element (z-direction).

4 Fig. 7.7-5 LIGA acceleration sensor a) general design, b) modified design for temperature compensation, c) SEM micrograph of a electroplated sensor element, d) details of the structure (height 100 pm, width of the cantilever beam 10 pm, gap width 4 pm).

356

7 The LIGA Process

Electrostatic Comb Drive An electrostatic comb drive can be realized by an arrangement where a movable, comb-like structure is immersed in a similar comb-like structure thus forming several capacitors [Burb91]. Therefore, the movable structure is that it can move only parallel to the comb fingers. When a voltage is applied between both combs, the system strives for a state of maximum capacity and therefore maximum potential energy. The possible displacement results from the depth of the comb-structure as well as from the restoring force of the spring element on which the structure is dependent, and the applied voltage. Figure 7.7-7a (complete view) and Fig. 7.77b (detail of the comb-alignment) shows a 60 pm high electrostatic comb drive suspended on four double springs, which are 4 mm long and 10 pm wide. The comb consists of 70 finger structures each with a length of 200 pm and a width of 50 pm. The clearance between the finger structures is 5 pm. From Fig. 7.77c, in which the measured displacement is drawn as a function of applied voltage, it can for example be deduced that a shift of 50 pm can be realized for such an arrangement at 70 V. In order to increase the usealte travel range, the condenser structure can be conically designed. Figure 7.7-8 shows such an actuator which is the component of an optical construction plate and with which an optical bypass-circuit was constructed (see Section 7.7.4). In this case, regulating distances of 90 pm are attained with voltages of about 45 volts. Electromagnetic Linear Actuators Electromagnetic microactuators were of lower importance in the past than electrostatic actuators, which can be attributed to the fact that electrostatic actuators are in a structural plane and therefore can be produced relatively easily, whereas efficient magnetic actuators require a three dimensional structure. However, the electromagnetic actuators are better suited with respect to maximum energy density and power. Although the electrical field strength is limited to 100 V/pm because of the risk of arcing and the magnetic current density is limited to about 1.4 Tesla because of saturation of the magnetic material (see Section 7.4.5), the maximum energy density of the magnetic field is about 20 times higher than that of the electric field. Comparing power

and

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7.7 Examples of Applications

357

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voltage 1 v 1 Fig. 7.7-7 Electrostatic comb drive. The movable part is suspended by elastic beams. When an electrostatic force is applied, the movable part dives into the comb structure of the stator. a) SEM micrograph of the total arrangement, b) detail of the finger structures (200 pm long, 5 pm wide, gap 5 pm), c) influence of the driving voltage on the displacement.

show that with a voltage of 100 V in the electrostatic case or a current strength of about 300 mAIN in the magnetic case, the same power can be achieved. Already with a moderate number of turns N , the value of the current is small enough for magnetic actuators to be operated with standard-bipolar circuits. As the achievable power is proportional to the structure height, the LIGA process can produce particularly effective electromagnetic actuators [Rogg96a]. However, for an integrated solution there is a need for coils with vertical axes, in order to produce a

358

a

7 The LIGA Process

b

Fig. 7.7-8 Electrostatic linear actuator with conical digit structure for an optical switching system a) total view, b) detail of the conical fingers.

microtechnically manufactured Permalloy core. This is achieved in the production process shown in Fig. 7.7-9, based on 5 mask layers. In the first step the conducting layer is structured by optical lithography and etching. A sacrificial layer is then deposited, which contains the openings to connect the core and the coil columns to the substrate. In the following X-ray lithography step with subsequent electroplating of Permalloy, the core, the anchor and the spring suspension of the anchor are produced. Finally, the Permalloy layer is covered with an X-ray resist and a structured electroplating seed layer is applied for upper connector mask strips of the coils. In the successive lithography and electroplating steps the coil columns are produced. When reaching the resist surface along the structured seed layer, they grow laterally and therefore the coil turns are completed [Rogg96b]. A linear actuator produced using this process is shown in Fig. 7.7-10. The rectangular coil winds itself around the Permalloy core. Likewise the anchor which is suspended from the coil is made of Permalloy. The copper coil has 40 turns, the total structure has a surface of 3.5 x 4.5 mm2 including bond pads. The structure height amounts to about 120 pm with the width of the elastic springs being 10 pm. With a current of 170 mA a displacement of 190 pm can be achieved using this actuator. Using a design which has been optimized in terms of a high retention force, a force of 17 mN is achieved with a current of 440 mA. As the described process is very complex, other concepts are followed where the coil is assembled with the LIGA structure [Guck96]. Fig. 7.7-11 shows such a coil. It consists of a core made of permalloy which is also fabricated with the LIGA process and detached from the substrate after electroforming. It has a length of 1.X mm, a width of 400 pm and a height of 180 pm. The core is surrounded by a copper wire 15 pm in diameter. This allows 90 windings per layer which results in an excitation of 1 Amp winding for a current of 11.1 mA. For example, for 5 layers the resistivity is about 50 Ohm resulting in a power loss of 6.2 mW. This

7.7 Examples of Applications

359

Patterning of the electrical layer on the substrate (CrlAu)

Patterning of sacrificial layer (Ti)

First aligned X-ray exposure, electroforming of the core from FeNi alloy

Deposition of PMMA, patterning of the seed layer on the PMMA, second X-ray exposure

Electroforming of the coils, removal of PMMA and the sacrificial layer Fig. 7.7-9 Process steps for the manufacture of an electromagnetic microactuator with ferromagnetic core.

is much more in excitation and much less in power loss compared to the integrated version. This coil is part of an electromagnetic micro chopper shown in the upper part of Fig. 7.7-12. The core is fixed in the LIGA structure by springs which are part of the yoke. The yoke has an opening which attracts the movable anchor when a current is applied to the coil. The anchor is fixed to a massive block by two parallel springs. This allows an excellent parallel movement of the anchor, which is necessary to avoid contact of the anchor to the yoke. The anchor is designed to act as an aperture which is oscillating between the end faces of two fibers fixed in the groove formed by the walls in the lower part of the picture. Thus due to the oscillating anchor the whole setup will act as a chopper for light of an optical

360

7 The LIGA Process

Fig. 7.7-10 SEM micrograph of an electromagnetic actuator fabricated as indicated i n Fig. 7.7-9.

Fig. 7.7-11 Microcoil: A copper wire 15 pm in diameter is wound around a nickell iron core structure (length 1.8 mm, width 0.4 mm, height 0.18 mm).

fiber [Krip99]. The size of this micro chopper is 3 X 3.2 mm2, the total height is 280 pm. Fig. 7.7-13 shows the amplitude per mA as well as the phase shift near the resonance frequency which is about 1100 Hz. For the resonance frequency the amplitude per mA is about 16 pm. As can be seen from Fig. 7.7-14 the amplitude can be increased to about 160 pm by increasing the current. Nevertheless for higher currents, no further increase is observed although the saturation current is not reached. This is because of the limited width of the yoke which is only 100 pm at the position of the anchor.

7.7 Examples of Applications

361

Fig. 7.7-12 A LIGA micro chopper made of Permalloy to be used in a fiber optical set-up.

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Microturbines, Flow Sensors In case of the acceleration sensor and the comb drive, besides the freely movable parts, there are still other parts fixed to the substrate. With rotating microstructures the rotors must however, be fully detached from the substrate, whilst with integrated structures the axes must remain fixed to the substrate [Wa1191]. Integrated microturbines driven by gases or liquids, can in principle be used as flow sensors. Figure 7.7-15a shows a nickel microturbine, whose diameter of 130 pm is smaller than its height of 150 pm. The gap between the rotor and axis is about 5 pm in this particular case. Since conventional lubrication of the rotors would be difficult due to the very narrow gaps, air sliding bearings are used. To determine the number of revolutions, a shaft is designed in which a glass fiber can be embedded,

7 The LIGA Process

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from which the number of rotations can be determined optically (Fig. 7.7-15b). The light emanating from the fiber is reflected on the front end of the turbine blades. The number of rotations can be measured with great precision by counting the number of light pulses. Figure 7.7-1% shows for example, the increase in the number of rotations during a start up process. One can see that the microturbine can accelerate from the stationary state to 2500 rotations per second in less than 6 rotations. In long-duration tests with this experimental setup it is demonstrated that a total number of rotations of about 100 million is possible with the existing bearings [Himm92]. Turbine structures with dimensions of a few millimeters and with free shaped turbine blades, which can be produced by the LIGA process, are an essential component of micromillers used in minimally invasive medical instruments. The entire milling head is produced by a mounting technology, in which the fluidic part is manufactured with a mold form, which was manufactured using mechanical micromanufacturing (see Chapter 8) [Wa1196]. With these turbines, flows of about 750 ml/min, torques of 10 pNm and 1000 to 2000 rotations per second were achieved. These values are over one order of magnitude higher than values achieved with electrostatically or electromagnetically driven micromotors.

Micromotors The principle of an electrostatic motor is based on the attractive force of two oppositely charged electrodes forming a capacitor. If two electrodes are staggered opposite to each other, both plates attract not only in the direction to their normal (operation component at right angles to the surface), but also parallel to the surface, until they are exactly facing each other (operation component tangential to the surface). Therefore, if a rotor and a stator have partially staggered electrodes, the tangential drive component can be exploited to produce a torque leading to rotation. Figure 7.7-16a shows the principle of an electrostatic motor, in which a voltage is applied across two oppositely facing stators. On the surface of the conducting rotor, opposite charges are induced which results in a tangential propul-

7.7 Examples of Applications

363

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sion force. After the alignment of the offset plate and thereby the disappearance of the tangential force, a voltage is applied across the neighboring electrode pair, which again shows an offset. In principle, it is possible to apply a voltage between each stator pair and the rotor which would have to be contacted in this case. The resulting torque increases linearly with the height of the structure, so that with electrostatic micromotors a large structure height is desirable. This can be easily produced with the LIGA process [Wa1192]. A large structure height is also advantageous for smooth running characteristics (low wobbling movement). As the tangential propulsion force depends on the height, but not on the length of

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7.7 Examples

of Applications

365

the electrode, it is advantageous to design the rotor and the stators such that many small, parallel switching electrodes result, which is best accomplished by the use of a toothed surface. Figure 7.7-16b shows the view of a LIGA motor and Fig. 7.7-16c shows a detailed view, from which one can see the size of the large structure height of 120 pm. In Fig. 7.7-16d, the voltage necessary to maintain rotation is shown as a function of the rotational velocity. This increases with increasing rotational velocity. With a small rotational velocity it amounts to 65 V. The voltage, required to start the motor is about 30 % larger in order to overcome the static friction. A suitable choice of all geometry parameters ensures that the rotor does not slide but rolls on the axle, which leads to a decrease in the friction between the motor and the axle. A problem with the micro electrostatic motors, whose diameter lies in the region of 1 mm, is the low torque of a few nNm, which is not generally sufficient for a propulsion unit. If micromotors are to be constructed by microtechnological processes, which are then suitable for a propulsion unit, it is necessary to assemble individual microcomponents utilizing mounting techniques, as was the case with microturbines. As a result, considerably more degrees of freedom result with the configuration of the components and the choice of material. On this basis an electromagnetic reluctance motor was realized [Lehr96]. Coil packets are attached onto a weakly magnetic stator. The weakly magnetic rotor and conventionally produced roller bearings sit on an axle and across a spacer ring on a stator set-up. Stator, spacer ring and rotor are produced with the LIGA process. By optimizing the electro-deposited weak magnetic material a torque of 1 to 2 pNm is generated with a maximum number of rotations of 10000 rev./min. Additional enhancement of the torque value can be attained by optimizing the stator and minimizing the air slit to 1-2 pm. Higher torque values are to be expected with motors constructed using the concept of the permanent magnet motor with air gap winding.

366

7 The LIGA Process

7.7.3 Fluidic Microstructures The application of microfluidics is predicted to become a large market in the future. Application areas are dosing systems for medical purposes or chemical analytical systems as well as microfluidic devices for use in biomedical analysis in the nanoliter regime, to name just a few examples. Strong efforts have therefore been made, to realize micropumps and microvalves using different microtechnical methods [Zeng96].

Micropumps froin the LIGA Process Micropumps, which work according to the thermopneumatic principle are produced in a batch fabrication process (AMANDA process) [Scho99] using molding technology of the LIGA process and adhesive assembly techniques. As the mold insert is fabricated by precision machining, these products are described in chapter 8.1.

Microfluidic Switches To control fluid flows, bistable wall attachment elements produced using the LIGA process are suitable. These planar systems consist of a feed nozzle, two wall-like structures and two control nozzles (Fig. 7.7-17a). Because of the Coanda effect the fluid stream adheres to one of the two adhesive walls located directly beyond the feed nozzle. The stream is switched from one position to the other by a short control pulse, which is brought in above the steering nozzle, located behind the adhesive wall. The method of operation of the microfluidic switch can be deduced from Fig. 7.7-17b. The initial pressure is shown as a function of the control pressure on a control nozzle. At point A the stream sticks to the side of the control input being considered, the output pressure on the opposite output is roughly zero. If one raises the control pressure, the beam switches to the other position after the control pressure has exceed about 10 hPa (Point B); the output pressure increases (point C). If one raises the control pressure further, nothing changes and the stream remains stable. Also by lowering the control pressure the stream remains stable until about 10 hPa is attained (point D). At this point the stream is switched to its original position (point E) [Vo1193]. As the necessary control pressure for switching is relatively low, it can be produced by warming up a gas volume. Thus, the elements are fabricated on a silicon wafer, on which a self supporting heat element is structured over an etch groove (Fig. 7.7-17c). With this construction, the fluid currents can be switched with a power of about 500 mW, applied for about 1 ms. The switching time is approximately 40 ,us (Fig. 7.7-17d). In addition the switching pulse can also be produced by feeding back part of the output fluid stream into the control stream. In this case the fluidic switch works as an oscillator.

7.7 Examples of Applications

367

368

7 The LIGA Process

Microfluidic Linear Actuators The pneumatic operation principle can be used if microactuators with large displacement and large force have to be realized. A pneumatic linear actuator produced with the LIGA process is shown in Fig. 7.7-18. A movable plunger is mounted in the long channel in the middle of the structure. After inserting the plunger the entire structure is covered with a lid. On applying a pressure over one of the holes lying to the right and left of the channel, the plunger moves away from the corresponding hole [Gebh96]. Other structures are used as pressure compensation openings. As the plunger is lubricated by the fluid, 10 hPa is enough to initiate its motion. In order to be able to exert a pressure of 500 hPa on the plunger, using water, flow rates of about 1.36 llh are necessary. Thereby a force of 5 to 10 mN is achieved. The speed of the plunger lies in the region of 1 mlsec.

Fig. 7.7-18 Pneumatic linear actuator with reciprocating piston

7.7.4 LIGA-Structures for Optical Uses The LIGA process is well suited to fabricate microoptical components and systems because of four reasons: 0

The resist used with the LIGA process, PMMA (polymethylmethacrylate), possesses good optical transmission properties in the visible and the near infrared wavelength range. Therefore, it is possible to use microstructures produced in the first step of the LIGA process for optical applications [Gott92, GOtt911. Other optical materials are available by molding, such as polycarbonate which possesses a higher temperature stability (to about 150 "C) and a higher refractive index (n=1.6). This gives further possibilities for optical applications.

0

Side walls produced by X-ray lithography have a surface roughness of 30-40 nm, which gives the possibility to use these structures as reflecting elements.

7.7 Examples of Applications

369

0

The shadow printing process ensures the highly precise position of the structures fabricated either by X-ray lithography or in the molding process using an X-ray lithography fabricated mold insert. Thus, because of their height, the LIGA structures can be used as alignment elements for optical components. This results in a very precise micro optical bench.

0

By the process described in Chapter 7.6.3 light guiding structures with very smooth surfaces can be patterned.

Thus, beside the fabrication of simple optical elements like lenses and prisms two main concepts in micro optics are followed using the LIGA process: 0

Fabrication of precise micro optical benches with LIGA fabricated alignment structures to which optical components like fibers, lenses, diodes, wavelength selective filters, beam splitters, etc. can be added. These optical benches can also include mechanical structures like electrostatic linear actuators.

0

Fabrication of planar waveguides with sidewalls acting as optical elements. Light guiding is achieved either in a three layer system by total internal reflection or in a free space covered by ground and top plate by Fresnel reflection.

Simple Optical Elements, Lenses, Prisms Simple examples of microoptical elements are cylindrical lenses and microprisms made from PMMA, which are produced by direct structuring. The forms can be arbitrarily chosen. Also the structures can be exactly positioned with respect to each other. Of particular advantage is the parallel production of microoptical components and of mechanical mount- and guidance structures for glass fibers, as thereby the alignment and mounting costs are considerably reduced. Figure 7.7-19 shows a beam splitter for multi-mode fibers. In this case a transmission fiber (fiber l), a measurement fiber (fiber 2) and a detector fiber (fiber 3) are accurately positioned to a microprism with the aid of fiber grooves. The arrangement enables to use the measurement fiber in a bidirectional mode. The light from a light source is coupled by the coupling element into the fiber and transmitted to a fiber optical sensor head. The modulated sensor signal is transmitted back in the same fiber and coupled to the detector fiber by the coupling element. Any desired intensity splitting relation can be achieved by choosing the size and position of the prism. Such a beam splitter can for example, represent an essential component in the distance sensor, schematically shown in Fig. 7.7-20a. The intensity of the reflected light, which is coupled with the prism of the beam splitter in fiber 3, strongly depends on the distance between fiber end and the mirror i.e. from the distance to the lens. The measured light intensity of the reflected light is shown in Figure 7.7-2013 for an arrangement with and without a lens. Because of the gra-

310

7 The LIGA Process Fiber 3

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Fig. 7.7-19 Optical beam splitter made by X-ray lithography. a) principal design, b) SEM micrograph of the beam splitter.

dient of both curves, small changes could be detected with great precision. If the reflecting surface is, for instance, the membrane of a pressure element, the pressure can be very precisely measured with this method. Structures with spherical lens geometry can be produced by the process described in Section 7.6.2. As the focal length corresponds to the diameter of the lens, lenses with high aperture ratio are produced in this way [Gott95]. These lenses are either separated from substrates and integrated in an optical set-up or used as lens plates.

7.7 Examples of Applications

371

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Microoptical Bench The necessity of using a variety of material systems imposes limitations on the monolithic integration approach when constructing modules with complex optical function. Therefore, modular concepts based on microoptical benches become more important in photonics applications. Active and passive microoptical components made by a variety of techniques can be combined most readily and relatively inexpensively into a complete functional system by utilizing the modular concept based on microoptical benches. Such concepts can be readily realized with the LIGA process and with the aid of automatic assembly and bonding techniques.

372

7 The LIGA Process

The cost of the active alignment can be avoided by the microtechnical manufacture of a precise, and if necessary also stepped, microoptical bench, in which the mounting for the hybrid components are structured with sub-micrometer accuracy at the required location with respect to the beam path. This makes the process easier both technically and economically. Parallel to the passive mountings, moveable micromechanical devices can also be integrated into the bench, which can then be used, e. g., as an opto-mechanical switch. Bidirectional Transmission- and Receiving Module

An optical transmission- and receiving module for bidirectional wavelength multiplexing (WDM) is built by using a microoptical bench produced by molding technology with a stepped LIGA-tool. Ball lenses, a wavelength filter and the glass fibers are positioned next to each others by the fixing structures of the optical bench (Fig. 7.7-21a). In order to guarantee a defined height of the optical axes, the fiber is fixed on a plateau, which lies precisely 387.5 pm above the highest level of the remaining substrate because of the diameter of the ball lens of 900 pm. To avoid misalignment by temperature changes the fixing structures are molded on a ceramic substrate by hot embossing. The bench is mounted in a hous-

Fig. 7.7-21

Bidirectional transmission and receiving module fabricated by means of hot embossing of a polymer substrate. a) Polymer substrate (microoptical bench) with ball lens and alignment guidance for a beam splitter, b) assembled set-up including actively aligned laser and photodiode (by courtesy of ALCATEL SEL AG, Stuttgart, Germany).

7.7 Examples of Apptications

373

ing where the hermetically sealed active components are adjusted and laser welded relative to the optical setup. Figure 7.7-21b shows the fully encapsulated bidirectional transmission and receiving module. The light emitted from the laser diode (on the right side of the housing) is collimated by the first ball lens, runs through the wavelength filter and is focused with the second ball lens on the end of the mono-modal fiber. Light emitted at another wavelength from the glass fiber, is collimated with the ball lens and reflected by the wavelength filter to the photodiode, which is sitting on the housing. With such a construction the insertion loss between the fiber and photodiode is as low as 1 dB, the cross talk attennation exceeds 40 dB.

Microoptical Function Module with Active Devices - Laser-waveguide Coupling To take further advantage of the described concept it is desirable to integrate also non-encapsulated active components in the microoptical bench. For this, active components with so-called ‘alignment trenches’ are used [Jone95]. These alignment aids must be positioned accurately relative to the active semiconductor laser area. This can be done after having produced the active laser area by an additional photolithography and etch process with tolerances of less than 1 pm. The passive mounting can be carried out with the required precision if appropriate trenches utilizing the concept of molding with stepped molding tools are integrated in the optical mounting frame relative to the optical beam path. This was demonstrated in the example of modules for laser-waveguide coupling (Fig. 7.7-22). In the picture the ‘alignment trench’ on the laser chip as well as the stop edges can be seen in the polymer construction.

Fig. 7.7-22 SEM micrograph of a laser diode which was aligned with a microoptical bench. The alignment is performed by means of an alignment trench on the laser diode and an alignment stop integrated into the microoptical bench structure.

7 The LIGA Process

314

Microoptical Function Module with Active Devices - Heterodyne Receiver

A heterodyne receiver is another example of a microoptical bench with an active component fabricated by the LIGA process and assembly technique [Zieg99]. In case of a heterodyne receiver the signal to be detected is coherently superposed with the optical signal from a local laser source, whereas the local laser has a slightly different frequency (normally 1 GHz) compared to the incoming signal [Imai91]. Thus, the incoming signal can be amplified and by tuning the frequency of the local laser a specified signal can be selected out of a number of signals with different frequencies travelling in the incoming fiber. To create the interference signal both signals must have the same polarization. Thus, the receiver has to be build in a polarization-diversity arrangement [Ryu95]. The signal as well as the local laser light is coupled into the device by fibers, mounted on a fiber mount, collimated by ball lenses and separated into the two polarizations by a prism coated with a polarization sensitive layer (Fig. 7.7-23). The p-polarized light is transmitted, whereas the s-polarized part is reflected. The four light beams are divided in two equal parts at the opposite prism edge which results in two beams, one having a phase shift of p. In an exact optical setup the respective parts of the incoming and the local laser beam are superposed by 100% at the four photodiodes placed after the prisms. As can be seen from Fig. 7.7-24 the fibers, the lenses, the prisms as well as the photodiodes are fixed, assembled and aligned by polymer structures fabricated by the LIGA process on top of a ceramic substrate which carries the electrical bond pads to contact the photodiodes. The alignment tolerances of all the optical components are less than 1 pm which can be concluded from the achieved 95 % superposition of the two beams at the location of the photodiode. Fig. 7.7-25 shows the Heterodyne receiver on top of an SMD board which is fixed into an electro-optical housing [Zieg99a].

