Reaction mechanism for fluorine etching of silicon

A reaction mechanism is proposed for etching of Si surfaces by F atoms in which ${\mathrm{SiF}}_4$ is formed. We proposed that in the rate-determining step the F atom attacks the back side of a —${\mathrm{SiF}}_3$ moiety with simultaneous Si—F bond formation and Si—Si bond rupture (as in an $S_N$2 process). The transition state for this process involves charge transfer from the rupturing Si—Si bond to the attacking F atom and is consistent with recent observations that the etch rate is greater for n-type than for p-type silicon.