Wafer-Level Vacuum-Packaged High-Performance AlN-on-SOI Piezoelectric Resonator for Sub-100-MHz Oscillator Applications

A high quality factor ( $Q$ ) and low-impedance aluminum nitride (AIN) on silicon piezoelectric resonator is reported in this paper. The piezoelectric resonator is fabricated based on the AIN-on-cavity-Silicon-on-Insulator (SOI) platform and vacuum encapsulated using the wafer-level chip-scale packaging by aluminum–germanium eutectic bonding approach. The fabricated resonator achieves a loaded $Q$ of 9520 and a motional impedance of 32.64  $\Omega$ at its series resonant frequency of 27.19 MHz. Thermal cycles (TC) and highly-accelerated stress test (HAST) are performed to evaluated the packaging reliability of the fabricated devices. No obvious impedance change caused by the TC and HAST is observed. The TC results indicate that the packaged devices have an equivalent life of more than 11.65 years based on the Coffin–Manson model. The piezoelectric resonator-based programmable oscillators demonstrate an overall frequency drift of $\pm$ 3 ppm over the temperature range of $-$ 20  $^{\circ }$ C to 70  $^{\circ }$ C thanks to the passive and active temperature compensations. With this stable frequency reference, the programmable oscillator produces a 75.01-MHz frequency output. The measured integrated phase jitter is 2.02 ps (rms) from 12 to 10 MHz.