Molecular dynamics simulations of CF2, F, and Ar+ impacting silicon surfaces were conducted to understand the mechanisms controlling steady-state etching in typical fluorocarbon (FC) plasmas. The simulations reveal the central importance of a mixed amorphous silicon carbide (a-Si:C) top layer that forms due to ion impact and ion-induced mixing. This layer of a-Si:C forms to a depth that depends on ion energy and the composition of the radicals impacting the surface. With only thermal CF2 and 200 eV Ar+, the a-Si:C layer stops the etching of the underlying Si. Adding as little as 1 F per ion reduces the thickness and increases the permeability of this layer, resulting in steady etching of the underlying Si. A mixed Si–C layer forms whenever C sticks to the Si surface in the presence of energetic bombardment. The FC polymer and bare Si etch much faster than the a-Si:C layer, suggesting that the competition to form and destroy this layer is key in controlling the Si etch rate in FC plasmas under some conditions. The FC polymer that forms on Si surfaces, under the conditions studied in the simulations, plays an indirect role in etching by supplying both C and F that subsequently mix into the underlying layers due to the impact of energetic species such as Ar+. © 2004 American Institute of Physics.