Mechanism of pre-annealing effect on electromigration immunity of Al–Cu line

Abstract

In this work, we have investigated effects of pre-annealing, which means annealing performed prior to electromigration (EM) test, on EM lifetime of Al–Cu lines. We also investigated the relationships between void formation and size of Cu precipitated area in the line under various pre-annealing conditions. It is found that EM lifetime decreases while the size of the Cu precipitated area increases with lengthening of the pre-annealing period. However, no void is observed after this pre-annealing treatment. The results indicate that the tiny voids generated by formation of Cu precipitation do not move during the pre-annealing period. In the case of EM testing, Cu precipitation occurs followed by void formation at the cathode area, probably due to diffusion of vacancies which are generated by Cu atom movement by electron wind. As a result, resistance of the line increases and eventually it fails completely. It is demonstrated that pre-annealing helps Cu atoms to accumulate at the grain boundary forming the Cu precipitates. However, in samples with no pre-annealing treatment, the accumulation of Cu atoms at the grain boundaries begins just after the start of the EM testing and then the Cu precipitates diffuse toward the anode. Since EM test conditions are the same for samples with and without pre-annealing treatment, the only variation is the incubation time to accumulate Cu atoms at the grain boundaries. This is the reason why EM lifetime of pre-annealed samples is shorter than that of samples with no pre-annealing treatment.