Towards chemically neutral carbon cleaning processes: Plasma cleaning of Ni, Rh, and Al reflective optical coatings and thin Al filters for Free Electron Lasers and synchrotron beamline applications

TL;DR

This study investigates plasma cleaning methods to remove carbon contamination from reflective optical coatings and filters used in FEL and synchrotron applications, aiming for chemically neutral cleaning processes.

Contribution

It introduces a plasma cleaning approach using N2/H2 and N2/O2/H2 gases, optimizing for surface preservation and effective carbon removal on Ni, Rh, and Al optics.

Findings

N2/H2 plasma best preserves surface chemistry while removing carbon.

N2/O2/H2 plasma rapidly removes carbon but forms surface oxides.

Successful carbon cleaning demonstrated on ultrathin Al filters at FEL.

Abstract

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.