Micro texture dependence of mechanical properties of electroplated copper thin films used for thin film interconnection

Electroplated copper thin films have been used for interconnection of semiconductor devices. Both the mechanical and electrical properties of the films were found to be quite different from those of bulk material, and thus, the reliability of the devices is not so high as to be expected. The main reason for the difference was found to be their micro texture. When the films consist of fine columnar grains with weak grain boundaries, their mechanical properties show strong anisotropy and complicated fracture mode. Thus, the fatigue strength of the electroplated copper thin films was measured under uniaxial 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 due to their unique micro structure. The micro texture of each film was observed by using SEM (Scanning Electro Microscope) and SIM (Scanning Ion Microscope). The low-cycle fatigue strength varied drastically depending on their micro texture, while the high-cycle fatigue strength was almost same. 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 the other was brittle one even under the fatigue load higher than its yield stress. The crack seemed to propagate in the grains when the ductile fracture occurred since typical striations and dimples were observed clearly on the fractured surfaces. On the other hand, the crack seemed to propagate along grain boundaries of columnar grains when the brittle fracture occurred. No striations or dimples remained on the fractured surfaces. One of the reasons for this brittle fracture can be explained by cooperative grain boundary sliding of the films which consist of fine columnar grains with weak grain boundaries. These results clearly indicated that the fatigue strength of the electroplated copper thin films varies depending on their micro structure. 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. Not only the average grain size, but also the crystallographic structure of the films changed significantly depending on their thermal history, and thus, the fatigue strength of the films varied drastically. It is important, therefore, to control the micro texture of the films for assuring their reliability.