Although the graphitic carbon contamination of synchrotron beamline optics has been an obvious problem for several decades, the basic mechanisms underlying the contamination process as well as the cleaning/remediation strategies are not understood and the corresponding cleaning procedures are still under development. In this study an analysis of remediation strategies all based on in situ low-pressure RF plasma cleaning approaches is reported, including a quantitative determination of the optimum process parameters and their influence on the chemistry as well as the morphology of optical test surfaces. It appears that optimum results are obtained for a specific pressure range as well as for specific combinations of the plasma feedstock gases, the latter depending on the chemical aspects of the optical surfaces to be cleaned.

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An extended study on an advanced method for the cleaning of carbon contaminations on large optical surfaces using a remote inductively coupled low-pressure RF plasma source (GV10x downstream asher) is reported in this work. Technical as well as scientific features of this scaled up cleaning process are analysed, such as the cleaning efficiency for different carbon allotropes (amorphous and diamond-like carbon) as a function of feedstock gas composition, RF power (ranging from 30 to 300W), and source-object distances (415 to 840 mm). The underlying physical phenomena for these functional dependences are discussed.

Analyzing carbon in an electron microscope by EDS is difficult, since the pollution by hydrocarbons is responsible for a fast growing C peak when the beam is positioned in spot mode. Hydrocarbons from the chamber surfaces, vacuum pumps and sample surface migrate, react with the electron beam and form a black spot rich in carbon where the analysis is conducted. The analytical difficulty extends to N, which peak is partially overlapped by the growing adjacent C peak and strongly absorbed by the carbon layer that builds up at the surface.The Evactron Anti-Contaminator (A-C) removes Hydrocarbons from the SEM vacuum and from the sample surface [1]. The Evactron A-C is a small RF plasma device mounted on a chamber port and produces Oxygen radicals to oxidize off the hydrocarbons. Although a full clean-up of a dirty microscope chamber use for years cannot be expected in a few minutes, the following results show that the Evactron A-C makes low level carbon analysis possible just after installation.To test the improvement on removing C pollution, a pure Cu polished sample has been used. The microscope was a Gemini 982, fitted with a Noran thin window EDS. The sample had been prepared using the best standard procedure recognized by the operator to give the smallest pollution: polishing, ultrasonic cleaning and final cleaning with pure ethanol. Spectra have been acquired at 15 kV and the EDS system used to measure net intensity peaks on the elements found: C and Cu La. The spot was left static for 15 minutes, and spectra were run for 50 s. The first spectrum was acquired immediately and then three other after 3, 6 and 15 minutes. Results:The first series of spectra has been acquired before Evactron A-C cleaning. The spectrum shows immediately a C peak, and this carbon peak quickly grows up from a relative C/ CuLa intensity ratio of 0.76 % to 5.06 % after 15 minutes of static beam, as it can be seen on the spectrum in Figure 1. The Evactron A-C was then run 4 minutes to remove the hydrocarbons. The first spectrum acquired after cleaning still showed a small carbon peak ratio of 0.61 % that grew up to 1.39 % after 15 minutes. This improvement was impressive but not enough: you cannot expect to clean a sample and a large chamber that had been used for several years in 4 minutes. So a second 4 minutes cleaning was done and a third one two hours later. This gave time to the trapped hydrocarbons to diffuse back and be converted to H 2 O, CO and CO 2 by oxygen radicals from the Evactron A-C. After the third run, the EDS system did not find any carbon in the spectrum for the spectra acquired immediately and after 3 minutes. The spectrum acquired after 6 minutes and 15 minutes showed a small increase of the C intensity that allowed the software to detect C.

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