Infinite etch selectivity during etching of SiON with an extreme ultraviolet resist pattern in dual-frequency capacitively coupled plasmas

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/(CH ₂F₂ + N₂) gas flow ratio and low frequency source power (PLF) in CH₂F₂/N₂/Ar plasma and the O₂ flow rate in CH₂F₂/N₂/O ₂/Ar plasma. The CH₂F₂/N₂ 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 (CH _xF_y) 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 CH_xF_y layer was deposited on the EUV resist surface. The critical dimension and line edge roughness increased with increasing CH₂F₂/(CH₂F₂ + N₂) flow ratio due to an increased degree of polymerization.