Preparation And Study Of High Frequency SAW Device Based On AlN/c-BN

With the rapid development of mobile communication technology,surface acoustic wave(SAW)devices for its high-frequency,high power,high reliability and miniaturization of the device are widely used in wireless communications,radar and daily consumption fields.How to improve the center frequency of the SAW device is the key to solve the further development of communication technology.Diamond has the highest SAW velocity,so the multilayer structure based diamond is the first choice for the preparation of high-frequency SAW device.Since AlN and c-BN film of piezoelectric material have a higher surface acoustic wave velocity,while belonging to the III-V nitride system and having a similar lattice structure with diamond.Therefore,this paper investigated how to obtain the(002)-oriented AlN thin films by annealing.The AlN / c-BN composite thin film structures were prepared and the corresponding SAW devices were fabricated and tested.Firstly,AlN thin films with(002)orientation were prepared by annealing process.The effects of different annealing temperatures on the AlN thin films were investigated by XRD,AFM,PFM and so on.Crystallinity,morphology and piezoelectric properties of AlN thin films were investigated under different conditions.It was found that the in-situ annealing was better than off-situ annealing.When the annealing temperature was 500 °C,the films had the best(002)orientation and the highest relative piezoelectric coefficient d33=0.52 V.Meanwhile,the polarization maintaining properties of AlN films were studied in detail.It was found that the electric field of coercive field was about the same as that of 8V.Secondly,the structure of c-BN/AlN with different thickness and layers were prepared.The best d33=0.75 V were obtained when three-layer structure of the thin films with thickness of c-BN and AlN 80 nm,80nm respectively.The d33 is 44.2% higher than that of the monolayer AlN thin film,while the multilayer films maintaining good polarization characteristics.The device structure with 2?m interdigitated width was designed and optimized in this paper,effectively reduced the device insertion loss.The structure of the device was integral and the line was intact through ultraviolet lithography.Based on this,the center frequency of the IDT/AlN/SiO2/Si device is 495 MHz,the insertion loss is-24.15 dB,and the IDT/AlN/ c-BN/AlN/SiO2/Si have the center frequency of 592 MHz and insertion loss of-35.8dB.The center frequency of c-BN / AlN multilayer structure is improved by 19.6% compared with that of single-layer AlN thin film device,and a high-frequency filter of GHz is obtained on the diamond substrate.When based on IDT/Al N/c-BN/AlN/diamond,the center frequency is up to 1.13 GHz and the insert loss is-46.5dB,which proves that multilayer structure have a bright future in the high frequency SAW devices.