Compositions and surface energies of plasma-deposited multilayer fluorocarbon thin films

Abstract

Multilayer fluorocarbon/hydrocarbon/silicon films were deposited by using a radio-frequency plasma discharge onto stainless steel substrates in order to produce coatings with progressively lower surface energies. Surface energy was varied through the use of reactant gas mixtures of hexafluoroacetone (HFA) and 5–100% acetylene or butadiene. The surface energy and surface and bulk composition of the films were determined by means of contact angle measurements, X-ray photoelectron spectroscopy (XPS), forward recoil spectrometry (FRS) and Rutherford backscattering spectrometry (RBS). Variations in HFA/hydrocarbon reactant gas mixtures used for deposition of the fluorocarbon layer demonstrated that films with surface energies as low as 19 mN m −1 could be obtained. Increasing the ratio of HFA to hydrocarbon in the feed gas mixture gave films with higher fluorine content and lower surface energy, with acetylene giving films with lower surface energy and higher fluorine content than butadiene. The surfaces of the films were shown to be richer in fluorine than the bulk. The surface and bulk compositional differences were attributed to lower levels of ion bombardment experienced by the film surface. The final composition and properties of the multilayer structure were suggested to result from a balance of the distribution of radical species in the plasma and the dose of energetic ion bombardment at the growing film surface. For the fluorocarbon layer, these factors were determined by the amount and type of hydrocarbon in the feed gas and by the plasma deposition conditions.