Fabrication of (111) nanotwinned Cu and its applications in interconnects of microelectronic devices

As the miniaturization trend of microelectronic devices continues, the dimension of interconnects also decreases gradually. The dimensions of Cu lines and solder joints shrink dramatically. Several challenges arise due to the scaling. First, the diameter of solder joints reduces from 70 µm to 20 µm, and it will be further scaled down in the future. Due to the dramatic decrease in solder volume, the yield and the brittleness of intermetallic compounds will become challenges in the fabrication of microbumps. Therefore, solders might be removed from the joints in the future and Cu joints may replace the solder joints for fine-pitch packaging. Low-temperature Cu-to-Cu direct bonding appears to be one of the solutions for fine-pitch microbumps for 3D IC packaging. However, the high bonding temperature and pressure are the main problems of this approach. We achieve low-temperature Cu-to-Cu direct bonding at low pressure and ordinary vacuum by highly (111) nanotwinned Cu (nt-Cu). The bonding temperature can be lowered to 150°C at a compressive stress of 114 psi held for 60 min at 10−3 torr, or at 200°C for 30 min. Excellent bonding interface can be obtained by bonding two highly (111)-oriented nt-Cu films. Our breakthrough is based on the finding that the diffusivity of Cu atoms on (111) surfaces is approximately 3–4 orders higher than other major planes.