Substrate-Dependent Etching of Plasma-Deposited Carbon Films on Si and HfO₂ Tuned by C₄F₈/CH₄ Gas Ratio

Advanced transistor architectures such as gate-all-around field-effect transistors (GAAFETs) require material-selective passivation to protect sensitive layers during plasma processing. However, forming robust carbon passivation films using conventional C₄F₈/H₂ plasmas is challenging due to the limited carbon supply. To overcome this limitation, in this study, CH₄ gas was introduced into C₄F₈ plasmas to control carbon-based radical chemistry, such as CF₂, C₂, and CH, by tuning the gas ratio R = CH₄/(C₄F₈ + CH₄). Increasing R decreased CF₂ and C₂ radical densities, while increasing CH radical density resulted in substrate-dependent film growth. On the Si substrate, CF₂ and C₂ species primarily contributed to the formation of defect-rich sp²-dominant films, whereas on the HfO₂ substrate, CH and C₂ radicals predominantly formed cross-linked hydrogenated sp³-rich films with embedded sp² domains. These structural differences led to contrasting etching responses: under Ar ion exposure, films on Si etched up to five times faster than those on HfO₂ at high R, while oxygen radicals preferentially etched films on HfO₂ at low R. This gas-ratio-controlled approach enables substrate-selective passivation within a single plasma process, offering a practical route to mitigate plasma-induced damage in advanced 3D device fabrication.