X-ray photoelectron spectroscopy is used extensively for the surface analysis of materials. Elements (with the exception of hydrogen and helium) are identified from comparisons of the binding energies determined from photoelectron spectra with tabulated values. Information on chemical state can be derived from the chemical shifts of measured photoelectron and Auger-electron features with respect to those measured for elemental solids.
Calibrations of the BE scales of XPS instruments are required for four principal reasons. First, meaningful comparison of BE measurements from two or more XPS instruments requires that the BE scales be calibrated, often with an uncertainty of about 0.1 eV to 0.2 eV. Second, identification of chemical state is based on measurement of chemical shifts of photoelectron and Auger-electron features, again with an uncertainty of typically about 0.1 eV to 0.2 eV; individual measurements, therefore, should be made and literature sources need to be available with comparable or better accuracies. Third, the availability of databases (3) of measured BEs for reliable identification of elements and determination of chemical states by computer software requires that published data and local measurements be made with uncertainties of about 0.1 eV to 0.2 eV. Finally, the growing adoption of quality management systems, such as, ISO 9001:2000, in many analytical laboratories has led to requirements that the measuring and test equipment be calibrated and that the relevant measurement uncertainties be known.
The actual uncertainty of a BE measurement depends on instrument properties and stability, measurement conditions, and the method of data analysis. This practice makes use of tolerance limits ±δ (chosen, for example, at the 95 % confidence level) that represent the maximum likely uncertainty of a BE measurement, associated with the instrument in a specified time interval following a calibration (ISO 15472:2001). A user should select a value of δ based on the needs of the analytical work to be undertaken, the likely measurement and data-analysis conditions, the stability of the instrument, and the cost of calibrations. This practice gives information on the various sources of uncertainty in BE measurements and on measurements of instrumental stability. The analyst should initially choose some desired value for δ and then make tests, as described in 8.14 to determine from subsequent checks of the calibration whether BE measurements are made within the limits ±δ. Information is given in Appendix X1 on how to evaluate for a material of interest the uncertainty of a BE measurement that is associated with the uncertainty of the calibration procedure. This information is provided for four common analytical situations. It is important to note that some BE measurements may have uncertainties larger than δ as a result of poor counting statistics, large peak widths, uncertainties associated with peak synthesis, and effects of surface charging.
Instrument settings typically selected for analysis should be used with this practice. Separate calibrations should be made if key operating conditions, such as choices of analyzer pass energy, aperture sizes, or X-ray source, are varied. Settings not specified in this practice are at the discretion of the user, but those same settings should be recorded and consistently used whenever this practice is repeated in order that the current results will be directly comparable to the previous results.
All of the operations described in Section 8 should be performed the first time that the BE scale ......<......
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