Excitation site-specific ion desorption study of Si(1 1 1) surfaces fluorinated by XeF2 using photoelectron photoion coincidence spectroscopy

The excitation site-specific ion desorption of Si(1 1 1) surfaces fluorinated by XeF2 at room temperature was studied by Si:2p-F+ photoelectron photoion coincidence (PEPICO) and Si:2p photoelectron spectroscopy (PES) using synchrotron radiation. The PES components with a chemical shifts of ∼1, ∼2 and ∼3 eV were assigned as the SiF, SiF2 and SiF3 sites according to a previous PES study by Lo et al. The components with a chemical shift of ∼4 and ∼5.6 eV are called SiFx and SiFy, respectively. For exposures of 3200–320,000 L, the SiF + SiF2 PES component reached a constant intensity, while the SiF3 and SiFx PES components continued to increase. The SiF + SiF2 and SiF3 PEPICO components, on the other hand, decreased over 9600 L, while the SiFx and SiFy PEPICO components continued to increase over 9600 L. Based on these results, we assigned the SiFx and SiFy components to the surface SiFSi*F3 and SiF2Si*F3 sites, respectively. We concluded that fluorination of the top-most Si layer proceeded after the initial XeF2 exposures, while fluorination of the second Si layer started in 6400–9600 L. We estimated the F+ desorption probability per Si:2p ionization for the SiF + SiF2, SiF3 and SiFSi*F3 sites to be 3–5×10−4, 1–5×10−4 and 2–3×10−4 for 800–9600 L, respectively and the SiF3 and SiFSi*F3 sites to be 2–3×10−4 and 3–5×10−4 for 12,800–32,000 L, respectively. PEPICO spectroscopy proved to be useful to monitor the fluorination process of surfaces.