Elastic Constants of Nanometer Thick Diamond-like Carbon Films
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Elastic Constants of Nanometer Thick Diamond-like Carbon Films Marco G. Beghi, Carlo E. Bottani, Andrea LiBassi, Rosanna Pastorelli, Brian K. Tanner1, Andrea C. Ferrari2 and John Robertson2 INFM and Nuclear Engineering Department, Politecnico di Milano, I-20133 Milan, Italy, 1 Physics Department, University of Durham, Durham, DH1 3LE, UK 2 Engineering Department, Cambridge University, Cambridge CB2 1PZ, UK 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. For a 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. INTRODUCTION The storage density of magnetic hard disks is steadily increasing [1,2]. Technological developments are required to maintain the increasing trend. Namely, the distance between the recording head and the magnetic material has to be decreased, and this requires a reduction of the thickness of the protective coatings of the disk and the head. Carbon coatings are presently the standard choice: they provide protection against corrosion, grafting sites for the lubricant molecules and mechanical protection [1,2]. The continuity of the film is a crucial requisite. The achievement of a storage density of 100 Gbit/in2 requires a reduction of coating thickness down to 2-5 nanometers. The characterization of such thin films is still an open question. The elastic constants provide useful information: beside the direct characterization of the mechanical behavior, they give indirect indications on the quality and the continuity of the film. Although a film of few nanometers on a softer substrate cannot provide a strong mechanical protection [2], measurement of its elastic moduli provides
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