Hybridization and tribomechanical properties of ultrathin amorphous carbon films synthesized by radio-frequency low-pressure plasma discharges

• Synthesis of ultrathin amorphous carbon films by rf low-pressure plasma discharges
• Insight into carbon atom hybridization from deconvolution of C1s core level XPS peaks
• Tribomechanical properties vs type and percentage of carbon atom hybridization
• Interpretation of experimental results in the context of particle collision analysis
• sp3 fraction depends on Ar+ ion flux and sputter yield and kinetic energy of C atoms.

Ultrathin amorphous carbon (a-C) films were deposited on Si(100) substrates by low-pressure radio-frequency plasma discharges of varying substrate bias voltage in pure Ar atmosphere. The surface roughness and tribomechanical properties of the a-C films were measured with an atomic force microscope and a surface force microscope, respectively. Insight into tetrahedral (sp3) and trigonal (sp2) atomic carbon hybridization was obtained from the deconvoluted C1s core level peak of X-ray photoelectron spectroscopy spectra. Energetic particle collision theory was used to correlate hybridization and tribomechanical properties to low-pressure plasma discharge conditions. The results are interpreted in the context of Ar+ ion collisions with carbon atoms on the growing film surface, followed by collision cascades between excited carbon atoms and other surface carbon atoms, resulting in the removal of weakly bonded carbon atoms and dominance of sp3 hybridization or the development of thermal spikes that promote sp2 hybridization. Particle collision analysis shows that the sp3 fraction is a function of Ar+ ion flux, sputtering yield of target carbon atoms, and kinetic energy of surface carbon atoms, in good qualitative agreement with experimental results of the sp3 content of a-C films versus substrate bias voltage.