Multimode Coupling Of Modes Model For Analysis Of Surface Acoustic Wave

As the mobile communication technique develops,the number of communication band also increases.This makes the distance between adjacent bands quite narrow.Thus,frequency characteristics of the surface acoustic wave?SAW?filter should have steep transition band and high out-band rejection to avoid the interference between adjacent bands.The main factors affecting the performance of current SAW resonator are:transverse mode,lateral?aperture direction?energy leakage,and energy loss caused by beam steering.They have stringent requirements on the design of lateral waveguide.Accurate simulation model with high speed and efficiency optimization method are the main factors for the refined design and fabrication of high performance SAW resonator.Precise model?such as:FEM/BEM and FEM/SDA?generally assume that the field quantities are independent of the lateral coordinate.They cannot be used to analyze effect of lateral waveguide.And,it is very complicated or even impossible to extend the model to include lateral propagation effects.Thus,precise model including the effect of lateral waveguide is still not realized.The simplified models represented by the Coupling-of-Modes?COM?model are widely used in device optimization design due to the fast calculation speed.With combination of the scalar potential theory,it can be used to analyze the lateral propagation of SAW.Free surface or metalized surface boundary conditions are widely assumed in COM analysis and the anisotropy factor is obtained by paraxial parabola approximation.However,there are gap,bus-bar,and grating regions in lateral direction of real SAW device.The electrical excitation and mass loading effect of grating cause frequency dispersion.The above assumptions cause some errors.Most of all,COM model is only applicable to the case where only one SAW mode exists.Multiple SAW modes can exist simultaneously and couple with each other on some substrates.The coupling effect has become a key factor to limit device design.However,no simplified model can quickly and accurately simulate and analyze the coupling effects now.Thus,the aim of the thesis is to build a fast and precise model which can be used to analyze the propagation characteristics when mode coupling effect exists in longitudinal?main propagation direction?and lateral directions.The following 5 topics will be discussed in the thesis:?1?Based on the piezoelectric constitutive equation and proper assumption,a phenomenological model is built to analyze the propagation characteristics with mode coupling in longitudinal and lateral directions.In the model,appropriate number of wave components are used to simulate the case where multiple SAW modes exist.The coupling effect is described by proper parameter setting.It is found that mode coupling has great influence on the behavior of slowness curve.?2?There exist many parameters in the model which are used to describe the longitudinal propagation,lateral propagation and coupling characteristics.Thus,the model parameters setting has great influence on the accuracy of the model.They are determined by fitting the dispersion curves in the thesis.As for the dispersion curves,they are obtained by calculating quasi three dimensional structure under the Floquet condition.?3?To confirm the validity of the model,it is applied to the 42oYX-LiTaO₃ piezoelectric substrate and analyze the effect of mode coupling on the propagation characteristics of the shear horizontal?SH?SAW.It is found that the shape of wavenumber frequency dispersion curve changes from concave to convex with electrode thickness.By including electrical excitation and detection,the model is used to analyze resonance characteristics.Three dimension finite element method?FEM?is used to calculate the real structure and the result are compared with those of the model.It is indicated that the mode can describe the dispersion curve excellently and predict the location of transverse mode excitation point accurately.Besides,the above analysis is applied to LiNbO₃ substrate.The mode coupling effect between Rayleigh and SH SAWs is researched.It is found that the coupling effect has great influence on the excitation of spurious mode.And,the coupling effect also causes the appearance of parasitic stopband,the intersection of the dispersion relation curve of frequency and lateral wavenumber,and variation of anisotropy factor.The model is applied to the substrate and it is found that the above phenomena are caused by acoustic coupling.Besides,the model can simulate and explain these phenomena.?4?Then,the model is applied to investigate Piston mode structure.The mechanism of traditional Piston mode structure is analyzed with the model firstly and the structure parameters on the performance of transverse mode suppression with Piston mode is researched.This proposes a new direction for device optimization.Under proper assumption,the excitation condition of the Piston mode with mode coupling is investigated by the model.Real structure is designed and the structure is analyzed by the model and FEM.The results are compared to confirm the mechanism and condition of Piston mode with mode coupling.The results also indicate that the coupling Piston mode can suppress transverse mode for wide frequency range.This also provides a new idea for the design of high performance SAW device.?5?Finally,the above model is used to estimate the power flow angle?PFA?on periodic grating structure.The traditional calculation method of PFA is realized based on the slowness curve under free surface or metallized surface conditions.The mass loading and modulation effects of grating are ignored.The model is extended to include the effect of PFA and to estimate the PFA under grating condition.The complex power flows in lateral and longitudinal directions are derived and PFA is estimated with the model parameters.The extended model is applied to the Quartz and Langasite substrates with non-zero PFA.It is found that grating has great influence on the behavior of PFA.