CHARGE COMPENSATION FOR XPS ON POLYMERS

Line broadening, induced by differential charging under monochromated X-radiation, makes it difficult to separate adjacent chemical states in X-ray photoelectron spectroscopy (XPS) on insulators, particularly polymers. Electron flood guns, floating filaments, standard X-ray sources, UV lamps, grids and combinations of these have been used to eliminate or minimize the line broadening. Low dose, wide area X-ray irradiation before the measurement, plus a hot filament grounded at its positive lead and shielded by an LN2-cooled copper foil, yields excellent results. Once the optimum geometrical arrangement has been found for sample surface, shield and filament, no bias voltage on the shield or other electrode is required. The same filament emission current, plus the same cooling power, reproduces routinely a FWHM (full-width-at-half-maximum) of 0.9 eV on solid PMMA [poly(methyl methylacrylate)] or PET [poly(ethylene terephthalate)]; this has been shown to be optimum for all other insulators investigated so far. More than 6 h of continuous filament operation does not result in any alteration of the sample by radiation or contamination. There are indications that the system also works for nonmonochromated XPS and for tilt experiments.