Design, fabrication, and evaluation of a biologically-inspired piezoelectric MEMS microphone with in-plane directivity

This work examines the directional hearing capabilities of the fly Ormia ochracea and how they are applied to microelectronics. The fly can auditorily determine the direction of a cricket chirp, though the wavelength of the chirp is more than ten times longer than the length of the fly's hearing mechanism. The design, modeling, fabrication, and evaluation of a microphone that harnesses this fly's hearing ability are explored. The device consists of a two-sided cantilever beam that rotates about torsional pivots located along the y-axis. The result is two main frequency modes that can be used in the sound localization process for in-plane directivity in the x-direction: a gradient mode that causes opposite ends of the beam to move out-of-phase, and a symmetrical mode that causes the ends of the beam to move in-phase. Springs connect the free ends of the beam to the center pivot. Strain in these springs is converted to a voltage at the output. The microphones are fabricated on a silicon-on-insulator wafer with a 2 μm-thick device layer and 500 nm-thick aluminum nitride film as the piezoelectric transduction material. The active beam measures 500 x 250 μm², which approaches the dimensions of the fly's ear and is the smallest version of the microphone to date. The sensing modes are modeled with finite element analysis and confirmed in multipoint scans. Preliminary directivity measurements are made to demonstrate the directional capabilities