Fast deposition of amorphous carbon films by an expanding cascaded arc plasma jet

Abstract

Using an expanding cascaded arc plasma jet, amorphous hydrogenated and fluorohydrogenated carbon films were deposited on silicon, glass, and steel substrates at high rates of tens of nanometers per second and on large areas of up to 100 cm2. The present work was aimed at depositing amorphous carbon films suited for optical and protective applications. Films deposited with the common argon/methane or argon/acetylene mixture tend to delaminate from the substrate when the film is thicker than about 1 μm. For this reason, also trials using other compounds like C7H8 (toluene), CF4, and H2, and mixtures of these, were carried out. Using toluene, several-μm-thick films with good adhesion to the substrate were deposited. With spectroscopic ellipsometry and infrared absorption spectroscopy optical parameters were obtained. Appropriate numerical models were developed for analyzing the data, taking into account interference fringes in the spectra due to multiple reflections in the thin film. The hydrogen and oxygen content in the films were determined with nuclear recoil techniques. Films deposited with the use of methane and acetylene are diamondlike with mainly sp3 bonding types, and a hydrogen content ranging from 36 to 26 at. % (with a low oxygen contamination of 1–2 at. %). Films deposited with the use of toluene are more polymerlike, with also sp1 and sp2 bonding types. These films have a high hydrogen content (35 at. %), and can be partially oxidized (up to 13 at. %). In general, going from the polymerlike to the more diamondlike films, the refractive index increases from 1.3 to 2.2, and the band gap decreases from about 2 to 1 eV. By the admixture of hydrogen in the deposition plasma diamondlike films were produced with a larger band gap of 2.2 eV. The corrosion performance of the films was studied by storing them in a humidity cabinet. The corrosion resistance of films deposited with hydrocarbon/argon plasma mixtures appears to be limited. Thick films with a good corrosion resistance were produced by admixing a fluorine containing gas in the plasma. Analysis of the infrared absorption spectra showed that these films consist of amorphous fluorohydrogenated carbon. The presence of fluorine radicals in the plasma may lead to a chemically enhanced surface mobility, leading to a less porous film structure, and resulting in lower internal stresses. The growth rates and the corrosion performances of the films appear to be different for substrates of different types of steel. This may be attributed to different initial growth mechanisms, as a consequence of the difference in electrical and thermal conductivity of the two substrate types used here.