Elementary processes in plasma-surface interaction: H-atom and ion-induced chemisorption of methyl on hydrocarbon film surfaces

Low-temperature plasmas are an important tool of modern technology for deposition and erosion of thin films. In a plasma, an initial monomer is dissociated and ionized. A flux of radicals and ions emerges from the plasma and a thin film is growing or being etched by accommodation of incident radicals and ions on a substrate. Since dissociation of the initial monomer happens in the gas phase, the choice of the substrate temperature and the details of the surface structure are of minor importance. Thereby, thin films of various composition and structure on a large variety of substrates can be produced. Surface processes leading to film formation or etching are inherently heterogeneous. It is believed that the impinging flux of ions, radicals, and atoms promotes chemisorption of many species produced in the plasma. These reactions are discussed in this paper for the case of H-atom and ion-induced chemisorption of methyl radicals on hydrocarbon (C:H) film surfaces. This mimics plasma deposition of carbonaceous materials in commonly used methane plasmas, where the dominant reactive species are CH3, H, and low-energy ions. By performing particle-beam experiments it is shown that CH3 chemisorption can be quantitatively described by attachment of methyl to dangling-bond sites, which are produced either via hydrogen abstraction by incident H atoms or by ion-induced displacement of surface-bonded hydrogen. The derived absolute cross-sections are in excellent agreement with molecular dynamics calculations or predictions by computer codes based on the binary collision approximation. (C) 2004 Elsevier Ltd. All rights reserved.