Deposition mechanism of nanostructured thin films from tetrafluoroethylene glow discharges

Abstract

Nanostructured polytetrafluoroethylene (PTFE)-like thin films can be deposited, in certain experimental conditions, by modulated discharges fed with tetrafluoroethylene (TFE). These coatings are characterized by a unique morphology consisting of highly twisted micron-long ribbons, which leads to an extremely high water repellency of the surface. In the present work, the diagnostics of the plasma phase is presented, coupled with that of the coating, in order to understand the film growth mechanism in different discharge regimes. When the duty cycle (DC) is increased in modulated C2F4 plasmas, the monomer depletion increases, too, and many recombination reactions take place at progressively higher rates, resulting in the formation of CF4, C2F6, C3F6, C3F8, and C4F10; the formation of powders in the homogeneous phase, however, was never evidenced. The modulation of C2F4 plasmas strongly affects the morphology of the resulting coating, as revealed by atomic force microscopy (AFM), ranging from bumpy to ribbon-like structures. The latter, moreover, are found to be more PTFE-like with respect to the remaining part of the film. In the last part of the paper, a deposition mechanism is proposed, where low radical densities in the plasma and surface migration of the precursors are the keys for the growth of ribbon-like structures.