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/H2 and HF/H2 plasmas were investigated in a dual-frequency capacitively coupled plasma reactor with increasing an H2 percentage from 5 to 34%. The etch rate decreased by 35% in CF4/H2 and 10% in HF/H2. 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)2SiF6, an ammonia fluorosilicate (AFS) phase, when H2 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/H2 plasma, the decrease in etch rate was small, but the F density decreased significantly. In the CF4/H2 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.