Identification and management of diffusion pathways in polysilicon encapsulation for MEMS devices

Hermeticity and diffusion behaviors of ´epi-seal´ wafer-scale polysilicon thin-film encapsulation were investigated by monitoring quality factor of MEMS resonator inside the encapsulation. Various gas species were examined in a 400degC furnace, identifying hydrogen and helium as diffusive gas species and showing argon and nitrogen to be non-diffusive under these conditions. Also, a series of devices with modifications of encapsulation geometry were tested and silicon dioxide layer was identified as the diffusion pathway through the encapsulation. These experimental results and diffusion pathway models enable design optimization and new process development aiming at true hermiticity of wafer-scale thin-film encapsulation for MEMS devices.