Thermodynamic limits for the substrate temperature in the CVD diamond process

Although which of diamond and graphite is dominantly precipitated from the gas phase is a problem of kinetics or a kinetic barrier, the condition under which diamond or graphite is not stable with respect to the gas phase can be specified by thermodynamics. The thermodynamic limit of the substrate temperature that diamond can deposit is evaluated in the C-H and C-H-O systems by assuming that the limit is defined by the metastable CVD diamond phase diagram. In the C-H system, the pressure of the CVD reactor is the most important parameter for the lower limit of the substrate temperature. For the gas mixture of 1% CH4-99% H-2, the lower limits are similar to 1400 K, similar to 900 K and similar to 600 K under 760 torr, 7.6 torr and 76 mtorr, respectively. Increasing the methane concentration to 5% decreases the limit only slightly. In the C-H-O system, the thermodynamic lower limit can decrease as low as room temperature. Although decreasing the O/C ratio is favorable for the low-temperature deposition, the non-diamond is expected to form according to Bachmann's C-H-O diagram. (C) 2000 Elsevier Science S.A. All rights reserved.