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 s oft-x-ray photoelectron spectroscopy (SXPS) and photon-stimulated deso rption (PSD). The exposures encompass the entire range from chemisorpt ion to steady state etching. By taking into account the different surf ace sensitivities of SXPS and PSD, the microscopic structure of the su rface fluorosilyl reaction layer is obtained as a function of exposure , and the reaction process is modeled. It is found that the reaction-l ayer structure passes through four distinct exposure regimes. Steric h indrance between the F atoms of neighboring fluorosilyl groups and def ects in the substrate are responsible for the evolution of the reactio n-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.