This section will describe major MEMS-based sensor developments in the United States and compare them with similar activities in Japan. This section also will show that, based on published information, there is greater diversity of MEMS sensor types and applications in the United States, due in part to the strength of U.S. university research. It may also be due to different approaches to publication and publicity, and the panel's imperfect knowledge of Japanese work. Also, Japanese industry focuses on a smaller number of sensors and applications that are deemed of greatest commercial importance. Finally, as noted from the discussion in a previous section, there are other areas of solid-state sensors such as gas, chemical, and humidity sensing, where Japanese efforts are greater than those of the United States.
There are major MEMS sensor efforts in both the United States and Japan on the development of advanced silicon pressure sensors and silicon accelerometers. These efforts are primarily driven by existing and potential high-volume applications in automotive, medical, commercial, and consumer products. The most important requirements are typically for moderate performance at very low cost. The batch fabricated nature of MEMS sensors addresses these needs, along with advantages of small size and low power. Development efforts also address high-performance needs in military and industrial systems. Many pressure sensors and accelerometers with significant levels of electronic integration are under development or even in production in both countries, while others may be expected in the marketplace in the next three to five years.
While bulk micromachined silicon diaphragm pressure sensors have been commonplace for many years, recent developments are resulting in significantly smaller and potentially less expensive devices. These devices are most suitable for low to moderate performance applications where cost and perhaps size are critical, such as disposable medical sensors and consumer products. One technique developed in the United States at the University of Wisconsin uses thin film polysilicon for the sensor diaphragm (Guckel et al. 1987). Because the polysilicon diaphragm is only about one-tenth the thickness of conventional silicon pressure sensor diaphragms, the polysilicon devices are correspondingly smaller. Thus there are many times more sensor dies per wafer, and the cost is much lower. Developments at Toyota using thin film nitride diaphragms were described above (Shimaoka et al. 1993).
Another technique, originated by NovaSensor in the United States, employs high temperature fusion bonding of silicon wafers to form inward tapering cavities under single crystal silicon diaphragms (Petersen et al. 1988). This technique can also result in much smaller sensor dies than standard bulk micromachining techniques. These devices are used in medical catheters, where size and cost are critical. No comparable work has appeared in Japan, although many industrial laboratories have wafer bonding technology.
Many pressure sensing applications may require measurement of low pressures in the range of 1,000 Pa, but adequate sensitivity to such low pressure is not easily accomplished with conventional bulk micromachined sensors. However, by using advanced MEMS micromachining technology to add corrugations or bosses to the diaphragm, areas of stress concentration can be formed that facilitate these low pressure measurements (Mallon et al. 1990). Several such low pressure micromachined sensors are commercially available in the United States.
Technology for miniature resonant strain sensors has been demonstrated both in the United States (Guckel et al. 1992; Petersen et al. 1991) and Japan (Ikeda et al. 1988, 1990a, 1990b), that may replace capacitive or piezoresistive readout of silicon diaphragm pressure sensors for improved performance. The devices employ a miniature beam driven into mechanical resonance by feedback electronics. The resonant frequency is a sensitive measure of strain, but is relatively insensitive to temperature or to the electrical properties of the device. The devices are located on a conventional silicon pressure sensor diaphragm where they function as frequency output strain gauges. There are no critical analog gain stages, and the frequency output is easily interfaced to digital signal processing circuitry. In the Guckel and Ikeda approaches, a vacuum shell can be integrally formed around the beam to allow operation in any media. The mechanical complexity and very small size required is made possible only by advances in MEMS technology. Japan is ahead in this technology and its commercialization, as described above in the work by Yokogawa.
Because of the large potential markets in automotive applications for sensors for air bag deployment and active suspensions, there are many efforts under way in the United States and Japan to develop miniature silicon accelerometers. MEMS technology is key to achieving the required performance combined with the low sensor costs demanded by these applications. Several accelerometers using bulk silicon micromachining to form a single crystal silicon proof mass and supporting flexures have been developed in the United States (Barth et al. 1988; Terry 1988) and Japan (Koide et al. 1992; Muro et al. 1992), and some are commercially available. Either piezoresistive or capacitive readout of proof mass displacement can be employed. These devices also use multiple wafer bonding technology to fabricate sandwich structures to provide overrange protection and damping. More recently, microminiature accelerometers fabricated in polysilicon by surface micromachining were reported by U.S. companies such as Analog Devices (Payne and Dinsmore 1991) and Motorola (Ristic 1992). No similar Japanese devices have been reported, but again, the technology capabilities are in place. These accelerometers may also be integrated with CMOS electronics. Several commercial products are available.
