Aqueous HF–HBrO4 solutions for wet-chemical etching of (100) silicon wafer surfaces

Abstract

Aqueous solutions of hydrofluoric acid (HF) and perbromic acid (HBrO₄) are investigated as nitrogen oxide (NO_x)-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 µm min⁻¹ at room temperature. Increasing the temperature leads to higher etch rates up to 7.8 µm 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.