Mechanisms for the behavior of carbon films during annealing

Abstract

The effect of annealing on $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ thick single and multilayer amorphous carbon $(a\text{\ensuremath{-}}\mathrm{C})$ films prepared by filtered cathodic arc is investigated. Single layer films, with a $s{p}^{2}$ to $s{p}^{3}$ bonding fraction of approximately $50%$ increase their level of compressive stress following annealing. Multilayer films---consisting of alternating layers of high $s{p}^{3}$ fraction (tetrahedral amorphous carbon, $\mathrm{ta}\text{\ensuremath{-}}\mathrm{C}$) and intermediate $s{p}^{3}$ fraction show a decrease in compressive stress following annealing. Using cross-sectional transmission electron microscopy, we show that the single layer films and the intermediate $s{p}^{3}$ layers in the multilayer films develop a strong preferred orientation with graphite-like layers aligned perpendicular to the film surface. The $\mathrm{ta}\text{\ensuremath{-}}\mathrm{C}$ layers in the multilayer films develop the opposite preferred orientation near their top interfaces. We conclude that these preferred orientation effects are linked to the stress profile of the structures. We propose an underlying mechanism for the annealing effects of $a\text{\ensuremath{-}}\mathrm{C}$ films based on ab initio calculations. In order to minimize total energy, intermediate $s{p}^{3}$ films will either decrease their $s{p}^{3}$ fraction and generate stress or increase their $s{p}^{3}$ fraction and relieve stress. On the other hand, high $s{p}^{3}$ films retain their high $s{p}^{3}$ fraction following annealing.