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Proceedings Paper

Krypton as filter for ions, debris, and useless radiation in EUV projection lithography systems
Author(s): Francesco Flora; Luca Mezi; Sarah Bollanti; Francesca Bonfigli; Paolo Di Lazzaro; Tommaso Letardi; Cheng En Zheng
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Paper Abstract

The life time of the first multilayer mirror (the condenser) in optical systems for projection Extreme Ultra-Violet (EUV) lithography based on Laser Produced Plasmas (LPP) is limited both by the debris bombardment and by the EUV radiation photoablation. These problems can be dramatically reduced by using krypton as debris stopper, exploiting its high atomic weight (21 times the helium value) and its high transparency in the EUV region only between 60 and 90 eV. We demonstrate both theoretically and experimentally that by filling the laser-target interaction chamber with krypton at low pressure (a few mbar) it is possible to stop efficiently the small debris (having a size smaller than 1 micron, including ions, neutrals and clusters) while keeping a reasonable transmission of the EUV radiation in the 60-90 eV range. In particular, this is experimentally demonstrated for a solid target LPP based on a tantalum or copper tape target and pumped by a large energy (4 J - 120 ns) XeCl excimer laser. The poor transmission of krypton, out of the above mentioned spectral window, significantly reduces the exposition of the condenser mirror to useless radiation. This new technique for stopping the debris has recently been patented by our group. As far as the larger size debris is regarded, a dramatic reduction of their flux has been obtained by decreasing the laser intensity on the target, still keeping a laser to EUV energy conversion as high as 0.7 percent/eV/sr at 70 eV, from a Ta target, together with a shot to shot EUV pulse energy stability better than 1 percent.

Paper Details

Date Published: 14 November 2001
PDF: 10 pages
Proc. SPIE 4504, Applications of X Rays Generated from Lasers and Other Bright Sources II, (14 November 2001); doi: 10.1117/12.448452
Show Author Affiliations
Francesco Flora, ENEA Frascati (Italy)
Luca Mezi, ENEA Frascati (Italy)
Sarah Bollanti, ENEA Frascati (Italy)
Francesca Bonfigli, ENEA Frascati (Italy)
Paolo Di Lazzaro, ENEA Frascati (Italy)
Tommaso Letardi, ENEA Frascati (Italy)
Cheng En Zheng, EL.EN SpA (Italy)

Published in SPIE Proceedings Vol. 4504:
Applications of X Rays Generated from Lasers and Other Bright Sources II
George A. Kyrala; Jean-Claude J. Gauthier, Editor(s)

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