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A hydrogen plasma cleaning technique to clean Sn (tin) off of EUV collector optics is studied in detail. The cleaning process uses hydrogen radicals and ions (formed in the hydrogen plasma) to interact with Sn-coated surfaces, forming SnH4 and being pumped away. This technique has been used to clean a 300mm-diameter stainless steel dummy collector optic, and EUV reflectivity of multilayer mirror samples was restored after cleaning Sn from them, validating the potential of this technology.
This method has the potential to significantly reduce downtime and increase source availability. Thus, an investigation into the fundamental processes governing Sn removal has been performed. These experiments have shown that the Sn etch rates scale with hydrogen ion energy at the surface. Incident ions upon the surface impart energy that weakens the Sn-Sn bond allowing the chemical etch by hydrogen to proceed at a faster rate. Due to this the plasma is able to be in a reactive ion etch (RIE) regime. Results showing etch enhancement due to ions in this particular chemistry, including threshold energy, are shown.
A concern for plasma based methods is the implantation of high energy hydrogen ions into the MLM, reducing reflectivity and possibly blistering. With a surface wave plasma (SWP) this concern is alleviated somewhat because of lower ion energies. Surface wave plasmas have lower electron temperatures than conventional sources in the range of 1 to 3 eV. In addition, SWP sources result in plasma densities on the order of 1011-12 cm-3, allowing for greater utilization of ion etch enhancement. Experiments measuring electron density and hydrogen radical density over large areas have been conducted and the results from these measurements are presented. Pressure has also been varied to illustrate the effect between etching with radicals and RIE etching with ions included. Etch rate radial profiles over pressures ranging from 30 mTorr to 1.3 Torr have been measured with peak etch rates of 94.9 ± 4.6 nm/min at 250 mTorr.
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