A high C-axial ZnO thin-film for piezoelectric sensor application

To effectively reduce the insertion loss of piezoelectric microsensors, this study aims to develop a high C-axis orientation ZnO piezoelectric thin film by magnetic RF sputtering system. The substrate temperature, RF power and argon/oxygen (Ar/O₂) flow ratio of sputtering process are varied and their influence on the grain size, pore density and X-ray diffraction (XRD) intensity of the eight sputtered ZnO thin films deposited on Si/Cr/Au substrate are investigated. Under the optimum condition obtained in this research (temperature: 100 °C, RF power: 350 W and Ar/O₂ flow ratio: 30sccm/13sccm), the average grain size of the optimized ZnO thin film is only about 74.7 nm and near zero pore density can be achieved. In addition, a very high XRD diffraction intensity (12,500 a.u.) and a 34.1° diffraction angle of the optimized ZnO thin film, which matches to the 34.422° diffraction angle of standard C-axis ZnO film, can be demonstrated in this work. Under the optimized ZnO deposition condition, a very low insertion loss (-11 dB) and center frequency (12.11 MHz) of the ZnO-based FPW device can be obtained.