The process window for infinitely high etch selectivity of silicon oxynitride (SiON) layers to extreme ultraviolet (EUV) resist and the variation in line edge roughness (LER) of etched EUV resist were investigated in a CH2F2/H2/Ar dual-frequency superimposed capacitively coupled plasma under various process parameters including the gas flow ratio and low-frequency source power (PLF). The CH2F2/H2 gas flow ratio was found to play a critical role in determining the process window for infinite selectivity of the SiON/EUV resist due to the change in the degree of polymerization. The preferential chemical reaction of hydrogen and carbon species in the hydrofluorocarbon (CHxFy) layer with nitrogen on the SiON surface led to the formation of HCN etch by-products, resulting in a thinner steady-state CHxFy layer. During continuous SiON etching, the thinner steady-state CHxFy layer was due to enhanced SiF4 formation, while the CHxFy layer was deposited on the EUV resist surface. In addition, the critical dimension and LER tended to increase with increasing Q(CH2F2) flow ratio.