MEMS Mechanical Sensors For a listing of recent titles in the Artech House Microelectromechanical Systems (MEMS) Series, turn to the back of this book. MEMS Mechanical Sensors Stephen Beeby Graham Ensell Michael Kraft Neil White Artech House, Inc. Boston • London www.artechhouse.com Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the U.S. Library of Congress. British Library Cataloguing in Publication Data Beeby, Stephen. MEMS mechanical sensors.— (Artech House MEMS library) 1. Microelectricalmechanical systems—Design and construction 2. Transducers I. Beeby, Stephen 621.3’81 ISBN 1-58053-536-4 Cover design by Igor Valdman © 2004 ARTECH HOUSE, INC. 685 Canton Street Norwood, MA 02062 All rights reserved. Printed and bound in the United States of America. 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International Standard Book Number: 1-58053-536-4 10987654321 Contents Preface ix CHAPTER 1 Introduction 1 1.1 Motivation for the Book 1 1.2 What Are MEMS? 2 1.3 Mechanical Transducers 3 1.4 Why Silicon? 4 1.5 For Whom Is This Book Intended? 5 References 5 CHAPTER 2 Materials and Fabrication Techniques 7 2.1 Introduction 7 2.2 Materials 7 2.2.1 Substrates 7 2.2.2 Additive Materials 11 2.3 Fabrication Techniques 11 2.3.1 Deposition 12 2.3.2 Lithography 17 2.3.3 Etching 21 2.3.4 Surface Micromachining 28 2.3.5 Wafer Bonding 29 2.3.6 Thick-Film Screen Printing 32 2.3.7 Electroplating 33 2.3.8 LIGA 34 2.3.9 Porous Silicon 35 2.3.10 Electrochemical Etch Stop 35 2.3.11 Focused Ion Beam Etching and Deposition 36 References 36 CHAPTER 3 MEMS Simulation and Design Tools 39 3.1 Introduction 39 3.2 Simulation and Design Tools 40 3.2.1 Behavioral Modeling Simulation Tools 40 3.2.2 Finite Element Simulation Tools 43 References 56 v CHAPTER 4 Mechanical Sensor Packaging 57 4.1 Introduction 57 4.2 Standard IC Packages 58 4.2.1 Ceramic Packages 58 4.2.2 Plastic Packages 59 4.2.3 Metal Packages 59 4.3 Packaging Processes 59 4.3.1 Electrical Interconnects 60 4.3.2 Methods of Die Attachment 63 4.3.3 Sealing Techniques 65 4.4 MEMS Mechanical Sensor Packaging 66 4.4.1 Protection of the Sensor from Environmental Effects 67 4.4.2 Protecting the Environment from the Sensor 71 4.4.3 Mechanical Isolation of Sensor Chips 71 4.5 Conclusions 80 References 81 CHAPTER 5 Mechanical Transduction Techniques 85 5.1 Piezoresistivity 85 5.2 Piezoelectricity 89 5.3 Capacitive Techniques 92 5.4 Optical Techniques 94 5.4.1 Intensity 94 5.4.2 Phase 95 5.4.3 Wavelength 96 5.4.4 Spatial Position 96 5.4.5 Frequency 96 5.4.6 Polarization 97 5.5 Resonant Techniques 97 5.5.1 Vibration Excitation and Detection Mechanisms 98 5.5.2 Resonator Design Characteristics 99 5.6 Actuation Techniques 104 5.6.1 Electrostatic 104 5.6.2 Piezoelectric 107 5.6.3 Thermal 107 5.6.4 Magnetic 109 5.7 Smart Sensors 109 References 112 CHAPTER 6 Pressure Sensors 113 6.1 Introduction 113 6.2 Physics of Pressure Sensing 114 6.2.1 Pressure Sensor Specifications 117 6.2.2 Dynamic Pressure Sensing 120 vi Contents 6.2.3 Pressure Sensor Types 121 6.3 Traditional Pressure Sensors 121 6.3.1 Manometer 121 6.3.2 Aneroid Barometers 122 6.3.3 Bourdon Tube 122 6.3.4 Vacuum Sensors 123 6.4 Diaphragm-Based Pressure Sensors 123 6.4.1 Analysis of Small Deflection Diaphragm 125 6.4.2 Medium Deflection Diaphragm Analysis 127 6.4.3 Membrane Analysis 127 6.4.4 Bossed Diaphragm Analysis 128 6.4.5 Corrugated Diaphragms 129 6.4.6 Traditional Diaphragm Transduction Mechanisms 129 6.5 MEMS Technology Pressure Sensors 130 6.5.1 Micromachined Silicon Diaphragms 130 6.5.2 Piezoresistive Pressure Sensors 132 6.5.3 Capacitive Pressure Sensors 137 6.5.4 Resonant Pressure Sensors 139 6.5.5 Other MEMS Pressure Sensing Techniques 142 6.6 Microphones 143 6.7 Conclusions 145 References 145 CHAPTER 7 Force and Torque Sensors 153 7.1 Introduction 153 7.2 Silicon-Based Devices 154 7.3 Resonant and SAW Devices 157 7.4 Optical Devices 159 7.5 Capacitive Devices 160 7.6 Magnetic Devices 162 7.7 Atomic Force Microscope and Scanning Probes 164 7.8 Tactile Sensors 166 7.9 Future Devices 168 References 168 CHAPTER 8 Inertial Sensors 173 8.1 Introduction 173 8.2 Micromachined Accelerometer 175 8.2.1 Principle of Operation 175 8.2.2 Research Prototype Micromachined Accelerometers 