Dual-mode vertical membrane resonant pressure sensor

Abstract

This paper presents a novel dual-mode resonant pressure sensor operating based on mass loading of air molecules on transversely resonating vertical silicon membranes. Two silicon bulk acoustic resonators (SiBAR) are acoustically coupled through thin vertical membranes, resulting in two high-g resonance modes with small frequency split, but large difference in pressure sensitivity. The membranes are designed to couple 180° out-of-phase vibrations of piezoelectrically-transduced SiBARs through pressure-insensitive extensional Lamb waves and without changing their resonance frequency. The in-phase vibrations, on the other hand, induce a high-order pressure-sensitive transverse flexural resonance in vertical membranes while slightly changing the resonance frequency of SiBAR due to stiffness and mass loading. A combinatorial of the two modes is used as a pressure sensor with an amplified sensitivity. A proof-of-concept device implemented on a 20 μm silicon substrate and activated by a thin aluminum nitride film shows a combinatorial beat frequency (fb) of 1.3 MHz with a linear pressure sensitivity of 346 ppm/kPa over 0-100kPa range.