High vacuum wafer level packaging for high-value MEMS applications

In this paper, our latest developments on high vacuum packaging technology at the wafer level are presented. The main objective of our works has been to demonstrate the feasibility of hermetic high vacuum wafer level packaging of MEMS (vacuum target is 10^-3 to 10^-4 mbar) using either anodic or eutectic bonding (AuSi). In the frame of this work, a test vehicle based on a MEMS resonator is used to characterize the vacuum inside the packaging by measuring the Quality factor (Q factor) of the resonator. The resonator is fabricated using 200 mm BSOI wafers. For capping, either 200 mm glass wafer or 200 mm silicon wafer are used depending on the bonding technology chosen. A deep cavity is created in the cap wafer. A getter is deposited inside the cavity to maintain a high vacuum level after bonding. The wafers are then bonded together using anodic or eutectic bonding and electrical interconnections (TSV) are finally carried out. First, the calibration curve i.e., Q Factor vs residual pressure has been achieved using our resonator with a non-hermetic package and a specific vacuum chamber that allowed us to measure the Q factor of the resonator at different residuals pressures (from 10^-1 to 10^-5 mbar). Then, anodic and eutectic bonding processes have been developed and optimized in order to reach vacuum level as low as 10^-3 mbar after interconnection. In the case of anodic bonding, it turned out that the main issue that has to be addressed was the permeation of noble gas through the glass wafer during interconnection process. For eutectic bonding, outgassing turned out to be the more significant parameter to control. Finally, residual pressure down to 10^-4 mbar has been achieved.