Non-planar diaphragm structures for high-performance silicon pressure sensors.

Abstract

The demand for micromachined solid-state pressure sensors has been escalating in many fields. Researchers are being called on to develop techniques for such devices allowing better performance. This dissertation reports research work aimed at the understanding and realization of high-performance silicon pressure sensors. The principal approach of this research has been to apply the finite element method to the integrated design and analysis of sensor diaphragms. A set of modeling and simulation techniques have therefore been developed and implemented in a software module, CAEMEMS-D. These techniques have been shown to be highly accurate in predicting the performance of complex diaphragm structures over a wide number of conditions. Two types of high-performance pressure sensors have been developed. The first type utilizes non-planar diaphragms having corrugated-bossed structures. These boron-doped silicon diaphragms, 1mm on each side, 3$\mu$m in thickness, and containing five 10$\mu$m-deep corrugations, produce deflections of more than 30$\mu$m at 760Torr. With thinner diaphragms and higher corrugation frequencies, deflections of more than 50$\mu$m should be achievable. Devices of the second type are ultra-sensitive capacitive pressure sensors which utilize an ultra-thin stress-compensated dielectric diaphragm with a silicon boss. The sensors exhibit pressure sensitivities of more than 5fF/mTorr (1370ppm/mTorr), at least five times greater than other recent devices. The operating range of the devices has been studied from 0 to 1000mTorr, which covers the range of sound pressure levels and complements existing barometric devices. The temperature coefficients of zero-pressure offset and pressure sensitivity of these devices are 910ppm/$\sp\circ$C and $-$2900ppm/$\sp\circ$C, respectively, and can be reduced to 15ppm/$\sp\circ$C and $-$63ppm/$\sp\circ$C using digital compensation techniques. The insights gained in this study of non-planar diaphragm structures have helped improve our understanding of the electro-mechanical behavior of these devices and should lead to further improvements in future pressure sensors. The FEM modeling and simulation techniques established in this study are expected to benefit the continuing development of pressure sensors having new structures and targeting new applications.