Synthesis of superhydrophobic PTFE-like thin films by self-nanostructuration in a hybrid plasma process

Superhydrophobic poly(tetrafluoro-ethylene) (PTFE) like thin films were grown on silicon wafers using a plasma-based hybrid process consisting on sputtering a carbon target in an Ar/CF₄ atmosphere. The influence of the bias voltage applied to the substrate (V_Bias) as well as of the gas mixture composition (%CF₄) on the chemical composition, the wettability and the morphology of the deposited thin films were evaluated.The chemical composition measured by X-ray Photoelectron Spectroscopy (XPS) has revealed that the F/C atomic ratio is always lower than for conventional PTFE (F/C = 2) and that it decreases when V_Bias increases (from F/C = 1 for V_Bias = ? 100 V to F/C = 0.75 for V_Bias = ? 200 V). This behavior is associated with the preferential sputtering of the fluorine atoms during the plasma-assisted growth of the films. Consecutively, a self-nanostructuration enhanced when increasing V_Bias is observed. As a consequence, the water contact angle (WCA) measurements range from 70° up to 150° depending on (i) the fluorine concentration and (ii) on the magnitude of the nanostructuration. In addition, for the films presenting the highest WCAs, a small hysteresis between the advancing and receding WCAs is observed (< 10°) allowing these films to fulfill completely the requirements of superhydrophobicity. The nanostructuration is probably due to the chemical etching by fluorine atoms of the fluorinated group.In order to get more understanding on the wettability mechanisms of these surfaces, the topography of the films has been evaluated by atomic force microscopy (AFM). The data have revealed, for all films, a dense and regular structure composed by conic objects (A_vH is their average height and A_vD is the average distance between them) for which the dimensions increase with V_Bias. A correlation between A_vH/A_vD, defined as the “morphological ratio”, with the WCA was established. Theoretical evaluations of the WCA using the Wenzel and Cassie equations with, as inputs, the features of the deposited thin film surfaces measured by AFM suggest that the wetting regime is intermediate between these two ideal situations.