Electroplated Cu films have been reported to show self-annealing, a microstructural evolution process that occurs at room temperature. As for the mechanism behind this phenomenon, redistribution of trapped additives prior to grain growth has been proposed as the trigger. We have controlled the occurrence of self-annealing by changing current densities (1, 7 A/dm(2)) and film thickness (0.1, 1 mum), and have investigated the impurity distributions. The impurity distributions have been measured just after deposition and after 3 weeks using secondary ion mass spectrometry. We find that the amount of impurities in electroplated Cu films is reduced throughout the film thickness only when self-annealing occurs, while no appreciable redistribution of impurities is observed when self-annealing does not occur. The redistributed impurities are mainly heavy molecular impurities. If impurities must diffuse out prior to self-annealing, we would expect significant impurity redistributions for all the samples after 3 weeks, regardless of the occurrence of self-annealing. Our results suggest that impurity redistributions hardly occur spontaneously at room temperature and do not play an important role in triggering self-annealing. Instead grain growth seems to force impurities to be redistributed.