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

• Notice
• Committee
• Acknowledgments
• Preface
• Contents
• Executive Summary

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

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.

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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.




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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 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 Nova-Sensor 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 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  BACKGROUND

Commercial Successes
Newly Introduced Products
Longer-Range Opportunities
Summary



2  INTEGRATED CIRCUIT-BASED FABRICATION TECHNOLOGIES AND MATERIALS

Strengths of the Integrated Circuit Process
Using Existing Integrated Circuit-Based Processes
Classifying Integrated Circuit-Based Technologies
Summary



3  NEW MATERIALS AND PROCESSES

Motivations for New Technologies
Materials and Processes for High-Aspect-Ratio Structures
Materials and Processes for Enhanced-Force Microactuation
Films for Use in Severe Environments: Silicon Carbide and Diamond
Surface Modifications/Coatings
Power Supplies
Summary



4 DESIGNING MICROELECTROMECHANICAL SYSTEMS

Metrology
Modeling
Computer-Aided Design Systems
Foundry Infrastructure
Summary



5 ASSEMBLY, PACKAGING, AND TESTING

Contrasts between Assembly, Packaging and Testing of Integrated Circuits and Microelectromechanical Systems
Interfaces
Packaging
Assembly
Standards, Testing, and Reliability
Failure Analysis
Summary



REFERENCES


APPENDICES

A World Wide Web Sites on MEMS

B Biographical Sketches of Committee Members





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Tables, Figures, and Boxes



TABLES
3-1  Potential Electroceramic Sensor Materials
5-1  Characteristics of Common IC Chip-Level Packages

FIGURES
1-1  Cross-section of an integrated thermal ink-jet chip
1-2  Evolution of ink-jet drop weight versus time
1-3  Schematic illustration of the sensing element of the ADXL50 accelerometer
1-4  Annotated photomicrograph of an ADXL50 single-chip accelerometer
1-5  Motorola accelerometer chip and electronics chip packaged together on a metal lead frame
1-6  Two pixels in the Texas Instruments mirror array
1-7  Scanning electron photomicrographs
1-8 Concepts for applications of automotive sensors and accelerometers
1-9 Potential MEMS to monitor the condition of the body remotely and actuate implanted MEMS devices to release controlled doses of medicine

2-1  Three-dimensional configurations that can be produced by combining directionally dependent and impurity dependent etching with photolithographic patterning
2-2  Generalized process flow for silicon diffusion bonding and deep reactive-ion etching (DRIE)
2-3  Torsional MEMS structure made possible by DRIE bulk micromachining processes
2-4  Multichannel neural probe with integrated electronics fabricated by the dissolved-wafer process
2-5  Deep reactive-ion etching (DRIE) depth as a function of feature width

3-1  Photomicrographs of HEXSIL tweezers
3-2  Schematic illustration of the steps in the basic LIGA process
3-3  Metal and plastic parts produced using LIGA
3-4  Microsurgical tool driven by piezoelectric materials

5-1  Block diagram of generic packaging requirements
5-2  Schematic diagram summarizing various input/output modalities for MEMS systems
5-3  Silicon pressure sensor
5-4  Accelerometer packaged in IC standard transistor outline (TO) package
5-5  Accelerometer packaged in IC standard dual in-line (DIP) package
5-6  Two-chip smart accelerometer
5-7  Detail of a multiplatform hybrid package showing feed-through, interconnect, and support features for an environmental monitoring cluster system
5-8  Flip-chip attachment of two die to form an integrated system
5-9  Assembled magnetic linear actuator
5-10  Packaged, normally-open microvalve and process flow for fabrication of a normally-open, thermopneumatically-actuated microvalve
5-11  Specifications at all levels of testing

BOX
1-1  Semantics: What's in a Name?





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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
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



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Executive Summary





As the twenty-first century approaches, the capacity to shrink electronic devices while multiplying their capabilities has profoundly changed both technology and society. Beginning in 1948, the vacuum tube gave way to the transistor, which was followed by a series of major strides leading to integrated circuits (ICs), which led to on-chip electronic systems, such as large-scale memories and microprocessors. Present silicon very-large-scale-integrated (VLSI) chip technology seems destined to continue developing for at least another 20 years based on smaller and smaller electronic devices that can operate faster and do more.

