Growth characteristics and formation mechanisms of Cu6Sn5 phase at the liquid-Sn0.7Cu/(111)Cu and liquid-Sn0.7Cu/(001)Cu joint interfaces

Abstract This work investigated the growth characteristics and formation mechanisms of the Cu6Sn5 phase at the liquid-Sn0.7Cu/(111)Cu and liquid-Sn0.7Cu/(001)Cu joint interfaces. As a result of contributions from the interfacial environments, regular arrays of the roof-type Cu6Sn5 grains with fixed intersecting angles were generated on both types of the Cu single crystals after soldering at 250 °C for 1 s–1 h. At the liquid-Sn0.7Cu/Cu6Sn5 interface, a hexagonal-rod-type growth mechanism for Cu6Sn5 phase was proposed on the basis of its anisotropy in surface energy and roughness. According to this mechanism, the Cu6Sn5 roofs formed on the (111) and (001) Cu pads would consistently elongate in the [0001] direction of Cu6Sn5 phase, regardless of whether they belonged to the minimum mismatch direction of Cu6Sn5 and Cu phases; and they would maintain the {11 2 ¯ 0} and {10 1 ¯ 0} planes as the side faces respectively, despite the reaction time being prolonged to 1 h. At the Cu6Sn5/Cu interface, two types of the three-dimensional placement rules for Cu6Sn5 roofs on the Cu single crystals were determined on the basis of the suitable Cu matches of these two phases. Specifically, the junction interfaces between Cu6Sn5 roofs and Cu single crystals were confirmed to be ‖{10 1 ¯ 0} ‖ {111} and {11 2 ¯ 0} ‖ {001}; and the parallel orientation relationships of {0001} ‖ {011} were always present on both types of interfaces. Our study can help to clarify the growth mechanism of Cu6Sn5 phase and to provide a scientific basis of Cu6Sn5 orientation design for three-dimensional integrated circuit interconnect applications.