Novel design method for sub-miniature MEMS microphones with enhanced acoustic SNR

Abstract

A well understood performance limitation in the conventional design of MEMS microphones relates to the dimensions of the back volume, which, along with the barometric vent, sets the ultimate limit of acoustic noise in the system, regardless of the microphone transduction principle. The acoustic noise in the back volume is generated from the heat flow within the thermal boundary layer near the isothermal walls. In this paper, a new and previously unused part of the back volume design space is explored, in which the primary loss mechanism is thermal (as opposed to viscous), and the effective thermal conductivity and heat capacitance of the gas in the volume is designed to achieve low acoustic noise in extremely small enclosures. In this new regime, the acoustic SNR in frequency bands of interest can be optimized to achieve values comparable or even larger than those in the conventional design space with much larger enclosures. The primary trade-off in design relates to achievable acoustic SNR versus necessary acoustic compliance with the selected transduction principle. Underlying principles and design examples are presented with supporting measurements from prototype devices, showing acoustic SNR in excess of 70 dB with a back volume of less than 0.2 mm3.