Growth mechanism of amorphous hydrogenated carbon

Abstract

Amorphous hydrogenated carbon (a-C:H) films offer a wide range of applications due to their extraordinary material properties like high hardness, chemical inertness and infrared transparency. The films are usually deposited in low temperature plasmas from a hydrocarbon precursor gas, which is dissociated and ionized in the plasma and radicals and ions impinging onto the surface leading to film growth. Final stoichiometry and material properties depend strongly on composition, fluxes and energy of the film forming species. The underlying growth mechanisms are investigated by means of quantified particle beam experiments employing radical sources for atomic hydrogen (H) and methyl (CH3) radicals as well as an argon ion beam. The interaction of these species with amorphous hydrogenated carbon films is investigated in real time by ellipsometry and infrared spectroscopy. The formation of hydrocarbon films from beams of CH3, H and Ar+ is considered a model system for growth of amorphous hydrogenated carbon films in low temperature plasmas from a hydrocarbon precursor gas. The growing film surface can be separated in a chemistry-dominated growth zone with a thickness of 2 nm on top of an ion-dominated growth zone. In the chemistry-dominated growth zone incident atomic hydrogen governs the surface activation as well as film stoichiometry. In the ion-dominated growth zone, especially hydrogen ions, displace bonded hydrogen in the film. Displaced hydrogen recombine and form H2 molecules, which desorb. This leads to a low hydrogen content of the films.