CHEMICAL AND PHYSICAL SPUTTERING OF FLUORINATED SILICON

Molecular dynamics simulations were performed on low-energy argon-ion bombardment (200, 50, and 20 eV) of silicon layers with varying amounts of fluorine incorporated. At low fluorine incorporation in the layers (F/Si<0.5), only physical sputtering was observed, although the physical sputtering yield increased compared to pure amorphous silicon. At higher levels of fluorine incorporation into the silicon layer, ion impact resulted in the formation of weakly bound SiFx (x=1-3) species in the layer. This phenomenon appears to be similar to chemical sputtering as defined by Winters and Coburn [H. F. Winters and J. W. Coburn, Surf. Sci. Rep. 14, 164 (1992)]. The overall yield, due to both physical and chemical sputtering, was found to follow a square-root dependence on ion energy. The threshold ion impact energy for the formation of weakly bound species in heavily fluorinated silicon layers extrapolated to ≤4 eV, and for physical sputtering to about 20 eV. The simulations imply that the source of the ion-neutral synergism in ion-assisted etching occurs on the collision cascade time scale (∼10-12 s) with the creation of these weakly bound species. The overall rate determining step for ion-assisted etching, however, is often a much slower process involving thermal desorption, chemical reaction, or diffusion. This difference between the source of the ion-neutral synergy (creation of weakly bound species in 1 ps or less) and the rate determining step (often on much longer time scales) has probably contributed to the confusion that has surrounded discussions of the mechanisms of ion-assisted etching. © 1995 American Institute of Physics.