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

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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.