Growth behavior and orientation evolution of Cu₆Sn₅ grains in micro interconnect during isothermal reflow

At present, three-dimensional (3D) packaging technology is one of the effective methods to extend Moore's law. In 3D stacking, micro interconnects (below 30 μm) have been widely used. As for Cu/Sn/Cu micro interconnect, after repeated or long-term reflow, the micro interconnect will be mainly composed by Cu-Sn intermetallic compounds (IMCs) because of the small bonding gap and fast IMC growth rate under size effect. In the present research, a Cu/Sn(12.5 μm)/Cu micro interconnect was reflowed at 260 °C. Laminar scallop-like Cu₆Sn₅ IMC was the dominant produce at both interfaces. The total thickness of the two Cu₆Sn₅ layers and the consumption of the bottom Cu layer both showed a parabolic law with reflow time, and their logarithmic format followed a good linear relationship. It is also found that as the reaction time extended, the number of the Cu₆Sn₅ grains decreased and the grain orientation tended to gathering towards certain directions, resulting in texture structure. The angle between the <0001> direction of the Cu₆Sn₅ grains and the growth direction of IMC (perpendicular to the interface) was analyzed to characterize the grain orientation evolution. The anisotropic growth of the Cu₆Sn₅ grains along the <0001> direction was discussed to reveal the mechanism of orientation evolution.