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.
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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