Elimination of voids in reactions between Ni and Sn: A novel effect of silver

Interfacial reaction under in a confined space is becoming an increasingly important issue, due to its applications for wafer bonding in three-dimensional integrated circuits. This study aims to uncover the space confinement effects on Ni/Sn and Ni/SnAg reactions. Space confinement causes the formation of voids near the center of Ni/Sn/Ni sandwiches. Adding Ag effectively eliminates these voids, which, if present, would undoubtedly degrade the reliability of wafer bonding joints. The mechanism for the formation of these voids is proposed and verified.