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Fig. 7.7-23 Principle of the microoptical bench of a hybrid heterodyne receiver.

7.7 Examples of Applications

375

Fig. 7.7-24 Hybrid setup of the heterodyne receiver in a microoptical LIGA bench.

Fig. 7.7-25 Microoptical bench of the heterodyne receiver on a SMB board mounted in a hermetically sealed housing.

376

7 The LIGA Process

Microoptical Bypass-switch Active optical switching elements for fiber networks are realized by integrating mechanical elements (microactuators) into the microoptical bench. As can be seen in Fig. 7.7-26 fibers and ball lenses are positioned and fixed in appropriate fixing grooves. A movable mirror which is part of the electrostatic actuator is placed at the cross-point of both collimated beam paths between the fibers. In the open state, the light from the lower left network fiber is transmitted to the upper right fiber, whereas the light from the upper left fiber is transmitted to the lower left network fiber, thus connecting a participating device to the fiber network. In the closed state the participant is bypassed by directly reflecting the light from the lower left network fiber to the lower right network fiber directly. Such elements are produced in metal with the LIGA process. For these elements a loss of 2 dB, was measured in the open state and without any optimization of the reflecting layer a loss of about S dB is achieved for the closed state.

Fig. 7.7-26 Microoptical bypass-switch with integrated linear actuator, as a component of an optical bench setup realized on a silicon wafer. Light Guiding Structures, Microspectrometer Light guiding structures based on total internal reflections are realized by the process described in Section 7.6.3. PMMA can be used as the core layer material for applications below 900 nm (Fig. 7.7-27). For wavelengths of about 900 nm and 1100-1250 nm PMMA shows high absorption because of resonance vibrations of the hydrogen atoms in the polymer molecule. If the hydrogen is completely replaced by the heavier deuterium (PMMA-d8), the resonances are shifted to larger wavelengths. Thus, using PMMA-d8 as the core layer, gives access to wavelength regions up to 1400 nm relevant for communication technology. An example of such a planar waveguide is a microspectrometer whose main features are shown in Fig. 7.7-28 [Ande88, Mii119Sal. The relevant structures of the device are a self focussing reflection grid and fiber alignment structures for coupling and decoupling fibers. The light is coupled into the planar waveguide by an optical fiber which is precisely positioned to the reflecting grid. The light hits the reflection grid and is diffracted into its spectral components, which are

7.7 Examples of Applications

377

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wavelength [nm] Fig. 7.7-27 Attenuation of light in dependence on the wavelength in a light-guiding

structure in PMMA, and in the case where the hydrogen in the PMMA is replaced by deuterium (PMMA-d8).

focused on the focal plane. Aberration is minimized by an optimized design of the individual teeth of the grid. The grid possesses about 1200 teeth, which show an average step width of 1.8 pm and an average step height of only 0.2 pm. The reflectivity of the grid is achieved by sputtering with a thin silver layer after structuring. The spectrometer can be easily combined with a suitable diode array if at the place of the outcoupling fibers a 45" edge is patterned by inclined exposure which changes the light path by 90" (Fig. 7.7-29). The diode array is evaluated by 16 bit electronics which fit optimally on the array, resulting in a spectrometer with a high dynamic range. The microspectrometer system includes all components of a complete macroscopic spectrometer with the volume of a cigarette box. With this construction, transmission values of 20 % can be achieved, the resolution lies at about 7 nm and the dynamic range can be up to 20,000. Such microspectrometers find applications in color measurement systems, in on-line process photometers and flow-injection systems [Mu1195b]. For building microspectrometers for the wavelength range above 1400 nm, multilayer waveguides based on polymer materials can no longer be used because of the low transmission of the polymers. To stay with the same concept of a planar spectrometer system the light has to be guided in a cavity by Fresnel reflection at the top and bottom cover. Such a cavity with diffraction grid and fiber fixing groove can be easily fabricated by molding techniques using a precise LIGA mold. Fig. 7.7-30 shows the whole setup where the molded part consists not only of the LIGA structures but combines also the lower housing of the spectrometer system [Krip99]. The light guiding part is covered by a metallized cover which has an opening into which the InGaAs detector ai-ray fits. The detector array is mounted on an electronic carrier and forms the second housing part.

7 The LIGA Process

378

400

500

600

700 800 Wavelenght I nm]

900

1000

1100

Fig. 7.7-28 Microspectrometer with self-focussing reflection grid in a trilayer set-up. a) principal set-up of the spectrometer, b) intensity distribution at the read-out slit measured with a photodiode at equidistant positions at the slit.

7.7 Examples of Applications

379

Fig. 7.7-29 Microspectrometer system including the spectrometer as shown in Fig. 7.6-28, the read-ont diode array, and the 16-bit evaluation electronics.

Cover of the spectrometer component with electronic

uiding structure

wer housing with optical structures

Fig. 7.7-30 Schematic view of the setup of the NIR microspectrometer system. The molded part consists of the optical LIGA structures and the lower housing of the spectrometer system. The second housing part is formed by an electronic carrier in which the detector array is mounted.

380

7 The LIGA Process

Fig. 7.7-31 Assembled NIR microspectrometer system. Only the electronic carrier is seen which covers the optical system. Underneath the white area the detector array is located.

Thus, the whole setup results in a very compact spectrometer system (Fig. 7.7-3 1) which has the size of half a cigarette box. The performance of the system was demonstrated for the wavelength range 0.95 pm to 1.75 pm. The resolution for a sensor element pitch of 52 pm is better than 20 nm, the sensitivity turned out to be better than 870 countshW. The noise equivalent power is minimized to 2.65 pW at 1.56 pm wavelength for an operating temperature of 42" which avoids a complex cooling system. With these values the system can easily be used in polymer characterization [Krip99].

8 Alternative Processes of Microstructuring

Besides the “great” technologies of microstructuring some processes were developed which alone or in combination with other methods, could offer valuable additions to optimized solutions of problems. Some of these alternative processes will be described in this chapter, to supply incentives for ideas of unconventional products, which can be derived from by combining different technologies.

8.1 Mechanical Micromanufacturing Mechanical micromanufacturing was developed originally at the former Nuclear Research Center, Karlsruhe (now Research Center Karlsruhe (FZK)) in collaboration with the former company Messerschmitt-Bolkow-Blohm (MBB), nowadays Daimler-Chrysler. The task was (as with the development of the LIGA technology) to be able to produce economical orifice ports with submillimeter cross-sections for enrichment of gaseous UF6 with the isotope U235.In the context of its new research emphasis of microtechnology, a new process was developed by Research Center Karlsruhe to produce microstructures mechanically, which is based on the surface structuring of films and substrates by precision machining with profiled microtools [BierSS, SchuSBa, Bier91al. Microstructure bodies can be produced for instance, by stacking and bonding structured metal films with numerous high precision microchannels. Furthermore, structured surfaces of thick substrates can be duplicated by different molding processes, which are also part of the LIGA technology. This provides the possibility for an economical mass production process. The smallest lateral dimensions of mechanical micromanufacturing is in the order of a few micrometers, whilst the structural Iength can be in the millimeter or even centimeter range. The cross-sectional shape of the microchannels and their arrangement is easily determined over a wide range. The manufacturing tolerances with lateral dimensions and with the parallel channel structures, are in the range of micrometers. All materials which can be machined with diamond, hard

382

8 Alternative Processes of Microstructuring

metal or ceramic tools can be considered, i.e. many metals and metal alloys as well as shape retaining plastics and semiconductors such as silicon [Bier92].

8.1.1 Production Process and Primary Structures Essentially the microstructuring of metal foils can be realized in two ways with precisely ground diamond tools. Figure 8.1-1 schematically shows the processing on a turning lathe. A metal tape is stretched around the circumference of a disk with the aid of a spring tension fixture, which is set up on the spindle of a lathe. The adjustable tool holding fixture with the microtool is located on the sliding rest below the disk. By CNC-control of the sliding rest and spindle-axes, parallel running grooves, with a shape predetermined by the microtool, are cut into the surface of the tape. Figure 8.1-2 is a schematic representation of the second method to structure films on a high precision milling machine, in which the metal foil is fixed onto a precision ground vacuum chuck. The chuck is mounted on the x-y-stage of the milling machine. The microtool (diamond miller) is located on the circumference of a high frequency spindle. The cutting speed of the diamond can be varied with the aid of the variable-speed high frequency spindle. Microstructuring in different directions is performed by using a revolving stage (not represented in Fig. 8.1-2). Groove cross-sections can be machined as rectangles, triangles or half circles depending on the shape of the profiled manufacturing diamond tools. Figure 8.1-3a shows the SEM-micrograph of a profiled micro-diamond with rectangular

Metal foil Chuck Clamping Spindle Cutting diamond Table

Fig. 8.1-1 Mechanical microfabrication using a lathe for manufacturing unidirectional structures (grooves) (Courtesy of Research Center Karlsruhe).

8.1 Mechanical Micromanufacturing

383

Tool holder Diamond

Foil Stopping face Sintered metal plate

-

Vacuum chuck to Vacuum pump

*

Table

Fig. 8.1-2 Mechanical microfabrication using a high precision milling machine (Courtesy of Research Center Karlsruhe).

cross-section and a cut width of 100 pm. One can see that the cutting contour of the diamond was shaped from a large raw diamond. Figure 8.1-3b shows a wedgelike shaped diamond with a nose angle of 20". Here the maximum cutting depth is 500 pm. A foil of 100 pm thick aluminum, which has been machined with a rectangular shaped diamond tool is shown in Fig. 8.1-4. The trench, which has been cut into the metal, has a cross-section of about 70 pm by 85 pm. The pitch size is 115 pm and the thickness of the remaining bottom is 30 pm. The surface still shows the structure of the original grinding and the edges carry fine burrs. If this burr is disruptive to further processing, then it can be removed in a second procedure with a flat-cutting diamond tool. The crosswise cutting of a smooth brass surface with the wedge-shaped diamond tool shown in Fig. 8.1-3b, results in a structure of four-sided pyramids as shown in Figure 1.1-3. In this sample the pyramids have a height of 250 pm with a pitch size of 100 pm in x- and y-direction. Therefore, 10000 micro pyramids are arranged on every square centimeter. The burr-free conical microstructures are especially well suited for the subsequent, repetitive molding process in polymers. With structured metal films, microstructure bodies were built and used in a variety of applications. Furthermore, plastic molding and subsequent electroplating opens yet more opportunities for a large number of new applications. The micromanufacturing is however, not limited to the processing of straight trenches. Microstructures can also be produced with very small drills and end-millers, which are subsequently used as molding tools for injection molding or hot embossing. Figure 8.1-5a shows a miller in hard metal for cutting rectangular grooves of 50 pm width [Scha99]. Figure 8.1-5b is displaying an arrangement

384

8 Alternative Processes of Microstructuring

Fig. 8.1-3 a) A profiled diamond for cutting grooves with rectangular cross section. The width of the cutting edge is 100 pm, and the maximum depth is about 300 pm, b) Picture of a wedge-shaped diamond for cutting V-grooves. The nose angle is 20°, and the maximum depth is 500 pm (Courtesy of Research Center Karlsruhe).

of circular channels cut into a brass plate. The channels have a width of 50 pm, the web is only 10 pm wide. The possibility to manufacture complex molding tools is shown in Fig. 8.1-6. Besides the very high surface quality, it is noteworthy also, that mechanical micromanufacturing offers the option to manufacture true three-dimensional dies, which as outlined in Chapters 6 and 7 is difficult if not impossible to fabricate by the “traditional” microstructure technologies. Therefore, the mechanical micromanufacturing is a valuable addition to these technologies, especially when manufacturing relatively large (> 10 pm) complex structures for molding processes.

8.1 Mechanical Micromanufacturing

385

Fig. 8.1-4 SEM picture of a microstructure fabricated on a lathe shown in Fig. 8.1-1. The grooves are 85 M r n wide and 70 pm deep. The remaining bottom has a thickness of 30 pin (Courtesy of Research Center Karlsruhe).

Fig. 8.1-5 a) Micro miller fabricated from hard metal. The diameter is about 40 pm, b) microstructure with 50 pm wide circular channels. The remaining web is 10 pm (Courtesy of Research Center Karlsruhe).

3 86

8 Alternative Processes of Microstructuring

Fig. 8.1-6 A complex structure as a molding tool, manufactured in mechanical microfabrication (Courtesy of Research Center Karlsruhe).

8.1.2 Examples of Applications Micro Heat Exchanger For the construction of micro heat exchangers in cross-flow mode, square pieces of structured metal foils (i. e. copper or stainless steel foils of 100 ,um thickness, see Fig. 8.1-4) were assembled as a cube in such a way, that every other foil was turned by 90". Subsequently, 100 of these foil pieces were stacked on top of each other. This resulted in a body, which consisted of about 8000 microchannels with rectangular cross-section, and from which half of them formed a passage for conducting heat emitting fluid and the other half for heat absorbing fluids. The heat exchange surface including fin-like bridges between the channels is about 150 cm2, the structured transfer volume 1 cm3, In a subsequent process step the stack of foils was clamped together and diffusion welded in a vacuum chamber. Figure 8.1-7 shows three copper heat exchangers of different size without the connecting pipes for fluid supply and delivery. The performance of these heat exchangers was tested with water. The water source at the entrance had a temperature of 95 "C, and the water to receive the heat had a feeding temperature of 13 "C. The water flow rate was varied between 0.25 and 12.5 Urnin, with the same value in both passages at any one time. The experimental data for the transferred heat and the heat transfer coefficient were plotted in Fig. 8.1-8 versus the flow rate of the water. A thermal exchange of about 20 kW was achieved with the highest flow rate and an average temperature difference between both fluids of 60 K was established. This result coiiesponds to a volumetric heat transition coefficient of 324 W/cm3K. This value rates about 1 to

8.1 Mechanical Micromanufacturing

387

Fig. 8.1-7 Photograph of three micro heat exchangers in copper. The smallest has an outside volume of 1 cm3, the actual exchange surface is 150 cm2. For higher throughput different sized were manufactured too. The size of the grooves though in all models was kept the same (Courtesy of Research Center Karlsruhe).

c

25 I

z

n

20

I

I

I

I

heat transition coefficient

10

rcI

rn

0

0

0

5 10 water flow [Vmin]

a,

c

15

Fig. 8.1-8 The transferred heat power and the heat transition coefficient versus the flow rate of the water (Courtesy of Research Center Karlsruhe).

3 8%

8 Alternative Processes of Microstructuring

2 orders of magnitude above the value obtained with conventional compact heat exchangers [Schu89b, Bier901. By using different materials (copper, stainless steel) consequences of the longitudinal thermal conduction can be clearly distinguished by the heat exchange behavior [Bier93b]. Applications for the micro heat exchangers are needed in areas, where high heat transfer using small weight and volume structures are required, e.g. in air- and space travel and chemical process technology and many other technologies [Schu98b, Wies97, Hage981.

Fig. 8.1-9 a) Schematic view of a static mixer (V-mixer) and b) SEM picture of the exit side of a micro mixer (Courtesy of Research Center Karlsruhe).

8.I Mechanical Micromanufacturing

389

Micro Reactor A further development of micro heat exchangers is seen in the micro reactor, to which a mixer element is added. The mixer is designed similarly to a crossflow heat exchanger, however at the output of the mixer, both fluid currents mix almost ideally into each other within a distance of 1-2 mm. Figure 8.1-9 shows a cross-section of a mixer in mechanical micromanufacturing. One clearly sees the microchannels at the output which are arranged at less than 90" to each other. After mixing, the fluid may enter a modified heat exchanger and can react with each other with precise temperature control. This proved to be necessary for strong exothermal reactions. By selecting the material of the exchanger, the reaction can be accelerated catalytically. Figure 8.1-10 shows a complete micro reactor comprising the two inputs for the media, the mixer, the reaction chamber, and finally the output of the reaction product. Despite the small dimensions of the reactor 1800 tons per year can be processed in a continual process with an output of 5 l/min and a density of the product of 1g/cm3. In many applications such a microsystem can be directly integrated into chemical production process [Schu98a].

Fig. 8.1-10 Photograph of a micro reactor including a mixer (the lower structure with

two inputs) and the reaction chamber (upper right part with a single output). The heat control is performed with the cross flow (lower right to upper left part) (Courtesy of Research Center Karlsruhe).

390

8 Alternative Processes of Microstructuring

Micro Containers for Cell Cultures A further potential application can be seen in the manufacture of arrays of small containers for biological cell cultures. In the example shown in Fig. 8.1-11 the micro containers have a depth of 100 ,urn and a with of 300 pm. A porous bottom is integrated into the structure. Therefore, nutrient solution can be added to cultures from the bottom without the risk of spreading cells into the neighboring containers.

a

b Fig. 8.1-11 a) Micro containers for biological cell cultures. The bottom of the cells is perforated to allow nutrition liquid to enter the container. The dimensions are 300 pm in width and 100 pm in depth. b) Embossing tool for microcontainers (Courtesy of Research Center Kai-lsnthe).

8.1 Mechanical Micromanufacturing

39 1

Micropumps In combination with other processes mechanical micromanufacturing opens a wide range of interesting applications. A micropump can serve as a representative of the potential of this technology [Maas941 (Fig. 8.1-12a). The dimensions of the pump lend themselves to a metal-cutting manufacturing of the molding tool. However, should the application of the pump need to be changed to smaller dimensions, at anytime the mold-making process could be replaced by other microstructuring technology, for instance the LIGA technology. The advantage is the time saved for prototyping a new application. For the final mass production optimized technologies can be chosen. The cross-section of the pump is shown in Fig. 8.1-12a. An actuator chamber, which is sealed hermetically with a membrane (polyimide, 2 pm thick), is inter-

b

input valve

utput:valve

Fig. 8.1-12 A micropump for gases, a) photograph of the product, the outside dimension is about 10 by 10 mm’, b) cross-section of the pump.

392

8 Alternative Processes of Microstructuring

mittently heated up by means of a meandered conducting line. The heated gas in the actuator chamber expands and the membrane is pushed into the pump chamber. The pressure in the pump chamber closes the inlet valve and opens the outlet valve. The medium in the actuator chamber is pushed out. When turning off the heater, the gas in the actuator chamber cools down, the membrane retracts from the pumping chamber. The low pressure in the chamber opens the input valve and closes the output valve, and the pump chamber is filled again with the medium to be pumped. The pump consists essentially of three components, firstly an upper molding part with fluidic in- and outlet and the pump actuator chamber, secondly a membrane with an integrated heater meander and thirdly a lower molding tool, which forms the actual pumping chamber. The upper and lower parts are manufactured from polysulfone by injection molding. The molding tools were produced by means of a metal-cutting micromanufacturing process. With each molding cycle a substrate with 12 pumps is produced (Fig. 8.1-13). On assembling the three components to a pump, the upper molding part is glued to the membrane. Therefore, the adhesive is inserted into a system of adhesion chambers via a few inlet openings, which are arranged such that the adhesive is distributed over all 12 pump structures. Thereby all 12 pump bodies are mounted simultaneously. The polyimide membrane is manufactured on a silicon wafer by spin coating. By means of photolithography and by thin film processes, the heater meander is patterned for the actuator chamber, finally the valve openings are inserted into the membrane. After curing the adhesive, the structural unit “upper molding part and membrane”, is removed from the substrate. In the next step the lower molding membrane is fixed to the structural unit in basically the same process. The fluidic connections as well as the electrical contacts are mounted individually after separation of the pumps.

Fig. 8.1-13 Batch fabrication of micropumps. On one substrate, 12 pump bodies are integrated. Upper mold, lower mold, and membrane were assembled at the substrate level.

8.1 Mechanical Micromanufacturing

a

393

b

Fig. 8.1-14 Cross section of a) a conventional X-ray recording film with intensifying layer. The incoming X-radiation is transformed to visible light by the luminescent intensifier, which in turn exposes the photographic film, b) a microstructured intensifier confines the light within the micro chamber, thus keeping the resolution in the range of the pitch size of the microstructure. c) Micrograph of a cross section of a microstructured intensifier. The pitch size is 100 pm (Courtesy of Research Center Karlsruhe).

394

8 Alternative Processes of Microstructuring

X-ray Intensifying Screens X-rays are commonly used in medical diagnosis. On one hand one has to obtain as much information as possible, however, on the other hand the dosage deposited into the patient should be as low as possible. X-ray films in combination with intensifying layers are used exclusively in X-ray diagnosis, which permits a reduction of dosage due to their high sensitivity. Unfortunately the increase in sensitivity, or the increase in layer thickness of the intensifying material, result in a reduction in resolution, as can be seen in Fig. 8.1-14a. The luminescent light with its isotropic characteristic together with additional scattering of the light on the polycrystalline intensifying layer generally produces blurred spots at the recording field. With the microstructured intensifying screen [Bier93a, Bier941 the sensitivity can be increased by using a large intensifying layer thickness, and at the same time avoid the excessive blurring of the spot. With a structure as shown in Fig. 8.1-14b, the luminescent light is confined in the micro cavity and can not exceed the pitch size of the micro structure. The microstructure is fabricated by hot embossing, while the embossing tool is manufactured by crosswise cutting V-grooves into the master. After filling these cavities with an X-ray intensifier (i. e. Gadoliniumoxisulfide), a microstructured X-ray intensifying screen is produced (Fig. 8.1-14c).

8.2 Electro-Discharge Machining (EDM) Electro-discharge machining or spark erosion is a very powerful and versatile technique, which can be utilized to shape 3 -dimensional microstructures out of electrical conductive material, especially of steel, hard metal, but also of ceramics and silicon. EDM is well established in the tool-making business and for machining high precision parts for prototyping. Due to the nature of the machining it is less suitable for mass-fabrication, but is well established as a means to manufacture dies and molding inserts for injection molding and hot embossing. The value of this technique for microfabrication was acknowledged already a few years ago, and in the meantime a variety of products have been offered, especially in the medical business, where three-dimensional shaping of wires of titanium-nickel alloys are in demand.

