XEF2 ETCHING OF SI(111) - THE GEOMETRIC STRUCTURE OF THE REACTION LAYER

Si(111)-7X7 is exposed to XeF2 in ultrahigh vacuum and examined with soft-x-ray photoelectron spectroscopy (SXPS) and photon-stimulated desorption (PSD). The exposures encompass the entire range from chemisorption to steady state etching. By taking into account the different surface sensitivities of SXPS and PSD, the microscopic structure of the surface fluorosilyl reaction layer is obtained as a function of exposure, and the reaction process is modeled. It is found that the reaction-layer structure passes through four distinct exposure regimes. Steric hindrance between the F atoms of neighboring fluorosilyl groups and defects in the substrate are responsible for the evolution of the reaction-layer structure. When steady-state etching is reached, the reaction layer evolves to a ''tree'' structure of fluorosilyl chains terminated at the surface by SiF3 groups.