Etching silicon with hydrofluoric acid containing halogen - hydrogen halides - halogenoxide acids (HF-HX-HOnX-X2 mit X = Cl, Br)

• Applicant: Professor Dr. Edwin Kroke
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461795804

The wet chemical etching of silicon plays an important role in the targeted structuring of wafer surfaces in semiconductor and microsystem technology as well as photovoltaic industries. Depending on the morphology to be achieved, either alkaline etching solutions or acidic solutions containing hydrofluoric acid are used. Alkaline media, mostly based on KOH, have an anisotropic, crystal lattice-dependent etching characteristic, which e.g. generates a pyramidal structure on monocrystalline silicon wafers for photovoltaics. Acidic HF-HNO3 mixtures, on the other hand, are ideal for removing saw damage during the manufacturing of polished wafers for the semiconductor industry. It applies to both systems that the mechanisms of silicon dissolution and the reasons for the formation of different surface structures are not yet fully understood. In our group, the etching behavior of silicon in novel chlorine-containing aqueous hydrofluoric acid solutions was extensively examined in previous work. For the first time an acidic etching medium with an anisotropic etching characteristic was found. The work included preliminary studies on the reaction sequences, although the etching mechanism has not yet been fully clarified. For example, the influence of potential intermediate chlorine oxygen species and the influence of the halogen on the dissolution of the silicon are not understood. In this project, the etching behavior of silicon in aqueous mixtures of HF, HCl, HOnCl and Cl2 or mixtures of HF, HBr, HOnBr and Br2 will be investigated regarding etching rates and the resulting surface morphology. In order to be able to include as many parameters as possible and still keep the number of tests within limits, design of experiments (DOE) will be used. Also, a preselection of acids or their salts regarding their stability is to be made. The solutions, the gas phase and the silicon surfaces will be comprehensively characterized. Therefore, XPS and DR-FT-IR spectroscopy are available for surface analysis, FT-IR and Raman spectroscopy for the analysis of the gas phase and the etching mixtures are investigated via NMR spectroscopy. Species that may occur as intermediates should also be determined using in-situ IR spectroscopy. The aim is to develop a universal proposal for the mechanism of the dissolution of silicon in different variants of compositions of HF-HX-HOnX-X2 solutions (with X = Cl, Br). We hope to get information about isotropic and anisotropic effects, the relationship between the dissolution mechanism and morphology (polishing, texture) and the halogen used in each case. For selected systems, the suitability of the processes for the solar, microsystem technology and / or semiconductor industry should be evaluated.

• DFG Programme: Research Grants