Effect of Grain Size on Cu-Cu Bonding Quality for Fine-Pitch Hybrid Bonding Application

With the continuous scaling of the IC dimensions, fabricating interconnections for heterogeneous integration has become increasingly challenging. Various Cu microstructures, such as nanotwinned Cu (NT-Cu), are approaching their limitations, whereas nanocrystalline Cu (NC-Cu) is less affected by the scaling effects of the damascene process. Additionally, due to its high grain boundary density, NC-Cu exhibits strong potential for Cu bonding by leveraging grain boundary diffusion paths and internal grain boundary energy to facilitate atomic movement under specific thermal treatments. Hence, the effect of grain size on bonding quality has become highly critical. In this study, we employed electroplated Cu films with different grain sizes in random orientation to analyze their effect on bonding quality. Since grain growth at the interface during bonding plays a key role in determining final bonding outcomes, the thermal stability of the films was evaluated through resistance measurements. Subsequently, FIB cross-sectional imaging and die shear testing provide a comprehensive analysis of the grain size effect, offering a novel perspective for enhancing Cu bonding in future packaging technologies.