Improvement of the crystallinity of electroplated copper thin films for highly reliable 3D interconnections

The degradation process of the crystallographic quality of copper thin films, which are used for interconnections and micro bumps for 3D integration, during electromigration and stress-induced migration tests is dominated by the diffusion along grain boundaries and the diffusion constant of copper varies drastically depending on the crystallinity of the films. The degradation process was visualized clearly by applying an electron back-scatter diffraction method. The copper atoms in the electroplated copper thin films migrated mainly in the area with low crystallinity, in other words, the area with high defect density. Since the crystallinity of the films was found to be dominated by the lattice mismatch between copper and the seed layer material used for electroplating, the integrity of the interface structure was improved by minimizing the lattice mismatch. It was validated that the introducing the thin layer with fine grains and random orientation is effective for minimizing the lattice mismatch and thus, improving the crystallographic quality of the electroplated copper thin-film interconnections.