  The choice of a reflective optical coating or filter material has to be
adapted to the intended field of application. This is mainly determined by the
required photon energy range or by the required reflection angle. Among various
materials, nickel and rhodium are standard materials used as reflective
coatings for synchrotron mirrors. Conversely, Aluminum is one of the most
commonly used materials for extreme ultraviolet (EUV) and soft X-ray filters.
However, both of these types of optics are subject to carbon contamination,
being increasingly problematic for the operation of the high-performance free
electron laser (FEL) and synchrotron beamlines. For this reason, an inductively
coupled plasma (ICP) source has been used in conjunction with N2/O2/H2 and
N2/H2 feedstock gas plasmas. Results from the chemical surface analysis of the
above materials before and after plasma treatment using X-ray photoelectron
spectroscopy (XPS) are reported. We conclude that a favorable combination of an
N2/H2 plasma feedstock gas mixture leads to the best chemical surface
preservation of Ni, Rh, and Al while removing the carbon contaminations.
However, this feedstock gas mixture does not remove C contaminations as rapidly
as, e.g., a N2/O2/H2 plasma which induces the surface formation of NiO and
NiOOH in Ni and RhOOH in Rh foils. As an applied case, we demonstrate the
successful carbon removal from ultrathin Al filters previously used at the
FERMI FEL1 using a N2/H2 plasma.
