It was recalled that the electromigration activation energy in multi-grained lines was often about 2 times lower than that for grain-boundary diffusion, while the pre-exponential factor in the electromigration rate expression was several orders of magnitude smaller than that which was characteristic of electromigration along grain boundaries. By analyzing published data (especially on drift velocities) it was shown that grain boundaries were the most likely major electromigration diffusion pathways in Cu interconnects. On the basis of recent advances in the theory of grain-boundary grooving with an arbitrary grain-boundary flux, and by using a specific model which applied the general theory to electromigration, the major features of electromigration in Cu were explained in terms of the extension of slit-like grooves along the interconnect line; followed by their merging. Upon incorporating the electromigration activation energy that had been reported for pure Cu into the model, it was concluded that surface diffusion along freshly created groove walls was slow; with an activation energy of more than 2eV. It was thought that this was probably due to trace surface contaminants. This added a new key element to the grain-boundary grooving model, in which surface diffusion acted as a so-called healing mechanism rather than as an independent pathway that was parallel to the grain-boundary diffusion. By using empirical surface diffusion parameters for Cu, it was possible to rationalize the main features of Cu electromigration behavior which had been reported. Such features included the values of the electromigration frequency factors, the island-like morphology of the electromigration displacement region, the current density exponent, and the cause of the effect of Sn upon the electromigration parameters. It was concluded that the full advantages of Cu could be obtained if trace contaminants, which were responsible for suppressing diffusion along freshly created surfaces, were identified and eliminated. This was expected to allow unalloyed multi-grained Cu interconnects to have an electromigration resistance that was 4 orders of magnitude larger than that of Al-2%Cu at 100C.

E.Glickman, M.Nathan: Journal of Applied Physics, 1996, 80[7], 3782-91