Effect of anisotropic grain growth on improving the bonding strength of <111>-oriented nanotwinned copper films

To overcome the scale-down dilemma of solder-based microbumps, copper-to-copper direct bonding has emerged to be one of the most promising approaches to replace solder joints in the high-end packaging technology. However, the bonding interface remains as bonding temperature is below 300 °C. We use highly <111>-orientated nanotwinned Cu films in the direct bonding due to its ability to transform into extremely large <100>-oriented grains after annealing. The extremely anisotropic grain growth can be completed at 250 °C for 90 min, 300 °C for 20 min, and 350 °C for 5 min. By eliminating the bonding interface, the bonding strength can be increased from 46.1 MPa to 57.1 MPa. We calculated the activation energy of extremely anisotropic grain growth, which is 82 kJ/mol (0.85 eV/atom). We suggest that the reason for the extremely anisotropic grain growth is due to the strain energy induced by thermal stresses.