Calibration Checks – Routinely Check BE Calibration Values – Validation Check
Weekly or Monthly
Routinely Check BE Calibration Values – Monthly
- Check BE Calibration by using the routine conditions used to separate and identify different chemical states. If more than one pass energy and step size (eV/step) is routinely used, then consider checking those settings at least one time.
- Check the Energy Scale by measuring the Cu 2p3 peak (932.62 eV) and either the Cu 3s peak (122.45 eV) or Cu 3p3 peak (75.13 eV) from pure (>99.9%) Copper metal that was freshly cleaned (e.g. scraped) and strongly ion-etched.
- If any BE is wrong by more than >0.15 eV (practical conditions), we can use the measured BEs to adjust the work function (energy offset), energy scale factor (DAC) or pass energies of that XPS instrument.
- Each time one instrument voltage is adjusted, the Cu 2p3 BE and Cu 3p3 (or Cu 3s) must be re-measured.
Various handbooks and standards on XPS recommend the use of two or more pure elements to check, and if necessary, adjust the reference energy scale of an XPS system. Various methods to check the accuracy of atom % results usually involve complicated checks of the transmission function or instrument response function of an XPS instrument, and often involve the use of additional materials.
While existing methods are valid, most are tedious, time consuming, expensive and seldom used. The end result of this conundrum is that published BEs and atom % values are unnecessarily inaccurate and, too often, overly deviant. As evidence of this still ongoing problem, please review the BE data, derived from the most recent version of the NIST XPS BE Database SRD20, v3.4 – shown as Figure 1.
This poster will present fast and simple methods to, not only check BE calibration, but also at the same time, check atom % accuracy by using a single piece of ~99.9% pure copper foil. The pure copper piece, just like all reference materials, must have minimal amount of surface contamination on both the top and bottom surfaces that ensures good electrical contact.
The sample mount, sample stage, mounting screws, mounting clips and those parts that connect the copper sample to the spectrometer, and to ground, must provide good electrical conductivity. This is the standard requirement for everyday analytical work, as well as the complicated calibration checks. Good electrical conductivity, that extends from the surface of a clean piece of copper on a sample mount all the way to the spectrometer, and to ground, sounds easy to achieve, but, from time to time, can be complicated by unexpected surface contamination/corrosion of sample mounts, sample stages and pressure-based electrical contacts inside the main analytical chamber.
This method expects that the copper foil sample has a Fermi level contact to a conductive sample mount and that the sample stage inside the main UHV chamber has a Fermi level contact to the instrument.
Look at the top and bottom of the copper sample and decide if it is or is not clean enough. If either looks brown, green or blue, then clean it using fine sandpaper. Don’t use metal polish as these have various chlorine agents that allow the copper to re-oxidize quickly. Surface cleanliness is more important than smoothness, although smoothness will affect the counts in the signals to be measured.
The time needed to remove a modest layer of oxidized copper is typically 2-3 minutes when using Argon ions and a 2-4 kV argon beam with a 5-10 mA current setting. Ion etch a region of the copper sample until the oxygen signal has dropped to near zero or zero. If possible, use a piece of copper foil that provides regions that will not be repeatedly ion etched clean. Store this sample in a closed box or keep it wrapped in aluminum foil to minimize the time needed to remove the oxidized copper layer. If you are able to store the sample in the prep lock or the main chamber then do that. If you only have an old piece of copper that is heavily oxidized and looks brown in color, then grab a knife or a razor blade and scrape the surface at different angles until you produce a bright copper surface. The roughness that will result has no effect on the BEs, but will the sample can more easily oxidize due to the roughness of the surface.
Adjust the sample to the optimum position for the instrument and begin measurements. This position may or may not be the optimum position for the instrument. If you suspect there may be some variation in BE as a function of sample height or pass energy, then you might want to run a series of measurements to decide if the instrument needs adjustment by a service engineer.
Variables that may Affect BE Calibration Checks
- Bad electrical grounding
- BE scale calibration
- Beam size vs BE
- Copper for energy reference
- Dirty sample mount
- Drift problem
- Excessive contamination on calibration metals
- Focus vs BE (does it vary?)
- Oxides on Copper or Silver
- Pass energy vs BE
- Reference energies
- Run chart for reference BEs
- Weekly or Monthly BE check