Preparation And Research Of Flexible Surface Acoustic Wave Device Based On AlN Thin Film

The potential application of intelligent wearable electronic devices in human health monitoring and nursing systems has attracted wide attention.Flexible wearable devices with good compatibility and stability with human body has been developed better.The rapid development of real time communication wearable devices has put forward higher requirements for flexible high frequency communication devices.At present,the main problems faced by flexible surface acoustic wave(SAW)devices are the preparation of piezoelectric materials on flexible substrates and the improvement of the performance of high frequency SAW devices.In this paper,based on polyimide(PI)thin film substrate,the deposition process optimization of AlN piezoelectric thin film material with high transverse sound velocity is mainly carried out.Combined with interdigital transducer(IDT)design,a flexible high frequency SAW device is constructed,which belongs to one of the research hotspots in the field of flexible high frequency devices.In this thesis,reactive magnetron sputtering was used to deposit AlN thin films on PI substrates.The effects of growth temperature,sputtering power,sputtering pressure and nitrogen and argon ratio on the physical properties of AlN thin films was mainly optimized.XRD,AFM,EDS and other test results show that the sputtering power is the key factor at room temperature,and the AlN thin film(002)had a good orientation with a surface roughness(RMS)of 3.2 nm and an Al/N element ratio of 1/1when the sputtering power is 240 W.The optimized process conditions are as follows:sputtering power 240 W,sputtering pressure 0.3 Pa,N/Ar ratio 2/1;The electrical characteristics test of AlN piezoelectric film shows that the butterfly curve window is full and the relative piezoelectric coefficient d*₃₃=8.01 pm/V;And the piezoelectric properties of AlN thin films remained stable after bending radius was 25.47 mm and10,000 cycles.In this thesis,"Ti/AlN/Mo/PI"flexible SAW device is constructed based on"electron beam exposure system and lift off process",and the effects of different logarithm,line width and aperture of IDT on the transmission performance of the device are studied.The test results of electrical characteristics of SAW devices show that with the increase of logarithm,the central frequency intensity of SAW devices increases,the relative bandwidth of main peak decreases,the insertion loss decreases,and the stray peak decreases.With the increase of line width,the higher the center frequency decrease,but the relative bandwidth and insertion loss of the device will also increase.When the aperture increases,the center frequency remain unchanged,and relative bandwidth and insertion loss decreases,but the clutter around the main peak of the center frequency increases.When the line width is 200 nm,the center frequency of SAW device is 4.95GHz and the insertion loss is-35.26d B.The cyclic bending experiments of SAW devices show that the center frequency and insertion loss of the devices remain stable after 10,000 cycles of cyclic bending.In this thesis,AlN thin films were grown and optimized on a flexible substrate,and SAW devices with a center frequency of more than GHz were fabricated.