Abstract
This work reports the design of a monolithic oscillator based on a low motional impedance (Rm) CMOS-MEMS resonator array with high-stiffness driving scheme in a standard 0.35 μm CMOS. Combined with polysilicon release process and the proposed “contact-array-assisted” gap spacing design, a tiny equivalent transducer’s gap (deff) of only 190 nm is successfully attained. We also discussed the effectiveness of the high-stiffness driving scheme to the proposed MEMS resonator. Traditional capacitive CMOS-MEMS resonators often exhibit high motional impedance (Rm) in a range of 0.1 to 10 MΩ owing to their large gap spacing and insufficient transduction areas. Such a high Rm not only makes oscillator design very difficult, but also introduces additional thermal-mechanical noise in the oscillation spectrum. This work attempted to fabricate a MEMS resonator with deep-submicron gaps in the CMOS process, and the feature of the small motional impedance of the resonator makes the implementation of the oscillator easier. To address the nonlinearity issue for the narrow-gap resonators, the designed resonator is formed by multiple high-velocity coupled clamped-clamped beams with a high-stiffness driving scheme, thus greatly improving power handling and reducing motional impedance of the resonator. The dummy resonator and fully-differential transimpedance amplifier (FD-TIA) are also employed for active feedthrough cancellation. Based on this feature, a low Rm of 10 kΩ is achieved under a medium bias voltage (VP) of only 30 V for a 4.15-MHz resonator in vacuum with the measured Q of 1,000. The combination of the mechanically coupled array and high-stiffness driving scheme significantly enhances oscillator performance. The 4.22-MHz single-chip CMOS-MEMS oscillator exhibits the phase noise of -90 dBc/Hz at 1-kHz offset and -121 dBc/Hz at 1-MHz offset, respectively.