Dual-frequency piezoelectric MEMS microphones based on AlSc30%N thin films for aeroacoustic measurements

Dual-frequency piezoelectric microelectromechanical systems (MEMS) microphones based on highly doped AlSc30%N thin films are developed for aeroacoustic measurements with high sensitivity, wide bandwidth, and large dynamic range. Comparing with the previous version based on lead zirconate titanate (PZT) thin films, the sensitivity of AlSc30%N based dual frequency MEMS microphones is significantly improved and no direct current bias is required for the new AlSc30%N material. The microfabrication, packaging, and testing of MEMS microphones with two pairs of triangular cantilevers were conducted using the cleanroom facilities and characterization tools. The MEMS die structure was characterized by a focused ion beam scanning electron microscopy (FIB-SEM). The resonant frequencies and mode shapes of large and small cantilevers were measured by laser Doppler vibrometer (LDV) to evaluate their mechanical performance. The aeroacoustic characterizations were conducted at the BACCHUS test bench in Airbus. The sensitivity of 3.2mV/Pa@30kHz, signal-to-noise ratio (SNR) of 60.2 dB (ref. 94 dB SPL, 30kHz), frequency range up to 55 kHz, and dynamic range up to 147 dB SPL were obtained in the first preliminary test. The MEMS microphone samples were also acoustically characterized within high-speed air flow. The overall performance of the dual-frequency piezoelectric MEMS microphones based on AlSc30%N thin films is better than the state-of-the-art microphones taking into account key parameters such as form factor, acoustic performance, and product price.