Research On High Frequency SAW Device Based On ScAlN Film

With the rapid development of communication technology,the upgrade of network communication has become more and more demanding for RF front-ends including SAW filters,duplexers,power amplifiers,switches,and low-pass filters.Among them,the filter is an important component of the RF front-end,and the surface acoustic wave(SAW)filter has gradually become the mainstream choice by virtue of its own advantages,and is developing toward miniaturization,high frequency,large bandwidth,and low insertion loss.Among many piezoelectric materials,aluminum nitride(Al N)films have the highest acoustic velocity,and their longitudinal sound velocity reaches 10400 m/s,which is the preferred piezoelectric film material for fabricating high frequency SAW filters.However,due to the limitations of the Al N film itself,the piezoelectric coefficient is lower than that of piezoelectric materials such as zinc oxide(Zn O),lithium niobate(Li Nb O₃),and lead zirconate titanate(PZT).In recent years,the doping of Al N by metal scandium(Sc)has gradually become a research hotspot to improve the piezoelectric properties of Al N films.In this paper,the ScAlN film is deposited by DC magnetron sputtering method.The process parameters are optimized by varying the sputtering power,working pressure,the ratio of nitrogen gas to argon gas flow,and then X-ray diffraction analysis and atomic force microscopy and piezoelectric microscopy tests comprehensively characterize film quality from crystal orientation,surface morphology and roughness,and piezoelectric properties.The growth law of ScAlN film under different parameters was studied to obtain the best growth conditions.The results show that under the process parameters of sputtering power of 180w,working pressure of 0.3Pa and nitrogen to argon flow ratio of 15:15,the C-axis optimal orientation of ScAlN film is the strongest,and a piezoelectric shape variable of 43.7 nm was obtained at a polarization voltage of 10 V,and good piezoelectric properties were obtained.On the basis of the optimal ScAlN film growth process,a high frequency SAW resonator was fabricated.On the one hand,an IDT structure with a reflective gate was designed on the platinum substrate.The interdigital finger width was 500 nm and the operating frequency was1.887 GHz.The insertion loss of the device is reduced to-50.935d B.On the other hand,a 300nm line-width interdigital electrode was fabricated on a diamond substrate with a resonant frequency as high as 7.48 GHz.The influence of the interdigitated pair and the dummy structure on device performance was investigated.In conclusion,the piezoelectric properties of Al N films are effectively improved by the doping of scandium elements,and it provides a certain reference value for the application of ScAlN films in SAW devices.