Fabrication and characterization of an indium tin oxide acoustoelectric hydrophone

Clinical ultrasound (US) imaging and therapy require a precise knowledge of the intensity distribution of the acoustic field. Although piezoelectric hydrophones are most common, these devices are limited in terms of, for example, type of materials, cost, and performance at high frequency and pressure. As an alternative to conventional acoustic detectors, we describe acoustoelectric hydrophones, developed using photolithographic fabrication techniques, where the induced voltage (phase and amplitude) is proportional to both the US pressure and bias current injected through the device. In this study a number of different hydrophone designs were created using indium tin oxide (ITO). A constriction of the current path within the hydrophone created a localized "sensitivity zone" of high current density. The width of this zone ranged from 30 to 1000 μm, with a thickness of 100 nm. A raster scan of the US transducer produced a map of the acoustic field. Hydrophones were evaluated by mapping the pressure field of a 2.25 MHz single element transducer, and their performance was compared to a commercial capsule hydrophone. Focal spot sizes at -6 dB were as low as 1.75 mm, comparing well with the commercial hydrophone measurement of 1.80 mm. Maximum sensitivity was 2 nV/Pa and up to the 2nd harmonic was detected. We expect improved performance with future devices as we optimize the design. Acoustoelectric hydrophones are potentially cheaper and more robust than the piezoelectric models currently in clinical use, potentially providing more choice of materials and designs for monitoring therapy or producing arrays for imaging.