Low Temperature Diffusion Bonding of Si Chips Sputtered with High Density (111)-Ag Nanotwinned Films

In this study, the microstructural evolution of Ag films with different amounts of (111)-oriented grains and coincident site lattice & sigma;3 (CSL-& sigma;3) twin boundaries after low-temperature direct bonding was demonstrated. The highly (111)-oriented surface grains provided the rapid surface diffusion paths and the special twin boundaries contributed to structural stability of the Ag films during bonding. Combining both, perfect bonding can be achieved at a low temperature of 150 & DEG;C for 60 min in the bonding of Ag films with 99.4% (111) grains. Owing to the low recrystallization temperature, bonding interfaces with only a few small voids were observed after bonding at 250 & DEG;C for 30 min. Annealing twins grew across the bonding interface and extended to the other side of the Ag films, implying a strong bonding strength. Though increasing the bonding temperature and period could help to achieve a bonding interface with the least voids formed, it was suggested that increasing the amount of (111) grains on the Ag film bonding surfaces is the key point to achieved perfect bonding at temperatures lower than 250 & DEG;C. This would provide great potential for Ag as a suitable material for applications in three-dimensional integrated circuit (3D-IC) interconnections.