RF MEMS and Their Applications RF MEMS and Their Applications Vijay K. Varadan K.J. Vinoy K.A. Jose Pennsylvania State University, USA Copyright  2003 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (+44) 1243 779777 Email (for orders and customer service enquiries): cs-books@wiley.co.uk Visit our Home Page on www.wileyeurope.com or www.wiley.com Reprinted April 2003 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, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London W1T 4LP, UK, without the permission in writing of the Publisher. 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Other Wiley Editorial Offices John Wiley & Sons Inc., 111 River Street, Hoboken, NJ 07030, USA Jossey-Bass, 989 Market Street, San Francisco, CA 94103-1741, USA Wiley-VCH Verlag GmbH, Boschstr. 12, D-69469 Weinheim, Germany John Wiley & Sons Australia Ltd, 33 Park Road, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809 John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada M9W 1L1 Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Library of Congress Cataloging-in-Publication Data Varadan, V.K., 1943 – RF MEMS and their applications / Vijay K. Varadan, K.J. Vinoy, and K.A. Jose. Includes bibliographical references and index. ISBN 0-470-84308-X (alk. paper) 1. Radio circuits–Equipment and supplies. 2. Microelectromechanical systems. 3. Microwave circuits. I. Vinoy, K.J. (Kalarickaparambil Joseph), 1969– II. Jose K. Abraham. III. Title. TK6560.V33 2002 621.384  13–dc21 2002071393 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-470-84308-X Typeset in 10/12pt Times by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain by Biddles Ltd, Guildford and King’s Lynn This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production. Contents Preface xi 1 Microelectromechanical systems (MEMS) and radio frequency MEMS 1 1.1 Introduction 1 1.2 MEMS 2 1.3 Microfabrications for MEMS 5 1.3.1 Bulk micromachining of silicon 5 1.3.2 Surface micromachining of silicon 8 1.3.3 Wafer bonding for MEMS 9 1.3.4 LIGA process 11 1.3.5 Micromachining of polymeric MEMS devices 13 1.3.6 Three-dimensional microfabrications 15 1.4 Electromechanical transducers 16 1.4.1 Piezoelectric transducers 18 1.4.2 Electrostrictive transducers 20 1.4.3 Magnetostrictive transducers 22 1.4.4 Electrostatic actuators 24 1.4.5 Electromagnetic transducers 27 1.4.6 Electrodynamic transducers 29 1.4.7 Electrothermal actuators 32 1.4.8 Comparison of electromechanical actuation schemes 34 1.5 Microsensing for MEMS 35 1.5.1 Piezoresistive sensing 35 1.5.2 Capacitive sensing 37 1.5.3 Piezoelectric sensing 37 1.5.4 Resonant sensing 38 1.5.5 Surface acoustic wave sensors 38 1.6 Materials for MEMS 42 1.6.1 Metal and metal alloys for MEMS 42 1.6.2 Polymers for MEMS 42 1.6.3 Other materials for MEMS 44 1.7 Scope of this book 44 References 45 vi CONTENTS 2 MEMS materials and fabrication techniques 51 2.1 Metals 51 2.1.1 Evaporation 51 2.1.2 Sputtering 53 2.2 Semiconductors 54 2.2.1 Electrical and chemical properties 54 2.2.2 Growth and deposition 57 2.3 Thin films for MEMS and their deposition techniques 61 2.3.1 Oxide film formation by thermal oxidation 61 2.3.2 Deposition of silicon dioxide and silicon nitride 62 2.3.3 Polysilicon film deposition 64 2.3.4 Ferroelectric thin films 64 2.4 Materials for polymer MEMS 67 2.4.1 Classification of polymers 67 2.4.2 UV radiation curing 74 2.4.3 SU-8 for polymer MEMS 80 2.5 Bulk micromachining for silicon-based MEMS 84 2.5.1 Isotropic and orientation-dependent wet etching 84 2.5.2 Dry etching 88 2.5.3 Buried oxide process 88 2.5.4 Silicon fusion bonding 89 2.5.5 Anodic bonding 90 2.6 Silicon surface micromachining 91 2.6.1 Sacrificial layer technology 91 2.6.2 Material systems in sacrificial layer technology 92 2.6.3 Surface micromachining using plasma etching 93 2.6.4 Combined integrated-circuit technology and anisotropic wet etching 94 2.7 Microstereolithography for polymer MEMS 94 2.7.1 Scanning method 95 2.7.2 Two-photon microstereolithography 96 2.7.3 Surface micromachining of polymer MEMS 97 2.7.4 Projection method 97 2.7.5 Polymeric MEMS