Solid state reaction of Sn3.0Ag0.5Cu solder with Cu(Mn) under bump metallization

In flip chip technology, Cu thin-film is a widely used under bump metallization (UBM). However, the major disadvantages of Cu UBM are fast consumption of copper, rapid growth of IMCs and easy formation of Kirkendall voids. Many efforts have been focused on suppression of Kirkendall voids which are detrimental to solder joints reliability in the microelectronics industry. In this study, a novel Cu(Mn) UBM design was provided by sputtering to improve the conventional Cu metallization. For the higher Mn concentration (10 at.% Mn) in the Cu(Mn) UBM, a new Sn-rich phase formed between Cu₆Sn₅ and the Cu(Mn) UBM. However, a layer of crack formed after aging. For the lower Mn concentration (2 at.% Mn), the growth of Cu₃Sn and Kirkendall voids was significantly suppressed during thermal aging. The interfacial reaction was analyzed by a field emission electron probe microanalyzer (FE-EPMA) to evaluate the composition distribution. Kinetic analysis and X-ray color mapping gave the evidence that Mn diffusion into Cu₃Sn slowed down the diffusivity of Cu in Cu₃Sn layer. The Mn-enriched Cu₃Sn might serve as a diffusion barrier to reduce interfacial reaction and Kirkendall voids formation. These results suggest the Cu-Mn UBM with a low Mn concentration is beneficial to retarding the Cu pad consumption in solder joints.