Paper AS-WeA7
Application and Optimization of Depth Profile Reconstruction from XPS Data using the Maximum Entropy Method

Wednesday, November 11, 2009, 4:00 pm, Room C2

Angle resolved X-ray Photoelectron Spectroscopy (ARXPS) is a useful method for obtaining nondestructive quantitative information about the depth distribution of chemical components in thin (2– 8 nm) films. Modern instrumentation makes the collection of large amounts of data straight forward but determining the depth distribution of elements is more challenging. One numerical method commonly applied to this type of data is the maximum entropy method (MEM). For the MEM to be successfully applied and a depth profile reconstructed, the experimental data must be consistent with the physical model describing the electron transport in the sample. The precision of the reconstructed depth profile depends both on the noise in the experimental data and on the accuracy of the physical model. The Beer-Lambert law of electron transport which is used for routine analysis of the experimental data provides inadequate description of the elastic scattering effects and doesn't account for the finite analyser acceptance angle.

We present a more refined model of the MEM algorithm based on a depth distribution function which allows the consistent inclusion of the aforementioned effects of scattering and analyser acceptance angle into the analysis of depth profiles. Direct incorporation of the depth distribution algorithm into the MEM model also allows the utilization of the results of Monte Carlo simulations of electron transport instead of various approximate attenuation length parameters.

The effect of data collection conditions, in terms of instrument operating mode; analyser collection angles; and signal to noise, on the reliability of reconstructed profiles is also investigated. A set of optimised conditions and minimum data quality for successful depth profiling of the materials under investigation are suggested.