Gold-tin eutectic bonding for hermetic packaging of MEMS devices with vertical feedthroughs

This paper presents a new method for wafer-level hermetic encapsulation of MEMS devices using low-temperature (280 to 300°C) Au-Sn eutectic bonding applied to the recently developed advanced MEMS (A-MEMS) process of the METU-MEMS Research Center, which uses an SOI cap wafer with vertical feedthroughs that does not need any complex via-refill or trench-refill process steps. The Au-Sn eutectic bonding process is achieved at 300°C with a bond pressure of 2 MPa by using a sealing alloy thickness less than 1.5 μm. The package pressure is characterized to be around 250 mTorr, without any getter activation. The remelting temperature of the Au-Sn bonding interface is measured by using differential scanning calorimetry (DSC) analysis and found to be around 280°C, verifying that the bonding is achieved at the desired eutectic composition (80% Au and 20% Sn), also confirmed by the energy dispersive X-ray spectroscopy (EDS) analysis. The shear strengths of several packages are measured to be above 20 MPa, indicating a mechanically-strong bonding. The robustness of the packages is also tested by subjecting them to high temperature storage at 200°C for 24 hours, and no degradation is observed in the hermeticity of the packages at the end of this period.