The Cu-Cu interaction energy for Cu 29 on both W 50-F and W 100 was calculated (see Table ). We find that the Cu-Cu interaction energy is significantly larger than the the Cu-substrate interaction and that the magnitude of these interactions are similar on both surfaces. For W 50-F this is consistent with migration of Cu atoms into a third layer. However, there are no Cu atoms migrating into a third layer on W 100 . The Cu-Cu interaction is much more favourable than the Cu-substrate interaction for Cu 29 on Co-and W-doped TaN (Table ). Given the large number of possible Cu-Cu interactions compared to the Cu-substrate interactions, it is likely that this is directly reflected by the large Cu-Cu interaction energy. This means that for very large Cu structures, the competition between these two interactions becomes essentially meaningless and is no longer necessarily a predictor of film morphology.