The roughening mechanism of ArF photoresist during etching was investigated to find out why CF3I gas reduces the line edge roughness (LER) in the photoresist pattern better than CF4 gas. Since the plasma of reactive ion etching (RIE) consists of ultraviolet (UV) photons, radicals, and ions, the authors used a UV lamp and a neutral beam source for evaluating the effect of different plasma compositions on the photoresist roughness. The roughness was found not to increase only by UV photons or F radicals, but increase under the CF4 RIE plasma which has both UV photons and F radicals. A C-F modified layer was generated on the resist surface because the UV damaged C=O bonds reacted with F radicals and the resist surface became softer and shrank. Since CF3I plasma has a lower UV intensity and fewer F radicals compared with CF4 plasma, the shrinkage on the sidewall of the photoresist was suppressed and resulted in a smaller LER when this plasma was used. (C) 2009 American Vacuum Society. [DOI:10.1116/1.3196785]