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Stability of optical interference coatings exposed to low-fluence 193 nm ArF radiation

: Heber, J.; Thielsch, R.; Blaschke, H.

Hove, L. van den ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Optical microlithography XI
Bellingham, Wash.: SPIE, 1998 (SPIE Proceedings Series 3334)
ISBN: 0-8194-2779-9
Optical Microlithography Conference <11, 1998, Santa Clara/Calif.>
Conference Paper
Fraunhofer IOF ()
dielectric coating; optical property; UV-radiation degradation

We report on our investigations on the long-term behaviour of optical coatings under 193 nm laser irradiation in dependence on coating materials, radiation conditions, and substrate properties. A wide variety of different highly reflective dielectric mirrors and antireflective coatings, deposited by an ultra low loss evaporation process onto calcium fluoride and fused silica, has been tested. Irradiation experiments with highly reflective coatings show that fluoride coatings exhibit nearly no changes of their optical function in air as well as in argon atmosphere due to low initial absorption levels. Temporal atmospheric contaminations can be removed by using appropriate irradiation conditions. Oxide layers tend to post-oxidize during 193 nm exposure in air and the DUV absorption level will be reduced. Effectively, reflectance of multilayer coatings on the basis of oxide materials can be improved through laser irradiation. Irradiation experiments with antireflective coatings point out the dominant role of bulk and surface properties of the substrate for prolonged laser irradiation. In addition, we present laser induced damage thresholds to demonstrate upper limits of laser radiation resistance that can be achieved nowadays with several types of coatings.