Synthesis of ultrasmooth nanostructured diamond films by microwave plasma chemical vapor deposition using a He/H2/CH4/N2 gas mixture

Ultrasmooth nanostructured diamond (USND) films were synthesized on Ti-6Al-4V medical grade substrates by adding helium in H-2/CH4/N-2 plasma and changing the N-2/CH4 gas flow from 0 to 0.6. We were able to deposit diamond films as smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 mu m(2). Grain size was 4-5 nm at 71% He in (H-2 + He) and N2/CH4 gas flow ratio of 0.4 without deteriorating the hardness (similar to 50-60 GPa). The characterization of the films was performed with AFM, scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond films. The plasma species during deposition were monitored by optical emission spectroscopy. With increasing N2/CH4 feedgas ratio (CH4 was fixed) in He/H-2/CH4/N-2 plasma, a substantial increase of CN radical (normalized by Balmer H alpha line) was observed along with a drop in surface roughness up to a critical N-2/CH4 ratio of 0.4. The CN radical concentration in the plasma was thus correlated to the formation of ultrasmooth nanostructured diamond films.