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.