Plasma Cleaning for Synchrotron Mirrors

Synchrotron light sources have become an essential tool for researchers in materials, life sciences, semiconductors and other fields. High-speed electrons in the synchrotron storage ring create high intensity X-ray and extreme ultraviolet light, which is then directed by specialized mirrors to experimental outstations where the measurements are taken.

Problem of Cleaning Synchrotron Mirrors

It is important that most of the optical elements are kept and operated under very high vacuum where carbon containing molecules such as hydrocarbons are ubiquitous contaminants. As the high intensity synchrotron light impinges on the specialized mirrors in the vacuum, any carbon on these mirrors will react to form a non-volatile layer.

The reflectivity of the mirror will be reduced even if this layer is only a few nanometers. Eventually the reflectivity will be so low and the layer thickness so large that the mirror is no longer useful for certain experiments.

It is time-consuming and expensive to remove the mirrors, clean and then remount. An in situ method of mirror cleaning will be very useful. Other technologies requiring clean vacuum systems have been employing ways to ameliorate the carbon contamination problem.

Specifically, a similar contamination issue is present in scanning electron microscopes (SEMs), where energetic electron beams cause carbon residing on the surface to be analyzed to chemically change and form a non-volatile layer of carbon. This layer is often seen as a “black square” in the SEM image.

GV10x Downstream Asher for Carbon Removal from SEM Chambers

For reducing the carbon in chambers in SEMs, an apparatus needs to be attached to the chamber which through downstream plasma process removes the carbon and improves SEM imaging. The commercially available GV10x Downstream Asher is highly effective in quickly removing carbon from SEM chambers while being gentle enough not to harm the sensitive components inside. The GV10x Downstream Asher makes use of a patented, inductively coupled plasma source to create energetic cleaning gas to remove contamination from SEM chambers. The question is whether the same technology can be used for synchrotron mirrors.

Ongoing Experiment with Synchrotron Mirrors

This solution is being tested by a group at the Cells-Alba Synchrotron in Barcelona, Spain, headed by Dr. Eric Pellegrin and in collaboration with ibss Group in San Francisco, CA, USA. Dr. Pellegrin and his colleagues, Dr. Igors Šics and Dr. Juan Reyes-Herrera, are determining the efficacy of the commercially available version of the GV10x. They are also measuring what impact the cleaning has on the optical surface of the mirror itself. They are working on tests with different gas mixtures that include oxygen, hydrogen, and noble gases, operating pressures, and RF powers.

The GV10x is also being compared with a capacitively coupled antenna source, a conventional method of introducing energetic cleaning gas into a chamber.

Electron beam deposition is used to deposit contamination on samples. The test chamber as shown in Figure 1 has both the GV10x and the antenna source attached.

Experimental apparatus used by the Cells-Alba group to test removal of carbon from mirror samples by plasma cleaning. The GV10x Downstream Asher Source is attached to the right side of the chamber.

Figure 1. Experimental apparatus used by the Cells-Alba group to test removal of carbon from mirror samples by plasma cleaning. The GV10x Downstream Asher Source is attached to the right side of the chamber.

Furthermore there is a quartz crystal microbalance (QCM), sample manipulator, gas mixing apparatus and a residual gas analyzer. An XPS is used to perform surface analysis on the samples for the measurement of surface chemistry and an interference microscope is used to measure surface roughness. Cells-Alba are also working with a prototype GV10x Asher system provided by ibss Group that operates at higher RF powers, which has been shown to be more effective at creating hydrogen radicals.

Conclusion

Initial data obtained is highly encouraging. When plasma gas is pure oxygen, the GV10x shows a more than 2x increase in cleaning rates over the “traditional” antenna source at the same power and pressure. The GV10x source also shows increasing promise while using hydrogen gas mixtures. For these cases, the cleaning is done without further oxidation of the sample surface. These experiments are ongoing and the results are anticipated in the near future.

This information has been sourced, reviewed and adapted from materials provided by IBSS Group

For more information on this source, please visit IBSS Group.

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