Preparation And Characterization Of Surface Acoustic Wave (SAW) High Temperature Pressure Sensor Based On Aluminum Nitride Film

Surface Acoustic Wave(SAW)sensor is a kind of energy-exchange filter made of piezoelectric effect and the physical characteristics of SAW propagation.However,the traditional piezoresistive Si-based MEMS pressure sensor uses the semiconductor characteristics of Si,which leads to poor temperature stability and cannot work stably at high temperature above 100°C.Therefore,it is difficult to meet the increasing pressure measurement in high temperature scenes.As a piezoelectric material,AlN film has excellent physical and chemical properties.However,the piezoelectric constant d₃₃and the electromechanical coupling coefficient K² are still low.This paper will focus on the study of modified AlN film process and technology to produce SAW piezoelectric sensor based on AlN piezoelectric film.The research content is to deposit AlN piezoelectric film with(002)orientation on sapphire substrate by reactive magnetron sputtering.The optimal experimental conditions for the growth of AlN piezoelectric film with(002)orientation are found by controlling the deposition temperature and working pressure.AlN film was characterized by X-ray diffraction(XRD);Scanning electron microscopy(SEM);X-ray photoelectron spectroscopy(XPS)and step profiler etc.The interdigital transducer was designed and patterned by lithography technology.The Pt electrode and Ta adhesion layer were deposited by DC magnetron sputtering.The SAW sensor with Pt/Ta/AlN/Sapphire structure was characterized by optical microscope and scanning electron microscope,and its characteristic frequency was measured by network analyzer.The results show that the substrate temperature and working pressure have great influence on the crystallization quality and preferred orientation of AlN piezoelectric films.When the substrate temperature was raised from 300°C to 400°C,the crystal quality of AlN first grew better along the(002)crystal plane and then deteriorated.When the working pressure decreases from 0.45 pa to 0.25 pa,the crystal quality of AlN(002)crystal plane first becomes better and then becomes worse.The optimum conditions for AlN(002)preferred orientation are:working pressure 350 W,Ar:N₂=20:4 sccm,substrate temperature 375°C,working pressure 0.35 Pa.The piezoelectric coefficient of AlN thin films prepared under the optimum process parameters is 25 p C/N.The characteristic frequency of SAW sensor prepared under the optimum process is 258 MHz,which is close to the design frequency of 250 MHz.S11 parameter values range from-4.1 d B to-13.89d B.In summary,this paper studied the preparation of piezoelectric film materials of SAW sensors,and proposed clear solutions to some problems encountered in the application of SAW sensors,which provided theoretical and technical basis for the subsequent application of high temperature sensors.