Higher performance accelerometer applications such as inertial navigation are being addressed at several laboratories. Closed loop silicon accelerometers have been reported in the United States and Japan. Several use bulk silicon structures (Henrion et al. 1990; Tsuchitani et al. 1991; Matsumoto and Esashi 1992), another employs a surface micromachined single-crystal structure (Boxenhorn and Greiff 1990), and still another consists of a polysilicon accelerometer integrated with CMOS detection circuitry (Yun, Howe, and Gray 1992). Open-loop accelerometers using resonant strain sensors are under development at Sundstrand.
For critical applications such as air bag deployment, efforts are under way in the United States to develop self-test capability in accelerometers. In one such device (Allen, Terry, and DeBruin 1990), an input signal heats an actuation beam that applies a known force to the device structure. Another concept uses electrostatic forces to apply the test signal to the accelerometer (Pourahmadi, Christel, and Petersen 1992). If proper response is obtained, the functionality of the accelerometer is confirmed. Self-test concepts have not been reported by Japanese researchers, but could easily be developed.
Several other types of sensors for mechanical variables are being developed in the United States, driven by military applications. No equivalent developments appear in Japanese publications. Vibratory silicon gyroscopes have been demonstrated by Draper Lab (Greiff et al. 1991) and by Sundstrand (Hulsing and MacGugan 1993). The Draper gyro is a doubly gimbaled structure supported by torsional flexures. The gimbaled mass is driven into torsional vibration, and Coriolis forces during rotation transfer energy into vibration in the orthogonal torsional mode. The amplitude of this second vibration is a measure of input rotation rate. The Sundstrand gyro concept uses Coriolis effects on a dithered pair of precision accelerometers, and is projected capable of measuring rates approaching 1°/hr. A micromachined silicon condenser hydrophone has been reported (Bernstein 1992). This device uses a surface micromachined silicon plate that is deflected by the acoustic wave, causing a capacitance change with respect to an overlying micromachined electrode. High acoustic sensitivity is obtained in a device only 1 mm on a side, comparable to ferroelectric hydrophones many times larger.
Complex, micromachined structures can be designed and fabricated to have unique thermal properties that can be useful for sensors: (1) Microminiature versions of thermal mass flowmeters have been demonstrated to be more sensitive and faster responding, and require lower power than macroscopic devices (Ohnstein et al. 1990; Tai, Muller, and Howe 1985). While the basic thermal principles of operation are similar, the miniature devices have significantly better performance due solely to their small size. (2) A fully integrated, miniature Pirani vacuum gauge (Mastrangelo and Muller 1991) measures absolute pressure between 10(1) to 10(4) Pa by the pressure dependent change in thermal conductivity. The MEMS device is very low in power, and is fully integrated with NMOS circuitry giving a digital output. (3) Microstructure bolometer devices have useful sensitivity to infrared radiation and can be easily fabricated as arrays of miniature devices. Thermal infrared detectors and small arrays have been developed in the United States (Choi and Wise 1986) and in Japan (Tanaka et al. 1992; Asahi et al. 1993). Work in the United States is far ahead, where very large imaging arrays of 200,000 pixels with high sensitivity and low noise-equivalent temperature difference, and complete integrated readout electronics have been produced (Wood, Han, and Kruse 1992).
MEMS technology is also being advantageously combined with optics for sensing in the United States. One example is the fabrication of a miniature Fabry-Perot interferometer for optical measurements in the near infrared spectral region (Jerman 1990). Another demonstration is use of light beams to excite a microminiature resonant beam strain sensor and sense the beam's vibrational motion (Guckel et al. 1993).
In an advanced concept, micromachined structures have been fabricated as extremely sensitive displacement transducers using electron tunneling concepts. The devices employ a microfabricated tunneling tip in close proximity to a surface. Resolution of 10(-2) A displacements between tip and surface by variation in the tunnel current have been demonstrated (Kenney et al. 1991). Applications to Golay cell infrared detectors, accelerometers, and seismometers are being explored. Such extremely high displacement sensitivity raises questions of dynamic range and stability that must be addressed. Japanese researchers have not reported on sensing applications to the tunnel sensor, but they are using silicon microstructures for scanning-tunneling microscopes (STMs), and are easily capable of developing such sensors.
As described above, there are many significant developments under way both in the United States and Japan for improved sensors and sensors for new variables, with new structures and operating principles, with improved performance, and reduced cost. However, many issues remain for developments in both countries in addressing potential practical applications. The specialized nature of many MEMS fabrication processes makes cost-effective fabrication an issue for low to moderate volume applications. High reliability and stability in the real operating environment are critical requirements for many sensors. These requirements are difficult to achieve, and continued development is required for many applications and products.
The important questions of cost, reliability, and performance go beyond the basic sensor element itself. The sensor package can have a dominant effect. New packaging and assembly schemes, including integrated wafer-level packaging, are being developed to address these issues, and are described in another section of this report. Finally, integration of sensors and electronic circuitry into more sophisticated instrumentation subsystems is an increasing trend. The smart sensors can provide compensation, linearization, output ranging, two-way communication, and many other functions.