Elastic Constants of Nanometer Thick Diamond-like Carbon Films

Abstract

Carbon films of thickness down to 2 nanometers are necessary to achieve a storage density of 100 Gbit/in² in magnetic hard disks. Reliable methods to measure the properties of these ultrathin films still have to be developed. We show for the first time that combining Surface Brillouin Scattering (SBS) and X-ray reflectivity measurements the elastic constants of such films can be obtained. Tetrahedral amorphous carbon films were deposited on silicon, by an S bend filtered cathodic vacuum arc, which provides a continuous coverage on large areas free of macroparticles. Films of thickness down to 2 nm and density of ∼3 g/cm³ were produced and characterized. The dispersion relations of surface acoustic waves are measured by SBS for films of different thickness and for the bare substrate. Waves can be described by a continuum elastic model. Fitting of the dispersion relations, computed for given film properties, to the measured dispersion relations allows the derivation of the elastic constants. Fora 8 nm thick film we find a Young’s modulus E around 400 GPa, with a shear modulus G lying in the 130 - 210 GPa interval. For a 4.5 nm thick film, E is around 240 GPa, with G lying in the 70 - 130 GPa interval. Results for even thinner films become highly sensitive to the precision of the substrate properties, and indicate that the above values are lower bounds. We thus show that we can grow and characterize nanometer size tetrahedral amorphous carbon films, which maintain their density and mechanical properties down to the nm range.