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Proceedings Paper

Investigations regarding the prevention of depolarization of ArF excimer laser irradiation by CaF2 laser optics
Author(s): Ute Natura; Dietmar Keutel; Martin Letz; Lutz Parthier; Konrad Knapp
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Paper Abstract

Crystalline calcium fluoride is one of the key materials for 193nm lithography and is used for laser optics, beam delivery system optics and stepper/scanner illumination optics. In comparison to fused silica it shows a much higher laser durability. However, even in pure calcium fluoride the irradiation by ArF excimer laser (193nm) can cause transmission loss and depolarization. Short time and long time tests of radiation induced changes of optical properties of CaF2 were carried out. Within short time tests initial and radiation induced absorption as well as the measurement of laser induced fluorescence and the measurement of laser induced depolarization are adequate methods for characterization of the material under ArF laser irradiation. Previous investigations were done by Burnett to prevent depolarization caused by spatial dispersion. Nevertheless an important challenge is the prevention of depolarization of the polarized laser beam by CaF2 laser optics caused by a temperature gradient. The dependence of depolarization on the direction of temperature gradient in comparison to the direction of the laser beam and the orientation of the CaF2 crystal was investigated. In the present work different paths to prevent or mitigate the depolarization by CaF2 due to a temperature gradient are discussed resulting in a special chance to mitigate depolarization by a laser window.

Paper Details

Date Published: 26 March 2007
PDF: 9 pages
Proc. SPIE 6520, Optical Microlithography XX, 652030 (26 March 2007); doi: 10.1117/12.712108
Show Author Affiliations
Ute Natura, SCHOTT AG (Germany)
Dietmar Keutel, SCHOTT AG (Germany)
Martin Letz, SCHOTT AG (Germany)
Lutz Parthier, SCHOTT AG (Germany)
Konrad Knapp, SCHOTT AG (Germany)

Published in SPIE Proceedings Vol. 6520:
Optical Microlithography XX
Donis G. Flagello, Editor(s)

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