Remote plasma cleaning of optical surfaces: Cleaning rates of different carbon allotropes as a function of RF powers and distances

Abstract

An extended study on an advanced method for the cleaning of carbon contaminations on large optical surfaces using a remote inductively coupled low-pressure RF plasma source (GV10x downstream asher) is reported in this work. Technical as well as scientific features of this scaled up cleaning process are analysed, such as the cleaning efficiency for different carbon allotropes (amorphous and diamond-like carbon) as a function of feedstock gas composition, RF power (ranging from 30 to 300W), and source-object distances (415 to 840 mm). The underlying physical phenomena for these functional dependences are discussed.

Low-pressure RF remote plasma cleaning of carbon-contaminated B4C-coated optics

“…Typical cleaning times required for an O 2 /Ar or pure O 2 plasma were in the range of 2.5 hours, whereas the corresponding cleaning times for an Ar/H 2 plasma were in the range of 12.5 hours, starting from a-C coatings with the same thickness. This significant difference in terms of carbon cleaning rate between oxygen-and hydrogen-based downstream plasma by a factor of up to roughly seven could already be observed in previous studies 8,9 . Fig.…”

“…In this paper, we describe an experimental approach based on the low-pressure RF downstream plasma cleaning of various B 4 C-coated test objects, which includes a variation of the plasma chemistry by a variation of the plasma feedstock gases. Although the employed experimental procedure is similar to the one described in previous studies 7,8 , the present results turn out to be completely different from those based on the cleaning of noble and non-noble metallic reflective optical coatings. In more detail, the results obtained give evidence for a satisfactory cleaning performance only for pure O 2 plasma, while an O 2 /Ar feedstock gas mixture results into an efficient carbon cleaning as well, but combined with significant degradation of the B 4 C layer.…”

“…As in the previous studies 8,9 a standard quartz crystal microbalance (QCM) including gold-coated and a-C contaminated quartz crystals was used to determine the carbon removal rate during the plasma process for optimizing the plasma operation parameters towards the required optimum carbon cleaning rates.…”

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

“…In Table 6 , we give a brief summary of the experimentally derived a-C cleaning rates that have been determined in the present study on a-C contaminated B 4 C-coated test objects presented here (extrapolated values) and on Au-coated QCM crystals that have been achieved in a previous study 7 , 8 .…”

“… 1.7 Å/min. All values were measured using 100 W plasma RF power at a plasma vacuum pressure of 0.005 mbar (*data taken from refs 7 , 8 ). …”