Solutions containing hydrofluoric acid (HF), chloric acid (HClO3), and hydrochloric acid (HCl) are investigated as novel acidic, nitrogen oxide (NOx)-free mixtures for wet-chemical etching of silicon wafer surfaces. HF-HClO3-H2O solutions exhibit etching rates of only up to 0.12 µm min‑1 at room temperature, as the oxidizing agent chlorate (ClO3-) is not very reactive. In contrast, aqueous HF-HClO3-HCl mixtures show etching rates of up to 10.78 µm min‑1 at room temperature, as chlorine (Cl2) and chlorine dioxide (ClO2) are formed, which also serve as oxidizing agents. Depending on the HCl content isotropic or anisotropic etching behavior occurs, resulting in polished or textured surfaces with upright, inverted, and rounded pyramids. The solutions and gas phases were analyzed using Raman spectroscopy. In addition, the resulting silicon surfaces were analyzed using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM).

In this work, we present a novel approach to metal-assisted chemical etching (MACE) of silicon using copper(II) ions in mixtures of hydrofluoric acid (HF) and hydrochloric acid (HCl) without the addition of an oxidizing agent. While the standard redox potential of Cu2+ is typically considered too low for silicon oxidation in HF, we observed anisotropic etching, yielding pyramidal and inverted pyramidal surface structures, with etching rates up to 14.18 mu m h-1. The presence of HCl is crucial as it prevents the significant copper film deposition seen in HCl-free solutions. Our analysis, including cyclic voltammetry and XPS, reveals that the Cu2+ ions act as a catalyst for silicon oxidation by dissolved oxygen, O2. The HCl stabilizes the reduced species, Cu+, as chloro-complexes of copper(I), maintaining the copper in solution and enabling a divalent dissolution mechanism. This work demonstrates a unique MACE regime where the metal catalyst remains dissolved, providing new insights into the complex mechanisms of silicon dissolution.

Aqueous solutions of hydrofluoric acid (HF) and perbromic acid (HBrO4) are investigated as nitrogen oxide (NOx)-free mixtures for wet-chemical etching of (100) silicon wafer surfaces. For the high amount of HBrO4 needed in this work, an improved synthesis for HBrO4 with less consumption of fluorine (F2) is reported. We investigated etching mixtures containing HF in the range of 14-22 mol L-1 and HBrO4 in the range of 0.25-0.5 mol L-1. These mixtures are polishing silicon surfaces with high etch rates of up to 2 & micro;m min-1 at room temperature. Increasing the temperature leads to higher etch rates up to 7.8 & micro;m min-1. Resulting morphologies on treated silicon surfaces are investigated by scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results indicate a reaction pathway with silicon surfaces oxidized by inserting oxygen into Si-Si-bonds by HBrO4. The reduction of HBrO4 leads to multiple Br-species in equilibria, thus altering the etching behavior and leading to different surface morphologies.

Alkaline, wet-chemical silicon etching is an essential part in the manufacturing of silicon solar cells, because the anisotropy of the etching process enables texturization of the wafer surface. Anisotropic etching in alkaline systems is traditionally attributed to the nucleophilic attack of OH- on Si-H, forming a pentacoordinated Si-atom as a transition state. This leads to (111) crystal planes being etched much slower, thus generating pyramidal structures on (100) wafer surfaces. In aqueous HF solutions with halogens as oxidants, similar surface structures are observed, even though the reactants are completely different from the alkaline solutions. The data show that in HF-HCl-Cl2 solutions, the presence of Cl3 - plays a decisive role. Trihalide ions and hydroxide ions are very similar: both are negatively charged, nucleophilic and relatively small. This suggests a similar reaction behavior. In this article, it is shown that trihalides in acidic wet-chemical silicon etching systems represent a functional analogy to hydroxide ions in alkaline systems and are responsible for anisotropic etching. The findings are supported by spectroscopic and surface analytical data.

Wet chemical etching processes are an essential part of silicon treatment in the photovoltaic and semiconductor industry. A commonly used system is HF-HNO3. In order to avoid NO x -formation, silicon can also be etched with HF-(HCl)-Cl2-mixtures. Thorough investigations into perchloric acid indicate that even Si-H terminated surfaces are inert against this very strong oxidizing agent.

