Monitoring of hydrocarbon concentrations in dust-producing RF plasmas

In Ar and He radio-frequency (RF) plasmas with admixtures of C 2 H 2 and CH 4 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 C 2 H 2 or 1 sccm CH 4 . The power was P = 15 W. Using TDLAS, the temporal evolution of the concentrations of the methyl radical and of four stable molecules, C 2 H 2 , CH 4 , C 2 H 4 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 10 11 molecules cm −3 . The concentrations of all hydrocarbon species were strongly correlated with the dynamic of the dust formation. Fragmentation efficiencies of acetylene ( R F (C 2 H 2 ) = 3.2 × 10 16 molecules J −1 ) and of methane ( R F (CH 4 ) = (0.16–2.5) × 10 16 molecules J −1 ) and conversion efficiencies to the produced hydrocarbons ( R C = (0.23–8.5) × 10 14 molecules J −1 ) could be estimated in dependence on the discharge conditions in the RF plasma.