It was recalled that, in polycrystalline Al-Cu, mass transport was well-known to occur in sequential stages. These were an incubation period (attributed to Cu depletion beyond a critical length), followed by Al drift. The drift behavior of bamboo reactive ion etched and damascene samples was analyzed here. By using Blech-type test structures, mass transport in reactive ion etched lines was shown to proceed via both lattice and interfacial diffusion. The predominant mechanism depended upon the Cu distribution in the line; as revealed by comparing as-patterned (lattice electromigration) and annealed (interface electromigration) samples. Interfacial electromigration occurred only at metallic interfaces. In this case, Cu alloying was observed to retard Al interfacial mass transport; thus giving rise to an incubation time. Although the activation energy for the incubation time was found to be similar to that which controlled Al lattice-drift (for which no incubation time was observed), lattice electromigration was preferred over interfacial electromigration because it was insensitive to enhancement by geometrical effects upon scaling. Upon comparing interfacial electromigration in reactive ion etched samples with bamboo damascene Al-Cu, with the incubation time controlling both, the higher electromigration threshold which was observed for damascene was shown to be insufficient to compensate for its significantly increased Cu depletion rate. This was contrary to the case of polycrystalline Al-Cu interconnects. It was demonstrated that 2 factors were involved. Firstly, there were more metallic interfaces and this was intrinsically related to the use of wetting or barrier layers. Secondly, additional formation of Al3Ti at the trench side-walls further enhanced the Cu depletion rate and reduced the rate-controlling incubation time. A separate drift study of reactive ion etched via-type test structures indicated that it was very difficult to suppress interfacial mass transport in favour of lattice electromigration in the presence of Al3Ti formation.

Electromigration-Induced Drift in Damascene and Plasma-Etched Al(Cu) - Mass Transport Mechanisms in Bamboo Interconnects. J.Proost, K.Maex, L.Delaey: Journal of Applied Physics, 2000, 87[1], 99-109