Study On Miniaturized And High Temperature Stable Thin Film Bulk Acoustic Filter

With the rapid development of mobile communication and defense electronic equipment system technology,the communication frequency band has been further extended to higher frequencies（S and C band）.The Film Bulk Acoustic Resonator filter（FBAR filter）is the key device to filter out other frequency bands and malicious interference signals.In the face of increasingly complex operating environment,military and civilian communication systems,radar,navigation and other equipment put forward higher requirements on the temperature stability and volume of FBAR filters.Due to the low temperature stability of the conventional FBAR filter（large frequency temperature coefficient）,it is necessary to increase the protection bandwidth of the system signal,increase the interference signal entering the receiving end,deteriorate the signal-to-noise ratio and sensitivity,increase the bit error rate of the whole machine,shorten the communication and detection distance,so that the communication system and equipment system can not work normally in harsh environments.At the same time,the volume of the conventional FBAR filter is large,so that the volume of the system is also large,which cannot be further miniaturized.Therefore,it is urgent to solve the problem of temperature stability and volume of FBAR filter.In this dissertation,by adding a temperature compensation layer with positive frequency temperature coefficient,the frequency temperature coefficient of FBAR filter is significantly reduced.By establishing the finite element simulation model,Mason model,equivalent circuit model,miniaturized wafer level package model and full-wave co-simulation models of different temperature conditioning structures,a complete simulation platform of temperature compensation thin film bulk acoustic resonator（TC-FBAR）is established to provide theoretical guidance for the development of TC-FBAR.Based on conventional FBAR,the preparation methods of composite films with different temperature compensation structures were systematically studied,and the high-quality composite films meeting the requirements of TC-FBAR is prepared.The performance of TC-FBAR with twin-sandwich temperature compensation structures is analyzed.In this dissertation,model building,design simulation,composite film preparation,process production,packaging and testing were conducted.The main achievements and innovations are as follows:Firstly,the Finite Element Modeling（FEM）simulation model of TC-FBAR with different temperature compensation structures is established;The effects of materials,thickness and structure of the film on its effective electromechanical coupling coefficient and Q value of the resonator are analyzed.A miniaturized Wafer Level Package（WLP）model of TC-FBAR is established,and the effects of Surface Mounted Device（SMD）on the performance are analyzed.The full-wave co-simulation model of TC-FBAR filter is obtained,and the WLP design of TC-FBAR filter is realized.Secondly,a variety of high-performance single-layer functional films and the final low-stress composite films is prepared.The main research results are as follows:（1）Before preparing Mo film at the bottom electrode,the substrate was pretreated and a seed layer of Al N film was grown to achieve Mo film preparation with high preferred orientation and low roughness;（2）The effects of different orientation,sputtering power and thickness of the Al N seed layer on the orientation of the piezoelectric film are analyzed.The c-axis（002）orientated growth of the piezoelectric Al N film is realized.（3）The preparation methods with different temperature compensation layer films are studied.The PECVD deposition method is used to prepare Si O₂ films of temperature compensation layer.The test and analysis showed that the stress of Si O₂ films is compressive stress.（4）Al N film is used as the adhesive layer when growing the Si O₂film,which ensures the adhesion ability of the temperature compensation layer film;（5）In the preparation of other films,the film structure is optimized by modifying the deposition process parameters,changing the film stress to tensile stress,compressive stress and tensile stress in a single layer film compensation effect,and achieving the low stress composite film preparation,preparation of composite film stress（|P|）in less than150 MPa.Lastly,the manufacturing processes of TC-FBAR with twin-sandwich temperature compensation structures is designed,and the manufacture of low-temperature compensation coefficient（TCF）and miniaturized TC-FBAR filters were realized.（1）Low TCF TC-FBAR filter with a central frequency of 2.493 GHz,an insertion loss of-2.94 d B,an-1d B bandwidth of 11.2 MHz,an out-of-band rejection（2.2～2.43&2.55～2.8 GHz）greater than 40 d Bc,and a TCF of frequency at only 0.96 ppm/℃in the whole temperature range（-55～125℃）.The measured curve of TC-FBAR filter is in good agreement with the simulation curve,which verifies the accuracy of the simulation model and the feasibility of the manufacturing process.（2）Miniaturized FBAR filter by using the silicon wafer gold bonding to form a cavity,so that air tight wafer-level miniaturization package is achieved.The minimum size of FBAR filter is 0.8×0.8×0.4mm³,which is 3 times smaller than that in a conventional surface mounted device.The feasibility of the design method and process path of miniaturized wafer level packaging is verified.