Advanced MEMS Process for Wafer Level Hermetic Encapsulation of MEMS Devices Using SOI Cap Wafers With Vertical Feedthroughs

This paper reports a novel and inherently simple fabrication process, so-called advanced MEMS (aMEMS) process, that is developed for high-yield and reliable manufacturing of wafer-level hermetic encapsulated MEMS devices. The process enables lead transfer using vertical feedthroughs formed on an Silicon-On-Insulator (SOI) wafer without requiring any complex via-refill or trench-refill processes. It requires only seven masks to fabricate the hermetically capped sensors with an experimentally verified process yield of above 80%. Hermetic encapsulation is achieved by Au-Si eutectic bonding at 400 °C, and the pressure inside the encapsulated cavity has been characterized to be as low as 1 mTorr with successfully activated thin-film getters. The pressure inside the encapsulated cavity can also be adjusted in the range of 1 mTorr-5 Torr by various combinations of outgassing and gettering options in order to satisfy the requirements of different applications. The package pressure is being monitored for the selected chips and is observed to be stable below 10 mTorr since their fabrication about 10 months ago. The shear strengths of several packages are measured to be as high as 30 MPa with average shear strength of 22 MPa, indicating a mechanically strong bonding. The robustness of the packages is tested by thermal cycling between 100 °C and 25 °C, and absolutely no degradation is observed in the hermeticity and the package pressure. The package pressure is also verified to remain unchanged after storing the packages at a high storage temperature of 150 °C for 24 h. Furthermore, the packaged chips are observed to withstand a high temperature shock test performed at 300 °C for 5 min, at the end of which the characteristics of the encapsulated sensor indicates that the package still remains hermetic (no detectable leaks) and also the package pressure remains constant at ~20 mTorr.