We have investigated the ripple topography on silicon surfaces developed by 2-10 keV O-2(+) bombardment at an impact angle of 45degrees. The ripple amplitude increased with the depth of erosion and finally became comparable with the ripple wavelength. This caused the local impact angle of ripples facing the oxygen-ion beam to reach the critical angle Of SiO2 formation, promoting the phase separation between silicon dioxide and silicon. As observed before, the critical depth for SiO2 formation decreased with decreasing the primary ion energy. This appears to be related to the amount of erosion necessary to reach the critical angle of SiO2 formation. The decrease of wavelength at lower primary ion energies is interpreted as the result of reduced surface diffusivity which is enhanced by ion bombardment. (C) 2002 Elsevier Science B.V. All rights reserved.