Tin is the preferred fuel in EUV sources due to its higher conversion efficiency (3%) compared to Xe (1%.)
However, there are several critical challenges to overcome before Sn can be used. Sn is a condensable fuel,
which deposits on nearby surfaces. The light is collected in this technology using reflective collector
mirrors, which are placed near to the plasma pinch area. Collection efficiency of these mirrors and their
ability to direct EUV light to the intermediate focus depends heavily on its reflectivity, which in turn
depends on the surface morphology and composition. Tin contamination reduces the reflectivity of the
mirror surfaces. High energy tin ions or neutrals, contaminate the surface, makes it rougher and also erode
it away. Due to these effects mirrors would need to be changed frequently, which increases the cost of
ownership. The Center for Plasma Material Interactions at the UIUC is expanding efforts to develop
cleaning methods for Sn off of EUV compatible surfaces. Reactive ion etching methods are developed as
an effective tool for this process. An in-house RIE chamber is used to investigate Sn etching by Ar/Cl2
plasma. Gas flow rates, chuck bias, sample temperatures and the chamber geometries are being analyzed to
optimize the etching. Results are very promising and encouraging towards an extended collector life time.
Etch rates are measured for Sn and its selectivity is studied over SiO2 and Ru, which shows that the method
adopted at UIUC for Sn etching is a potential solution to this problem. Additional experiments for cleaning
Sn off a mock collector mirror geometry, shows the potential to integrate this method in real technology.