Characterization of Carbon-Contaminated B4C-Coated Optics after Chemically Selective Cleaning with Low-Pressure RF Plasma

TL;DR

This study investigates low-pressure RF plasma cleaning methods for carbon-contaminated B4C-coated optics, finding pure O2 plasma effectively removes contamination while preserving coating integrity, crucial for FEL applications.

Contribution

It provides a comparative analysis of different RF plasma cleaning processes, highlighting the effectiveness of pure O2 plasma for maintaining B4C optical coating integrity.

Findings

Pure O2 plasma selectively removes carbon contamination.

Other plasma mixtures cause morphological or chemical damage.

Pure O2 plasma maintains B4C coating integrity.

Abstract

Boron carbide (B4C) is one of the few materials that is expected to be mostly 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 modern light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. These contaminations represent a serious issue for the operation of high performance 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 from these comparative plasma processes that pure O2 feedstock plasma only exhibits the required chemical selectivity for maintaining the integrity of the B4C optical coating.