Gas-phase etching mechanism of silicon oxide by a mixture of hydrogen fluoride and ammonium fluoride: A density functional theory study

We report the selective etching mechanism of silicon oxide using a mixture of hydrogen fluoride (HF) and NH₄F gases. A damage-free selective removal of native oxide has been used in semiconductor manufacturing by forming and removing the ammonium fluorosilicate [(NH₄)₂SiF₆] salt layer. A downstream plasma of NF₃/NH₃ or a gas-phase mixture of HF and NH₄F was used to form (NH₄)₂SiF₆. We modeled and simulated the fluorination of silicon oxide and the salt formation by density functional theory calculation. First, we simulated the successive fluorination of silicon oxide using SiO₂ slab models. The fluorination reactions of SiO₂ surfaces by the mixture produced a volatile SiF₄ molecule or a surface anion of -OSiF₄⁻* with an NH₄⁺ cation with low activation energies. Unlike HF, NH₄F produced surface salt species consisting of a surface anion and an ammonium cation. Next, we simulated the (NH₄)₂SiF₆ formation from the two reaction products on fluorinated SiO₂ surfaces. (NH₄)₂SiF₆ can be formed exothermally with low activation energies (0.27 or 0.30 eV). Finally, we compared silicon with SiO₂ to demonstrate the inherently selective etching of silicon oxide. The fluorination reactions of silicon by the mixture showed the activation energies significantly higher than the SiO₂ cases, 1.22-1.56 eV by HF and 1.94-2.46 eV by NH₄F due to the less stable transition state geometries. Therefore, the selective salt formation on silicon oxide, not on silicon, is expected in near-room temperature processing, which enables selective etching of silicon oxide.