Effects of residual stress on MEMS direction-finding sound sensors

The hearing mechanism of the fly Ormia Ochracea has been studied and mimicked using micro-electromechanical system (MEMS) technology as an acoustic direction-finding sensor. A typical MEMS sensor consists of two wings connected by a bridge with the entire mechanical structure connected to a substrate. The sensors employed in this work were fabricated on a silicon-on-insulator (SOI) substrate using MEMSCAP’s SOIMUMPS process which found to generate a residual-stress induced curvature on the wings. This reduces the overlap between the wing and substrate comb fingers, which affects the electronic signal output. An in-depth study was performed to understand the stress dependence on geometry and substrate and device configurations. A set of sensors fabricated with different configurations were systematically studied using optical profilometry and scanning electron microscopy. The measured profiles show offsets between the edge of the wings and substrates as high as 40 um for metalized devices and as low as 10 um for non-metallized devices. The curvatures of the sensors were fitted using finite element models to provide the stress values for each configuration and designing parameters to minimize this effect in future designs.