In this study, we introduce an innovative approach to vacuum-encapsulation of MEMS resonators using Silicon Migration Seal (SMS) technology, a novel wafer-level vacuum packaging method. SMS utilizes silicon reflow phenomena under high-temperature ( $>$ 1000 $^{\circ}$ C) hydrogen environments to seal release holes effectively. We successfully demonstrated this technique on a MEMS resonator made on a standard SOI wafer, commonly used in inertial sensors and timing devices. After the encapsulation, hydrogen diffusion from the sealed cavity was performed through annealing at 430 $^{\circ}$ C for 27 hours in a nitrogen environment. Further analysis using focused ion beam (FIB) penetration outside the resonating element confirmed an impressive vacuum level improvement in the sealed cavity, estimated at $\sim$ 60 Pa. Notably, after additional air-baking at 145 $^{\circ}$ C, the maintained high Q factor suggests a potential vacuum level below 10 Pa. These findings not only illustrate the efficiency of SMS in wafer-level vacuum packaging but also open up possibilities for optimizing sealing pressure in MEMS packaging. 2024-0014

This paper reports the experimental estimation of the mechanical quality factor ( $Q_{m})$ of polymer films, which can be used as damping materials for MEMS. Considering the application to MEMS devices, polymer/PZT composite actuators using two thick photoresists (TMMR-NA1000 and SU8) and PDMS were fabricated and the Q factors were evaluated in a vacuum environment. The comparison between the measured and simulated Q factor confirmed a $Q_m$ range of 14-18 for TMMR-NA1000, 13-20 for SU8, and 5-8 for PDMS, indicating the superior damping capability of PDMS. Additionally, it was also found that the PZT thin film used in this study exhibited $Q_m$ of 200-220 under the driving voltage of 2 V $_{\mathrm{pp}}$ with $+$ 1V DC offset. The evaluation approach developed for assessing $Q_m$ of polymer materials is straightforward, easily implementable, and has broad structure applicability, offering a promising tool for assessing a wide array of materials. 2024-0096

Silicon migration seal (SMS) wafer‐level packaging is proposed for high vacuum encapsulation of MEMS. The sealing of vent holes is possible based on silicon surface migration effect at 1100°C in 100% hydrogen ambient without deposition, if the size of the vent holes is properly designed. The feasibility of SMS packaging was experimentally demonstrated using 4‐inch wafers. Hermetic sealing was confirmed after 168 hours from the packaging process by diaphragm displacement in the cap wafer.