XPS - A Tool for the Chemical Characterization of Coatings and Interfaces
August 1, 2009
XPS (X-ray Photoelectron Spectroscopy) is a powerful method for the quantitative chemical characterization of thin films and solid surfaces. In this paper the XPS process and instrumentation will be described briefly, followed by examples that illustrate the power of the technique in the analysis of coated surfaces.
Why is Surface Analysis Necessary?All solid materials interact with their surroundings via their surfaces. It is therefore crucial that the surface properties of any material are suitable for the proposed application. Modern, high-performance materials will only be suitable for their intended purpose if their surface properties are correctly engineered. Surfaces are very important in a wide range of materials. A few examples are: glasses, painted surfaces, metals, polymers, biomaterials and semiconductor devices.
The need for the engineering of surface properties gives rise to the requirement for the detailed chemical characterization of the surface. XPS provides a method for quantitative chemical analysis of surfaces and thin films and consequently is a valuable tool for the surface engineer.
What is XPS?As the name implies, XPS relies upon Einstein’s photoelectric effect in which an X-ray photon removes an electron from an atom or molecule provided that photon has sufficient energy. The kinetic energy of that electron depends upon the photon energy and the energy required to remove the electron (the binding energy) according to the equation:
Binding energy = Photon energy – Kinetic energy
In XPS, the kinetic energy of the emitted electron is measured and so, knowing the photon energy, the binding energy can be calculated. From the binding energy it is possible to determine not only the elemental composition of the surface, but also the chemical state of the elements in the material. The basic XPS process is illustrated in Figure 1.
Why is XPS Surface Specific?When X-rays strike a solid, they penetrate into the material to a great depth causing photoemission along the whole of their path. However, as the photoelectrons travel through the material, they undergo collisions in which they lose energy. These electrons do not contribute to the peak in the photoelectron spectrum and many do not escape the solid. Only photoelectrons originating close to the surface (within the top 5 to 10 nm, depending on the material) stand a chance of escaping without energy loss. These are the ones than can be detected in the spectrometer.
Components of an XPS SpectrometerFigure 2 shows a schematic diagram of a modern XPS instrument, the Thermo Scientific K-Alpha. An XPS spectrometer requires a source of X-rays. In modern instruments, an X-ray monochromator delivers Al K-Alpha X-rays to the surface (photon energy = 1486.6 eV). Using this energy it is possible to detect all elements except hydrogen and helium. Non-monochromatic X-ray sources continue to be used, and the photon energy produced by the source is dependent upon the material used for the X-ray anode.
The kinetic energy of the emitted electrons is normally measured using a hemispherical electron energy analyzer, illustrated in Figure 3. In such an analyzer, the emitted electrons are focused by means of electrostatic or electromagnetic lenses into the gap between two concentric hemispherical electrodes. By applying voltages to the electrodes it is possible to ensure that electrons having only a small energy range can reach the detector. By scanning these voltages a binding energy spectrum can be produced.
A typical XPS measurement is in two parts, the survey spectrum and a set of narrow (or high-resolution) spectra. The survey spectrum is used to determine which elements are present at the surface of a solid material. The example shown in Figure 4 is a survey spectrum from an oxidized fluoro-polymer from which it can be seen that carbon, fluorine and oxygen are present.
A quantification table derived from this spectrum is shown in Table 1.
The narrow scans are then collected at a higher resolution in order to determine the chemical states in which each element exists. Figure 5 shows, as an example, the narrow scan from the C 1s region of Figure 4 in which the various chemical states of carbon present in the sample have been labeled.
Using XPS in combination with an ion beam allows concentration depth profiles to be constructed. To do this, analysis and etching steps are alternated, as illustrated in Figure 6. The number of cycles and the etch time per cycle depend upon the etch depth required. The range of depths accessible using a typical XPS instrument is up to about 1 µm.
Examples of the Use of XPS in Coatings and Interfaces
There is a range of problems that can affect painted surface production. These include gross faults such as flaking or delamination of the paint film, and more subtle effects such as mottling or crater defects. Failures can occur for a variety of reasons. For example, contamination on the surface prior to coating can result in poor adhesion of the paint film. Alternatively, problems could occur in the coating itself, such as segregation of the components once applied. XPS is a very surface-sensitive technique and the ideal analysis tool for identifying the cause of a particular failure. In this example, the defect under investigation is a crater defect, which appears as a shallow bowl in the film with a raised centre.
An optical image from XPS system optics is shown in Figure 8 (crater center indicated with an arrow). XPS images of the defect were collected by rastering the stage under the X-ray beam and collecting 128-channel snapshot spectra at each point.
The defect was investigated by acquiring wide scan survey spectra from the centre and edges to identify the elements present, and then mapped to generate XPS images for the chemistries present. The atomic concentration images are shown in Figure 9. It can be seen that there is tin present at the center of the crater but not anywhere else on the surface. In contrast there is depletion of the N1s and O1s signals at the same point. By peak fitting the spectra associated with each pixel of the image, chemical state images such as those shown for the C1s chemical states can be obtained. Again, it can be seen that there is a difference in the carbon chemistry of the defect center compared with the rest of the crater. The C-C/C-H component is in greater concentration at the defect center.
Spectra can be retrospectively extracted from images. In Figure 10 the red spectra are created by averaging the area marked with a red box on the O1s image in Figure 9. The blue spectra were obtained by averaging the spectra in the pixels enclosed by the blue box. The differences between the individual areas can be clearly seen again. In particular the increase in intensity of the C-C/C-H peak in the C1s spectrum (indicated by an arrow) is very obvious. Reductions in the intensity of the N1s peaks and the low binding energy O1s component (again marked with an arrow) are also easily evidenced from this retrospective spectroscopic procedure.
This suggests that the presence of the tin particle not only causes the physical defects (i.e., the crater structure), but also affects the chemistry of the paint film at that location.
ConclusionXPS is a powerful analytical tool that can provide quantitative, spatially resolved, chemical-state analysis of surfaces and thin films. In this paper it has been shown that quantitative chemical state information can be obtained from coated surfaces. A combination of imaging and small area spectroscopy can be used to establish the nature of features and defects within coatings. Combining XPS spectroscopy with ion beam sputtering provides valuable information about the structure and composition of the film along with the chemistry of the interface region. The K-Alpha represents the latest state-of-the-art XPS spectrometer designed to resolve a wide range of surface and interface materials characterization problems and is uniquely suited for the analysis of coated surfaces.
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