Wafer level hermetic encapsulation of MEMS inertial sensors using SOI cap wafers with vertical feedthroughs

This paper reports a new, inherently simple, and high-yield wafer-level hermetic encapsulation method developed for MEMS inertial sensors, enabling lead transfer using vertical feedthroughs that do not require any complex via-refill or trench-refill processes. The process requires only seven masks to complete both the sensor and cap wafers, whereas the combined yield for the sealing and lead transfer is experimentally verified to be above 90%. Hermetic encapsulation is achieved by Au-Si eutectic bonding, and the pressure inside the encapsulated cavity has been characterized to be as low as 1 mTorr with a thin film getter. MEMS resonators packaged with the proposed method demonstrated quality factors over 125,000. The pressure inside the encapsulated cavities has been monitored for 4 months since the first prototypes, and it is observed to be stable throughout this period.