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 C 4 F 8 /H 2 plasmas is challenging due to the limited carbon supply. To overcome this limitation, in this study, CH 4 gas was introduced into C 4 F 8 plasmas to control carbon-based radical chemistry, such as CF 2, C 2, and CH, by tuning the gas ratio R = CH 4 /(C 4 F 8 + CH 4 ). Increasing R decreased CF 2 and C 2 radical densities, while increasing CH radical density resulted in substrate-dependent film growth. On the Si substrate, CF 2 and C 2 species primarily contributed to the formation of defect-rich sp 2 -dominant films, whereas on the HfO 2 substrate, CH and C 2 radicals predominantly formed cross-linked hydrogenated sp 3 -rich films with embedded sp 2 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 HfO 2 at high R, while oxygen radicals preferentially etched films on HfO 2 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.