8.2.1 The Basics of EDM EDM is a process, in which a tool is imaged onto a workpiece. The physical effect which is used in electro-erosion is the evaporation of matter due to an electrical discharge between tool-electrode and workpiece-electrode. The two electrodes are submerged in a dielectric liquid. Both electrodes are positioned at close

8.2 Electro-Discharge Machining (EDM)

395

distance, and a voltage is supplied. When the breakdown voltage is exceeded, a plasma channel builds up between the electrodes, and as a consequence of this, material of the electrodes is evaporated [Reyn97, KOni971. This process can be divided into three elementary steps: Starting phase, discharge phase, and closing phase. In the starting phase the plasma channel is generated. In this period the main current is concentrated at the surface area of the channel. By electron bombardment of the anodic side of the channel, a small amount of material is vaporized. In the discharge phase the plasma channel is fully developed and the main current is confined to a small cross section within the channel. Due to the thermal energy of the plasma discharge certain amounts of material are melted and vaporized at both electrodes. In the dielectric fluid an ever increasing bubble takes up the metal vapor. In the closing phase, the current is shut down, the plasma channel together with the bubble collapse and the evaporated material is ejected from between the two electrodes. The whole process relies on the fact that the material erosion is highly unbalanced between anode (workpiece) and cathode (tool). Otherwise a precise shaping of a workpiece would not be feasible. The erosion resistance index C, is a measure for the tool life. It is calculated using the formula: C, = ACT:,

(8.1)

where A is the heat conductivity (W K-l m-'), c is the specific heat (J mP3 K-I) and T, the melting temperature (K). Materials with high erosion resistance are suitable for EDM tools, whereas materials with low resistance index serve best as workpiece. The following table lists a few materials, which can either be used as tool or as workpiece. Table 3.2-1 Erosion resistance index of different materials [Reyn97] Electrode material

Erosion resistance index (lo1' Jz m-' s-' kg-')

Tungsten Copper Steel Silicon

2.99 2.79 0.23 0.0075

By appropriate setting of the process parameters and by the right choice of materials for tool and workpiece, the erosion on the workpiece relative to the erosion on the tool can be increased by an order of magnitude and more. There are generally two types of EDM: 0 0

the die sinking, and the wire erosion.

396

8 Alternative Processes o j Microstructuring

Both types are sketched in Fig. 8.2-1. In the first case a preformed tool, usually made of graphite or copper, is copied by electro-erosion into the workpiece, in the second case a wire is used as a kind of cutting tool to shape the workpiece into the desired form. Since the wire is subjected to erosion too, in wire erosion machines the cutting wire, usually made of tungsten or copper from 25 pm to 300 ,um in diameter, is fed continuously onto the workpiece to guaranty always a fresh and defined cutting surface and to improve the accuracy of the shaping process. Variations of the process are seen in EDM milling, where a tool is moved in x-, y-, and z-direction to generate the desired shape into the workpiece, and EDM drilling, where a wire is used as a sink tool to cut a hole into the workpiece (Fig. 8.2-2). As dielectric fluid usually mineral oil or synthetic oil is used. Sometimes deionized water is applied especially with wire erosion in microdimensions. The dielectric fluid serves three different purposes during the erosion process: 0 0 0

the contraction of the plasma channel, and therefore the increase in energy density, the removal of the erosion particles, and the cooling of the electrodes.

Fig. 8.2-1 The EDM process: a) die sinking, b) wire erosion.

8.2 Electvo-Discharge Machining (EDM)

397

Fig. 8.2-2 Variations of the two general EDM methods can be seen in a) the EDM milling of complex 3-dimensional shapes, and b) in machining small holes into the surface of the workpiece by means of a piece of wire.

8.2.2 Applications of EDM for Microsystems Contrary to lithography based microstructure technologies, with EDM complex 3 -dimensional microstructures can be manufactured and utilized as tools for injection molding or embossing. There is also the possibility to manufacture intricate instruments by wire erosion for minimally invasive therapy [Menz94]. The material for the following examples was superelastic NiTi alloy. A rod of 500 pm diameter was cut with a 30 pm tungsten wire in order to fabricate a micro gripper as seen in Fig. 8.2-3. The two jaws of the gripper were bend apart to keep the gripper in the open position. A plastic tube is slipped over the jaws to close the gripper. This is demonstrated in Fig. 8.2-4 where a gripper is used to assemble a microlens to a medical catheter. By moving cutting wire and workpiece with respect to each other during the erosion process more complex structures can be achieved, as shown in Fig. 8.2-5.

398

8 Alternative Processes of Microstructuring

Fig. 8.2-3 a) A method of fabricating a micro gripper by wire erosion, b) SEM-picture of the finished gripper made of superelastic NiTi-wire 500 pm in diameter. The object between the jaws is a human hair.

Fig. 8.2-4 SEM micrograph of a gripper handling a micro lens for medical application.

Fig. 8.2-5 Another gripper made with wire erosion by moving both cutting wire and workpiece with regard to each other during the cutting process.

8.3 Laser Micromachining

399

8.3 Laser Micromachining Laser micromachining is an attractive alternative for the fabrication of miniaturized parts in cases precluding the use of mechanical machining approaches such as drilling, milling, and cutting. Shaping hard and brittle materials such as silicon, diamond, quartz, diamond-like carbon (DLC), glasses, ceramics, and hard metals can be done readily. Soft materials which are subject to plastic flow when machined using conventional techniques, such as soft polymers, can be, machined precisely. Delicate workpieces and objects with inconvenient geometry can be handled, since laser micromachining exerts no large force. Surfaces can be modified at submicron depths. Holes and cavities can be realized with geometries and dimensions unachievable using classical methods. Laser micromachining operates by thermally ablating material from the workpiece surface. The laser beam is focussed, using an optical system, into a spot with a diameter of typically a few to a few tens of micrometers. Due to the absorption of the laser power at the surface, material is locally melted and vaporized, and dissipated as a plume of ionized and neutral atoms. Left behind on the substrate surface is a crater with geometry depending on laser parameters and operation mode, the material properties and the gas assisting the process. Figure 8.3-1 schematically shows the result of the continuous or pulsed dissipation of photonic energy on a substrate, leading to the formation of a hole. Ejected molten material decorates the upper rim and forms what is referred to as recast. Vaporized atoms and compounds re-deposit on surfaces near the machining zone and form so-called debris. At the surface of the heat affected zone (HAZ), the material has recrystallized, leaving the sample locally in a stressed state. Because of shadowing and beam shape effects, the hole walls are usually tapered. A typical laser micromachining setup is shown in Fig. 8.3-2. The workpiece is moved under the focussed laser beam using an x-y-table with encoders for posi-

Fig. 8.3-1 Schematic cross-section of a laser micromachined hole.

400

8 Alternative Processes of Microstructuring

Monitoring Focus control Ablation zone

Fig. 8.3-2 Schematic view of a laser micromachining setup.

tion control. Optimal focus is maintained by adjusting the position of the focussing objective using a z-motion stage. Focus is sometimes controlled by an independent inexpensive laser system. Initial alignment and ablation progress is monitored using a video camera. In some systems, the workpiece is enclosed in a chamber which can be flushed with reactive gases or inert protecting atmosphere. The entire setup is mounted on a rigid support guaranteeing the stable and repeatable positioning of the laser focus. Scanning speeds of a few centimeters per second are typical. Depending on the scanning speed, laser power, and absorption spectrum, thermal conductivity, and heat capacity of the material in question, depths of roughly 10 nm to a few micrometers are ablated at each pass of the laser beam. The setup is controlled by a computer program translating appropriate CAD/CAM data into stage motions. Professional systems cost 100 BUS$ upward. A range of different lasers is in use. The list includes various excimer lasers emitting light pulses in the ultraviolet (UV) wavelength range between 351 nm and 193 nm. An example is provided by KrF lasers with roughly 50 pJ output per pulse. Also popular are pulsed neodymium/yttrium aluminum garnet (Nd:YAG) lasers emitting at a fundamental wavelength in the near infrared (IR) of 1067 nm. Using frequency doubling, tripling, etc., the wavelength is folded into the visible and UV ranges, at wavelengths of 533 nm, 355 nm, and 266 nm. A third laser type is C 0 2 lasers emitting in the deep IR at 10.6 pm. Finally lasers operating in the continuous wave (cw) mode, such as argon-ion lasers emitting at 488 nm are in use as well. Fluences achievable using such photon sources are of the order of 10 J/cm2. The pulsed systems are operated at repetition rates of several kilo-Hertz, with pulse lengths from microseconds (ps) to picoseconds (ps). Ultrafast pulses are best for high precision laser machining. This is due to the limited diffusion of energy into the sample, enabling more efficient use of the laser energy to be made and the formation of recast and recrystallization zones to be minimized. Because a smaller depth of the sample is heated, therinomechanical stresses are lower and the tendency of charring, local microfracturing and chipping is reduced. Thus better surface quality is achieved. Beam focus determines resolu-

5.3 Laser Micromachining

401

tion and the minimal radius of curvature of edges. Accurate energy level control is required for predictable ablation. The choice of the appropriate laser is guided by the optical properties of the material to be laser machined. While many metals absorb over a broad range of frequencies including the visible range and thus can be machined using different lasers, glasses, semiconductors, quartz, diamond, and oxide-based ceramics show a transmission plateau in and around visible frequencies. In this case, excimer or C 0 2 laser may be suitable. Beside the simplest holes and channels, more complex cavities and prominent structures are feasible. In the machining process, one layer of material is removed after the other. By appropriately varying the two-dimensional scan shape of each ablated layer, with each layer scan being smaller than its predecessor, cavities with shapes can be obtained which are not feasible using mechanical techniques and silicon micromachining methods. The fabrication of hierarchical recesses in silicon would require repeated mask deposition, photolithography and etching. With laser micromachining, the process is performed in one step. To increase the ablation rate, the ablation process can be assisted by chemical etching gases. A significant increase in etch rate is achieved in laser micromachining of silicon using an argon-ion laser, if the reaction is accompanied by chlorine. This gas reacts with the silicon vapor into volatile silicon chlorides. As a consequence, the formation of recast and debris is reduced. Use of chlorine requires special precautions regarding chamber materials. The processing should be performed under a vented cabinet with regulated chlorine flow and at controlled temperature. At a removal rate of several lo5 pm3/s, the result is an etching process much faster than the fastest focussing ion beam method presently available. Applications of laser micromachining include MEMS, but go far beyond this field. The technique is routinely used for welding, cutting, drilling, milling, stripping, hardening, and heat treating, among others. Miniature holes are cut in tubes, isolating layers are stripped, flexible substrates are cut to complex shapes or are excised, and parts are micromarked using the laser technique. Three-dimensional micromachining is used in the fabrication of channels for biosensors, nozzles for ink-jet printers and wave-guides for THz applications. Laser micromachining is applied to obtain micromoulds, elements for diffractive optics, and holograms. The main disadvantage of laser micromachining is that in contrast with chemical etching it is a sequential technique. On the positive side, it demands less preparatory work in terms of mask fabrication than chemical techniques, since numerical data can be translated directly into ablation scans. This is particularly attractive in time-critical applications like the fabrication of small series or fast prototyping. In addition, laser processing in MEMS has found a broader range of applications through techniques such as photopolymerization, laser microchemistry, and laser assisted chemical vapor deposition (LCVD). The latter technique is well known to complement laser micromachining used in microelectronic circuit post-fabrication repair work. While laser micromachining is able to cut through erroneous metal connections or cut vias through dielectric layers to metal interconnect layers, LCVD is able to bridge metal lines with missing direct connection.

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9 Packaging and Interconnecting Techniques (PIT)

Packaging and interconnecting techniques occupy a central position within microsystem technology, and it is possible to combine components of microsystem technology and of microelectronics with each other, even though they are technologically incompatible during their individual production. PIT must be able to solve various problems of assembly and of materials combination within the system and to the outside world. Consider for instance the application in automobiles. A microsystem must be able to work here in a temperature range of -40 to 125 "C, in special applications up to 200 "C, it must be able to withstand accelerations of up to 50 g (for instance at the injection pump), it must be resistant to water spray, salt mist and corrosion against oil, gasoline, alcohol and detergents. In addition one expects a lifetime of 10 years or more for such systems for economical use. Here the highest demands are placed on the materials and methods of PIT. Similarly difficult conditions are encountered in medical technology, as all systems which come into contact with biological matter must be sterilizable. On implantation, the biological compatibility, as well as the reliability and the long term stability will be extremely important. One also frequently overlooks the fact that the system is to be protected against the corrosive influences of living organisms. Blood, for instance, is genetically programmed to dissolve foreign matter or, if this is not feasible, to encapsulate the foreign body by a passivation layer. However, in order to implant sensors into a body, the organisms defense system must be overcome by methods, which in the broadest sense are to be allocated to PIT. On manufacture of microsystems the PIT forms a decisive role for low cost, competitive production. In other words: PIT will play the key role in any further industrial expansion. If it is not possible to develop a suitable manufacturing process for widely different interfaces of a microsystem, then this microsystem technology will remain industrially unsuccessful.

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9 Packaging and Interconnecting Techniques (PIT)

9.1 Hybrid Technology Hybrid circuit integration means: the joining of microstructure elements of different materials and production technologies onto a common substrate. Screen printing is an essential technology of microelectronic hybrid technology. During the process the printing layers are applied preferentially onto a ceramic carrier and subsequently fired. This technology is relatively well known. A1,0, -ceramic substrates and various screen printing pastes are used as standard materials, which are introduced below.

9.1.1 Substrates and Pastes Substrates are usually dielectric carriers, on which the hybrid components are deposited and electrically connected by a wiring circuitry. Aluminum oxide ceramics are predominantly used for screen printing circuits, beryllium oxide ceramic was used as an alternative only in the area of power electronics. As beryllium oxidedust is highly toxic, it has been increasingly replaced over the last years by new substrate materials e. g. aluminum nitride. A variant of the “standard” ceramic substrate is the multilayer substrate (multilayer ceramics = MLC). These are used mainly in the packaging technology as wiring planes of highly complex integrated circuits. They consist of stacked layers of ceramic, interconnecting conductors and vias (vertical connectors to the adjacent circuit planes). The technology of the Green TapeTMLow Temperature Cofired Ceramics (LTCC) has a potential for mesoscale mechanical systems and therefore is treated in more detail in Section 9.6.1. The screen printing pastes, which are used for printing, consist of inorganic powders, which are mixed with a paste-like organic carrier material. The typical components of a screen printing paste are: 0 0 0 0

fritted glass, solvent- and wetting agent, organic binders, additives to modify the rheological properties.

Depending on the application specific paste components are added: 0

0 0

metal powder with conducting pastes, metal oxide with resistance pastes, glass or ceramics with dielectric pastes.

The exact formulation of the rheological properties of a paste is of prime importance, as the printing result is dependent on a number of parameters, which are associated directly and indirectly with the rheological properties, like for instance the viscosity, the surface tension between paste and substrate as well as the inner

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405

shear, which is present in the paste during the process of printing. By addition of certain gels to the printing paste, the material becomes thixotropic. The thixotropic material displays a high viscosity when undisturbed, but becomes more fluid when shear stress is applied, such as stirring, shaking or squeezing through a mesh, as it happens in screen printing. Therefore the paste flows easily through the pores of the screen and adheres to the underlying substrate. In the first instant the structure of the mesh of the screen is smoothed out and then the thixotropy prevents a further flow of the printed pattern and the paste solidifies (Fig. 9.1-1). Interconnects are printed with pastes of high conductivity. The commonly used conducting pastes consists of gold or silver, or alloys of gold or silver with platinum or palladium. Pastes based on copper are nowadays preferentially used due to the high price of nobel metals. Typical components of conductive pastes are: Metal particles (50-70 %) with a particle size of 0.5-10 pm, solvents (12-25 %; alcohols and terpineols are preferred, the rheological parameters of the paste are adjusted with additives, fritted glass (10-20 %), this is glass powder with low melting point (copper-bismuth oxide), it effects the paste adhesion of the substrate.

0 0 0

The specific resistance of fired interconnects is about 10 times higher than that of the bulk material, because the conducting particles are dispersed together with other additives in the glass. Although pure silver paste is the least expensive of all the nobel metal bases and in addition displays a high conductivity, it is rarely used. The disadvantages are: Low adhesion to the substrate and minor stability towards corrosion. Com-

lo4

I

'?

loa

2

I

lo2 10

-

'/

I

transfer of paste through screen time

-

Fig. 9.1-1 Rheological behavior of the screen printing paste.

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9 Packaging and Interconnecting Techniques (PIT)

pared with silver, gold pastes have properties which act positively on conductivity and corrosion resistance. Because of the high costs of nobel metal pastes the packaging industry is searching at great expense for inexpensive conductive pastes. Copper pastes have a high conductivity, excellent solderability and a good leaching resistance when exposed to tin-lead solders. However, they need to be fired in an inert or reducing atmosphere. An interesting variant are the resinate pastes. Resinates are salts of resin acids, which are dissolved in an aromatic oil. Screen printing circuits with gold resinates after firing, have a layer thickness of only 0.1-3 pm. Similarly, also layers from silver, palladium, platinum, iridium or rhodium can be produced. The area resistance depends strongly on the surface roughness of the substrate, due to the small layer thickness. It should be pointed out here, that the distinction between thick film technology and thin film technology is not primarily the thickness of the layer produced, but the technology involved to fabricate the layer, such as screen printing, tampon printing and the like for thick films, and chemical vapor deposition (CVD) or physical vapor deposition (PVD) for thin films. The parameters of resistance pastes must be very precisely controlled in their parameters, as small geometric differences on printing can have a large influence on the ultimate resistance value. The firing of resistance pastes is critical, as the fast oxidation process in the paste, which strongly influences the electrical properties, requires precise temperature control. Mainly bismuth-borosilicate and zirconate are used together with different oxides. Ethyl cellulose and terpineol are used as solvents. The standard resistance system consists of: 0 0 0 0

palladium oxidehilver, iridium oxide/platinum, ruthenium oxide, ruthenates.

The dielectric pastes can be divided into the following three groups according to the prevalent application areas : 0 0 0

pastes for protective glazes, pastes for cross over conductors and multilayer circuits, pastes for capacitors.

Protective glazes should show a low dielectric coefficient and low melting temperature. They should be gas-tight and innocuous to the underlying structures e. g. circuit components like resistors and capacitors. Insulating pastes for cross over conductors and multilayer circuits consist essentially of glass. Crystallizable glass behaves like normal glass on firing, but looses its glass character with decreasing temperatures and finally transforms into a crystalline structure. As a result the melting point rises by up to 100°C

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407

on re-heating. This is a significant advantage in order not to melt the fired layer again if successive layers on top are to be added.

9.1.2 Layer Production Screen printing is a thousand year old technique to decorate textiles and other materials. However, this old technology has not much in common with modern screen printing, which in the course of microelectronics was developed to great perfection. Structures were produced with the screen printing circuit on a ceramic plate, whose lateral dimensions are of some centimeters, whilst the layer thickness varies in the region of 0.3 to 80 pm. Isolated silicon chips (integrated circuit or ICs) are fixed onto this screen printing circuit and are electrically connected to the landing pads on the substrate via wire-bonds or other processes. As can be seen in Fig. 9.1-2, with the printing process the paste is pressed onto the substrate by means of a squeegee (a kind of rubber lip) through a screen. This

-

I squeegee

leao

Fig. 9.1-2 The principle of screen printing. Due to the thixotropic behavior of the paste it has a low viscosity when pushed through the mesh of the screen. Immediately after printing, due to the surface tension, the printed pattern is smoothed out. After that the viscosity increases and the pattern remains stable.

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9 Packaging and Intevconnecting Teclzniques (PIT)

fine-meshed screen, which gave the process its name, is in the first instance completely covered with photoresist. The desired pattern is then transferred by means of photolithography onto the screen, resulting in a mask which is supported by the screen. The photoresist serves as a stencil for the paste. In many cases, due to increased requirements on packing density, a single circuit plane is no longer sufficient, and the necessity for multiple printing layers arise. In this case isolating intermediate layers must be printed to separate the individual circuits. Openings or vias in this isolation layer serve as connectors to the other circuit. In order to obtain a flat surface after each printed functional layer, isolation layers with complementary patterns are applied. For complex structures up to 20 layers are build up on top of each other. Since every layer has to be fired individually, the lowest layer of such a stack has to survive 20 firing cycles without changing any physical characteristics. Drying and Firing of Pastes The printing is followed by a pre-drying of the paste for about 10 min at room temperature. Finally the paste is dried at a temperature between 80 and 150°C. The volatile solvent gradually vaporizes on drying. One of the essential process steps on production of a screen printing circuit is the firing process, because with this the electrical (but also the mechanical and chemical) properties of the layers are finally established. In mass production continuous furnaces are used to precisely control the required temperature rise- and fall rates. The printed substrates are transported with uniform speed on a conveyer belt through the different temperature zones of the furnace. At high temperatures the activators decompose and prepare the surface of the metallic particles for the subsequent sintering process. At temperatures of greater than 800 "C the actual sintering process occurs. Finally the substrate is cooled down, the glass components solidify and form a solid mechanical compound with the substrate.

9.1.3 Placement and Soldering of the Circuit Components Soldering is a well established process to join two metallic or metallized components by means of a third partner, the metallic solder. Usually the two components to be joined are brought into close contact, and the molten solder is pulled into the gap between the joining partners by capillary force and surface tension. For uncritical joints with respect to corrosion, flux is used to clean the surface prior to soldering. Flux consists of an acid to reduce the oxide films on the surface to be prepared. Very common is colophonium which carries an organic acid. Unfortunately flux has an adverse influence on corrosion. Therefore it is to be removed after soldering. After solidification of the solder the partners are fixed to each other mechanically and electrically. Whereas there is a wide variety of different solder techniques reflow soldering is the most commonly used process. Reflow soldering is exclusively used with SMD (surface mounted devices) technique. SMDs are miniaturized electronic

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devices, which are supplied with solder plated contact surfaces. The devices are placed onto the “landing pads” of the substrate. This is done with so-called pick-and-place-robots. The components are temporarily fixed using adhesive and then in a subsequent solder reflow process electrical and mechanically integrated into the circuit. For this the assembly is heated beyond the melting temperature of the solder by 20 to 30 K (usually in the temperature range of 270 to 300 “C). The reflow-soldering with a heated stamp is a frequently used single soldering process. A solder stamp is pressed with a defined force onto the solder points. The contact pads in this case are either pre-tinned, or a punched preform of solder is placed between the joining partners. After attaining the predetermined contact pressure a current impulse is released, which heats up the stamp and maintains the required temperature during the soldering process. The heat is transferred by conductivity into the contact area. For high throughput the whole circuit board is assembled and then heated in an oven or by continuous flow through a furnace. By controlling the speed of the belt and the temperature of the individual heat zones, precise temperature-time profiles can be maintained. Another method to precisely control the soldering temperature is with vapor phase soldering (Fig. 9.1-3). In this case the circuit board is submerged into the saturated vapor above certain organic liquids with a boiling temperature around 300 “C. Due to the condensation of the vapor onto the circuit board the condensation enthalpy is released and the board is heated to exactly the boiling temperature of the liquid. A further rise in temperature is inhibited due to the re-evaporation of the liquid film and releasing the evaporation enthalpy until the boiling temperature is reached again.

Fig. 9.1-3 Principle of vapor phase soldering. The workpiece is emerged into the saturated vapor zone above the boiling liquid. By condensation of the vapor, the released condensation enthalpy heats up the workpiece to exact the boiling temperature of the liquid.

