Degradation behavior of thin polystyrene films on exposure to Ar plasma and its emitted radiation

Spin-coated films of amorphous polystyrene (PS) were exposed to argon plasma for a few seconds to several minutes. The PS film was either in direct contact with the plasma or was shielded from the direct plasma contact by filters with different cutoff wavelengths in the vacuum UV region or by a Faraday cage (FC) made from metal mesh to prevent the impinging of charged species. Only energy-rich neutrals and plasma radiation may be operative in presence of the FC. lithium fluoride (LiF) filter protects the sample from direct contact with the plasma. Wavelengths of plasma radiation shorter than c. 105nm (approximate to 11.8eV) were cut off. Glass filters made of fused SiO2 have a cutoff at approximate to 175nm completely the vacuum UV radiation of plasma (ca. 175nm approximate to 7.0eV). These energies are sufficient to produce CC, CH bond scissions in case of direct Ar plasma exposure and Ar plasma exposure with use of the LiF filter. Only quartz glass shielding did not produce significant effects on the polymer surface in comparison to the reference PS, either in surface energy or O/C ratio or in IR spectra. Oxygen plasma has worked most aggressive and had etched the PS film, thus establishing a steady state between introduction of new oxygen functionalities and polymer etching. Ar plasma exposure produces also oxidation and etching of the polymer films as the oxygen plasma. Using of a FC during Ar plasma exposure or the LiF filter a slightly weaker oxidation was observed.