Research On Thin Film Bulk Acoustic Wave Devices For Millimeter Waves

The commercialization of 5G technology in China has promoted the continuous trend of miniaturization and integration of wireless radio frequency terminals.Different from traditional resonators and surface acoustic wave filters(Surface Acoustic Resonator,SAW),thin film bulk acoustic wave filters(Thin Film Bulk Acoustic Resonator,FBAR)have a high quality factor,small size and easy integration,low power consumption and high power,High out-of-band suppression and low insertion loss,and is compatible with integrated circuit technology Therefore it has received widespread attention and has become a research hotspot in the industry.The current 5G technology has two frequency bands,one is the Sub-6GHz frequency band,the other is the millimeter wave(mm Wave)frequency band,which is above 24 GHz.5G technology is rapidly spreading in the Sub-6 GHz frequency band in China,but few reports about millimeter wave frequency band have been found.On the other hand,the domestic research on FBAR started late,and most of the commercial FBAR devices are concentrated in low frequency bands.The independent production of millimeter-wave frequency FBAR devices are still confronted with great challenges.Therefore,this paper proposes to design and prepare a thin film bulk acoustic wave filter that fits the millimeter wave frequency band.The main innovations and research results in the work are as follows:First,the basic structure and equivalent model of the FBAR device are analyzed,and a simulation library for the layers of the composite structure is established based on the Mason model.Studies have been made about the effection of several factors on the performance of the device through ADS electrical simulation,including the types,thickness,area of piezoelectric materials and electrode materials.The design of the device is constantly adjusted based on the simulation results,and various parameters is coordinated to reach a balance which leads to some regular patterns about the capabilities especially the operating frequency of the device.Mo was selected as the electrode layer material and AlN as the piezoelectric layer material.And the feasibility of the device structure to achieve millimeter wave frequency operation was demonstrated.Second,the complete process flow of the film bulk acoustic resonator facing the millimeter wave frequency band is realized,from the etching of the substrate sacrificial layer,to the thinning of the sacrificial layer,the growth of the bottom electrode,the growth of the piezoelectric layer,and the top electrode The growth,and finally the release of the sacrificial layer.For the growth of the piezoelectric layer,in order to ensure the crystalline quality of the piezoelectric film,a reverse photolithography mask is used,that is,a layer of AlN is sputtered uniformly and completely on the bottom electrode,and then the exposed area AlN is removed by an ion etching process.And here,for the etching of the piezoelectric layer,a self-aligned method is proposed,instead of photoresist protection,the top electrode of the upper layer is used as a mask as a result of its high hardness and excellent etching selectivity for etching,thereby exposing the etching pits of corrosion through holes.Third,the piezoelectric film which is based on AlScN doped withScis used as a contrast to the FBAR device based on the AlN film.As an emerging material developed on the basis of AlN,AlScN introduces scandium(Sc)into aluminum nitride to improve the electromechanical coupling coefficient and piezoelectric properties of the material,thereby increasing the bandwidth and the conversion efficiency of mechanical energy and electrical energy.The electrode materials of both groups of FBAR devices are 40 nm Mo,and the piezoelectric layers are 100 nm AlN and 100 nm AlScN respectively.S-parameter tests are performed on both sets of devices,and the results show an oscillating effect in the highfrequency region.