In this article, the authors focus on the profiles formed by high-aspect-ratio deep Si etching with SF6/O2 plasma mixtures. One of the most serious problems for deep Si etching processes is lateral etching in the upper regions of sidewalls. This lateral etching seems to depend on time (or etched depth) rather than aspect ratio (depth/width). Reducing the SF6:O2 ratio and lowering the temperature not only reduced the scope of lateral etching but also produced features that were more strongly tapered with depth. Results of simulation indicated that the distribution of radicals during the formation of a hole plays a significant role in determining the characteristics of the hole. In fact, the addition of SiFx radicals from the top of the hole is markedly effective in reducing lateral etching without sacrificing shape in the vicinity of the bottom of the hole. However, the distribution of radicals alone cannot explain the time dependence of lateral etching. The results of experiments with masks having various shapes shows that scattered ions at the facets of the mask are mainly responsible for lateral etching. The time dependence of lateral etching is due to expansion of the mask facets over time.