Etching Silicon with HF–HNO₃–H₂SO₄/H₂O Mixtures – Unprecedented Formation of Trifluorosilane, Hexafluorodisiloxane, and Si–F Surface Groups

Abstract

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.

On The Mechanism of the Anisotropic Dissolution of Silicon in Chlorine Containing Hydrofluoric Acid Solutions

“…Other silicon-fluorinespecies like HSiF 3 ,which were for example found in the system HF-HNO 3 -H 2 SO 4 , were not detected. 23 This indicates an abstraction of hydrogen atoms from the silicon surface atoms during the oxidation step of the etching process. Characterization of the silicon wafer surface.-To determine the oxidation step during the etching of silicon surfaces, H-terminated silicon wafers were treated with solutions containing chlorine in absence of hydrofluoric acid.…”

Highly Conductive PEDOT:PSS Transparent Hole Transporting Layer with Solvent Treatment for High Performance Silicon/Organic Hybrid Solar Cells

“…Finally, the samples were immersed in a diluted HF (5%) solution for 5 min to remove the native oxide to obtain H-Si surfaces. The cleaned Si were then transferred into a diluted HNO 3 (10%) solution to form a SiO x film to act as a passivation layer [25,26]. Highly conductive PED-OT:PSS (Clevios PH1000) uniformly mixed with 5 wt% DMSO and 1 wt% Triton X-100 was spin-coated onto the surface of the SiO x -terminated Si substrate at a spin speed of 1500 rpm in air for 60 s. Following that, the samples were annealed at 140°C for 10 min under nitrogen atmosphere.…”

Stretchable and colorless freestanding microwire arrays for transparent solar cells with flexibility

“…2a). For this purpose, a 70-nm-thick SiO 2 thin film was formed on the tips of the SiMWs as an etching mask, and the SiMWs were chemically etched using a hydrofluoric acid−nitric acid–acetic acid (HNA) mixture solution 58–61 . Silicon etching using the HNA solution proceeds in three steps as follows:The overall reaction is initiated by breaking the covalent bonds between the silicon atoms by H + ions supplied from HNO 3 , producing SiO 2 by recombination of Si 2+ ions with hydroxide ions.…”

“…However, the SiMWs were still extensively etched by the solution, leaving only the Al 2 O 3 layer behind. To reduce the etching rate and change the behavior of HF-based etching, various additives, including organics, were introduced into the HF solution 61,62 . Surprisingly, the etching rate was significantly reduced with the etching solution in which Dimethylformamide (DMF) was added instead of acetic acid, and this dramatically changed the shape of the SiMW tip (Fig.…”