Comprehensive stoichiometric studies on the reaction of silicon in HF/HNO₃ and HF/HNO₃/H₂SiF₆ mixtures

The stoichiometry of the wet chemical etching of silicon in concentrated binary and ternary mixtures of HF, HNO₃ and H₂SiF₆ was comprehensively investigated. A complete quantification of both dissolved and gaseous reaction products was carried out for a variety of different acid mixtures. It could be shown that the total nitric acid consumption is directly determined by the concentration of undissociated HNO₃ in the mixture and can be attributed to the consumption in subsequent reactions with increasing concentration. Furthermore, a critical minimum concentration of undissociated HNO₃ of q(HNO₃, undiss) ≥ 0.35 mol kg^-1 could be determined, which is required to start the reaction at 20 °C with agitation, irrespective of the composition of the mixture (binary/tertiary). The simultaneous determination of the nitrogen oxides in the gas phase supports the theory that NO is the only direct reduction product of HNO₃ in the reaction with Si. Furthermore, the amount of formed hydrogen is determined by both the HF and the HNO₃ concentration in the mixture. For binary mixtures, the H₂ formation can be quantitatively described as a function of the concentration of HNO₃, HF and H₂O. The most important finding from comparative investigations between binary and ternary mixtures is that the overall reaction is largely determined by the formation of the reactive intermediate HNO₂ as a result of complex reaction pathways. Both the formation and the accumulation of this intermediate are determined by the water content of the etching mixture. The consumption of HNO₃ and also the formation of the reaction products NO and H₂ can therefore be functionally described on the basis of the H₂O content in the etching mixture, regardless of a binary or ternary mixture.