In the late 1980s, the design and manufacturing tool set developed for VLSI was adapted for use in a field called microelectromechanical systems (MEMS). These systems interface with both electronic and nonelectronic signals and interact with the nonelectrical physical world as well as the electronic world by merging signal processing with sensing and/or actuation. Instead of handling only electrical signals, MEMS also bring into play mechanical elements, some with moving parts, making possible systems such as miniature fluid-pressure and flow sensors, accelerometers, gyroscopes, and micro-optical devices. MEMS are designed using computer-aided design (CAD) techniques based on VLSI and mechanical CAD systems and are typically batch-fabricated using VLSI-based fabrication tools. Like ICs, MEMS are progressing toward smaller sizes, higher speeds, and greater functionality.

MEMS already have a track record of commercial success that provides a compelling case for further development (e.g., pressure sensing, acceleration sensing, and ink-jet printing). Like any developing field, however, commercial successes in the MEMS field coexist with products still under development that have not yet established a large customer base (e.g., MEMS display systems and integrated chemical-analysis systems).

The U.S. Department of Defense and the National Aeronautics and Space Administration requested that the National Research Council conduct a study (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 areas that would lead to the necessary advances in materials and fabrication processes for MEMS. The Committee on Advanced Materials and Fabrication Methods for Microelectromechanical Systems, under the auspices of the National Materials Advisory Board, was convened to undertake this study and write this report.

The committee concluded that the MEMS field faces a number of challenges to the establishment of an environment that promotes healthy and vigorous growth. These challenges are presented in this Executive Summary along with recommendations for meeting them. Because of the broad perspective with which the MEMS field is viewed in the report, the findings and recommendations are not prioritized.




LEVERAGING AND EXTENDING THE INTEGRATED
CIRCUITS FOUNDATION

A great deal of the excitement and promise of MEMS has arisen from the demonstrated ability to produce three-dimensional fixed or moving mechanical structures using lithography-based processing techniques derived from the established IC field. Conventional IC materials can continue to be used in new ways in MEMS, and much of the needed MEMS-specific hardware can still be leveraged from IC-technology. Such MEMS developments are most likely to be accepted in traditional IC-fabrication facilities and therefore most likely to succeed commercially.

In the microelectronics world, major steps forward have sometimes resulted from inspired looks backward at technologies and materials that were already known and well categorized. For MEMS, this "cleverness research" can take on a special character by posing mechanical problems to technologies that originally responded only to the demands of electrical design. A wide field of opportunity for creative work in MEMS could be based on what is already known about IC processing, particularly in the re-evaluation of the vast knowledge compiled during the history of IC development (e.g., transistor-transistor logic; integrated-injection logic; analog; bipolar; n-channel metal-oxide semiconductors).

Conclusion. The expertise and advanced state of the current microelectronics industry provides an enormous advantage for the development of MEMS. Leveraging and extending existing IC tools, materials, processes, and fabrication techniques is an excellent strategy for producing MEMS with comparable levels of manufacturability, performance, cost, and reliability to those of modern VLSI circuits.

Recommendation. Efforts to stimulate solutions to the challenges of producing MEMS should capitalize on the families of relatively well understood and well documented IC materials and processes. These solutions may be found in current IC practices but may also result from creatively re-establishing older IC technologies. This recommendation calls for continuing strategic investment.




ENLARGING THE SUITE OF MATERIALS SUITABLE
FOR INTEGRATED-CIRCUIT-LIKE PROCESSING

Although there may be commercial advantages to leveraging the present suite of IC-process materials, they will not be able to meet all of the demands that a growing number of users and applications will place on MEMS. Easily foreseen requirements (e.g., higher forces, stability in harsh and high-temperature environments, and robust high-aspect-ratio structures) will compel the application of new materials and extend the MEMS field beyond the boundaries of the IC world.

Materials that are not usually used in IC processes include magnetic, piezoelectric, ferroelectric, and shape-memory materials. Actuating-force requirements for valve closures and motor drives, for example, are already drawing attention to the advantages these materials would bring to MEMS. Other developments, such as MEMS for optics, biological purposes, chemical-process controls, high-temperature applications, and other hostile environments, will inevitably draw attention to the need for an even broader range of materials.

