Dimensional Effects on Grain Size and Surface Orientation of Nanocrystalline Cu in SiO2 Micro-Vias

Nanocrystalline copper (NC-Cu) has emerged as a potential candidate for interconnect material due to its capability of achieving low-temperature bonding. In this study, NC-Cu films and SiO2 vias were electroplated under various direct current densities of 1∼8 ASD. The vias had diameters of $3 \sim 8\ \mu \mathrm{m}$. NC-Cu was effectively filled into the micro-vias. By reducing the current density, the grain size of the NC-Cu increased and the -oriented ratio increased. As the diameter of the Cu via increased, the sidewall effect diminished, leading to an equiaxial crystallographic orientation around the sidewalls. This study showed that NC-Cu in micro-vias still remains at the nanoscale and without voids, the high energy state of the grain boundaries gives a driving force to eliminate the bonding interface and triggers grain growth during the low-temperature bonding process.