AFM and XPS study of ion bombarded poly(methyl methacrylate)

An in situ XPS analysis was performed on nearly monodisperse poly(methyl methacrylate) (PMMA) samples (M-p = 31 kDa) deposited on silicon substrate. The samples have been bombarded with 5 keV Ar+ in a fluence range from 10(12) to 5 X 10(15) cm(-2). The experimental evidence inferred from the XPS findings is consistent with a loss of oxygen-rich moieties, connected to main chain scission of the polymer under bombardment. In addition to X-ray photoelectron spectroscopy we used the atomic force microscopy (AFM) technique in order to gain information on the changes in surface topography under ion irradiation. The main AFM result is that the bombardment induces a topographic reorganisation towards a less complex surface and a loss of the pristine high frequency features. We explain these data basing upon the fact that at high fluence (> 10(14) cm(-2)) the PMMA (a positive resist) turns to a negative tone resist and cross-linking becomes the most probable effect giving rise to a 3D network. This heavy modification of the polymer skin under ion beam shrinks the polymer surface and is responsible for the topographic reorganisation detected by AFM.