ASTM E2108-10

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 is calibrated or after any substantial modification of the instrument. For later checks of the calibration, to be performed on a regular schedule, only the operations in 8.2-8.5, 8.10, 8.11, and 8.14 need to be performed. While the measurements described in 8.7-8.9 for the first calibration require moderate time and effort, they are essential for ensuring that realistic tolerance limits ±δ have been chosen. The control chart, described in 8.14, is a simple and effective means of demonstrating and documenting that the BE scale of the instrument is in calibration, that is, within the tolerance limits, for a certain period of time.

The average energy of the X-rays incident on the specimen for instruments equipped with a monochromated Al X-ray source will generally be slightly higher, by up to about 0.2 eV, than the average X-ray energy for instruments equipped with an unmonochromated Al X-ray source (4). The actual energy difference depends on the alignment and thermal stability of the X-ray monochromator. An optional procedure is given in Appendix X2 to determine this energy difference from measurements of the Cu L₃VV Auger-electron peak. This information is needed for the determination of modified Auger parameters and Auger-electron kinetic energies on instruments with the monochromated Al X-ray source.

1.1 This practice describes a procedure for calibrating the electron binding-energy (BE) scale of an X-ray photoelectron spectrometer that is to be used for performing spectroscopic analysis of photoelectrons excited by unmonochromated aluminum or magnesium Kα X-rays or by monochromated aluminum Kα X-rays.

1.2 The calibration of the BE scale is recommended after the instrument is installed or modified in any substantive way. Additional checks and, if necessary, recalibrations are recommended at intervals chosen to ensure that BE measurements are statistically unlikely to be made with an uncertainty greater than a tolerance limit, specified by the analyst, based on the instrumental stability and the analyst's needs. Information is provided by which the analyst can select an appropriate tolerance limit for the BE measurements and the frequency of calibration checks.

1.3 This practice is based on the assumption that the BE scale of the spectrometer is sufficiently close to linear to allow for calibration by measurements of reference photoelectron lines having BEs near the extremes of the working BE scale. In most commercial instruments, X-ray sources with aluminum or magnesium anodes are employed and BEs are typically measured over the 0–1000 eV range. This practice can be used for the BE range from 0 eV to 1040 eV.

1.4 The assumption that the BE scale is linear is checked by a measurement made with a reference photoelectron line or Auger-electron line that appears at an intermediate position. A single check is a necessary but not sufficient condition for establishing linearity of the BE scale. Additional checks can be made with specified reference lines on instruments equipped with magnesium or unmonochromated aluminum X-ray sources, with secondary BE standards, or by following the procedures of the instrument manufacturer. Deviations from BE-scale linearity can occur because of mechanical misalignments, excessive magnetic fields in the region of the analyzer, or imperfections or malfunctions in the power supplies. This practice does not check for, nor identify, problems of this type.

1.5 After an initial check of the BE-scale linearity and measurements of the repeatability standard deviation for the main calibration lines for a particular instrument, a simplified procedure is given for routine checks of the calibration at subsequent times.

1.6 This practice is recommended for use with X-ray photoelectron spectrometers operated in the constant-pass-energy or fixed-analyzer-transmission mode and for which the pass energy is less than 200 eV; otherwise, depending on the configuration of the instrument, a relativistic equation could be needed for the calibration. The practice should not be used for instruments operated in the constant-retardation-ratio mode at retardation ratios less than 10, for instruments with an energy resolution above 1.5 eV, or in applications for which BE measurements are desired with tolerance limits of ±0.03 eV or less.

1.7 On instruments equipped with a monochromated aluminum Kα X-ray source, a measurement of the position of a specified Auger-electron line can be used, if desired, to determine the average energy of the X-rays incident on the specimen. This information is needed for the determination of modified Auger parameters.

1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.