In semiconductor and photovoltaic industries numerous process steps deal with etching and silicon surface modification. The present study focuses on the reactivity of HF-H2O2-based mixtures toward silicon surfaces in a wide range of concentrations. The generally very moderate reactivity is investigated regarding kinetic aspects and the silicon dissolution reactions. The activation energy of silicon dissolution in HF-H2O2 mixtures is determined to be ∼50 kJ/mol, which supports a surface reaction controlled mechanism. This interpretation is checked by oxidation experiments of Si surfaces with HF-free H2O2 solutions. Resulting silicon surfaces were characterized by means of diffuse reflection Fourier transform infrared spectroscopy and photoelectron spectroscopy. Surface properties give hints for an “electrochemical” silicon oxidation. Furthermore, the oxidation behavior of different H2O2 solutions is compared to that of HNO3 solutions. All results suggest kinetically limited silicon dissolution in HF-H2O2 mixtures and hole injection into the silicon surface to be the rate-determining part of the reaction process.

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The wet-chemical treatment of silicon wafers is an important production step in photovoltaic and semiconductor industries. Solutions containing hydrofluoric acid, ammonium peroxodisulfate, and hydrochloric acid were investigated as novel acidic, NOx-free etching mixtures for texturization and polishing of monocrystalline silicon wafers. Etching rates as well as generated surface morphologies and properties are discussed in terms of the composition of the etching mixture. The solutions were analyzed with Raman and UV/vis spectroscopy as well as ion chromatography (IC). The silicon surfaces were investigated by scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), diffuse reflection infrared spectroscopy (DRIFT), and X-ray photoelectron spectroscopy (XPS). Surprisingly, pyramidal surface structures were found after etching SiC-slurry as well as diamond wire-sawn monocrystalline Si(100) wafers with hydrochloric acid-rich HF-(NH4)2S2O8-HCl mixtures. Acidic etching solutions are generally not known for anisotropic etching. Thus, the HNO3-free mixtures might allow to replace KOH/i-propanol and similar alkaline solutions for texturization of monosilicon wafers at room temperature with less surface contamination. Besides, common HNO3-based etching mixtures may be replaced by the nitrate-free system, leading to significant economic and ecological advantages.

The etching behaviour of sulfuric‐acid‐containing HF–HNO3 solutions towards crystalline silicon surfaces has been studied over a wide range of H2SO4 concentrations. For mixtures with low sulfuric acid concentration, NO2/N2O4, N2O3, NO and N2O have been detected by means of FTIR spectroscopy. Increasing concentrations of nitric acid lead to high etching rates and to an enhanced formation of NO2/N2O4. Different products were observed for the etching of silicon with sulfuric‐acid‐rich mixtures [c(H2SO4) > 13 mol L–1]. Trifluorosilane and hexafluorodisiloxane were identified by FTIR spectroscopy as additional reaction products. In contrast to the commonly accepted wet chemical etching mechanism, the formation of trifluorosilane is not accompanied by the formation of molecular hydrogen (according to Raman spectroscopy). Thermodynamic calculations and direct reactions of F3SiH with the etching solution support an intermediate oxidation of trifluorosilane and the formation of hexafluorodisiloxane. The etched silicon surfaces were investigated by diffuse reflection FTIR and X‐ray photoelectron spectroscopy (XPS). Surprisingly, no SiH terminations were observed after etching in sulfuric‐acid‐rich mixtures. Instead, a fluorine‐terminated surface was found.

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The reaction behavior of HF-HNO 3 -H 2 O etching mixtures, which are frequently used for texturing silicon surfaces, is significantly influenced by the addition of sulfuric acid. For high concentrations of sulfuric acid, nitronium ions NO 2 + ions have been detected by means of 14 N NMR spectroscopy, and results of Raman spectroscopic investigation have allowed the quantification of the nitronium ions. Maximum etching rates of 4000 -5000 nm s −1 are reached for HF (40 %)-HNO 3 (65 %)-H 2 SO 4 (97 %) mixtures with w (40 %-HF)/w (65 %-HNO 3 ) ratios of 2 to 4 and w (97 %-H 2 SO 4 ) < 0.3. For higher concentrations of sulfuric acid, H 2 SO 4 can be considered as a diluent. In order to investigate the influence of the sulfuric acid at constant HF and HNO 3 quantities, fuming HNO 3 (100 %) was used and the water in the mixtures successively replaced by H 2 SO 4 . A sudden increase of etching rates was found for sulfuric acid concentrations around 6 mol L −1 correlating with the characteristic color of the etching solutions. Decreased reaction rates at > 7 mol L −1 H 2 SO 4 are attributed to high solution viscosities and the formation of fluorosulfuric acid. Generally, in HF-HNO 3 -H 2 SO 4 /H 2 O etching mixtures a reduced dissociation of nitric acid, the formation of nitronium ions, the solubility of neutral nitrogen intermediates (e. g. NO 2 , N 2 O 3 ), as well as other effects influence the attack of silicon surfaces. The structure of etched silicon surfaces was investigated by means of scanning electron (SEM) and laser scanning microscopy (LSM). The morphologies are influenced most significantly by the relative amounts of sulfuric acid. Unexpectedly, in nitronium ion-containing mixtures rough surfaces with pore-like etching pits are generated.

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