Epitaxial Growth of (111) Nanotwinned Ag on (111) Nanotwinned Cu Films for Low-Temperature Cu–Cu Bonding

Abstract

Copper joints have replaced solder interconnects in integrated circuits due to their great electrical properties and lower-temperature processing. To isolate Cu from oxidizing during bonding processes, a (111)-oriented nanotwinned Ag (NT-Ag) thin layer was electroless-deposited on a (111)-oriented NT-Cu film. Such a method outperforms the sputtering approach in terms of expenditure, environmental impact, and deposition rate. The microstructures of the Ag films were then analyzed. Results show that columnar NT-Ag grains epitaxially grew along the columnar NT-Cu grains. Additionally, two types of joints (Cu–Ag and Ag–Ag) were fabricated and characterized. We found that the bonding strength of the Cu–Ag joints was higher than that of the Ag–Ag joints. This could be attributed to the greater diffusion rate of Ag atoms in Cu than the self-diffusion of Ag.