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.

Date Published: 5 April 2007
PDF: 9 pages
Proc. SPIE 6518, Metrology, Inspection, and Process Control for Microlithography XXI, 65184N (5 April 2007); doi: 10.1117/12.712372

Published in SPIE Proceedings Vol. 6518:
Metrology, Inspection, and Process Control for Microlithography XXI
Chas N. Archie, Editor(s)