A study was made of 0.15 to 10μ wide, and 0.3μ thick, lines which had been deposited via physical vapour deposition. Resistance and edge-displacement techniques were used at 255 to 405C. In the case of wider (greater than 1μ) polycrystalline lines, the predominant diffusion mechanism was a mixture of grain boundary and surface diffusion. In narrower lines, the predominant mechanism was surface transport. The activation energy for grain-boundary transport was about 0.2eV higher than that for surface transport. That is, the activation energies for surface and grain-boundary diffusion were 0.9 and 1.1eV, respectively. These values were in good agreement with reported values of 0.78 to 0.90eV for the activation energy of surface diffusion and of 0.88 to 0.95eV for grain-boundary diffusion. The apparent effective charge number could be estimated from the diffusivities and other data. By taking the surface diffusivity parameters to be 0.15cm2/s and 0.78eV, or 0.26cm2/s and 0.90eV, the apparent effective charge number was deduced to be -0.1 or -0.8, respectively, at 400C. The negative sign indicated that the Cu atoms drifted in the direction of the electron wind. Upon using the grain-boundary diffusivities, a grain-boundary charge number of -14 at 400C was obtained. These values depended upon the accuracy of published diffusion parameters, but were consistent with the theoretical prediction that the electron wind force decreased in going from the bulk, to grain boundary, to surface. The overall results showed that the fast-migration paths were the surface, in bamboo and near-bamboo structures, and were a mixture of surface and grain boundary paths in polycrystalline films.

C.K.Hu, R.Rosenberg, K.Y.Lee: Applied Physics Letters, 1999, 74[20], 2945-7