On the formation mechanism of solid-solution Cu-to-Cu joints in the Cu/Ni/Ga/Ni/Cu system
Highlights: • The mechanism of Cu/(Ni,Cu,Ga)/Cu solid-solution joint formation is elaborated. • Ga is the dominant diffusion species in the Cu/Ni/Ga/Ni/Cu bonding system. • Loose and columnar Ni{sub 3}Ga{sub 7} layer is formed by the (001)-textured Ni/Ga reactions. • Cu significantly accelerates the growth of Ni{sub 3}Ga{sub 7} layer by enhancing Ga diffusion. • Supersaturated Cu in Ni{sub 2}Ga{sub 3} phase promotes the formation of the final fcc phase. - Abstract: Copper (Cu)-to-Cu interconnection is crucial in electronic packaging for applications ranging from three-dimensional integrated circuits to die-attachment in wide band-gap devices. Solid-solution phases are innate with better ductility and electrical and thermal conductivity over intermetallic compounds, which are brittle, electrical resistant, and unstable; yet are commonly seen in electronic joints. The desired solid-solution Cu-to-Cu joints have recently been demonstrated using transient-liquid-phase bonding with gallium (Ga) as the filler material to bond Cu substrates with electroplated nickel (Ni) as under-bump-metallurgy. The Cu/Ni/Ga/Ni/Cu couples can fully transform into the Cu/face-centered cubic (fcc)-(Ni,Cu,Ga)/Cu solid-solution joints. However, why and how the solid-solution joints were formed remains unclear. Here we proposed the formation mechanism of the fcc-(Ni,Cu,Ga) solid-solution joints based on five sets of Cu-Ni-Ga couples, namely the Ni/Ga, Ni/Cu/Ga, Ni/Ni/Ga, (Cu,Ni)/Ga, and Cu/Ni/Ni{sub 3}Ga{sub 7}/Ni{sub 3}Ga{sub 7}/Ni/Cu reactions. The roles of each element in the Cu-Ga-Ni interactions as well as the reaction progressions are elaborated.