Foundations of MEMS

Chang Liu  
Total pages
March 2011
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For courses in Micro-Electro-Mechanical Systems (MEMS) taken by advanced undergraduate students, beginning graduate students, and professionals.

Foundations of MEMS is an entry-level text designed to systematically teach the specifics of MEMS to an interdisciplinary audience. Liu discusses designs, materials, and fabrication issues related to the MEMS field by employing concepts from both the electrical and mechanical engineering domains and by incorporating evolving microfabrication technology - all in a time-efficient and methodical manner. A wealth of examples and problems solidify students' understanding of abstract concepts and provide ample opportunities for practicing critical thinking.


  • Concise background information from several engineering domains:
    • Makes students conversant with unfamiliar concepts and practices that are needed to solve MEMS problems.
    • Presents exciting new opportunities for a student and practitioner of MEMS to become involved in specific application domains, such as bioengineering, chemistry, nanotechnology, optical engineering, power and energy, and wireless communication.
  • Systematic teaching of materials, design, and fabrication issues, in an ascending and widening spiral introduces topics in an ordered and logical progression.
  • Critical-thinking challenges foster a deeper understanding of the subject matter and show students how to think like engineers.
  • Extensive examples and homework problems help teachers explain difficult concepts and assist students in practicing these concepts.
  • Current data and up-to-date materials keep students and researchers abreast of the latest technologies.

New to this Edition

  • New content, concepts, and insight. The MEMS field has changed dramatically in the past five years. This book captures new contents (generated in academia and industry), new concepts (e.g., packaging and integration), and insights. This should provide more value for the reader.
  • New homework problems. New homework problems have been added to facilitate teaching and student learning. Homework solutions can be provided to teachers upon request.
  • Added analytical examples for design and process selection. This new edition provides teachers with new materials to discuss design and process analytically.¿
  • New beginner-friendly materials for teaching processes. Beginning students may be amazed by the array of processing related information. A number of new tables are provided to make it easier for students to climb the learning curve. These tables (in the appendix section) provide first time students a simplified summary of the most commonly encountered materials and etching methods. An easy-to-understand table summarizing their interactions is also provided.
  • Deeper case studies added to challenge the readers understanding about the subject. The overall structural of the book is maintained. A new chapter (Chapter 15) is added, dealing with in-depth case discussion of successful MEMS products in the market place. The author believes these commercialized MEMS devices, conceived for and tested in the real-life business world, are good examples to illustrate principles of design, fabrication, and integration. A discussion of most essential fabrication technology is added in Chapter 2. The discussion is meant to provide essential and qualitative review of processing methods. Other changes can be found in various chapters, especially Chapters 1, 2, 12, 15, and 16.
  • A new dedicated companion website for instructors and students The website is a permanent home to the book and will serve the readers of this book in the new era of internet and on-line communication. On this website, a reader can find supplemental chapters, supplemental teaching materials, links to resources pertaining to the MEMS field, and errata. Instructors will find teaching aid materials such as PowerPoint files, figures, homework solutions, etc. The website serves a number of important purposes. It is originally driven by the desire to not make this book too large while still maintain its ability to satisfy a varied audience.¿ It will help the user community in a way that is more enduring than a single print.
  • Chapter line-up and flow is streamlined. The chapter on optical MEMS is now moved to the website as a supplement. This and other chapters dealing with specialty topics (such as RF MEMS, BioMEMS) will be hosted in the website so that the book can be kept small and still satisfy the needs of teachers who wish to discuss about these exciting areas in class. Moving the chapters to the website also makes it possible to update frequently. ¿

Table of Contents

Preface to Second Edition
Preface to First Edition
Note to Instructors
About the Author
Notational Conventions

Chapter 1: Introduction
1.0. Preview    
1.1.  The History of MEMS Development    
1.1.1. From the Beginning to 1990    
1.1.2. From 1990 to 2001    
1.1.3. 2002 to present    
1.1.4. Future Trends    
1.2. The Intrinsic Characteristics of MEMS    
1.2.1. Miniaturization    
1.2.2. Microelectronics Integration    
1.2.3. Parallel Fabrication with Precision    
1.3. Devices: Sensors and Actuators         
1.3.1. Energy Domains and Transducers         
1.3.2. Sensors Considerations         
13.3.  Sensor Noise and Design Complexity         
1.3.4. Actuators Considerations         

