Silicon carbide MEMS-resonator-based oscillator

An oscillator employing a polycrystalline 3C silicon carbide (3C-SiC) folded-beam microelectromechanical resonator as a frequency-setting component with silicon discrete electronics has been designed and demonstrated. Due to the residual strain gradient in the 3C-SiC thin film, fabricated resonators exhibit a warped proof mass, thus causing vertical misalignment of the electrostatic comb-drive fingers. The folded-beam resonators, therefore, can generate three resonant modes, namely the lateral, rocking and vertical resonances, under electrostatic actuation. The lateral, rocking and vertical resonances exhibit a measured resonant frequency of 27.1 kHz, 30.3 kHz and 24.2 kHz; a phase shift of 0 degrees, 0 degrees and 180 degrees; a quality factor (Q) of 13 550, 10 300 and 9480; and a small-signal motional resistance of 98 M Omega, 25 M Omega and 39 M Omega, respectively, under a 10 V dc bias and 1 mTorr pressure. The rocking resonant mode of the resonator is chosen for the prototype oscillator design due to its lower motional resistance than that of the other two modes and an adequate quality factor. With a MEMS resonator exhibiting closely spaced multiple resonances, sustaining electronics must be designed properly with an accurate small-signal loop gain and phase shift to ensure the excitation of the rocking mode as the single resonance in a closed-loop oscillator with a well-defined steady-state oscillation amplitude. Based on a proper design, the prototype oscillator outputs a desired sinusoidal waveform at 30.2 kHz with an output power of -17 dBm under 10 V dc bias and achieves a minimum phase noise of -78 dBc Hz(-1) at a 12 Hz offset frequency from the carrier limited by the power developed in the resonator and noise floor from the interface electronics, which closely matches with the analytical result of -80 dBc Hz-1 defined by the signal-to-noise-floor ratio (SNfR). The oscillation frequency exhibits 16 ppm stability at room temperature under 1 mTorr pressure observed over a period of 100 h including all environment effects. The demonstrated oscillator design technique can be effectively applied to various MEMS resonators, particularly to the resonators exhibiting multiple resonant modes, for achieving a desired single-tone performance.