INFLUENCE OF REACTANT GAS-COMPOSITION ON SELECTED PROPERTIES OF N-DOPED HYDROGENATED AMORPHOUS-CARBON FILMS

We report a study of N-doped hydrogenated amorphous carbon films depos ited using the r.f. (13.56 MHz) self-bias method. While maintaining a constant r.f. power density (3.2 W cm(-2)), total feed-gas flow rate ( 45 sccm), initial substrate temperature (87 degrees C), and deposition gas pressure (69 mu bar), we examined the effect of changing the nitr ogen-to-carbon ratio (0.0-4.8) in the feed gas on the film properties: specifically the resistivity, compressive stress, Knoop microhardness , density, index (eta) of refraction and chemical composition of the f ilm. As the N-to-C ratio in the feed gas was increased from 0.0 to 4.8 , up to 15.6 at.% N was incorporated in the film. Nitrogen substituted for carbon and hydrogen in the amorphous matrix and the hydrogen cont ent decreased from similar to 16.6 to 10.0 at.%. Fourier transform IR spectra of the films showed an increasing concentration of nitrogen-co ntaining functional groups (-NH and nitrile) in the film as the nitrog en concentration [N] in the film increased. At [N] between 0 and 2 at. %, no significant change was seen in eta, mechanical properties or ato m number density rho(N) but the resistivity decreased fifty- to eighty -fold. At [N] > 2 at.%, dramatic reductions were seen in the hardness, stress, rho(N), eta and resistivity. Over the entire doping range, th e resistivity of the films decreased nearly 6000-fold, while the intri nsic stress decreased 56% and hardness was reduced by 52%. The change in resistivity at high [N] appears more correlated with hydrogen loss and enlargement of graphitic microclusters induced by nitrogen ion bom bardment during film growth rather than with the N content in the film .