Balanced Drive and Sense CMOS Thermal Piezoresistive Resonators and Oscillators

In this work, we report a novel fully balanced drive and sense thermal-piezoresistive resonator (TPR) in 2P-4M $0.35~\mu \text{m}$ CMOS technology with maskless post-release process. Through the use of the Wheatstone bridge arrangement for both the thermal drive heater resistors and sense piezoresistors, the proposed design greatly simplifies the overall system by removing the need for bias-tee or noisy and power consuming current sink/source. This ensures that the differential terminals of drive and sense ports are at the same DC potential, thus allowing the use of standard differential amplifiers as interface circuits. In addition, this design utilizes bimorph like actuation which enables in-plane motion. An in-plane clamped-clamped beam (CCB) TPR was fabricated, demonstrating a quality factor ( ${Q}{)}$ of 1,600 at 2 MHz with transconductance ( ${g}_{m}{)}$ of $46.8~\mu \text{S}$ for a dc-power of 1.7 mW in a vacuum (1 mTorr). To go one step further, a double-ended tuning fork (DETF) TPR was designed, featuring a transconductance of 62.7 $\mu \text{S}$ with ${Q}$ of 4,400 at 1.9 MHz under the same conditions as CCB. The CCB thermal-piezoresistive oscillator (TPO) implemented using a discrete interface circuit had phase noise of −89 dBc/Hz and −100 dBc/Hz at 1 kHz and 1 MHz respectively while that for DETF TPO was −100 dBc/Hz at 1 kHz and −100 dBc/Hz at 1 MHz in air. Frequency stability was found to be 180 ppb and 125 ppb for CCB and DETF TPOs respectively in air.