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 160 C to 300 C were examined. For reactions at temperatures lower than 240 C, the reaction path is Cu/γ ₃-Cu₉Ga₄/θ-CuGa ₂/liquid, where the γ ₃-Cu₉Ga₄ and θ-CuGa₂ phases are thin planar and thick scalloped layers, respectively, while for the reactions at 280 C and 300 C, the scalloped γ 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 160 C were investigated. The original Cu/liquid/Cu couples isothermally transformed to Cu/γ ₃-Cu ₉Ga₄/ θ-CuGa₂/γ ₃-Cu₉Ga₄/Cu couples as the reaction progressed. However, cracks were observed in the θ-CuGa₂ phase regions after metallographic processing. The brittle θ-CuGa₂ phase is undesirable for Ga-based TLP bonding.