In the IC world, new materials are typically incorporated as thin films and are produced by a limited number of techniques (e.g., low-pressure chemical-vapor deposition or sputtering). Many of these materials either do not show optimal mechanical properties in thin-film form or are difficult to deposit by typical IC-fabrication methods or are incompatible with the microelectronic IC process. For some MEMS designs, it is possible to apply these specialized materials either by incorporating them in a step prior to more-conventional processing or by adding them as a final step. Either option raises the possibility that the technology will be substantially different from better known processing techniques. Materials that are incompatible with the IC-processes might have to be handled by a specialized foundry.

Conclusion. Extending the list of materials that have useful MEMS properties and can be processed using lithography-based, IC-compatible techniques will be beneficial to MEMS development.

Recommendation. Research and development should be encouraged to develop new materials that extend the capabilities of MEMS. The new materials should be integrable, at some level, with conventional IC-based processing. This recommendation calls for continuing strategic investment.

Recommendation. Research should be encouraged to develop techniques to produce repeatable, high-quality, batch-processed thin films of specialized materials and to determine the dependence of their properties on film-preparation techniques. For some materials, it may be advisable to establish "foundries" that are available to the entire MEMS community and can serve as repositories for equipment and know-how. This recommendation calls for new strategic investment.




CHARACTERIZING MEMS MATERIALS

The IC industry has been built on an extensive, constantly expanding body of knowledge about the behavior of silicon and related materials as they are scaled down in size. No comparable resource has been established for MEMS, however. For example, although a great deal is known about the electrical properties of polysilicon thin films, not much is known about their micromechanical properties or about specific details of the long-term reliability of mechanically stressed polysilicon or the surface mechanics related to friction, wear, and stress-related failure. There is a similar lack of fundamental knowledge about other thin-film materials borrowed from the electrical domain that are now exercised mechanically (e.g., silicon nitride, silicon dioxide, and thin-film metals). Many thin-film materials that are used in the IC industry (e.g., aluminum, silicon dioxide, amorphous silicon, porous silicon, various other deposited and plated metals, and polyimide) have still not been extensively studied and evaluated for their applicability to MEMS.

Conclusion. A thorough understanding of the micromechanical properties of the materials to be used in MEMS at appropriate scales is not available.

Recommendation. The characterization and testing of MEMS materials should be an area of major emphasis. Studies that address fundamental mechanical properties (e.g., Young's modulus, fatigue strength, residual stress, internal friction) and the engineering physics of long-term reliability, friction, and wear are vitally needed. It is important that these studies take into account fabrication processes, scaling, temperature, operational environment (i.e., vacuum, gaseous, or liquid), and size dependencies. Studies of the size effects of physical elements, on a scale comparable to the crystallite regions in a polycrystalline material, are required. This recommendation calls for continuing strategic investment.




UNDERSTANDING SURFACE AND INTERFACE EFFECTS

The properties of materials can differ at the small scales at which individual MEMS devices are configured, causing effects that can influence their behavior. At these tiny scales, material behavior is more influenced by surface-driven effects than by volume or bulk effects. For example, frictional effects take on overwhelming importance, in contrast to inertial effects, in small mechanical systems. If the interfaces act as electrical contacts (e.g., in MEMS microrelays), additional wear, corrosion, frictional effects, and contact forces are present. Surface-to-surface sticking (stiction) is also likely to be important in surface-driven processes. During the drying process and after the final cleaning of MEMS devices, the surface tension of the meniscus of liquids can pull suspended mechanical structures toward nearby surfaces, causing the structures to become stuck. Stiction can also occur during the operation of actuated MEMS if shock, electrostatic discharge, or other stimuli cause moving components to touch either each other or to touch another surface.

The MEMS operating environment and the interfaces of this environment on individual MEMS devices can influence performance. Signals admitted to the MEMS package may have electrical, thermal, inertial, fluid, chemical, optical, and possibly other origins. Output can be electrical, optical, mechanical, chemical, hydraulic, or magnetic signals. MEMS applications to liquid systems, for example, would raise interface questions about the use of wetting and dewetting agents and the nature of fluids in micrometer-sized channels and cavities. The high precision of some MEMS sensing devices also makes them sensitive to gas/solid interactions.

Conclusion. Further development of moving elements in MEMS demands a more complete understanding of (1) the effects of internal friction, Coulomb friction, and wear at solid/solid interfaces and (2) the influence of interfaces on performance and reliability. This understanding should lead to the development of suitable coatings, lubricants, and wetting agents, as well as improved designs that take these effects into account.

