Chemical State Mapping of Polymers with XPS

The latest XPS system from Thermo Fisher Scientific is the K-Alpha. It is equipped with a multi-channel detector and a microfocusing monochromator, and this combination makes the instrument suitable for chemical state mapping. The maps spatial resolution is determined on the user-selected, X-ray spot size. The multi-channel detector allows snapshot spectra to be obtained at each pixel of the map, much quicker than scanned acquisition. K-Alpha’s new charge compensation system enables chemical state maps to be created very easily from insulating materials.

Sample Preparation

After attaching a copper grid to a substrate comprising of silicon, coated with an acrylic acid plasma polymer, the substrate was exposed to plasma comprising of a fluorocarbon monomer. This led to the polymerization of the fluorocarbon on the regions of the substrate subjected to plasma exposure. Removing the grid from the substrate, after plasma exposure, leaves a patterned, polymeric fluorocarbon. Figure 1 illustrates the preparation technique and the sample area that was analyzed in the Thermo Scientific K-Alpha X-ray Spectrometer.

An illustration of the sample preparation, the grid used in the preparation and the area of the sample analyzed in K- Alpha

Figure 1. An illustration of the sample preparation, the grid used in the preparation and the area of the sample analyzed in K- Alpha

Analysis Conditions

The monochromated X-ray spot size was fixed at 30 µm. Snapshot XPS spectra of C1s and F1s were acquired into 64 channels for each element. Scanning the sample stage resulted in the acquisition of spectra at each pixel, in an array of 67 x 94 pixels with a step size of 10 µm.

Peak Fitting Spectrum

The C 1s spectrum that was acquired by summing the C 1s signal from the whole of the image can be seen in Figure 2. This spectrum evidently shows the presence of both fluorocarbon and ester species.

Peak fitting of the C 1s spectrum obtained by summing the spectra in each of the pixels of the map.

Figure 2. Peak fitting of the C 1s spectrum obtained by summing the spectra in each of the pixels of the map.

Image Construction

Just by displaying the image produced at a selected binding energy, a map can be generated. The images formed at 10 of the 64 binding energies obtained for this measurement are depicted in Figure 3.

Maps constructed at 10 of the 64 binding energies collected.

Figure 3. Maps constructed at 10 of the 64 binding energies collected.

In Figure 4a an image constructed from a binding energy of 284.7 eV (hydrocarbon) is displayed, while in Figure 4b an image of the peak at a binding energy of 291 eV (a fluorocarbon peak) is displayed. An overlay of these two images is illustrated in Figure 4c.

(a) Map obtained from the signal at a binding energy of 284.7eV (the hydrocarbon peak) (b) Map obtained from the signal at a binding energy of 291eV (a fluorocarbon peak) and (c) an overlay of the maps shown in (a) and (b).

Figure 4. (a) Map obtained from the signal at a binding energy of 284.7eV (the hydrocarbon peak) (b) Map obtained from the signal at a binding energy of 291eV (a fluorocarbon peak) and (c) an overlay of the maps shown in (a) and (b).

The full set of the individual spectra constituting the map can be fitted with the same set of peaks by performing a peak fit, as illustrated in Figure 2, generating images from any of the fitted peaks displaying atomic concentration or fitted peak areas. Figure 5 illustrates the atomic concentration images, where the atomic concentration maps were created for the two components by applying non-linear least squares fitting to the data set.

(a) Atomic concentration map from the hydrocarbon peak following peak fitting at every pixel. (b) Atomic concentration map from the fluorocarbon peak.

Figure 5. (a) Atomic concentration map from the hydrocarbon peak following peak fitting at every pixel. (b) Atomic concentration map from the fluorocarbon peak.

Reconstructed Spectra

A spectrum is present in each pixel of the image, so the sum of the spectra can be derived from defined areas on the image to create a small area spectrum. Two spectra built from the indicated areas of the image are depicted in Figure 6. From this, it is clear that the substrate peaks can be observed in the fluorocarbon region; however fluorocarbon is not observed in the substrate region.

Figure 6. C1s spectra reconstructed from the areas indicated on the map.

Thickness Map

The peaks from the substrate can be identified in the sample areas covered by the fluorocarbon material, suggesting that the thickness of the fluorocarbon layer is just a few nanometers. A thickness map of the sample can be constructed using the ‘Overlayer Thickness Calculator,’ which is an essential part of the Avantage data system. The only hypothesis made in the calculation was that the density of each polymer is equal to its bulk density. The results of the measurement are illustrated in Figure 7.

Thickness map of the fluorocarbon layer.

Figure 7. Thickness map of the fluorocarbon layer.

Advantages of Stage Scanning

The sample stage is scanned to create a map using the K-Alpha system. This technique of map acquisition has several benefits over other techniques:

  • Spatial resolution is established just by the X-ray spot size, which is stable throughout the acquisition. The image quality at the edges shows no degradation.
  • The spatial resolution is independent of the settings of the transfer lens, therefore the spectrometer is constantly operated at its highest transmission
  • Having a spectrum at every pixel allows thickness maps and quantitative maps of chemical states to be constructed easily
  • The point on the sample being mapped is constantly in the optimum analysis position, so there can be no alterations in the sensitivity across the field of view
  • Very large field of view possible - up to 60 x 60 mm with the K-Alpha spectrometer
  • X-ray energy and intensity do not depend on the position on the sample

Conclusions

High-quality images can be produced using the Thermo Scientific K-Alpha X-ray Spectrometer. The large number of channels obtained in each spectrum indicates the possibility of reliably performing quantitative, chemical state mapping and the parallel acquisition of spectral data enables rapid acquisition. The superior features available in the Avantage data system offer precise chemical state data, and reveal the distribution of the chemical states in two dimensions. The results have also revealed the ability of the K-Alpha system to non-destructively measure and map the thickness of a thin overlayer.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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