Chapter 2: First-Pass Introduction to Microfabrication           
2.0. Preview         
2.1. Overview of Microfabrication         
2.2. Essential Overview of Frequently Used Microfabrication Processes    
2.2.1. Photolithography         
2.2.2. Thin film deposition         
2.2.3. Thermal oxidation of silicon         
2.2.4. Wet Etching         
2.2.5. Silicon anisotropic etching         
2.2.6. Plasma etching and reactive ion etching         
2.2.7. Doping         
2.2.8. Wafer dicing         
2.2.9. Wafer bonding         
2.3. The Microelectronics Fabrication Process Flow         
2.4. Silicon-based MEMS Processes         
2.5. Packaging and Integration         
2.5.1. Integration Options         
2.5.2. Encapsulation         
2.6. New Materials and Fabrication Processes         
2.7. Process Selection and Design         
2.7.1. Points of Consideration for Deposition Processes    
2.7.2. Points of Consideration for Etching Processes         
2.7.3. Ideal Rules for Building a Process Flow         
2.7.4. Rules for Building a Robust Process         

Chapter 3: Review of Essential Electrical and Mechanical Concepts         
3.0 Preview         
3.1. Conductivity of Semiconductors         
3.1.1. Semiconductor Materials         
3.1.2. Calculation of Charge Carrier Concentration         
3.1.3. Conductivity and Resistivity         
3.2. Crystal Planes and Orientations         
3.3. Stress and Strain         
3.3.1. Internal Force Analysis: Newton's Laws of Motion         
3.3.2. Definitions of Stress and Strain         
3.3.3. General Scalar Relation between Tensile Stress and Strain         
3.3.4. Mechanical Properties of Silicon and Related Thin Films         
3.3.5. General Stress - Strain Relations         
3.4. Flexural Beam Bending Analysis under Simple Loading Conditions         
3.4.1. Types of Beams         
3.4.2. Longitudinal Strain under Pure Bending         
3.4.3. Deflection of Beams         
3.4.4. Finding the Spring Constants         
3.5. Torsional Deflections         
3.6. Intrinsic Stress         
3.7. Dynamic System, Resonant Frequency, and Quality Factor         
3.7.1. Dynamic System and Governing Equation         
3.7.2. Response under Sinusoidal Resonant Input         
3.7.3. Damping and Quality Factor         
3.7.4. Resonant Frequency and Bandwidth         
3.8. Active Tuning of Spring Constant and Resonant Frequency         
3.9. A List of Suggested Courses and Books         
Chapter 4: Electrostatic Sensing and Actuation
Section 4.0. Preview         
Section 4.1.  Introduction to Electrostatic Sensors and Actuators         
Section 4.2. Parallel Plate Capacitor         
4.2.1. Capacitance of Parallel Plates         
4.2.2.  Equilibrium Position of Electrostatic Actuator under Bias         
4.2.3. Pull-in Effect of Parallel-Plate Actuators         
Section 4.3. Applications of Parallel-Plate Capacitors         
4.3.1. Inertia Sensor         
4.3.2. Pressure Sensor         
4.3.3. Flow Sensor         
4.3.4. Tactile sensor         
4.3.5. Parallel-plate actuators         
Section 4.4. Interdigitated Finger Capacitors         
Section 4.5. Applications of Comb-Drive Devices         
4.5.1. Inertia Sensors         
4.5.2. Actuators         

Chapter 5: Thermal Sensing and Actuation
5.0.     Preview         
5.1. Introduction         
5.1.1. Thermal Sensors         
5.1.2. Thermal Actuators         
5.1.3. Fundamentals of Thermal Transfer         
5.2. Sensors and Actuators Based on Thermal Expansion
5.2.1. Thermal Bimorph Principle         
5.2.2. Thermal Actuators with a Single Material         
5.3. Thermal Couples         
5.4. Thermal Resistors         
5.5. Applications         
5.5.1. Inertia Sensors         
5.5.2. Flow Sensors         
5.5.3. Infrared Sensors         
5.5.4. Other Sensors         

