Synchrotron photoemission investigation of the initial stages of fluorine attack on Si surfaces: Relative abundance of fluorosilyl species

The nature of fluorosilyl species existing on silicon surfaces during the initial stages of fluorine attack has been investigated with use of high-resolution soft-x-ray photoemission spectroscopy. In this study, several different silicon surfaces [Si(111)-(2×1), Si(111)-(7×7), Si(100)-(2×1), and silicon rendered amorphous by argon-ion bombardment] were exposed to fluorine via the dissociative chemisorption of Xe${\mathrm{F}}_2$. For fluorine coverages in the monolayer regime we observed Si $2p$ levels chemically shifted by approximately 1, 2, and 3 eV corresponding, respectively, to Si${\mathrm{F}}_1$, Si${\mathrm{F}}_2$ and Si${\mathrm{F}}_3$. The relative abundance of the specific fluorosilyl species varied significantly among the different silicon surfaces. Recent experiments indicate atomic fluorine is the primary agent in reactive ion etching (RIE) of silicon by ${\mathrm{CF}}_4$/${\mathrm{O}}_2$ plasmas. Based on such experiments several reaction models hypothesizing the existence of specific surface fluorosilyl species have been proposed, although no direct evidence for any of these surface species has been presented. The results presented here provide the first direct measurement of the composition and relative abundance of the fluorosilyl species remaining on silicon surfaces after exposure to fluorine.