Etching characteristics of hydrogenated amorphous carbon with different sp²/sp³ hybridization ratios in CF₄/O₂ plasmas

Amorphous carbon is used as a hard mask for dielectric etching with a high aspect ratio in the fabrication processes of semiconductor devices. The dependence of the etching characteristics of hydrogenated amorphous carbon (a-C:H) films on the sp²/sp³ hybridization ratios was studied for CF₄/O₂ plasma mixtures. The etch rate of sp³-rich a-C:H is 33.7 times higher than that of sp²-rich a-C:H in a plasma comprising 50% CF₄ and 50% O_2. The etch rate of the sp²-rich a-C:H exhibits a linear correlation with the ion density of CF₄/O₂ plasma, whereas that of the sp³-rich a-C:H exhibits a second-order exponential correlation with O radical density. A combined etch rate model was suggested to explain the etch rates of a-C:H. Ion-enhanced etching is identified as the dominant etching mechanism for the sp²-rich a-C:H, whereas spontaneous chemical etching is the main reaction mechanism for the sp³-rich a-C:H in CF₄/O₂ plasmas.