An Introduction to Surface-Micromachining
An Introduction to S urface-micromachining by Robert W. Johnstone M. Parameswaran Engineering Science Simon Fraser University Kluwer Academic Publishers Boston/DordrechtiLondon
Distributors for North, Central and South America: Kluwer Academic Publishers 101 Philip Drive Assinippi Park Norwell, Massachusetts 02061 USA Telephone (781) 871-6600 Fax (781) 871-6528 E-Mail <kluwer@wkap.com> Distributors for all other countries: Kluwer Academic Publishers Group Post Office Box 322 3300 AH Dordrecht, THE NETHERLANDS Telephone 31 78 6576 000 Fax 31 786576474 E-Mail <orderdept@wkap.nl>... ~ " Electronic Services <http://www.wkap.nl> Library of Congress Cataloging-in-Publication Title: An Introduction to surface-micromachining Author (s): Robert W. Johnstone & M. Parameswaran ISBN 978-1-4757-1077-9 ISBN 978-1-4020-8021-0 (ebook) DOI 10.1007/978-1-4020-8021-0 Softcover reprint ofthe hardcover 1 st edition 2004 Copyright 2004 by Kluwer Academic Publishers All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photo-copying, microfilming, recording, or otherwise, without the prior written permission ofthe publisher, with the exception of any material supplied specifically for the purpose ofbeing entered and executed on a computer system, for exclusive use by the purchaser of the work. Permissions for books published in the USA: permissions@wkap. com Permissions for books published in Europe: permissions@wkap.nl Printed on acid-free paper.
This book is dedicated to all things flat and small.
Contents Dedication Preface Acknow ledgments v xi xiii 1. INTRODUCTION 1 1.1 Why miniaturize? 2 1.2 Why surface-micromachining? 3 1.3 History 4 5 Part I Fabrication 2. MICROFABRICATION PROCESSES 9 2.1 Photolithography 9 2.2 Thermal Growth 14 2.3 Deposition 15 2.4 Etching 18 2.5 Doping 21 2.6 Chemical Mechanical Polishing (CMP) 22 2.7 Wafer Bonding 23 2.8 Release 24 2.9 Wire Bonding 25 28
Vlll AN INTRODUCTlON TO SURFACE-MICROMACHINING 3. MICROMACHINING TECHNOLOGIES 3.1 Bulk Micromachining 3.2 LIGA 3.3 Surface-Micromachining 29 31 34 35 41 Part 11 Design 4. MICROSCALE PHYSICS 4.1 Scaling Laws 4.2 Casimir Force 4.3 Surface Forces 4.4 Friction 4.5 Continuum Mechanics Approximation 5. MECHANICS 5.1 Beam Equations 5.2 Torsion 5.3 Cantilevers 5.4 Euler-columns 5.5 Pseudo-rigid body Modelling 45 45 51 52 55 57 61 63 63 66 68 70 71 73 6. 7. DESIGN RULES 6.1 Minimum Geometry Rules 6.2 Maximum Geometry Rules 6.3 Minimum Enclosure Rules 6.4 Minimum Overlap Rules 6.5 Design Rule Checks (DRC) NON-IDEAL PROCESSES 7.1 Topography 7.2 Dimensional Tolerances 7.3 Thin-film Stresses 75 75 76 77 78 78 81 81 85 87 92
Contents IX 8. DESIGN TECHNIQUES 93 8.1 Ground Planes 93 8.2 Double Thickness Parts 94 8.3 Conducting Paths 95 8.4 Tethers 96 8.5 Dimpies 97 8.6 Process Variations 98 99 Part III Devices 9. ELECTRICAL COMPONENTS 103 9.1 Bonding Pads 103 9.2 Resistors 106 9.3 Capacitors 108 9.4 Inductors 110 112 10. MECHANICAL COMPONENTS 115 10.1 Compliant Devices 115 10.2 Springs 116 10.3 Suspensions 119 10.4 Flexure Joints 124 10.5 Levers 126 133 11. ELECTROSTATIC ACTUATORS 135 11.1 Parallel-Plate Actuators 136 11.2 Interdigitated Comb Actuators 141 11.3 Curved Electrode Actuators 145 11.4 Scratch-drive Actuators 147 150 12. ELECTRO-THERMAL-COMPLIANT ACTUATORS 153 12.1 Thermal Bimorph Actuators 154 12.2 Guckel Actuators 154 12.3 Bent-beam Actuators 159
x AN INTRODUCTION TO SURFACE-MICROMACHINING 12.4 Topological Optimization 13. RAISED STRUCTURES 13.1 Hinges 13.2 Assembly 162 164 165 165 168 172 Part IV Processes 14. POLYMUMPS PROCESS 14.1 Design Rules 14.2 Photo Gallery 15. SUMMIT PROCESS 15.1 Design Rules 15.2 Photo Gallery 16. MUSIC PROCESS 16.1 Design Rules 16.2 Photo Gallery Index 177 178 178 179 181 182 182 183 184 184 184 189
Preface In the process ofteaching several courses on surface-micromachining, it has become apparent that there is a severe gap in the existing literature. In articles and in books, authors approach the subject from one of two directions: processing knowledge or device knowledge. These two approaches neglect an important bridge between the fabrication process and actual device implementation. A large set of skills exists that is required to design surface-micromachined devices. Knowledge of a specific micromachining process is not enough. Theory specific to a particular device is not enough. Designers must connect the manufacturing process, microscale phenomena, and design data to physical form and function. Intuition, developed by experience with macroscale systems, will not help with microscale systems. These are the basic ideas that designers must understand if they are to successfully implement any device design. We hope that this book, while it cannot be complete, will provide a unified view of surface-micromachining. Building up from the basic building block of microfabrication techniques, this book will discuss surface-micromachining design in general, and it will finish with the theory and design of concrete components.
Acknowledgments We would like to thank the following people for their help in the writing of this book: Sunjaya Djaja, Linnea Duke, lan Foulds, and Bonnie Gray. We would also like to thank all the students who took the surface-micromachining course at Simon Fraser University, their successful and less than successful experiments played a role in shaping this text.