Sensitivity and Directivity Analysis of Piezoelectric Ultrasonic Cantilever-Based MEMS Hydrophone for Underwater Applications

In this paper, we report on the characterization of the sensitivity and the directionality of a novel ultrasonic hydrophone fabricated by micro-electro-mechanical systems (MEMS) process, using aluminum nitride (AlN) thin film as piezoelectric functional layer and exploiting a stress-driven design. Hydrophone structure and fabrication consist of four piezoelectric cantilevers in cross configuration, whose first resonant frequency mode in water is designed between 20 kHz and 200 kHz. The MEMS fabricated structures exploit 1 mu m and 2 mu m thick piezoelectric AlN thin film embedded between two molybdenum electrodes grown by DC magnetron sputtering on silicon (Si) wafer. The 200 nm thick molybdenum electrodes thin layers add a stress-gradient through cantilever thickness, leading to an out-of-plane cantilever bending. A water resistant parylene conformal coating of 1 mu m was deposited on each cantilever for waterproof operation. AlN upward bent cantilevers show maximum sensitivity up to -163 dB. The cross configuration of four stress-driven piezoelectric cantilevers, combined with an opportune algorithm for processing all data sensors, permits a finer directionality response of this hydrophone.