Reseach On The Cavity Structure Of Bulk Acoustic Wave Filter And Its Wideband Technology

Fifth generation mobile communication?5G?technology has an urgent need for small size,high frequency,wideband and high performance film bulk acoustic wave resonator filter?FBAR filter?.It combines acoustic,radio frequency and MEMS tech-nology.At present,there is no costomized design platform,and it is difficult to realize high quality film deposition and complex processes.At the same time,FBAR filter is a typical narrowband device.How to increase its bandwidth to meet the requirements of wideband signal processing is also a research hotspot in recent years.By establishing FEM model,equivalent circuit model and full-wave simulation model,this paper systematically expounded the influence of material and structure on the performance of FBAR,and the influence of piezoelectric material,packaging,para-sitic vibration and circuit structure on the performance of FBAR filter.A high-precision simulation platform was established,which provides theoretical guidance for the reali-zation of wideband FBAR.Through the study of the microcosmic?such as structure,morphology?and macroscopic?such as stress,uniformity?characteristics of the key processes such as cavity structure,piezoelectric and electrode film deposition,frequen-cy adjustment,etc.,the material and process were realized,which can meet the re-quirements of excellent piezoelectric properties,high structural stability and low energy dissipation of FBAR.Finally,the accuracy of the simulation model was verified by the development and testing of conventional structure,high Q structure,high effective elec-tromechanical coupling coefficient(kt²_eff)structure FBAR and narrowband,high Q,wideband FBAR filter.In this paper,a series of achievements and innovations have been achieved through simulation,material deposition,process developing and measurement.1.Modeling and Simulation of FBAR and FBAR filterThe finite element model of the resonator is established.The effects of film thick-ness ratio,electrode area,structure and shape on kt²_eff,vibration mode and Q value of the resonator were simulated and analyzed.The optimal design method of FBAR with low parasitic vibration and high Q value was realized.The MBVD model was improved to take into account the effects of substrate and parasitic vibration.Full-wave simulation model was established to realize the cooperative simulation of FBAR chip and package,and the simulation accuracy of filter was improved.After that,the realization of wide-band FBAR filters was studied in terms of circuit structure and piezoelectric materials.2.Research on Key processesThe key processes,including cavity structure developing,piezoelectric composite films deposition and FBAR filter frequency trimming were studied.The main achieve-ments are as follows:?1?Three kinds of CMP methods,namely direct polishing,two-step polishing and reverse etching,were compared and analyzed to achieve small silicon groove dish,high uniformity and low surface roughness.?2?The effects of carri-er gas N₂ flow rate on the etching rate of amorphous silicon were analyzed by XeF₂ dry chemical vapor etching?CVE?process.The cavity structure can be fabricated efficiently.?3?The effects of plasma cleaning substrate and Mo electrode orientation on the orien-tation of AlN films and the effects of gas flow rate on the stress of AlN films were ana-lyzed.The 150 nm and 1?m thick AlN films can meet the requirements of FBAR filters in the range of 2⁹ GHz.?4?A method of splicing target is proposed to realize the arbi-trary adjustment of Sc content.The effects of seed layer induction on film orientation and Ar flow rate on film stress were analyzed.Then,high coupling ScAlN films were deposited.?5?The effects of seed layer and substrate film roughness on the preferred orientation growth of Mo electrodes were analyzed and the optimal deposition condition was achieved.?6?The thickness uniformity optimization of piezoelectric thin films and frequency tuning of filters were studied by ion beam scanning etching to achieve high consistency and accuracy of frequency on wafer-range.3.FBAR fabricationFBAR process flow was designed and high frequency,high Q value,and wide band FBAR devices were developed.?1?High frequency FBAR devices:the highest working frequency of narrow-band FBAR filter chips reached 9 GHz.Packaged FBAR filter working frequency reached 5.6 GHz.Its'IL is-1.7 dB,and rejection is greater than 30 dB.The measured and simulated results of the filters are in good agreement,which verifies the accuracy of the full wave model and the manufacturability of the process.?2?High Q value FBAR devices:the Q value of concave-convex structure in-creased by about 400 at f_s.The IL of high Q structure filter is about 0.27 dB smaller than that of conventional structure,and the shape factor(BW_-40dB/BW_-1dB)is reduced from 4.1to 2.72.The validity of high Q structure design method is verified.?3?Wide band FBAR devices:2.8GHz FBAR filters with BW_-1dB 3.92%were developed based on AlN thin films by matching method;2GHz FBAR filters with BW_-1dB 5%were developed using Sc_0.12Al_0.88N thin films and matching circuit;FBAR filters with Sc_0.2Al_0.8N thin films were developed with 3.935 GHz working frequency and BW_-1dB 5.39%.The valid-ity of wide band filter design method is verified.