Mechanistic Predictions for Fluorine Etching of Si(100)

We present results of highly correlated ab initio electronic structure calculations on embedded silicon clusters containing 0–4 fluorine atoms that are designed to mimic the Si(100) surface in the initial stages of the fluorine etching reaction. We predict that fluorine atoms initially saturate all the dangling bonds with no activation barrier and with a large release of heat into the solid (6.1–6.4 eV per Si-F bond formed). Above θ_F = 1.0 ML (ML = monolayer), Si-Si bonds start to break, with the reaction still exothermic by 2.9 eV up to θ_F = 1.25 ML. Reaching a coverage of 1.5 ML is either downhill or activated, depending on how the F atoms are deposited. Beyond a coverage of 1.5 ML, we predict that adjacent SiF₂ groups are highly destabilized and should be preferentially etched. These results are consistent with recent experiments involving F atom adsorption on Si(100) and offer the first ab initio heats of reaction for elementary steps in silicon etching by atomic fluorine.