TUNABLE DIODE-LASER SPECTROSCOPY MEASUREMENT OF CH3 AND C2H2 DENSITIES IN A H2O CH3OH RADIO-FREQUENCY CHEMICAL-VAPOR-DEPOSITION DIAMOND SYSTEM/

We present preliminary results from our methanol-water plasma system u sing a radio frequency (rf) discharge reactor without a substrate. Sec ond-harmonic, tunable diode laser absorption spectroscopy (TDLAS) was used to detect the number densities of the dilute molecular species, C H3 and C2H2. TDLAS was also used to measure the dissociation fractions of CH3OH and H2O. The plasma pressure was 1.0 Torr, and the molecular density fraction, [CH3OH]/[CH3OH+H2O], was 0.6 to give the atomic rat io of C, H, and O most favorable for diamond film growth. We found tha t about 73% of the water was dissociated with 1500 W of rf power while more than 98% of the methanol was dissociated with only 500 W of powe r. The CH3 and C2H2 concentrations gradually increased with rf power u p to 900 W. The ranges of the number densities were 0.56 x 10(12)/cm(3 )-4.34 x 10(12)/cm(3) for CH3 and 0.50 x 10(13)/cm(3)-3.18 x 10(13)/cm (3) for C2H2. The number density of CH3 showed an abrupt decrease at 1 000 W, while a sudden increase of C2H2 occurred at the same power. The se sudden changes are due to a transition of the plasma from a low-den sity state plasma to a high-density, more localized state, caused by a change in the plasma coupling mechanism. This transition also occurs in the dissociation of water. (C) 1997 American Vacuum Society.