XeF₂ etching of Si(111): The geometric structure of the reaction layer

Si(111)-7×7 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.