Full intermetallic joints for chip stacking by using thermal gradient bonding

Solid-liquid interdiffusion bonding is a promising process for three dimensional chip stacking, but nevertheless is plagued with low throughput. A new solid-liquid interdiffusion bonding process, with much higher bonding speed, is proposed in this study. Instead of using a homogeneous bonding temperature, a temperature gradient is superimposed across the joint in the new process, which possesses many advantages. Firstly, the new process is 3-10 times faster, depending on the bonding parameters. Secondly, columnary Cu6Sn5 grains grow from the cold-end to the hot-end, with the crystallographic orientation of Cu6Sn5 preferentially aligned along the (0001) pole. Lastly, the new process consumes very little Cu substrate at the cold-end due to the fact that most of the Cu atoms are from the hot-end. The mechanism for the new process is proposed and experimentally verified in this study.