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9 Packaging and Interconnecting Techniques (PIT)

For very delicate and temperature sensitive components the laser soldering is a suitable process. The areas to be soldered are locally heated using a laser beam. The joining pads must however be designed to be reached easily with a laser beam. YAG-Neodymium lasers (A = 1064 nm) or even better C0,-lasers (1 = 10 pm) are used for this process. The advantage of the process is the low thermal load which is put onto the circuit in total. A drawback of this process is the fact, that the temperature is dependent on the emission coefficient of the surface irradiated by the laser. The local thermal conduction too is a parameter influencing the temperature. To facilitate the soldering process and also other bonding techniques so-called bumps have to be provided at one surface of the joining partners. These bumps are solder dots which stick out of the surface in a semispherical shape. These bumps are ductile so that they can compensate for any gap variations between the partners. Since bumps are a prerequisite in the flip-chip technique, which is used in mass fabrication of microelectronic devices, a large amount of development work has gone into the bumping technique. Frequently the problem arises that the bumps are not already applied in the semiconductor factory, but at the user’s place for special applications such as microsystem devices. In this case another photolithography process and a subsequent galvanic process has to be applied to the otherwise finished wafer, as seen in Fig. 9.1-4a [Simo90]. SEM-micrographs are shown in Fig. 9.1-4b of Pb40Sn60-solder bumps, which can be used for flip-chip technology [Wolf96]. The solder material plays a crucial role for reliable solder connections. There is a wide variety of different materials, always tailored to a special application. Widely used in the semiconductor industry is an alloy of the composition: Sn62Pb36Ag2 with a melting temperature of T, = 179°C. The small silver content is to reduce the silver solubility in the solder and to avoid the leaching of silver in the landing pads. Another frequently used solder is Sn63Pb37 with T, = 183°C.

9.1 Hybrid Technology

4 11

Fig. 9.1-4 a) Steps for fabricating bumps on processed silicon wafers, b ) SEM-picture of arrays of bumps. Remarkable is the high degree of uniformity (Courtesy of Fraunhofer Institute €or Reliability and Microintegration (IZM), Berlin).

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9 Packaging and Interconnecting Techniques (PIT)

9.1.4 Mounting and Contacting of Silicon Dies In contrast to housed devices, which are connected to the substrate by a soldering technique, two processes are required for the assembly of “naked” chips or dies. The first process serves to mechanically fix the semiconductor to the (gold plated) substrate, to dissipate the heat off the die, and to keep the bulk of the die at a defined electrical potential. This process is called die-bonding. The die is pressed with a defined pressure onto the gold-plated substrate, a low frequency grinding movement of the crystal supports the wetting of the phase borders and accelerates the alloy process. As can be seen from the Fig. 9.1-5, gold and silicon have a distinctive eutectic at Au31Si69. TEuis then 370 “C. The bond is usually tempered for some hours. The subsequent process takes care of the individual electric contacts between the integrated circuit on the die and the landing pads of the substrate (usually the ceramic substrate with contact pads manufactured by screen printing technique). This bonding process is discussed in the following section in detail.

1600 I

1400

-

1200

$J 1000 Y

$

800

ICI

E

E“

r-”

600 400

200

00 Au

Atom-%

Si

Si

Fig. 9.1-5 Au-Si diagram. The eutectic at 31 atom-% of Au with the liquidus-solidus phase at 370°C is used for die bonding.

9.2 Wire-Bonding Techniques

4 13

9.2 Wire-Bonding Techniques The wire-bonding is a joining technique to produce discrete electrical contacts, in general, from the die onto the substrate, where the lateral bridging as well as the height difference between the two surfaces have to be overcome [Lind89]. To wire-bond, the components to be joined must show suitable contact surfaces (so-called “landing pads”). The requirements of a suitable junction technique for the hybrid technology are a good, stable electrical junction, small space requirement for the junction points, low mechanical and thermal loading of the component, as well as cost effectiveness and compatibility to the manufacturing processes. For the development of a mature bond technology, expensive metallurgical experiments and the development of special apparatus to handle the junction wires were necessary. The wires used are normally made of gold or aluminum alloy with diameters of down to 10 pm. All wire-bonding processes are similar in that they rub off the oxide skin of the wire by introducing pressure, heat and ultrasonic energy. The joining partners (wire-contact surface) are brought into such a close contact that the van der Waals forces become effective and a stable junction is made possible. To wire-bond in practice, the following processes have proven to be effective.

9.2.1 Thermocompression Wire-bonding (Hot-Pressure Welding Bonding) On thermocompression wire-bonding or hot-pressure welding the bonding pressure and heat is supplied with an electrode to the joining partners. With the plastic molding of the wire, the oxide film, which is usually always present and prevents a “cold bonding” of the wire with the contact surface, bursts and brings the pure surface of the junction reactants into atomic contact. In addition to this oxide peeling process, a temperature of 280 “C is required on the contact points. In general, the substrate is preheated by means of a heating plate to about 150°C to 170°C whilst the bond tool itself supplies the additional temperature rise to the joining partners. Tungsten and tungsten- or titanium-carbide are suitable materials for the tool for impulse heating. For continuous heating over a long period, mostly ceramic tools are used, which are less expensive and show a higher resistance to wear. Despite its higher price, ruby is also used successfully as it possesses a still higher wear resistance than ceramic. Only a few materials are suitable for the hot-pressure welding, because ductility and the absence of thick oxide layers on the surface are critical parameters. Therefore, gold is used exclusively, as it does not need any (expensive) protective gas atmosphere. The material cost of the bond wire is negligible compared to other process expenses. Power, temperature and time are the three most important parameters for a reliable junction and must be balanced with the plastic.flow behavior of the material. Typical process parameters for a gold wire with 25 pm diameter

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9 Packaging and Interconnecting Techniques (PIT)

are a bonding force of 0.3 to 0.9 N, a bonding temperature of 280 to 3 5 0 ° C a substrate temperature of 240 to 280°C and a bonding time of 0.3 to 0.6 s. The parameters influence each other mutually and must be optimized experimentally. Other parameters to be considered are the hardness of the material as well as the dynamics of the bonding machine and the design of the tool (capillary, wedge).

9.2.2 Ultrasonic Wire-bonding (Ultrasonic Bonding) With ultrasonic wire-bonding the parameters, force and friction, play a critical role within the joining process. The ultrasonic energy is introduced into the contact region of the joining partners by a sonotrode. The oscillation amplitude is kept tangential to the direction of force (and therefore tangential to the substrate surface). The frequency range is between 15 and 60 kHz depending on the material and thickness of the wire, as well as according to the type of bonder. The oxide skin of the surfaces of the joining partners is disrupted by ultrasonic oscillation and the clean surfaces to be joined are brought within atomic distance to each other. The oxide layers are broken down, but remain in the joining region. Therefore, it is important that these remnants constitute only a small fraction of the contact area. The power introduced by the sonotrode enables the plastic deformation of the wire and the approach of the joining partners down to atomic distances. The bonding parameters (ultrasonic energy, time, pressure) must be controlled very carefully for repeatable results. A good contact between sonotrode and joining partners is essential for the loss free introduction of the ultrasonic power into the bond area. In addition, the substance (thickness) and the ductility of the materials involved play an important role. Sonotrodes with rough surfaces could improve the effectiveness of the ultrasonic transfer. A contact pressure which is too high may weaken the bond, a contact pressure which is too low may damage the surface of the sonotrode and the contact area due to frictional heat dissipation.

9.2.3 Thermosonic Wire-bonding (Ultrasonic Hot-pressure Welding) Whilst with ultrasonic bonding power, ultrasonic energy and time represent the essential process parameters, with ultrasonic hot-pressure welding temperature is the essential parameter. The ductility of the bond wire is increased by additional heating, which is advantageous for a complete introduction of ultrasonic energy into the junction, and also the contact areas are cleaned by outgassing. Thermosonic wire-bonding with gold wire improves the reliability of the bonds. The wire-bond processes can be classified not only by the kind of process parameter, but also according to handling of the wire and tool.

9.2 Wire-Bonding Techniques

4 15

9.2.4 Ball-Wedge Bonding The ball-wedge bonding process is the most frequently used process. The movement process is shown in Fig. 9.2-1. A wire is fed (preferentially a gold wire) through a tube-shaped tool with a central capillary, with the wire sticking out at the lower end of the capillary. In the first cycle of the bond process the end of the wire is melted by an electrical discharge. Because of the surface tension, the melt takes the shape of a sphere with 2 to 3 times the diameter of the wire. Before this drop resolidifies, it is pressed with the capillary onto the bond pad and welded with the surface. The capillary is then raised and run over to the second landing pad, where the tool is lowered again. This time the wire is pressed onto the landing pad with the edge of the capillary, the wire is plastically deformed and welded with the landing pad of the substrate. Due to the special form of the bond tool, a predetermined breaking point is introduced into the wire. On raising the capillary, the wire breaks at this point and the connection is completed. The broken end of the wire is re-melted onto the capillary and the bonder is prepared for another working cycle. The advantage of the process lies in the fact, that after placing the first bond, the direction of the second bond can be arbitrarily chosen. With mass production the throughput is increased by the fact that the substrate is not to be rotated for bond connections in different direction. The drawback of the ball-wedge bonding however is, that essentially only gold wire can be used, because aluminum wire would oxidize with the melting of the wire tip. Fig. 9.2-2 (below) shows two SEM micrographs of a flawless ball-wedge bond connection.

/

bonding wire bond capillary

a) molten ball

1. bond (nailhead)

2. bond (stitch)

wire broken off

Fig. 9.2-1 Principle of the ball-wedge bonding process.

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9 Packaging and Interconnecting Teclzniq~ies(PIT)

Fig. 9.2-2 Micrograph of a ball-joint (left) and a wedge-joint (right).

9.2.5 Wedge-Wedge Bonding Another possibility for the wire-bond technology is the wedge-wedge bonding (Fig. 9.2-3). In this connection both bonds are placed wedge-like. This has the consequence, that the first bond already determines the direction of the second bond. Depending on orientation, the substrate must therefore be rotated under the bond tool, which of course is a time consuming process. The advantage of the process is the use of aluminum wires, as the melting is not involved in the process. Furthermore, with this process smaller bond pads and pitch sizes can be achieved. In addition the wire loops are smaller with a positive effect on parasitic capacitance of the bond. Wedge-wedge bonds are therefore especially suited for high frequency applications. Instead of wires also tapes with high current carrying capabilities can be used which is essential for high power applications.

wire and tool at the start

2. bond pressure, ultrasound

Fig. 9.2-3 Principle of the wedge-wedge banding process.

9.2 Wire-Bonding Techniques

417

9.2.6 Advantages and Disadvantages of the Wire-bond Processes In thermocompression wire-bonding almost exclusively gold wire is used. On deciding about bond connections one must take into consideration that the hardness of the wire is reduced by heating, which can have an adverse effect if larger distances or height differences of the wire loop are to be overcome. Also many bonds put onto a small area may lead to a deterioration of the device due to excessive heat loading. However, pulse heating may reduce this problem considerably. With circuits of high frequency technology the wedge-wedge process is used, as the loop is more a flat arch and better suited for controlling the geometry. Furthermore, the parasitic inductance and capacitance of the connection of the circuit is smaller than with the high loop of the ball-wedge bond. The landing pads can be designed smaller (less than 50 pni) with the wedge-wedge process. This may bring a considerable saving of real estate on the chip for ICs with some hundred connections. Ultrasonic bonding is predominantly used with aluminum wires. Normal diameters vary in the region of 17 to 500 pm. Very thin wires from pure aluminum are too soft, therefore generally alloys with about 1 % silicon are used. Pure aluminum (A1 99.9) is used with wires of 100 pm diameter or more, because here higher conductivity is preferable to mechanical strength. Process automation though is more expensive because of the above mentioned limitation in bond direction. The thermosonic wire-bonding is mainly used with gold wires of 17 to 100 pm in diameter. It represents the standard process to contacting ICs, isolated diodes and transistors on ceramic substrates with screen printing- or thin layer circuits. The advantage is seen in the resistance to corrosion, especially on applications in harsh atmosphere (automotive, environment, medical). A disadvantage of this process (but also of any other process with contacts between gold and aluminum) is the diffusion of gold atoms in aluminum at higher temperatures. This can lead to voids in the contact region due to the migration of gold atoms, which considerably weakens the mechanical and electrical contact (Kirkendal-voiding).

9.2.7 Test Procedures and Alternatives For these bonding processes numerous destructive and non-destructive test procedures were developed. Most of the information about the quality of a bond can be obtained by optical evaluation of the shape of a wire-loop, of the appearance of the bonds, and of the position relative to the landing pads. Besides that there are micro-stress- and micro-shear-tests in use, which work partly destructively and partially non-destructively. The investigation of the surface of the fracture is an important indication of the nature of the failure mechanism. For basic research on bond problems the whole range of surface analyses is being used. Numerous tests on reliability of bond connections were carried out with respect to temperature change, mechanical impact, vibration as well as storage under extreme humidity or aggressive atmosphere.

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9 Packaging and Interconnecting Techniques (PIT)

The highly reliable and economical wire-bond processes were developed especially for microelectronics. This development involved the standardization of parameters and materials. Therefore, the wire-bond processes that are available in the microelectronics manufacturing industry, are dedicated to limited applications. In microsystem technology, which is still at the beginning of it’s industrial implementation, these standard processes are not always able to solve existing problems. Therefore wire bonding is to be developed further with respect to materials variety, material pairing and special applications (e. g. the contacting of three dimensional structures). As well as thermosonic bonding, also laser beamsoldering and welding will be developed further. Economic manufacturing of microsystems depends heavily on the availability of many dedicated connection techniques. Research and development of new techniques in bonding are therefore vital for further success of MEMS.

9.3 New Contacting Technologies The wire-bond processes have several disadvantages, which are more pronounced when dealing with larger integration densities and higher signal frequencies. On the one hand the serial operation limits the output and becomes critical for still higher numbers of connections. Even though the wire bond process runs very reliably, several thousand bonds on one device is a potential risk especially in applications with extreme environmental conditions. On the other hand, the dimensions of the bond pads cannot be micro-miniaturized to the same degree as the minimum structures within the circuit. With several hundreds of landing pads on one die, the predetermined real estate for bonding pads takes up a considerable fraction of the area of a chip and therefore a considerable fraction of the manufacturing costs. With high frequency circuits, the parasitic inductance and capacitance of the wire-bonds add significantly to the signal distortion. The tape-automated-bonding (TAB) and the flip-chip technology are processes to avoid some of these disadvantages, and will be described in the following.

9.3.1 The TAB Technology With the TAB-technology (TAB = tape automated bonding) the function of the individual wire during the wire-bonding process is replaced by metallic strips on a polymer substrate. The contacting is accomplished by simultaneous bonding of all connections of a chip to the ceramic substrate or to the conducting plate. The technological difficulty of the process can be seen in the preparation of the contact areas and the application of equally shaped bumps on the landing pads of the chips. The bumps are connected with the underlying circuitry by vias in the passivation layer of the die. The semi-spherical shaped bumps must clearly stick out from the passivation layer (usually P-Glass or Si,N,), to ensure a reliable contact

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419

with the metal strips of the film carrier (Fig. 9.3-1). To fulfill the requirements on corrosion resistance, minimized leaching, good wettability and reliability in soldering gold, but also other complex metallurgical combinations are used for bumping. Gold bumps have to be separated by diffusion barrier layers from the underlying aluminum conductors, since otherwise, by interdiffusion at elevated temperatures, the bond could suffer from mechanical instabilities and increased contact resistance or even from a complete failure of the contact (see Section 9.2.6). The gold bumps are applied by electroplating onto the aluminum pads which are covered by thin films of Ti, Wand Au. To achieve a high bump density, precise patterns in a relatively thick photoresist have to be achieved. A bump geometry of 20 x 20 x 12 pm3 with a pitch size of 50 pm has been demonstrated. After electro-

TAB-bonded hybrid

Fig. 9.3-1 Tape automated bonding.

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9 Packaging and Interconnecting Techniques (PIT)

plating of the bumps, the resist is removed and the bumps are reshaped to the semi-spherical shape in a reflow treatment. The eutectic gold-tin soldering (eutectic temperature TM= 280 "C) and the thermocompression bonding is usually used for contacting the die with the tape (innerlead bond). The contacting of the outer leads (tape/conducting plate) is carried out mostly with a tin-lead solder. The bump technology opens a wide field of new applications and consequently can be considered as a key technology for microelectronics and microsystem technology. An improvement of the infrastructure (technology- and equipmeiit-supplier) is though necessary for further proliferation. The TAB technique provides the possibility to test the dies prior to installation into a system. In addition, the front- and backside of the die is directly accessible. This may be of particular advantage with the integration of MEMS components into microelectronics (e. g. thermal coupling).

9.3.2 The Flip-Chip Technology The principle arrangement of the flip-chip contacting is shown in Fig. 9.3-2. In contrast to the TAB-process, the mounting of the chips onto an intermediate substrate (film) is avoided and the chip is positioned head first on the substrate. With the aid of an infrared microscope, the chip can be aligned (silicon is transparent to infrared light) and connected directly to the landing pads of the substrate. In order

chip is flipped over to the substrate

9.3-2 Principle of flip-chip technology.

----GI

9.3 New Contacting Technologies

421

to compensate mechanical stress due to different thermal expansion coefficients of die and substrate, very soft and ductile solders are needed. The bond pads are opened on the passivated wafer, and films of Ti/Cu, Cr/Cu or Cr/Ni are deposited to improve the adhesion and to prevent interdiffusion with the subsequent layers. The bump material (Sn/Pb) is deposited by electroplating and shaped by a reflow process. The pads 011 the substrate must also be prepared with a solderable metal layer (e. g. AgPd). A problem with the flip-chip process arises from different coefficients of expansion of the joining partners and the low thermal contact to the substrate. Overheating the chips may result in large sheer stress on the bond pads, which could finally cause the fracture of the connection and total failure of the circuit. A considerable advantage of the flip-chip technique is the high bond density and the fact that the bond pads are not limited to the outer borders of the die. Substrates with matching thermal coefficients of expansion make this process highly economical for the mass production of circuits with low dissipation energy. In Table 9.3 -1 different bonding processes are compared [ReicSS]. However, for the application of a method on a special problem, the relevant subject matter must be consulted in detail.

Table 9.3-1 Comparison of different contacting techniques Properties

c+w

FCT

TAB

BTAB

Preparation requirements Hermetic passivation Reliability Surface requirements Testability Optical inspection Suitable for ASCIs Encapsulation necessary Number of connections Flexibility cost

No No Different Average No Yes No Yes Low Very good Low

Yes Yes Very good Small No No No No High Very good High

Yes Yes Very good Average Yes Yes Yes No High Bad Very high

No No Different Average Yes Yes Yes Yes Low Bad High

Explanation of the abbreviations: C+W = Chip-and-wire techniques (wire-bonding) FCT = Flip-chip technology TAB = Tape-automated-bonding BTAB = Bumped tape (TAB-technique with bumps on the tape)

422

9 Packaging and Interconnecting Techniques (PIT)

9.4 Adhesion The adhesion technique is an essential component for the connection and assembly of MEMS and is a successful means in the area of SMD techniques [Ke1191, Habe90, Henn901. In the meantime the adhesion technique is also established in microsystem technology. Single or double component epoxy resins are used predominantly for the application.

9.4.1 Isotropic Adhesion Single component systems already contain all the necessary components for curing and are activated by heat. With double component systems the components are mixed prior to use. There are cold- and warm curing systems. The epoxy resins are available on the market as electrically conducting or heat conducting adhesives. The electrically conducting adhesives are mostly filled with silver laminate (average diameter 25 pm). The filling fraction is between 60 and 80 wt%. The contact surface of the single particles under each other is critical for the electrical conductivity. One achieves specific resistances of lop3 to 2.10-5 Ohm cm. The curing conditions also exert a large influence on the electrical conductivity of the adhesion layer. As the polymerized adhesive forms the matrix in which the metal particles are incorporated, mechanical stress of this matrix could have a large influence on the conductivity. The heat conducting adhesives are used to fix electronic components to the substrate and to guide the dissipation of heat from the electronic device into the substrate. Filling materials used for these problems are mostly aluminum oxide or boron nitride. Heat conductivity of 0.7 to 1.5 W/mK, with metallic filler 1.5 to 3.5 W/mK are common (unfilled epoxy resins have a heat conductivity of about 0.3 W/mK). The adhesive is applied with automatic dispensers, with stamps or in screen printing. The curing of the adhesive within certain limits can be chosen at different temperature-time combinations; the typical temperature range is from 80 to 180 "C. An important requirement on the adhesive layer is the compensation of mechanical stress in adjacent parts, which is caused by different thermal expansion coefficients of the partners to be joined, by shrinkage of the adhesion layers, and by alternating thermal load on the joining components. For the estimation of the mechanical stress behavior of the adhesive layers, knowledge of the elastic properties is an important requirement. If the stress temperature is below the glass transition temperature of the adhesive, then a linear relation exists between the Young's modulus and temperature. A relatively stiff structure with maximum stability in the adhesion layer is the result. If the adhesive layer is heated above the glass transition temperature, a viscoelastic behavior can be assumed for the adhesive. At the Research Center in Karlsruhe, the adhesion technique has been developed further for mounting microfluidic components and is already successfully

9.4 Adhesion

423

used in “pre-industrial” short runs. The adhesive is applied via a system of channels in one of the joining partners onto numerous contact points simultaneously. The adhesive is introduced at one orifice with a dispenser into the channel system and is distributed over a large area. Excess adhesive flows out of other orifices and can be discarded. With this method, micropumps are manufactured in batches of 12 at a time [Maas94]. Another method makes use of the capillary force between to components to be joined. By suitable construction, adhesive can be guided into the narrow gap between the joining surfaces. Adjacent deeper trenches can prevent the adhesive from reaching unwanted places in the inside of the component as outlined in Fig. 9.4-1. Only one drop of the adhesive is placed near to the cleft, the right dose is administered by the existing capillary force and the viscosity of the adhesive alone. Both the channel- as well as the capillary adhesion process, therefore avoid any precise and time consuming apportioning.

Fig. 9.4-1 Principle of capillary adhesion bonding.

424

9 Packaging and Interconnecting Techniques (PIT)

9.4.2 Anisotropic Adhesion In anisotropic adhesion a film is placed between the components to be joined [Shio94, Schm941. This film consists of an polymeric carrier, with a suspension of conducting spheres. The concentration is low to prevent the spheres from contacting each other. In this state the film is an insulator. After the film is heated and in some areas deformed and compressed, the conducting particles are compacted and come into contact, and the film becomes electrically conducting at these isolated areas. This film is described as an anisotropic conductive film (ACF) as it conducts only in the direction normal to the surface and not in the plane of the film. The application of anisotropic adhesion is outlined in Fig. 9.4-2.

Fig. 9.4-2 The principle of anisotropic adhesion bonding. A polymer film is filled with conductive particles which do not touch each other. The conductivity across this film is zero. By compressing the film between two bonding partners the conducting particles touch each other and the conductivity is increased across the film.

