Plasma Sn cleaning integrated in EUV source system

Extreme ultraviolet lithography (EUVL) is a potential candidate for the next generation lithography techniques, which will use Xe or Sn as a main fuel to produce EUV light. However, the industry has favored to use Sn as main fuel in EUVL systems because of its high conversion efficiency over Xe. Sn has an advantage of producing more light, but on the other hand its condensable nature is a real threat to the reflective mirrors which are used to collect the EUV light at intermediate focus. Center for Plasma Material Interactions (CPMI) at the University of Illinois has studied plasma etching as a potential method of Sn removal from collector optics. RF-driven chlorine plasma is used to etch Sn from mirror samples. Previously we reported high selectivity of Sn over several EUV compatible mirror materials. The increased confidence in this technology had led us to perform cleaning experiments on real Sn contaminated samples exposed in an EUV source and the results obtained have been very encouraging. Small mock up shells (same as in the grazing incidence collector optics system) were constructed at CPMI and chlorine etching was performed at different samples placed at different locations on multi-shell collector mock up in ICP-RIE chamber. Post cleaning material characterization results of samples shows that chlorine can potentially clean Sn off of collector optics (Ru was used in this study as a mirror sample). Realizing this as a viable cleaning solution, we have stepped further and performed a full size cleaning test in the Xtreme's XTS 13-35 EUV source. Large mock up with appropriate dimension was placed in the EUV source chamber and the cleaning system was installed to etch Sn away from Ru surface. This study compares the cleaning results in a real system scale with the previous simulated system. The comparison shows how to improve the Sn cleaning system in the EUV source chamber. Results are encouraging and may enable source suppliers to integrate this technology in their respective sources. Cleaning rate was measured as >100nm/min using ion sputtered Sn samples.