Toward a micro-scale acoustic direction-finding sensor with integrated electronic readout

Abstract

Several advances are made toward a microelectromechanical (MEMS) acoustic direction-finding sensor based on the Ormia ochracea fly’s ear. First, linear elastic stiffness models are presented and then validated by using a nanoindenter to measure the sensor’s stiffness directly. The measured stiffness is highly linear, and the resonant frequencies are correctly predicted by the models presented. Additional nanoindenter results suggest that the sensor can be exposed to at least 162 decibel sound pressure level with no loss of function. Next, an improved capacitive readout system using branched comb fingers is presented. This design is shown to double electrical sensitivity to motion. Finally, it is shown that residual stress-induced curvature in the sensors greatly reduces their sensitivity by effectively shrinking the readout capacitors. A simple model of this curvature is presented and then verified by measurements. This model offers an extremely straightforward means of predicting curvature in similarly fabricated structures. It is also shown that perforations in the sensor’s structure have no effect on curvature. The results presented here provide several essential tools for the continued development of the MEMS acoustic direction-finding sensor.