We present simulations of Si etching with F and Cl radicals in the presence of inert ion bombardment. Si etch yields predicted by the simulation are in good agreement with experiments. The atomic-scale mechanisms of ion-enhanced etching are classified as enhanced spontaneous etching, chemically enhanced physical sputtering, and chemical sputtering. ne primary effects of ions are to increase the local surface coverage of etchant species by increasing the sticking coefficient of arriving radicals and by mediating diffusion of etchant into the subsurface during impact, and create volatile products by inducing chemical reactions within the halogenated surface layer. Ion-assisted effects are most pronounced at low neutral-to-ion ratio and decline as this ratio increases. Explicit ion enhancements to the etch yield are greater for Cl than for F. (C) 2005 American Vacuum Society.