architecture with silicon, metal and ceramics 102 2.7.6 Microstereolithography integrated with thick-film lithography 105 2.8 Conclusions 105 References 105 3 RF MEMS switches and micro relays 109 3.1 Introduction 109 3.2 Switch parameters 111 3.3 Basics of switching 115 3.3.1 Mechanical switches 116 3.3.2 Electronic switches 117 CONTENTS vii 3.4 Switches for RF and microwave applications 117 3.4.1 Mechanical RF switches 118 3.4.2 PIN diode RF switches 119 3.4.3 Metal oxide semiconductor field effect transistors and monolithic microwave integrated circuits 123 3.4.4 RF MEMS switches 124 3.4.5 Integration and biasing issues for RF switches 125 3.5 Actuation mechanisms for MEMS devices 127 3.5.1 Electrostatic switching 128 3.5.2 Approaches for low-actuation-voltage switches 141 3.5.3 Mercury contact switches 146 3.5.4 Magnetic switching 148 3.5.5 Electromagnetic switching 148 3.5.6 Thermal switching 151 3.6 Bistable micro relays and microactuators 152 3.6.1 Magnetic actuation in micro relays 152 3.6.2 Relay contact force and materials 156 3.7 Dynamics of the switch operation 157 3.7.1 Switching time and dynamic response 158 3.7.2 Threshold voltage 160 3.8 MEMS switch design, modeling and evaluation 162 3.8.1 Electromechanical finite element analysis 163 3.8.2 RF design 165 3.9 MEMS switch design considerations 174 3.10 Conclusions 175 References 178 4 MEMS inductors and capacitors 183 4.1 Introduction 183 4.2 MEMS/micromachined passive elements: pros and cons 184 4.3 MEMS inductors 184 4.3.1 Self-inductance and mutual inductance 185 4.3.2 Micromachined inductors 188 4.3.3 Effect of inductor layout 194 4.3.4 Reduction of stray capacitance of planar inductors 198 4.3.5 Approaches for improving the quality factor 200 4.3.6 Folded inductors 211 4.3.7 Modeling and design issues of planar inductors 212 4.3.8 Variable inductors 215 4.3.9 Polymer-based inductors 215 4.4 MEMS capacitors 215 4.4.1 MEMS gap-tuning capacitors 217 4.4.2 MEMS area-tuning capacitors 224 4.4.3 Dielectric tunable capacitors 228 4.5 Conclusions 229 References 235 viii CONTENTS 5 Micromachined RF filters 241 5.1 Introduction 241 5.2 Modeling of mechanical filters 244 5.2.1 Modeling of resonators 244 5.2.2 Mechanical coupling components 251 5.2.3 General considerations for mechanical filters 257 5.3 Micromechanical filters 258 5.3.1 Electrostatic comb drive 258 5.3.2 Micromechanical filters using comb drives 260 5.3.3 Micromechanical filters using electrostatic coupled beam structures 265 5.4 Surface acoustic wave filters 268 5.4.1 Basics of surface acoustic wave filter operation 269 5.4.2 Wave propagation in piezoelectric substrates 270 5.4.3 Design of interdigital transducers 271 5.4.4 Single-phase unidirectional transducers 274 5.4.5 Surface acoustic wave devices: capabilities, limitations and applications 275 5.5 Bulk acoustic wave filters 276 5.6 Micromachined filters for millimeter wave frequencies 278 5.7 Summary 282 References 283 6 Micromachined phase shifters 285 6.1 Introduction 285 6.2 Types of phase shifters and their limitations 286 6.2.1 Ferrite phase shifters 287 6.2.2 Semiconductor phase shifters 287 6.2.3 Ferroelectric thin-film phase shifters 288 6.2.4 Limitations of phase shifters 288 6.3 MEMS phase shifters 289 6.3.1 Switched delay line phase shifters 289 6.3.2 Distributed MEMS phase shifters 289 6.3.3 Polymer-based phase shifters 296 6.4 Ferroelectric phase shifters 298 6.4.1 Distributed parallel plate capacitors 299 6.4.2 Bilateral interdigital phase shifters 301 6.4.3 Interdigital capacitor phase shifters 304 6.5 Applications 305 6.6 Conclusions 305 References 306 7 Micromachined transmission lines and components 309 7.1 Introduction 309 7.2 Micromachined transmission lines 310 7.2.1 Losses in transmission lines 311 7.2.2 Co-planar transmission lines 313 [...]