Radioactive tracer technique was applied to examine the migration of Cu impurity from ArF and KrF photoresists onto silicon-based underlying substrates of Si(100), Si(110), Si(111), poly-Si, SiO 2 and Si 3 N 4 . The effects of the polymer constituent in the photoresists and the baking temperature were studied. Results showed that the migration ratio of Cu from ArF photoresist was always exceeded that from KrF photoresist, independently of the substrate types and baking temperatures. The benzene ring in the KrF photoresist confined the Cu migration, while the alicyclic constituent in the ArF photoresist could not. The migration process of Cu from the bulk region into the interface region was discussed. The residual solvent in the bulk region influenced the Cu migration. The electronegativity of Cu explained the accumulation of Cu on wafer surface. Simple diffusion model was tried to explain the migration ratios, but the model did not explain it well due to the change of the photoresist medium during baking. A correlation function was proposed to modify the diffusion coefficient for describing the experimental observation of migration ratios.