Comprehensive stoichiometric studies on the reaction of silicon in HF/HNO 3 and HF/HNO 3 /H 2 SiF 6 mixtures.

The stoichiometry of the wet chemical etching of silicon in concentrated binary and ternary mixtures of HF, HNO 3 and H 2 SiF 6 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 HNO 3 in the mixture and can be attributed to the consumption in subsequent reactions with increasing concentration. Furthermore, a critical minimum concentration of undissociated HNO 3 of q (HNO 3 , 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 HNO 3 in the reaction with Si. Furthermore, the amount of formed hydrogen is determined by both the HF and the HNO 3 concentration in the mixture. For binary mixtures, the H 2 formation can be quantitatively described as a function of the concentration of HNO 3 , HF and H 2 O. 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 HNO 2 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 HNO 3 and also the formation of the reaction products NO x and H 2 can therefore be functionally described on the basis of the H 2 O content in the etching mixture, regardless of a binary or ternary mixture.