XPS for Quantitative Analysis of Surface Nano-structures

Abstract

X-ray photoelectron spectroscopy (XPS) is one of the most widely used techniques for surface analysis. In the technique, which has been applied by industry for more than 30 years, atom core electrons are excited by soft X-rays and the kinetic energy of the emitted electrons is measured. Peaks occur corresponding to the binding energy of the core electrons and their position gives therefore direct identification of atoms. Since the inelastic mean free path for electrons in the energy range studied (typically ~ 0.1 to 1.5 keV) is 0.5 -3 nm, the peak intensity originates from electrons excited from atoms that are within a 2-5 nm thin surface layer. The core electron binding energy varies slightly depending on the chemical environment of the atom, and this is used to get information on the chemical bonds. Scanning XPS has been increasingly more used in the past decade driven by substantial improvements in lateral resolution. The interpretation of XPS has largely been based on simple peak intensity measurements and this strongly limits the quantitative accuracy of XPS. The depth resolution of XPS has usually been done by ion-etching the surface. In the past decade, significant advancements has however been done in the development of improved algorithms for quantitative XPS and non-destructive atom depth distribution analysis in the 0-10 nm depth range [1]. Lateral resolutionThe lateral resolution in XPS-mapping has in general been inferior to scanning techniques based on electrons or ions partly because these are easier to focus to a small spot-size and partly because of a low photoelectron yield. Improvements have been done by new designs of the X-ray source and the electron energy analyzer and the resolution has improved gradually from ~1 mm resolution 20 years ago to ~ 3 µm today. With the construction of the third generation synchrotron radiation sources, which provide very intense photon beams it is now possible to get better than 100 nm resolution and this is expected to be further reduced to better than 5 nm in the near future [2]. Depth resolution.Since the electron inelastic mean free path is ~1nm, the XPS signal attenuates strongly with the depth on the nano-meter scale. It is this property that makes XPS interesting and powerful because it gives the technique its high surface sensitivity. However this is also the source of the high uncertainty in quantitative interpretation of XPS unless it is accounted for. The fundamental problem is illustrated in fig.1 which shows spectra for different depth distributions of Cu in a Au matrix. Due to the attenuation, the Cu2p XPS-peak intensity from all four different surface morphologies is exactly identical although the surface compositions are very different. Quantification based on the peak intensity alone is thus subject to a large error. However, the energy distribution in a wider energy range below the peak depends critically on the depth distribution of atoms. These are electrons that originate from the peak but lose energy on their way out of the solid. It is th...

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