Characterization of ZnO piezoelectric films prepared by rf planar-magnetron sputtering

Abstract

ZnO films with an excellent crystal orientation and surface flatness have been prepared by high-deposition-rate rf planar-magnetron sputtering. A detailed study of these films has been carried out using x-ray diffraction, scanning electron microscopy, reflection electron diffraction, optical measurement, and electromechanical measurement. These films have the c-axis perpendicular to the substrate. The value of the standard deviation angle σ of the c-axis orientation distribution is smaller than 0.5°, and the minimum value of σ is 0.35°, where the sputtering conditions are that the gas pressure is 5×10−3–3×10−2 Torr of premixed Ar (50%)+O2(50%) and the substrate temperature is 300–350 °C. ZnO films with a thickness up to 48 μm have been reproducibly prepared without the decreases of film quality and surface flatness. The surface flatness of these films is similar to that of a glass substrate. An optical waveguide loss for the TE0 mode of the He-Ne 6328-Å line is as low as 2.0 dB/cm in a 4.2-μm-thick film, without postsputtering treatment. The effective surface wave coupling factors are greater than 95% of the theoretical values of the ZnO/glass structure, both in the case of the interdigital transducer (IDT)/ZnO/glass and ZnO/IDT/glass structure.