A 100-MHz ultrasonic transducer array using ZnO thin films

摘要

A 100-MHz ultrasonic linear transducer array made from anpiezoelectric zinc oxide thin film on a sapphire substrate was developednand evaluated. Epitaxial, high-acoustic quality 10-Μm-thick ZnO filmnlayers were produced by rf-magnetron sputter deposition onto an(111)-oriented gold film (with a chromium adhesion layer) that wasnvacuum-evaporated onto a (0001) sapphire surface. We found that, innwell-oriented growth of gold, it is important to control the chromiumnsublayer thickness (less than 5 nm). An array was constructed bynphotolithography with an appropriate etch. V-shaped grooves betweennadjacent elements were formed by using an anisotropic etchant (HCl andnHNO3-based) that preferentially etched the c-plane of ZnO.nTypical array elements were 90 Μm wide, 3.2 mm long, and 10 Μmnthick, and the pitch of an array was typically 100 Μm. Our finenuniform array resulted in uniform ultrasonic response of individualnelements throughout the array. For a 32-element array, the ultrasoundnbeam in the azimuth plane in water could be electronically focused innthe 100 MHz range to obtain a half-amplitude width of 60 Μm at thenfocal depth, agreeing well with theoretical predictions. Besides the usendemonstrated with this present transducer, piezoelectric thin filmsnshould also lead to fabrication of various other kinds of ultrasonicntransducers that can operate at high frequencies and should providenopportunities for miniaturizing transducers and making integratednultrasonic devices