New Aspects on the Reduction of Nitric Acid during Wet Chemical Etching of Silicon in Concentrated HF/HNO₃ Mixtures

Abstract

The reduction of nitric acid that occurs during wet chemical etching of silicon using HF/HNO3 mixtures had been studied under different reaction conditions. The proof of the generation of considerable amounts of ammonium shows that the reduction of nitric acid is more complex than assumed so far and that the current model of NO as final product of nitric acid reduction during silicon etching does not hold. A qualitative model of the nitric acid reduction during silicon etching in HF/HNO3 mixtures that bases on experimentally verified and also assumed nitrogen intermediates is presented and explains the formation of ammonium out of nitric acid by four successively proceeding two-electron reduction steps. The formed ammonium shows an unexpected temperature-dependent decomposition behavior in concentrated etch solutions that has to be discussed in light of a previous study on the generation and decay of N(III) intermediates, mainly dissolved N2O3 and NO+, during the etching. Below 8 °C, ammonium is found to be stable in concentrated etch solutions and in the presence of N(III) intermediates. The observed time-dependent decay of ammonium that occurs at temperatures ≥8 °C is directly related to the decay of N(III) intermediates. Consequently, the decay of ammonium was found to be finished immediately after all N(III) intermediates are completely decomposed. As one possible explanation, a model that links the decomposition of ammonium with the thermal and oxidative decomposition of the N(III) intermediates is suggested.

Preparation of small silicon carbide quantum dots by wet chemical etching

“…Most of the authors agree that the active agent of silicon etching is not NO 3 À , but an intermediate nitrogen species in the oxidation state 13. [25][26][27][28] We suppose that the active agent is similar in the etching of SiC and the accelerated etching could be due to the higher intermediate stability in the closed system. In an open system, the gas phase reaction product of nitrogen reduction can escape and this can shift the equilibrium, which lowers the probability of etching.…”

Experimentally Derived Catalytic Etching Kinetics for Defect-Utilized Dual-Porous Silicon Formation

“…The reason for the increasing oxygen diffusion rate, confirmed from the increasing dissolution rate, is considered as the existence of so-called 'autocatalyst'. 29 oxidation is performed mainly by the reactive oxidant [29][30][31][32]35,37 rather than direct HNO 3 . The induction period prior to an observable macroscopic reaction is direct evidence, 21,[29][30][31][32]37,55 and the oxidation only by HNO 3 requiring very long duration (several hour) and certain thermal condition (∼120°C) to form thin oxide film (<30 nm) is indirect evidence.…”

Porous Silicon Formation by Stain Etching

“…Acker and co-workers (Acker et al 2012;Hoffmann et al 2011;Steinert et al 2005Steinert et al , 2006Steinert et al , 2007Steinert et al , 2008Henssge and Acker 2007;Acker and Henssge 2007;Weinreich et al 2007;Jadzinsky et al 2007) The rate of stain etching is enhanced by the presence of defects. This characteristic has been used to create a hybrid amorphous porous/defect-followed mesoporous structure (Woo et al 2012a, b).…”

“…Unfortunately, the reduction of NO 3 À and the myriad other nitrogen-containing species that are formed as by-products are extremely complex. Only recently have Acker and co-workers been able to establish the role of these various species in the electropolishing regime (Acker et al 2012;Hoffmann et al 2011;Steinert et al 2007Steinert et al , 2008Henssge and Acker 2007;Acker and Henssge 2007;Steinert et al 2005Steinert et al , 2006. This complexity (i.e., high sensitivity to composition, temperature, extent of reaction, various dissolved gases, etc.)…”