As a basis for comparison with activities in Japan, the following examples of activities in the United States illustrate the degree to which monolithic sensor-circuit integration has been employed and what it has been used for. As noted above, many U.S. efforts are still hybrid, especially in smaller companies. This approach somewhat simplifies the associated process technologies and allows the use of undisturbed circuit processes, often realized using foundries. In the United States, there is considerable interest in the establishment of MEMS foundry capabilities, with efforts at MOSIS and MCNC among the most prominent. There are no comparable efforts known in Japan. Monolithic sensor-circuit integration is being studied at several U.S. universities that have substantial circuit fabrication facilities. Sensor readout itself is also becoming an important research focus, with traditional difference amplifiers, capacitive oscillators, and switched-capacitor integrators being joined with efforts involving force-balanced feedback schemes or the use of tunneling or atomic force feedback to detect microstructure motion with extreme sensitivity.
A few representative U.S. university efforts will be described here to indicate levels of integration being achieved and functions being integrated. Figure 5.2 shows a bulk-micromachined mass flowmeter chip developed at the University of Michigan (Yoon and Wise 1992). The chip contains transducers for measuring gas flow velocity, direction, and type along with sensors for absolute temperature and pressure, allowing the computation of mass flow. On-chip CMOS circuitry allows addressable signal readout and the ability to measure the thermal time constant of the flow sensors to detect the buildup of surface films well in advance of the levels required to alter calibration. On-chip ADC is not included, but all needed readout amplifiers and servo circuits for temperature stabilization and control are. The chip size is 3.5 mm x 5 mm in 3 mm features using a simple 8-mask double-poly single-metal p-well CMOS process with five additional masks for the transducers. The on-chip actuators here are limited to the heaters used in conjunction with the flow sensors.
Figure 5.2. Monolithic mass flowmeter with on-chip CMOS interface circuitry.
Figure 5.3 shows a surface-micromachined high-Q microelectromechanical resonator recently realized at the University of California at Berkeley (Nguyen and Howe 1992). Electrostatic feedback is used to control the quality factor, independent of the ambient operating pressure. The chip achieves Qs of more than 50,000. Figure 5.4 shows a surface-micromachined digitally-force-balanced accelerometer with CMOS detection circuits (Yun, Howe, and Gray 1992), also developed recently at Berkeley. The chip employs a sigma-delta modulator in a feedback control loop to provide a large dynamic range and a direct digital output. The chip contains about 500 transistors in a die size of 2.5 mm x 5 mm. Still other university efforts in the United States have reached integration levels on sensor chips of several thousand devices, involving a mix of analog and digital circuit elements (Mastrangelo and Muller 1991; Tanghe and Wise 1992).
Figure 5.3. Surface-micromachined micromechanical resonator chip formed as a high-Q mechanical filter fabricated at the University of California at Berkeley.
Figure 5.4. Top view of a force-balanced accelerometer chip formed by surface micromachining together with CMOS detection electronics realized at the UC Berkeley.
There are many examples of sensor-circuit integration from U.S. industry, including efforts at Ford, General Motors, Honeywell, Johnson Controls, Rosemount, Texas Instruments, Motorola, Analog Devices, and other companies. Some of these are monolithic designs and some are hybrid. Figure 5.5 shows a surface-micromachined accelerometer chip recently developed by Analog Devices (Payne and Dinsmore 1991) and intended for automotive applications. The chip contains extensive CMOS signal processing electronics, which allow the reliable detection of capacitance at the subfemtofarad level using the differential lateral capacitance between the interleaved fingers of a micromachined comb structure. Still higher levels of circuit integration have been obtained in the micromachined uncooled infrared imager reported by Honeywell (Wood, Han, and Kruse 1992) and in the micromirror-based color projection display chip (Sampsell 1993) of Texas Instruments. The devices contain arrays of over 80,000 and over 440,000 elements with monolithically-integrated selection and readout electronics. These devices represent the largest known examples of integrating transducers and electronics on a common substrate, excluding visible imagers, where integration levels of several million have been demonstrated in both the United States and in Japan. Both would be classed as MEMS devices.
Figure 5.5. Surface-micromachined accelerometer with on-chip signal processing electronics from Analog Devices.