Single-crystal silicon is being increasingly employed in a variety of new commercial products not because of its well-established electronic properties, but rather because of its excellent mechanical properties. In addition, recent trends in the engineering literature indicate a growing interest in the use of silicon as a mechanical material with the ultimate goal of developing a broad range of inexpensive, batch-fabricated, high-performance sensors and transducers which are easily interfaced with the rapidly proliferating microprocessor. This review describes the advantages of employing silicon as a mechanical material, the relevant mechanical characteristics of silicon, and the processing techniques which are specific to micromechanical structures. Finally, the potentials of this new technology are illustrated by numerous detailed examples from the literature. It is clear that silicon will continue to be aggressively exploited in a wide variety of mechanical applications complementary to its traditional role as an electronic material. Furthermore, these multidisciplinary uses of silicon will significantly alter the way we think about all types of miniature mechanical devices and components.

Silicon as a mechanical material

Steep subthreshold swing transistors based on interband tunneling are examined toward extending the performance of electronics systems. In particular, this review introduces and summarizes progress in the development of the tunnel field-effect transistors (TFETs) including its origin, current experimental and theoretical performance relative to the metal-oxide-semiconductor field-effect transistor (MOSFET), basic current-transport theory, design tradeoffs, and fundamental challenges. The promise of the TFET is in its ability to provide higher drive current than the MOSFET as supply voltages approach 0.1 V.

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Low-Voltage Tunnel Transistors for Beyond CMOS Logic

The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electromechanical-systems (MEMS) that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research.

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Micro-electro-mechanical-systems (mems) and fluid flows

Since the late 1980s there have been spectacular developments in micromechanical or microelectro-mechanical (MEMS) systems which have enabled the exploration of transduction modes that involve mechanical energy and are based primarily on mechanical phenomena. As a result an innovative family of chemical and biological sensors has emerged. In this article, we discuss sensors with transducers in a form of cantilevers. While MEMS represents a diverse family of designs, devices with simple cantilever configurations are especially attractive as transducers for chemical and biological sensors. The review deals with four important aspects of cantilever transducers: (i) operation principles and models; (ii) microfabrication; (iii) figures of merit; and (iv) applications of cantilever sensors. We also provide a brief analysis of historical predecessors of the modern cantilever sensors.

Cantilever transducers as a platform for chemical and biological sensors

A device is described which permits high- Q frequency selection to be incorporated into silicon integrated circuits. It is essentially an electrostatically excited tuning fork employing field-effect transistor "readout." The device, which is called the resonant gate transistor (RGT), can be batch-fabricated in a manner consistent with silicon technology. Experimental RGT's with gold vibrating beams operating in the frequency range 1 kHz 0 Q 's as high as 500 and overall input-output voltage gain approaching + 10 dB have been constructed. The mechanical and electrical operation of the RGT is analyzed. Expressions are derived for both the beam and the detector characteristic voltage, the device center frequency, as well as the device gain and gain-stability product. A batch-fabrication procedure for the RGT is demonstrated and theory and experiment corroborated. Both single- and multiple-pole pair band pass filters are fabricated and discussed. Temperature coefficients of frequency as low as 90- 150 ppm/°C for the finished batch-fabricated device were demonstrated.

The resonant gate transistor

The tunistor is a new mechanically resonant device which utilizes piezoelectric film transducers and a free-free flexural mode of resonance. Monolithic silicon tunistors have been demonstrated at frequencies up to 500 kHz with Qs to several thousand and temperature coefficients of frequency of 40 ppm/°C or less. Frequencies to several MHz should be feasible, and extensional modes should permit the frequency range to be increased even further. Metal tunistors which are compatible with hybrid microcircuits have been demonstrated down to several hundred Hz, and may be made relatively immune to external vibration. This paper describes the results obtained to date, and the technique used to fabricate tunistors.

The tunistor: A mechanical resonator for microcircuits

Singly resonant silicon tuning transistors have been reported. Multiresonant mechanical structures, properly constructed, can result in filters with flat passband and steep rejection slopes. We report a batch-fabricated twin cantilever RGT containing a coupling web near the clamped end. Under one beam is the electrostatic input electrode; under the other beam is the channel of the MOS detector. The beams can vibrate collectively in two modes. If the web width is adjusted properly, the two mode frequencies will be about fo/Q apart resulting in approximately flat doubly-tuned bandpass properties.

W.E. Newell

Papers

The resonant gate transistor

The tunistor: A mechanical resonator for microcircuits

A doubly-tuned coupled resonant gate transistor