Formation mechanisms for profile anomalies such as surface roughness and residue have been investigated numerically and experimentally for Si etching in Cl(2)/O(2) plasmas. The numerical simulation employed an atomic-scale cellular model (ASCeM) based on the Monte Carlo algorithm, which reproduced the feature profile evolution experimentally observed during etching at increased O(2) concentrations. A comparison between simulation and experiment indicated that the local surface oxidation induces surface roughness at the bottom of the feature during etching; then, synergistic effects between surface oxidation and ion scattering in microstructural features on roughened surfaces increase the surface roughness, which in turn causes a number of significant residues or micropillars on bottom surfaces of the feature. In practice, in roughened microstructural features, geometrical shadowing effects for neutral oxygen suppress the surface oxidation at the local feature bottom, where the energetic ion incidence is increased owing to ion scattering on sidewalls of the local feature. (C) 2009 Elsevier B.V. All rights reserved.