Investigating the Plasma Surface Modification of Polystyrene at Low Ion Power Densities

Abstract

A new grid-separated, two-chamber low-pressure plasma reactor [Dhayal, M.; Forder, D.; R. Short, D.; Bradley, J. W. Vacuum 2003, 70, 67] 1 has been developed to study the surface modification of polymeric materials under low ion power density bombardment. Through external biasing of the grids, the ion flux r and energy E of the bombarding plasma ions at the substrate can be controlled independently (within certain experimental limits). Importantly, this can be achieved without changing the VUV component of the plasma. In the present arrangement, ion power densities (qΓE) in the range 0.7-15.5 W m - 2 can be achieved. This allows a low power density regime to be explored, not possible in typical glass barrel RF reactors often used for surface modification of polymeric materials, in which ion power densities are typically 10-160 W m - 2 . The effect of ion energy and flux on the surface modification of polystyrene (PS) was quantified by the ratio of atomic oxygen to carbon in the post-treated samples exposed to air (measured by X-ray photoelectron spectroscopy, XPS). It was found that, over the whole available range in ion fluxes, (1.7-5 x 10 1 8 m - 2 s - 1 ) and energies (1.5-23 eV), the O/C ratios did not change with O/C about 0.1 to 0.12 for plasma exposure times of 90 s. With elimination of the ion species from the processing region of the discharge through grid biasing, the O/C ratios were also found not to change, indication that below a certain threshold of ion power (at least 15.5 W m - 2 ) ions play little role in the surface modification. By replacing the grids with a LiF window (cutoff wavelength λ c 104 nm) having a transparency in the VUV region (<200 nm) at least as good as the geometric transmission area of the two grid system we observe again little change in O/C, indicating that the dominant species in production of free radical sites is the VUV, with any excited neutral species and radicals in the plasma playing a minor role.