Substrate-Dependent Etching of Plasma-Deposited Carbon Films on Si and HfO ₂ Tuned by C ₄ F ₈ /CH ₄ 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.