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

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.