The effects of microwave electron cyclotron resonance (ECR) plasma oxidation on smooth as well as purposely roughened Si surfaces were studied using atomic force microscopy with fractal analysis and the results were compared to thermal oxidation. In addition, spectroscopic immersion ellipsometry was employed to obtain information on theSi/SiO2interface without the necessity of physically removing the oxide overlayer. The results are concordant and show that during both the ECR plasma and thermal oxidation processes, theSi/SiO2interface roughness decreases for the purposely roughened Si surfaces, but increases for initially smooth Si surfaces. Besides the initial surface roughness dependence, the change of this interface roughness depends mostly on the extent of oxidation, but also on temperature. The decreasing roughness for plasma oxidation is thought to be driven by the Kelvin equation, but enhanced by an applied external electric field. Ion bombardment, localized defects, and nucleation are eliminated as causative for roughening of initially smooth Si surfaces, and the results suggest a chemically enhanced roughening transition.