Role of Thin Sn Layer for Low Temperature Al-Al Thermo-compression Bonding of Wafer-Level Hermetic Sealing

This paper reports low temperature hermetic wafer bonding using Al/Sn/Al/Sn/Al as a bonding layer. The Al surface of the bonding layer was oxidized in air, but hermetic sealing was demonstrated without surface treatment at 370 degrees C to 390 degrees C, which was lower than the maximum temperature of complementary metal oxide semiconductor (CMOS) backend process (400 degrees C). For the successfully sealed samples, the bonding layer was considerably compressed and squeezed, and the remaining thickness was only <16% of the initial one, that is, the reduction rate was >84%. On the other hand, the samples without Sn layers inserted, that is, using a pure Al bonding layer, were not hermetically sealed at similar temperatures, showing the reduction rate smaller than 70%. A clear correlation between the reduction rate and the yield of hermetic sealing was observed. Taking account of analytical results and Al-Sn phase diagram, it is suggested that Al-Sn liquid phase in Al grain boundaries enhances the grain slip deformation of the bonding layer and fractionates the surface Al oxide layer during the bonding process. The function of Sn for Al-Al bonding suggested in this paper is useful for wafer-level hermetic micro electro mechanical systems (MEMS) packaging at low temperature.