Influence of CF4 addition for HFCVD diamond growth on silicon nitride substrates

This work presents a study of CVD diamond growth on silicon nitride-based ceramics with the addition of carbon tetrafluoride (CF4) in a hot filament-assisted reactor (HFCVD). Silicon nitride substrates were hot pressed under a nitrogen atmosphere for 90 min at 1750°C, giving specimens of very high density and good mechanical properties. The CF4 addition is known to bring several advantages to diamond growth and, in particular, in this work, an important interaction of the CF4-containing gas phase with the silicon nitride (Si3N4) substrates has been proven to be very beneficial for nucleation, growth and adherence of the diamond films. A basic gas mixture of H2/1.5 vol.% CH4/0.5 vol.% CF4 was used in the growth experiments. The nucleation study reveals a strong interaction of the halogen-containing gas phase with the vitreous phase on the substrate surface. A strong erosion of the surface has been observed, which induced a high nucleation density (Nd) of the order of 108 particles cm−2, without any surface pre-treatment. Silicon nitride surface analysis was performed with Raman and infrared specular reflectance spectroscopy. Results suggest the erosion of the vitreous phase, mainly the silica (SiO2) component, and the formation of silicon carbide, prior to diamond growth. Raman spectra and scanning electron microscopy (SEM) show better quality film grown with CF4 addition. Indentation tests with a Rockwell C tip, at variable charge, show a better film adherence if grown with CF4 addition.