We investigated the processing window for the etch selectivity of silicon oxynitride (SiON) layers to extreme ultraviolet (EUV) resists and the variation in line edge roughness of EUV resists during etching of SiON/EUV resist structures in a dual-frequency superimposed capacitively coupled plasma etcher. We varied the processing parameters of the CH2F2/(CH2F2 + N-2) gas flow ratio and low frequency source power (P-LF) in CH2F2/N-2/Ar plasma and the O-2 flow rate in CH2F2/N2/O2/Ar plasma. The CH2F2/N-2 flow ratio was found to play a critical role in determining the processing window for infinite etch selectivity of SiON/EUV resists due to disproportionate changes in the degrees of polymerization on SiON and EUV resist surfaces. The preferential chemical reaction between hydrogen and carbon in the hydrofluorocarbon (CHxFy) polymer layer, and the nitrogen and oxygen in the SiON layer, presumably led to the formation of HCN, CO, and CO2 etch by-products and resulted in smaller steady-state hydrofluorocarbon thicknesses on SiON. As a result, continuous SiON etching due to enhanced SiF4 formation occurred while the CHxFy layer was deposited on the EUV resist surface. The critical dimension and line edge roughness increased with increasing CH2F2/(CH2F2 + N-2) flow ratio due to an increased degree of polymerization. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3251010] All rights reserved.