A surface-micromachined capacitive microphone with improved sensitivity

We present a surface-micromachined capacitive microphone with a membrane center hole and back-plate supports. The proposed membrane center hole reduces air damping at the center of the membrane and increases the sensitivity and frequency response. The back-plate supports allow for a deep back-chamber and prevent deformation of the back-plate. The proposed microelectromechanical-system (MEMS) microphone is fabricated using fully CMOS-compatible processes. The fabricated MEMS microphone has a membrane 500 mu m in diameter and a center hole 30 mu m in diameter. A deep back-chamber with a depth of 100 mu m is formed by the back-plate supporting structures. During fabrication, the residual stress of the membrane is minimized using PECVD silicon nitride inserted in the metal membrane. The measured residual stress of the sensing membrane is 14.8 MPa. Acoustic measurements show that the sensitivity of the microphone is -49.1 dBV Pa-1 @1 kHz at a 12 V dc bias voltage, which is in good agreement with the calculated value.