Investigations regarding the prevention of depolarization of ArF excimer laser irradiation by CaF<sub>2</sub> laser optics

Ute Natura; Dietmar Keutel; Martin Letz; Lutz Parthier; Konrad Knapp

doi:10.1117/12.712108

26 March 2007 Investigations regarding the prevention of depolarization of ArF excimer laser irradiation by CaF₂ laser optics

Ute Natura, Dietmar Keutel, Martin Letz, Lutz Parthier, Konrad Knapp

Author Affiliations +

Proceedings Volume 6520, Optical Microlithography XX; 652030 (2007) https://doi.org/10.1117/12.712108
Event: SPIE Advanced Lithography, 2007, San Jose, California, United States

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.

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Ute Natura, Dietmar Keutel, Martin Letz, Lutz Parthier, and Konrad Knapp "Investigations regarding the prevention of depolarization of ArF excimer laser irradiation by CaF₂ laser optics", Proc. SPIE 6520, Optical Microlithography XX, 652030 (26 March 2007); https://doi.org/10.1117/12.712108

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