Microstructure and bending piezoelectric characteristics of AlN film for high-frequency flexible SAW devices

Flexible surface acoustic wave (SAW) device is one of hot point areas for wearable communication technology because of their ductility and bendability, but the deterioration problem of device performance after bending experiments is limiting its wide application for future wearable communication devices. In this work, the effect of sputtering power on microstructure properties and bending piezoelectric characteristics of aluminum nitride (AlN) film on polyimide (PI) with molybdenum (Mo) buffer layer for high-frequency flexible SAW device was discussed in detail from 200 W to 280 W. Microstructure results show that the high oriented (002) AlN films with narrow full width at half maximum (FWHM) of 0.29 degrees and low roughness of 3.2 nm are obtained under 240 W sputtering power. Piezoelectric and frequency results of SAW devices after cyclic bending indicate that the relative piezoelectric coefficient d*(33) is about 8.01 pm/V, and the center frequency of the flexible SAW devices is as high as 4.95 GHz, especially the piezoelectric and frequency characteristics can maintain high satiability after 10,000 bending cycles. This work may be of useful for the flexible high-frequency SAW devices in the wearable communication area.