Interfacial Reactions in Cu/Ga and Cu/Ga/Cu Couples

Cu-to-Cu bonding to connect through-silicon vias in three-dimensional integrated-circuit packaging is the most important interconnection technology in the next-generation semiconductor industry. Soldering is an economic and fast process in comparison with diffusion bonding methods. Ga has high solubility of up to 20 at.% in the Cu-rich face-centered cubic (FCC) phase and high mobility at moderate temperatures. In this work, an attempt has been made to evaluate Ga-based Cu-to-Cu interconnection by transient liquid-phase (TLP) bonding. The Cu/Ga interfacial reactions at temperatures ranging from 160A degrees C to 300A degrees C were examined. For reactions at temperatures lower than 240A degrees C, the reaction path is Cu/gamma (3)-Cu9Ga4/theta-CuGa2/liquid, where the gamma (3)-Cu9Ga4 and theta-CuGa2 phases are thin planar and thick scalloped layers, respectively, while for the reactions at 280A degrees C and 300A degrees C, the scalloped gamma (3)-Cu9Ga4 phase is the only reaction product. The phase transformation kinetics, reaction mechanisms, and microstructural evolution in the Cu/Ga couples are elaborated. In addition, reactions of Cu/Ga/Cu sandwich couples at 160A degrees C were investigated. The original Cu/liquid/Cu couples isothermally transformed to Cu/gamma (3)-Cu9Ga4/ theta-CuGa2/gamma (3)-Cu9Ga4/Cu couples as the reaction progressed. However, cracks were observed in the theta-CuGa2 phase regions after metallographic processing. The brittle theta-CuGa2 phase is undesirable for Ga-based TLP bonding.