Diagnostic and analytical study on a low-pressure limit of diamond chemical vapor deposition in inductively coupled CO–CH4–H2 plasmas

Abstract

The role of neutral radicals and charged ions in a low-pressure limit of plasma-enhanced chemical vapor deposition of diamond has been studied by plasma diagnostics and a kinetic rate analysis for radicals. The fluxes of atomic hydrogen (H), methyl radicals (CH3), and ionic species were determined by optical absorption spectroscopy and mass spectrometry. The ion-bombardment energy was estimated by measuring plasma potentials and ion energy distributions. The deposits were obtained on Si and diamond substrates with a mean ion energy of a few eV. At 10 mTorr, nanocrystalline diamond could be deposited on a diamond substrate, while not on a Si substrate, as confirmed by Raman spectroscopy. In this limiting condition, attempts were made to lower the pressure limit by increasing fluxes of carbonaceous species and/or reducing an ion-to-adatom flux ratio. It was found that low radical fluxes rather than a high ion-to-adatom flux ratio limited the nucleation. The calculated growth rates with the measured H and CH3 densities from a simplified growth model for a set of reactions of adsorbed H and CH3 were compared to the experimental ones, and then the creation of radical sites was likely to limit the growth. A transition from diamond to no diamond growth in the pressure limit of growth was demonstrated by comparing kinetic reaction rates for desorption and incorporation of adsorbed CH3. The results show how the nucleation and the growth are limited by radical fluxes and modified to a degree by dynamic effects of energetic ions.