Effect of intermetallic compound formation on electrical properties of Cu/Sn interface during thermal treatment

Cu6Sn5 and Cu3Sn intermetallic compounds are commonly found in the Sn-Cu bimetallic system. Due to the distinct resistivity of these two compounds, the electrical properties of Cu/Sn interfaces, e.g., solder joints on Cu metallization, may be impacted by the formation of Cu-Sn compounds. In this study, the kinetics of Sn-Cu compound formation was investigated by in-situ resistivity measurement, x-ray diffraction, and scanning electron microscopy (SEM). The interfacial reaction of the Cu-Sn bimetallic thin film specimen was monitored by the resistivity change of the specimen during thermal treatment. The activation energy of formation of Cu-Sn compounds was determined to be 0.97 +/- 0.07 eV. It is proposed that the Cu6Sn5 compound first forms at Sn/Cu interfaces and then reacts with Cu, forming the Cu3Sn compound at elevated temperatures during the thermal ramping process. The effect of thin film thickness on the sequential formation of Sn-Cu compounds is also discussed.