Characterization of aluminum nitride film properties for high signal-to-noise ratio piezoelectric micro-electromechanical system microphones

The Signal-to-Noise Ratio (SNR) of piezoelectric microphones is significantly dependent on the material properties of the piezoelectric film, namely the piezoelectric coefficient, the dielectric constant, and the dielectric loss. This work presents a metrological approach for the characterization of Aluminum Nitride (AlN) films used as sensing elements in high SNR piezoelectric Micro-Electromechanical System (MEMS) microphones. A dynamic measurement method based on micro-Laser Doppler Vibrometer (LDV) was adopted to evaluate the piezoelectric d33 coefficient of 500 nm thick AlN films deposited by reactive magnetron sputtering over conductive cantilever test samples. The LDV measurement results were also compared against the d33 value determined by a Piezo Evaluation System coupled to a single point laser vibrometer, exploiting the converse piezoelectric effect. The accurate and precise evaluation of the piezoelectric properties, together with the proper mechanical design of the MEMS microphone, is fundamental to provide reliable estimations of the electroacoustic performances, in terms of SNR, dynamic range, and frequency response. Furthermore, a metrological approach for the evaluation of the measurement uncertainty of the piezoelectric coefficient allows predicting its contribution to the uncertainty associated with the electroacoustic characteristics of the MEMS microphone.