The mechanism of modified layer formation for 193 nm photoresist (PR) during a short time (up to similar to 10 s) fluorocarbon plasma exposure was investigated. We employed a shutter approach to achieve rapidly steady-state plasma condition when processing PR surfaces. The time evolution of the optical constants and the thickness of the modified layer on the PR surface were obtained using two layer optical modeling of ellipsometric data for the processed PR material. This enabled us to determine the time-resolved etching rate of the PR and the kinetics of modified layer formation. The change in the surface chemical composition of the PR materials was determined by x-ray photoelectron spectroscopy (XPS). A graphitic layer with a higher refractive index as compared to the bulk PR material was formed on the PR surface within a few (similar to 3 s) seconds of plasma exposure. The XPS data revealed that before a fluorinated surface developed, cleavage at ester groups of the side chain in the polymer and dangling bond formation took place, leading to cross-linking. To investigate the influence of the oxygen content of the polymer on surface roughness formation, we compared the surface evolution of oxygen-rich 193 and 248 nm PRs, which have a smaller oxygen content. Remarkable differences in the etching behavior during the initial plasma interaction period were observed for the two materials. Whereas for 193 nm PR, etching was observed immediately and the material exhibited higher surface roughness, for the 248 nm PR material fluorocarbon film deposition took place initially. Once a fluorinated surface had developed, steady-state etching took place, but the 248 nm PR exhibited lower surface roughness than the 193 nm material. XPS measurements showed that when comparing the fluorine content of the surface layer to the oxygen content, the fluorine content was relatively more important for the 248 nm PR than for the 193 nm PR. For the latter, oxygen in the side groups of the bulk PR enhanced the PR etching rate initially and led to a rapid surface roughness formation. This coincides with the development of a fluorinated surface.