9.5 Anodic Bonding

425

By varying the material parameters, one attains numerous different products and potential applications. This process is widely used already with the contacting of flat displays (liquid crystal display = LCD), with thousands of contacts on a glass carrier to be connected reliably. The processing is performed in two steps. First the ACF, which has a protective film on the upper side, is appropriately trimmed and applied to one of the joining partners with a hot stamp of 100 "C and a pressure of 1 N/mm2. Then the protective film is removed and the second partner is attached and positioned. In the second sequence the bonding partners are joined by applying 180 "C and 2 N/mm2 of pressure[Schm94]. The contact pressure must be maintained until the polymeric carrier has been cooled down to below its glass transition temperature. For uncritical parts (e.g. parts with only a few widely spaced electrical contacts) the process can be reduced to one step only, so that joining of the partners and deformation of the ACF is performed simultaneously. The pitch size of the conducting contact pads as well as the pad area is established by the density of conducting spheres. Since the distribution of the spheres in the polymer is stochastic, for the sake of reliability the pitch size should not be below 100 pm and the contact pads should have at least an area of 0.025 mm2. Epoxy resin, polyimide, polyester-urethane, butadiene-styrol copolymer and others are used for the polymer carrier. Agglomerates of nickel, copper, tin and lead are used as conducting particles, as well as plastic balls, which are coated with nickel or gold. The dimensions of the spheres vary between 5 and 30 pm.

9.5 Anodic Bonding 9.5.1 Wafer-to-Glass Bonding The anodic bonding process evolved in the recent years to a valuable tool for packaging and protection of microcomponents or even complete systems. Anodic bonding facilitates the irreversible mounting of a cover glass on top of a microstructure. Microstructures are, by definition, three dimensional bodies of micrometer dimensions, consisting of flexible elements (e. g. bending beams, oscillating mirrors, linear motors) and completely freed structures (rotors, gears, pistons). These structures are produced, similarly as in microelectronics, on substrates (wafers) in large numbers and high packaging density. In order to use them separately, the component must be cut out from the wafer. In semiconductor manufacturing the dies are scribed and then broken, or they are cut by a diamond saw. Semiconductor devices are protected by a passivation layer. Cutting the wafer into dies does not harm the devices. This is very different in MEMS. Here the devices to be separated are usually of intricate design with narrow gaps and open trenches. The cutting procedure with a diamond saw and the use of cooling liquid would damage

426

9 Packaging and Interconnecting Techniques (PIT)

these microstructures if they are not protected. The application of a passivation layer is not feasible due to the 3 dimensionality. By applying a cover glass onto the wafer, the chips can be separated into individual components with full protection of the delicate structure. In microelectronics the anodic bonding is commonly used for attachment of dies over the whole of the wafer for electric contacting to an interconnect structure on the substrate. In MEMS the task of anodic bonding is manifold. It is not only protection against dirt, which makes covering necessary, but the function of the microstructure or of the microsystem demands a cover, if for example it is to produce hermetically sealed cavities to build up a pressure sensor or to provide opposing electrodes to moving seismic masses to measure acceleration. In principle the process of anodic bonding is simple: glass plates and silicon wafers are heated to a temperature of about 400°C in close contact with each other and are supplied with a voltage of about 1000 V. Because of the fact that glass at these elevated temperatures becomes electrical conductive due to the mobility of some ions. Positively charged sodium ions, which are present in a large number in the glass, migrate to the cathode opposite the glass-silicon interface and are neutralized (Fig. 9.5-1). The mobility of the negatively charged Si02--molecule is less by orders of magnitude. As a consequence of the migration of sodium ions, a space charge

cathode

Na, 0

Nat

anode

t

I silicon

\

(heater)

Pyrex

Fig. 9.5-1 Silicon-glass anodic bonding. Due to the migration of Naf ions under the influence of heat and an external applied electrical field an eletrostatical field is generated at the border of silicon-glass. The attractive force and subsequent chemical processes at the interface will lead to a very strong bond between the partners.

9.5 Anodic Bonding

427

Fig. 9.5-2 Cross-section of an anodic bond between a structured silicon wafer and a

glass cover. is generated which attracts the wafer surface to the glass. The result of such a bond process is shown in Fig. 9.5-2 [Ke1191]. In order to achieve a reliable bond between glass and silicon, certain requirements such as flatness of both surfaces as well as a matching of the different thermal expansion coefficients must be fulfilled. A suitable material is pyrex-7740 glass, which has an expansion coefficient close to that of silicon. There are several modifications of this process. Instead of the bulk glass cover, a thin layer of sputtered glass serves the same purpose. Using this approach, two silicon wafers may be bonded with a glass film in between the bonding area.

9.5.2 Wafer-to-Wafer Bonding A technique of great promise is the wafer to wafer bonding without the auxiliary glass layer in between [Shim86, Tong99, Plol3991. Wafer bonding happens when two very flat and very clean surfaces are brought into close contact. Either van der Waals forces, or chemical bonds are the reason for the attraction of the two surfaces even at room temperature. The effect of bonding was known already before the “silicon age” by toolmakers with the “wringing on“ (German: ansprengen) of so-called “end pieces”, which are optically polished metallic precision measurement length scales. In such cases the bonding is a kind of unwanted side effect. Wafer bonding as a technique for joining two wafers face to face irreversibly is only known since the middle eighties. Reliable wafer bonding appears for highly polished wafers with a surface roughness in the order of a few nanometers. Even very small particles in between the two surfaces to be joined may disturb this process considerably, since a particle of only 1 pm in diameter may leave an unbonded area of 1 cm2. The bonding process therefore requires a clean room environment for good results. The preparation of the wafer surface prior to bonding is imperative, since the surface under normal conditions is covered with a layer of oxide, followed by a

428

9 Packaging and Interconnecting Techniques (PIT)

layer of water molecules and depending on the pre-treatment, some polar and nonpolar organics and finally layers of adsorbed gas. Usually these layers must be removed to avoid contamination due to molecules trapped in the oxide layer. The oxide layer is taken off by a dip in diluted hydrofluoric acid or buffered ammonium fluoride solution. A mixture of H,SO, and H20, combined with a small amount of hydrofluoric acid leads to a very thin (1-2 nm) native oxide layer. The native oxide layer is terminated by Si-OH groups. These so called silanol groups render the surface hydrophilic. These silanol groups react with each other according to the following:

2 (Si - O H ) +Si

-

0

-

Si + HzO

(9.1)

A treatment of the wafer in hydrofluoric acid or ammonium fluoride takes off the native oxide layer and the resulting bare silicon surface is terminated by hydrogen atoms which render the surface hydrophobic. Even more convenient in the process is the realization of silicon-silicon bonds by means of a thermally grown Si0,-layer. An additional advantage in comparison to the described method, is the absence of alkali-ions, which have a negative effect on the function of MOS-circuits. The bonds occur by heating the wafer to 1100-1200°C and by applying a voltage of 20 V. The build-up of an electrostatic field is produced by migrating of H- and OH-ions. On subsequent thermal treatment the adhesion increases and after tempering at lOOO"C, the breaking limit of single crystal silicon is achieved (-5 kg/cm2). A longer tempering causes a break-up of the natural oxide layer. The oxygen atoms diffuse into the interior of the substrate and a pure Si-Si bond is generated on the surface.

9.6 Low Temperature Cofired Ceramics (LTCC) A technology of great potential for the cost-effective manufacture of simple microsystems in the mesoscale dimension can be seen in the DuPont Green TapeTM LTCC technique. The process sequence is basically the following (and may be modified for special applications): Ceramic powder (A1,0,) and fritted glass is mixed with an organic binder to a high viscosity slurry, which is cast onto a continuous tape of several centimeters or decimeters in width and fractions of a millimeter in thickness. After being dried this tape can be handled like a rubber elastic tape. This technology was developed by DuPont, the tape is supplied under the trade name "green tape". This material can be cut by a knife or a pair of scissors and can be used as a flexible substrate for screen-printing with gold, silver or copper. The printed sheet can be cut to size or punched out with a punching die. The substrates are now stacked on top of each other with alignment holes to guaranty precise adjustment and pressed to a stack of up to 40 layers (Fig. 9.6-1). This device is then fired at temperatures up to 850 "C for at least 15 min. Subsequently the upper layer

Low Temperature Cojired Ceramics (LTCC)

429

Fig. 9.6-1 Representative cross section of a Green TapeTMpackage.

Fig. 9.6-2 Cross-section view of fabricated Green Tape basic flow sensor [Gong991 (by permission fo M. Gongora-Rubio, lnstituto de Pesquisas Tecnologicas (IPT), Sao Paulo, Brazil).

430

9 Packaging and Interconnecting Techniques (PIT)

can be supplied with SMDs or dies, which in turn are fixed to the multilayer ceramic by flip-chip, TAB or wire bonding technique. The layer can be cut individually to shape within the stack buried caverns, meanders for cooling liquids or devices for advanced packaging of microsystems. An example is shown is Fig. 9.6-2: The main assets of this technology are:

0

mesoscale structures can be manufactured by inexpensive processes, mechanical structures and microelectronics can be integrated within one device, package density is high due to vertical build-up, all fabrication processes are matured and cost-effective.

Interesting and low cost problem solutions for mesoscale systems are to be expected in the future. An overview of Green Tape Technology is given in the paper of M. R. Gongora-Rubio et a1 [GongOO].

System Technology

10.1 Definition of a Microsystem Essentially up to now only microstructure technology and those basic processes have been discussed, which enabled mechanical, fluidic, optical single structures or components with very small dimensions to be achieved on one substrate. Microstructure technology is in fact a necessary, but not sufficient pre-requisite for the microsystem. If one considers the history of microelectronics, one can not refrain from stating, that the technological and economical breakthrough has not come about through the discovery of the transistor, which was of course necessary for the further development, but was introduced with the availability of the microprocessor. However, the microprocessor is the combination of many single elements, which are produced economically and with a high packing density on the substrate, and only the integration of these single elements and the sophisticated connection of element and subsystems to a whole system creates a device, which is able to fulfill the manifold tasks of intelligent signal processing. This concept follows for the microsystem too. The above statement could even serve as the definition of a (micro)system:

A system is a device which is more than the sum of the individual elements of which it is composed. For microsystem technology to be economically successful the same requirement holds true: economical fabrication of components in small dimensions. These elements are to be joined intelligently into the system. In the past, in microsystem technology the emphasis was put to the microstructure technologies and to the development of microcomponents. To be successful on the market the emphasis in research and development should be switched to the systems aspect. In the manufacture of microcomponents certain standard processes have been established, unfortunately in the development of systems the aspect of quasi-standard concepts have not even been touched. Nevertheless, the potential of microsystems is high and so are the expectations. Applications in many areas can be thought of,

432

10 System Technology

which could not be achieved using conventional methods and which far exceed the possibilities of microelectronics only. In general, it is true for a microsystem, that the functional capability of a microelectronic circuit is greatly enlarged by adding the abilities of an integrated sensor and microactuator technique. The spectrum of performance capabilities of the microsystems technology is expected to be large and as a consequence so are the potential applications. It is important that actuators as well as sensors, can be produced using microstructure technology. As they are produced in the broadest sense utilizing the methods of microelectronics, also the dimensions, the function density and the production costs should be compatible with the components of microelectronics. Just as connecting intelligently, large numbers of similar elements has elevated microelectronics to a more powerful regime, so the microsystem is enriched by sensors and actuators and thereby brought to a higher level of potency. In other words: the microsystem technique extends the possibilities of microelectronics beyond electronics to other areas of physics, chemistry and biology. If microelectronics has in some ways changed our lives, then this is to be expected even more so from microsystem technology. However, technologically, there are still some hurdles to be overcome in areas of research and development to reach complete control of microsystem technology. In most cases a microsystem consists of an arrangement of components and subsystems manufactured by different technologies, which are incompatible with each other. The process to join 3 -dimensional microbodies is both technologically as well as economically of the highest importance in microsystem technology. The economics (and therefore the industrial breakthrough of microsystem technology along a wide front) will be evaluated, on the basis on whether suitable, economical processes of packaging- and joining technologies are available. For this it is necessary to define and produce not only suitable standard elements but also their interfaces to the system. An international cooperation seems to be essential in this area. To produce economic microsystems, an infrastructure must be developed, which offers standard Components on the market. From this, specific solutions could be developed and manufactured using suitable combinations. Even if it is too early, maybe even counter productive, to plan premature rigorous standardization, decisions must be made at the right time about the joining of different microcomponents economically. Only in a few special cases involving large numbers of pieces, will a monolithic microsystem i. e. a system that is completely manufactured within one technology on a single silicon chip, be the economical solution. With monolithic microsystems, small and average sized companies would be excluded from the market and rapid solutions deduced from small numbers of pieces are impractical. Opinions about the definition of a microsystem are extremely diverse, the discussions come from completely different motivations. However, it is essential to agree that a microsystem consists of several building blocks. As can be seen in Fig. 10.1-1, a complete microsystem has a sub-region of sensors, another sub-region of actuators, a signal processing, eventually other mechanical structures such

10.2 Sensors

433

Bus systems

Fig. 10.1-1 The different functional sections of a complete microsystem.

as alignment stops, mounting devices, tools and the like and a region of interfaces to the macroscopic environment. Next the four functions of which the microsystem is made up, will be discussed in detail. The sub-units are: 0 0 0

sensors, actuators, signal processing and interface to the environment.

10.2 Sensors The high potential of microsystem technology, compared with conventional technology, are obvious especially in sensor techniques. Still sensors are considered the weakest member in the chain of metrology systems. There is an ever increasing demand on reliable sensors, with high requirements on accuracy, sensitivity, selectivity, long life time, and stability against drifting over long periods. Especially with chemical or biochemical sensors the problem arises that the sensors have to be exposed to the unknown medium as unhindered as possible, but at the same time be protected against spurious influences. A well known problem illuminating this is in the automotive field, namely the poisoning of lambda probes

434

10 System Technology

in the exhaust system by the lead content of the gasoline. In the medical field implantable sensors are difficult to maintain over longer periods. Bio-compatibility does not only mean protecting the human body against the foreign matter of the implant, but also vice versa to protect the implant against the hostile environment of the biological system, which during evolution was programmed to reject or dissolve any foreign body in the system or a least to encapsulate it with passivating tissue. The general development in sensors changed with microsystem technology from highly sophisticated single sensor elements with analogous signal analysis, to the sensor array with microprocessor-controlled data processing. At the same time the quality of the measuring system could be considerably improved, by using for instance statistical methods of averaging multiple signals or by gradation of the sensors in their region of sensitivity, to improve on the dynamics of the system. An example for this is a system, where the acceleration could be distinguished between amplitude and arbitrary direction in space. The single acceleration sensor elements have a preferred direction in sensitivity due to their design. By appropriate alignment of several sensor elements, an impressed acceleration can be divided into components, for instance to the x- and y-direction. The sensor elements, which are produced in the LIGA-technology, could measure accelerations tangentially to the substrate surface. By aligning several sensor elements at 90" to each other on the substrate surface, allows accelerations which occur tangentially to the substrate surface to be determined not only in their amplitude, but also in their direction. Since all sensor elements in the initial lithography step are simultaneously positioned on the substrate, the highest precision in the alignment is guaranteed. For a three dimensional acceleration sensor system other sensor elements must be introduced, which can measure acceleration normal to the substrate surface. Sensors in silicon-micromechanics are particularly suitable for this purpose. Such a construction is shown in Fig. 10.2-1 as a model. The possibilities, which arise from the application of intelligent signal processing, are demonstrated in a test system, in which accelerations can be recorded in two directions. The signal processing is performed in two levels. In level 1 plausibility tests are carried out with experimental data from the redundant elements, in order to recognize defect sensors and to exclude them from further signal processing. Then the experimental results of the remaining sensors are averaged and an acceleration value for each direction in space is given. Figure 10.2-2 shows the comparison of corrected and non-corrected experimental results using an example. The experimental data prepared in such a way could be processed further in level 2 using Fourier-transformation or other algorithms, particular suited for the task. This example shows clearly both advantages of using sensor arrays compared with isolated sensors: 0

The improvement of reliability. Even on failure of two sensors from an array of six, the proper function is still secured.

10.2 Sensors

435

Direction of Sensitivity

LlGA Sensors

Silicon Sensors

E ~ ~ l u ~ ~Circuit ion

Fig. 10.2-1 Model of a three-dimensional acceleration sensor with built-in redundancy for improved reliability.

so0

,

Acceleration (arb. units)

Acceleration (arb. units) I I

X-direction 0.5 I

i

c_=_

i^

b

-1.5

I~

,~

Y-ciirection

25

50

Time [s]

Fig. 10.2-2 Experimental data from a 2-dimensional acceleration sensor system with 3 redundant sensors each. a) Data received directly from the experimental set-up. b) Data after elimination of faulty sensors and averaging the signal for each set of sensors.

436 0

10 System Technology The improvement of the measurement quality. By using three sensors which are arranged by 90” to each other, one can determine by measuring, not only the amplitude but also the direction of the acceleration in space.

If this should happen in real time on-board the microsystem, then considerable requirements are made on the computing capacity and the efficiency of the software. In this area there is a strong need to catch up in research and development. Also on developing physical and chemical sensors even larger efforts must be made. Many applications in the chemical, biochemical and the medical field fail, due to the lack of selectivity and insufficient long-term stability of the sensors. An example of the capability of an array of chemical sensors is the “electronic nose”. Chemical sensors based on ion sensitive field effect transistors (ISFET) or conductivity sensors are usually not very selective. An unknown chemical substance in most cases cannot be identified uniquely with a single sensor element. By using two sensor elements with different sensor characteristics, the identification capabilities for the substance are already somewhat improved. When using an array of numerous sensors, each one having different sensor characteristics, then complex chemical substances can be accurately identified. Therefore, such an array, in conjunction with suitable signal processing for analysis, is referred to as an “electronic nose”. The potential for such electronic noses can be seen in the food sector, in medical diagnosis, and in the area of hazardous systems. The food industry has great demand for quality assurance of consumer products such as wine, cheese, or other products. The olfactory quality control is therefore an important factor and a promising field for the microsystem technology. In Fig. 10.2-3 the experimental set-up of such a measuring device is shown [Alth96]. The

Fig. 10.2-3 The central component of an electronic nose, comprising of 40 sensor elements with each having an individual sensor characteristic (Courtesy of Institute for Instrumentation Analysis, Research Center Karlsruhe).

10.2 Sensors

431

microsystem consists of an array of sensor elements based on layered metal oxideconductivity detectors. 40 sensor elements are accommodated on a 100 mm2 large silicon substrate. The principle of metal oxide conductivity gas sensors is based on the reaction of oxygen ions near the surface of the detector with gases in the surrounding atmosphere. By different cover layers and by operating at different temperatures for each element distinctive sensitivities to gas are achieved. This on exposure with the atmosphere to be analyzed leads to characteristic signal patterns. The signal patterns can be compared and determined, using a specially developed neural network program with stored calibration patterns. Some signal patterns for baking flavors are shown in Figure 10.2-4. As can be seen, a high selectivity between the individual patterns exist in such a way, that even mixtures of flavors can be analyzed in a reliable way. With large arrays of sensor elements and further refining of analysis, the spectrum in the future will be improved even more, although it seems questionable whether the human sense of smell can be matched or even surpassed in its sensitivity and versatility by such an arrangement.

Almond

lemon

Rum

VanilIa

Fig. 10.2-4 Signal patterns for different baking flavors obtained from the device shown in Fig. 10.2-3 (Courtesy of Institute for Instrumentation Analysis,

Research Center Karlsruhe).

438

10 System Technology

10.3 Actuators The second functional group of a microsystem consists of one or many actuators. The actuator can be understood in principle as the counterpart of the sensor. Whilst the sensor on input of a physical or chemical parameter responds with the output of an electrical or optical signal, the actuator on input of an electrical, optical or thermal signal gives an output of a physical parameter such as force, torque, dimensional- or phase changes. With the aid of actuators specific manipulations are controlled from the sensors and vice versa. Actuators enable the entire microsystem to move around, so that measurements or monitoring tasks carried out locally, for instance for environmental protection, can be performed independently and sequentially. Whilst a large number of sensors have been presented in the scientific literature and some have already been manufactured in industry, microactuator techniques still lag behind considerably. Presumably, this is because numerous sensor principles can be realized by relatively small modifications of microelectronic components, like for example ISFETs, whilst actuators could hardly be developed from microelectronic building elements. Furthermore, whereas silicon has excellent electronic and mechanical properties, it exhibits no great potential in the context of physical principles underlying actuators. Principles which depend on ferromagnetism, ferroelectricity, phase changes and the like, must be realized with other materials in other technologies. The special adaptation of LIGA technology for the realization of microactuators has been detailed in Chapter 7. It is emphasized once again, that special requirements are necessary with actuators in microtechnology. Already in Chapter l it was mentioned, that in microstructure technology surface effects show a strong dominance. Tribological problems are difficult to control in microtechnology, so components with mechanical friction should be avoided when possible. Membrane movements, or better still resonant oscillations of elastically suspended elements are better served using components with rolling or sliding friction. If we recall once more the motivation of microsystem technology, namely to transfer the “design philosophy” of microelectronics over to non-electronic systems. In this context the question is which element of microsystem technology could eventually take over the role of the transistor, i.e. the element which could lead to a higher system performance by duplicating many times and which has a high packing density as well as appropriate interconnections. In sensor technology we have found many such examples, however in microactuator technology such analogues presumably do not exist. Consider an array of electrostatically driven micromotors. An “intelligent combination” of these motors, which could lead to a higher system performance, is difficult to imagine. Such a concept is most likely best carried out with fluidic elements. The fluidic switch, a version of which was discussed under LIGA technology in Section 7.7.3, has a fluidic input, two control leads and two outputs. The analogy is close to electronic switches. A linear amplifier can be constructed by joining the output of one element with the control input of a boosted second element,

10.4 Signal Processing

439

a Fig. 10.3-1 Fluidic switches as elements for subsystems. (a) Three switches connected as linear amplifier, (b) fluidic switch with feed-back loop as oscillator.

whilst a bistable switch element results on feedback of the output on the control input of the same element, (Fig. 10.3-1). In principle, each logical switching element of microelectronics can be realized with fluidic switches. In earlier times developments were made, which were aimed at designing fluidic computers with such switches. Fluidic elements, especially switches, have their indisputable advantages also for other reasons. They work without mechanical friction, can be driven with inert gases or liquids, which is a considerable advantage in medical applications. In addition, they possess a high power density, and the attainable power extends continually into the macroscopic region.

10.4 Signal Processing The third functional group of a microsystem depends on signal processing. In this context the tasks of data processing are manifold. First of all, the flow of experimental data from the sensor array must be processed in parallel and could be used to control the actuators, or has to be transmitted to the outside via the interface. Signal processing in microsystems opens another broad field for research and development. Complex problems of matrix operations, approximations, regressions and characteristic diagram adjustments must be solved with the highest computing efficiency and in real time with the reduced possibilities of microprocessors in the system. The reliability of a system plays a crucial role in safety relevant tasks in the automotive field, or in air- and space travel, and especially in medical technology. The microsystem technology can contribute considerably especially to these re-

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10 System Technology

quirements. By incorporation of self-test routines as well as by redundant components or subsystems, the reliability can be increased to such a degree, that applications become feasible, which up until now have not been tackled just because these features have been lacking. As pointed out in Section 10.2, chemical sensors and especially biochemical sensors usually leave a great deal to be desired in their long-term stability. If the sensor is subjected to aggressive media, biological tissue or thermal loading, the characteristics of the sensor changes and the useful operating time decreases. The “intelligence” of the microsystem makes possible the recording of the operating time or of the “thermal history” for the sensor. From written-in performance characteristics the operation time of the corresponding compensation factors could be selected and used for the correction of the experimental values. Therefore, the useful operation time can be extended. Also neural concepts for information processing in microsystems are increasingly discussed (see in addition Section 10.4.2). However, the new concept which is copied from nature will not completely replace the classical von-Neumann architecture, because efficient “neuro-chips” are still missing on the market. Surely problems, like for instance pattern recognition, are however predestined for neural structures.

