INFLUENCE OF OXYGEN ON THE NUCLEATION AND GROWTH OF DIAMOND FILMS

The addition of oxygen to a diluted methane/hydrogen gas mixture (2% C H4 in H-2) activated by a microwave discharge during diamond film depo sition has been studied. We have observed that oxygen addition at very low concentrations (< 2%) to the methane/hydrogen gas mixture makes t he nucleation of diamond crystals more difficult, by causing a sharp d ecrease in the deposition rate of the diamond film. For increasing oxy gen concentrations, in the [O-2] = 0.25-1% range, the diamond and grap hite deposition rates remain nearly constant, showing a slight decreas e in the graphite deposition rate for [O-2] = 1%. By contrast, for hig her oxygen concentrations (1% < [O-2] < 2.5%) thinner films of a high quality are deposited (diamond content > 84%). These facts have been e xplained by an abrupt change in the chemical processes when the oxygen is fed to the CH4 + H-2 mixture, even in a small concentration We ass ume that the role of the atomic oxygen is two-fold: (i) formation of O H radicals, which etch the diamond and graphite phases at high rates. and (ii) direct etching of the initial carbon layer formed during the nucleation stage, producing CO molecules. However, for [O-2] greater t han or equal to 2.5% the carbon etching rate (for all the phases) is s o high that no continuous film can be deposited, In this paper we pres ent the relative variation of the formation, in the 0-2.5% oxygen rang e. for both the diamond and non-diamond phases, as determined by Raman spectroscopy and scanning electron microscopy. The results have been related to the changes in the plasma composition (mainly the OH, O and CO species), as detected by optical emission spectroscopy and mass sp ectrometry. (C) 1997 Elsevier Science S.A.