A revised model of silicon oxidation during the dissolution of silicon in HF/HNO₃mixtures

Abstract

This article presents a comprehensive quantification of the gaseous and dissolved reaction products formed during the etching of Si in HF/HNO3 mixtures and provides a revised model of HNO3 reduction during the oxidation of Si.

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

“…9 However, the discussed relationship between N 2 O 3 and the intermediate N 4 O 6 2+ , which led to the assumption that NO + is the determining reactive N(III) species in the etching process, 9,11 was disproved in a recent study. 12 The formation of N 4 O 6 2+ is exclusively caused by the accumulation of N 2 O 4 in the etching mixture and has no influence on the etching rate. 12 Furthermore, in this study, 12 the assumption that NO 2 is a direct reaction product of the reaction of Si and a mixture of HF/HNO 3 8 could be disproved.…”

“…12 The formation of N 4 O 6 2+ is exclusively caused by the accumulation of N 2 O 4 in the etching mixture and has no influence on the etching rate. 12 Furthermore, in this study, 12 the assumption that NO 2 is a direct reaction product of the reaction of Si and a mixture of HF/HNO 3 8 could be disproved. The entire quantification of the gas phase, including H 2 , as well as dissolved N species and nitrogen oxides proves that the crucial charge transfer step is the reduction of HNO 2 to NO, 12 which is in agreement with the work of Balbaud et al 44 The regeneration of the reactive species HNO 2 could be described as a series of side and consecutive reactions of the reduction product NO within the bulk solution.…”

“…However, current studies show that these simplified models neither adequately describe the exact reaction process nor the stoichiometry of the reaction. For example, the reduction of nitric acid leads to a variety of gaseous and dissolved nitrogen oxides (NO, N 2 O, 6,7 NO 2 , 8 N 4 O 6 2+ and N 2 O 3 8,9 ) and even elemental nitrogen, N 2 , 12 as well as ammonium ions, NH 4 + . 11 And even the amount of nitric acid needed to dissolve silicon varies with the HF : HNO 3 mixing ratio.…”

“…While the reaction step of SiO 2 dissolution in HF has been extensively studied, 14,15 the oxidation of Si by H 2 formation and reduction of the used oxidant is still discussed in the literature. 6–34 In case of HNO 3 as oxidant, the situation is complicated due to the variety of different reduction products, acting itself as oxidising agents. This concerns the reduction of HNO 3 and the associated question of the reactive species.…”

The Role of the Molecular Hydrogen Formation in the Process of Metal-Ion Reduction on Multicrystalline Silicon in a Hydrofluoric Acid Matrix

“…In the context of calibration, these values were related to the respective molar amounts of molecular hydrogen, which are in a 1:1 mol:mol stoichiometric ratio to the amount of zinc. This method follows Rietig et al [78]. The amount of molecular hydrogen ∆n H 2 (g, t = 3600 s) or ∆n H 2 (g, t = 14,400 s) in the metal deposition experiments was calculated inversely from the integrals of the ion current at 2 amu of t = 0 s to t = 3600 s (Ag, Cu, Au) or to t = 14,400 s (Pt) via the calibration (+/− confidence interval).…”