Sidewall roughening of fluorinated polyether waveguides fabricated by reactive-ion etching in pure oxygen plasma was investigated. Variation of sidewall roughness (SWR) was observed by scanning electron microscopy, and was measured and confirmed by atomic force microscopy. Lower-pressure SWR variation along the vertical depth is due to the combination of both the shadowing and the first-order reemission effect, and higher-pressure SWR variation is postulated to be due to a continuous shadowing effect. It was discovered that the autocorrelation length (ACL) calculated from the experiment varies in a similar manner as the SWR does along the depth of the waveguides. This variation of ACL suggests that the surface-roughness evolution at the sidewalls during plasma etching possesses the same etch dynamics as that of planar thin-film etching. With the increase of pressure, the average value of roughness exponent increases. (c) 2006 American Vacuum Society.