A multilevel resist (MLR) structure can be fabricated based on a very thin amorphous carbon (a-C) layer (congruent to 80 nm) and Si3N4 hard-mask layer (congruent to 300 nm). The authors investigated the selective etching of the Si3N4 layer using a physical-vapor-deposited (PVD) a-C mask in a dual-frequency superimposed capacitively coupled plasma etcher by varying the process parameters in the CH2F2/H-2/Ar plasmas, viz., the etch gas flow ratio, high-frequency source power (P-HF), and low-frequency source power (P-LF). They found that under certain etch conditions they obtain infinitely high etch selectivities of the Si3N4 layers to the PVD a-C on both the blanket and patterned wafers. The etch gas flow ratio played a critical role in determining the process window for infinitely high Si3N4 /PVD a-C etch selectivity because of the change in the degree of polymerization. The etch results of a patterned ArF photoresisit/bottom antireflective coating/SiOx/PVD a-C/Si3N4 MLR structure supported the idea of using a very thin PVD a-C layer as an etch-mask layer for the Si3N4 hard-mask pattern with a pattern width of congruent to 80 nm and high aspect ratio of congruent to 5. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3268624]