Low-temperature aluminum thermo-compression wafer bonding with tin antioxidation layer for hermetic sealing of MEMS

Hermetic Al-Al thermo-compression bonding was demonstrated at the lowest temperature ever reported, 370–390°C, using a thin antioxidation capping layer of Sn. A key factor of hermetic sealing is bonding pressure enough to compress the bonding interlayer metal. From the same point of view, a narrower sealing frame for stress concentration, thicker Al and a higher bonding temperature within the allowable range (<400°C) are favorable for high yield hermetic sealing. Judging from Al-Sn phase diagram, Sn should uniformly and sparsely exist among Al grains as Al-Sn eutectic, which was also supported by the cross-sectional observation of the bonding interlayer. In such a microstructure, Al-Al direct metal bonding, which is stable at Pb-free solder reflow temperature, should be created. Al is the standard metal of CMOS backend, free from the risk of metal contamination and inexpensive, and thus the bonding technology described in this paper is useful for MEMS-CMOS integration.