Elastic, mechanical, and thermal properties of nanocrystalline diamond films

Nanocrystalline columnar-structured diamond films with column diameters less than 100 nm and thicknesses in the range of 1–5 μm were grown on silicon substrates by chemical vapor deposition (CVD) in a microwave plasma reactor with purified methane and hydrogen used as the reactants. Uniform conformal nucleation densities in excess of ${10}^{12}$ ${\mathrm{cm}}^{\text{−2}}$ were accomplished prior to growth by seeding with explosively formed nanodiamonds, which resulted in good optical quality films. The film thickness was measured in situ by the laser reflectometry method. The grain size and optical quality of the films were characterized by scanning electron microscopy and Raman measurements. Broadband surface acoustic wave pulses were used to measure the anomalous dispersion in the layered systems. The experimental dispersion curves were fitted by theory, assuming the diamond film as an isotropic layer on an anisotropic silicon substrate, to determine mean values of the density and Young’s modulus of the diamond films. The density was close to the density of single crystal diamond or polycrystalline diamond plates grown by the CVD technique, whereas the Young’s modulus varied strongly with the nucleation density between 517 and 1120 GPa. Young’s moduli close to the single crystal values were obtained for films grown with a nucleation density ${\text{⩾10}}^{12}$ ${\mathrm{cm}}^{\text{−2}}.$ The thermal diffusivity in these films was measured by the traveling wave technique. The value for ∼3.5-μm-thick nanocrystalline diamond films with nucleation densities ${\text{⩾10}}^{12}$ ${\mathrm{cm}}^{\text{−2}}$ was ∼7.2 ${\mathrm{cm}}^2/\mathrm{s},$ whereas those with lower nucleation densities showed a value of ∼5.5 ${\mathrm{cm}}^2/\mathrm{s}.$