10.4.1 SignaI Processing for Sensors in Microsystems Signal processing in the context of microsysteins means the development and the use of system oriented solutions to detect, analyze and produce signals. Furthermore, the optimization of single tasks is not the first priority, but rather the optimization of the whole system regarding efficiency, reliability and cost-benefitratio. These criteria are associated with questions of failure compensation, data reduction, consideration of sample scattering, calibration and self monitoring. In this connection the decentralized signal pre-processing in the system components are of particular importance, which as subsystems take over specific tasks of signal processing, in order to achieve more clearly represented system structures as well as simpler testability. The potentials of a sensor array can only be realized with effective software, which considers the limited possibilities of the on-board computers. This leads to digital processing for the following reasons: 0

Calibration and compensation for aging and external influences in a sensor array can be managed by means of stored characteristics mappings.

0

Digital processing makes possible error tolerant, interference proof data transfer over large distances and in strongly electromagnetically disturbed environment.

The acquired signals with the sensor element which is generally analogue, is preamplified in its analog form and subsequently converted to digital form. A multi-

10.4 Signal Processing

441

plexer allows the accumulated signal flow from the array to be processed quasiparallel by the microprocessor. By comparing the experimental data during a calibration run with the required data, which can originate from a PROM, fabrication tolerances of the sensors could be individually compensated, undesired effects eliminated (e. g. temperature effects) and arithmetic operations carried out (for instance, the non-linear connection of experimental parameters to complex chemical or biochemical measurements). Many sensors age because of thermal exposure i. e. the parameter like sensitivity and selectivity change in mathematically describable ways compared with the virgin state (as manufactured). By recording the “thermal history” of a sensor element compensation terms can be calculated, so that the long term stability of a system can be raised significantly. In complex optical and chemical measurement tasks methods of pattern recognition can be used. However, the expenditure for signal processing can very quickly become so large that a small on-board microcomputer is overtaxed. However, neural networks offer promising possibilities, as described in Section 10.4.2. Following the outlines given by Najafi and Wise [Naja94j, sensors could be categorized by several generations. The plain sensor, with no electronics involved and no signal processing whatsoever, is considered the first generation, with the bimetallic strip serving as an example. The second generation involved remote electronic amplification of the analog signal and perhaps some temperature compensation. All electronics are remote from the sensor. The data is analog and the flow is one-way from the sensor to the display. The third generation (which is the state of most of the current devices) has at least some amplification and signal buffering occurring in-module using discrete or hybrid electronics. The sensor transmits the data into a remote signal-processing package consisting of an analog-todigital converter (ADC) and a microcomputer. The communication link is still one-way from the sensor to the evaluation circuitry. In the simplest case the system contains besides the actual sensor element the microelectronic circuit for signal processing [Tran88] i. e. 0 0

0

the analog signal pre-amplifier, the analog digital conversion (A/DC), the digital signal pre-processing in the microprocessor.

The structure suitable for microsystems is first of all the “chain structure”, in which the above mentioned functional building blocks are sequentially ordered (Fig. 10.4-la). The signal pre-amplification is connected directly to the sensor element. The kind of signal processing depends, of course, on the type of sensor. Voltage amplifiers, Am-converters and frequently also voltage-frequency-converters (V/F) are used with sensor elements with voltage output. In piezoelectric sensor elements the use of charge amplifiers is preferred. The amplified signal is then further processed with an A/D-converter or voltage-frequency converter. Sensor elements with frequency output are particularly favored, because of the convenient signal

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Fig. 10.4-1 Different arrangements of integrated sensors in microsystems. a) chain structure, b) parallel structure, c) circular structure with active feed-back by means of actuators. A/D = analog-digital converter, V/F = voltage-frequency converter, BI bus interface, SE = sensor element, AE = actuator element.

=

processing, in particular due to the simple means of converting the signal into the digital form. Usually sensor systems in microsystems are operated in parallel structures (Fig. 10.4-lb). In some cases a chain arrangement in differential mode is appropriate for linearization of the signal and the suppression of disturbing signals. A special type of sensor-arrangement is the circular structure (Fig. 10.4-lc). The physical parameter is fed back by means of actuators. Very stable systems with a-high dynamic range and protection against overloading result from this configuration, The technical requirement for sensor systems in circular structures is the availability of actuators which are suitable for compensation e. g. electrostatically moved cantilevers for feedback in acceleration sensors. The sensors and signal processing elements must be combined on an integral microsystem. The reason for this being: 0

the signal processing must be fitted to the related sensor structure,

0

the coupling to a standardized digital interface is only possible if the sequences of signals and the associated control commands are exactly maintained, parallel signal processing from sensor arrays is only possible, if the wide band transfer paths are short.

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443

The microsystem technology provides the ideal conditions for new kinds of measuring concepts by utilizing the advantages of sensor arrays in connection with error compensation, the electronic characteristic calibration, self-testing of functions, and other intelligent tasks to raise the reliability of the system. The value of sensor arrays in microsystems will be demonstrated in the following example. A single sensor element exposed to an environment of multiple unknown parameters a,b, c, d will result in a value, which is a function of all the parameters f (a,b,c,d). In other words: If one measures the pressure (parameter a),usually the temperature (parameter b), the type of gas (parameter c) and others are affected. It is not possible to measure only the pressure, unless there are other sensors to measure the temperature, the type of gas and so on. In general, to measure n parameters of a medium one needs n sensors of independent parameter functions: W =fi (a,b,c,d, ..., n> W, = f i (a,b,c,d, ..., n> = f 3 (n,b,c,d, ..., n>

w, w,= f n

...

(a,b,c,d, ..., n> with Wi the output from the sensor i. To solve the system of equations, the n parameter functions f, ... f, have to be known, e.g. by calibration of the sensors. Even then the task is probably not realistic with the limited computing power of the on-board microprocessor of a microsystem (Fig. 10.4-2) [Poin89]. To overcome this problem of solving a system of equations with a large number of unknowns, other methods have been developed. The task of evaluating the data flow from large arrays of sensors is very common in nature. The sense of seeing, hearing, feeling is a similar problem of evaluating the data flow from large sensor

Fig. 10.4-2 The principle of a sensor array and its evaluation.

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arrays, usually much larger than in MEMS. Nature pursues data processing in a totally different way than our data processing in so-called von-Neumann architecture. In our computers so far, data processing is basically a serial process, whereas in nature as far as we understand the function of a brain data processing is performed basically parallel, since speed of processing is more important for the survival of the individual than precision. Neural data processing therefore is discussed in the next section. It should be pointed out though, that this has, up to now, only a slight relationship to nature’s performance, but at least, neural data processing is trying to follow nature’s example.

10.4.2 Neural Data Processing for Sensor Arrays Although microsystem technology offers the possibility of utilizing whole arrays of sensors for one measurement, the efficient management and analysis of the data flow from the array requires computer power, which is difficult to achieve in microsystems. As always with complex tasks of signal processing, nature, during its evolutionary stages, has developed optimal concepts, which are worthwhile emulating. Information processing of living beings by the nervous system is fundamentally organized differently from that used in digital technical data processing, as commonly used nowadays. The information unit in the brain is the neuron. It consists of three main components; dendritic structure, cell body and axons (Fig. 10.4-3a). Across the dendrites, which one can view as data input units in analogy to technical data processXI

Synapses

/ Input

Y, = f ( l j )

endrites x2

Axon

n /

Weighting

x3

a

b

Fig. 10.4-3 Principle of a biological neuron a and its microelectronic analogy b.

10.4 Signal Processing

445

ing, signals are transferred from other neurons as electrical potentials by complex physiological processes. From these individually received signals one integrated signal is guided into the cell body. If this integrated potential exceeds a particular limit, then the neuron “fires”, i. e. the cell nucleus produces an electrical impulse, which is transmitted across the output circuit through the axon, onto the dendrite structure of other neurons. The transfer happens not across solid electrical contacts, but so-called synapses, which are basically non-conducting contact points, but are made to electrically conduct utilizing complex physiological electrolytes. A simple analogue to a technical component would be a coupling condenser, whose displacement current would be controlled by dielectric changing with time. This “neural displacement current” one also calls “synapsis strength”. The control of the synapsis strength is the condition for learning in neural systems. In nature these neural networks are extremely complicated and the regularity in the networks is still essentially unknown. The human brain has about 10“ neurons. One neuron has up to lo4 connections with 4 bytes each. In comparison to the main memory of an efficient computer, the brain possesses a capacity which is about lo8 times larger! How can one transfer such a biological structure to technically related problems? The analogy to the biological system is schematically shown in Fig. 10.4-3b. The model can be mathematically described in a simple form: (10.1) where

Y, = output signal of the neuron, X j = input signal of the neuron, Wu= synapsis strength with which the input signal can be multiplied.

f is the transfer function, which gives information about the signal height which the neuron “fires”, i. e. an impulse of a certain amplitude is emitted. This output signal Y, can be supplied again to many other neurons as the input signal. Usually technical neural networks are represented in layers (Fig. 10.4-4). The simplest configuration consist of an input layer, an output layer and an inner layer in-between (hidden layer). A certain signal pattern, which is placed on the input layer, results in a signal pattern on the output layer. The inner layer transforms by distribution and weighting the input signal onto the output pattern. As the information about the transformation is distributed over the whole network, the system is relatively insensitive to errors or even failure of single elements. By “training” the inner layer, i. e. by adjusting the weighting factors Wv, one can optimize the system to particular tasks. There are rules to train the inner layer, three of which will be sketched out [Hopf82, Koho88, Amit891.

Delta Learning Rule (10.2)

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output Layer

Inner Layer

Input Layer

Fig. 10.4-4 Example of a neural network comprising of an input layer, an output layer, and one inner layer.

where X j Y,,, Y, E

= input value =

desired output value

= actual output value = so-called learn parameter, where

0

<E

<1.

Hopfield’s Learning Rule The output of every processor element is fed to all others again as input:

(10.3) AWij = (2Xi - 1 ) . ( 2 4 - 1) It is repeated until there is no change in state by further running. A constraint is:

w..= w.. CI

.7l

Back Propagation The learning rule for the case that several inner neural layers are stacked on top of each other, to minimize the quadratic imaging defect E, with integrating to all patterns p :

Si(Xv)is the output signal of the output layer, if the input layer is fed the pattern X“. For the change of weight in the Zth layer then the following equation is valid: (10.5)

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447

Next using a relatively simple example, the advantages of neural analysis are shown. In practice sensor arrays are subjected to interference which can be divided into two groups: 0

falsification of sensor signals by time dependent disturbance signals, faulty function of sensors because of manufacturing errors or total failure.

A sensor array to measure the acceleration in the x- and y-direction is shown in Fig. 10.4-5. From six sensors, which are arranged in a circle, one obtaines a known redundancy of the test set-up arrangement. As an example the total failure of sensor number 1 is considered, which measures only the x-component of an acceleration. The error associated with conventional averaging of data of all sensors results in an error of 33.3 % for all accelerations in the x-direction. Signals of an acceleration in the y-direction only remain exempt. The error which is averaged over all sensors by the total failure of a sensor thus amounts to 17%. The neural network now interpolates this (error) pattern between the known trained patterns, whereby the error is distributed over the total network. The error is 24% and thus, greater than that with conventional averaging, but the largest error of a single result is clearly reduced, as shown in Fig. 10.4-6. Just as a total failure or some other “hardware” error can be reduced by neural methods so can the noise of a system. In this case the network is trained in such a way, that a given input signal produces the same output signal. It is trained with the back propagation algorithm to the point where no further improvement of the pattern results in the output. After optimization the noisy input signals are fed into

Acceleration 1

4

6

5 Fig. 10.4-5 Test set-up of 6 acceleration sensors tilted to each other by 60”.

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10 System Technology 0.04 0.03

P Y

-lv

c w

0.01

G b u)

0

$

-0.01

C

neural network

0.02

-0.02 -0.03

4

5

6 Sensor #

I

2

3

Fig. 10.4-6 Result of the neural treatment of this set-up. The failure of one sensor is partially compensated by the remaining sensors.

the network. How well now these noisy input signals of the network can be smoothed out, depends on the number and the “training” of the neuron in the inner layer. First hand knowledge in constructing neural networks, so-called neuro-chips is commercially available. These switches can cover input values with changeable weighting and can be added up. These values so obtained could be transferred again to other storage components. Such neuro-chips are presently available with 256 neurons. The clock rate lies in the 20 MHz region. As in such networks the neurons of every layer operate in parallel, the processing time of an analysis depends only on the number of layers and is independent of complexity. As soon as an operation is passed into the next highest level, the lower layer can again tackle a new task. For reasons of clarity, a very simple constellation of sensors was chosen as an example. However, the big advantages of neural networks first become significant with large and complicated systems. Therefore, a future direction lies in the design and analysis of the behavior of neural networks for signal processing with systems having large numbers of sensors. In order that the output signals of the neurochips could be optimally evaluated, suitable arrangements and wiring, as well as efficient methods for training these networks must be found. This must be carried out and monitored by a computer, i. e. the chips at this stage are coupled with a computer. After determination of the network configuration and the training phase, the chips could again be decoupled and fulfil their tasks independently.

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Some applications for neural networks have been achieved by intense development efforts in this area, e.g. sorting of packing waste, discovery of explosives during clearance procedures at airports, exchange rate prognosis on the stock market and others. Despite all the successes of the above mentioned applications, there still exists a good deal of scepticism. With specific tasks, in which a certain predictable (i. e. deterministic) behavior is expected, the use of neural networks is not possible or at best only to a limited extent. An analysis of what is going on in detail within the network structure, is only feasible in exceptional cases. All tasks, which require 100% accuracy, for instance safety tasks in complex industrial areas such as nuclear power plants, are therefore excluded from neural methods.

10.5 Interfaces of Microsystems The interface with the outside world has to be discussed as the final functional group of a microsystem. The interface can be, for example, an error tolerant data interface. With certain applications the requirements of a data transfer are extremely demanding. Consider for instance the noisy electromagnetic environment of the ignition system of the automobile or the transcutaneous transfer of data with intelligent implants in the medical area. Summorizing the interface of a microsystem, then we have to consider the total variety of all connections to and from the macroworld. Every microsystem is embedded in a macroscopic environment. Not only data and information must be exchanged but also physical parameters, for example the coupling to external energy sources of the thermal, optical, mechanical and fluidic kind (Fig. 10.5-1). In microelectronics these interfaces, which are limited to electrical signals, can be realized by relatively simple means and by technologies, which are well established and stable. In microsystem technology however, many problems must still be solved. These interfaces play an important role especially in medical technology, since here the microsystem must communicate with a highly complex “macrosystem”, the homo sapiens. Every microsystem consists of (quasi-two-dimensional) microelectronics, (three-dimensional) microcomponents, and interfaces to the macroworld. The concept of interfaces and couplings of the microsystem with the surrounding macro devices is an important matter in designing microsystems. Micro-/ macrocoupling incorporates all kinds of communication between microsystems and their macroscopic environment. The task involves a defined transfer of information, energy or substance (IES). This correlation is shown in Fig. 10.5-2. A typical microsystem consisting of sensors, actuators and other units, which are joined together across an internal bus system, is schematically represented. The components, which are required to ensure the IES-transfer, are shown as the shaded area inside and outside of the microsystem. The design of the micro4macrocoupling of microsystems does

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i

Sensor

internal Bus

Power supply .. Substances

I'

control Fluid .

c

1.r ~

Actuator

I

Fig. 10.5-1 The typical interfaces of a microsystem to the outside world. The types of interfaces can be distinguished by information, energy, and substances.

Fig. 10.5-2 The IES-transfer between microsystem and macroscopic environment.

10.5 Interj4uces of Microsystems

451

not only involve the manufacturing of the transfer component, inside and outside the microsystem, but also the development of suitable PCT technology applicable to these components, and finally the provision of standardized transfer protocols, guidelines, and requirements for a failure proof transfer. For microsystems it is characteristic, that the tasks of their micro-/macro coupling could be based on very different disciplines such as microelectronics, micromechanics or microoptics, but also chemistry or biology. A systematic comprehension of all IES principles represents a considerable aid for conceiving micro-/ macro coupling. A summary of some available transfer principles and corresponding transfer components is given in Table 10.5-1. Table 10.5-1 IES Transfer principles and transfer components

I

Transfer principle

Source

Medium

Receiver

Electric Magnetic Electromagnetic

Conductor Magnetic tape Space

Register Reading head HF-receiver antenna Photocell Ultrasound receiver Power supply

(Optical) Mechanical

Driver Driver HF-transmitting antenna Laser, LED Ultrasound transmitter Chargeholtage source Current loop, electron spins HF-transmitting antenna Laser, LED Tool

Thermal Fluidic

Heat source Micropump

Material, Space Tube

Particle beam

Radiation source

Space

(Optical) Mechanic a1 E

Electrical Magnetic Electromagnetic

S

Glass fiber Material Conductor Space Space Glass fibers Space

Current loop, Electron spins HF-transmitting antenna Photocell Object of the surrounding Heat sink Reservoir, Microturbine Surface

Additional help in microsystem design comes from a systematic classification of basic operations and physical effects, of available technologies and material data, but also from procedures and tools designed to find optimum solutions, The specific problem of microsystems design is represented by the range of required expertise, which is usually no longer available with one person because of the multitude of associated disciplines. The knowledge-based development of methods is therefore an important requirement.

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Other problems specific to microsystems result from the miniaturization and the package density of IES transfer components, which could lead to cross talk and unwanted interferences to an extent of making the systems inoperable. Because of the variety of methods and tools inherent with the design of IES-components, heterogeneous design environments are required, which need standard data format for integration in microsystems design. As the optimization of the whole system is the final target, the conception of micro-/macrocoupling too is to be included in the whole design concept.

10.5.1 The IE-Transfer Electrical Micro-/Macrocoupling Electrical micro-/macrocoupling incorporates all coupling between microsystems and their macro environment, whereby the IE-transfer is carried out using electrical connections. Interfaces in microelectronics, which have been realized and matured in large volume production, are of particular interest for electrical micro-/ macrocoupling. Here intelligent sensors and actuators which are presently under development together with their ability to communicate over bus systems represent an important basis for the development of microsystems and their electrical micro-/macrocoupling . Analog signals are generally processed in digital computers. The conversion of analog signals from sensors is performed by A/D-converters with ever increasing improvements in recent years. Actuator are mostly driven by analog power, using digital/analog converters D/AC. The I-transfer often is associated with voltage signals between 0 and 1OV. The transfer of voltage signals compared to the transfer of current signals have in fact the advantage of a lower dissipation. However, a considerable disadvantage is the sensitivity to interfering voltage which can be problematic depending on the input impedance. Clearly better properties with immunity to noise in mind can be achieved by FM or PDM transfer of frequency or time analog signals. The schematic construction of microelectronic systems nowadays and of those which are currently under development, are compared in Fig. 10.5-3. By using multiplex-technology more analog signals can be transferred on one wire, whereby the amount of A/D- and D/A-transformers is reduced. In conventional centralized systems, see Fig. 10.5-3a, A/D-, and D/A-transformers and connected multiplexers or de-multiplexers, are located within the central computer. Today decentralized systems gain importance due to the reductions in size, power and performance. With this also parts of the signal processing are relocated from the central computer to the process periphery. This concept leads to the development of intelligent sensors and actuators, which in addition can communicate over bus systems equipped with analog signal processing units, A/D-, or D/Atransformers and microcomputers. In the context of the sensor generations mentioned in Section 10.4.1 these devices can be considered as sensors of the fourth generation.

10.5 Interjaces of Microsystems

453

Fig. 10.5-3 a) Schematic construction of existing microelectronic systems, b) design of systems under development. SE = sensor element, AE = actuator element, AA = analog amplifier, F/D = frequency digital converter

Optical Micro-/Macrocoupling In optical micro-/macrocoupling, all types of signal transfer by electromagnetic waves are included. The transfer is carried out either by a transparent lightwave conducting medium (waveguides) or wireless in space. Recently optical waveguide structures gained increasing importance for microsystems.

The IE-transfer by optical waveguides possesses a number of inherent advantages compared to electrical transfer principles, which seem very attractive for many microsystems of future application areas. To these belong: large signal band widths, high signal transfer rates, transmission reliability, electromagnetic immunity to noise, electromagnetic immunity to interference, ideal electric isolation, large temperature application range, small space requirement. Important aspects for the development of micro-/macrocoupling, based on optical waveguides, are represented in existing microoptical components, integrated optical circuits and optical bus systems, which are being developed. Their functionality, quality and fabrication costs depend greatly on further development of packaging

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and connection technology (PCT), in order to fulfil the demanding requirements of tolerance and long term stability. Parallel to this, the development and application of effective simulation tools becomes more important, in order to achieve the design of complex, optical coupling tuned to the microsystem components.

Mechanical Micro-Nacrocoupling The definition of mechanical micro-/macrocoupling includes all components of the mechanical IE-transfer between microsystems and their macro environment. IE-transfer is carried out on one hand by direct contact using motion units e. g. microtools to manipulate objects of the macro environment, devices for positioning, moving, and navigation, or microsensing devices i. e. feelers. Other mechanical concepts of micro-/macrocoupling are based on acoustic principles like the I-transfer by ultrasound. The development of mechanical micro-/macrocoupling is still in its early stage. As a starting base however, an extensive amount of expertise from macromechanics is available. It is therefore advisable to transfer macroscopic mechanical concepts by scaling to smaller dimensions. The use of these processes on the whole microsystem can only be accomplished with restrictions, since modifications such as changing the hierarchy of the power could have the consequence of changing the systems properties altogether. Miniaturization down to the dimensions in the submicrometer region are in principle questionable. For these reasons additional concepts must be developed, which are custommade for small dimensions. Novel concepts can be adapted from the area of bionics, where the study of biological systems and the technical conversion of biological principles is also the aim.

