Enhancing Si₃N₄ Selectivity over SiO₂ in Low-RF Power NF₃-O₂ Reactive Ion Etching: The Effect of NO Surface Reaction

Highly selective etching of silicon nitride (Si₃N₄) and silicon dioxide (SiO₂) has received considerable attention from the semiconductor community owing to its precise patterning and cost efficiency. We investigated the etching selectivity of Si₃N₄ and SiO₂ in an NF₃/O₂ radio-frequency glow discharge. The etch rate linearly depended on the source and bias powers, whereas the etch selectivity was affected by the power and ratio of the gas mixture. We found that the selectivity can be controlled by lowering the power with a suitable gas ratio, which affects the surface reaction during the etching process. X-ray photoelectron spectroscopy of the Si₃N₄ and QMS measurements support the effect of surface reaction on the selectivity change by surface oxidation and nitrogen reduction with the increasing flow of O₂. We suggest that the creation of SiO_xN_y bonds on the surface by NO oxidation is the key mechanism to change the etch selectivity of Si₃N₄ over SiO₂.