High Performance Piezoelectric MEMS Microphones.

Abstract

Piezoelectric microphones have been designed, fabricated using MEMS techniques, and tested. These designs are aimed at minimizing the noise floor as this has limited the applicability of previously constructed piezoelectric MEMS microphones. In order to minimize the noise floor, models of both cantilever and diaphragm based microphones are developed. This work presents a closed form solution for the sensitivity of a cantilever based microphone without making the small piezoelectric coupling assumption and provides criteria for determining the validity of this assumption. The complete microphone model includes a model of the amplifying electronics as well and, after examining the complete system, an optimization parameter is presented for the optimization of the MEMS transducer which can be applied to any amplifying electronics. This optimization parameter is then used to determine the optimal device material and geometry for a piezoelectric MEMS microphone. This optimization is used to design two generations of devices. The first generation device was a simple, 4 mask device used to work out processing issues mostly associated with aluminum nitride, the selected piezoelectric material. While these first generation devices did not have outstanding performance, measurements of the performance validated the models and highlighted the most relevant fabrication issues. The second generation device was a more complex, 7 – 9 mask process which allowed for further optimization over the first generation device. The fabrication issues experienced in the first generation device were solved by this design. Devices of various bandwidths and sizes were built but a design with a 790 μm x 790 μm area and an 18.4 kHz resonant frequency exhibited a 37 dBA noise floor and a design with a 960 μm x 960 μm area and a 12.4 kHz resonant frequency exhibited a 34 dBA noise floor. Compared to the lowest noise piezoelectric MEMS microphones previously reported, these microphones have a smaller area and noise levels more than 10 times lower making them appropriate for a wide range of applications.