Research On Bulk Acoustic Wave Filter Design Methods

The bulk acoustic wave(BAW)filter is composed of thin film bulk acoustic resonators(FBARs),which is an IC process-compatible new type filter and is crucial for the miniaturization,multi-functionality and integration for the RF front-end of the new generation personal mobile terminals.However,two difficulties of BAW filter R&D recognized are: precise design,precise process.This paper researched on BAW filter design method.The research results of the design method are shown by designing a WiFi band BAW filter.A method to modify FBAR Mason model is proposed to get an accurate modeling of FBAR.FBAR finite element model added mechanical loss and dielectric loss of piezoelectric layer is used to simulate.Admittance curve data in simulation is imported into the FBAR EM model by using equivalent permittivity method.The parameters of MBVD model are extracted by fitting impedance curve.FBAR Mason model is modified by MBVD Model parameters.A method for determining the order of a BAW ladder filter is proposed by studying on relationships among the out-of-band rejection and capacitance ratio(parallel/series FBAR),with order N.The out-of-band rejection and passband performance of the three topological structures(L-type,T-type,and ?-type)in the ladder filter are analyzed and compared.Wi-Fi band BAW filter is designed to research on BAW filter design method.The combined acoustic-electromagnetic method is used to simulate to verify filter performance.Aiming at the phenomenon that left transmission zero of BAW filter offset to the left in simulation result,we put forward the concept of ‘common ground inductance' effect and verified the existence of this effect in simulation and analyzed influence of common ground inductance on filter performance.An auto-layout tool is developed to realize the auto-layout and build the 3-D model of BAW filter for combined acoustic-electromagnetic simulation of BAW filter.The users only need to input the number of FBARs,FBARs area and cascade relationship,initial layout of the filter can be completed automatically.Then,users input FBARs thickness and other parameters,the filter 3-D model is automatically generated,which is imported into the EM simulation software for EM simulating.