Analysis of silicon surfaces etched in aqueous HF-(HBr)–Br2-mixtures

Solutions containing hydrofluoric acid (HF), hydrobromic acid (HBr) and bromine (Br₂) were investigated as novel acidic, NO_x-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.