Deposition of organic thin films by plasma and photochemical techniques
The work detailed in this thesis concerns organic thin films synthesised either using R.F. inductively coupled plasmas excited in unsaturated monomers containing either fluorine or a nitrile group, or else irradiating the said monomers in vacuo using ultraviolet light. The effect of the following parameters on the composition and structure of the resultant films was determined using ESCA/XPS : a) power input to the R.F. plasma system, b) photon flux during UV irradiation, c) monomer type (including structural isomerism), and d) monomer flow rate. Relative system deposition rates were rationalised in terms of Yasuda's parameter, W/FM, which was found to hold true qualitatively, if not quantitatively. Introduction of halogen vapour to the plasma system in the presence of nitrile monomers physically decreased the glow volume. Analysis by ESCA and UV absorption spectroscopy revealed the presence of ionic halogen species in the resultant films. An overall decrease in deposition rate of the system was also observed. A similar result for the latter was seen for UV irradiation in the presence of iodine. The results were rationalised by assigning a free radical mechanism for both plasma and photochemical film deposition which is inhibited by halogens. Films formed by irradiation at >200 nm were found to have differing chemical compositions compared to those obtained in the vacuum ultraviolet (<200 nm). This result was attributed to the differing photochemistries occurring in the two wavelength regions. Reference to the gas-phase photochemical literature enabled identification of the likely intermediates and term states involved, including 1,1 and 1,2 molecular elimination from ethylenic monomers in the vacuum UV to give the respective ethynes, together with secondary photolysis products. Consequently a mechanism for surface photopolymerisation was outlined which was compared with that proposed for plasma polymerisation, both of which involve vibrationally excited ground states for the monomers studied.