Effect of C ₄ F ₈ Isomers on High Aspect Ratio Contact SiO ₂ Etching 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.