Ultraminiature AlN diaphragm acoustic transducer

Piezoelectric acoustic transducers consisting of a circular aluminum nitride and silicon nitride unimorph diaphragm and an encapsulated air-filled back cavity are reported. Analytical and finite element analysis models are used to design the transducer to achieve low minimum detectable pressure (MDP) within chosen size restrictions. A series of transducers with varying radii are fabricated using microelectromechanical systems (MEMS) techniques. Experimental results are reported for a transducer with a 175 mu m radius on a 400 x 500 x 500 mu m(3) die exhibiting structural resonances at 552kHz in air and 133kHz in water. The low-frequency (10Hz-50kHz) sensitivity is 1.87 mu V/Pa (-114.5dB re 1V/Pa) in both air and water. The sensor has an MDP of 43.7 mPa/root Hz (67dB SPL) at 100Hz and 10.9 mPa/root Hz (55dB SPL) at 1kHz. This work contributes a set of design rules for MEMS piezoelectric diaphragm transducers that focuses on decreasing the MDP of the sensor through size, material properties, and residual stress considerations.