High-sensitivity pressure sensing via frequency modulation in layer-transferred AlN membrane resonators

Abstract

This work reports on a novel fabrication approach for the realization of high-sensitivity frequency modulated (FM) pressure sensors based on aluminum nitride drumhead resonators with fully integrated piezoelectric actuation and readout. Our approach eliminates sacrificial layers and vacuum-sealed reference cavities by using metal–metal flip-chip bonding, where the cavity depth is defined by the thickness of the bonded metal. The shallow cavity, in equilibrium with the ambient environment, enhances the modulation of the frequency via the squeeze-film effect. Multiple resonance modes are characterized, with the (0, 1) mode achieving a high linear sensitivity of 13.2 ppm/Pa over a pressure range from vacuum to 33 kPa. This is the highest sensitivity reported for FM pressure sensors to date. The (1, 1) mode maintains a linear sensitivity of 2.3 ppm/Pa up to atmospheric pressure. This work also marks the first fully integrated FM pressure sensor to demonstrate spring stiffening with increasing pressure using electrical excitation and readout. The static and dynamic pressure sensing performance is validated in a closed-loop system using phase-locked loop measurements, enabling precise tracking of pressure-induced frequency shifts.