Stress-induced and electro-migration of electroplated copper thin film interconnections used for 3D integration

Electroplated copper thin films have started to be applied to not only interconnections in printed wiring boards, but also thin film interconnections and TSV (Through Silicon Via) in semiconductor devices because of its low electric resistivity and high thermal conductivity. Thus, the electrical reliability of the electroplated copper interconnections was investigated experimentally. Self-made electroplated copper thin film interconnections were used for the evaluation. Electroplating conditions are as follows. The thin film interconnections were made by damascene process for electromigration tests. The applied current density during the test was varied from 1 MA/cm² to 10 MA/cm². Abrupt fracture caused by the local fusion was often observed in the as-electroplated interconnections within a few hours during the test. Since there were a lot of porous grain boundaries in the as-electroplated thin films, the local high Joule heating should have caused the fusion at one of the porous grain boundaries. Actually, it was confirmed that the failure rate increased linearly with the square of the amplitude of the applied current density. However, the diffusion of copper atoms caused by electromigration was enhanced significantly when the film was annealed at 400°C. Many voids and hillocks were observed on their surfaces. This change of the fracture mode clearly indicates the improvement of the crystallographic quality of the annealed film. It was also observed that the stress-induced migration was activated substantially in the annealed film. Large voids and hillocks grew during the custody of the film even at room temperature without any application of current. This stress-induced migration was caused by the increase of residual tensile stress of about 200 MPa in the annealed film. It was also found that sulfur atoms segregated in the grown hillocks, though no sulfur atoms were found by EDX in the initial as-electroplated interconnections or other area in the annealed thin film interconnections. Thus, the hillock formation in the annealed interconnections was enhanced by the segregation of sulfur atoms. These sulfur atoms should have been introduced into the electroplated films during electroplating. Therefore, it is very important to control the micro texture, the residual stress and the concentration of sulfur in the electroplated copper thin film interconnections to assure the stable life, in other words, to eliminate their sudden brittle fracture and time-dependent degradation caused by the residual stress in the thin film interconnections.