Kinetics and role of C, O, and OH in low-pressure nanocrystalline diamond growth

A simple low-pressure condition at 80 mTorr has been employed to study the kinetics and role of C, O, and OH in diamond growth by using inductively coupled CO/CH₄/H₂ and O₂/CH₄/H₂ plasmas. Vacuum ultraviolet absorption spectroscopy (VUVAS) and actinometric optical emission spectroscopy (OES) were used to examine the densities of ground-state C atoms and emissive species such as OH. C₂, and O, respectively. Diamond films consisting of nanocrystallites with sizes as small as 20 nm were obtained on positively biased Si substrates only when CH₄ was fed. Both diamond and nondiamond growth were enhanced with increasing CO for a fixed CH₄ concentration of 5%. while diamond growth was suppressed with increasing O₂. Comprehensive discussion along with the VUVAS and OES results suggested that the C atoms resulting mainly from CO by electron impact dissociation had a close relation with the formation of C₂ or still larger species as the precursors to nondiamond phase. while the OH radicals resulting predominantly by loss reactions of the byproduct O atoms with H₂ and CH₄ were highly responsible for the enhanced diamond growth. A large amount of O atoms from O₂ was shown to affect the initial nucleation stage seriously. The results support the growth chemistry of diamond from H-hybridized carbon radicals fragmented from CH₄ rather than from H-stripped carbon radicals.