A ±0.3 ppm Oven-Controlled MEMS Oscillator Using Structural Resistance-Based Temperature Sensing

This paper presents a 77.7-MHz silicon microelectromechanical-systems oven-controlled oscillator (MEMS OCXO) that uses the structural resistance ( $R_{{{\text {Struc}}}}$ ) of the resonator as an embedded temperature sensor. The $R_{{\text {Struc}}}$ exhibits a large temperature coefficient of resistance and is used as a self-temperature sensor to accurately and locally monitor the temperature of the resonator. A high-Q capacitive cross-sectional Lame-mode resonator fabricated using the nanogap high aspect-ratio combined poly- and single-crystal silicon process (HARPSS) is used as the frequency selective element. A silicon resistor micro-oven is implemented on the MEMS die adjacent to the resonator and the ensemble is wafer-level packaged in vacuum to yield a 2 mm $\times2$ mm MEMS die. The micro-oven resistor is automatically controlled by the analog loop to provide active temperature stabilization for the resonator. A resistance temperature detector (RTD) circuit, high-gain loop filter, and heater amplifier are implemented as the analog micro-oven control loop. To further boost the frequency stability, a digital feedforward calibration path which uses the digitized RTD output to fine tune the phase shift of the sustaining amplifier is added to the system. The silicon MEMS OCXO achieves ±0.3-ppm frequency stability from −25 °C to 85 °C. The microresonator is interfaced with a sustaining amplifier implemented in Taiwan Semiconductor Manufacturing Company 0.35-s $\mu \text{m}$ CMOS process, consuming 16 mA from a 3.2-V supply.