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

We report on the microstructure and mechanical properties of diamond films grown using varying nitrogen additions to a plasma with a high-CH₄ fraction of 15% (in hydrogen) and an operating pressure of 125 Torr. Films were grown at N₂/CH₄ ratios ranging from 0 to 0.30 by fixing the CH₄ flow rate and changing only the N₂ 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 cm^-1, while the crystalline diamond peak at 1332 cm^-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 N₂/CH₄ 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 C₂ in the plasma is responsible for the formation of tetrahedral amorphous carbon (indicated by the Raman band at 1550 cm^-1). © 1999 American Institute of Physics.