A curved-beam piezoelectric MEMS resonator featuring multiple temperature plateaus with enhanced stability

Silicon-based Micro-electromechanical Systems (MEMS) resonators face stability challenges due to their temperature susceptibility, which impedes the advancement towards high-precision applications. This work reports a multi-modal curved-beam piezoelectric resonator that enhances frequency stability through engineered irregular geometry and mechanical nonlinearity. Utilizing a standard Silicon-On-Insulator (SOI) - Aluminum Nitride (AlN) process, the proposed device engenders 17 vibration modes within 5 MHz, addressing the detection constraints and information extraction of non-inplane modes for capacitive counterparts and circumventing the needs of vacuum encapsulation. In addition, owing to the inimitable temperature coefficient of frequency (TCf) insensitive plateaus discovered in high-frequency modes (Mode 16 and Mode 18), a novel stability enhancement strategy has emerged. Through open-loop sweeps and phase-locked loop (PLL) tracking, it is suggested that these plateaus likely stem from modal coupling triggered by mechanical nonlinearities. This behavior, reminiscent of frequency veering, potentially enables energy redistribution between modes, leading to the resonance frequency deviating from the intrinsic thermal drift trajectory of silicon. Experimental results show that Mode 16 achieves a stability of 17.4 ppb at 46.390 s at a 62 °C plateau under ±1 °C fluctuations and reaches 2.0 ppb under stringent temperature regulation. Mode 18 reaches a peak stability of 37.9 ppb at 43 °C. This innovative approach presents a promising avenue for developing high-precision frequency references without the need for intricate TCf compensation, providing considerable potential for the evolution of next-generation silicon micro-resonators and enhancing their capabilities as stable timing sources in various applications.