Monitoring of hydrocarbon concentrations in dust-producing RF plasmas

• Published in: Plasma sources science & technology Vol. 21; no. 5; pp. 55001 - 1-9
• Main Authors: Hempel, F, Lopatik, D, Sikimic, B, Stefanovic, I, Winter, J, Röpcke, J
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.10.2012; Institute of Physics
• Subjects: absorption spectroscopy; Basic studies of specific kinds of plasmas; Dust; Dusty or complex plasmas ; plasma crystals; dusty plasmas; Exact sciences and technology; Fourier transforms; Hydrocarbons; Infrared spectroscopy; Methane; Physics; Physics of gases, plasmas and electric discharges; Physics of plasmas and electric discharges; Plasma diagnostic techniques and instrumentation; plasma reactions; Plasmas; Radio frequencies; Reactors; time-resolved spectroscopy of radicals; Methane; Absorption spectroscopy; Fourier transform spectroscopy; Hydrocarbons; Time evolution; Dusty plasmas; Experimental study; Acetylene; Concentration measurement; Methyl radicals; Plasma diagnostics; Infrared laser; Tunable lasers; Laser spectroscopy; Capacitive coupling; Parallel plate
• ISSN: 0963-0252; 1361-6595
• DOI: 10.1088/0963-0252/21/5/055001

In Ar and He radio-frequency (RF) plasmas with admixtures of C2H2 and CH4 the hydrocarbon chemistry has been studied in relation to dust particle formation by means of infrared tunable diode laser absorption spectroscopy (TDLAS) combined with Fourier transform infrared (FTIR) spectroscopy. The experiments were performed in a RF capacitively coupled parallel plate reactor at a frequency of f = 13.56 MHz, a pressure of p = 0.1 mbar and a flow rate of Φ = 8 sccm of Ar or He with admixtures of 0.5 sccm C2H2 or 1 sccm CH4. The power was P = 15 W. Using TDLAS, the temporal evolution of the concentrations of the methyl radical and of four stable molecules, C2H2, CH4, C2H4 and CO, was monitored in the plasma. Simultaneously, the growth process of the dust particles was analysed by FTIR spectroscopy. The degree of dissociation of the acetylene precursor was found to be nearly constant in the range of 96% under stabilized conditions for both the Ar and He plasmas. In contrast, the degree of dissociation of the methane precursor varied between 45% and 90% depending (i) on the appearance of dust particles in the reactor volume and (ii) on the Ar or He plasma conditions. The methyl radical concentration was found to be in the range of 1011 molecules cm−3. The concentrations of all hydrocarbon species were strongly correlated with the dynamic of the dust formation. Fragmentation efficiencies of acetylene (RF (C2 H2) = 3.2 × 1016 molecules J−1) and of methane (RF (CH4) = (0.16-2.5) × 1016 molecules J−1) and conversion efficiencies to the produced hydrocarbons (RC = (0.23-8.5) × 1014 molecules J−1) could be estimated in dependence on the discharge conditions in the RF plasma.