Microelectromechanical Systems

Advanced Materials and Fabrication Methods

Committee on Advanced Materials and Fabrication Methods for Microelectromechanical Systems

National Materials Advisory Board

Commission on Engineering and Technical Systems

National Research Council

NMAB-483
NATIONAL ACADEMY PRESS
Washington, D.C.
1997



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--> Microelectromechanical Systems Advanced Materials and Fabrication Methods Committee on Advanced Materials and Fabrication Methods for Microelectromechanical Systems National Materials Advisory Board Commission on Engineering and Technical Systems National Research Council NMAB-483 NATIONAL ACADEMY PRESS Washington, D.C. 1997

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--> NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an advisor to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William Wulf are chairman and vice chairman, respectively, of the National Research Council. This study by the National Materials Advisory Board was conducted under Contract No. MDA972- 92-C-0028 with the Department of Defense and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the organizations or agencies that provided support for the project. Library of Congress Catalog Card Number 97-80865 International Standard Book Number 0-309-05980-1 Available in limited supply from: National Materials Advisory Board 2101 Constitution Avenue, N.W. Washington, DC 20418 202-334-3505 nmab@nas.edu Additional copies are available for sale from: National Academy Press Box 285 2101 Constitution Avenue, N.W. Washington, DC 20055 800-624-6242 202-334-3313 (in the Washington metropolitan area) http://www.nap.edu Copyright 1997 by the National Academy of Sciences. All rights reserved. Printed in the United States of America. Cover: Rotating grating on a 200 µm diameter gear that allows 180 degrees of positioning. The grating is 185 µm x 200 µm with 2 µm wide lines and spaces. The device has the potential to be used as a beam splitter or as a diffractive element in a microspectrometer. The system was designed by Major John Comtois and Professor Victor Bright, U.S. Air Force, and fabricated by the DARPA-sponsored MCNC MUMPs program. Courtesy of J.H. Comtois and V.M. Bright, U.S. Air Force.

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--> COMMITTEE ON ADVANCED MATERIALS AND FABRICATION METHODS FOR MICROELECTROMECHANICAL SYSTEMS RICHARD S. MULLER (chair), University of California, Berkeley MICHAEL ALBIN, The Perkin-Elmer Corporation, Foster City, California PHILLIP W. BARTH, Hewlett-Packard Laboratories, Palo Alto, California SELDEN B. CRARY, University of Michigan, Ann Arbor DENICE D. DENTON, University of Washington, Seattle KAREN W. MARKUS, MEMS Technology Applications Center at MCNC, Research Triangle Park, North Carolina PAUL J. MCWHORTER, Sandia National Laboratories, Albuquerque, New Mexico ROBERT E. NEWNHAM, Pennsylvania State University, University Park RICHARD S. PAYNE, Analog Devices, Inc., Cambridge, Massachusetts National Materials Advisory Board Staff ROBERT M. EHRENREICH, Senior Program Officer PAT WILLIAMS, Senior Project Assistant CHARLES HACH, Research Associate JOHN A. HUGHES, Research Associate BONNIE A. SCARBOROUGH, Research Associate Technical Consultants GEORGE M. DOUGHERTY, U.S. Air Force, Wright Patterson Air Force Base, Ohio JASON HOCH, MEMS Technology Applications Center at MCNC, Research Triangle Park, North Carolina HOWARD LAST, Naval Surface Warfare Center, Silver Spring, Maryland NOEL C. MACDONALD, Defense Advanced Research Projects Agency, Arlington, Virginia Liaison Representatives KEN GABRIEL, Defense Advanced Research Projects Agency, Arlington, Virginia CARL A. KUKKONEN, Jet Propulsion Laboratory, Pasadena, California WILLIAM T. MESSICK, Naval Surface Warfare Center, Silver Spring, Maryland DAVID J. NAGEL, Naval Research Laboratory, Washington, D.C. JOHN PRATER, Army Research Office, Research Triangle Park, North Carolina RICHARD WLEZIEN, NASA Langley Research Center, Hampton, Virginia National Materials Advisory Board Liaison LIONEL C. KIMERLING, Massachusetts Institute of Technology, Cambridge

