Surface reactions during growth and erosion of hydrocarbon films

Abstract

The chemical reactions and physical processes which occur at the surface of hydrocarbon films during deposition from low-temperature hydrocarbon plasmas are reviewed. Special emphasis is placed on the chemical reactions of atomic hydrogen, the interaction of energetic particles with the solid, and the synergistic effects between them. The interaction of energetic particles with the surface of the growing film has been simulated in the binary collision approximation by means of the TRIM.SP computer code. The sputtering and displacement yields were calculated for hydrogen and carbon ions in the energy range from 25 to 500 eV. In addition, the depth distributions of the damage and implantation profiles are shown. The validity of this binary collision approach is discussed and compared with molecular dynamics simulations. The dominant ion-induced effect in this energy range is displacement of bonded hydrogen atoms. The microscopic processes of atomic hydrogen interacting with a carbonaceous surface, such as adsorption, abstraction, addition, and etching, are briefly reviewed and summarized. Furthermore, investigations of synergistic effects using thermal, atomic hydrogen and low-energy hydrogen ion beams are discussed. The film growth of hydrocarbon films from hydrocarbon plasmas was investigated experimentally by real-time, in-situ ellipsometry and a variety of ex-situ analyses. The real-time possibilities and the submonolayer sensitivity of ellipsometry allow detailed investigation of the growth process in the plasma environment. The temperature dependence of the growth and the interaction of atomic hydrogen and energetic particles with the film surface were thoroughly investigated. The experimentally observable net deposition rate represents a competition between a temperature-independent deposition process and the temperature-dependent erosion by atomic hydrogen. At very low ion energies the synergistic interaction between atomic hydrogen and the ions leads to `ion-assisted chemical erosion'. The interaction of ions with the surface generates a modified layer on top of the film surface. This modified layer is an intrinsic property of the deposition as well as the erosion process at ion energies above about 30 eV. The deposition results are discussed on the basis of the reviewed microscopic processes and a framework for understanding film growth is presented.