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

Abstract

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...

About determining reliable etching rates and the role of temperature in kinetic experiments on acidic wet chemical etching of silicon

“…Depending on the amount and thus the heat capacity of the etching mixture, the strongly exothermic reaction causes a more or less strong temperature increase, which can lead to a self-acceleration of the reaction during the etching rate determination. A temperature increase of the etching bath is inevitably accompanied by a change in the stoichiometry of the reaction and, in particular, by a change in the conversion to reactive intermediates 15 and their stability, 17,20,21,23 and thus also influences the underlying etching mechanism. A clear separation between the impact of temperature and reaction mechanism on the etching rate is therefore experimentally unattainable.…”

“…This higher consumption of HNO 3 is attributed to a higher conversion in the side reaction eqn (2) at increased reaction temperature. 15 Effects of the texture of the silicon and the induction period During texturing of untreated sawn (''as-cut'') solar wafers in HF/HNO 3 /H 2 SiF 6 mixtures, Meinel et al observed very high etching rates 25 in the immediate initial phase of etching and thus in the region of saw damage, typically with a depth of 2 to 5 mm. 26,27 With increasing reaction time and the advancing removal of the saw damage, the determined etching rates decrease until finally a reaction rate is observed that is attributed to a constant bulk etching (Fig.…”

“…As a result, the side reaction described in eqn (2) results in an over-stoichiometric consumption of the HNO 3 used, which is not represented in eqn (1). 5,15…”

“…In analogy to the formation of H 2 , comprehensive studies on binary and ternary mixtures of HF, HNO 3 , and H 2 SiF 6 over a wide range of mixture variations show that the formation of excessive HNO 2 is substantially controlled by the H 2 O content in the etching mixture. 15 Although the formation of NO is determined by the initial key species of undissociated HNO 3 under all conditions, the stability of dissolved N 2 O 4 in the etching mixture is limited by the H 2 O content, 22 so that excessive N 2 O 4 outgases from the mixture as NO 2 . 5,15 The unravelling of the reaction mechanism and the formation conditions for the reactive N(III) species HNO 2 5,15 provides now the basis for a fundamental kinetic investigation of the dissolution of silicon in HF/HNO 3 and HF/HNO 3 /H 2 SiF 6 acid mixtures.…”

“…15 Although the formation of NO is determined by the initial key species of undissociated HNO 3 under all conditions, the stability of dissolved N 2 O 4 in the etching mixture is limited by the H 2 O content, 22 so that excessive N 2 O 4 outgases from the mixture as NO 2 . 5,15 The unravelling of the reaction mechanism and the formation conditions for the reactive N(III) species HNO 2 5,15 provides now the basis for a fundamental kinetic investigation of the dissolution of silicon in HF/HNO 3 and HF/HNO 3 /H 2 SiF 6 acid mixtures. The present study is focussed on the reliable and valid determination of the reaction rate, i.e.…”