Investigation On Surface Acoustic Wave Devices Based On Multilayered Structures

Surface acoustic wave?SAW?device is an indispensable RF signal processing device in modern communication system due to its small size,excellent performance,good uniformity,and well reliability.The SAW filter fabricated using traditional piezoelectric single crystal faces the problems of low applicable frequency range and poor temperature stability.In this thesis,the SAW properties of two multilayered structures?piezoelectric film/diamond and SiO₂/LiTaO₃?were theoretically investigated and optimized in detail.Moreover,the super high frequency?SHF?SAW filters based on piezoelectric fim/diamond multilayered structure were designed and fabricated.The following are the research contents and results:Using stiffness matrix theory,the effective surface permittivity expressions for the piezoelectric film/semi-infinite substrate layered structures were given firstly under four excitation conditions.Then the method was extended to calculate and optimize the SAW properties for the piezoelectric multilayered structures with arbitrary number of layers and arbitrary thicknesses of the layers under different excitation conditions.The related calculation software was programmed for the optimized design of the SAW devices based on piezoelectric multilayered structures.The characteristics of SAW in multilayered structures of ZnO/polycrystalline diamond and AlN/polycrystalline diamond were systematically investigated and optimized using the self-owned software.The dependences of SAW phase velocity,electromechanical coupling factor and temperature coefficient of frequency?TCF?on the thickness of the piezoelectric layer were calculated under different electrical boundary conditions,different excitation modes and different piezoelectric material parameters.According to the theoretical optimized results,two types of SHF SAW filters based on IDT/ZnO/nancrystalline diamond and IDT/AlN/nancrystalline diamond structures were designed and fabricated.The central frequency of the fabricated SAW filter is up to 4.6 GHz at the interdigital electrode width of 0.5?m.The experimental measured data are in agreement with the design parameters,which verifies the correctness of theoretical calculation.The SAW properties in the novel periodic piezoelectric structure of?AlN/ZnO?_N/diamond?N is the periodicity of AlN/ZnO piezoelectric composite film?were theoretically investigated for the first time.It is found that this novel structure has better trad-off relationship between the SAW velocity,the electromechanical coupling coefficient and the temperature coefficient of frequency?TCF?,and thus can obtain better SAW properties than single layer/diamond structure in theory.Finally,the leaky SAW?LSAW?properties of multilayered structure of SiO₂/IDT/?°YX-LiTaO₃??is the rotation cut angle of LiTaO₃ substrate?were theoretically investigated.Furthermore,the two-dimensional optimizations were performed for the LSAW characteristics under the free and metallized electrical boundary conditions repectively.