High Q low inpedance WLCSP resonator for sub-100 MHZ programmable oscillator application

A high performance aluminum nitride (AlN) on silicon piezoelectric resonator is reported in this paper. The resonator is fabricated based on the AlN on cavity-silicon-on-insulator (SOI) platform and vacuum encapsulated using the wafer level chip scale packaging (WLCSP) via aluminum-germanium (Al-Ge) eutectic bonding approach. After dicing, the resonator has an attractive compact size of 0.6×0.6×0.7 mm 3 . The fabricated resonator achieves a quality factor (Q) of 9517 and an impedance of 51 Ohm at its resonant frequency of 27.19 MHz. The measured frequency drift is within 30 ppm after 1000 thermal cycles (−45 °C to 85 °C). No obvious impedance change caused by the thermal cycles is observed. The piezoelectric resonator based programmable oscillator shows an overall frequency drift of 3 ppm over the temperature range of −20 °C to 70 °C, thanks to the passive and active temperature compensation approaches. With this stable frequency reference, the programmable oscillator produces a 76.4 MHz frequency output with an integrated phase jitter of 2.21ps from 12 kHz to 10 MHz.