Influence of Bonding Parameters on the Interaction Between Cu and Noneutectic Sn-In Solder Thin Films

Thermocompression bonding of through-layer copper interconnects is of great interest for fabrication of three-dimensional (3D) integrated circuits. We have investigated interactions of Cu films with noneutectic Sn-In at length scales of 1 μm to 5 μm. The effects of bonding time, bonding temperature, and post- bonding annealing temperature on intermetallic compound (IMC) formation, joint microstructure, and shear strength were investigated using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometry (XRD), and shear testing. It is shown that bonding temperature plays an important role in increasing the true contact area, while the postbonding annealing temperature affects the formation of a single IMC, the η-phase [Cu6(Sn,In)5]. Both of these phenomena were found to contribute to the shear strength of the joints. It is shown that two-step bonding processes, involving short bonding times and longer postbonding annealing, can be used to optimize the bond formation for increased throughput.