Abstract
Electromigration in unpassivated copper metal lines was directly observed at room temperature in ultrahigh vacuum by in-situ transmission electron microscopy. Electromigration study of Cu grains with specific crystal orientations was performed when surface diffusion is a dominant migration mechanism, and the EM-related reliability considerations were also discussed. Atomic resolution imaging of Cu electromigration was conducted on (211) and (110) crystal planes under an electric current density of 2 × 106 A/cm2. It was found that the EM-induced atomic migration appeared to be anisotropic, and some atomic surface steps were observed on (111) crystal planes. The combination of {111} planes and <110> directions was suggested to be the most favored electromigration paths for crystalline copper. The relatively intensity of (111) peak in the Cu thin film was found to degrade after electric current stressing by X-ray diffractometry, indicating that {111} crystal planes are more sensitive to EM damage. The atomic migration leading to a smoothing of the Cu thin film was also observed by atomic force microscopy, which suggests a feasible way to reduce the roughness of the Cu thin film. Furthermore, the influence of twin boundaries on atomic-scale electromigration was also investigated at room temperature in ultrahigh vacuum by in-situ transmission electron microscopy. It was found that some atomic surface steps are driven by an electric current to move along specific crystal planes and are blocked at the {111}/<112> type twin boundaries in a Cu grain for a period of time (~ 5s). The results indicate that the twin boundaries serve as barriers to EM-induced atomic transport, and hence reduce the voiding rate in the Cu conductors under electric current stressing. The study not only helps in understanding the fundamental EM mechanism in the twin-structured crystal, but also provides a possible means of improving the reliability of Cu interconnects by introducing a high density of twins in the Cu wires. In addition, it was found that the twin region in the Cu grain can be eliminated by introducing an electric current through the sample. The grain boundary structure may be held responsible for the observed different interactions between electric currents and twin boundaries.