Fabrication of c-axis oriented ZnO/AlN thin films prepared by radio frequency reactive sputtering and development of ZnO/AlN layered structure surface acoustic wave devices

Abstract

ZnO/AlN films have been continuously deposited on silicon wafers by rf reactive magnetron sputtering method. The c-axis preferred orientation of ZnO film with respect to the properties such as crystallinity and interface smoothness of AlN film at bottom has been examined by x-ray diffraction (XRD). The surface acoustic wave (SAW) filters with a wavelength of 52.8 μm using these ZnO/AlN bilayered structure have been fabricated for the first time and their SAW velocities and electromechanical coupling coefficients have been measured by a network analyzer. It is observed that as the thickness of AlN film increases, the average roughness of AlN film increases and the degree of the c-axis preferred orientation of ZnO film reduces. From these results, it is suggested that the factor determining the preferred orientation of top ZnO layer is the surface smoothness of bottom AlN layer. By measuring SAW characteristics of the ZnO/AlN bilayered structure, the phase velocity is calculated to be 4300 m/s and the electromechanical coupling coefficient is 3.23%, which are high enough for commercialization. It is also found that the velocity and the coefficient follow the better one of two films. The SAW filter fabricated in this work reveals good characteristics as the thickness of ZnO film increases and that of AlN film decreases.