Absolute number density and kinetic analysis of the CF radical in pulsed CF4 + H2 radio-frequency plasmas

The infrared tunable diode laser absorption spectroscopy (IR-TDLAS) experimental technique was employed to study CF radical kinetics in pulsed capacitively coupled CF4/H2 rf plasmas. In particular, the special data acquisition approach (so-called 'burst mode') was adapted and applied to monitor the absolute number density of CF with a temporal resolution of better than 1 ms. This enabled a proper kinetic analysis of the radical during both 'plasma on' and 'plasma off' phases. Thus, during the 'plasma off' phase, the CF species was found to be rapidly consumed, mostly in the reactor volume. Possible surface losses (sticking) of CF were shown to be of minor importance, and became notable only at total pressures lower than 10 Pa. During the 'plasma on' phase the CF concentration was measured to increase rapidly, reach a maximum and then decrease to its steady-state value. The observed overshoot in the CF density trace was shown to correlate with the corresponding decay of the C2F4 species also measured in the 'plasma on' phase. The electron impact fragmentation of C2F4 was supposed to contribute significantly to the effective production of CF at the beginning of the 'plasma on' phase, which might have caused the observed overshoots. (paper)