Long-Term Frequency Stability of a Dual-Oven MEMS Oscillator Based on a High-Q Lateral Extensional Resonator

Abstract

This work presents a 5 MHz microelectromechanical systems (MEMS) oscillator based on a high-quality factor ( $Q_{\mathrm {unloaded}} \gt 50$ ,000) lateral extensional (LE) mode Aluminum Nitride (AlN)-on-silicon (Si) resonator for stable clock applications. The resonator is designed to have a turn-over temperature (ToT) above $100~^{\circ }$ C and is fabricated on a $350~\mu $ m thick single-crystal Si substrate to achieve high $Q$ while its lateral dimensions are optimized to obtain a low motional impedance. To improve frequency stability, the oscillator is operated under high vacuum and evaluated using two thermal-control configurations. The single-oven configuration utilizes an integrated heater on the resonator and applies a fixed integrated-heater bias near the ToT, while the dual-oven configuration additionally stabilizes the environmental temperature fluctuations within the vacuum-chamber using controlled external heating. Both configurations reach an Allan deviation (ADEV) of approximately $1.7\times 10^{-10}$ at $\tau =1$ s, demonstrating strong short-term stability enabled by the high- $Q$ resonator. However, the dual-oven configuration suppresses the increase in ADEV at long averaging times, maintaining an ADEV of approximately $1.6\times 10^{-10}$ at $\tau =1000$ s compared with $9.5\times 10^{-9}$ for the single-oven operation, and exhibiting an average net drift rate of 0.85 ppb/h over 32 h. The results distinguish the roles of the high- $Q$ resonator and external chamber thermal stabilization in achieving short- and long-term frequency stability, respectively.[2026-0044]