This paper presents a review of silicon micromachined accelerometers and gyroscopes. Following a brief introduction to their operating principles and specifications, various device structures, fabrication, technologies, device designs, packaging, and interface electronics issues, along with the present status in the commercialization of micromachined inertial sensors, are discussed. Inertial sensors have seen a steady improvement in their performance, and today, microaccelerometers can resolve accelerations in the micro-g range, while the performance of gyroscopes has improved by a factor of 10/spl times/ every two years during the past eight years. This impressive drive to higher performance, lower cost, greater functionality, higher levels of integration, and higher volume will continue as new fabrication, circuit, and packaging techniques are developed to meet the ever increasing demand for inertial sensors.

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Micromachined inertial sensors

The paper critically reviews the state of the art in the field of experimental techniques possible to be applied to the study of conductometric gas sensors based on semiconducting metal oxides. The used assessment criteria are subordinated to the proposed R&D approach, which focuses on the study, and subsequent modelling, of sensors’ performance in realistic operation conditions by means of a combination of phenomenological and spectroscopic techniques. With this viewpoint, the paper presents both the to-date achievements and shortcomings of different experimental techniques, describes – by using selected examples – how the proposed approach can be used and proposes a set of objectives for the near future.

Metal oxide-based gas sensor research: How to?

Part I: Background and Fundamentals Introduction, Mohamed Gad-el-Hak, University of Notre Dame Scaling of Micromechanical Devices, William Trimmer, Standard MEMS, Inc., and Robert H. Stroud, Aerospace Corporation Mechanical Properties of MEMS Materials, William N. Sharpe, Jr., Johns Hopkins University Flow Physics, Mohamed Gad-el-Hak, University of Notre Dame Integrated Simulation for MEMS: Coupling Flow-Structure-Thermal-Electrical Domains, Robert M. Kirby and George Em Karniadakis, Brown University, and Oleg Mikulchenko and Kartikeya Mayaram, Oregon State University Liquid Flows in Microchannels, Kendra V. Sharp and Ronald J. Adrian, University of Illinois at Urbana-Champaign, Juan G. Santiago and Joshua I. Molho, Stanford University Burnett Simulations of Flows in Microdevices, Ramesh K. Agarwal and Keon-Young Yun, Wichita State University Molecular-Based Microfluidic Simulation Models, Ali Beskok, Texas A&M University Lubrication in MEMS, Kenneth S. Breuer, Brown University Physics of Thin Liquid Films, Alexander Oron, Technion, Israel Bubble/Drop Transport in Microchannels, Hsueh-Chia Chang, University of Notre Dame Fundamentals of Control Theory, Bill Goodwine, University of Notre Dame Model-Based Flow Control for Distributed Architectures, Thomas R. Bewley, University of California, San Diego Soft Computing in Control, Mihir Sen and Bill Goodwine, University of Notre Dame Part II: Design and Fabrication Materials for Microelectromechanical Systems Christian A. Zorman and Mehran Mehregany, Case Western Reserve University MEMS Fabrication, Marc J. Madou, Nanogen, Inc. LIGA and Other Replication Techniques, Marc J. Madou, Nanogen, Inc. X-Ray-Based Fabrication, Todd Christenson, Sandia National Laboratories Electrochemical Fabrication (EFAB), Adam L. Cohen, MEMGen Corporation Fabrication and Characterization of Single-Crystal Silicon Carbide MEMS, Robert S. Okojie, NASA Glenn Research Center Deep Reactive Ion Etching for Bulk Micromachining of Silicon Carbide, Glenn M. Beheim, NASA Glenn Research Center Microfabricated Chemical Sensors for Aerospace Applications, Gary W. Hunter, NASA Glenn Research Center, Chung-Chiun Liu, Case Western Reserve University, and Darby B. Makel, Makel Engineering, Inc. Packaging of Harsh-Environment MEMS Devices, Liang-Yu Chen and Jih-Fen Lei, NASA Glenn Research Center Part III: Applications of MEMS Inertial Sensors, Paul L. Bergstrom, Michigan Technological University, and Gary G. Li, OMM, Inc. Micromachined Pressure Sensors, Jae-Sung Park, Chester Wilson, and Yogesh B. Gianchandani, University of Wisconsin-Madison Sensors and Actuators for Turbulent Flows. Lennart Loefdahl, Chalmers University of Technology, and Mohamed Gad-el-Hak, University of Notre Dame Surface-Micromachined Mechanisms, Andrew D. Oliver and David W. Plummer, Sandia National Laboratories Microrobotics Thorbjoern Ebefors and Goeran Stemme, Royal Institute of Technology, Sweden Microscale Vacuum Pumps, E. Phillip Muntz, University of Southern California, and Stephen E. Vargo, SiWave, Inc. Microdroplet Generators. Fan-Gang Tseng, National Tsing Hua University, Taiwan Micro Heat Pipes and Micro Heat Spreaders, G. P. "Bud" Peterson, Rensselaer Polytechnic Institute Microchannel Heat Sinks, Yitshak Zohar, Hong Kong University of Science and Technology Flow Control, Mohamed Gad-el-Hak, University of Notre Dame) Part IV: The Future Reactive Control for Skin-Friction Reduction, Haecheon Choi, Seoul National University Towards MEMS Autonomous Control of Free-Shear Flows, Ahmed Naguib, Michigan State University Fabrication Technologies for Nanoelectromechanical Systems, Gary H. Bernstein, Holly V. Goodson, and Gregory L. Snider, University of Notre Dame Index

