Theoretical Investigation On Longitudinal Surface Acoustic Wave Resonator Based On Lithium Niobate/Silicon Carbide Acoustic Heterostructure

In recent years,with the widespread deployment of 5G mobile communication networks,new challenges have been presented to the design of surface acoustic wave（SAW）filters in the RF front-end.In order to meet 5G communication standards,SAW filters need to have a higher operating frequency（3～6GHz）and a larger bandwidth.In response to the low working frequency band and relatively small bandwidth of SAW filters,this paper proposes a new type of heterogeneous acoustic layer structure"IDT/LiNbO₃ directions/SiC"to guide the conversion of longitudinal leakage surface acoustic waves into non-leakage longitudinal surface acoustic wave.Utilizing the characteristics of high sound velocity of longitudinal surface wave,large electromechanical coupling coefficient and non-leakage,a 5GHz ultra-high frequency,wideband,high Q single-ended SAW resonator was designed,laying an important technical foundation for the development of high-performance SAW filters.The specific research contents of this paper are as follows:This paper analyzes the propagation characteristics of three kinds of bulk waves（slow-shear bulk waves,fast-shear bulk waves and longitudinal bulk waves）in semi-infinite LiNbO₃ substrates with different cut directions,and finds the cut angle when the electromechanical coupling coefficient of the longitudinal body wave is relatively large.And then the conversion mechanism of longitudinal body wave and longitudinal leakage surface wave is explored,and the energy leakage mechanism of LLSAW in the optimized tangential LiNbO₃ substrate is analyzed.Using the effective dielectric constant method,the relationship between the LiNbO₃ tangential and longitudinal surface wave propagation characteristics in the LiNbO₃/SiC structure was calculated,and the optimized LiNbO₃ tangent was obtained,and the LLSAW energy leakage mechanism in the corresponding tangential LiNbO₃/SiC structure was further analyzed.Using COMSOL multiphysics finite element calculation software,a 2.5-dimensional periodic model of IDT/LiNbO₃/SiC structure was established,and the optimized cut-down LiNbO₃/SiC structure was calculated.The relationship between LiNbO₃ film thickness,electrode structure parameters,and LLSAW propagation characteristics and attenuation the coefficients.By optimizing the LiNbO₃ film thickness and electrode structure parameters,the longitudinal leakage surface wave was successfully converted into the non-leakage longitudinal surface wave,the attenuation coefficient was greatly reduced,and the Q value was increased.According to the theoretical optimization results of the LiNbO₃ film thickness and the electrode structure parameters,the admittance of the IDT/LiNbO₃/SiC structure was calculated,the parasitic mode caused by the electrode was found,the reason was analyzed,and the parasitic mode suppression scheme was given.Finally,using COMSOL software,the IDT/LiNbO₃/SiC structure quasi-three-dimensional finite-length single-port resonator model was established,its admittance characteristics were calculated,and the influence of the number of reflection grids and the number of IDT fingers on its electrical characteristics was analyzed.The simulation results show that the effective electromechanical coupling coefficient of the single-port resonator made by longitudinal surface wave can reach up to 22%,and when the electrode width is 0.3μm,the resonant frequency can reach up to 5.1GHz.