Effect of micro-texture of electroplated copper thin-films on their mechanical and electrical reliability

Both the static and fatigue strengths of electroplated copper thin films were measured under uni-axial stress. The mechanical properties such as the yield stress, fracture elongation and Young's modulus of each film were quite different from those of bulk copper depending on their micro structure. The fracture surfaces were observed by SEM after the fatigue test. It was found that there were two fracture modes under the fatigue test. One was a typical ductile fracture, and another was brittle one even under the fatigue load higher than its yield stress. Since the initial micro texture was found to change significantly even after the annealing at temperatures lower than 300°C, the effect of the thermal history of them after electroplating on both their micro texture and fatigue strength was investigated quantitatively. In addition to the mechanical properties, the electrical properties of the films varied significantly depending on their micro texture. The resistivity of the films was much higher than that of bulk material, and the increase ratio varied from a few to a hundred depending on the conditions of electroplating. This increase can be also attributed to the variation of their micro texture. The fracture mode of the films under electromigration tests also varied from the local fusion cutting caused by the localized Joule heating to the conventional electromigration caused by the diffusion of copper atoms. This change can be explained by the characterization of grain boundaries of the films. When the grain boundaries were rather porous, copper atoms could not cross the adjoining grains. Since the resistivity of the porous grain boundaries is very high, fusion cutting may occur due to the highly localized Joule heating. This fusion cutting occurs unexpectedly without clear increase of the resistance of interconnections.