Ripple formation on silicon due to oblique (ultra-)low-energy cesium bombardment was studied by measuring the degradation in depth resolution during sputter profiling of an antimony doped delta layer sample (six Sb deltas with an interlayer spacing of 5 nm). Additional insight into the evolution of surface roughness was obtained by recording depth dependent changes in matrix negative-ion signals. The surface topography of the sputtered crater bottoms was investigated ex situ by scanning electron microscopy. At beam energies between 0.25 and 1 keV, ripple formation was found to take place at "critical" angles exceeding 50-60degrees. Depending on the impact angle theta, the onset of ripple growth could be observed at depths as small as about 2 nm, i.e. immediately after saturating the sample with cesium. The rate of ripple growth increased with increasing theta. Almost complete loss of resolution of the Sb deltas occurred at angles about 10degrees above the critical angle, at depths as low as. 15-20 nm. (C) 2002 Elsevier Science B.V. All rights reserved.