Micromachined sensors and actuators have been investigated and industrially produced for more than twenty years. The most widely used sensing principle, especially for pressure sensors, is piezoresistive effect of the silicon, due to its simple fabrication process, measurement circuit and so on. To date, devices based on piezoresistive strain sensors are mostly realized on single-crystal (c-Si) substrates. However, a major problem of this approach is that significant drift is produced by the leakage current of the pn junctions used to isolate the piezoresistors. Therefore, the low-pressure chemical vapor deposited polycrystalline silicon (poly-Si) was used to solve this problem using insulator, such as oxide, but not pn junctions to isolate the piezoresistors. However, because of the in...[ Read more ]

Micromachined sensors and actuators have been investigated and industrially produced for more than twenty years. The most widely used sensing principle, especially for pressure sensors, is piezoresistive effect of the silicon, due to its simple fabrication process, measurement circuit and so on. To date, devices based on piezoresistive strain sensors are mostly realized on single-crystal (c-Si) substrates. However, a major problem of this approach is that significant drift is produced by the leakage current of the pn junctions used to isolate the piezoresistors. Therefore, the low-pressure chemical vapor deposited polycrystalline silicon (poly-Si) was used to solve this problem using insulator, such as oxide, but not pn junctions to isolate the piezoresistors. However, because of the inferior electrical properties of the material, integration of mechanical sensing and electronic signal-processing devices on such poly-Si has not been seriously considered.

In our work, the metal-induced lateral crystallization (MILC) of amorphous silicon (a-Si) is developed to form poly-Si with significantly improved material properties. Further enhancement is obtained by subjecting MILC poly-Si to a high-temperature re-crystallization (RC) step. RC-MILC poly-Si piezoresistors with improved gauge factor, reduced noise, lower temperature coefficient of resistance and transistors with performance approaching those built on c-Si have been obtained. The nickel concentration effect in transistors was also investigated.

An RC-MILC poly-Si piezoresistive pressure sensor with integrated amplifier has been designed, fabricated, and tested. The integrated one-stage amplifier has DC gain of about 30dB. Airborne sound sensitivity of 50μV/Pa and a flat frequency response within ±3dB between 100 Hz and 8 kHz has been measured. Many methods are proposed to further increase the sensitivity by reducing the residual stress of the sensing diaphragm.

At last, another device, silicon capacitive microphone, is developed. The equivalent circuit of the microphone was developed, including acoustic, mechanical and electrical parts. A very simple fabrication is developed. The device shows sensitivities of 300μV/Pa with bias voltage 8V.