Why does an SF$_{\mathrm{6}}$ plasma etch silicon much faster than any other fluorine atom generating plasma?

It has long been known that F atoms are the reactive species responsible for etching of silicon in all fluorine containing plasma. Despite this, SF$_{\mathrm{6}}$ plasmas are widely found to etch silicon up to 100 times faster than the other fluorine-containing plasmas. We have found that this is due to the presence of adsorbed sulfur that catalyzes the fast reaction of F with Si. F atom reaction probabilities are \textasciitilde 30-fold higher in SF$_{\mathrm{6}}$ plasmas compared with values in NF$_{\mathrm{3}}$ plasmas. Addition of only 10{\%} SF$_{\mathrm{6}}$ to an NF$_{\mathrm{3}}$ plasma produced a much higher reaction probability (\textasciitilde 10-fold) than in a pure NF$_{\mathrm{3}}$ plasma. By allowing sulfur in isopropyl alcohol to evaporate on masked Si samples, sulfur could be preferentially deposited in relatively high concentrations in selected regions. When this sample is placed side by side with one not exposed to sulfur, the sulfur-dosed sample etched several times faster at the center of each bead, while sulfur-free surfaces exhibited the expected slower rate. Discrepancies among previous published studies will be resolved. Mechanisms for the catalytic behavior, such as enhanced chemisorption of F and electronic effects caused by S mid-bandgap states in Si, will be discussed.