The modification of organic materials by plasmas and ion beams as studied by ESCA

Abstract

X-ray photoelectron spectroscopy (XPS or ESCA) has been employed to investigate the changes in the surface chemistry of polymers produced by their exposure to plasmas and ion beams and the chemistry of thin polymeric films produced by the ion beam irradiation of films of condensed organic material. Modification by hydrogen plasmas of fluoropolymers is shown to result in extensive defluorination and hydrogenation of the surface regions of the polymer to a depth of ~20Å. In the initial stages of the reaction ESCA analysis reveals the presence of a smooth reaction gradient within the ESCA sampling depth. The modification is accompanied by extensive crosslinking of the polymer surface. A model is also proposed to account for these observations. In order to try and gain an insight into the contributions of the various components of a plasma with polymer surfaces the interaction of low energy (l-5keV) argon ions with fluoropolymer and polymers containing cajbon hydrogen and oxygen were studied. The reaction leads to the depletion of the surface region in the most electronegative components of the polymer. The modification, irrespective of the beam energy used, is homogeneous within the ESCA sampling depth using Mg(_ka1,2) radiation. The polymers all lead to the formation of a highly crosslinked, unsaturated carbonaceous surface layer on bombardment. This defunctionalisation is marked in the ESCA spectra by modifications in the lineshape of the C(_1s) photoionisation peak, the asymmetry of which has been found to, in a qualitative sense at least, reflect the presence of unsaturation in the systems. The exposure of condensed films of hexafluorobonzene to relatively low doses of argon ions has been shown to result in the formation of a thin film of an involatile polymeric residue. The product bears a striking resemblance in its chemistry to that of plasma polymerised films of hexafluorobenzene. Apart from the potential ability of such films, e.g. as dielectric films in microelectronics fabrication, they also illustrate the link between the interactions of ion beams with organic materials and plasmolysis of such materials. The thermal stability of films produced by the ion beam induced polymerisation and plasma polymerisation of hexafluorobenzene is also investigated.