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M. Lippold, U. Böhme, C. Gondek, M. Kronstein, S. Patzig-Klein, M. Weser, E. Kroke:
"Etching Silicon with HF-HNO₃-H₂SO₄/H₂O Mixtures - Unprecedented Formation of Trifluorosilane, Hexafluorodisiloxane, and Si-F Surface Groups";
European Journal of Inorganic Chemistry, 2012 (2012), 34; 5714 - 5721.

The etching behaviour of sulfuric-acid-containing HF-HNO3 solutions towards crystalline silicon surfaces has been studied over a wide range of H₂SO₄ concentrations. For mixtures with low sulfuric acid concentration, NO₂/N₂O₄, N₂O₃, NO and N₂O have been detected by means of FTIR spectroscopy. Increasing concentrations of nitric acid lead to high etching rates and to an enhanced formation of NO₂/N₂O₄. Different products were observed for the etching of silicon with sulfuric-acid-rich mixtures [c(H₂SO₄) > 13 mol L^-1]. Trifluorosilane and hexafluorodisiloxane were identified by FTIR spectroscopy as additional reaction products. In contrast to the commonly accepted wet chemical etching mechanism, the formation of trifluorosilane is not accompanied by the formation of molecular hydrogen (according to Raman spectroscopy). Thermodynamic calculations and direct reactions of F₃SiH with the etching solution support an intermediate oxidation of trifluorosilane and the formation of hexafluorodisiloxane. The etched silicon surfaces were investigated by diffuse reflection FTIR and X-ray photoelectron spectroscopy (XPS). Surprisingly, no SiH terminations were observed after etching in sulfuric-acid-rich mixtures. Instead, a fluorine-terminated surface was found.

http://dx.doi.org/10.1002/ejic.201200674