Enhancement of the nucleation of smooth and dense nanocrystalline diamond films by using molybdenum seed layers
J. G. Buijnsters, L. Vázquez, G. W. G. van Dreumel, J. J. ter Meulen, W. J. P. van Enckevort, and J. P. Celis

Abstract:
A method for the nucleation enhancement of nanocrystalline diamond (NCD) films on silicon substrates at low temperature is discussed. A sputter deposition of a Mo seed layer with thickness 50 nm on Si substrates was applied followed by an ultrasonic seeding step with nanosized detonation diamond powders. Hot-filament chemical vapor deposition (HF-CVD) was used to nucleate and grow NCD films on substrates heated up at 550 °C. The nucleation of diamond and the early stages of NCD film formation were investigated at different methane percentages in methane / hydrogen gas mixtures by atomic force microscopy, micro-Raman spectroscopy, scanning electron microscopy, and grazing incidence x-ray analyses in order to gain specific insight in the nucleation process of NCD films. The nucleation kinetics of diamond on the Mo-coated Si substrates was found to be up to ten times higher than on blank Si substrates. The enhancement of the nucleation of diamond on thin Mo interlayers results from two effects, namely, (a) the nanometer rough Mo surface shows an improved embedding of ultrasonically introduced nanosized diamond seeds that act as starting points for the diamond nucleation during HF-CVD and (b) the rapid carbonization of the Mo surface causes the formation of Mo₂C onto which diamond easily nucleates. The diamond nucleation density progressively increases at increasing methane percentages and is about 5 x10¹⁰ cm⁻² at 4.0% methane. The improved nucleation kinetics of diamond on Mo interlayers facilitates the rapid formation of NCD films possessing a very low surface roughness down to ~6 nm, and allows a submicron thickness control.