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
$(75\ ^\circ \mathrm{C})$
, 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
$75^{\circ}\mathrm{C}$
. In addition, the current density was demonstrated to dominate the microstructure of Cu film, with a high current density
$(0.015 \mathrm{A}/\text{cm}^{2})$
, the electroplated nanocrystalline Cu exhibited a uniform grain size distribution while the low current density group
$(0.005 \mathrm{A}/\text{cm}^{2})$
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
$7 5^{\circ}\mathrm{C}$
. 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. Keywords=Nanocrystalline Cu, Electroplated Cu film, Low temperature self-annealing