Electroplated low temperature self-annealing Cu film for Cu-Cu bonding

Cu-Cu bonding is a critical technology in three dimension integrated circuit (3D IC) integration. In this study, thermal instability Cu film, in which grain self-annealing was carried out at a relatively low temperature (75 degrees C), was prepared through direct current (DC) electroplating. Experimental results indicated the self-annealing behavior of electroplated Cu film was significantly affected by the electrolyte aging time and electroplating current density. The Cu film electroplated with a fresh electrolyte could not derive self-annealing, but after 1 day of aging, the electroplated Cu film showed an obvious self-annealing performance at 75 degrees C. In addition, the current density was demonstrated to dominate the microstructure of Cu film, with a high current density (0.015 A/cm(2)), the electroplated nanocrystalline Cu exhibited a uniform grain size distribution while the low current density group (0.005 A/cm(2)) showed a bottom-to-up increasing grain size distribution. Meanwhile, the high current density electroplated Cu film shows a fast self-annealing velocity, the as-deposited nanocrystalline grew to microcrystalline within 10 min at 75 degrees C. As the electroplated Cu grain is nanocrystalline, the excessive energy stored in the grain boundary could be the main driving force for rapid self-annealing. Considering the Cu-Cu bonding is a solid-solid diffusion process, the Cu diffusion process could be accelerated with a rapid self-annealing rate. Hence, the thermal instability Cu film has a great potential to achieve low temperature Cu-Cu bonding.