High-frequency ultrasonic transducer arrays using ZnO thin films

The 100 MHz linear transducer arrays described here were produced by rf-magnetron sputter deposition of a 10 μm thick ZnO film onto a (111)-oriented gold film (with a chromium sublayer) vacuum-evaporated onto (0001) sapphire surface. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCl and HNO₃-based) that preferentially etched the c-plane of ZnO. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused to obtain a half-amplitude width of 60 μm at the focal depth, agreeing well with theoretical predictions