Vacuum-Sealed MEMS Resonators Based on Silicon Migration Sealing and Hydrogen Diffusion

Abstract

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°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°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 ~60 Pa. Notably, after additional air-baking at 145°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]