For fabrication of a multilevel resist (MLR) structure with silicon nitride (Si3N4) and amorphous carbon (a-C) layers, highly selective etching of the Si3N4 layer using a chemical vapor-deposited (CVD) a-C etch mask was investigated by varying the following process parameters in CH2F2/H-2/Ar plasmas: etch gas flow ratio, high-frequency source power (P-HF), and low-frequency source power (P-LF) in a dual-frequency superimposed capacitively coupled plasma etcher. The results of etching the ArF photoresist/bottom antireflective coating/SiOx/CVD a-C/Si3N4 MLR structure showed the possibility of obtaining an infinitely high selective etch process for the Si3N4 layer using a thin CVD a-C etch mask for high aspect-ratio pattern formation. The CH2F2/H-2 gas flow ratio was found to play a critical role in determining the process window for infinite Si3N4/CVD a-C etch selectivity, due to the change in the degree of polymerization on Si3N4 and CVD a-C surfaces. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3430551]