Cl 2 plasma etching of Si(100): Nature of the chlorinated surface layer studied by angle-resolved x-ray photoelectron spectroscopy

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Abstract
The interaction of a Cl2 plasma with a Si(100) surface has been investigated by angle resolved x-ray photoelectron spectroscopy (XPS). It was found that the amount of chlorine incorporated into the near-surface region of Si increases with ion energy, and does not change with long exposure to the plasma. Chlorine is present as SiClx (x=1–3) with average relative coverages (integrated over depth) of [SiCl]:[SiCl2]:[SiCl3]≅1:0.33:0.13 at −240 V dc bias (mean ion energy ≈280 eV) and 1:0.34:0.087 at 0 V dc bias (mean ion energy ≈40 eV), at x-ray photoelectron spectroscopy (XPS) binding energies of 100.2, 101.2 and 102.3 eV, respectively. Moreover, there is a substantial amount of disordered Si within the chlorinated layer at high ion energy, reflected in a broadening of the 99.4 eV Si peak and the appearance of a shoulder at 98.8 eV, ascribed to Si with a dangling bond. In addition, bulk Si plasmon losses associated with the Cl(2p) and Cl(2s) core levels indicate that roughly one-third of the Cl in the near-surface region is surrounded by bulklike Si at the high ion energy. Modeling of the dependence of the relative concentration of Cl on the take-off angle was used to estimate the Cl content and thickness of the surface layer. From an inversion of the observed take-off angle dependence of the relative Cl and Si XPS signals, depth profiles were derived for the near-surface region. Cl content falls off in a graded fashion, over a depth of about 25 and 13 Å for a mean ion energies of 280 and 40 eV, respectively. The Cl areal density (coverage integrated throughout the layer) increases with increasing mean ion energy from 1.8×1015 Cl/cm2 at 40 eV to 3.5×1015 Cl/cm2 at 280 eV. From a similar inversion of the take-off angle dependence of the SiClx signals, SiCl2 and SiCl3 are found to be largely confined at the top ∼5 Å, while below the surface, disordered Si and SiCl are present.