Hydrophones based on piezoelectric bulk materials are used for decades, mostly lead zirconate titanate (PZT) in form of cylinders or spheres are employed which are protected against water by a polymeric layer e.g. polyurethane, rubber, and neoprene . The main focus of our work is the development of thin film hydrophones especially for array applications with a high spatial resolution. Therefore, the sensors need to be small by showing similar properties as bulk hydrophones despite their small active volumes.
The first sensors were based on PZT produced by a chemical solution deposition. The sensors are tested in laboratory conditions as well as open water conditions with a bulk hydrophone as reference. PZT thin film sensor of 2 µm thickness showed a sensitivity around 8.9 µV/Pa (-221 dB re 1V/µPa), which is about a factor 2 to 8 worse than bulk hydrophones. To improve the sensitivity different materials were tested. Aluminium nitride (AlN) and aluminium scandium nitride (AlScN) were chosen, both are deposited by reactive magnetron sputtering [2,3].
Calculations using the measured piezo- and dielectric coefficients result in an improvement
of the sensitivity for AlN by a factor of 3 and for AlScN by a factor of 6 which was confirmed by acoustic measurements. AlN and AlScN sensors with a thickness of 2 µm are showing sensitivities between 17 and 44 µV/Pa (-215 and -207 db re 1V/µPa), which is in a similar range as the millimetre-sized PZT bulk hydrophones with sensitivities from 14 to 71 µV/Pa (-217 to -203 dB re 1V/µPa) .