180 8.2.3 Commercial Micromachined Accelerometer 192 8.3 Micromachined Gyroscopes 195 8.3.1 Principle of Operation 195 8.3.2 Research Prototypes 199 8.3.3 Commercial Micromachined Gyroscopes 204 Contents vii 8.4 Future Inertial Micromachined Sensors 206 References 207 CHAPTER 9 Flow Sensors 213 9.1 Introduction to Microfluidics and Applications for Micro Flow Sensors 214 9.2 Thermal Flow Sensors 217 9.2.1 Research Devices 219 9.2.2 Commercial Devices 225 9.3 Pressure Difference Flow Sensors 229 9.4 Force Transfer Flow Sensors 232 9.4.1 Drag Force 232 9.4.2 Lift Force 235 9.4.3 Coriolis Force 236 9.4.4 Static Turbine Flow Meter 238 9.5 Nonthermal Time of Flight Flow Sensors 239 9.5.1 Electrohydrodynamic 239 9.5.2 Electrochemical 240 9.6 Flow Sensor Based on the Faraday Principle 241 9.7 Flow Sensor Based on the Periodic Flapping Motion 242 9.8 Flow Imaging 243 9.9 Optical Flow Measurement 245 9.9.1 Fluid Velocity Measurement 245 9.9.2 Particle Detection and Counting 246 9.9.3 Multiphase Flow Detection 246 9.10 Turbulent Flow Studies 247 9.11 Conclusion 248 References 250 About the Authors 257 Index 259 viii Contents Preface The field of microelectromechanical systems (MEMS), particularly micromachined mechanical transducers, has been expanding over recent years, and the production costs of these devices continue to fall. Using materials, fabrication processes, and design tools originally developed for the microelectronic circuits industry, new types of microengineered device are evolving all the time—many offering numerous advantages over their traditional counterparts. The electrical properties of silicon have been well understood for many years, but it is the mechanical properties that have been exploited in many examples of MEMS. This book may seem slightly unusual in that it has four editors. However, since we all work together in this field within the School of Electronics and Computer Science at the University of South - ampton, it seemed natural to work together on a project like this. MEMS are now appearing as part of the syllabus for both undergraduate and postgraduate courses at many universities, and we hope that this book will complement the teaching that is taking place in this area. The prime objective of this book is to give an overview of MEMS mechanical transducers. In order to achieve this, we provide some background information on the various fabrication techniques and materials that can be used to make such devices. The costs associated with the fabrication of MEMS can be very expensive, and it is therefore essential to ensure a successful outcome from any specific produc- tion or development run. Of course, this cannot be guaranteed, but through the use of appropriate design tools and commercial simulation packages, the chances of failure can be minimized. Packaging is an area that is sometimes overlooked in text - books on MEMS, and we therefore chose to provide coverage of some of the meth - ods used to provide the interface between the device and the outside world. The book also provides a background to some of the basic principles associated with micromachined mechanical transducers. The majority of the text, however, is dedi - cated to specific examples of commercial and research devices, in addition to dis - cussing future possibilities. Chapter 1 provides an introduction to MEMS and defines some of the com - monly used terms. It also discusses why silicon has become one of the key materials for use in miniature mechanical transducers. Chapter 2 commences with a brief dis - cussion of silicon and other materials that are commonly used in MEMS. It then goes on to describe many of the fabrication techniques and processes that are employed to realize microengineered devices. Chapter 3 reviews some of the com - mercial design tools and simulation packages that are widely used by us and other researchers/designers in this field. Please note that it is not our intention to provide critical review here, but merely to indicate the various features and functionality ix [...]... 