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--> NATIONAL MATERIALS ADVISORY BOARD ROBERT A. LAUDISE (chair), Lucent Technologies, Inc., Murray Hill, New Jersey REZA ABBASCHIAN, University of Florida, Gainesville JAN D. ACHENBACH, Northwestern University, Evanston, Illinois MICHAEL I. BASKES, Sandia-Livermore National Laboratory, Livermore, California JESSE (JACK) BEAUCHAMP, California Institute of Technology, Pasadena FRANCIS DISALVO, Cornell University, Ithaca, New York EDWARD C. DOWLING, Cyprus AMAX Minerals Company, Englewood, Colorado ANTHONY G. EVANS, Harvard University, Cambridge, Massachusetts JOHN A.S. GREEN, The Aluminum Association, Inc., Washington, D.C. JOHN H. HOPPS, JR., Morehouse College, Atlanta, Georgia MICHAEL JAFFEE, Hoechst Celanese Research Division, Summit, New Jersey SYLVIA M. JOHNSON, SRI International, Menlo Park, California LIONEL C. KIMERLING, Massachusetts Institute of Technology, Cambridge HARRY LIPSITT, Wright State University, Yellow Springs, Ohio RICHARD S. MULLER, University of California, Berkeley ELSA REICHMANIS, Lucent Technologies, Inc., Murray Hill, New Jersey KENNETH L. REIFSNIDER, Virginia Polytechnic Institute and State University, Blacksburg EDGAR A. STARKE, University of Virginia, Charlottesville KATHLEEN C. TAYLOR, General Motors Corporation, Warren, Michigan JAMES WAGNER, Johns Hopkins University, Baltimore, Maryland JOSEPH WIRTH, Raychem Corporation, Menlo Park, California BILL G.W. YEE, Pratt & Whitney, West Palm Beach, Florida ROBERT E. SCHAFRIK, Director

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--> Acknowledgments The Committee on Advanced Materials and Fabrication Methods for Microelectromechanical Systems gratefully acknowledges the information provided to the committee by the following individuals: Rolfe Anderson, Affymetrix; Ian Getreu, Analogy, Inc.; Joseph Giachino, Ford Motor Company; Michael Hecht, Jet Propulsion Laboratory; Larry Hornbeck, Texas Instruments, Inc.; William Kaiser, University of California-Los Angeles; Gregory T.A. Kovacs, Stanford University; Dennis Polla, University of Minnesota; Calvin F. Quate, Stanford University; Yu-Chang Tai, California Institute of Technology; George M. Whitesides, Harvard University; and Mark Zdeblick, Redwood Microsystems. We thank George Dougherty, Jason Hoch, and Howard Last for their excellent contributions as technical consultants. Sincere appreciation is also expressed to the staff of the National Materials Advisory Board for its unswerving support. Robert M. Ehrenreich, senior program officer, showed unfailing patience and dedicated much time and energy to bringing the report into being. Pat Williams very effectively handled many issues as the senior project assistant. The three research associates who worked on the report, Jack Hughes, Charles Hach, and Bonnie Scarborough, also made important contributions to its completion. The committee chair especially thanks the committee members for their dedication to a task that seemed daunting at times. Without their freely given time and efforts, this report would have been impossible. Special acknowledgment is due to Professor Noel MacDonald who made many contributions to the project until he was required to resign his committee membership upon being selected director of the Electronics Technology Office at the Defense Advanced Research Projects Agency.

