Research On Preparation Of High-quality Bulk Acoustic Wave Filter Based On AlN Film Grown By Two-step Method

The mainstream frequency bands of global mobile communication are mainly in the range of 0.9～6GHz.With the rapid commercialization of the fifth generation mobile communication technology（5G）around the world,the scale of the mobile information market is constantly expanding,and the service forms are constantly changing.More and more functional modules are added to wireless terminals,and the overall utilization frequency band is constantly developing towards high frequency.Compared with traditional cavity filter,dielectric filter and surface acoustic wave filter（SAW）,bulk acoustic wave filter（BAW）has the advantages of smaller volume,lower insertion loss,integration,high quality factor,high out-of-band rejection,high operating frequency and high power tolerance,etc.It is the most potential RF filter in 5G medium and high frequency（>3GHz）communication.At present,the thin film bulk acoustic resonator（FBAR）filter based on polycrystalline aluminum nitride（Al N）is the most successfully used in BAW filters,and has been widely used in the 4G communication era.However,there are still many problems in chip structure design,Al N material growth,chip process,which restrict the development of FBAR filter towards higher frequency and higher power capacity.The key research direction of FBAR filter in the 5G era is how to establish a more effective equivalent model of FBAR filter,further improve the quality of Al N piezoelectric film,simplify the preparation process of FBAR filter and improve the product yield.Therefore,this thesis focuses on the design and design inspection method of Al N-based FBAR filter on Si substrate,the growth of high-quality single-crystal Al N film,and the micro-machining（MEMS）technology of FBAR filter.The main achievements are as follows:（1）The design of FBAR filter and the method of design inspection are studied,in which ADS software based on Mason equivalent circuit model is used.According to the application requirements and key indicators,combined with the theoretical basis,the simulation model is established and optimized,and then the initial sample is prepared.By testing the prepared samples,the design parameters are deduced,and then compared with the design parameters before preparation,the relationship between design and samples is established.Namely design and design inspection.In this paper,through the design of Band40 and N79 filters,the thickness ratio between electrodes and piezoelectric films,effective electromechanical coupling coefficient,cascade mode and the influence of inductance on resonators and filters are studied,and a systematic and effective design inspection methods are established.Combined with device preparation,the circular verification between chip design and preparation is carried out to realize the optimization and improvement of filter design.（2）High quality aluminum nitride（Al N）films were grown by metal organic chemical vapor deposition（MOCVD）and physical vapor deposition（PVD）.Firstly,the preparation and characterization of Al N bulk acoustic resonators grown by PVD were studied,including the effects of RF power,argon flow and nitrogen flow on Al N（002）orientation,surface roughness and stress,as well as the effects of film repair on Al N surface roughness and stress.Secondly,in order to overcome the bottleneck of further improving the quality of sputtering Al N,a two-step growth method combining MOCVD and PVD technology was designed to grow Al N films on Si substrates.By systematically researching the surface morphology and crystallization quality of Al N films grows by two-step method,the growth mechanism of Al N which combines with Si substrate at high temperature and grows at low temperature is revealed.The high temperature environment of MOCVD can provide enough energy for the reaction atoms to reach the energy window which is suitable for the growth of single crystal materials,thereby preparing high quality single crystal Al N thin film buffer layer with low defect density.Then continue to grow Al N using PVD.Because the single crystal Al N buffer layer has smooth surface and low defect density,it can improve the wettability of Al N surface,weaken the surface migration barrier of Al atom,improve the mobility,and promote the two-dimensional growth of Al N thin film,and then get high quality Al N piezoelectric thin film.At the same time,the half aspect ratio of Al N（0002）orientation is reduced by half only by magnetron sputtering.Finally,a high quality Al N film with half peak width of 0.68°of Al N（0002）swing curve is prepared.（3）The FBAR resonators and filters with the high quality Al N films were prepared by MEMS process,meanwhile their transmission performance was researched.Thus getting the optimal conditions of each key preparation process.The Q value of that resonator reaches 3456,which is138%higher than that of the resonator prepared by ordinary sputtering Al N film,and the k²_eff is 5.3%higher.The reason lies in the defects in the high quality Al N films prepared by the two-step method and the grain boundaries are much lower than those of PVD sputtering Al N films,so the loss of BAW reduces.The overall research route of this paper is the design of FBAR filter and the design test method,the preparation of high quality Al N film,MEMS fabrication process optimization of devices,the device test and analysis.By optimizing the design process,analyzing the mechanism of the two-step growth method of high quality Al N piezoelectric film,debugging and optimizing the fabrication process of MEMS,high-quality FBARs and filters are fabricated,which provides an important reference for the growth of high quality Al N film on Si substrate and the preparation of filters.