A 100-MHZ ULTRASONIC TRANSDUCER ARRAY USING ZNO THIN-FILMS

A 100-MHz ultrasonic linear transducer array made from a piezoelectric zinc oxide thin film on a sapphire substrate was developed and evaluated. Epitaxial, high-acoustic quality 10-mu m-thick ZnO film layers were produced by rf-magnetron sputter deposition onto a (111)-oriented gold film (with a chromium adhesion layer) that was vacuum-evaporated onto a (0001) sapphire surface. We found that, in well-oriented growth of gold, it is important to control the chromium sublayer thickness (less than 5 nm). An array was constructed by photolithography with an appropriate etch. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCI and HNO3-based) that preferentially etched the c-plane of ZnO. Typical array elements were 90 mu m wide, 3.2 mm long, and 10 mu m thick, and the pitch of an array was typically 100 mu m. Our fine uniform array resulted in uniform ultrasonic response of individual elements throughout the array. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 mu m at the focal depth, agreeing well with theoretical predictions. Besides the use demonstrated with this present transducer, piezoelectric thin films should also lead to fabrication of various other kinds of ultrasonic transducers that can operate at high frequencies and should provide opportunities for miniaturizing transducers and making integrated ultrasonic devices.