New aspects on the reduction of nitric acid during wet chemical etching of silicon in concentrated HF/HNO3 mixtures

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 T, 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 degrees 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.