Direct ion-beam deposition of amorphous hydrogenated carbon films

Abstract

Amorphous, hydrogenated carbon (a-C:H) films were prepared by direct ion-beam deposition using a broad-beam Kaufmann-type ion source fed with methane as the process gas. The optical properties of a-C:H films deposited at different ion energies (100–1600 eV) were characterized. The properties of the a-C:H films were found to depend strongly on the energy of the film-forming hydrocarbon ions. Increase in the ion energy from 100 to 1600 eV led to a decrease in the optical gap from 1.3 to 0.3 eV and to an increase in the refractive index from 1.9 to 2.3. By comparing these results with data from r.f. plasma deposited a-C:H films, it is concluded that the optical gap is primarily determined by the average ion energy, whereas the refractive index shows a more complex dependence on the energy distribution of the film-forming hydrocarbon ions. To elucidate the interaction between energetic hydrocarbon ions and the growing a-C:H films, the ion current density at the substrate, the mass distribution of the hydrocarbon ions, the growth rate and the mass density of the a-C:H films were measured. These data indicated that less than 60% of the deposited mass was provided by the ion beam. Hence there is a considerable contribution from neutrals to the film growth. It is suggested that this neutral contribution comes from the background methane gas and not from radicals created in the ion source.