Ion-Enhanced Etching Characteristics of sp2-Rich Hydrogenated Amorphous Carbons in CF4 Plasmas and O2 Plasmas

The sp²-rich hydrogenated amorphous carbon (a-C:H) is widely adopted as hard masks in semiconductor-device fabrication processes. The ion-enhanced etch characteristics of sp²-rich a-C:H films on ion density and ion energy were investigated in CF₄ plasmas and O₂ plasmas in this work. The etch rate of sp²-rich a-C:H films in O₂ plasmas increased linearly with ion density when no bias power was applied, while the fluorocarbon deposition was observed in CF₄ plasmas instead of etching without bias power. The etch rate was found to be dependent on the half-order curve of ion energy in both CF₄ plasmas and O₂ plasmas when bias power was applied. An ion-enhanced etching model was suggested to fit the etch rates of a-C:H in CF₄ plasmas and O₂ plasmas. Then, the etch yield and the threshold energy for etching were determined based on this model from experimental etch rates in CF₄ plasma and O₂ plasma. The etch yield of 3.45 was observed in CF₄ plasmas, while 12.3 was obtained in O₂ plasmas, owing to the high reactivity of O radicals with carbon atoms. The threshold energy of 12 eV for a-C:H etching was obtained in O₂ plasmas, while the high threshold energy of 156 eV was observed in CF₄ plasmas. This high threshold energy is attributed to the formation of a fluorocarbon layer that protects the a-C:H films from ion-enhanced etching.