Surface charge neutralization of insulating samples in x-ray photoemission spectroscopy

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Abstract
Obtaining high resolution x-ray photoemission spectroscopy (XPS) spectra of insulating samples has long been a problem because of difficulty controlling sample surface potentials. A flood of low energy electrons has traditionally been used to control surface potential, but as monochromatized instruments with small, intense x-ray beams have become available, control has become much more difficult, particularly on larger samples. Increasing the current or energy from the flood gun does not improve the control appreciably. To understand the charging effect in a quantitative way, we have conducted some experiments with a test sample, configured to approximate the geometry present in several commercial XPS instruments using focused x-ray sources. These results show that, because of the energy spread of the flooding electrons, a negative potential is induced by the flood gun in the region surrounding the x-ray beam. This negative potential repels electrons, inhibiting the adequate neutralization of many samples. Based upon an analysis of these results, a neutralizing system is described in which a high current density flood gun with a narrow energy spread (to reduce potential variation on the sample) is used in combination with a source of low energy positive ions (to neutralize the negative potential in the peripheral region). This system has been tried on a wide variety of insulating samples in three commercial instruments, and found to give reproducible spectra with narrow linewidths. We present here spectra taken with the instrument offering the smallest and brightest x-ray beam, the most challenging case of the instruments tried.

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