Etching Silicon with HF-HNO3-H2SO4/H2O Mixtures - Unprecedented Formation of Trifluorosilane, Hexafluorodisiloxane, and Si-F Surface Groups

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. Vibrational spectroscopic investigations of the acidic wet chemical silicon etching process with HF–HNO3–H2SO4 mixtures indicate that trifluorosilane (HSiF3) and hexafluorodisiloxane (F3SiOSiF3) are generated. At the same time fluorine‐terminated silicon surfaces are formed as proved by X‐ray photoelectron spectroscopy (XPS).