Fabrication of mems z axis accelerometer for vibration sensing

Abstract

Abnormal vibrations in rotating machines are indicative of undue newlinestresses in critical structural elements potentially leading to catastrophic newlinefailures. Consequently, measurement and characterization of vibration levels newlineprovides strong cues to detect anomalies and prevent failure. This work focuses newlineon development of a z-axis MEMS capacitive accelerometer for vibration newlinemeasurement. Optimal design and design-realization are two major steps to newlinebuild sensors for effective use in target application. Considering the design newlineaspect, planar MEMS structures for sensors usually have a large area compared newlineto thickness or gap. In such structures, squeeze film damping properties of the newlinegas significantly affects the performance of the device. Damping effects can be newlinereduced by perforations in the structure to reduce path-lengths of gas movement newlinein the narrow gap. But perforations in the proof mass reduce sensitivity, newlineTherefore, parameters enable trade-off between sensitivity and damping newlinecoefficient. This work studies different configurations of perforations for a newlinesquare accelerometer structure on a typical Silicon-on-Insulator substrate with newlineone µm air gap in the released structure. The ratio of perforation size versus newlineperforation pitch, and#414;, is used as a basis for comparing sensitivity and the damping newlinecoefficient. It is observed that and#414; in the range 0.3lt and#414; lt0.55 where the damping newlinecoefficient reduction is on the order of 90% to 97% while reduction in change newlinein capacitance is limited within 10% to 25%. Considering design-realization newlineprocesses, micro-machining of planar structures using photo-lithography and newlinewet-etching provides a simple and cost-effective approach. But wet-etching of newlinemono-crystalline Silicon exhibits anisotropic etch profile with erosion of newlineconvex corners of the planar structure. The resultant structure significantly newlinedeviates from the design leading to degradation in device performance. newline