Demonstrating 170 °C Low-Temperature Cu-In-Sn Wafer-Level Solid Liquid Interdiffusion Bonding

The wafer-level solid liquid interdiffusion (SLID) bonds carried out for this work take advantage of the Cu-In-Sn ternary system to achieve low-temperature interconnections. The 100-mm Si wafers had \mu -bumps from 250~\mu \text{m} down to 10~\mu \text{m} fabricated by consecutive electrochemical deposition of Cu, Sn, and In layers. The optimized wafer-level bonding processes were carried out by EV Group and Aalto University across a range of temperatures from 250 °C down to 170 °C. Even though some quality-related process challenges were observed, it could be verified that high strength bonds with low defect content can be achieved even at a low bonding temperature of 170 °C with an acceptable 1-h wafer-level bonding duration. The microstructural analysis revealed that the bonding temperature significantly impacts the obtained phase structure as well as the number of defects. A higher (250 °C) bonding temperature led to the formation of Cu 3 Sn phase in addition to Cu 6 (Sn,In) 5 and resulted in several voids at Cu 3 Sn|Cu interface. On the other hand, with lower (200 °C and 170 °C) bonding temperatures, the interconnection microstructure was composed purely of void-free Cu 6 (Sn,In) 5 . The mechanical testing results revealed the clear impact of bonding quality on the interconnection strength.