A ScAlN-based piezoelectric breathing mode dual-ring resonator with high temperature stability

In this work, a scandium-doped aluminum nitride (ScAlN)-based piezoelectric breathing mode dual-ring resonator with high temperature stability is presented. The designed resonator consists of two identical rings and a coupling straight beam in between. A combination of highly doped silicon and composite structure using silicon oxide is implemented to improve the frequency-temperature stability of the resonator. The dual-ring resonator is fabricated based on a ScAlN-based thin-film piezoelectric-on‑silicon (TPoS) platform. The measurement results show that the fabricated dual-ring resonator has a loaded quality factor ($Q_l$) of 6889 and an insertion loss of 13.898 dB at its resonant frequency of 16.766 MHz, corresponding to a motional resistance of 395 $\Omega$, and an unloaded quality factor ($Q_{\mathrm{un}}$) of 8681. The resonator's $Q_{\mathrm{un}}$ is almost constant within the pressure range of less than 300 Pa, indicating a good process tolerance in the vacuum packaging process. With the aid of the passive temperature compensation, the reported resonator exhibits an overall frequency variation of less than ±70 ppm over the entire temperature range of 20 °C to 105 °C, which agrees well with the predicted value obtained by finite element method (FEM) analysis. Moreover, Allan deviations of the resonator-based oscillator frequency are collected.