The Ultrasound Transfer Wireless IE-transfer principles are interesting for microsystems applications with difficult physical access, e. g. medical implants. The ultrasound transfer is one possibility to wireless I-transfer. The E-transfer by ultrasound is however not yet realistic. Ultrasound-transfer systems, which are for example used successfully for industrial communication in machines, could be used as the starting point for further development in the microsystem technology. Custom-made FEM-models were further developed to simulate specific acoustic sound field characteristics. Possible principles of ultrasound transfer are based on piezoelectric, electromagnetic or magnetostrictive sound conversion. Piezoelectric sound transmitters and receivers could be realized by conducting pads on piezoelectric substrates with standard planar technology and are therefore the simplest to transfer to microsystem applications. Typical frequencies lie in the range of 20 kHz to 1 GHz. Depending on the ultrasound frequency, relatively large I-transfer rates are possible, although damping- and dispersion mechanisms on the transfer channel have to be taken into account. An advantage is the high interference stability in comparison to the electromagnetic I-transfer, because generally acoustic interfer-

10.5 Inteqaces of Microsystems

455

ence can be easily suppressed in that particular frequency region. The ultrasound transfer is compatible with all modulation- and encoding procedures. One advantage of transcutaneous (transfer without cutting the skin) I-transfer to implanted microsystems is the low damping of the transfer channel in biological tissue. In the soft tissue the average ultrasound damping is 1 [dB/cm.MHz], so that at typical ultrasound frequencies (about 1 MHz) the propagation is only weakly dampened in the tissue. Low-cost piezo-ceramic transformers could be used as transmitters and receivers, whose acoustic impedance matches that of the tissue. On transfer a low-loss yield is achieved with a directional transmitterand receiver characteristic of the ultrasound transformer. The miniaturization of the transformer is limited by the minimal required signal power at the receiver. Typical dimensions of realized transformers in this frequency range are in the area of 1 cm. An average intensity of 0.1 W/cm2 in the tissue is considered innocuous. For directional ultrasound transfer over short distances, voltages of some volts suffice on the piezoelectric transformer.

10.5.2 The S-Transfer Fluidic Micro-/Macrocoupling All couplings for the S-transfer are described as fluidic micro-/macrocoupling for gases or liquids. An important special case is the fluidic E-transfer which in substance transport occurs simultaneously e. g. in hydraulic or pneumatic systems, whereby the compressive force of the gas or the kinetic force of the fluid are put to use. Only those fluidic substances are acceptable which do not interfere chemically or in any other way, i.e. by adsorption or desorption, with the fluidic devices of the microsystems. Besides pure fluids, e. g. coolants, rinsing fluids, colloidal dispersed medical systems are of particular interest to transfer solid body particles e. g. body liquids. The development of fluidic micro-/macrocoupling is still in its starting phase. At the center of development work is the preparation of suitable transfer components and their integration by PCT. Fluidic Microcomponents Until now only a few fluidic microcomponents are commercially available, except for microcapilliary polyimide tubes and stainless steel microtubes. Minimum inner diameters are between 80 and 150 pm. Fluidic micro4macrocouplings components such as microvalve, microswitches, micropumps, and microconnectors have yet to be developed on an industrial scale. The design of fluid microcomponents is not confined to a choice of suitable materials and processes but also to the simulation and optimization of fluid flow conditions. For fluidic applications only those materials are considered, which do not interact with the substance to be transferred or which only interact with them in a specific way by chemical reactions, adsorption or desorption. For example, typical

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LIGA-materials like copper and nickel, or ion containing glasses and swelling adhesives like cyanoacrylate are not suitable for medical applications. To realize fluidic microcomponents and their connections, hybrid devices are required. A typical construction of membrane driven micropumps or -valves consist of one or several actuator chambers, micromembranes and horizontal or vertical flow channels. In order to produce those kinds of 3D structures by means of 2D structure technologies, it is sensible to arrange several structured layers on top of each other. One problem with this process is the requirement for imperviousness of all the seals. In order to take into account the above described aspects in the design and construction of a microsystem, corresponding design tools must be made available. Initially the microsystem must be theoretically described and approximated in iterative simulation runs for the optimal solution. Originating from this total concept, the design concepts of microstructuring of information processing, and of microelectronic equipment and finally the construction- and junction techniques are developed.

10.6 The Module Concept of Microsystem Technology By integrating microcomponents to the microsystems, not only building elements, which are fabricated by different technologies, but also components based on different physical concepts have to be connected with each other. So for example, in certain cases fluid components with optical or mechanical components must be connected to electrical components. The necessary PCT technique involves other physical disciplines besides microelectronics, so that much experience and many different processes are necessary to manufacture such systems. The expense of developing- and manufacturing may extend to such a degree, that small and medium sized enterprises can not handle the full complexity of microsystems technology. In addition, many applications require a high flexibility in manufacturing due to the small size of individual lots for very specialized applications (e. g. medical applications). A way out of this problem may be the concept of a modular systems design for microsystems. An analogy for such an approach can be seen in microelectronics in the gate array technology. These gate arrays can be fabricated in large quantities and later, depending on the application, tailored by soft- or hardware modifications to meet their final performance requirements. The disadvantage is a certain reduction in performance and functional density compared to a custom made circuit (ASIC), and the cost of such a device is considerable, when only small quantities are required. This approach should also be adopted in microsystem technology. Especially in MEMS there are many dedicated applications where only small quantities of modules are needed. As in microelectronics, certain modules (e. g. power supplies, fluidic control units, sensor arrays) can be fabricated efficiently in large quantities,

10.6 The

457

Fig. 10.6-1 The principle of developing a modular microsystem. The complete micro-

system is partitioned into functional modules which can be tailored to their desired performance by minor alterations of the hardware design or of their driving electronics. and the modules are then tailored by minor software- or hardware modifications to meet the requirements of a particular system. The principles of the development path are shown in Fig. 10.6-1. As a consequence the microsystem must be partitioned into functional subsystems (modules), which can then be assembled in different combinations to fabricate microsystems with a minimum of investments and without highly skilled workers. The high development costs for microsystems with a large range of uses are then divided among different suppliers of modules and manufacturers of microsystems. The pre-requirement for this is the development of a simple assembly technique for the modules and an early standardization of the interfaces to other modules or to the outside world. The module concept will be exemplified next, as demonstrated in the Research Center, Karlsruhe. The starting point is a microanalysis system which is schematically shown in Fig. 10.6-2 designed to analyze heavy metal ions [Ache95]. The material which is to be investigated, is pumped into a cuvette equipped with an optochemical sensor. In the presence of certain ions (Hg+, Cd+), this sensor undergoes a change in color, which is measured by a microspectrometer and subsequently evaluated in a circuit. After this measurement, the sample is discarded, and the cuvette is refilled with a reference sample to calibrate the system. Afterwasds the cuvette is rinsed and thus being prepared for a new measurement. The handling of the liquid for the analysis is canied out by a “fluid management device”. This complex system made up of optical, fluidic and microelectronic components, was designed by a modular construction system, as depicted in Fig. 10.6-3.

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Fig. 10.6-2 The schematic set-up of an optochemical microsystem for the analysis of water polluted with heavy metal ions.

Fig. 10.6-3 The fluid management subsystem manufactured from modular components.

10.6 The

459

Fig. 10.6-4 Components to build either a) the fluid management device or b) a new electrochemical microanalysis system (ELMAS) [Ache95].

Fig. 10.6-5 Photography of the electrochemical microanalysis system (ELMAS) built from components shown in Fig. 10.6-4b.

10 System Technology

460

With the exchange of a few modules, other systems with different tasks can be assembled conveniently. The assembly of modules and components is shown in Fig. 10.6-4 and the corresponding technical version by Fig. 10.6-5. The advantage of this concept becomes immediately obvious. Only relatively few components need to be developed specifically for this application. A large part can be accepted as standard building elements, which could thereby be produced “as stock” by an inexpensive quantitative manufacturing process. A more universal example is shown in Fig. 10.6-6. This device is supposed to transport two liquids A and B by means of two micropumps and two flow sensors into a mixing chamber. The product is then analyzed in a subsequent module and further processed or discarded. The modular concept would look like a stack of modules sketched in Fig. 10.6-6b. One can distinguish the individual modules (Fig. 10.6-Bc) connected by a common “fluidic bus”. Each module has a “fluidic distributor” to guide the required fluid into the individual module. The modules can be arranged in parallel or in series with regard to the fluidic bus. If such standard components are to be made by a large number of producers, the interfaces must be standardized. Standardization may even be the key issue of such a concept, since the availability of modules from international suppliers is essential for the economic success. The advantage of this modular concept could be seen in the fact that the considerable development costs are distributed among the manufacturers of the modules and the system manufacturer. The system manufacturer could chose the required modules from different suppliers and assemble the systems without the need of special equipment (clean room, process line) and without especially skilled manpower. This modular concept could be a powerful tool to proliferate the MEMS idea among the small and medium sized enterprises with limited development capacity.

Pump 1

PUrnP 2

Rinsing

I chamber

sensor1

+

+

Analysis chamber Flow sensor2

Analysis chamber

I

c Flow sensor 1 + 2 ~ump1+2

Fig. 10.6-6 a) A typical set-up of a microsystem in the chemical industry. b) the verification of the set-up in the modular concept. Each modul is connected with the “fluidic bus” by means of “fluidic distributors”. c) The concept of a module. The microstructure is fabricated by hot embossing or injection molding. Each module is supplied with an electrical and fluidic interface.

10.7 Design, Sinzul&on, Iiitegrution, und E s t of Microsystems

46 I

10.7 Design, Simulation, Integration, and Test of Microsystems The starting point o f innovative solutions for microsystems is the analysis and synthesis of principles and functions with the aim of finding new functional principles. For this, simulation processes are of great importance. The inl'lucnce of a multitude of different electrical and non-electrical parameters in complex three dimensional geometries innst be calculated as a function o f time. For the synthesis of systems and for problem optimization, suitable calculation processes and procedures should be developed ;is well as design tools lor intcrfaces. MEMS simulation tools should provide the integration of manufacturing, materials and functional requirements [BMFTO2]. In contrast to conventional system design, lor instance with inicroclcctronics, the design of microsystems is characterized by the necessity to consider a number 01physical, chemical and biological dimensions, different mechanisms and para.. .. sitic sensitivities. An integral microsystem design without using computer--aided processes is economically not viable because of its high complexity of design, verilication and testing. Unlortunately, at the moment only tools for the design of single componcnts are available. Because of numerous possible solutions and because of mutual incompatibilities with different fabrication processes, the modeling support in the early phase of the microsystems design, namely the study-, specification- and planning phase, takes an evcr increasing importance. It is necessary to simulate the microsystem in its entirety from the point of view of functional, electrical, physical-chemical, time and application dependencies due to cross-talk, parasitic sensitivity of the individual micro components. In addition, digital-, analog- and conduction simulation has to be co-ordinated with the simulation of mechanical, optical, chemical, thermal and other components. A serious problem for development and manufacture is the test and diagnosis of microsystems. In contrast to microelectronics, because of the multitude of nonelectrical parameters, it will be difficult to define standardized test systems. Therefore, universal interfaces to the microsystem hardware must be developed, in order to test description languages, and to design manufacturing tools. The task for a system design is such that the components have to be arranged, that a system performance is possible, which is optimized from the point of view of function and economics, and which, so to speak, represents an energy minimum. With this requirement a microsystem design does not differ from a macroscopic design. In order to get results two extremes in the procedure can be pursued: the bottom-up design and the top-down design. With the bottom-up design one starts with the (optimized) element, for instance a sensor element, combines this to a functional module (sensor array, actuator array and microprocessor), and finally combines several functional modules to a system. The advantage of this procedure is that the expenditure for the simulation

462

10 System Technology

is comparatively low, as one can consider assembled elements or functional groups as a black box, whose “step response” can be measured or analytically calculated. The disadvantage with the bottom-up design is that the resulting system usually is neither functionally nor economically optimized. An element can in fact be optimized for a predicted task e. g. with a sensors highest sensitivity for a measurement, however that does not mean that the system in total is optimized. With mutual interferences of the elements for instance due to electromagnetic or thermal coupling, the intended effect (namely the highest precision in measuring) can be spoiled. The theoretically correct formulation for an optimal system design would be the top-down concept. Here, first of all, the systems specifications are defined, from which follow the specifications for the functional groups and finally of the elements. As such an approach is in principle carried out with microelectronics, and the relevant simulation tools are available, one had the expectation, this could be easily transferred to microsystem design too. However, the conditions inherent in microsystem technology, differ from those in microelectronics. With microsystems one expands the regime of microelectronics to a multitude of physical, chemical, and biological regimes and their interdependencies. This diversity cannot yet be completely mastered, with present day computing power and simulation tools. Already the different physical arrangement of the components leads to unpredictable solution for systems performance. Consider for instance the difficulty, to calculate the mechanical, electromagnetic and thermally optimized structure of a single acceleration element from the system specification of a three dimensional acceleration sensor system in the top-down process. As in many situations in real life, the “middle of the road” approach is the way to go. In this context, one works with macromodels i. e. with simplified models of the components, which describe the characteristics to such a decree in the different physical areas, so that a simulation at the systems plane is possible with justifiable costs. For instance, besides the initial, electronic function of a component, this macromodel must also be able to describe thermal and mechanical properties. The system simulation must be able to process this macromodel and the corresponding coupling. Depending on the position of the components to each other, one achieves different system performances. One can now use optimization routines, to calculate the component position according to certain criteria. These criteria could be: 0 0 0 0

optimizing the package density, minimization of the interconnect length, avoiding thermal overload and “hot spots”, minimization of the parasitic cross talk.

Some of these optimization routines could be taken from microelectronics in particular with regard to the package density and the interconnect lengths and crossovers. Others must be especially developed for the microstructure technology. The

10.7 Design, Simulation, Integration, and Test of Microsystems

463

calculation and optimization of parasitic cross talk among different physical regimes is a demanding task. System integration means the assembly of different functional units, where the problems of assembly, interfaces, housing and connections are solved. All of these activities should aim for a system that is more than the sum of its components. However, it is not sufficient to add up the physical parameters of the functional units, but also the interaction of the individual components with respect to electrical, mechanical, thermal, and other parameters must be taken into account. Because of the increasing packaging density of the components with advanced integration, the interference - desired or not - of the physical parameters has increasing influence on system performance. The interference of the components may be even the limiting factor for further miniaturization of microsystems.

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Index

A accelerometer 232, 245, 252, 257, 260 Acetic acid 253 activation energy 234 actuators 438 - fluidic switch 438 - linear amplifier 439 - oscillator 439 ADFET 281-282 adhesion 422 - anisotropic adhesion 424 - anisotropic conductive film (ACF) 424 - capillary adhesion bonding 423 - electrically conducting adhesives 422 - heat conducting adhesives 422 - Isotropic Adhesion 422 alignment 179 alkali hydroxide 217, 219, 221 aluminum hydroxide 253 Aluminum Micromachining 253 ammonium hydroxide 225 analysis of crystal structure 55 analysis of thin films 164 - Auger Electron Spectroscopy (AES) 166 - Electron Probe Microanalysis (EPM) 165 - Ion Scattering Spectroscopy (ISS) 169 - microprobe 165

Rutherford Back Scattering Spectroscopy (RBS) 169 - Scanning Tunneling Microscope 170 - Secondary Ion Mass Spectroscopy (SIMS) 168 - Secondary Neutral Particle Mass Spectroscopy (SNMS) 169 - X-Ray Photoelectron Spectroscopy (XPS) 168 anisotropy 217, 223, 226 - isopropyl alcohol 223 anodic bonding 284, 425 - process of anodic bonding 426 - Wafer-to-Glass Bonding 425 - Wafer-to-Wafer Bonding 427 applications 349 - Filters for the Far Infrared 349 - fluidic microstructures 366 - LIGA-structures for optical uses 368 - microactuators 353 - Microcoils 351 - Microgears 352 - microsensors 353 - Rigid Metallic Microstructures 349 array 275 arrays 273 Arrhenius’ law 239 arsine 250 ASIC, see application specific -integrated circuit 2 14 -

486

Index

B back-end 213 basic processes 26 - chip 26 - connecting 26 - dicing 26 - housing 26 batch fabrication 5 batch process 218 beam 248, 253 bipolar technology 210, 213, 279 black silicon method 242 bolometer 274 bonding, themal 214 boron etch stop 235 boundary layer 216 Bragg’s condition 56 - elastic scattering 57 - intensity 61 - interference 57 - reciprocal lattice 58 Bragg-Method 60 Bravais 47 Bridge 233 Bridgman method 24 - vertical Bridgman method 24 bulk micromachining 214 bump 253, 270, 273 buried layer 214

C Cantilevers 23 1 carbon dioxide 255 cathodic metal deposition 74 - convection 77 - crystallographic growth 75 - diffusion 77 - diffusion layer thickness 78 - diffusion limited reaction 77 - diffusion overpotential 77 - electron transfer 75 - embedded microelectrode 79 - Fick’s first law 77 - forced convection 80 - inhibitor 76 - linear diffusion 78

material transport 80 migration 75-76 - Nernst diffusion layer 77 - reaction limited 77 - rotating disc electrodes 77 - surface energy 75 CCD, see Charge-coupled device 210 ceramics 84 - aluminum nitride (AlN) 84 - aluminum oxide (A203) 84 - barium titanate BaTiOs 88 - beryllium oxide (BeO) 84 - electromechanical coupling factor 88 - gas sensors 88 - louse 90 - phased array 91 - piezoelectric actuators 90 - piezoelectric effect 87 - Sn02 sensor 88 - ultrasonic micromotor 91 - ultrasonic scanner 91 - zirconium oxide ZrOz 88 channel 211 CHEMFET 281 chemical etching technology 162 - Barrel Etching (BE) 162 - barrel reactor 163 - downstream etcher 163 chemical sensor 232 chemical vapor deposition (CVD) ff 27 - low pressure (LPCVD) 27 - plasma enhanced (PECVD) 27 - threshold energy 27 ciliary actuator 254 clean room technique 39 - dean bench 43 - Clean room classification 40 - grey room 39 - I S 0 standards 14644 41 - laminar flow 39 - particle density 39 - retention period 43 - size distribution 41 - standard ambient environment 42 -

Index

turbulent flow 43 US federal standard 209E 40 - volume flow 43 CMOS, see complementary metaloxide-semiconductor 210 CMP, chemical-mechanical polishing 218 CO 282 coating technique 133, 143, 146 - chemical vapor deposition = CVD 133 - CVD Processes 143 - epitaxy 146 - Oxidation 148 - physical vapor deposition = PVD 133 - Plasma Polymerization 148 coil 286 columnar growth 250 comb-drive 247, 252, 261, 285 combined etch process 160 - advanced silicon etch process (ASE-process 160 - characteristics of IBE 162 - characteristics of RIE 162 - normalized lateral etching 160 combined etch technology 158 - Chemically Assisted Ion Beam Etching (CAIBE) 159 - Cryo-etch Technology 159 - Reactive Ion Beam Etching (RIBE) 159 - Reactive Ion Etching (RIE) 159 compensation structure 230 complementary metal oxide semiconductor 223, 238, 253, 269, 271, 273, 276, 286 complementary oxide metal semiconductor 242, 248 computer aided design (CAD) 176 - aliqument pattern 178 - CAD-layout 177 - Calma GDS I1 format 181 - circular element 177 - logical layer 181 - organization of design 18 1 -

-

487

polygon 177 test structure 178 - Text 177 concentration gradient 2 16 conformal 254 contact pad 223, 253 contacting technology 41 8 - adhesion 422 - anodic bonding 425 - bump technology 420 - comparison of different contacting techniques 421 - eutectic gold-tin soldering 420 - Flip-Chip Technology 420 - TAB-technology 418 contamination 2 18 cooling, cryogenic 244 coriolis force 263 critical point drying 255 crystal 45 crystalline structure 62 - Debye-Scherrer camera 63 - Debye-Scherrer method 63 - Electron Beam Diffraction 64 - X-ray Diffraction 62 crystallography 45 - bases 46 - body centered cubic 48 - Bravais 47 - cubic lattice 48 - face centered cubic 48 - lattice 46 - lattice vector 46 - parallelepiped 46 - primitive unit cell 46 - simple cubic 48 - translation vector 46 - unit cell 46 - Wigner-Seitz 46 CVD (chemical vapor deposition) 143, 212 - adsorption 145 - APCVD = atmospheric pressure chemical vapor deposition 144,250 - coverage 145 - desorption 145 -

-

488

Index

flow behavior 145 LPCVD = low pressure chemical vapor deposition 144, 212, 250, 277 - PECVD = plasma enhanced chemical vapor deposition 144, 250, 274 - surface migration 145 Czochralski method 18, 23 - doping 18 - ingot 20 - mechanical set-up 19 - mechanical traps 23 - nucleating crystal (seed crystal) 18 - oxygen 23 - pulling speed 18 -

D 3D-structuring 344 - conical structures 346 - inclined structures 345 - lens arrays 347 - spherical surfaces 346 - stepped structures 344 Debye-Scherrer Method 60 decoupling 238 - thermal 238 definition of a Microsystem 431 denuded zone 21 1 design, simulation, integration, and test of microsystems 461 - “middle of the road” approach 462 - bottom-up design 461 - top-down concept 462 diffusion 32, 249 - concentration profile 32 - constant-surface-concentration diffusion 32 - constant-total-dopant diffusion 32 - dopant impurity 32 - doping layer 32 - Fick’s second law of diffusion 32 - impurity concentration 32 diode 265, 273 dipole layer 222, 282 doping 211, 235

drain 211 dry etch process 151 - active etch particles 154 - anisotropic etching 152 - anisotropy 152 - BE = Barrel Etching 154 - CAIBE = Chemical Assisted Ion Beam Etching 154 - chemical etching technology 162 - combined etch technology 158 - directionality 152 - IBElIBM = Ion Beam EtchinglIon Beam Milling 154 - IE = Ion Etching 154 - isotropic etching 152 - PE = Plasma Etching 154 - physical etch technology 155 - RIBE = Reactive Ion Beam Etching 154 - RIE = Reactive Ion Etching 154 - selectivity 152 dry etching 212, 241

E electro-discharge machining (EDM) 394 - applications 397 - basics of EDM 394 - closing phase 395 - die sinking 395 - dielectric fluid 396 - discharge phase 395 - EDM milling 396 - erosion resistance index 395 - micro gripper 397 - Starting phase 395 - wire erosion 395 Electrochemical Etching 237 electrode-electrolyte interface 69 - diffuse double layer 72 - double layer 70 - Galvani-potential 70 - inner Helmholtz plane 72 - interfacial region 72 - outer Helmholtz plane 71 electron beam diffraction 64

Index electron beam lithography 182 beam deflection 184 - beam step size 186 - data flow 189 - Fowler-Norheim equation 183 - Gaussian beam 182 - jobfile 190 - post processor 189 - principle construction I82 - proximity effect 190 - Richardson equation 183 - screen scan process 185 - shaped beam 187 - vector scan process 185 - writing on the fly 186 electroplating 66, 253-254 - conductivity 68 - dissociation 66 - electrical mobility 66 - electrolyte 66 - electrolytic tank 67 - hydratation 69 - hydrate molecules 66 - hydrate sheath 69 - resistivity 68 - solvatation energy 69 epitaxial layer 2 11 Epitaxy 28, 146 - diborane B2H6 147 - GaAs-epitaxy 147 - Hetero-epitaxy 147 - Homo-epitaxy 146 - phosphine PH3 147 - reaction gas 29 - standard reactor types 28 - temperature control 28 - temperature gradients 29 etch rate 217, 239 etch stop 234 etching 35, 215, 223, 225-226, 24 1 - alkali hydroxide 221 - alkaline 215 - aluminium 271 - aluminum 223, 253 - ammonium hydroxide 225 -

