Noise minimization in micromachined piezoelectric microphones

Abstract

Piezoelectric MEMS microphones have been built for more than 30 years and they offer some advantages over other technologies such as improved linearity, simple construction, no need for a bias voltage or charge, and the ability to withstand high temperatures. Despite these advantages, a relatively high noise floor has always limited their utilization. Traditionally, the noise of these sensors has been minimized by viewing the pre-amplifier or amplifier as a black box with fixed gain and noise. This leads the designer to minimize noise by maximizing microphone sensitivity. By viewing the microphone-amplifier system together, we develop a different method of optimization, leading to lower noise. Further, by including the back cavity compliance of the package in the optimization, we can determine absolute limits on the minimum achievable noise floor with very few assumptions. To date, we have built piezoelectric MEMS microphones utilizing aluminum nitride with a 32 dBA noise floor. We can compute a minimum achievable noise floor of 24 dBA for the same sensing structure with a 2 mm3 back cavity volume.