Chemical modification of sputtered amorphous-carbon surfaces

Methods to chemically passivate the surfaces of amorphous-carbon films (a-C ) produced by dc magnetron sputtering were studied. The chemical compositio n of carbon surfaces produced via sputtering are dependent upon the environ ment to which the carbon is exposed immediately following deposition. When the sputtered film is vented to ambient conditions, free radicals produced at the surface during the deposition process are quenched by reaction with oxygen and/or water to form an oxidized, hydrophilic surface. If the sputte red carbon film is, however, exposed to a reactive gas prior to venting to ambient, the chemical nature of the resulting surface can be modified subst antially. Specifically, a less highly oxidized and much more hydrophobic ca rbon surface is produced when the surface free radicals are quenched via ei ther an addition reaction (demonstrated with a fluorinated olefin) or a hyd rogen abstraction reaction (demonstrated with two alkyl amines). Chemical m odification of amorphous-carbon films can also be accomplished by performin g the sputtering in a reactive plasma formed from mixtures of argon with mo lecular hydrogen, amines, and perfluorocarbons. The elemental composition o f these films, and the relative reactivity of the surfaces formed, were inv estigated via x-ray photoelectron spectroscopy and contact-angle goniometry , respectively. In the case of sputtering with a mixture of argon and hydro gen, increasing the hydrogen flow results in an increase in the amount of h ydrogen incorporated into the carbon film and a decrease in the surface fre e energy. Sputtering in diethylamine produces an amorphous-carbon film into which nitrogen is incorporated. The free energies of the a-C:N surfaces pr oduced in this process are similar to those of the a-C:H films. Sputtering in a fluorocarbon vapor results in the incorporation of fluorine into the f ilm structure and the formation of very low free-energy surfaces. Increasin g the concentration of the fluorocarbon in the sputtering plasma increases the amount of F incorporated into the film. At the highest fluorocarbon flo w rates employed, a-C films were produced with stoichiometries and surface free energies comparable to those of bulk Teflon. (C) 2001 American Institu te of Physics.