The MEMS Handbook

If you've not been involved in MEMS (MicroElectroMechanical Systems) technology or had the cause to use MEMS devices, then you may wonder what all the fuss is about. What are MEMS anyway? What's the difference between MEMS and MST (MicroSystems Technology)? What are the advantages over existing technologies? If you have ever found yourself pondering over such questions, then this book may be for you. As the title suggests, the main aim is to provide an introduction to MEMS by describing the processes and materials available and by using examples of commercially available devices. The intended readership are those technical managers, engineers, scientists and graduate students who are keen to learn about MEMS but have little or no experience of the technology. I was particularly pleased to note that Maluf has dedicated a whole chapter to the important (and often difficult) area of packaging. The first three chapters provide a general overview of the technology. Within the first three pages we are introduced to the MEMS versus MST question, only to discover that the difference depends on where you live! The United States prefer MEMS, while the Europeans use the handle MST. (Note to self: tell colleagues in MEMS group at Southampton). A good account is given of the basic materials used in the technology, including silicon, silicon oxide/nitride/carbide, metals, polymers, quartz and gallium arsenide. The various processes involved in the creation of MEMS devices are also described. A good treatment is given to etching and bonding in addition to the various deposition techniques. It was interesting to note that the author doesn't make a big issue of the differences between bulk and surface micromachined devices; the approach seems to be `here's your toolbag - get on with it'. One of the great strengths of this book is the coverage of commercial MEMS structures. Arising as they have, from essentially a planar technology, MEMS devices are often elaborate three-dimensional creations, and 2D drawings don't do them much justice. I have to say that I was extremely impressed with the many aesthetic isometric views of some of these wonderful structures. Pressure sensors, inkjet print nozzles, mass flow sensors, accelerometers, valves and micromirrors are all given sufficient treatment to describe the fundamental behaviour and design philosophy, but without the mathematical rigour expected for a traditional journal paper. Chapter 5 addresses the promise of the technology as a means of enabling a new range of applications. The concept of using MEMS devices as key elements within complex systems (or even microsystems!) is explored. The so-called `lab-on-a-chip' approach is described, whereby complex analytical systems are integrated onto a single chip together with the associated micropumps and microvalves. The design and fabrication of MEMS devices are important issues by themselves. A key area, often overlooked, is that of packaging. Painstaking modelling and intricate fabrication methodologies can produce resonator structures oscillating at precisely, say, 125 kHz. The device is then mounted in a dual-in-line carrier and the frequency shifts by 10 kHz because of the additional internal stresses produced. Packaging issues can't be decoupled from those of the micromachined components. Many of these issues, such as protective coatings, thermal management, calibration etc, are covered briefly in the final chapter. Overall, I found this book informative and interesting. It has a broad appeal and gives a good insight into this fascinating and exciting subject area. Neil White

