Analytical Ion Microbeam

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Method

Normally ion beams have a size in the mm range but they can also be focussed to µm beamsizes using slits as object and a set of magnetic quadrupole lenses. The ion beam current for ion microbeams is much less than for mm-sized ion beams because of the small size of the object slits, needed for a small beamsize. The ion beam can be scanned over the sample magnetically of electrostatically to obtain laterally resolved information, e.g. elemental distribution maps.

All Ion Beam Analysis (IBA) methods can also be applied with a ion microbeam but PIXE is most often used because PIXE is a very sensitive technique when 2-3 MeV proton beams are used. It is also important to have large solid angles for the detectors or use multiple detectors to improve the sensitivity because of the low beam currents available. Multiple techniques like PIXE, RBS, PIGE are often used simultaneously to obtain a maximum of information from one measurement.

Advantages

• Quantitative determination of concentrations based on IBA techniques is available at the micrometer scale.
• Light elements can be analysed by PIGE, NRA or RBS in the energy region of elevated non-Rutherford cross sections.
• Some of the methods like RBS and NRA are depth sensitive.

Limitations

• The beamsize is typically several µm and it is difficult to focuse sub-µm because MeV ions are much more rigid then keV electrons.
• Measurements in vacuum, in air is possible but with larger beam size of tens of µm.
• Some damage can occur, especially for fragile samples.

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Analytical Ion Microbeam setup at HZDR

The ion microbeam is connected to the 3 MV Tandetron of the Ion Beam Center and typically a 3 MeV proton beam is used, which is very suitable for PIXE and non-Rutherford RBS. The following methods are available: PIXE, RBS, PIGE, NRA, STIM (Scanning Transmission Ion Microscopy). Quantitative elemental concentration maps can be obtained from PIXE measurements using the software package GeoPIXE, available at the HZDR.

Samples

A wide variety of samples from many fields of application can be analysed, such as minerals for Geoscience, aerosol filters for environmental science, coatings and thin films in tribology (science of wear, friction and lubrication), biomedical devices like stents.

• Flat samples with size of max. 25x25 mm² and a maximum thickness of 5 mm can be mounted with double sided tape on the sample holder.
• Samples with a diameter of ≤ 10 mm and a maximum thickness of 15 mm can be mounted at recesses on the side of the sample holder.
• Point measurements can be performed on irregular samples, whereas a flat (small) surface is required for elemental imaging.

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Application 1: Analysis of individual aerosol particles

• Aerosol particles have a direct impact on air quality, cloud nucleation, radiation balance, public health, etc.
• These particles can be collected on filter materials with an impactor.
• Particle Induced X-Ray Emission (PIXE) is a non-destructive and undemanding technique for elemental analysis that is suitable for offline filter analysis.
• Rutherford Backscattering Spectrometry (RBS) is a non-destructive technique that can provide offline elemental depth information.
• With an ion microbeam, laterally resolved elemental maps can be made, making the analysis of individual particles possible.

Samples:

• Mineral/Saharan dust particles collected on filter tape of quartz fibres coated with Teflon using a Aethalometer AE33.

Measurements:

• Ions: H⁺ (protons)
• Energy: 3 MeV
• Beam size: 6x4 μm²
• Maps: 256x256 points with a step size of 1 µm
• Detection: characteristics X-rays with a Ketek Silicon drift detector
                   backscattered protons with a silicon strip detector

Spectra with regions for maps:

Elemental maps:

• Large high intensity areas in the Fe map correspond to the large areas in the Mn map.
• The small Fe containing particles (red points in the Fe map) do not correspond to Ca containing particles.

• Three elements (Ba, K, Zn) show predominantely a fibre structure that is similar for each element.
• The rightmost graph shows a RGB composite with each element assigned a basic colour. This graph shows a good correlation between all elements (white) but also fibres with only K (green) and fibres with Ba (L-lines) and Zn (pink).

• These three maps show the teflon coated quartz fibres of the filter tape at different depths. These fibres are most clearly visible in the maps based on carbon because RBS with 3 MeV protons has a high sensitivity for carbon.
• The rightmost graph shows a RGB composite with each RBS map assigned a basic colour.The graph clearly shows the fibre structure, even though they are at different depths.

Conclusion

• PIXE provides good elemental identification and sensitivity but is not depth sensitive. It can be used to identify elements in aerosol particles.
• RBS provides depth information and using a 3 MeV H-ion beam is especially useful for detecting light elements like C.
• Elemental maps are a useful tool to assign elements to individual particles or fibres.

This work is part of the Helmholtz European Partnering Project CROSSING (PIE-0007)

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Application 2: Analysis of wear processes in tribological ta-C coatings

Objective: Develop solid lubricants with the aim of eliminating liquid lubricants. This eliminates the consumption of lubricants, which results in

• cost savings and
• reduced environmental impact.

First tribological wear tests

• ta-C (hydrogen-free, tetraedic, amorphous carbon) as a new and exciting solid lubricant coating on steel
• Counter bodies of various materials, e.g. brass, steel, SiC
• Apply tribological tests by rotating the counter body over the coating

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Analyse wear track and counter body for

• Loss of coating
• Transfer of material from counter body

=> Apply Ion Beam Analysis
Measurement conditions:

• Use the ion microbeam because wear areas and tracks are small
  Focus: 5-10 µm
• Different methods
  • PIXE
  • RBS
• Different ions and energies
  • H⁺, 3 MeV
  • He²⁺, 2 MeV

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Wear track on the ta-C coating made by the SiC counter body

• PIXE and RBS with a 2 MeV He-ion beam

• Maps of the wear track obtained from the regions in the PIXE and RBS spectra

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Contact area on a steel counter body

• PIXE and RBS with a 3 MeV H-ion beam

• Maps of the area on the counter body that was in contact with the ta-C coating

• RBS spectra are extracted from selected regions to do a detailed analysis

Conclusion

• Optimising measurement conditions can provide information for both
  • a light matrix, the ta-C coating, using a 2 MeV He-ion beam and
  • a heavy matrix, the counter body, using a 3 MeV H-ion beam.
• RBS provides depth information and is sensitive to the surface and
  • using a 3 MeV H-ion beam is especially useful for detecting light elements like C and O in a heavy matrix.
• PIXE provides good elemental identification and sensitivity but is not depth sensitive,
  • using a 2 MeV He-ion beam increases the sensitivity for light elements.
• Elemental maps are a useful tool to study transfer of material.

This work is part of the Deutsche Forschungsgemeinschaft (DFG) Projekt TRIGUS (415726702)

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