Research On Layered Surface Acoustic Wave Devices For Future Mobile Communication Systems

With the fast development of mobile communication systems,surface acoustic wave?SAW?devices are widely employed as key elements in smart mobile devices.To be capable of working at different countries or regions,smart devices need to support many communication standards,such as UMTS,HSPA,LTE,and so on.Besides,wireless LAN standards,positioning and navigation standards should also be supported,which means more than 40 frequency bands in one smart device.Moreover,with the widely application of carrier aggregation?CA?and multi-input-multi-output?MIMO?solutions,more than100 micro acoustic filters or duplexers will be embedded in one smart phone.The great demand has led to fierce competitions,not only between SAW manufacturers,but also between SAW and film bulk acoustic wave resonator?FBAR?manufacturers.The competitions are so fierce that improvement of current filter characteristics are made at pace of 0.05dB insertion loss/quarter or 0.1dB out band rejection/quarter.To satisfy the need for filters with low-insertion loss,more frequency bands and capability in analyzing transversal spurious modes,the following 3 topics will be discussed in the dissertation,?1?Novel substrate with adjustable SAW velocity and effective electro-mechanical coupling factor?K_e²?should be developed to take place of traditional 42^oYX-LiTaO₃substrates for applications in different frequency bands with different band width.By using an alloy target and adjusting the sputtering conditions,this dissertation successfully sputtered high quality ScAlN films on single crystal Si?100?substrate.However,measured Sc content was significantly lower than that of the target.Influence of the N₂content in the sputtering gas was also investigated,and the result indicated that nitridation of the target surface is at least one of the major reasons causing the reduction of the Sc content in the deposited films.Variation of deposited film qualities was observed with the accumulated sputtering time.Besides,due to the lack of accurate material constants,we deposited ScAlN films with different Sc concentrations,compared the experimental and simulation results,firstly verified the reliability of Caro's material constants in the world.?2?The traditional finite element method?FEM?/boundary element method?BEM?is no longer suitable for layered SAW device with complex configurations,establishment of a fast and accurate model for layered SAW devices is a necessary.This dissertation illustrates the common methods used for SAW device simulations by using the commercial software COMSOL.First,a brief introduction to the finite element analysis theory is given,then modal analysis combined with energy depth is employed for dispersion analysis.Analysis on mass loading effects is also given by using the aforementioned methods,where the K_e² can be enhanced by appropriately adjusting the relationship between the acoustic field and electric field.Then,traditional coupling-of-modes?COM?model is modified to consider the coupling between SAW and bounded BAW.The modified model is fulfilled in the PDE module of COMSOL,experimental devices verified the effectiveness of the modified model.?3?Transversal analysis is a key element in designing modern SAW devices,where unproper design of waveguides will lead to transverse modes,energy leakage and mode conversions.For the first time,the dissertation established a fast and accurate model for lateral SAW propagation.While the fast and accurate analysis method is the bottleneck for current SAW theory,this dissertation proposes a novel simplified model for SAW transverse propagation analysis,where it is derived from the piezoelectric constitution equations,equal to the COM model in the longitudinal direction,and equal to the improved scalar potential theory in the lateral direction.Moreover,this simplified model can be extended to analyze SAW propagation when several modes are coupled with each other.The parameters needed in the model can be extracted in the similar way with the traditional COM model,where relevant procedures are given in the dissertation.Furthermore,by using longitudinal resonance condition,the frequency dispersive anisotropic factor for SAW obliquely propagating under periodic gratings can be extracted.Last,the simplified model is fulfilled in the PDE module of COMSOL,and analysis for transverse on Al/128^oYX-LiNbO₃ and Cu/ScAlN/Si is given,which proved the effectiveness of the proposed model.