Electric current-induced abnormal Cu/γ-InSn₄ interfacial reactions

The electromigration effect upon the γ-InSn₄/Cu interfacial reactions have been studied by examining the γ-InSn₄/Cu/γ-InSn₄ couples annealed at 160 °C with and without current stressing. Scallop-type η-Cu₆(Sn,In)₅ phase layers are formed in the couples without current stressing and at the γ-InSn₄/Cu interface where electrons are flowing from the γ-InSn₄ to the Cu. The reaction path is Cu/η-Cu₆(Sn,In)₅/γ-InSn₄. However, very large η-Cu₆(Sn,In)₅ compounds are found at the Cu/γ-InSn₄ interface where electrons are from Cu to the γ-InSn₄. Although the melting points of both γ-InSn₄ and Cu are higher than 160 °C, the liquid phase is formed at 160 °C in the electrified couple at the downstream γ-InSn₄ phase near the Cu/γ-InSn₄ interface. The reaction path is Cu/η-Cu₆(Sn,In)₅/liquid/γ-InSn₄. The liquid phase propagates along the grain boundaries of the γ-InSn₄ matrix. The very large η-Cu₆(Sn,In)₅ compounds are the coupling results of the liquid phase penetration and the Cu transport enhancement due to electromigration.