Effect of C₄F₈Isomers on High Aspect Ratio Contact SiO₂Etching and Greenhouse Gas Emission

In the semiconductor industry, dynamic random access memory and three-dimensional NAND (3D NAND) processes commonly employ polymer-rich perfluorocarbons (PFC) and hydrofluorocarbon (HFC) gases for high aspect ratio contact (HARC) SiO₂ etching such as C₄F₈-cyclo (octafluorocyclobutane; CF₂CF₂CF₂CF₂) having four branches of CF₂ arranged in a circular structure. In this study, the effect of a C₄F₈ isomer, C₄F₈-linear (octafluoro-2-butene; CF₃CF═CFCF₃) having two CF₃ branches in a linear structure, compared to conventionally used C₄F₈-cyclo, on the plasma characteristics and HARC SiO₂ etch characteristics was investigated using a capacitively coupled plasma (CCP) etcher with gas mixtures composed of the C₄F₈ isomer (C₄F₈-cyclo or C₄F₈-linear), C₄F₆, Ar, and O₂. The C₄F₈-linear showed a little higher SiO₂ etch rate and higher etch selectivity over the amorphous carbon layer (ACL) compared to C₄F₈-cyclo, and ACL-patterned HARC SiO₂ etching also showed less enlarged SiO₂ hole size for C₄F₈-linear at the same etch conditions. The improved etch properties with C₄F₈-linear were related to the differences in dissociation characteristics of C₄F₈ isomers by showing less high-mass ions above 90 amu while showing less CF⁺, less CF₂⁺, and higher CF₃⁺ for C₄F₈-linear, which leads to heavier ion bombardment for C₄F₈-cyclo even with more polymerizing species to the material surfaces. This demonstrates that even with identical chemical compositions, variations in molecular structure can lead to distinct etch and plasma characteristics. Additionally, an analysis of greenhouse gas emissions was conducted to evaluate the environmental impact. While C₄F₈-linear produced higher amounts of CF₄ and C₂F₆ upon plasma dissociation, the overall million metric ton carbon equivalent (MMTCE) was lower ∼62% for C₄F₈-linear compared to C₄F₈-cyclo. This indicates that under the same process conditions, the use of C₄F₈-linear instead of C₄F₈-cyclo can reduce overall greenhouse gas emissions while improving the etch characteristics.