RF plasma cleaning of mirror surfaces: characterization, optimization, and surface physics aspects of plasma cleaning

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

“…As a first significant difference, we note the strongly non-linear behaviour of the cleaning rates shown in Fig. 7(b) with respect to the RF power, which exhibit a saturation starting at about 250 W. Regarding the absolute cleaning values, it is obvious that the cleaning obtained from the H 2 /Ar plasma are about a factor of about 6 to 7 lower as compared to the O 2 /Ar cleaning rates as has been observed previously [1]. In the present case of the H 2 /Ar plasma, we thus also do observe a correlation between the applied RF power and the OES line intensities on one hand as well as the cleaning rates on the other hand, albeit the saturation in both is in sharp contrast to the corresponding linear behaviour in the case of the O 2 /Ar plasma.…”

“…As discussed in ref. [1], we rather attribute this saturation to the recombination of hydrogen HI radicals into H 2 molecules by their mutual interaction and/or via the interaction with vacuum recipient walls as has already been discussed by several authors [17,18]. This effect appears to be much more intense in the case of the hydrogen radicals as they have a higher reactivity than the oxygen OI radicals.…”

“…Oxygen/argon and hydrogen/argon gas mixtures were used as feedstock gases for the plasma during the cleaning experiments, with a total pressure of 5.0×10 -3 mbar. These parameters were derived from the optimized concentrations and pressures (i.e., for highest cleaning rates) as found in our previous studies [1]: 95%/5% and 7%/93% for O 2 /Ar and H 2 /Ar, respectively. Relative concentrations were measured and systematically adjusted using the optical emission spectrum (OES) from the plasma as a reference.…”

“…Previous publications in the field of low-pressure radiofrequency (RF) plasma cleaning of optical surfaces have shown that it is possible to efficiently and safely remove carbon contaminations at the molecular scale [1][2][3][4][5][6][7] by using O 2 /Ar or H 2 /Ar mixtures as plasma feedstock gases. In our earlier study [1], the main motivation was to report on a new, safe, and efficient method with well-defined sets of process parameters capable to perform an in-situ cleaning of carbon-contaminated optical elements that are typically enclosed within UHV chambers, such as synchrotron optics, extreme ultraviolet (EUV) optics, high-power laser optics, SEM/TEM setups, etc. In order to achieve this purpose, we did perform a comparative work using several different direct capacitive coupled (d-CCP) as well as commercial remote inductively coupled (r-ICP) model GV10x plasma sources operating at different RF powers.…”

Mechanism and model of atomic hydrogen cleaning for different types of carbon contamination on extreme ultraviolet multilayers

“…However, the basic chemical kinetics mechanism for atomic hydrogen cleaning technology is not clear for each of the different types of carbon contamination [11]. Therefore, further research of the cleaning mechanism is necessary to build the cleaning model.…”