Theory of a double-diaphragm MEMS ultrasonic transducer

Conventional capacitive MEMS transducer designs feature a single flexible diaphragm acting as a moving plate and an electrode patterned on the substrate acting as a stationary plate. Such devices generally function adequately as receivers. However, such devices show poor transmission efficiency when the gap dimension is limited by the manufacturing process and when the diaphragm would suffer "pull-in" collapse if deflected electrostatically beyond a critical fraction of the gap dimension. We describe a novel design, one that can be easily fabricated within an existing multi-layer polysilicon process, to achieve more efficient emission. The design features two flexible diaphragms to be actuated by a coordinated series of pulses. We report predictions of the improvement using dimensions characteristic of a multi-user surface-machined MEMS process.