Depth resolution and inhomogeneity of the sputtering dose with sample rotation and ion beam rastering

Sample rotation during sputter depth profiling can improve the measured depth resolution. We examine some of the practical issues of particular relevance to rotation conditions in SIMS. There are many ways of arranging the rotation for sputtering and, to illustrate the issues, one configuration is studied. Through simulations of the spatial distribution of ion dose using a rastered ion beam and sample rotation, we demonstrate that significant variations in the distribution of ion dose across the surface can occur during the profile. With rotational frequencies much lower than raster frequencies, as used for Auger and XPS, these variations are typically small and have the same periodicity as the rotation. If rotational frequencies are similar to, or larger than the raster frequency, then large spatial variations in dose can occur. In this case, extreme care must be taken to ensure that the two frequencies are not related by a simple rational number and that the sputtering ion beam size should be significantly broader than the line spacing of the raster, but significantly smaller than the raster size. Specific recommendations are provided for setting both the rotation frequency and the size of the sputtering ion beam in order to ensure that the relative standard deviation of the ion dose across the analysis area remains lower than 1%. However, the best method may be a stepwise rotation of 90° after each analysis and sputtering cycle. © Crown copyright 2011. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.