Recommendation. Surface and interface studies should be pursued to address questions associated with contact forces, stiction, friction, corrosion, wear, lubrication, electrical effects, and microstructural interactions at solid, liquid, and gaseous interfaces. Engineering design and manufacturing solutions to the problems associated with MEMS surfaces and interfaces should also be pursued. This recommendation calls for continuing strategic investment.




ETCHING TECHNOLOGIES

At the heart of MEMS is the ability to construct extremely small mechanical devices, preferably using batch processing. Wet etching has historically dominated the MEMS field because (1) structures can be micromachined from silicon in a short time and (2) chemical-etch equipment is well established, simple, and inexpensive. The disadvantages of wet-chemical processing are its inability to achieve vertical sidewalls and nonorthogonal linear geometries in d silicon and its reaction with films on the wafer surface. Because of the lateral spread of etching, patterned features must also be spaced relatively far apart so that adjacent features do not merge, and the features on the mask and pattern-transfer layer must be biased or reduced (and sometimes even distorted) to achieve the desired size and shape at the completion of the wet-etch process. Although dry etching is a mainstay of IC processing and gas-phase dry-etching techniques are currently a subject of research for MEMS production, the etch depths for MEMS are often significantly greater than those commonly employed in IC-fabrication. Therefore, etching for MEMS may present different or additional challenges.

Conclusion. Because controlled etching is so important to the fabrication of three-dimensional structures and, therefore, to progress in MEMS, methods of etching in a controlled fashion and ways of tailoring the isotropic or anisotropic etch-rates of various materials are of great value.

Recommendation. Further research and development should be undertaken to improve etches, etching, and etching controls for MEMS. This work should take into account the status, potential development, and limitations of manufacturing-process equipment. This recommendation calls for continuing strategic investment.




ESTABLISHING STANDARD TEST DEVICES AND METHODS

Standard test devices and methods are required to determine the mechanical properties of MEMS devices, to demonstrate the repeatability and reliability of mechanical devices, and to facilitate quality-control practices. Package-level testing is currently the most common way to measure MEMS performance, but the development of in-process wafer-level testing will be necessary for low cost manufacturing. Wafer-level testing of MEMS presents special challenges that are often product dependent. Nevertheless, generic test structures that indicate basic mechanical properties of MEMS materials at the wafer level should be developed and characterized. As more and more industries, universities, and other research groups enter the MEMS field, it is also becoming increasingly important to provide accepted standards that can be used for comparison.

Conclusion. Test-and-characterization methods and metrologies are required to (1) help fabrication facilities define MEMS materials for potential users, (2) facilitate consistent evaluations of material and process properties at the required scales, and (3) provide a basis for comparisons among materials fabricated at different facilities.

Recommendation. Standard test methods, characterization methods, and test devices should be developed and disseminated that are suitable for the range of materials and processes of MEMS. Ideally, metrology structures will be physically small, simply designed, easily replicated, and conveniently and definitively interrogated. MEMS engineering standards should be similar to those already established for materials and devices in conventional sizes by organizations such as the National Institute of Standards and Technology (NIST), the American Society for Testing and Materials (ASTM), and the Institute of Electrical and Electronics Engineers (IEEE). This recommendation calls for new strategic investment.




MEMS PACKAGING

Packaging a device, interfacing it to its operating domain, and assembling it as a part of a larger system are critical final production steps and can easily represent up to 80 percent of the cost of a component. Although considerable attention continues to be paid to innovative applications of MEMS processing techniques and devices, "back-end" processes have historically been approached on a specialized, case-by-case basis. The lack of publicly available technology or information to support packaging has meant that each organization has essentially had to invent and reinvent solutions to common problems. Possible extensions of batch processing to back-end processes could substantially reduce costs.

Conclusion. Packaging, which has traditionally attracted little interest compared to device and process development, represents a critical stumbling block to the development and manufacture of commercial and military MEMS. The imbalance between the ease with which batch-fabricated MEMS can be produced and the difficulty and cost of packaging them limits the speed with which new MEMS can be introduced into the market. Expanding the small knowledge base in the packaging field and disseminating advances aggressively to workers in MEMS could have a profound influence on the rapid growth of MEMS

Recommendation. Research and development should be pursued on MEMS packaging and assembly into useful engineering systems. The goal should be to define, insofar as possible, generic, modular approaches and methodologies and to extend batch-processing techniques into the various back-end steps of production. This recommendation calls for new strategic investment.




