This appendix is a brief review of the development of microelectromechanical systems in the United States. It was prepared by this JTEC panel as a baseline against which to compare Japanese activities in this area. Indeed, this appendix will review the recent approaches taken in the United States, and U.S. perceptions of the challenges that confront the continued growth of this area, as a basis for understanding the approaches taken in Japan and the important role that organizations there have played and continue to play in this field. The views expressed here necessarily represent those of the panel members alone, but are nonetheless thought to accurately reflect the current situation in the United States in both academia and industry.
The field of MEMS has been recognized internationally only within the last few years, although it is rooted in efforts on sensors and actuators that go back thirty years or more. The field has been driven by the rapid global progress in the field of microelectronics, where solid-state microprocessors and memory have revolutionized many aspects of instrumentation and control, and have facilitated explosive growth in data processing and communications for three decades. Many of the emerging application areas for microelectronics, however, deal with nonelectronic host systems and thus require that parameters such as pressure or flow be converted to electrical signals that can be processed by computer. After the necessary control decisions are made electronically, the resulting electronic signals can be fed to actuators to control the parameters of the host system. Figure D.1 shows the general arrangement of this control loop. The peripheral functions of sensing and actuation represent the principal bottlenecks today in the application of microelectronics to such systems, including those in automotive areas (both vehicular and smart highways), environmental control (HVAC), global environmental monitoring, health care (monitoring, diagnostics, and instruments for microsurgery and prosthetics), defense, automated manufacturing (including those for use in the microelectronics industry itself), and important consumer products. It is thus evident that sensors, actuators, and MEMS are likely to exert considerable leverage on the microelectronics industry beyond the considerable direct markets for them, since they will enable the use of microprocessors and memory that otherwise could not be applied.
During the past thirty years, considerable strides have been made in the realization of sensors and actuators using solid-state technology, and most efforts have come to concentrate on this approach (Petersen 1982). Beginning with visible image sensors in the mid-sixties and then pressure sensors in the 1970s, most efforts to realize sensors have drawn extensively from integrated circuit technology and have been silicon-based. In the 1980s, accelerometers emerged as additional high-volume product targets, driven primarily by needs in the automotive industry. Both microactuators and MEMS were born during this decade. Today, visible image sensors are approaching the resolution of photographic film (Nomoto 1993) and offer the promise of automatic electronic processing of both video and still images. Infrared imaging has similarly resulted in large area arrays, and more recently has been demonstrated in several uncooled implementations based on micromachining (Wood, Han, and Kruse 1992). Solid-state pressure sensors have been demonstrated over a broad range of applications, from ultrasensitive devices capable of serving as solid-state microphones (Kim, Kim, and Muller 1991) or capacitive manometers (Cho and Wise 1993) to rugged devices used in electronic transmissions and in the hydraulic control of heavy equipment, spanning at least eight orders of magnitude in pressure. A variety of accelerometers are being merged with on-chip circuitry for high-volume applications (Payne and Dinsmore 1991), and inertial navigation systems based on integrated gyroscopes (Bernstein et al. 1993) are in development in a number of companies. Microflowmeters are emerging for industrial process applications, and still other devices are being designed for chemical sensing and for applications in decoding the human genome. Many of these emerging applications are potentially very high in volume and very important to global society.
From the perspective of this particular study, it is important to define what we mean by MEMS, since there is as yet no clear definition of this term. It clearly includes many sensors and actuators, but by no means all of them. For purposes of this report, "MEMS" are batch-fabricated devices that involve the conversion of physical parameters to or from electrical signals and that depend on mechanical structures or parameters in important ways for their operation. Thus, this definition includes batch-fabricated monolithic devices such as microaccelerometers, pressure sensors, microvalves, and gyroscopes fabricated by micromachining or similar processes. Also included are microassembled structures based on batch-fabricated parts, especially when batch assembly operations are used, but this study will not focus on individually-fabricated devices that are unlikely to see wide use. It is expected that an interface to electronic signal processing will exist in most MEMS, which implies that they will include sensors, actuators, or (in most cases) both. Image sensors, chemical sensors, and purely thermal or magnetic devices, however, will not be covered in detail in this study even though they may involve technology and generic microstructures that are similar to those used in MEMS. New materials and processes such as LIGA, however, are an important part of the study along with testing, packaging, and many issues associated with the design and development infrastructures needed for MEMS.
Figure D.1. Structure of a sensor/actuator control loop typical of evolving microelectromechanical systems.
