Shallow arsenic implantation distributions and narrowly spaced antimony delta markers in silicon were used to explore near-surface changes of the sputtering yield under ultra-low-energy Cs+ bombardment (0.25-1 keV) at impact angles theta between 0degrees (normal incidence) and 60degrees, i.e. below the critical angle for ripple formation. In all cases, the profiles were found to be shifted towards the surface, with shifts ranging from about 1 nm (250 eV, 0-15degrees) up to 4 nm (1 keV, 0degrees). These artefacts are attributed to unexpectedly high sputtering yields during Cs build-up in silicon. In most cases, the yields became stationary at sputtered depths of less than 5 nm. Using the mean erosion rate between 0 and 5 nm and the stationary rate above 5 nm to define sputtering yields, the initial yield enhancement eta = / Y-infinity for 1 keV Cs+ was found to be as large as 3.2 at 15degrees, decreasing to about 1.6 at 60degrees. Between 30degrees and 60degrees the eta-values for 250 and 500 eV were only slightly less than at 1 keV. Significantly lower values derived at angles theta < 30degrees are probably due to the fact that the respective transition depths are significantly smaller than 5 nm. The results imply that depth profiles previously observed under the conditions of this study suffer from serious depth calibration errors. (C) 2002 Elsevier Science B.V. All rights reserved.