Solutions containing hydrofluoric acid (HF) and bromic acid (HBrO 3 ) are investigated as nitrogen oxide (NO x )‐free mixtures for wet‐chemical etching of (100) silicon wafer surfaces. Isotropic etching behavior with high dissolution rates of up to 10 µm min −1 is observed at room temperature, leading to polished surfaces. Anisotropic etching is observed when bromine (Br 2 ) is added to HF‐HBrO 3 solutions. Therefore, monocrystalline (100) silicon wafer surfaces are covered with random upright pyramids with edge lengths of about 5 µm. The etch rate is strongly dependent on the concentration of HBrO 3 . Silicon surfaces are analyzed by scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The gas phase is analyzed using Raman and infra red (IR) spectroscopy. The oxidation of the silicon surface by bromic acid likely takes place via oxygen insertion into the rearward silicon bonds. During etching, multiple active Br‐species are formed which alter the etching behavior.

Thorough investigations into perchloric acid indicate that even Si–H terminated surfaces are inert against this very strong oxidizing agent.

Solutions containing hydrofluoric acid (HF) and bromic acid (HBrO3) are investigated as nitrogen oxide (NOx)-free mixtures for wet-chemical etching of (100) silicon wafer surfaces. Isotropic etching behavior with high dissolution rates of up to 10 mu m min-1 is observed at room temperature, leading to polished surfaces. Anisotropic etching is observed when bromine (Br2) is added to HF-HBrO3 solutions. Therefore, monocrystalline (100) silicon wafer surfaces are covered with random upright pyramids with edge lengths of about 5 mu m. The etch rate is strongly dependent on the concentration of HBrO3. Silicon surfaces are analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The gas phase is analyzed using Raman and infra red (IR) spectroscopy. The oxidation of the silicon surface by bromic acid likely takes place via oxygen insertion into the rearward silicon bonds. During etching, multiple active Br-species are formed which alter the etching behavior.

Solutions containing hydrofluoric acid (HF), hydrobromic acid (HBr) and bromine (Br2) were investigated as novel acidic, NOx-free mixtures for wet-chemical etching of silicon wafers. HF–Br2-mixtures exhibit isotropic etching behaviour towards silicon, etch rates up to 4.0 ​μm ​min−1 were observed at room temperature, which are higher than the etch rates of commercially used alkaline solutions. HF–HBr–Br2-mixtures show anisotropic etching behaviour, texturing the surface of monocrystalline silicon wafers with random upright or random inverted pyramidal structures. Etch rates up to 2.4 ​μm ​min−1 were observed, the etch rate increases linearly with the concentration of Br2 in the etching solution. Silicon surfaces were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The light trapping efficiency of wafers etched by HF–HBr–Br2 solutions was compared to commercially available textured wafers by UV/Vis-reflectivity measurements indicating lower reflectivities for the HF–HBr–Br2-treated samples. A reaction scheme for the anisotropic dissolution of silicon in bromine-containing aqueous HF-solutions is proposed, which involves bromine as oxidizing agent.

A new approach for the texturing of monocrystalline silicon solar wafers was developed. By using a spray etching process with aqueous HF-HCl-Cl 2 mixtures, textured wafers were generated and processed to PERC solar cells and compared to cells textured by a conventional alkaline texturing process. Also, a novel saw damage removal (SDR) process using aqueous HF-Cl 2 mixtures was tested against the typical NaOH SDR. A possible dependency of cell parameters on the wafer reflectivity is discussed. The etch rates of the spray etching process are compared to other chlorine-containing hydrofluoric acid-based processes and alkaline potassium hydroxide-based mixtures. Computational fluid dynamic analyses were performed to identify possible reasons for marks occurring on the spray textured wafers. The influence of the marks on the electrical properties were examined with electroluminescence measurements. As cell performance data show promising results, we discuss whether the spray etching process with HF-HCl-Cl 2 mixtures is a viable way for the implementation of an inline texturing process for solar cell production.

29Si enrichment would be advantageous for many NMR studies of the structural properties of sol-gel derived materials. The required starting materials (29Si enriched alkoxysilanes), however, are expensive and difficult to provide. Here we present a scalable, reliable synthesis of 29Si enriched tetraethoxysilane starting from silicon dioxide or elemental silicon with a total yield up to 75%.

