Gas composition optimization of reactive ion beam etching for Ta2O5–SiO2 nanostructured diffraction gratings

Ta2O5–SiO2 multilayered thin films offer excellent optical properties for nanostructured diffraction gratings, but a major fabrication challenge is optimizing multiple parameters of the etching process (e.g., etch rate, etch selectivity, and etch profile angle) simultaneously. In this study, reactive ion beam etching (RIBE) was applied to fabricating Ta2O5–SiO2 nanostructured diffraction gratings on nonconductive glass substrates, and the effects of controlling the reactive gas composition (Ar:CHF3:CF4) used as a source of etching radicals on the RIBE process parameters were investigated. Among the five initial gas compositions evaluated, Ar:CHF3:CF4 = 7:14:4 achieved a sidewall angle of 87.5° but limited etch selectivity of 0.80, restricting the deep etching required for multilayer grating structures. Optical emission spectroscopy was used to identify reactive species and their roles: F radicals from CF4 increased the etch rate while H and CF2 radicals from CHF3 controlled polymer formation and the etch profile angle. Based on this analysis, the gas composition was optimized to Ar:CHF3:CF4 = 5:20:2, which improved the etch selectivity to 1.19 while maintaining a sidewall angle of 89°. The improved etch selectivity and profile control enable stable deep etching using only a nanoimprint resin mask under the optimized conditions examined in this work, suggesting a viable route to reducing reliance on metal hard masks for Ta2O5–SiO2 multilayer patterning on insulating substrates. This technique provides a basis for increasing the etch depth toward practical Ta2O5–SiO2 multilayer diffraction grating structures.