Mechanisms for the behavior of carbon films during annealing

The effect of annealing on 1 μm thick single and multilayer amorphous carbon (a-C) films prepared by filtered cathodic arc is investigated. Single layer films, with a sp² to sp³ bonding fraction of approximately 50% increase their level of compressive stress following annealing. Multilayer films-consisting of alternating layers of high sp ³ fraction (tetrahedral amorphous carbon, ta-C) and intermediate sp³ 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 sp³ layers in the multilayer films develop a strong preferred orientation with graphite-like layers aligned perpendicular to the film surface. The ta-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-C films based on ab initia calculations. In order to minimize total energy, intermediate sp³ films will either decrease their sp³ fraction and generate stress or increase their sp ³ fraction and relieve stress. On the other hand, high sp³ films retain their high sp³ fraction following annealing.