Deposition of diamond films on Si by microwave plasma CVD in varied CH4-H2 mixtures: Reverse nanocrystalline-to-microcrystalline structure transition at very high methane concentrations

Microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films were synthesized on Si substrates by a microwave plasma chemical vapor deposition in methane-hydrogen gas mixtures with a wide range of methane concentrations of 0.5–40%. The samples were investigated with scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. We observed an unusual transition of the produced material structure from MCD to NCD and again to MCD with the increase in CH4 content in the gas. The deposition rate, grain size, and the roughness of the films exhibited extrema in the middle of the methane concentration range ([CH4] = 20%). Further increase of CH4 percentage leads to improvement of the diamond film quality, as confirmed by Raman spectra and SEM images while keeping a high deposition rate. These results may be used for the growth of the wide range of PCD films to be used as multipurpose protective layers, hard coatings for the cutting tools, and the diamond base for composite MCD/NCD structures. [Display omitted] •The PCD films were synthesized in a microwave plasma CVD with methane concentration in hydrogen varied from 0.5% to 40%;•The shape of plasma gradually changes in between 20 and 30% [CH4], which influences the composition of PCD films;•The average grain size and film roughness have a minimum at [CH4] = 20% and increase at higher concentrations.