Etching Mechanism Based on Hydrogen Fluoride Interactions with Hydrogenated SiN Films Using HF/H₂ and CF₄/H₂ Plasmas

The etch characteristics of SiN films using CF4/H-2 and HF/H-2 plasmas were investigated in a dual-frequency capacitively coupled plasma reactor with increasing an H-2 percentage from 5 to 34%. The etch rate decreased by 35% in CF4/H-2 and 10% in HF/H-2. F density, measured by optical emission actinometry, decreased by approximately 70% in both plasmas, but it alone could not explain the etch rate reduction. Surface analysis revealed the formation of (NH4)(2)SiF6, an ammonia fluorosilicate (AFS) phase, when H-2 was added to both plasmas. A model is proposed where anhydrous HF gas directly reacts with a hydrogenated SiN surface to form the AFS phase. In the HF/H-2 plasma, the decrease in etch rate was small, but the F density decreased significantly. In the CF4/H-2 plasma, HF etchants released from the fluorocarbon layer can still react with the hydrogenated SiN surface, even with a deficiency of F radicals. The observations suggest that the formation of AFS does not necessarily inhibit etching and can assist in SiN etching with a sufficiently high voltage bias. These results highlight the significant role of HF formation and its reactions with the hydrogenated SiN surface in SiN etching using hydrogen and fluorine-containing plasmas.