Synthetic and formation mechanism of Cu6Sn5 single-crystal layer on Cu pad for UBM application

Grain boundaries and/or solder channels between the neighboring Cu₆Sn₅ grains at the Sn/Cu interface have long been thought to have the negative effects on the interconnect reliability of the solder joints. In this paper, the single-crystal Cu₆Sn₅ seed is synthesized and then assembled on Cu pad via 1 min reflow at 250°C. By combining the EBSD analysis, the traditional polycrystalline Cu₆Sn₅ monolayer, which is always generated at the Sn/Cu interface after reflow, has been replaced by a single-crystal monolayer without intergranular structures (solder channels or grain boundaries). The structural reliability of this single-crystal layer is also established by nanoindentation, and the formation mechanism behind this single-crystal-forming procedure is finally confirmed to be a grain boundary migration. This study may deepen the grow behavior of the interfacial Cu₆Sn₅ phase, and also provide a good starting point for further synthetic of Cu₆Sn₅ under-bump metallization (UBM) layer.