| With the development of wireless communication technologies,such as 5G,Wi Fi,Bluetooth.Filters with high frequency,high power,low insertion loss,high passband roll down coefficient and small volume,have become the development trend in the future.Among common filters,Low Temperature Cofired Ceramic(LTCC)filter can achieve high frequency and high power,but its volume is slightly big and can't be integrated into RF modules.And the performance of LTCC filter is mediocre.Surface Acoustic Wave(SAW)filter has better performance than LTCC filter,but it's difficult to achieve high frequency and high power,due to its resonator's cross finger electrode width limitation.Bulk Acoustic Wave(BAW)filter stands out from kinds of filters,due to its advantages of high frequency,high performance and high power.At the same time,with the popularization of 5G,the number of communication frequency bands is increasing.As a result,isolation specifications between bands are increasing,and isolation requirements between bands are getting higher and higher.Therefore,filters are required to have higher passband roll down coefficient and greater suppression ability out of passband.To achieve these goals,BAW resonator with high Q value is required.In this thesis,Y43-cut single crystal Lithium Niobate(LN)was used as the piezoelectric layer of BAW resonator.Solidly Mounted Resonator(SMR)was selected as the research object,and studied the loss and performance of it.The main research contents include the following aspects:(1)By using Y43-cut single crystal LN as the piezoelectric film of the resonator,completed the design of C-band 7.5GHz resonator film thickness.Then,using Mason model with MBVD model loss term,the relationship between impedance,Q value of resonator and different loss was investigated qualitatively.(2)Two-dimensional piezoelectric effect simulation was carried out,aimed at researching the influence of the step structure on the resonator loss and performance.The effect of step structure on impedance curve of resonator was obtained.The impedance of parallel resonant point was increased by 4 to 9 d B,and the parallel resonant frequency was reduced.Step structure improves parallel resonant point impedance by reducing acoustic leakage loss at the frequency.The regularity of step was obtained through multiple sets of simulation: increase of step thickness has marginal effect on impedance increase;increase of step width has an optimal value for impedance increase;impedance increase effect of step is more effective for resonator with smaller area.(3)Make piezoelectric effect and solid heat transfer couple,that is,a piezoelectric thermal two-dimensional finite element simulation.Explored the method of reducing electrode loss by design.The distribution of loss in resonator was obtained by simulation.Then,proposed several design methods to reduce electrode loss: increasing the width of signal in/out terminals;parallel split of resonators;reducing electrode adhesion layer Ti thickness;interconnecting wire is connected with resonator electrode multiple edge,and balances the width of resonator signal in/out terminals.In addition,a smooth connection method was proposed,to solve dense ohmic loss at the junction of resonator electrode and interconnection wire.(4)The control resonator of electrode loss optimization method was prepared,and used Vector Network Analyzer(VNA)to test these devices.The resonator using optimization method reduced impedance at the series resonant point by 2.2d B,and improved Q value at the series resonant point by 70.The test result proved that the electrode loss optimization method is effective to improve series resonant point Q value. |
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