Chapter 6:  Piezoresistive Sensors         
6.0.     Preview         
6.1.     Origin and Expression of Piezoresistivity         
6.2.     Piezoresistive Sensor Materials         
6.2.1. Metal Strain Gauges         
6.2.2.     Single Crystal Silicon         
6.2.3. Polycrystalline Silicon         
6.3. Stress Analysis of Mechanical Elements         
6.3.1. Stress in Flexural Cantilevers         
6.3.2. Stress and Deformation in Membrane         
6.4. Applications of Piezoresistive Sensors         
6.4.1. Inertial Sensors         
6.4.2. Pressure Sensors         
6.4.3. Tactile sensor         
6.4.4. Flow sensor         

Chapter 7: Piezoelectric Sensing and Actuation    
7.0. Preview    
7.1. Introduction    
7.1.1. Background   
7.1.2. Mathematical description of piezoelectric effects    
7.1.3. Cantilever piezoelectric actuator model    
7.2. Properties of Piezoelectric Materials    
7.2.1. Quartz    
7.2.2. PZT    
7.2.3. PVDF    
7.2.4. ZnO    
7.2.5. Other Materials    
7.3. Applications    
7.3.1. Inertia Sensors    
7.3.2. Acoustic Sensors    
7.3.3. Tactile Sensors    
7.3.4. Flow Sensors    
7.3.5. Surface Elastic Waves    

Chapter 8: Magnetic Actuation    
8.0. Preview    
8.1. Essential Concepts and Principles    
8.1.1. Magnetization and Nomenclatures    
8.1.3. Selected Principles of Micro Magnetic Actuators    
8.2 Fabrication of Micro Magnetic Components    
8.2.1. Deposition of Magnetic Materials    
8.2.2. Design and Fabrication of Magnetic Coil    
8.3. Case Studies of MEMS Magnetic Actuators   

Chapter 9: Summary of Sensing and Actuation Methods
9.0. Preview    
9.1. Comparison of Major Sensing and Actuation Methods   
9.2. Other Sensing and Actuation Methods   
9.2.1. Tunneling Sensing    
9.2.3 Optical Sensing    
9.2.4. Field Effect Transistors    
9.2.5. Radio Frequency Resonance Sensing    

Chapter 10: Bulk Micromachining and Silicon Anisotropic Etching    
10.0.      Preview    
10.1.     Introduction    
10.2.     Anisotropic Wet Etching    
10.2.1. Introduction    
10.2.2. Rules of Anisotropic Etching-Simplest Case  
10.2.3. Rules of Anisotropic Etching-Complex Structures    
10.2.4. Forming Protrusions  
10.2.5. Interaction of Etching Profiles from Isolated Patterns    
10.2.6. Summary of design methodology   
10.2.7. Chemicals for Wet Anisotropic Etching    
10.3. Dry Etching and Deep Reactive Ion Etching    
10.4. Isotropic Wet Etching  
10.5. Gas Phase Etchants    
10.6. Native Oxide    
10.7. Special Wafers and Techniques    

Chapter 11: Surface Micromachining    
11.0. Preview    
11.1. Basic Surface Micromachining Processes    
11.1.1.     Sacrificial Etching Process    
11.1.2. Micro Motor Fabrication Process-A First Pass    
11.2.3. Micro Motor Fabrication Process-A Second Pass    
11.1.4. Micro Motor Fabrication Process-Third Pass    
11.2. Structural and Sacrificial Materials    
11.2.1. Material Selection Criteria for a Two-layer Process   
11.2.2. Thin Films by Low Pressure Chemical Vapor Deposition    
11.2.3. Other Surface Micromachining Materials and Processes    
11.3. Acceleration of Sacrificial Etch    
11.4. Stiction and Anti-stiction Methods    

Chapter 12: Process Synthesis: Putting It all Together    
12.0.     Preview    
12.1. Process for Suspension Beams    
12.2. Process for Membranes    
12.3. Process for Cantilevers    
12.3.1. SPM Technologies Case Motivation    
12.3.2. General Fabrication Methods for Tips   
12.3.3. Cantilevers with Integrated Tips    
12.3.4. Cantilevers with Integrated Sensors    
12.3.5. SPM Probes with Actuators    
12.4. Practical Factors Affecting Yield of MEMS    

Chapter 13: Polymer MEMS    
13.0. Preview    
13.1. Introduction    
13.2. Polymers in MEMS    
13.2.1. Polyimide    
13.2.2. SU-8    
13.2.3. Liquid Crystal Polymer (LCP)    
13.2.4. PDMS    
13.2.5. PMMA   
13.2.6. Parylene  
13.2.7. Fluorocarbon   
13.2.8. Other Polymers    
13.3. Representative Applications    
13.3.1. Acceleration Sensors    
13.3.2. Pressure Sensors    
13.3.3. Flow sensors    
13.3.4. Tactile Sensors    

