Boron carbide (B4C) - due to its exceptional mechanical properties - is one of the few existing materials that can withstand the extremely high brilliance of the photon beam from 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 modern accelerator-, plasma-, or laser-based light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. These contaminations - that are presumably produced via cracking of CHx and CO2 molecules by photoelectrons emitted from the optical components - represent a serious issue for the operation of the pertinent high performance beamlines due to a severe reduction of photon flux and beam coherence, not necessarily restricted to the photon energy range of the carbon K-edge. Thus, a variety of B4C cleaning technologies have been developed at different laboratories with varying success . Here, we present a study regarding the low-pressure RF plasma cleaning of a series of carbon-contaminated B4C test samples via an inductively coupled O2/Ar and Ar/H2 remote RF plasma produced using the IBSS GV10x plasma source following previous studies using the same RF plasma source [2, 3]. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coatings and surfaces before and after the plasma cleaning process are reported.
In this study, we present the results from an analysis of the various aspects of the low pressure RF plasma cleaning process for the removal of graphitic carbon contamination layers deposited on different test objects. After determining the optimum parameters for a time-minimized cleaning process using an inductively coupled plasma source as well as oxygen/argon mixtures as feedstock gas, a special emphasis has been put on the characterization of the cleaned surface (Au, Ni, Rh) both in their pristine as well as in their cleaned state. This includes the physical as well as the chemical properties of the surfaces involved. Last but not least, an effort has been made to characterize/monitor the RF plasma during the cleaning procedure in order to allow for an improved process control.