Hydrophones are becoming increasingly important in today's world, especially in areas like cost-effective protection of port infrastructure and underwater traffic monitoring in port environments.

Current state-of-the-art bulk hydrophones are made of PZT (lead-zirconate-titanate) ceramics with several millimeters of edge lengths, which makes them not only bulky but harder to fit into existing structures. Also, the material contains lead, which should be avoided in underwater applications due to environmental issues. Another problem is the non-form-fitting positioning of the hydrophones. The aluminum nitride (AlN)-based thin-film hydrophones offer solutions to both problems. They are fabricated without lead with a typical thickness of 2 μm of the piezoelectric AlN and can be applied to surfaces in a form-fitting manner.

The AlN layers are deposited in a reactive pulsed DC magnetron sputtering process without additional substrate heating, which allows deposition on heat-sensitive layers and substrates [1]. The sputtered films are then characterized structurally and functionally, for which XRD (x-ray diffraction) rocking curves are used to determine the structural properties of the layers, DBLI (double beam laser interferometry) measurements to determine the piezoelectric and dielectric properties, and tan(δ) measurements using an impedance analyzer. A low FWHM (full width at half maximum) of less than 2° of the rocking curve confirms a preferred columnar c-axis growth of the AlN layer. A high clamped piezoelectric coefficient d33,f of 5.7 pm/V illustrates the good piezoelectric properties of the AlN thin films. By optimizing the presented sputtering process, it is possible to achieve tan(δ) values below 0.1 % with low internal film stress.

The presentation will introduce the properties of these thin-film hydrophones and compare them to those of conventional hydrophones. In particular, approaches will be presented to improve the SNR of the thin-film hydrophones.