Chapter 14: Micro Fluidics Applications   
14.0. Preview    
14.1. Motivation for Microfluidics    
14.2. Essential Biology Concepts    
14.3. Basic Fluid Mechanics Concepts   
14.3.1. The Reynolds Number and Viscosity    
14.3.2. Methods for Fluid Movement in Channels    
14.3.3. Pressure Driven Flow    
14.3.4. Electrokinetic Flow    
14.3.5. Electrophoresis and Dielectrophoresis    
14.4. Design and Fabrication of Selective Components    
14.4.1. Channels    
14.4.2. Valves    

Chapter 15: Case Studies of Selected MEMS Products    
15.0. Preview    
15.1. Case Studies: Blood Pressure (BP) Sensor   
15.1.1. Background and History   
15.1.2. Device Design Considerations    
15.1.3. Commercial Case: NovaSensor BP Sensor    
15.2. Case Studies: Microphone    
15.2.1. Background and History    
15.2.2. Design Considerations    
15.2.3. Commercial Case: Knowles Microphone    
15.3. Case Studies: Acceleration Sensors    
15.3.1. Background and History    
15.4.2. Design Considerations    
15.4.1. Commercial Case: Analog Devices and MEMSIC   
15.4. Case Studies: Gyros    
15.4.1. Background and History    
15.4.2. The Coriolis Force  
15.4.3. MEMS Gyro Design    
15.4.4. Single Axis Gyro Dynamics   
15.4.4. Commercial Case: InvenSense Gyro    
15.5 Summary of Top Concerns for MEMS Product Development    
15.5.1. Performance and Accuracy    
15.5.2. Repeatability and Reliability    
15.5.3. Managing the Cost of MEMS Products    
15.5.4. Market Uncertainties, Investment, and Competition    

Appendix 1: Characteristics of selected MEMS material
Appendix 2: Frequently Used Formula for Beams, Cantilevers, and Plates
Appendix 3: Basic Tools for Dealing with a Mechanical Second-order Dynamic System
Appendix 4: Most Commonly Encountered Materials
Appendix 5: Most Commonly Encountered Material Removal Process Steps
Appendix 6: A List of General Compatibility between General Materials and Processes
Appendix 7: Comparison of Commercial Inertial Sensors
Answers to selected problems


Chang Liu received his M.S. and Ph.D. degrees from the California Institute of Technology in 1991 and 1995, respectively. His Ph.D. thesis was titled Micromachined sensors and actuators for fluid mechanics applications. In January 1996, he joined the Microelectronics Laboratory of the University of Illinois as a postdoctoral researcher. In January 1997, he became an assistant professor with major appointment in the Electrical and Computer Engineering Department and joint appointment in the Mechanical and Industrial Engineering Department. In 2003, he was promoted to the rank of Associate Professor with tenure. In 2007, Chang Liu joined Northwestern University (Evanston, Illinois) as a full professor of engineering. He established the MedX Laboratory to conduct advanced engineering research for medicine and health care.

Dr. Liu has 20 years of research experience in the MEMS area and has published 200 technical papers in journals and refereed conference proceedings. He teaches undergraduate and graduate courses covering broad-ranging topics, including MEMS, solid-state electronics, electromechanics, sensor technology, circuits, dynamics, and heat transfer. He won a campus "Incomplete list of teachers ranked as excellent" honor in 2001 for developing and teaching the MEMS class, a precursor to this book. He received the National Science Foundation's CAREER award in 1998 for his research proposal of developing artificial haircells using MEMS technology. He is currently a Subject Editor of the IEEE/ASME Journal of MEMS, and was an Associate Editor of the IEEE Sensors Journal. His work has been cited in popular media. Dr. Liu is a cofounder of Integrated Micro Devices (IMD) Corporation and a member of the scientific advisory board of NanoInk Corporation (Chicago, IL). In 2004, he won the University of Illinois College of Engineering Xerox Award for Faculty Research. In the same year, he was elected a Faculty Associate at the Center for Advanced Studies at the University of Illinois, to pursue research in large-format integrated sensors. He is a Fellow of the IEEE, the world's largest professional association for the advancement of technology.