Guide to Analysis Using CasaXPS for Kratos Aquired Data

Introduction

Firstly, thank you for using the EPSRC service here at Cardiff, and we hope that your visit was fruitful and informative as it was for us. However, XPS analysis, although relatively straight forward, should be treated with care, epscially if you are unawre of the features present in the spectra, together with transmission function correction and which sensitivity factors etc to use.

Here we will give a (brief!) guide to help you in the right direction on the analysis of your data, using CasaXPS software the demo version of which is available from the CasaXPS website, and will allow you to read and manipulate/analyse your saved data.

Should you wish to know more about XPS (and surface analysis in general), CasaXPS or quantification issues etc, we recommend the following texts/databases:

An Introduction to Surface Analysis by XPS and AES	J.F.Watts and J.Wolstenholme	(ISBN: 9780470847138)
Surface Analysis - The Principle Techniques	J. Vickerman	(ISBN: 9780470017647, second edition)
Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy	Ed. D Briggs and J.T. Grant	(ISBN: 1901019047)
Surface Analysis of Polymers by XPS and Static SIMS	D. Briggs	(ISBN: 9780521352222)
High Resolution XPS of Organic Polymers: The Scienta ESCA 300 Database	G. Beamson and D. Briggs	(ISBN: 0953784843)
Handbook of Surface and Interface Analysis: Methods for Problem-Solving	J.C Riviere and S. Myhra	(ISBN: 0824700805 first edition, 2nd ed now in press)

Analysing Data

Important things to remember for the analysis of XPS spectra are the corrections for the relative sensitivity factors of the elements/orbitals present in the sample and also the transmission function of the spectrometers analyser. Proper appreciation of these, will yield the true surface composition (within experimental certainty).

(1) Transmission Correction

The VAMAS format files supplied to you have the Transmission function of the Kratos Axis Ultra-DLD instrument (measured in all spectroscopic modes on a clean gold foil) stored within it, so no further work is required for this step. To ensure that at transmission function is present, call up the Region Properties dialogue (F5) and if the Automatic tick box is ticked, then there is a transmission function present, and this is being used. It is recommended to use this transmission function if using the Kratos modified Wagner sensitivity factors (see below).

Should you wish to use theoretically calculated sensitivity factors, or to use a Tougaard background subtraction, the spectrometer intensity response function must be determined absolutely and you should therefore use the transmission function as that developed by the NPL, and as seen in the paper by Walton and Fairley, then you may use our NPL derived transmission function. It is recommended to use this transmission function if you wish to use Scofield's relative sensitivity factors (see below).

To use the NPL derived transmission function, firstly download the spectra (here - Will be added shortly) and extract to directory of your choice. We offer this transmission function for both 160 and 80 eV Pass energies, and therefore recommend it used with the appropriate survey spectra for quantification.
Now load the VAMAS data file and select all the objects in the browser window that the changes are to be applied to.
Open the "Quantification" window from the options menu and in the Intensity box enter a value of -0.6 for the energy dependancy of the IMFP, and click update.
Open the "Processing" window from the options menu and select "intensity calibration". Click "add transmission function from file" button and select the appropriate transmission function file you downloaded. NOTE: For quantification DO NOT click the apply button. Only click this button if you wish to modify the spectra with the transmission function. We use the transmission function correction parameters appended to the end of the transmission function data. Now the quantification can be carried out by measuring photoelectron peak areas in the usual way. If you wish to modify the data with the transmission function, be aware that the statistics of the spectra will change, and that you should also uncheck the automatic intensity calibration box in the quantification window.

(2) Sensitivity Factors

The sensitivity factors supplied with CasaXPS are those tabulated by Scofield (Journal of Electron Spectroscopy and Related Phenomena, Volume 8, Issue 2, 1976, Pages 129-137) , although widely used they require further correction, typically for angular and dependency and also the mean free path (MFP) of the photoelectrons, which is not always overly evident, especially if the geometry of the system is unknown. To overcome this the author of CasaXPS supplies a ready-made database of Kratos sensitivity factors, which are based on those by Wagner (Sensitivity factors for XPS analysis of surface atoms (Journal of Electron Spectroscopy and Related Phenomena, Volume 32, Issue 2, 1983, Pages 99-102) but modified to account for the geometry of the system (60 degrees). Since Wagner (and therefore modified Wagner) factors are based on measurements of many compounds, then the MFP dependence is already accounted for. To load the modified factors, you must do the following:

Download the Zip file from this link
Unzip the file directly to your CasaXPS directory overwriting the default CasaXPS.lib file which contains the Scofield factors. Should you wish to use these also, rename this original file to CasaXPS_Scofield.lib or something similar before extracting the new Kratos library.

Should you wish to use the NPL transmission function from section (1) and therefore use the Scofield parameters, then together with the MFP dependency discussed above, you must account for the angular dependence of 60 degrees. This is done as follows:

Select the Element Library Function (F10)
On the first tab, click "Edit Source Analyser Angle" and enter 60 as the new value and select OK. If you have already created regions, force an update by adding # to the start of the region name to cause CasaXPS to load the modified Scofield value

Surface Analysis @ Cardiff

Guide to Analysis Using CasaXPS for Kratos Aquired Data

Introduction

Analysing Data

(1) Transmission Correction

(2) Sensitivity Factors