Quantitative XPS analysis of surface nano-structures

Sven Tougaard

Physics Institute, University of Southern Denmark, DK-5230 Odense M, Denmark

svt@fysik.sdu.dk

The main objectives of quantitative XPS are to determine

· the amount of atoms in the outermost ~ 10 nm of a solid

· and the in-depth distribution of these atoms

For practical application of XPS as a routine technique for analysis of surface nano-structures it is important to have analysis procedures that are

· robust and accurate

· user friendly and application oriented

· in operator rather than expert mode

· apt for automatic data handling

To achieve the latter three properties, it is necessary to make simplifying assumptions which, if they are not made with care, may affect the robustness and accuracy. It is therefore of utmost importance to make clever approximations. This essentially means first to identify the leading factors for accurate quantification procedures and ignore the less important factors. Next to make simplifications for description of the leading factors which result in procedures that are still accurate and robust in the sense that operator induced differences in data handling results in minimal variation in the quality and reliability of the analysis.

The most important physical parameters required for analysis are the inelastic electron mean free path and the photoionization cross section. Besides, the by far most important physical effect that must be taken care of is the fact that 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 highest uncertainty in quantitative interpretation of XPS unless it is accounted for. Different methods have been suggested. One possibility is to measure the variation in peak intensity with the angle of emission and another is to analyze the energy distribution in an energy range around the peak energy.

In the talk, algorithms with various degrees of complexity will be discussed and compared in relation to their ability to fulfill the points in the two lists given above. Several practical examples of nano-structure analysis will be presented.