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--> Preface Many people in the field of microelectromechanical systems (MEMS) share the belief that a revolution is under way. As MEMS begin to permeate more and more industrial procedures, not only engineering but society as a whole will be strongly affected. MEMS provide a new design technology that could rival, and perhaps even surpass, the societal impact of integrated circuits (ICs). Is this fact or fiction? If it is fact, then several questions must be asked. •   What precisely is the nature of this "revolution"? •   What should be done to exploit MEMS in the most advantageous way? •   Are lessons learned from the development of other fields applicable to the future of MEMS? •   What are the risks of various strategies? •   What steps can be taken to provide an environment in the U.S. that promotes healthy and vigorous growth for MEMS? A brief consideration of the nature of the revolution can provide a focus for further discussion. Although the revolution may seem to be nothing more than the "miniaturization of engineering systems" to some observers, the authors of this report believe that much more is involved. Miniaturization per se is more of an evolutionary than a revolutionary process. Building systems as compactly as possible has been a theme of engineering practice for many years, and progress toward this goal is typically measured in terms of countless refinements in design and manufacturing techniques. MEMS is a new and revolutionary field because it takes a technology that has been optimized to accomplish one set of objectives and adapts it for a new, completely different task. The industry, of course, is the silicon-based IC process, which is now so highly refined that it can produce millions of electrical elements on a single chip and define their critical dimensions to tolerances of 100-billionths of a meter. Countless hours and dollars were invested in this technology over the past 30 years to develop a superb method for fabricating overwhelmingly complex electrical systems. The MEMS revolution arises directly from the ability of engineers to harness IC know-how and use it to build working microsystems from micromechanical and microelectronic elements. Because the committee believes that this adaptation is the revolutionary aspect of MEMS, this report will strongly emphasize those "lithography-based" processing methods that have been well established through the IC experience. MEMS is a multidisciplinary field that involves challenges and opportunities for electrical, mechanical, chemical, and biomedical engineering, as well as for physics, biology, and chemistry. Papers describing developments in MEMS are being presented more and more frequently at research meetings that have traditionally focused on other fields, such as the large and respected annual International Electron Devices Meeting of the Institute of Electrical and Electronics Engineers (IEEE). Articles about these conferences in trade publications indicate the importance of MEMS to ICs in the gigabit era. One finds "evening discussion sessions," for example, that explore the impact of MEMS on the design of control systems, displays, optical systems, fluid systems, instrumentation, medical and biological systems, robotics, navigation, and computers, among other fields. Universities worldwide are incorporating MEMS research into their programs. To accommodate the interdisciplinary features of the field, many universities are creating cross-departmental and cross-college programs. New graduate courses are being introduced using new materials for teaching, and several books on the subject are nearing completion. A significant number of government programs supporting MEMS development are in place around the world (e.g., Japan, Switzerland, Germany, Taiwan, and Singapore), and the list is growing. This suggests that development will accelerate as new applications and product opportunities become evident. One can see a similarity to the parallel, independent development of ICs that coalesced in the early 1970s, after a decade or so of intense development had led to processes and designs suitable for use in marketable products. Early federal support for MEMS research in the United States came from the National Science Foundation, which recognized the field as an emerging area of opportunity. This very limited support (less than $1 million per year) was only for prototype demonstrations, however. In recent years, a major additional source of federal funds has been the U.S. Department of Defense, which currently supports a program at a level of more than $50 million per year. Only now are established industries in the United States becoming aware of the potential effects of MEMS on their products, and a "show me" attitude has arisen in many quarters. Interest has been steadily increasing with the success of

