Revealing the nature of surface charging during X-ray photoelectron spectroscopy analysis of thin film insulators with metal capping layers

Abstract

X-ray photoelectron spectroscopy (XPS) analysis of chemical bonding in electrically insulating samples is seriously complicated by sample charging. Recently, capping an insulator with a few nm thick metallic layer with low affinity to oxygen was shown to eliminate charging in several common insulators. Here, results of the follow-up study aiming at a better understanding of the mechanisms behind this effect are reported. SiO₂ films, used as model insulators, are grown by magnetron sputtering with the thickness $d_{{\mathit{SiO}}_2}$$d_{{\mathit{SiO}}_2}$ in the range 30–3000 nm to study phenomena operating on different length scales. Metal caps, with different photoelectron yields (W and Al), are applied either as a global layer grounded on top with Cu clips or as dots with 1- or 5-mm diameter connecting to the ground only through the silica film. Charging elimination irrespective of $d_{{\mathit{SiO}}_2}$$d_{{\mathit{SiO}}_2}$ takes place in samples with global grounded W caps, and surprisingly also for films capped with 5 mm W dots, provided they are not thicker than 500 nm. Moreover, if the area irradiated by the X-ray beam is larger than that under the metal cap (realized here for samples with 1 mm W dots), charging elimination is observed only for$d_{{\mathit{SiO}}_2}$$d_{{\mathit{SiO}}_2}$ = 30 nm. Material properties that control the effect include the metal photoelectric yield (with respect to that of an insulator), X-ray-induced conductivity, secondary electron (SE) yield, SE inelastic mean free paths, and X-ray attenuation lengths. Experimental variables such as the size of the irradiated area with respect to that of the metal cap and the type of grounding connection are also crucial. Although the study is based on thin films the conclusions give insights into critical factors that govern charging phenomena in any other type of insulating samples.