The wet-chemical acidic treatment of silicon wafer surfaces is very important in photovoltaic, microelectronic and further industries. Recent works report on new mixtures for acidic anisotropic etching mixtures based on hydrofluoric acid HF and hydrochloric acid HCl with an added oxidant. The aim of this work was to get an insight into the reactions during the etching process of silicon in the system HF-HCl-Cl-2. The etching mixtures, gaseous reaction products, as well as the generated silicon surfaces were investigated by F-19, Si-29, and Cl-35 NMR, ion chromatography (IC), iodometric titration, FT-IR spectroscopy, diffuse reflectance FT-IR spectroscopy (DRIFT) as well as scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX). A reaction scheme for the anisotropic dissolution of silicon in chlorine containing aqueous HF-solutions is proposed, which involves dissolved Cl-2 as the oxidizing agent, coordination of fluoride/chloride ions and formation of a hydrophilic surface. These steps are similar to the well known alkaline anisotropic etching of silicon. (C) The Author(s) 2017. Published by ECS.

Solutions for the wet chemical treatment of silicon wafer surfaces were investigated using mixtures which are based on hydrofluoric acid (HF), hydrochloric acid (HCl), and chlorine (Cl2). We used a DoE‐test plan (Design of Experiments) to evaluate the effects of five selected parameters: concentrations of HF and HCl, gas flow rate of Cl2, stirring, and saw type of the wafer material. High etch rates of up to 0.63 μm min−1 were observed at room temperature, which are comparable to the etch rates of KOH‐IPA solutions. The silicon surface was investigated by reflectivity measurements and scanning electron microscopy (SEM), indicating pyramidal textured and polished surfaces for diamond wire‐ and SiC‐slurry‐sawn wafers. Using an optimized parameter set, random inverted pyramidal surface structures are formed. These random inverted structures show a significant increase in light absorption compared to standard random upright pyramid textures, for example produced by KOH‐IPA solutions.

Aqueous acidic ozone (O3)-containing solutions are increasingly used for silicon treatment in photovoltaic and semiconductor industries. We studied the behavior of aqueous hydrofluoric acid (HF)-containing solutions (i.e., HF–O3, HF–H2SO4–O3, and HF–HCl–O3 mixtures) toward boron-doped solar-grade (100) silicon wafers. The solubility of O3 and etching rates at 20 °C were investigated. The mixtures were analyzed for the potential oxidizing species by UV–vis and Raman spectroscopy. Concentrations of O3 (aq), O3 (g), and Cl2 (aq) were determined by titrimetric volumetric analysis. F–, Cl–, and SO42– ion contents were determined by ion chromatography. Model experiments were performed to investigate the oxidation of H-terminated silicon surfaces by H2O–O2, H2O–O3, H2O–H2SO4–O3, and H2O–HCl–O3 mixtures. The oxidation was monitored by diffuse reflection infrared Fourier transformation (DRIFT) spectroscopy. The resulting surfaces were examined by scanning electron microscopy (SEM) and X-ray photoelectron spectrosc...

Solutions containing hydrofluoric acid (HF), hydrochloric acid (HCl), and hydrogen peroxide (H2O2) were investigated as novel acidic, NOx-free etching mixtures for texturing of monocrystalline silicon wafers. High etch rates of up to 13.3nms−1 were observed at room temperature, which are comparable to the etch rates of KOH-IPA solutions. The silicon surface was investigated by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM), indicating pyramidal textures for diamond wire and SiC-slurry sawn as well as saw-damage etched (polished) wafers. Non-stirred baths generate random pyramidal structures while constantly stirred solutions generate novel random inverted pyramidal surface structures. The light trapping efficiency of wafers etched by the HF-HCl-H2O2 solutions was compared by UV/vis-reflectivity measurements to KOH/i-propanol specimens indicating lower reflectivities for the HF-HCl-H2O2-treated samples. Using the ‘wafer ray tracer’ (pvlighthouse.com) the light absorption properties of monomodal and random inverted pyramid structures were simulated and compared to well-known random and monomodal textures for PERC solar cells, clearly indicating the best performance for random inverted pyramids. Besides, simulation of a PERC solar cell on a roof top at our university was performed, indicating improved performance, especially for random inverted pyramid textures.