| The thin fim bulk acoustic wave filters are widely used in radio frequency（RF）front-end system in the field of communication because of their advantages of high frequency,high power and small size,and play an important role in both civil and military scenarios such as mobile phones,pilotess aircrafts,vehicle communications,base station equipments,individual equipment,space-borne and missile-borne communication.With the development of 5G communication technology and the increasing demand for information integration of national defense equipment,the frequency band for communication is widening day by day,and the number of thin film bulk acoustic filters used in multifunction communication equipment is increasing sharply.Therefore,the RF front-end system has higher and higher requirements for the bandwidth of thin film bulk acoustic filter.However,the current Al N based thin film bulk acoustic filters have a small bandwidth and is usually used as narrowband filters,which are difficult to meet the needs of wide band applications.In view of this situation,the design and preparation of wideband filter based on single crystal LiNbO3（LN）material was carried out in this work.The main work are as follows:（1）The finite element model of the resonator and the equivalent circuit model of the filter were established.The structure parameters of the device and the topology structure of the circuit are systematically studied.The design and simulation of the material,the resonator and the filter were realized.The effect of single crystal LN cutting orientation on electromechanical coupling coefficient and sound velocity was analyzed by material simulation.The results indicated that 43°Y-cut can achieve the optimal comprehensive performance of resonator.Through the simulation and analysis of the longitudinal and transverse structure of solidly mounted resonator（SMR）,the influences of functional layer thickness,electrode material,electrode edge effect,resonant area and electrode shape on the performance of the resonator were obtained.It provides guidance for the structure design of resonator.The relationship between the design parameters of the basic topological unit and the transmission characteristics of the ladder type filter was analyzed by the Mason model with modified loss terms.An initial structure design flow of the filter was established,which laid a foundation for the structural design of the filter.（2）The fabrication process of SMR based on Bragg reflector structure was established and the influence of reflector material combination on the figure of merit of the resonator was studied.Through the simulation design of LN single crystal ion implantation energy parameters,reflection layer combination and number of layers,benzocyclobutene（BCB）bonding layer was selected to form the SMR process.SMR structure was studied systematically in two classes of reflector,which were metal-containing reflectors of SiO2/W,SiO2/Mo and all-dielectric reflectors of SiO2/HfO2、SiO2/Ta2O5,respectively,on the transmission and reflection characteristics of sound waves.The results showed that SiO2/W structure had large stress and roughenss,and small effective electromechanical coupling coefficient(Keff2)with figure of merit of 10.65;and the SiO2/Mo structure had small stress and large roughenss,and large Keff2 with figure of merit of 19.27;and the SiO2/HfO2 structure has large stress and small roughenss,and small Keff2,and the figure of merit reached 38.42.（3）A wideband filter topology circuit with metal-containing reflector of SiO2/Mo was designed,and the influence of the parasitic capacitor introduced by metal layer of Mo on the bandwidth and insertion loss was studied.A filter sample was prepared by unpatterned reflector technology.The test results showed that the parasitic effect was obvious in this structure,and the filter relative bandwidth can reach 6%,but the loss is high.The seventh-order and ninth-order wideband filter topology circuits of SiO2/HfO2all-dielectric reflector structure were designed,and the effects of order on filter insertion loss,bandwidth and out-of-band attenuation were studied.Based on the analysis of the effects of LN thickness on the frequency shift,surface roughness on the insertion loss and Al electrode thickness on the bandwidth,a filter sample was prepared by using the unpatterned reflector process.The test results showed that the bandwidth of the filter in this structure could exceed 10%,and the insertion loss was 2.2 dB,which realized the wideband and low loss of the filter.（4）In order to realize the ultra-wideband filter of solidly mounted type,the effect of on-chip inductance on the effective electromechanical coupling coefficient of the resonator,the combination of reflected layer thickness on the resonator quality factor and the electrode step structure on the transverse parasitic mode of the resonator were studied.The results showed that the effective use of of the parasitic inductance in the resonator can significantly increase Keff2,up to 43.28%.The longitudinal leakage of shear wave can be weakened and Q value can be increased by adjusting the layer thickness of all-dielectric reflector.The step structure at the edge of the electode can reduce the intensity of the transverse acoustic resonant mode and restrain the generation of parasitic mode.By extracting the parameters of the resonator model,a design method of ultra-wideband and low loss filter based on the parasitic inductor was established.Two kinds of filter with the bandwidth of 200 MHz and 500 MHz were designed,with the relative bandwidth of7%and 14%and the insertion loss of 2 dB and 1.459 dB,respectively.It provides a new idea for the application of solidly mounted type bulk acoustic wave filter in N78frequency band. |
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