In this paper, we report on the characterization of the sensitivity and the directionality of a novel ultrasonic hydrophone fabricated by microelectromechanical 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 stressgradient through cantilever thickness, leading to an outofplane 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 stressdriven piezoelectric cantilevers, combined with an opportune algorithm for processing all data sensors, permits a finer directionality response of this hydrophone.

中文翻译：

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基于压电超声波悬臂的MEMS水听器在水下应用中的灵敏度和方向性分析

在本文中，我们报告了通过微机电系统（MEMS）工艺，使用氮化铝（AlN）薄膜作为压电功能层并采用应力驱动设计的新型超声波水听器的灵敏度和方向性的表征。水听器的结构和制造由四个交叉配置的压电悬臂组成，其在水中的第一共振频率模式设计为​​20 kHz至200 kHz。MEMS制成的结构利用嵌入在两个钼电极之间的1 µm和2 µm厚的压电AlN薄膜，该两个钼电极通过在硅（Si）晶片上的直流磁控溅射法生长。200 nm厚的钼电极薄层增加了悬臂厚度的应力梯度，导致悬臂外弯曲。在每个悬臂上沉积1 µm的防水聚对二甲苯保形涂层，以进行防水操作。AlN向上弯曲的悬臂显示出最高灵敏度，最高可达−163 dB。四个应力驱动压电悬臂的交叉配置，结合用于处理所有数据传感器的适当算法，使该水听器具有更好的方向性响应。