Stress-induced migration of electroplated copper thin films used for 3D integration

Since electroplated copper thin films have started to be applied to not only interconnections in printed wiring boards, but also thin film interconnections in semiconductor devices, their electrical reliability was investigated experimentally. Self-made electroplated copper thin films were used for the evaluation. The test film was electroplated on stainless steel under the constant current density of 30 mA/cm² at 30°C. In addition, some films were annealed in pure argon gas at temperatures from 100°C to 400°C for 30 minutes. 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 films within a few hours during test. Since there were a lot of porous grain boundaries in the as-electroplated film, 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. Though the grain coarsening occurred in the annealed film, the grain boundaries of the annealed film still consisted of the high angle grain boundaries. Since the high angle grain boundaries have high grain boundary energy, the diffusion of copper atoms along the grain boundaries should have been enhanced remarkably during the test. This change of the fracture mode clearly indicates the improvement of the crystallographic quality of the annealed film. However, it was also observed that stress-induced migration was activated substantially in the annealed film. Large voids and hillocks grew during the c- - ustody 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 films or other area in the annealed films. Thus, the hillock formation in the annealed films was enhanced by the segregation of sulfur atoms. These sulfur atoms should have been introduced into the 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 films 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 films.