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 using a radio frequency (rf) discharge reactor without a substrate. Second-harmonic, tunable diode laser absorption spectroscopy (TDLAS) was used to detect the number densities of the dilute molecular species, CH3 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 ratio of C, H, and O most favorable for diamond film growth. We found that 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 power. The CH3 and C2H2 concentrations gradually increased with rf power up 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 1000 W, while a sudden increase of C2H2 occurred at the same power. These sudden changes are due to a transition of the plasma from a low-density 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.