Oxidation of HF-treated Si wafer surfaces in air

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Abstract
The change in the chemical surface state of polished Si wafers [p‐type, (100) oriented] during storage in air at room temperature was investigated for storage times up to half a year. Measurements were performed by x‐ray Photoelectron Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS). Immediately after the HF treatment (1 min 5% HF, 2 min water rinse) vibrational spectroscopy (HREELS) shows a predominant coverage of the surface with hydride groups (80%–90% of a ML), which can be inferred from the presence of the stretching (2100 cm−1), scissor (900 cm−1) and bending (640 cm−1) vibrations in the spectra. A slight additional coverage with oxygen is proved by XPS and originates from Si‐OH groups (3670 cm−1) and oxygen‐related hydrocarbon groups (XPS). These Si‐OH groups result from an exchange reaction of Si‐F with water during the two‐minute water rinse. The development of an oxygen coverage during subsequent storage in air occurs extremely slowly and shows a logarithmic behavior. A monolayer coverage of oxygen (7×1014/cm2) is reached after approximately 7 days of storage in air. HREELS spectra exhibit the concurrent development of the asymmetric Si‐O‐Si vibration, which indicates that oxygen penetrates the lattice and breaks Si—Si bonds. During this period the Si‐O‐Si frequency shifts from about 1060 to 1100 cm−1. The penetration of backbonds of Si—H gets evident by broadening of the Si‐H stretching vibration and finally by a shift to higher wavenumbers. Chemically shifted components of the Si 2p line (partially oxidized Si) are present with the SiO2−x component (chemical shift ≳3.4 eV) becoming dominant after roughly a week. Further oxidation proceeds essentially by an increase of the SiO2 peak in combination with a steeper slope of the logarithmic growth curve. The SiO2 thickness after half a year is about 8 Å. The frequency of the Si‐O‐Si vibration shifts up to 1120 cm−1, which can be related to a growing angle of the Si‐O‐Si bridge. Si—H groups are still present, the final peak position is about 2220 cm−1. The measurements show an extended induction period until the monolayer range of oxide coverage is attained. We ascribe this to the passivation of the surface by hydrogen and to a HF treatment according to Very‐Large‐Scale‐Integration standards.