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

• Published in: Journal of synchrotron radiation Vol. 25; no. 6; pp. 1642 - 1649
• Main Authors: Moreno Fernández, Harol, Zangrando, Marco, Sauthier, Guillaume, Goñi, Alejandro R., Carlino, Vincent, Pellegrin, Eric
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01.11.2018; John Wiley & Sons, Inc
• Subjects: Aluminum; Angle of reflection; Carbon; carbon contamination; Cleaning; Coatings; Contamination; FEL and synchrotron radiation beamline optics; Foils; free‐electron lasers (FELs); Gas plasmas; Inductively coupled plasma; Laser beams; Materials selection; Nickel; Nitrogen plasma; Optical coatings; Organic chemistry; Plasma; plasma cleaning chemistry; Raw materials; Rhodium; Surface analysis (chemical); X ray spectra; carbon contamination; free-electron lasers (FELs); plasma cleaning chemistry; FEL and synchrotron radiation beamline optics; inductively coupled plasma
• ISSN: 1600-5775; 0909-0495; 1600-5775
• DOI: 10.1107/S1600577518014017

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 common materials used as reflective coatings for (soft) X‐ray mirrors. Similarly, aluminium is one of the most commonly used materials for extreme ultraviolet and soft X‐ray transmission filters. However, both of these types of optics are subject to carbon contamination, which can be increasingly problematic for the operation of the high‐performance free‐electron laser and synchrotron beamlines. As an attempt to remove this type of contamination, an inductively coupled plasma 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 are reported. It is concluded 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 contamination. However, this feedstock gas mixture does not remove C contamination as rapidly as, for example, an N2/O2/H2 plasma which induces the surface formation of NiO and NiOOH in Ni and RhOOH in Rh foils. As an applied case, the successful carbon removal from ultrathin Al filters previously used at the FERMI FEL1 using an N2/H2 plasma is demonstrated. A chemically neutral low‐pressure RF plasma cleaning technique for Ni, Rh and Al reflective coatings is presented. The successful carbon contamination cleaning of thin Al filters (100 nm thickness) for FEL and EUV applications is also demonstrated.