Photolysis of SF6 adsorbed on Si(111)-7 × 7 by monochromatic soft X-ray

Continuous-time photoelectron spectroscopy (PES) and photon-exposure-dependent photon-stimulated desorption (PSD) were employed to investigate the monochromatic soft X-ray-induced dissociation of SF6 molecules adsorbed on Si(111)-7 × 7 at 30 K (SF6 dose = 3.4 × 1013 molecules/cm2, ∼ 0.5 monolayer). The photon-induced evolution of adsorbed SF6 was monitored at photon energies of 98 and 120 eV [near the Si(2p) edge], and sequential valence-level PES spectra made it possible to deduce the photolysis cross section as a function of energy. It was found that the photolysis cross sections for 98 and 120 eV photons are ∼ 2.7 × 10− 17 and ∼ 3.7 × 10−17 cm2, respectively. The changes in the F− and F+ PSD ion yields were also measured during irradiation of 120 eV photons. The photon-exposure dependencies of the F− and F+ ion yields show the characteristics: (a) the dissociation of adsorbed SF6 molecules is ascribable to the substrate-mediated dissociations [dissociative attachment (DA) and dipolar dissociation (DD) induced by the photoelectrons emitting from the silicon substrate]; (b) at early stages of photolysis, the F− yield is mainly due to DA and DD of the adsorbed SF6 molecules, while at high photon exposure the F− formation by electron capture of the F+ ion is likely to be the dominant mechanism; (c) the F+ ion desorption is associated with the bond breaking of the surface SiF species; (d) the surface SiF is formed by reaction of the surface Si atom with the fluorine atom or F− ion produced by scission of S–F bond of SFn (n = 1–6) species.