Characterization of gaseous species in scanning atmospheric rf plasma with transmission infrared spectroscopy

A scanning atmospheric radio-frequency (rf) plasma was analyzed with transmission infrared (IR) spectroscopy. The IR analyses were made for the plasmas used for hydrophobic coating deposition and superhydrophobic coating deposition processes. Since the rf plasma was generated in a small open space with a high gas flow rate in ambient air, the density of gas-phase molecules was very high and the plasma-generated reactive species seemed to undergo various reactions in the gas phase. So, the transmission IR spectra of the scanning atmospheric rf plasma were dominated by gas-phase reaction products, rather than plasma-generated intermediate species. In the CH4/He plasma used for hydrophobic coating deposition, C2H6, C2H2, and a small amount of C2H4 as well as CO were detected in transmission IR. The intensities of these peaks increased as the rf power increased. The CO formation is due to the activation of oxygen and water in the air. In the CF4/H2/He plasma used for deposition of superhydrophobic coatings, C2F6, CF3H, COF2, and HF were mainly detected. When the H2/CF4 ratio was ∼0.5, the consumption of CF4 was the highest. As the H2/CF4 ratio increased higher, the C2F6 production was suppressed while the CF3H peak grew and the formation of CH4 were detected. In both CH4/He and CF4/H2/He plasma systems, the undissociated feed gas molecules seem to be highly excited vibrationally and rotationally. The information on plasma-generated reactive species and their reactions was deduced from the distribution of these gas-phase reaction products