Abstract
Boron carbide (B4C) is one of the few materials that is expected to be most resilient with respect to the extremely high brilliance of the photon beam generated by free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. Carbon contaminations represent a serious issue for the operation of FEL beamlines due to severe reduction of photon flux, beam coherence, creation of destructive interference, and scattering losses. A variety of B4C cleaning technologies were developed at different laboratories with varying success. We present a study regarding the low-pressure RF plasma cleaning of carbon contaminated B4C test samples via inductively coupled O2/Ar, H2/Ar, and pure O2 RF plasma produced following previous studies using the same ibss GV10x downstream plasma source. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coating before and after the plasma cleaning are reported. We conclude that among the above plasma processes only plasma based on pure O2 feedstock gas exhibits the required chemical selectivity for maintaining the integrity of the B4C optical coatings.
Highlights
We report on different approaches regarding the chemically selective low pressure RF downstream plasma cleaning of B4C-coated and amorphous carbon-contaminated optics by the variation of the plasma feedstock gas and the inherent plasma chemistry
We thereby report on the first successful plasma cleaning of such optics, i.e., without incurring damage of the B4C optical coating and its surface resulting into a fully maintained optical performance
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Summary
It can be noted that there is no Fe2p line visible in the survey spectra of the plasma-processed samples It is clear from these numbers that while going from Au to B4C coatings, the cleaning rates for the O2 and O2/Ar-based plasma are strongly reduced (roughly by a factor of 2 to 2.5), whereas the cleaning rates for the H2/Ar-based plasma is basically maintained
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