Long-term 193-nm laser irradiation of thin-film-coated CaF2 in the presence of H2O

V. Liberman; M. Switkes; M. Rothschild; S. T. Palmacci; J. H. C. Sedlacek; D. E. Hardy; A. Grenville

doi:10.1117/12.601468

12 May 2005 Long-term 193-nm laser irradiation of thin-film-coated CaF₂ in the presence of H₂O

V. Liberman, M. Switkes, M. Rothschild, S. T. Palmacci, J. H. C. Sedlacek, D. E. Hardy, A. Grenville

Proceedings Volume 5754, Optical Microlithography XVIII; (2005) https://doi.org/10.1117/12.601468
Event: Microlithography 2005, 2005, San Jose, California, United States

Abstract

The final projection lens element in a 193-nm immersion-based lithographic tool will be in direct contact with water during irradiation. Thus, any lifetime considerations for the lens must include durability data of lens materials and thin films in a water ambient. We have previously shown that uncoated CaF₂ is attacked by water in a matter of hours, as manifested by a substantial increase in AFM-measured surface roughness.¹ Thus, CaF₂ lenses must be protected, possibly by a thin film, and the coatings tested for laser durability in water. To address the above lifetime concerns, we have constructed a marathon laser-irradiation system for testing thin film exposure to water under long-term laser irradiation. Coated substrates are loaded into a custom water cell, made of stainless steel and Teflon parts. Ultrapure water is delivered from a water treatment testbed that includes particle filtration, deionization and degassing stages. In-situ metrology includes 193-nm laser ratiometry, UV spectrophotometry and spectroscopic ellipsometry, all with spatial profiling capabilities. In-situ results are coupled with off-line microscopy, AFM measurements and spatial surface mapping with spectroscopic ellipsometry at multiple incidence angles. A variety of laser-induced changes have been observed, from complete adhesion loss of protective coatings to more subtle changes, such as laser-induced index changes of the thin films or surface roughening. Implications of the study on expected lifetimes of the protective coatings in the system will be discussed.

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V. Liberman, M. Switkes, M. Rothschild, S. T. Palmacci, J. H. C. Sedlacek, D. E. Hardy, and A. Grenville "Long-term 193-nm laser irradiation of thin-film-coated CaF₂ in the presence of H₂O", Proc. SPIE 5754, Optical Microlithography XVIII, (12 May 2005); https://doi.org/10.1117/12.601468

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