Deposition of high-quality zinc oxide thin films on diamond substrates for high-frequency surface acoustic wave filter applications

Surface acoustic wave (SAW) device based on diamond is attracting much interest because diamond has the highest elastic constant of all substrates and the SAW velocity is more than 10,000 m/s. Although diamond is not piezoelectric, its high acoustic propagation makes it a desirable substrate for SAW device when coupled with piezoelectric thin films such as zinc oxide (ZnO) thin film. It is well known that, for these devices, performance is critically dependent on the quality of ZnO films. In this paper, highly-oriented, dense, and fine-grain polycrystalline ZnO thin films with excellent surface flatness and high resistivity were produced on diamond substrate by reactive magnetron sputtering. The properties of ZnO films were characterized by X-ray diffraction, high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The influences of deposition condition are discussed. Interdigital-transducer/ZnO/diamond layered structure was fabricated, which are expected to be capable of wide bandwidth application in SAW devices at a high frequency of about 2.5 GHz.