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

In this work, we have investigated effects of pre-annealing, which means an nealing performed prior to electromigration. (EM) test, on EM lifetime of A l-Cu lines. We also investigated the relationships between void formation a nd size of Cu precipitated area in the line under various pre-annealing con ditions. It is found that EM lifetime decreases while the size of the Cu pr ecipitated area increases with lengthening of the pre-annealing period. How ever, no void is observed after this pre-annealing treatment. The results i ndicate that the tiny voids generated by formation of CU precipitation do n ot move during the pre-annealing period. In the case of EM testing, Cu prec ipitation occurs followed by void formation at the cathode area, probably d ue to diffusion of vacancies which are generated by Cu atom movement by ele ctron 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 g rain boundary forming the Cu precipitates. However, in samples with no pre- annealing treatment, the accumulation of Cu atoms at the grain boundaries b egins 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 incuba tion time to accumulate Cu atoms at the grain boundaries. This is the reaso n why EM lifetime of pre-annealed samples is shorter than that of samples w ith no pre-annealing treatment. (C) 2001 Elsevier Science Ltd. All rights r eserved.