Mechanical Performance of SiC based MEMS Capacitive Microphone for Ultrasonic Detection in Harsh Environment

In this project, SiC based MEMS capacitive microphone was developed for detecting leaked gas in extremely harsh environment such as coal mines and petroleum processing plants via ultrasonic detection. The MEMS capacitive microphone consists of two parallel plates; top plate (movable diaphragm) and bottom (fixed) plate, which separated by an air gap. While, the vent holes were fabricated on the back plate to release trapped air and reduce damping. In order to withstand high temperature and pressure, a 1.0 mu m thick SiC diaphragm was utilized as the top membrane. The developed SiC could withstand a temperature up to 1400 degrees C. Moreover, the 3 mu m air gap is invented between the top membrane and the bottom plate via wafer bonding. COMSOL Multiphysics simulation software was used for design optimization. Various diaphragms with sizes of 600 mu m(2), 700 mu m(2), 800 mu m(2), 900 mu m(2) and 1000 mu m(2) are loaded with external pressure. From this analysis, it was observed that SiC microphone with diaphragm width of 1000 mu m(2) produced optimal surface vibrations, with first-mode resonant frequency of approximately 36 kHz. The maximum deflection value at resonant frequency is less than the air gap thickness of 8 mu m, thus eliminating the possibility of shortage between plates during operation. As summary, the designed SiC capacitive microphone has high potential and it is suitable to be applied in ultrasonic gas leaking detection in harsh environment.