Si is very reactive to normal plasma etchants such as fluorine (F) based chemicals and the reactions are inherently isotropic. To fabricate small and/or high aspect ratio nanoscale structures in Si, an anisotropic etching process is necessary. SF6 combined with C4F8 has been demonstrated as a good gas combination for anisotropic Si etching. In this study, Ar gas was introduced into the etching chamber to improve Si etching rate. In addition to mixing Ar with F etching gases directly, an alternating Ar and F gas flow process is proposed. It is interesting to see that not only Si etching rate but also etching selectivity are improved by alternating Ar bombardment and SF6/C4F8 etching steps. The Si etching rate is determined by the Ar treatment step in this new alternating Ar and F two step process.