Surface roughness of diamond-like carbon films prepared using various techniques

Diamond-like carbon (DLC) films have been deposited onto mirror-finish silicon surfaces using three different techniques: (a) RF glow discharge from methane, (b) DC magnetron sputtering from a graphite target, with substrate RF bias; and (c) an ion beam generated from a cathodic arc discharge. Nanoscale atomic force microscopy (AFM) (tapping mode) surface roughness analysis has been carried out on as-deposited and post-deposition treated DLC films. It was found that, in terms of the effect on surface roughness, there is a threshold value of the ion impingement energy. Below this ion energy, which has a value of approximately 50 eV for each of the three techniques studied, the DLC surface is very rough on a nanoscale, while above it the roughness drops sharply. This threshold is close to previously reported atomic displacement threshold energies for graphite and diamond. These observations have been explained in terms of the tendency for surface diffusion to generate sp2-rich surface clusters which roughen the surface, whereas high impingement energies lead to ion implantation, and hence less efficient surface diffusion, and to preferential sputter removal of protruding regions, which also favours low roughness. Observed effects of substrate temperature and exposure to atomic hydrogen have also been explained in terms of these mechanisms of roughening and smoothing.