Thin-film piezoelectric-on-substrate resonators with Q enhancement and TCF reduction

In this paper we report on lateral mode thin-film piezoelectric-on-substrate (TPoS) resonators with techniques to enhance the quality factor (Q) and reduce the temperature coefficient of frequency (TCF). Such techniques utilize a highly or degenerately doped Si substrate layer as the ground electrode, and reduce the thickness and volume ratio between the AlN piezoelectric layer and the Si substrate. By patterning the AlN and eliminating the bottom metal electrode, a record quality factor of over 30,000 is observed in air at 27 MHz (>66,000 in vacuum). The highly-doped Si brings the resonator average TCF to around -12 ppm/°C.