FOUNDRY AND COMPUTER-AIDED DESIGN
INFRASTRUCTURE FOR MEMS

Rapid development in the IC industry has been aided by the establishment of a foundry infrastructure that ensures that industry and government users will be able to manufacture IC products at competitive rates and enables companies that do not have wafer-processing capabilities to enter the field. One of the key factors in the development of the IC foundry infrastructure was the development of a CAD infrastructure that became the backbone of foundry operations. Design methods were implemented that allowed IC designers to develop systems independently and have them manufactured by submitting only a design-language file. The MEMS field is more complicated because of the broad range of electrical and mechanical applications, including consumer, automotive, aerospace, and medical products. Thus, several standard-process MEMS foundries would have to be available and accessible, as well as custom, flexible fabrication facilities for users who require access and manipulation of the process to produce and optimize their products.

The committee recognizes that realizing the concept of MEMS foundries may be difficult because many commercial companies have difficulty seeing "what's in it for them." Besides the danger of compromising proprietary know-how, companies offering a foundry service will have to commit to specific processes and reasonable turnaround schedules. In the instances where small industries have tried to accommodate MEMS foundry runs so far, the results have not been warmly received. A more feasible road to at least moderate success at the present juncture appears to be using academic and government laboratories to provide foundry services. The recent expansion of the National Nanofabrication Laboratory to sites at several universities and the capabilities of national laboratories, like Sandia and Livermore, may provide opportunities for MEMS foundries of a different nature, where direct hands-on work can be done by the MEMS researcher. This kind of operation could not be as widely extended as the more traditional foundry approach of MCNC, which interacts with users only through exchanges of software, but it may provide an interim avenue until specific areas in the MEMS field are further developed.

Conclusion. Establishing standard CAD and foundry infrastructures for MEMS is essential in the near future to support the growth of MEMS from the prototype and low-volume commercial level to the volume-driven, low-cost commercial level. The development of a MEMS foundry-technology base, similar to the base that supports ICs, would ensure that MEMS products could be manufactured at competitive rates and would enable more small companies and research organizations to enter the field.

Recommendation. A MEMS CAD-infrastructure that extends from the processing and basic modeling areas to full system-design capabilities should be established. A process-technology infrastructure (e.g., supporting electrical, mechanical, fluid, chemical, and other steps and their integration to form complete systems) that is widely available to MEMS designers and product engineers should be developed. This recommendation calls for new strategic investment.




ACADEMIC STRUCTURE TO SUPPORT MEMS

The field of MEMS rests on multidisciplinary foundations. Practitioners who are poised to advance MEMS must have knowledge and skills in several fields of engineering and applied sciences. The participation of motivated, well trained young researchers is probably the single most important driver for success in MEMS. Some of these researchers will come from the ranks of trained IC engineers, who are already familiar with tools, materials, and procedures that are useful for MEMS. In general, however, these practicing engineers will have to learn new aspects of mechanical design, materials behavior, computing techniques, and systems design. Providing learning opportunities and educational materials for practicing engineers is important. But for future engineering students, effective instruction in MEMS will require major changes in curricula. A high priority should be placed on establishing an academic infrastructure that conveys the excitement and promise of the field, offers a sound and thorough education for MEMS researchers, and facilitates development of and access to new and innovative ideas across and among various disciplines.

Conclusion. Contributors to MEMS can be recruited both from practitioners already active in the IC field and from newly trained engineers. To facilitate the entry of practicing engineers into the field, opportunities to learn material that is special to MEMS should be encouraged through stimulating short courses and specialized text materials. For engineering undergraduates entering MEMS, programs and industrial procedures should be encouraged that stimulate multidisciplinary university education and enhance the skill and knowledge base of those training for or contributing to the development of MEMS. New MEMS engineers will require a broad understanding of several fields (e.g., electrical, mechanical, materials, and chemical engineering).

Recommendation. MEMS short courses and instructive materials that introduce practicing IC engineers to MEMS should be encouraged. Teaching institutions should be encouraged to see the benefits to their students and to their programs of emphasizing a broad, basic foundation in materials, production techniques, and engineering needed for MEMS. This recommendation calls for new strategic investment.