Surface Acoustic Wave Humidity Sensors Based on ZnO Piezoelectric Films Sputtered on R-Sapphire Substrates**Supported by the National Natural Science Foundation of China under Grant Nos 11174142, 11304160 and 11404147, the National Basic Research Program of China under Grant No 2012CB921504, the PAPD Project, the Natural Science Foundation of Jiangsu Higher Education Institutions of China under Grant No 13KJB140008, and the

Abstract

ZnO films on R-sapphire substrates are prepared and characterized by x-ray diffraction and scanning electron microscopy, which indicate that the thin films are well crystallized with texture. Love wave and Rayleigh wave are used for fabrications of humidity sensors, which are excited in and [0001] directions of the ZnO piezoelectric films, respectively. The experimental results show that both kinds of sensors have good humidity response and repeatability, and the performances of the Love wave sensors are better than those of the Rayleigh wave sensors at room temperature. Moreover, the theoretical calculations of the mass sensitivity of the sensors are also carried out and the calculated results are in good agreement with the experimental measurements.