Formation and nanomechanical properties of intermetallic compounds in electrodeposited Cu–Sn–Co multilayers

Abstract

The properties of the intermetallic compounds (IMCs) formed at the interface between the solder and the Under Bump Metallization (UBM) are crucial for the reliability of solder joints. Cobalt (Co) is a potential candidate for UBM since it can act as a diffusion barrier and hence can improve the performance of tin-based solder joints. Growth kinetics, mechanism of formation, and properties of IMC in the copper-tin-cobalt (Cu–Sn–Co) system that is encountered with Co UBM are presented in this paper. Cu–Sn–Co systems were prepared by electrodeposition of Cu, Sn and Co multilayers on Cu substrates followed by reflow at 250 °C for varying durations (1–30 min). Microscopic observation revealed the formation of defect-free (Co,Cu)Sn₃, crack-containing (Cu,Co)₆Sn₅ and thin irregular Cu₃Sn IMCs after 1 min reflow. As reflow duration increased, (Co,Cu)Sn₃ and Cu₃Sn grew at the expense of (Cu, Co)₆Sn₅. The consumption of the entire Sn layer after 1 min and the calculated effective interdiffusion coefficient of (Co,Cu)Sn₃ suggested that the growth of (Co,Cu)Sn₃ after 1 min is controlled by solid-state diffusion of Sn. The average Young’s Modulus values of (Co,Cu)Sn₃, (Cu,Co)₆Sn₅ and Cu₃Sn are 99.5 ± 3.2 GPa, 110.8 ± 7.3 GPa and 109.4 ± 0.3 GPa, respectively, while the nanohardness values are 4.15 ± 0.34 GPa, 6.74 ± 0.62 GPa and 4.96 ± 1.09 GPa, respectively. Increasing the reflow duration in the Cu–Sn–Co system is expected to improve the performance of the solder joint, through the replacement of the crack-containing (Cu,Co)₆Sn₅ by the defect-free (Co,Cu)Sn₃.