Key Technologies Of High Frequency SAW Filter With Zno/diamond Multilayer Structure

Due to its small volume, good reliability and multifounction, surface acoustic wave (SAW) device has been widely used in radar, wireless and optical communication system. The rapid development of mobile communication system has increased the demand of high frequency filters with piezoelectric film/diamond substrate multilayer structure. In this thesis, the study is carried out on the key technologies of the preparation of the SAW filter with ZnO/diamond multilayer structure.Free-standing polycrystal diamond films were deposited using DC arc plasma jet CVD and the surfaces of the diamond were polished mechanically. The polished diamond films are with the high crystal quality, high diamond phase purity, large thermal conductivity (13.48 W/(cmK)) and low root-mean-square (RMS) roughness (0.2 nm). Those properties can meet the requirements of the high frequency and large power SAW devices.To obtain high quality ZnO films for high frquency SAW devices, the influence of the substrate sputter-etching and the post annealing with O2 on the morphology , microstructure, component, defect and electrical properties of the ZnO films was invageted by AFM,SEM,XPS,EDS,XRD,Raman,FTIR,PL and high-resistance instrument. The RMS roughness and defect of the films were both reduced, and the crystal quality and c-axis orientation of the films were improved by sputter-etching of the substrate. The internal stress of the ZnO films is reduced and the resistivity is increased to 1-2 orders by annealing of the film with O2.Simultaneous imaging of surface morphology, as well as the magnitude and phase of the out-of-plane and in-plane piezoresonse, was performed by piezoresponse force microscopy (PFM) on the c oriented ZnO films. The distribution histograms of the magnitude and phase of the piezoresponse were calculated to obtain a statistical description. The stronger out-of-plane piezoresponse with two primary antiparallel spontaneous polarization orientation (upward and downward), which is perpendicular to the surface of the film, has been found, and the spontaneous polarization orientation of most grains is upword. Meanwhile, the weaker in-plane piezoresponse has also been detected in some of the grains. Moreover, the relation of the morphology, piezoresponse amplitude and polarization orientation of the individual grains is discussed by drawing cross-sectional profiles from the topography and piezoresponse images. The relation of the phase of the piezoresponse and the spontaneous polarization direction (or crystal direction) is discussed by transferring the piezoelectric eqution from crystal coordinate to lab coordinate using Euler angle. The influence of the spontaneous polarization orientation on the micro piezelectricity of the film is explained. In addition, the influence of substrate temperature on the piezoelectricity of the c oriented ZnO film was also investigated using PFM. As the increase of the temperature, the spoutanous polarization of the piezoresponse is tranfacted from upward (O-face) to downward (Zn-face).The manufacture process of SAW filter, including the preparation of diamond substrate, the deposition of ZnO piezoelectric film and metal Al film and the etching of IDT, was studied. SAW filters with IDT/LiTaO₃ sturctrue and ZnO/IDT/Diamond multilayer structure were successfully fabricated. The central frequency and phase velocity of the SAW filter with ZnO/IDT/Diamond sturctrue is up to 2 GHz and 13600 m/s, respectively. Using the wider finger width of 1.7μm, the SAW filter with ZnO/IDT/Diamond sturctrue obtains two times higher central frequency than the traditional SAW filter the finger width of 1μm.