By combining substrate-free structures with anodic bonding technology, we present a simple and efficient micro-electro-mechanical system (MEMS) thermal shear stress sensor. Significantly, the resulting depth of the vacuum cavity of the sensor is determined by the thickness of silicon substrate at which Si is removed by anisotropic wet etching process. Compared with the sensor based on a sacrificial layer technique, the proposed MEMS thermal shear-stress sensor exhibits dramatically improved sensitivity due to the much larger vacuum cavity depth. The fabricated MEMS thermal shear-stress sensor with vacuum cavity depth as large as 525m and vacuum of 5×10 -2 Pa exhibits a sensitivity of 184.5 mV/Pa and a response time of 180 μs. We also experimentally demonstrate that the sensor power is indeed proportional to the 1/3-power of the applied shear stress. The substrate-free structures offer the ability to precisely measure the shear stress fluctuations in low speed turbulent boundary layer wind tunnels._____________________________ a) Author to whom correspondence should be addressed. Electronic mail: xiechangqing@ime.ac.cn.

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A substrate-free microelectromechanical system (MEMS) focal plane array (FPA) has been successfully developed. Since there is no silicon substrate beneath the FPA's pixels, the infrared radiation (IR) loss has been significantly reduced. However, at ambient pressure, it is difficult to obtain a clear visible image due to the sensitivity loss caused by the thermal conductance of air. To improve the packaged FPA's performance, its thermal sensitivity and response time were investigated under varying air pressure. The high-contrast IR thermal images of a human hand were captured at room temperature and under an air pressure of 0.01 Pa, and the noise-equivalent temperature difference of the FPA was also estimated to be about 373 mK compared with the 1.4 K under a pressure of 10 Pa. What is important is that the FPA's response time was increased from 50.1 to 336.6 ms by the decrease in air pressure. In addition, we present an approach to package the MEMS FPA at a low-pressure vacuum with two optical windows, which is an effective way to increase FPA's sensitivity.Index Terms-Focal plane array (FPA), microelectromechanical systems (MEMSs), response time, thermal sensitivity, uncooled, vacuum package.

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