Lamb-wave resonator for microphone application

We study the pressure sensitivity of AlN thin-film resonators on a poly-Si membrane sublayer. The studied resonators operate on the S₀-mode Lamb wave at 750 MHz. The resonators are fabricated on a released membrane which consists of a polysilicon sublayer, molybdenum electrodes, and AlN as the piezoelectric layer. Operation of the test structures is simulated using 2D and 3D finite element method models. Simulated pressure sensitivity at pressures below 1 Pa is 0.035 ppm/Pa. Pressure sensitivity of the fabricated test devices was measured by probing the devices on-wafer and measuring their electric frequency response while applying a variable differential pressure over the wafer. To remove the noise and the outliers, the data was filtered with a 3-point median filter prior to analysis. The linearized pressure sensitivity of frequency below 1 Pa is around 1 ppm/Pa. With minimized residual stress, optimal materials and modified membrane design, the pressure sensitivity can be improved.