Integrated High-frequency Reference Clock Systems Utilizing Mirror-encapsulated BAW Resonators

This work introduces an integrated reference clock system using a 2.5 GHz mirror-encapsulated BAW (bulk acoustic wave) resonator as the frequency source. With acoustic mirrors also placed on top of the resonator, this symmetrical DBAR (dual-Bragg acoustic resonator) structure offers extra design freedoms to alter the resonator’s acoustic properties. A DBAR requires no cavity on either side of the resonator yet still immune to mass loading effect from contamination and assembly. This advantage enables cost-effective system integration by directly marrying a DBAR and a CMOS die in a plastic package. System chips utilizing DBAR as the high-frequency reference clocks are demonstrated here. With compensation algorithm and careful design of the package, these DBAR-based clocking systems can achieve ±25 ppm stability over the entire operation temperature range (including aging, solder shift, etc.), resulting industry’s first crystal-less BLE (Bluetooth Low Energy) radio chip.