Design and Fabrication of Si-Diaphragm, ZnO Piezoelectric Film-Based MEMS Acoustic Sensor Using SOI Wafers

This paper reports a simpler technique for fabricating an microelectromechanical system acoustic sensor based on a piezoelectric zinc oxide (ZnO) thin film, utilizing silicon-on-insulator wafers. A highly c-axis-oriented ZnO film of thickness 2.4 μm, which is covered with 0.2-μm-thick PECVD SiO₂, is sandwiched between two aluminum electrodes on a 25- μm-thick silicon diaphragm. This diaphragm thickness has been optimized to withstand sound pressure level range of 120-160 dB. Stress distribution studies using ANSYS have been performed to determine the locations for placement of capacitor electrodes. This paper also reports a technique for the creation of a positive slope of the ZnO step to ensure proper coverage during Al metallization. In order to maximize yield, process steps have been developed to avoid the microtunnel blockage by silicon/glass particles. The packaged sensor is found to exhibit a sensitivity of 382 μV/Pa (RMS) in the frequency range from 30 to 8000 Hz, under varying acoustic pressure.