An Introduction to Microelectromechanical Systems Engineering

Surface micromachining is characterized by the fabrication of micromechanical structures from deposited thin films. Originally employed for integrated circuits, films composed of materials such as low-pressure chemical-vapor-deposition polycrystalline silicon, silicon nitride, and silicon dioxides can be sequentially deposited and selectively removed to build or "machine" three-dimensional structures whose functionality typically requires that they be freed from the planar substrate. Although the process to accomplish this fabrication dates from the 1960's, its rapid extension over the past few years and its application to batch fabrication of micromechanisms and of monolithic microelectromechanical systems (MEMS) make a thorough review of surface micromachining appropriate at this time. Four central issues of consequence to the MEMS technologist are: (i) the understanding and control of the material properties of microstructural films, such as polycrystalline silicon, (ii) the release of the microstructure, for example, by wet etching silicon dioxide sacrificial films, followed by its drying and surface passivation, (iii) the constraints defined by the combination of micromachining and integrated-circuit technologies when fabricating monolithic sensor devices, and (iv) the methods, materials, and practices used when packaging the completed device. Last, recent developments of hinged structures for postrelease assembly, high-aspect-ratio fabrication of molded parts from deposited thin films, and the advent of deep anisotropic silicon etching hold promise to extend markedly the capabilities of surface-micromachining technologies.

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Surface micromachining for microelectromechanical systems

A new generation of production-ready yaw rate sensor, based on silicon micromachining, is presented. The sensor is designed for mass production and high performance applications. A combination of surface and bulk micromachining leads to an advantage in design, signal evaluation and packaging. This paper discusses the design of the sensing element: two bulk micromachined oscillating masses each of which supports two surface micromachined accelerometers for detection of the Coriolis force. Mechanical balancing of the sensor is avoided by implementation of a new dry etching process and precise photolithography. The electrodynamic actuation and high Q-value of the oscillator allow packaging at atmospheric pressure. Characterization results of the device are presented.

A precision yaw rate sensor in silicon micromachining

A semiconductor plate having an epitaxial layer of a conductivity type opposite to that of the substrate on which it is formed has a depression, including one or more elongate channels. The depression is etched into a depth passing entirely through the epitaxial layer to isolate at least one tongue extending from a tongue pedestal into the etched depression and having parallel major sides which are perpendicular to the principal planes of the semiconductor plate. The tongue is under-etched so that it will be free to vibrate by motion in directions parallel to the principal planes of the plate. One of the major sides of the tongue faces a stationary electrode across a gap and the electrode and the tongue are insulated from each other, at least in one embodiment, by the fact that the etched depression extends all the way through the epitaxial layer in its depth. Deflection or vibration of the tongue changes the capacitance between the electrode and the tongue and contacts are provided for measuring the capacitance. Various embodiments utilizing multiple tongues and one or more electrodes are shown.

Acceleration and vibration sensor and method of making the same

From a silicon block or wafer a stationary frame is shaped and a seismic mass which is displaceable in rotation is mounted within the frame. The seismic mass is symmetrically suspended in the frame by two pairs of oppositely located flexible strips and either piezoresistive or capacitive detection of rotation is provided by the strips, the capacitive detection beeing provided with the help of parallel stationary electrodes connected to and insulated in the frame. In another embodiment an anchor stud in the center of the frame has two flexible interlaced spirals extending therefrom and their respective outer turns carry radial disposed masses with finger structures extending circumferentially in both directions. Stationary finger structures are provided to provide interfitting variable capacitors sensitive to rotary displacements.

Silicon-mass angular acceleration sensor

A car is skidding, and stabilizes itself without driver intervention; a laptop falls to the floor, and protects the hard drive by parking the read/write drive head automatically before impact; an airbag fires before the driver/occupant involved in an impending automotive crash impacts the steering wheel, thereby significantly reducing physical injury: - all systems involved are based exclusively on MEMS sensors. These crucial MEMS-sensor components of electronic control systems are making system reaction to human needs more intelligent, precise, and at much faster rates than humanly possible.

MEMS for automotive and consumer electronics

SUMMARY A new generation of production-ready yaw rate sensor, based on silicon micromachining, is presented. The sensor is designed for mass production and high performance applications. A combination of surface and bulk micromachining leads to an advantage in design, signal evaluation and packaging. This paper discusses the design of the sensing element: two bulk micromachined oscillating masses each of which supports two surface micromachined accelerometers for detection of the Coriolis force. Mechanical balancing of the sensor is avoided by implementation of a new dry etching process and precise photolithography. The electrodynamic actuation and high Q-value of the oscillator allow packaging at atmospheric pressure. Characterization results of the device are presented.

Jiri Marek

Papers

A precision yaw rate sensor in silicon micromachining

Acceleration and vibration sensor and method of making the same

Silicon-mass angular acceleration sensor

MEMS for automotive and consumer electronics

3B1.03 A Precision Yaw Rate Sensor in Silicon Micromachining