Figure D.2 summarizes many of the sensor development activities in the United States since their beginnings in the 1950s in materials-oriented research at Bell Telephone Laboratories, Honeywell, and Westinghouse. As part of the development of beam-lead (air-isolated) integrated circuits at Bell Telephone Laboratories in the 1960s, precision silicon etching technology was developed, and by the mid-seventies this had been utilized in important ways by the sensor community and had been rechristened "micromachining." The earliest university work in this area was probably that started at Stanford University in the mid-sixties, followed by important efforts at Case Western Reserve University, the University of California (Berkeley), the University of Wisconsin, Massachusetts Institute of Technology, the University of Michigan, the University of Utah, and elsewhere. Indeed, the development of sensors in the United States was pioneered primarily in academia, with commercialization efforts relatively slow in coming. The field was enhanced significantly with the development of surface micromachining at the University of California (Berkeley) and the University of Wisconsin in the mid-eighties, which made possible a wide array of additional microstructures that could be realized using silicon technology. Many of these new devices were microactuators, and the concept of marrying sensors, actuators, and electronics to collapse entire microinstrumentation systems to the level of a single chip emerged as a result. It was in the late 1980s that the term "MEMS" was born to describe one portion of the expanding sensor-actuator area.
Figure D.2. The development of solid-state sensors, actuators, and MEMS in the United States.
The 1980s saw the demonstration of many new devices in prototype and increasing efforts at commercialization. With the establishment of many international and regional conferences during the 1980s, solid-state sensors and actuators emerged as a recognized and important field, centered in microelectronics but extending broadly into mechanical engineering, robotics, and a variety of additional fields as well. It appears that the decade of the nineties will be devoted to merging devices and control electronics to form integrated microsystems, implemented in either hybrid or monolithic form, and to applying these microsystems to meet a number of application needs.
While increasingly recognized as important in the implementation of a wide variety of emerging systems, however, enthusiasm over the realization of sensors, actuators, and MEMS using microelectronic technology must be tempered by the realization that many such devices have existed in various forms for a long time. The commercialization of these devices has been slow, and in many areas technology push has been considerably stronger than market pull. In some cases, the control system hierarchy into which these devices must work must change considerably in order to accommodate them, and the general lack of synergy between the sensing and control areas has undoubtedly retarded progress. Simply replacing earlier devices with solid-state versions of them has not fulfilled the potential of this area, and going beyond simple component replacement into integrated microsystems has required interdisciplinary cooperation that has been difficult to achieve, at least in the United States. Has the technology associated with MEMS really matured to the point where high-volume devices can be realized using batch processes with high yields? If not, when will it do so? Can the specialized processes required for the realization of microelectromechanical microstructures really be successfully merged with circuit processes to form microsystems on a chip? Is this necessary, or will hybrid systems do just as well? Where are the markets for these devices that will demand continuous technology improvements similar to what the memory has done for microelectronics? Where are the high-volume markets that will fuel the sensor/actuator/MEMS industry and significantly benefit society? These are some of the questions to be considered in this study. They are being addressed on a global scale with an increasing sense of urgency.
In order to answer such questions, funding for MEMS research has increased substantially on a worldwide basis over the past decade. Funding for MEMS in the United States began with early efforts funded by the mission-oriented agencies of the federal government, especially the National Institutes of Health and the National Science Foundation (NSF). In the late eighties, efforts intensified with a very visible program run at NSF under their Emerging Technologies Initiative. This was the forerunner of support by the Advanced Research Projects Agency, which has further enhanced funding in this area. While the total funding for MEMS and MEMS-related activities in the United States is not known, university funding is probably no more than $20 million per year. The effort in Europe is felt to be somewhat larger. The Japanese effort under MITI has been the most visible effort globally, and yet how much of this effort is directed specifically at MEMS and how MEMS is interpreted under this program has not been entirely clear. Japanese programs have been significant in the development of sensors, actuators, and MEMS through programs at a number of universities and companies, and the leadership of Japanese industry in consumer products puts them in an excellent position to benefit from MEMS technology.
Microelectromechanical systems have the potential to leverage microelectronics into important additional areas that could be revolutionized by low-cost electronic signal processing, computing, and control. These microsystems could have a profound effect on society, but will require a synergism among many different disciplines that may be slow in coming. Global leadership and cooperation will be absolutely required if the benefits of MEMS are to be realized in a timely way. Only understanding the potential of this emerging field and working together to overcome its challenges will ensure the early utilization of MEMS to benefit mankind.