Effect of nitrogen addition on the microstructure and mechanical properties of diamond films grown using high-methane concentrations

Abstract

We report on the microstructure and mechanical properties of diamond films grown using varying nitrogen additions to a plasma with a high-${\mathrm{CH}}_4$ fraction of 15% (in hydrogen) and an operating pressure of 125 Torr. Films were grown at ${\mathrm{N}}_{\mathrm{2}}{\mathrm{/CH}}_4$ ratios ranging from 0 to 0.30 by fixing the ${\mathrm{CH}}_4$ flow rate and changing only the ${\mathrm{N}}_2$ flow rate. With increasing nitrogen addition, we observe an increase in intensity and a decrease in the full width at half maximum (FWHM) of the Raman band at 1550 ${\mathrm{cm}}^{\text{−1}}$ , while the crystalline diamond peak at 1332 ${\mathrm{cm}}^{\text{−1}}$ decreases in intensity and increases in the FWHM. X-ray diffraction confirms that the film crystallinity and diamond grain size decrease rapidly with increasing nitrogen additions up to a ${\mathrm{N}}_{\mathrm{2}}{\mathrm{/CH}}_4$ ratio of 0.10, but then do not change significantly above this ratio. A similar trend is observed for film surface roughness. In addition, we find from indentation testing that all films exhibit high hardness values ranging from 70 to 90 GPa and that the toughness of the films improves with increasing nitrogen addition. Optical emission spectroscopy reveals that an increase in CN species relative to ${\mathrm{C}}_2$ in the plasma is responsible for the formation of tetrahedral amorphous carbon (indicated by the Raman band at 1550 cm⁻¹).