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--> a number of MEMS pioneer companies (e.g., Analog Devices, Inc., EGG IC Sensors, and NovaSensor) in developing commercially rewarding products. More than 80 U.S. firms currently have activities in the MEMS area, a high proportion of which (65 percent) can be classified as "small businesses" (i.e., annual revenues of less than $10 million-in most cases less than $5 million). About 20 large U.S. companies have also incorporated MEMS into their products (e.g., Honeywell, Motorola, Hewlett-Packard, Texas Instruments, Xerox, GM Delco, Ford Motor Company, and Rockwell). According to Kurt Petersen (1996), a founder of NovaSensor and a recognized pioneer in the field, total sales of MEMS in the United States by 1994 were about $630 million, with pressure sensors for medicine ($170 million), automotive use ($200 million), and industrial/aerospace applications ($200 million) completely dominating the scene. The rest of the market was divided among pressure sensors for non-medical applications ($20 million), accelerometers for air bag deployment ($15 million), auto suspension ($2 million), fuel injectors ($20 million), and microvalves ($2 million). Although developments were anticipated in all of these areas, as well as in wholly new areas, Petersen notes that the pace of commercial development was very slow before the 1990s. MEMS pressure sensors were first commercialized in the 1960s, and ink-jet nozzles in production printers have been evolving since 1974. In response to the growing interest in MEMS, various trade groups and technical-assessment organizations have surveyed the field and attempted to predict its course. As is customary with predictions and especially with economic punditry, the outcome values of these assessments vary substantially. Although the committee neither reviewed nor compared the various predictions, it did believe that noting some general statements from these sources would be valuable. Projections began to appear in the early 1990s when, for example, a Battelle survey predicted about $8 billion in MEMS products worldwide by the usually quoted target year of 2000. Other predictions since 1990 have generally been more bullish, between $12 and $14 billion. In 1994, the U.S. trade group SEMI (Semiconductor Equipment and Materials International) conducted a survey of commercial opportunities (Walsh and Schumann, 1994). These predictions were based on information from MEMS manufacturers, users, suppliers, and researchers. This feature does not, of course, validate the study, and committee members had different views of "best guesses" for the field. We repeat here only a few of the SEMI report conclusions starting with its prediction of a year 2000 MEMS world market of more than $14 billion, of which medical and transportation applications for pressure-sensing could provide about 30 percent. SEMI's report also predicts major markets (totaling $2.7 billion) for inertial sensors, including accelerometers for auto-crash safety systems, auto suspensions and braking systems, munitions, pacemakers (which can use accelerometers to sense bodily activity), and machine control and monitoring. Other MEMS areas targeted for strong growth in the SEMI survey were fluid regulation and control, optical switching and routing, mass-data storage, displays, and analytical instruments. Based on a fairly general consensus that lithography-based technologies are the key to low-cost MEMS developments and on the shared desire for "foundry processing," some MEMS foundries are now in operation, notably at MCNC in Research Triangle Park, North Carolina, but also through runs sponsored by the Defense Advanced Research Projects Agency (DARPA) at Analog Devices, Inc., and by special arrangement at Sandia National Laboratories. For specialized uses, such as for space applications, more expensive customized processing techniques like LIGA may be needed, and MCNC is also exploring possibilities in this area. A growing number of examples show that MEMS fabrication could be possible by adding processing steps to conventional IC production lines. In a recent paper entitled MEMS: What Lies Ahead?, Kurt Petersen (1995) states that "without exception, every company involved in electronics and miniature mechanical components should have programs to familiarize themselves with the capabilities and limitations of MEMS. Instrumentation companies that are not fluent in MEMS in the coming years will experience severely threatening competition." Petersen continues that, as MEMS evolves, it is becoming "less an industry unto itself and more of a critical discipline within many other industries." This means that application-specific MEMS processes will undoubtedly evolve as producers discover the best way to use MEMS for their products. Just like production for ICs, processes for MEMS will probably be limited by economic factors, and designers will attempt to satisfy their needs with the simplest, most economical technology. The purpose of this report is (1) to review, current and projected MEMS needs based on projected applications, (2) to identify shortcomings in present and developing MEMS technologies, (3) to recommend how MEMS can best use advanced materials and fabrication processes to overcome these shortcomings, and (4) to recommend research and development (R&D) areas that would lead to the necessary advances in materials and fabrication processes for MEMS. The first chapter provides background information on the development of the MEMS field and future prospects. Chapter 2 examines the strengths of the various IC-based technologies for fabricating MEMS and their potential for producing even more innovative devices. Chapter 3 focuses on the rationale for introducing new materials and processes that can extend the capabilities and applications of MEMS and that are compatible with IC-based, batch fabrication processes. Chapter 4 extends the discussion of MEMS to the information and manufacturing infrastructure needed to favor the development of MEMS. The final chapter of the report examines the

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--> major challenges facing the assembly, packaging, and testing of MEMS. This report concentrates on MEMS technologies and designs that either derive from or are applicable to those of the IC industry. In the view of the committee, these areas hold the greatest opportunity for the immediate future. Discussions of technologies, fabrication tools, and properties for microsystems made solely from non-IC-based materials (e.g., glasses, plastics, or semiconductors other than silicon) have been necessarily omitted. The committee believes that there are important opportunities for these microsystems, but they are beyond the scope of this report. Richard S. Muller, chair Committee on Advanced Materials and Fabrication Methods for Microelectromechanical Systems

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--> Contents     EXECUTIVE SUMMARY   1 1   BACKGROUND   6     Commercial Successes,   7     Newly Introduced Products,   9     Longer-Range Opportunities,   13     Summary,   13 2   INTEGRATED CIRCUIT-BASED FABRICATION TECHNOLOGIES AND MATERIALS   14     Strengths of the Integrated Circuit Process,   14     Using Existing Integrated Circuit-Based Processes,   15     Classifying Integrated Circuit-Based Technologies,   20     Summary,   22 3   NEW MATERIALS AND PROCESSES   23     Motivations for New Technologies,   23     Materials and Processes for High-Aspect-Ratio Structures,   23     Materials and Processes for Enhanced-Force Microactuation,   27     Films for Use in Severe Environments: Silicon Carbide and Diamond,   30     Surface Modifications/Coatings,   31     Power Supplies,   32     Summary,   32 4   DESIGNING MICROELECTROMECHANICAL SYSTEMS   34     Metrology,   34     Modeling,   35     Computer-Aided Design Systems,   35     Foundry Infrastructure,   35     Summary,   36 5   ASSEMBLY, PACKAGING, AND TESTING   38     Contrasts between Assembly, Packaging and Testing of Integrated Circuits and Microelectromechanical Systems,   38     Interfaces,   39     Packaging,   41     Assembly,   44     Standards, Testing, and Reliability,   47     Failure Analysis,   47     Summary,   49     REFERENCES   51     APPENDICES   57     A World Wide Web Sites on MEMS   59     B Biographical Sketches of Committee Members,   60

