SUITABLE GAS COMBINATIONS FOR PURE DIAMOND FILM DEPOSITION

In order to identify the appropriate gas combinations that realize diamond film growth in an excess atomic hydrogen environment, the relative atomic hydrogen concentrations C[H] of various source gas systems (CH4/H2, CO/H2, CO2/H2, CH4/O2 (or CO2)/H2 and CO/O2 (or CO2)/H2) were compared using plasma emission spectroscopy. C[H] was ranked in the following order: CO/O2/H2 > CO2/H2 > CH4/O2/H2 > CO/CO2/H2, CH4/CO2/H2 > CO/H2 > CH4/H2. When oxygen-containing molecules (CO, CO2 and O2) were present in the plasma, there was also an increase in the amounts of atomic oxygen, O2 and OH. These species had the same effect of eliminating non-diamond components and reproducing diamond-growing sites as atomic hydrogen. Thus enhancement of diamond-selective growth can be expected in the above-ordered gas systems. Diamond film synthesis was attempted using the CH4/H2, CO/H2, CO2/H2 and CO/O2/H2 systems. The atomic hydrogen concentrations in the microwave plasmas of these systems were correlated with the properties of the deposited films. Polycrystalline films could be synthesized at a growth rate of 0.93-1.2-mu-m h-1 in the CO(7%-10%)/H2 system. However, no deposits were confirmed within 2 h in the CH4(1%)/H2 system and only amorphous phases were deposited in the CH4(10%)/H2 system. The observed enhancement of diamond growth in the CO/H2 system is considered to be due to the larger amounts of atomic oxygen, O2, OH and atomic hydrogen than in the CH4/H2 system, as mentioned above. At this time, no deposition was observed in the CO2/H2 system, which was due to the high concentration of oxygen in the plasma removing both diamond and amorphous deposits faster than they grew. The CO/O2/H2 system was found to be promising for pure diamond synthesis because inclusion of the amorphous component was greatly suppressed with the addition of O2 (the optimized concentration was about 2%). Diamond film with good qualities was synthesized in the CO/O2 (2.2%)/H2 system. The full width at half-maximum of the diamond Raman peak was 4.1 cm-1, which is extremely close to that of natural diamond.