Comparison of MWPCVD diamond growth at low and high process gas pressures

Diamond growth by microwave plasma chemical vapour deposition (MWPCVD) at high temperature (∼1200 °C) and high process gas pressure (200 mbar) was studied and compared with growth at standard conditions (800 °C, 30 mbar). The growth rate was measured via laser reflection interferometry (LRI) and the gas phase was analysed by mass spectrometry (MS). The growth law was deduced from the variation of the growth rate with feed gas composition and additionally with the mass spectrometer signal of the relevant hydrocarbon species for all process conditions. It was found that etching of the surface by atomic hydrogen yields a significant modification of diamond growth at high gas pressure. One can treat it as process parallel to and independent of the growth which is controlled by the concentration of atomic hydrogen. At a substrate temperature of 800 °C the growth law shows a square root dependence on the CH4,feed gas concentration and a saturation for high methane concentrations for both gas pressures. This behaviour is in accordance with a methyl based growth mechanism. At 1200 °C a low process gas pressure (30 mbar) resulted in deposition of graphitic phases. Increase to 200 mbar facilitated again single crystal diamond growth. The growth rate showed a linear relationship with the methane gas concentration and the C2Hx MS signal over more than one order of magnitude without any indication of saturation behaviour up to 20% CH4. The change of the growth law indicates an acetylene based growth mechanism at high temperature. However, a growth mechanism based on a bimolecular reaction of CHx species cannot be excluded completely.