Photolysis of SF_6 adsorbed on Si(111)-7×7 by monochromatic soft X-ray

摘要

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 SF_6 molecules adsorbed on Si(111)-7×7 at 30K (SF_6 dose = 3.4×10~(13) molecules/cm~2, ～0.5 monolayer). The photon-induced evolution of adsorbed SF_6 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) cm~2, 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 SF_6 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 SF_6 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 SF_n (n = 1-6) species.