Research On Passive Broadband Integrated Bulk Acoustic Wave Filter

Bulk Acoustic Wave（BAW）and surface acoustic wave（SAW）are the critical cores in acoustic filters,which provide pass band with low insertion loss（IL）and small rectangular coefficient by the electric connection of resonator.Owing to small size and high working frequency,acoustic filters become an important part of radio frequency front end（RFEE）,widely used in 4G communication.As the development of communication system,higher frequency and wider pass band are important requirements from 5G communication.Laterally excited bulk acoustic wave resonator（XBAR）is a novel type of BAW resonator with high electromechanical coupling efficiency using free-standing lithium niobate（LN）film.Compared to thin film bulk wave resonator（FBAR）or SAW,it is easy to generate wide pass band.Hence,XBAR is paid much more attention in these years.But there are some challenges in the development of XABR,such as spurious modes,large temperature coefficient of frequency（TCF）and low power durability.Toward a better application,this thesis discussed XBAR integrated with Bragg acoustic mirror,called laterally excited solidly mounted resonator（XSMR）,different from traditional solidly mounted resonator（SMR）.Researches include high coupling coefficient,spurious suppression,high performance acoustic mirror,high speed simulation methods and filter.1.Based on the wide band requirement from the application of 5G communication,high frequency bulk acoustic wave resonator with higher coupling coefficient is researched.Firstly,the analytical model and circuit model are derived in the condition of one-dimensional laterally excitation,used for the high-efficient calculation of critical parameters,including of resonant frequency and coupling coefficient,k².Then the methods to enhance k² are studied by different methods,including of cut angle of piezoelectric material and the dispersion of Lamb wave.Then a deep analysis of mass loading effect of electrodes is given.Finally,a XBAR with k² more than 30%is realized based on one-dimensional model.2.To solve the problem of spurious modes in XBAR,the mechanism and suppression technique is researched.Firstly,eigen modes and frequency domain analysis are done based on 2D/3D FEM model,to investigate the relationship between spurious and structural size.For high order modes,they are suppressed by optimization with multiple parameter of electrode.For transverse modes and transverse leakage,a novel broadband piston mode（BPM）is proposed.This structure provides a free boundary with nearly zero phase physically.Transverse leakage and transverse modes are both suppressed.Compared to conventional piston structure,BPM works well in wide band and quality factor is enhanced significantly.By the electrode optimization and BPM design,most spurious can be suppressed in simulation.3.To overcome the bottleneck of low power durability and low temperature stability of XBAR,solidly mounted resonator（XSMR）is researched toward high performance stack layers.Metallization type acoustic mirror and dielectric acoustic mirror are discussed saparatedly.For metallization type,a distributed capacitance model is built up to demonstrate the lose efficacy.Then dielectric mirror is modeled with different high acoustic impedance materials,including of Hf N,WN,WO₃,Hf O₂,and Ta₂O₅,et al.,based on the analysis of Bragg reflection.And a XSMR structure with high coupling coefficient and high-quality factor is proposed.At last,BPM is applied in dielectric mirror,to evaluate the effect of the suppression of transverse modes and transverse leakage.Its effectiveness is proved.Besides,the depth to width ratio of BPM etching is not so strict,it means an easier fabrication of BPM.4.To solve the requirement of end loss analysis in IDT device,and the calculation difficulties with massive degree of freedom（DOF）,long running time and the memory limitation,end loss,including of end radiation/scattering and end leakage,is researched by using hierarchical cascading technology（HCT）modeling（numerical mode）and Coupling of Modes（COM）modeling（physical model）.Firstly,a hierarchical model is built up for IDT device to calculate the response of ultralong IDT using HCT.The different loss is distinguished by quality factor.And mathematic fitting formulas are given to characterized different type of loss.Then a modified COM model is built up to modeling end leakage and extended to nonlinear calculation.This model has good performance in linear/nonlinear fitting.Hence,three methods to express end loss are developed.Finally,the relative technologies are also applied to XBAR/XSMR.The intrinsic loss mechanism and the approaches to higher Q are studied with IDT.5.Aimed at the structure design and fabrication of resonator filter,wide band acoustic wave filters are designed,fabricated,tested and evaluated.Firstly,two methods are proposed to realize different frequency units:one is the change of IDT period,and the another is mass loading of Si O₂.Filter design is simulated with 128YX-LN toward band n77/78.It is indicated that silicon dioxide approach is better with less spurious and 12.75%relative bandwidth in simulation.Finally,a bulk acoustic wave filter based on X-cut LN is produced with similar structure of XSMR to verified the dielectric mirror and prementioned analysis.The filter realizes 8.45%relative bandwidth,which verified the wide band ability of bulk wave resonator.At last,spurious in passband and the impact of distributed parasitic parameters are investigated for further improvements.Over all,this dissertation aimed at wide band bulk acoustic wave filter,different models are built up in different dimensions toward a high-speed simulation and design.A high coupling coefficient design,a novel structure（broadband piston mode）,and a high-performance acoustic mirror,are proposed to realize a XSMR resonator with high coupling coefficient and less spurious.The mechanism of loss of interdigital device are analyzed based on lossy model in HCT/COM.Relative technology is also applied in XBAR and XSMR to realize high speed simulation and loss evaluation.Then two kinds of frequency tuning methods in XBAR/XSMR filter are comparatively study,and the relative bandwidth of filter is about 12.75%in simulation.At last,resonators based on Si O₂/Hf O₂ stack layer and X-cut LN are fabricated and maximum relative bandwidth is8.45%in experiment.It is proved that the filter based on Li Nb O₃ thin film is promising for wide band by the further improvements.