Research On AlN Thin Film Layered Surface Acoustic Wave Devices Based On Finite Element Method

Due to its excellent features such as high performance,low loss,minimization etc.,surface acoustic wave（SAW）devices are widely applied in radar,communication and sensing fields,in particular,the use as the key component of modern mobile communication system has been paid more and more attention.For development of SAW devices to meet high frequency and high performance requirements and to gain a high market share,SAW researchers are always seeking new piezoelectric materials and structures with a very high phase velocity（V）and a large effective electromechanical coupling factor（K²）,as well as small temperature coefficient of frequency（TCF）.Up to now,none of the conventional bulk piezoelectric single crystal can simultaneously possess the above advantages,while AlN piezoelectric thin film based layer structures make it possible and thus become the current mainstream study of SAW devices.Currently,numerous new kinds of complex layered structure continuously turn up,which requires a universal simulation tool to analyze SAW devices based on those complex layered structure.The finite element method（FEM）is quite feasible for such complex cases,however,since the scale difference between SAW devices and their wavelength is almost two orders of magnitude,it requires tens of millions degree of freedoms for a full three-dimension（3D）FEM model,which suffers from the challenge of large requirement for computer memory and CPU computation.Therefore,in this dissertation,considering the FEM advantage,a reduced（quasi-3D）FEM model is built assuming that the aperature of inter-digital tranducer is infinite long and field variables is unchanged along the aperature direction,which enables accurately and fast analyze the complex layered SAW devices by employing commercial FEM software COMSOL Multiphysics.This thesis centers on research on AlN thin film layered SAW devices and achieves the work in the following topics:（1）To seeking for the layered piezoelectric substrate that possessing multiple dood SAW characteristicses including high V,large K²and small TCF etc.,a general quasi-3D periodic model applicable to arbitrary complex layered structure is built for accurate and fast analysis of SAW properties.On basis of the validated model,SAW properties SAW modes propagating along the AlN thin film layered structure are theoretically calculated.Thorough research on the influences of AlN thin film thickness,combination of materials and structures as well as piezoelectric coupling configurations on SAW properties of AlN-based layered structure have been performed,and the investigation results suggest that the K² of Rayleigh mode for buried electrode configuration is larger than that of the top electrode configuration.Furthermore,we creatively propose a novel layered structure with embedded electrodes and Si O₂ filling layer.The proposed structure not only overcome the problem of crack in AlN film caused by the buried electrodes,but also offer passive temperature compensation effect.It is shown that high V,large K²and small TCF can be simultaneously obtained on the proposed layered structure.（2）FEM is unable to analytically remove static electricity and bulk acoustic wave contributions from acoustic fields and isolate SAW mode contribution like Green’s function analysis.In order to analyze SAW scatterirng from nonsynchronized or a small number of gratngs on layered structure,a general wavenumber domain analysis（WDA）quasi-3D finite-length model is proposed for accurate and fast calculating SAW scattering characteristics.Based on the validated WDA model,scattering analysis is applied to calculating Sezawa wave scattering from a few electrodes on AlN-based layered structure for the first time.The simulation results demonstrate that the proposed layered structure is quite favorable for application in high-frequency and wide-band SAW devices.In addition,the investigation results also show that there is no efficient reflection for Sezawa wave due to considerable mode conversion to Rayleigh wave.This feature also can be useful for design of some special SAW devices.（3）On basis of FEM Multiphysics coupling analysis tool,the models for analysis of temperature and pressure frequency behaviors are modeled by makeking the bias fields effects directly coupled to piezoelectric equation in form of coupling variable of initial stress and strain.The accuracy of the proposed model for sensitive behavior prediction is validated by comparing with experimental results.Moreover,a SAW pressure sensor employing slot-cantilever with bionic spider’s sensilla as pressure-sensitive structure and AlN-based layered structure as piezoelectric substrate is proposed and analyzed.The simulation results validate that the proposed SAW pressure sensor can achieve four times higher pressure sensitivity,meanwhile zero TCF and large K² are simultaneously obtainable.In conclusion,this thesis proposes and builts a general quasi-3D FEM models for accurate and fast analyzing arbitrary layered structure SAW devices,which provides theoretical foundation for development of SAW devices with high frequency and high performance.