In order to fabricate the structures with high aspect ratio (depth/width), it is necessary to develop plasma etching processes with a very accurate feature control and improvements in etching rates. We have developed an etching simulator which takes into account the main plasma-surface interactions in a SF6/O-2 plasma etching on silicon substrate process. In this article, the role of oxygen on final trench topography and etching rate evolution is discussed. The presented results show that the notion of balance between the passivation regime and the etching processes has great consequences in topographic and kinetic trench characteristics. In particular, a good correlation has been established between the roughness on the trench sidewalls and zones of underpassivation. (C) 2004 American Vacuum Society.