Interaction of low-energy ions and hydrocarbon radicals with carbon surfaces

Abstract

A review is given on the physical and chemical reactions that occur if atomic hydrogen, hydrocarbon radicals, and low-energy ions interact with carbonaceous surfaces. In a first set of experiments the surface loss probabilities of different hydrocarbon radicals are determined in low-temperature plasmas using the cavity technique. The following values were determined: β(C2H) = 0.90 ± 0.05, β(C 2H3) = 0.35 ± 0.15, and β(CH 3,C2H5) ≤ 10-2. Another set of experiments was carried out in an UHV-based system working with well-defined, quantified particle beams. This system was employed to measure the sticking coefficient of methyl radicals (CH3), the simultaneous interaction of CH3 radicals and atomic hydrogen or low energy ions leading to chemical sputtering and ion-induced deposition, respectively, and the simultaneous interaction of all tree species (CH3, H, and ions). The sticking coefficient of methyl radicals on a hydrocarbon surface at 340 K is of the order of 10-5 to 10-4. The temperature dependence of this process was determined in the range from 340 to 800 K. Simultaneous exposure of the surface to atomic hydrogen and CH3 leads to an increase of the sticking coefficient up to 10-2 depending on the H flux. Simultaneous interaction of CH3 and low-energy ions (E < 1 keV) also causes an enhancement of CH3 sticking to about 10 -2. Simultaneous interaction of atomic hydrogen and low-energy ions leads to chemical sputtering. Chemical sputtering occurs also at energies well below the threshold for physical sputtering. In addition, the rate of chemical sputtering is significantly higher than the sum of chemical erosion due to atomic hydrogen alone and physical sputtering due to ions. A microscopic model for the chemical sputtering mechanism is suggested which allows a quantitative description of the flux and energy dependence of the process.