Investigation of support transducer enabled higher-order radial bulk mode MEMS resonator and low phase noise oscillator

Abstract

This work reports the successful excitation of a novel bulk acoustic mode whose actuation and sensing are facilitated by the support transducer topology (STT). The design methodology concurrently supports low motional impedance and high energy confinement features which are crucial for frequency reference components in Radio Frequency communication. A conventional bulk mode operating in the width extension (WE) mode is deployed to efficiently excite the higher-order radial mode using the STT design feature of resonant frequency matching. The thin-film piezoelectric on substrate support transducers excited in a WE mode is mechanically coupled to the bulk silicon allowing the acoustic energy of the MEMS resonator to be stored maximally in the high-quality factor (Q) resonant tank, thereby alleviating the overall losses due to low-Q of the piezoelectric thin film and electrode material. The resonator exhibits a Q of 19 728 at 63.56 MHz resonant frequency with a motional resistance (R m) of 368 Ω when measured in vacuum at a power level of 0 dBm. Under cryogenic measurement conditions, the device recorded a Q of 24 153 at 15 K. A standalone WE resonator is studied to put a spotlight on the quality factor enhancement technique using the STT. The STT enabled novel bulk mode enhances the overall Q by 320% and halves the R m. When implemented as an oscillator, its performance exceeds the Global System for Mobile communication standards phase noise (PN) requirements. PN of −136.95 dBc Hz−1 and −161.52 dBc Hz−1 at 1 kHz and 1 MHz offset, respectively, were recorded when normalized to a carrier frequency of 13 MHz.