Design, fabrication and reliability study of piezoelectric ZnO based structure for development of MEMS acoustic sensor

This paper reports a piezoelectric zinc oxide (ZnO) based acoustic sensor for high sound pressure level (SPL) with wide bandwidth for audio and aeroacoustic applications. The proposed structure has been simulated using finite element method (FEM) based MEMS-CAD tool Coventorware and optimized the dimensions of the diaphragm for a large dynamic range of 100-180 dB sound pressure level (SPL) and large bandwidth (22 kHz). The resonant frequency of the simulated structure is 67 kHz. Optimized structure dimensions have been chosen and fabricated the structure. The diaphragm structure has been released using bulk micromachining wet etching process. The ZnO thin film of 1.71 mu m has been deposited using radio frequency (RF) sputtering technique and characterized using X-ray powder diffraction (XRD) and atomic force microscopy (AFM). The resonant frequency of the fabricated device is measured using laser doppler vibrometer (LDV) and found to be 65 kHz. The experimental sensitivity of the fabricated device is measured and is found to be 80 mu V/Pa. The reliability testing of the fabricated structure was performed. The maximum current that can be passed across aluminum (Al) and deposited ZnO edge was found to be 2.69 A without any damage. The electrical characterization such as capacitance, loss (tan delta) of fabricated device parameters were measured and the effects of frequency variation on both were also studied.