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

Basit Abdul,Francesco Rizzi,Antonio Qualtieri,Francesco Guido,Massimo De Vittorio,Vincenzo Mariano Mastronardi,Luciana Algieri

doi:10.3390/jmse8100784

Abstract

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 µm and 2 µm 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 µm 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.

Highlights

Hydrophones are underwater acoustic receivers that play an increasingly important role in submarine resources exploration, marine military, underwater noise monitoring, and sonar systems [1]
Hydrophone design and fabrication are based on micro-electro-mechanical systems (MEMS) technology, combining solid-state physics, mechanics, acoustics, and electronics [2] to detect underwater sounds
This is explained because a change on aluminum nitride (AlN) thickness leads to a shift in neutral axis position, due to a change in the whole beam stress due to different growth time and temperature

Summary

Introduction

Hydrophones are underwater acoustic receivers that play an increasingly important role in submarine resources exploration, marine military, underwater noise monitoring, and sonar systems [1]. Hydrophone design and fabrication are based on micro-electro-mechanical systems (MEMS) technology, combining solid-state physics, mechanics, acoustics, and electronics [2] to detect underwater sounds. The readout mechanism of micro-electro-mechanical system (MEMS) devices exploits piezoelectric or piezoresistive material as sensing element [3,4,5]. In 1996, a MEMS-based hydrophone was reported as the first directional underwater acoustic sensor [7]. Two T-shape vector hydrophone using MEMS technology inspired by fish lateral line, based on piezoresistive cantilever has been reported [10]. These hydrophones have receiving sensitivity up to −180 dB and −192 dB at 1 kHz, showing directivity pattern in the form of dipolar “8” shape. A micro-machined hydrophone based on piezoelectric PZT membrane was developed [13], and in order to improve the sensitivity, air backing was implemented

Methods

Results

Conclusion