Study of Sn Removal by Surface Wave Plasma for Source Cleaning

A hydrogen plasma cleaning technique to clean Sn off of EUV sources is studied in detail. The cleaning process uses hydrogen radicals ( 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. 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 radical density and etch rate profiles have been conducted and the results from these measurements are presented. These will help demonstrate scalability of SWP cleaning techniques for use in EUV sources.