Ongoing investigation of collector cleaning by surface wave plasma in the Illinois NXE:3100 chamber

A hydrogen plasma cleaning technique to clean Sn (tin) off 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 as it occurs in-situ. While previous experiments have been conducted using an RF capacitively coupled plasma, a microwave-generated surface wave plasma (SWP) is advantageous due to its high density, low electron temperature, and ability to be generated locally where etching is needed. Langmuir probe measurements of the surface wave plasma show electron temperatures of 1 to 5 eV and plasma densities on the order of 10^11-12 cm-3. Previous experiments have shown etch rates of greater than 10 nm/min over a 2 inch diameter circular plasma area with an unoptimized SWP launcher. Peak etch rates of 94.9 nm/min were measured in the immediate vicinity of the plasma source. A custom launcher was designed for the 3100 collector and tested in the NXE:3100 chamber at Illinois at standard operating conditions. Previous results at different pressures have been presented, but the flow rate dependency was never investigated. Etch rates at commercially viable pressures and different flow rates were explored and these results will be presented.