Elastic constants and structural properties of nanometre-thick diamond-like carbon films

Abstract

Carbon coatings of thickness down to 2 nm are needed to increase the storage density in magnetic hard disks and reach the 100-Gbits/inch2 target. We show that the combination of surface Brillouin scattering, X-ray reflectivity and Raman spectroscopy measurements allow us to measure the mechanical and structural properties of ultra-thin tetrahedral amorphous carbon films. Densities up to 2.8 g/cm3 are found indicating a significant sp3 content even in the thinnest films. Surface Brillouin scattering provides the dispersion relations of surface acoustic waves and from these the elastic constants of the films. The elastic constants of 2, 4 and 8 nm-thick films are measurably different. Our results show that these ultra-thin films reach the full stiffness of tetrahedral amorphous carbons when the thickness becomes of the order of 10 nm, but even the thinnest films have a Young's modulus of at least ∼100 GPa. The combination of surface sensitivity and resonant enhancement of UV Raman, with surface enhancement effects obtained by depositing the films on Al, allows the detection of the Raman spectrum, even for the thinnest films. This is the quickest non-destructive structural probe of nanometre-thick diamond-like carbon films.