... frequency (RF) MEMS The term RF MEMS refers to the design and fabrication of MEMS for RF integrated circuits It should not be interpreted as the traditional MEMS devices operating at RF frequencies MEMS devices in RF MEMS are used for actuation or adjustment of a separate RF device or component, such as variable capacitors, switches, and filters Traditional MEMS can be divided into two classes: MEMS actuators... (1998), the RF MEMS development to date can be classified into the following categories based on whether one takes an RF or MEMS view point: (1) RF extrinsic in which the MEMS structure is located outside the RF circuit and actuates or controls other devices in the RF circuit In this class, one would consider the example of a tunable microstrip transmission line and associated 2 MEMS AND RF MEMS phased... structural materials were expected for MEMS, Figure 1.6 Freeform three-dimensional microfabrication (additive) 16 MEMS AND RF MEMS and much progress has been made (Cohen et al., 1999; Ikuta and Hirowatari, 1993; Jiang, Sun and Zhang, 1999; Takagi and Nakajima, 1993; Taylor et al., 1994; Zhang, Jiang and Sun, 1999) On the other hand, most sensing and actuating structures used in MEMS are fabricated from thin... antenna, but also increases the bandwidth Many MEMS based microwave components are aimed at reducing insertion loss and increasing bandwidth This third aspect is valid for surface micromachined devices such as RF switches, tunable capacitors and micro inductors Conventional RF switching systems such as PIN diodes tend to be inefficient at higher frequencies MEMS based RF switches with very low actuation... MEMS phased shifters and arrays Microstrip lines are extensively used to interconnect high-speed circuits and components because they can be fabricated by easy automated techniques (2) RF intrinsic in which the MEMS structure is located inside the RF circuit and has both the actuation and RF- circuit function In this class, one could consider traditional cantilever and diaphragm type MEMS which can be... years, and we are increasingly leaning towards their applications in microwave and millimeter wave systems, and even in optical systems Apart from having the advantages of bulk production, and being miniaturized, these can often lead to more efficient systems compared with conventional ones The need for micromachining and MEMS based systems for RF and microwave applications arise from the inherent limitations... micromachining is 6 MEMS AND RF MEMS Concave corner Isotropic wet etching: agitation Convex corner SiO2 mask Isotropic wet etching: no agitation Top view Cantilever beam (a) Anisotropic wet etching: (100) surface (100) Surface orientation Masking film (111) Buried etchstop layer (100) Surface orientation (111) 54.74˚ Silicon Side view Silicon (c) (100) Surface orientation Anisotropic wet etching: (110) surface Dielectric... book and are greatly appreciated In particular we also wish to thank many of our colleagues and students, Taeksoo Ji, Yanan Sha, Roopa Tellakula, Hargsoon Yoon, and Bei Zhu, at the Center for Electronic and Acoustic Materials and Devices for their contributions in preparing the manuscript for this book We would like to thank Professors Vasundara V Varadan and Richard McNitt for their support and encouragement... moulding) 4 MEMS AND RF MEMS and ‘glue mechanism’ using the photoforming process to fabricate complicated structures by combining components, each of them made by its best fabrication process Batch processing of such hybrid silicon and polymer devices thus seems feasible The combined architecture may also result in sheets of smart skin with integrated sensors and actuators at the µm to mm scale For some applications. .. microelectromechanical systems (MEMS) in many current and future radio frequency (RF) , microwave and millimeter wave systems Such devices are termed RF MEMS, although it encompasses all miniaturized devices; whether they are operated micromechanically, or fabricated by micromachining, or both Fortunately enough, the processing techniques for MEMS systems have improved significantly over the years, and we are increasingly . RF MEMS and Their Applications RF MEMS and Their Applications Vijay K. Varadan K.J. Vinoy K.A. Jose. (RF) MEMS. The term RF MEMS refers to the design and fabrication of MEMS for RF integrated circuits. It should not be interpreted as the traditional MEMS