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--> Tables, Figures, and Boxes TABLES 3-1   Potential Electroceramic Sensor Materials,   30 5-1   Characteristics of Common IC Chip-Level Packages,   44 FIGURES 1-1   Cross-section of an integrated thermal ink-jet chip,   7 1-2   Evolution of ink-jet drop weight versus time,   7 1-3   Schematic illustration of the sensing element of the ADXL50 accelerometer,   8 1-4   Annotated photomicrograph of an ADXL50 single-chip accelerometer,   8 1-5   Motorola accelerometer chip and electronics chip packaged together on a metal lead frame,   9 1-6   Two pixels in the Texas Instruments mirror array,   9 1-7   Scanning electron photomicrographs,   10 1-8   Concepts for applications of automotive sensors and accelerometers,   11 1-9   Potential MEMS to monitor the condition of the body remotely and actuate implanted MEMS devices to release controlled doses of medicine,   12 2-1   Three-dimensional configurations that can be produced by combining directionally dependent and impurity dependent etching with photolithographic patterning,   16 2-2   Generalized process flow for silicon diffusion bonding and deep reactive-ion etching (DRIE),   17 2-3   Torsional MEMS structure made possible by DRIE bulk micromachining processes,   17 2-4   Multichannel neural probe with integrated electronics fabricated by the dissolved-wafer process,   18 2-5   Deep reactive-ion etching (DRIE) depth as a function of feature width,   21 3-1   Photomicrographs of HEXSIL tweezers,   25 3-2   Schematic illustration of the steps in the basic LIGA process,   26 3-3   Metal and plastic parts produced using LIGA,   26 3-4   Microsurgical tool driven by piezoelectric materials,   31 5-1   Block diagram of generic packaging requirements,   39 5-2   Schematic diagram summarizing various input/output modalities for MEMS systems,   39 5-3   Silicon pressure sensor,   41 5-4   Accelerometer packaged in IC standard transistor outline (TO) package,   41 5-5   Accelerometer packaged in IC standard dual in-line (DIP) package,   41 5-6   Two-chip smart accelerometer,   42 5-7   Detail of a multiplatform hybrid package showing feed-through, interconnect, and support features for an environmental monitoring cluster system,   45 5-8   Flip-chip attachment of two die to form an integrated system,   46 5-9   Assembled magnetic linear actuator,   47 5-10   Packaged, normally-open microvalve and process flow for fabrication of a normally-open, thermopneumatically-actuated microvalve,   48 5-11   Specifications at all levels of testing,   49 BOX 1-1   Semantics: What's in a Name?,   6

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--> Acronyms A/D analog-to-digital converter ADI Analog Devices, Inc. AP&T assembly, packaging, and testing ASIC application-specific integrated circuit BiCMOS bipolar complementary metal oxide semiconductor CAD computer-aided design CAE computer-aided engineering CMP chemical-mechanical polishing CNC computer numerical control CPU central processing unit CRT cathode-ray tube CVD chemical vapor deposition DARPA Defense Advanced Research Projects Agency DIP dual in-line package DLP digital light processing DMD digital micromirror display DRAM dynamic random-access memories DRIE deep reactive ion etching EDM electron-discharge machining FAMOS field-avalanched metal oxide semiconductor device FEA finite-element analysis HF hydrofluoric acid HP Hewlett-Packard IBSD ion-beam sputter deposition IC integrated circuit ICP inductively coupled plasma KOH potassium hydroxide LCD liquid-crystal display LED light-emitting diode LPCVD low-pressure chemical-vapor deposition MBE molecular-beam epitaxy MEMS microelectromechanical systems MOCVD metal-organic chemical-vapor deposition MOD metal/organic decomposition

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--> MOS metal oxide semiconductor MOSIS metal oxide semiconductor implementation system (now refers to a wider scope of technologies) MST microsystem technology NITINOL Ni/Ti thin-film material NMOS N-channel metal oxide semiconductor NSF National Science Foundation NVFRAM nonvolatile ferroelectric random access memory PCA portable clinical analyzer PLAD pulsed laser-ablation deposition PECVD plasma-enhanced chemical-vapor deposition PMMA polymethylmethacrylate PSD plasma sputter deposition R&D research and development. RIE reactive-ion etching SAM self-assembled monolayer SMA shape memory alloy TI Texas Instruments TO transistor outline VLSI very large-scale integration