Surface wettability control and fluorination modeling of amorphous carbon films fluorinated with CF4 plasma

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In this study, the wettability of amorphous carbon films was tuned by controlling the degree of plasma fluorination, and the degree of fluorination on the surface was characterized by varying the process variables in inductively coupled CF4 plasmas. Surface wettability was characterized in terms of the contact angle, whereas the degree of surface fluorination was estimated by measuring the F/C atomic ratio using X-ray photoelectron spectroscopy. The relationship between the contact angle and F/C atomic ratio was investigated. The contact angle decreased from 59° to 20° as the F/C atomic ratio increased from 0 to 0.1 with a low degree of fluorination. The contact angle subsequently increased to 114° as the F/C atomic ratio increased to 0.46 with increased surface fluorination. The hydrophilicity, represented by a low contact angle, can be attributed to the large fraction of polar CF bonds on the surface at low fluorination, whereas the hydrophobicity, represented by a high contact angle, can be attributed to the CF2 bonds on the surface at high fluorination. The effect of ions on surface fluorination was investigated in CF4 plasma. The surface fluorine fraction increased from 0.026 to 0.31 as the ion density increased from 2.6 × 109 to 1.3 × 1010 cm−3, while it increased from 0.026 to 0.30 with an increase in ion energy from 15 to 140.5 eV. The degree of surface fluorination was modeled in terms of the adsorption and desorption rates of fluorine atoms with different radical densities, ion densities, and ion energies. Ion bombardment can increase fluorine adsorption by creating active sites on the surface. The average number of active sites for F radical adsorption per ion bombardment was estimated from the surface fluorination model. The modeled surface fluorine fraction demonstrated favorable agreement with our experimental results for CF4 plasma fluorination at various ion densities and energies. This model indicates that the contribution of ions to the fluorination of the amorphous carbon surface is considerably larger than that of the F radicals.