Gas-phase reaction mechanism in chemical dry etching using NF₃ and remotely discharged NH₃/N₂ mixture

Modeling of dry etching processes requires a detailed understanding of the relevant reaction mechanisms. This study aims to elucidate the gas-phase mechanism of reactions in the chemical dry etching process of SiO₂ layers which is initiated by mixing NF₃ gas with the discharged flow of an NH₃/N₂ mixture in an etching chamber. A kinetic model describing the gas-phase reactions has been constructed based on the predictions of reaction channels and rate constants by quantum chemical and statistical reaction-rate calculations. The primary reaction pathway includes the reaction of NF₃ with H atoms, NF₃ + H → NF₂ + HF, and subsequent reactions involving NF₂ and other radicals. The reaction pathways were analyzed by kinetic simulation, and a simplified kinetic model composed of 12 reactions was developed. The surface process was also investigated based on preliminary quantum chemical calculations for ammonium fluoride clusters, which are considered to contribute to etching. The results indicate the presence of negatively charged fluorine atoms in the clusters, which are suggested to serve as etchants to remove SiO₂ from the surface.