XPS Info - Background

At each core level peak a rise in the base line (towards higher binding energy) can be observed. The cause is inelastic scattering of electrons. Because of this, these electrons will loose some of their kinetic energy. Electrons that have a lower kinetic energy will appear as electrons with a higher binding energy.

The working principle of XPS is that an X-ray will cause core electrons to be emitted. If electrons are removed from the sample, the sample becomes (positively) charged, unless the ejected electrons are being replaced. This will be achieved naturally in conducting materials. For insulators the flood gun has to be used in order to achieve this.

If the charging process can not be cancelled out, than this will show in the spectra. The peaks will shift to (much) higher energies.

An atom that is part of a chemical compound will share one or more electrons with another atom in covalent bonds. Or it will completely 'take' or 'give' one or more electrons from or to another atom in ionic bonds. Chemical bonds will change the electron density of the atoms.

Atoms with a higher electron density will display a lower binding energy. And atoms with a lower electron density will display a higher binding energy.

The XPS at ChemE is a ThermoFisher K-Alpha. The K-Alpha has two vacuum chambers. One for sample entry and one for analysis. Both chambers have a separate vacuum pump which both share a single backing pump.

The sample entry chamber, or load lock, has a hatch that can be opened to let the sample(s), which are placed on a sample holder, in. A camera is mounted above the hatch, for making overview pictures of the sample holder.

The sample entry chamber is connected to the analysis chamber with an interlock in between. With a transfer arm the sample can be moved from the sample entry chamber to the analysis chamber onto a stage.

An X-ray gun, a flood gun and an ion gun are mounted on the analysis chamber, as well as 2 cameras. The analysis chamber also holds a stage to move the sample holder in 3 directions. Furthermore the analysis chamber holds a titanium sublimation pump.

The X-ray gun uses an Al Kα source and a Rowland circle monochromator for radiating with a monochromated energy of 1486 eV. The cathode is set to 12 kV and the beam current to 3 mA. The (nominal) spot size by default is 400 µm but can be as small as 30 µm. The actual spot size, as seen on the phosphor reference, is about 770x380 µm. The flood gun is used to replace the electrons which have been emitted. It thus prevents charging of the sample.

The ion gun is used to etch the sample with ionized argon. Etching can be used either to clear the surface of the sample or to perform depth profiling. Or both.

Figure - Instrument overview as presented by ThermoFisher

Of course the XPS also contains a detector. This detector receives the electrons that have passed through electrostatic lenses and a semispherical analyzer. The voltage applied to the electrostatic lenses influence which electrons will hit the detector. The voltage between the inner and the outer shell of the analyzer largely determines the resolution of the detector.

The detector itself consists of 128 elements. The analyzer determines which electrons will hit the detector, based on their kinetic energy. The 'pass energy' is the width of the kinetic and thus binding energy range that will be measured. In 'snap mode' the pass energy is selected such, that it passes electrons within the entire range to be measured. In 'scan mode' the pass energy is usually set at a lower value and the central energy is scanned to match the binding energy range to be measured.

The snap mode is extremely fast but the electron count is relatvely low. A typical measurement in snap mode will take 5 seconds. The scan mode gives more defined spectra, with fixed steps in binding energy typlically 0.1 eV and a (much) higher electron count. A typical measurement in scan mode will take about 2 minutes.

Once the sample holder is in the entry chamber the pressure has to be below 2.10^-7 mbar before it may enter the analysis chamber. That is not the default instrument limit but ChemE policy! Depending on the bypass valve (between the entry chamber and the analysis chamber) the stand-by pressure in the analysis chamber can be low as 6.10^-9 mbar (valve open) or at about 6.10^-8 mbar (valve closed). While doing measurements the bypass valve is always open. Most experiments will be performed with the flood gun on, in which case the pressure rises to about 3.10^-7 mbar.

Pressure overview, values in mbar:

Flood gun settings: 1 V, 100 µA.

X-ray gun settings: cathode at 12 kV, beam of 3 mA (for a 400 µm spot)

The relative sensitivity factor (RSF) is the relative chance of electrons escaping from the core. It depends on the atomic weight, but also on electron configuration and it is a different value for electrons for different orbitals. The element info page will show you the various RSF values for all the elements (in the binding energy range that can be measured on our device e.g. with an Al source).

It is noteworthy that peaks with a small RSF may appear relatively insignificant, but they can represent a larger atomic percentage when compared to a (much) larger peak with a (much) larger RSF.

As a side note, heavier elements (on our machine starting from oxygen) will have multiple XPS peaks within a spectrum. These peaks tend to have different areas, but corrected with their respective RSF values will represent the same (relative) atomic percentages. For example if an element gives rise to peak A and B then [peak area A/RSF A] should be the same as [peak area B/RSF B]. Of course this is how the RSF values are obtained in the first place, by assuming that all peaks from one element should represent the same amount.