The properties of fluorine containing diamond-like carbon films prepared by plasma-enhanced chemical vapour deposition

Films of diamond-like carbon containing up to 39.2 at.% fluorine (F-DLC) were deposited on silicon substrates by radio frequency plasma-enhanced chemical vapour deposition (rf PECVD). A mixture of acetylene (C2H2) and carbon tetrafluoride (CF4) was used for the deposition of F-DLC films. The influence of fluorine content on self-bias voltage, deposition rate, and composition, bonding structure, surface energy, hardness, stress and surface roughness were investigated and correlated with fluorine content. The deposition rate was found to decrease linearly with increasing CF4 in the plasma due to the higher density of F+. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence C–C, C–CF and C–F for F-DLC films with low fluorine concentration (6.5 at.%), however for films with higher fluorine content (20.7 at.% and 39.2 at.%) an additional peak due to CF2 bonding was detected. The Raman G-band peak position shifted to higher frequency and can be attributed to the increased amount of sp2 bonds. Ion scattering spectrometry (ISS) measurements revealed the presence of fluorine atoms in the outmost layer of the F-DLC films and there was no evidence of surface oxygen contamination. The surface energy was found to reduce with increasing fluorine content and has been attributed to the change of the bonding nature in the films, in particularly increasing CF and CF2 bonds. The addition of fluorine into the DLC film resulted in lower stress and hardness values. The work of adhesion of the surface of the films for Pseudomonas aeruginosa bacteria was estimated to increase with fluorine content, which is indicative of an improved antibacterial activity for F-DLC films.