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

摘要

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 $mu{rm m}$, which is covered with 0.2-$mu{rm m}$-thick PECVD ${rm SiO}_{2}$, is sandwiched between two aluminum electrodes on a 25- $mu{rm m}$-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 $mu{rm V/Pa}$ (RMS) in the frequency range from 30 to 8000 Hz, under varying acoustic pressure.