489

anisotropic 215, 217, 239, 260, 283 - anisotropic etching 35 - barrel etching 35 - control 234 - dry 215, 241 - dry etching 35 - EDP 226, 242, 276 - Electrochemical 237, 258 - etch rate 239 - Etch Stop 234, 238 - HNA 220 - hydrazine 226 - isotropic 215, 217 - KOH 221 - Plasma etching 35 - porous silicon 239 - reactive ion etching (RIE) 35 - selectivity 35 - silicon 223 - silicon nitride 223 - Sputter etching ion milling 35 - wet 215, 220 - wet chemical etching 35 - xenon difluoride 241 etching, Shape 227 ethylene-diamine 219, 226 Ewald-construction 59 -

F FET, see field effect transistor 210 field effect transistor 280 field oxide 212, 252 film deposition ff 27 - chemical vapor deposition (CVD) ff 27 - spin coating 27 filter 285 Filters for the Far Infrared 349 - band pass filter 349 - Highpass filter 350 flip-chip bonding 270 float zone method 20 - diborane (B2H6) 22 - melt-zone 22 - Phosphine (PH3) 22

490

Index

seed crystal 20 segregation 20 flow sensor 268 fluidic microstructures 366 - bistable attachment wall element 367 - microfluidic linear actuators 368 - microfluidic switches 366 - micropumps 366 fluorocarbon coating 244 foundry technology 214 four-point configuration 266 Fresnel lens 252, 274 front-end 2 13 functional section 433 -

G GaAs single crystals 24 galvanic deposition 3 16 - “active carbon” purification 3 19 - alloys 322 - copper microstructures 322 - current density 3 17 - deposition of nickel 316 - diffusion layer thickness 323 - electroplating 3 16 - gold microstructures 322 - hydrogen deposition 3 18 - intrinsic stress 317 - microthrowing power 317 - Ni-Co alloys 323 - nickel sulfamate electrolyte 316 - nucleation agent 3 18 - oxidation of titanium 316 - permalloy 323 - Vicker’s hardness 3 18 - wetting agent 317 gas dynamics 115 - gas dynamical continuum theory 115 - Knudsen number 115 - molecular flow theory 115 gas flow sensor 232 GASFET 281 gate 211, 250, 281 gate oxide 2 11

gels 101 polyacrylates 101 - reversible swelling 102 - volume-phase transformation gettering 2 11 gradient freeze method 24 - quartz ampoule 24 grating 277 gyroscopes 253, 262 -

102

H Hall effect 279 Hall plate 279 HARM, see high aspect ratio micromachining 239 heat capacity 233 heat transfer 268 heater 253 heating resistor 269, 272, 275, 284 Hemispere 242 high aspect ratio microstructure 239 high aspect-ratio micromachining 242 hinge 252 hot embossing 333-334 - process steps 334 - processed silicon wafers 335 hot film anemometry 268 hot-wire anemometry 268 hybrid technology 37, 404 - bumping 410 - bumps 410 - dielectric paste 406 - firing of pastes 408 - insulating paste 406 - Interconnects 405 - laser soldering 38, 410 - multilayer ceramics = MLC 404 - placement of components 408 - reflow soldering 38, 408 - resinate paste 406 - resistance paste 406 - rheological properties 404 - screen printing 37, 404, 407 - soldering 408 - substrates and pastes 404 - surface mounted devices (SMD) 38

Index

thixotropic 405 vapor phase soldering 409 wire bonding 39 hydrazine 226 hydrochloric acid 253 hydrofluoric acid 2 17 hydrogen 282 hydrogen peroxide 253 hydroxyl 222 -

I IC see integrated circuit 210 inductor 285 infrared detector 273 injection molding 327 - Duroplastic 328 - feed plate 332 - functional principle 328 - glass transition temperature 329 - hydraulic pressure 330 - injection molding machine 327 - material parameters 329 - plasticization 328 - polyacrylnitrile butadiene styrol (ABS) 328 - polyvinyl chloride (PVC) 328 - pressure-time sequence 329 - screw path 331 - sprue bush 332 - suspended microstructure 333 - Thermoplastics 328 - volume shrinkage 330 integrated circuit - substrate 210 - technology 210 interfaces of microsystems 449 - electrical micro-/macrocoupling 452 - fluidic micro-/macrocoupling 455 - fluidic microcomponents 455 - IE transfer 452 - IES transfer principles 451 - IES-transfer 449 - information, energy or substance (IES) 449 - light-wave coupling 453

491

mechanical micro-/macrocoupling 454 - micro-lmacrocoupling 449 - optical micro-/macrocoupling 453 - piezoelectric sound transmitter 454 - S-Transfer 455 - Ultrasound Transfer 454 - transcutaneous 1-transfer 455 interfacial processe 10 ion implantation 33 - annealing 33 - channelling 33 - contamination 34 - doping profile 34 - implanter 33 - penetration depth 33 ISFET 281 -

K KOH 238 Krumm etch

253

L laser micromachining 399 - ablation process 401 - C 0 2 laser 400 - excimer laser 400 - heat affected zone (HAZ) 399 - (Nd:YAG) laser 400 - laser assisted chemical vapor deposition (LCVD) 401 - laser micromachining setup 399 Laue-Method 60 layer sacrificial 248 leakage and leak detection 128 - real leak 128 - virtual leak 128 LEC method 26 - barrier layer 26 - boron oxide 26 - dislocation 26 LEC method (liquid encapsulated crystal) 25 lens, electrostatic 246 LIGA 214

492

Index

LIGA Process 289 3D-Structuring 344 galvanic deposition 316 injection molding 327 light-conducting structures 347 mask production 291 mold insert fabrication 321 plastic molding 324 - process steps 290 - quality of the structure 310 - sacrificial layer technology 341 - second electroplating 337 - variations and additional steps 341 - X-ray lithography 301 LIGA technology 6 LIGA-structures for optical uses 368 - beam splitter 369 - bidirectional transmission- and receiving module 372 - heterodyne receiver 374 - laser-waveguide coupling 373 - lens 369 - microoptical bench 371 - microoptical bypass-switch 376 - microoptical components 368 - microspectrometer 376-377 - NIR-microspectrometer 377, 379 - prism 369 - self-focussing reflection grid 378 light valve 276 light-conducting structures 347 - applications 349 - cladding layer 347 - mono-modal structures 347 - multi-mode application 347 - reflecting layer 348 - wave guiding structures 347 liquid crystals 88 - cholesteric phase 89 - nematic phase 89 - smectic C phase 89 - smectic phase 89 lithography 30 - computer aided design (CAD) 176 - electromagnetic spectrum 30 - electron beam lithography 30, 182 -

general procedure 175 imaging process 30 - ion beam lithography 202 - optical lithography 192 - pattern for aligment 179 - photoresist 30 - shadow projection 30 - step and repeat 30 - synchrotron radiation 30 - test pattern for overetching 179 - X-ray lithography 202 - yellow rooms 31 litography, resist 171 LOCOS, see local oxidation of silicon 211 Lorentz force 279 low temperature cofired ceramics (LTCC) 428 LPCVD 212, 250, 277 lubrication 25 1 -

M magnetotransistor 279 magnetron ion etching 244 mask 217-218 mask production 291 - Absorber 291 - adhesion 299 - beryllium 299 - carrier foil 292 - comparison of different processes 297 - construction of a mask 291 - cyanide electrolytes 299 - electron beam lithography 296 - electron beam writer 296 - electroplating 298 - intermediate 292 - intermediate mask 294 - invar 294 - materials for mask carriers 293 - optical lithography 295 - polyimide layer 297 - process mask 292, 299 - proximity effect 296 - Reactive Ion Etching 296

Index structuring of the resist 295 sulfite gold electrolytes 299 titanium 293 titanium layer 297 titanium mask 294 transmission 292 wet chemical oxidation 299 X-ray transmission 292 materials 81, 87 - actuator application 90 - anisotropy-energy density 94 - electroheological fluids 105 - gels 101 - Joule magnetostriction 93 - liquid crystal polymers (LCP) 103 - liquid crystals 88 - magnetostrictive linear inch-wormmotor 95 - magnetostrictive linear motor 95 - magnetostrictive metals 92 - polarization 87 - quartz crystal 87 - sensor applications 87 - shape memory metals 96 - surface properties 81 - Terfenol-D 93 - transversal magnetostriction 93 materials for shaping microstructures a3 - ceramics 84 - polymers 84 - single crystals 83 mean free path 110 - doubling temperature 112 - impact parameter 111 - Southerland correction 112 mechanical micromanufacturing 3x1 - batch fabrication 392 - complex molding tool 384 - diamond tool 382 - heat transition coefficient 386 - injection molding 392 - micro container 390 - micro heat exchanger 386 - micro miller 385 -

493

micro mixer 388 micro pump 391 micro pyramid 383 micro reactor 389 - microstructured intensifier 393 - milling machine 382 - production process 382 - turning lathe 382 - V-mixer 388 - X-ray intensifying screen 394 - X-ray recording film 393 mechanical microproduction 8 membrane 223, 230, 236-238, 258, 269,271,273,283,286 memory technology 210 Mesa 230 metal 218 methanol 255 microactuator 353 - electromagnetic linear actuators 356 - electromagnetic reluctance motor 365 - electrostatic comb drive 356 - electrostatic motor 363 - micro chopper 359 - micromotors 363 - microturbines 361 - minimally invasive medical instrument 362 - optical switching system 358 microbridge 233, 248 microchannel 235, 248, 253 microelectronics 2, 15 microgear 248 microlens 243 micromachining 215, 242 micromirror 252-254, 276, 278 micromotor 248, 251, 257 microneedles 235 micropump 282 microsensor 232, 257, 353 - acceleration sensors 353 - accelerometer 232, 245, 252, 257 - arrays 273 - capacitive acceleration sensor 353 -

-

494

Index

chemical 232 CO 282 flow sensor 268, 361 gas flow 232 hydrogen 282 infrared detector 273 magnetic induction 279 mechanical 257 microturbines 361 pH 281 pressure 271 pressure sensors 257-258 radiation 273 radiation impared 232 temperature 263 temperature compensation 355 thermal converter 239 microsensors 280 - chemical 280 microstructure 1 microsystem 12 - communications 12 - self-test 12 - signal processing 12 - simulation 12 - system specification 12 microsystems 9 microvalve 283 Miller index 50 mirror 252-253, 276 mirrors 278 module concept 456 - electrochemical microanalysis system (ELMAS) 459 - fluid management device 457 - fluidic bus 460 - fluidic distributor 460 - interfaces 460 - microanalysis system 457 - optochemical microsystem 458 MOEMS, see microoptoelectromechanical systems 252 mold insert 321 - interfacial layer 322 - metal deposition 321 - mold insert fabrication 321 -

monolayer time 112 coverage 112 monotime 113 monotime table 113 mounting 38 - chip-and wire-technologies -

38

N nanotechnology 1 narrow-bandgap semiconductor 273 Neural data processing 444, 447 - back propagation 446 - biological neuron 444 - delta learning rule 445 - Hopfield’s learning rule 446 - neural displacement current 445 - neural treatment 448 - neuro-chip 448 - synapsis strength 445 nickel 263 nitric acid 217 nitric acidlhydrofluoric acid 2 15 noise-equivalent temperature difference 274

0 open-circuit potential 237 optical lithography 192 - clear defect 193 - contact printing 194 - DESIRE-process 199 - Fresnel diffraction 195 - full wafer exposure 197 - grey-tone lithography 20 1 - imaging projection 196 - mask 193 - mask repair 193 - modulation transfer function (MTF) 196 - numerical aperture 196 - opaque defect 193 - optical lithography for micromechanics 200 - phase shifting mask 198 - Poor Man’s LIGA 201 - proximity printing 195

Index resolution 195-196 shadow projection 194 - step and repeat process 197 - tri-level process 199 - UV-LIGA 201 orthosilicic acid 225 overpotential 73 - crystallization overpotential 74 - diffusion overpotential 74 - electron transfer overpotential 74 - reaction overpotential 74 oxidation 148 - CVD process 150 - dry oxidation 150 - oxide thickness 151 - TEOS 150 - wet oxidation 150 - wet process 150 oxidation, wet 212 oxide, native 226 -

P packaging 36 - failure fractures 36 - gate array 36 - mechanical stress 36 - power dissipation 36 - VLSI (very large scale integration) 36 packaging and interconnecting techniques (PIT) 403 - contacting of silicon dies 412 - contacting technology 4 18 - die-bonding 412 - eutectic 412 - hybrid technology 404 - low temperature cofired ceramics (LTCC) 428 - wire-bond techniques 413 palladium 282 parylene 254 passivation 2 13, 244 PECVD 250, 274 pH 281 phosphine 250 photoconductors 273

495

photolithograph 212 photolithography 2, 4 physical 133 physical deposition process (PVD) ff 29 physical etch technology 155 - angle of incidence 157 - Artifacts of physical etching 158 - characteristics of pure physical etching 157 - Ion Beam Etching (IBE) 156 - Ion Beam Milling (IBM) 156 - Ion Etching (IE) 155 - parallel plate reactor 155 - sputter etching 155 piezoresistor 258, 260 pinhole 223 Pirani gauge 271 planarization 254 plasma polymerization 148 plasma, inductirely compled 242 plastic molding 324 - hot embossing 324, 333 - reaction injection molding 324 polarization 73 polymer 218 polymer micromachining 254, 277 polymers 84 - Addition polymerization 85 - amorphous polymer 85 - as resists 104 - crystalline polymer 85 - glass region 86 - glass transition temperature 86 - melting region 86 - monomer units 84 - negative resist 105 - photoresist 105 - plastic range 86 - polymethylmethacrylate (PMMA) 104 - positive resist 105 - viscous region 86 polymethylmethacrylate (PMMA) 302 - Attenuation of light 377 - Chain scission 305

496

lvidex

Cross-linking of PMMA 305 direct polymerization 302 monomer MMA 302 scission of the polymer chain 303 polysilicon 212, 234, 248, 250, 266, 285 - micromachining 285 polysilicon micromachining 2 14, 250, 26 1 porous silicon 239 pressure sensor 257, 258, 271 proof mass 260 properties of thin films 129 - adhesive strength 132 - chemical bonding 132 - columnar 130 - columnar structure 130 - dendritic 130 - dendritic structure 130 - electrostatic bond 132 - measurement of adhesion 132 - movchan and demchishin 130 - polycrystalline 130 - polycrystalline structure 130 - punch 133 - punch test 133 - Scotchtape test 133 - scratch test 133 - structure zone model 129 - Thornton 130 - van der Waals bond 132 PSG, see phosphorous silicate glass 250 PTAT 265 pump 282 purification see also segregation 21 PVD (physical) vapor deposition - electron beam vaporization 134 - evaporation 133 - evaporation source 134 - high frequency sputtering 138 - inductively heated source 134 - ion cluster beam technology 140 - Ion plating 139 - magnetron sputtering 138 - neutral atom clusters 141 -

-

non-self-supported discharge

- plasma assisted deposition

137 139 138

radio frequency sputtering resistance source 134 - self-bias process 138 - self-supported 137 - self-supported discharge 137 - sputter etching 138 - Sputtering 135 - target 135 - TiAlNi 139 - TiN 139 pyramid, inverted 227 pyramid, tumcated 230 pyrazine 219 Pyrex 284 pyrocatechol 219, 226 pyrolysis 212, 250 -

Q

quadrupole

245

R radiant signal 273 radiation sensor 232 reaction injection molding (RIM) 324 - polymerization 327 - reaction shrinkage 325 - scheme of a RIM machine 324 - vacuum RIM machine 326 - vacuum-reaction injection molding 326 reciprocal lattice 55 resist 171 - carbene 173 - crosslinking 173 - diazonaphtoquinone (DQN) 173 - ketene 173 - negative resist 172 - Novolak resin 173 - photoinduced polymerization 173 - polymethyl methacrylate (PMMA) 172 - positive resist 172 - spin coating 172 - Wolff conversion 173

Index resist layer 302 adhesion 303 cast 303 casting 303 - developer 305 - direct polymerization 302 - molecular weight 304 - monomer MMA 302 - scission of the polymer chain resistor 253 resonator 232, 245, 286

three-dimensional acceleration sensor 435 sensor array 10 sensor process 215 sensor system 11 shape memory metals 96 - austenite lattice structure 96 - critical temperature 96 - martensite lattice structure 96 - one-way effect 96 - properties of shape memory metals 99 - Suppressed Shape Memory 98 - two-way effect 96 sidewall passivation 244 signal processing 439 - analog digital conversion (A/DC) 44 1 - analog signal pre-amplifier 441 - calibration 440 - chain structure 442 - circular structure 442 - first generation sensor 441 - long-term stability 440 - neural data processing 444 - parallel structure 442 - second generation sensor 441 - signal processing for sensors in microsystems 440 - thermal history 440 - third generation sensor 441 - value of sensor array 443 signal, neural 235 silane 212 silicon 15, 210, 223 - amorphous 250 - hemispherical 239 - physical properties 15 - polycrystalline, see polysilicon layers 250 silicon crystal 53 - stereographic projection 55 silicon MEMS 209 silicon micromachining 2 15 silicon micromechanics 6 silicon microsystem technology 209 -

-

303

S sacrificial aluminum etching 271 sacrificial layer 253-254, 271, 274, 277 sacrificial layer technology 34 1 - movable microstructures 344 - process steps 342 - sacrificial layer 342 - selectively etched 343 - selectively etching 343 - titanium layer 343 scene simulator, thermal 275 SCREAM 245 second electroplating 337 - carbon black 340 - conducting molding materials 339 - electrically conducting plastics 338 - metallic feed plate 337 - metallic microstructures 337 - Production variants of molding 339 - PVD-process 341 Seebeck coefficient 267 segregation coefficient 20 segregation effect 20 seismic masse 260 selectivity 218, 234 semiconductor 16 - compound semiconductor 16 - elemental semiconductor 16 sensor 433 - chemical sensor 436 - electronic nose 436 - signal pattern 437

497

498

Index

silicon nitride 212, 218, 223, 234, 274, 277, 281 silicon nitride bridges 268 silicon on insulator 214 silicon oxide 213, 218, 234, 250, 28 1 silicon oxynitride 2 13 silicon separated by implantation of oxygen 213 silicon single crystal 17 - Czochralski method 18 - electronic grade silicon 18 - fractional distillation 17 - metallurgical grade silicon 17 - polycrystalline silicon 18 - production process 17 - the Float Zone method 18 silicon substrate 2 11 Silicon Technology 210 silicon tetrafluoride 24 1 silicon, complexed 222 SIMOX, see silicon separated by implantation of oxygen 210 SIMPLE 246 single crystal wafers 15 - production 15 SOI, see silicon on insulator 210 solar cell 273 source 211 spin coating 27 stereographic projection 48 - Wulff's grid 49 sticking 255, 276 stiction 255 STM, see scanning tunneling microscope 245 strain gauge 258 stress 250 sublimation of frozen solvent 255 supercritical drying 255 surface micromachining 214, 248 surface modification 3 1 surface reaction 2 17 surface tension 255 synchrotron radiation 205 - aperture angle 205

application of X-ray lithography 208 - beam lead 207 - characteristic wavelength 205 - curvature radius 207 - spectral output 207 system technology 43 1 -

T tandem detection 263, 273 tape-automated bonding 270 TEAM, see tetraethylammonium hydroxide 225 technology 1 temperature coefficients of resistance 266 - aluminium 266 - Diffused silicon resistor 266 - negative 266 - Polysilicon 266 temperature sensor 263 TEOS, see tetraethoxysilane 250 tetraethylammonium hydroxide 225 thermal converter 239 thermal oxidation 3 1 - dry oxidation 31 - oxidation layer 31 - wet oxidation 31 thermistor 253, 266, 269, 274 thermocouples 266, 267 thermoelectric 267 thermoelectric effect 269, 273 thermopile 267, 269 thermopneumatic 284 thin film stress 250 three-electrode setup 237 tip 230, 235 TMAH, see tetramethylammonium hydroxide 225 transduce - mechanical 257 - pressure sensors 257 transducer 232 - accelerometer 232, 245, 252 - arrays 273 - chemical 232

Index

CO 282 electrical signal 285 - flow sensor 268 - hydrogen 282 - infrared detector 273 - light valve 276 - magnetic induction 279 - microfluidic 282 - micromirror 276, 278 - pH 281 - pressure 271 - pressure sensors 258 - radiation 273 - radiation, impared 232 - temperature 263 - thermopneumatic 284 transducer, gas flow 232 transducerscene Simulator, thermal 275 transmission gear 25 1 transport 215 - diffusion 216 trench 217, 244, 280, 286 trenche 245 -

-

U ultrasound 2 16 underetching 217, 233, 241 V V-Groove 227 vacuum 109 - mean free path 110 - monolayer time 112 vacuum measurement 125 - Bayard-Alpert gauge 127 - cold cathode ionization gauge (Penning Principle) 127 - friction type vacuum gauge 126 - penning ionization vacuum gauge 127 - pressure transducer 125 - thermal conductivity vacuum gauge 126 - thermionic ionization vacuum gauge 127

499

vacuum production 117 absorption pump 124 adsorption pump 122 cryo-pump 123 diffusion pump 121 displacement pump 118 ejector vacuum pump 121 gas ballast 119 gas transfer pumps 117 gas-bonding pump 121 getter pump 124 ion getter pump 124 kinetic pumps 117 sorption pump 121 sorption pumps 117 turbomolecular pump 119 Vacuum pumps 117 vane-type rotary pump 118 Working principle of a turbomolecular pump 120 - zeolite 122 vacuum technology 109, 116 - Avagadro’s constant 116 - barometric formula 109 - classification 116 - fine vacuum 116 - high vacuum 116 - rough vacuum 116 - ultrahigh vacuum 116 - vacuum production 117 valve 282 velocity of atoms and molecules 114 - average velocity 114 - distribution of velocity 114 - Maxwellian distribution 114 - table of velocities 115 -

W wafer 22 - flat 22 - inner diameter saw 22 - mechanical lapping 22 - wire cutting saw 22 wagon wheel 240 wet etching 220 Wheatstone bridge 258, 269

500

Index

wire-bond technique 413 advantages and disadvantages 417 - ball-wedge bonding 415 - hot-pressure welding 413 - sonotrode 414 - test procedures 417 - thermocompression wire-bonding 413 - thermosonic wire-bonding 414 - ultrasonic hot-pressure welding 414 - ultrasonic wire-bonding 414 - wedge-wedge bonding 416 -

x X-ray diffraction 62 X-ray lithography 7, 202, 289, 301 - adhesion 303 - Betatron oscillation 313 - characteristic wavelength 3 12 - damage dose D, 309 - distribution of the molecular weight 307 - divergence of radiation 313 - fluorescence radiation 3 13 - Fresnel-Diffraction 3 11

-

-

-

-

-

G-values 304 inclination of the absorber walls 313 limiting dose DG 308 molecular weight 304 penetration depth 308 photoelectrons 3 11 plasma source 204 polymethylmethacrylate (PMMA) 302 pre-absorber 309 production of secondary electrons 3 14 radiation dosage 306 resist layer 302 resolution limiting effects 311 surface dose Do 308 synchrotron radiation 205, 308 total error 312 X-ray mask 203 X-ray source 204 X-ray tube 204

Y yaw rate

262

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