Kinetic study of intermetallic growth and its reliability implications in Pb-free Sn-based microbumps in 3D integration

In this study, we investigated the kinetics of intermetallic compound (IMC) growth in Pb-free Sn-based microbumps with Cu or Ni under-bump metallization (UBM) in three-dimension (3D) integration. An analysis was formulated to study the multi-phase multi-component kinetics for solid-state phase transformation of intermetallic growth. A numerical optimization method called simulated annealing (SA) was applied to derive the diffusion coefficients required for the model analysis. The derivation was based on the kinetic data obtained from IMC formation in through-silicon-via (TSV) microbump structures at 170°C. The diffusion coefficients of Cu in Cu₃Sn and Cu₆Sn₅ were found to be 5.12 10^-16 m²/s and 9.42 10^-16 m²/s, respectively, while those of Sn were found to be 1.46 10^-16 m²/s and 9.44 10^-16 m²/s in Cu₃Sn and Cu₆Sn₅, respectively. The diffusion coefficients of Ni and Sn in Ni₃Sn₄ were found to be 1.36 10^-17 m²/s and 6.81 10^-18 m²/s, respectively. The deduced diffusivities were then applied to a finite difference model to predict the IMC growth kinetics, and the results were verified by the experiments. Finally, the effect of IMC formation on the reliability of microbumps in 3D structures was discussed.