Work function effects on the positive charging of supported insulating samples exposed to X-rays (as in XPS) and other irradiations

Abstract

Some aspects of the time dependence of positive charging of insulating films are evaluated when they are set on conductive substrates and are exposed to X-rays. This evaluation is based on the influence of the electrostatic surface potential due to the positive charges left by the effective secondary electron emission into the vacuum, δx, combined with some work function effects. Valid up to a surface potential of ∼3–5V, an expression for the nearly exponential decay of δx, time constant τ, is established as a function of the fluence of the irradiation and the geometry and characteristic of the sample and of its environment. The results being a function of the spectral and angular distributions of the emitted SEs, the order of magnitude of the corresponding factors are evaluated numerically for two extreme situations, that of inorganic insulators and that of floating conductors in order to be extended easily to other types of materials. The decay may be preceded by a dwell time, τs, particularly when an external collector is negatively biased. When flood guns are used, the work function effects limit the efficiency of the neutralization effects by the increasing reflection coefficient of the very low energy incident electrons. Finally, the mechanism of the degradation effects of electrons that are issued from the sample holder/sample interface is also developed. This approach may also be adapted other projectiles such as UV photons, ions or electrons.