1.3 Mechanical Transducers The market for micromachined mechanical transducers has, in the past, had the largest slice of the pie of the overall MEMS market This is likely to be the case in the immediate future as well The main emphasis of this text is on mechanical sensors, including pressure, force, acceleration, torque, inertial, and flow sensors Various types of actuation mechanism, relevant to MEMS, ... pressure sensors (Chapter 6), force and torque sensors (Chapter 7), inertial sensors (Chapter 8), and flow sensors (Chapter 9) These devices use many of the principles and techniques described in the earlier stages of the book Acknowledgments We authors express our thanks to all the contributing authors of this book They are all either present or former colleagues with whom we have worked on a variety of MEMS. .. United States, Asia, Europe, and Canada); SPIE hold many symposia on MEMS at worldwide locations In addition, there are several journals that cover the field of microsensors and sensor technologies, including: • • • • • • • • • • • Sensors and Actuators (A-Physical, B-Chemical); IEEE/ASME Journal of Microelectromechanical Systems (JMEMS); Journal of Micromechanics and Microengineering; Measurement,... These size limits turned out to be slightly too large and the motor was actually made using conventional mechanical engineering methods that did not require any new technological developments 1.2 What Are MEMS? MEMS means different things to different people The acronym MEMS stands for microelectromechanical systems and was coined in the United States in the late 1980s Around the same time the Europeans... micromachined mechanical sensors It also discusses ways to minimize unwanted interactions between the device and its packaging Chapter 5 presents some of the fundamental principles of mechanical transduction This chapter is largely intended for readers who might not have a background in mechanical engineering The remaining four chapters of the book are dedicated to describing specific mechanical microengineered... millennium, the number of microsensors evident in everyday life continues to increase From automotive manifold pressure and air bag sensors to biomedical analysis, the range and variety are vast It is interesting to note that pressure sensors and ink-jet nozzles currently account for more than two-thirds of the overall microtransducer market share Future predications indicate that the mechanical microsensor... studied for many years and are well understood and thoroughly documented Silicon also possesses many desirable mechanical properties that make it an excellent choice for many types of mechanical sensor Today there are many companies working in the field of microelectromechanical systems (MEMS) A quick search on the Internet in July 2003 revealed several hundred in the United States, Europe, and the... responds to the electrical signals generated within the circuit Both the sensor and the actuator could be MEMS devices in their own right For the purpose of this book, MEMS is an appropriate term as it specifically relates to mechanical (micro) devices and also includes wider areas such as chemical sensors, microoptical systems, and microanalysis systems There is also a wide variety of usage of terms... Transducers—International Conference on Solid-State Sensors and Actuators (held biennially and rotating location between Asia, North America, and Europe); Eurosensors (held annually in Europe); IEEE Sensors Conference (first held in 2002, annually United States and Canada); Micro Mechanics Europe—MME (held annually in Europe); 1 2 Introduction • • • • • IEEE International MEMS Conference (rotates annually between... of actuation mechanism, relevant to MEMS, will also be addressed together with examples of the fundamental techniques used for mechanical sensors The main methods of sensing mechanical measurands have been around for many years and are therefore directly applicable to microsensors There is, however, a significant effect that must be accounted for when considering mesoscale devices (i.e., those that . MEMS Mechanical Sensors For a listing of recent titles in the Artech House Microelectromechanical Systems (MEMS) Series, turn to. Cataloguing in Publication Data Beeby, Stephen. MEMS mechanical sensors. — (Artech House MEMS library) 1. Microelectricalmechanical systems—Design and construction