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26 June 2003 Accelerated damage to blank and antireflectance-coated CaF2 surfaces under 157-nm laser irradiation
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Successful insertion of 157-nm lithography into production requires that materials comprising the optical train meet the lifetime requirements of the industry. At present, no degradation of bulk fluoride materials has been observed for at least up to 109 pulses. However, last year we reported on the surface damage to fluoride materials that appeared after 3-4x109 pulses at moderate fluences of 3-4 mJ/cm2/pulse2. This damage manifested itself as a precipitous transmission drop of up to 50% at 157 nm and was accompanied by the formation of a porous rough surface layer about 0.20 μm thick. Understanding this surface damage is important for the durability of laser windows and beam delivery optics, and it may also help elucidate fundamental 157-nm photophysics of fluoride surfaces. To understand the underlying phenomena, we have designed and constructed a new accelerated damage test chamber. The chamber utilizes 157-nm light from a lithography-grade laser operating at 1000 Hz. Inside the chamber, light is focused onto the sample to a submillimeter spot size. The chamber allows us to test in-situ transmission of multiple spots on a given sample over a range of fluences up to 140 mJ/cm2/pulse without breaking purge. We have used this chamber to understand the scaling of the damage mechanism for both uncoated and antireflectance (AR) -coated CaF2 samples as a function of laser repetition rate and fluence. Substrate damage appears to be governed by a complex set of mechanisms, both thermal and non-thermal in origin. Preliminary damage studies of AR-coated substrates show that AR-coating related degradation occurs well before the onset of the substrate surface damage.
© (2003) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Vladimir Liberman, Mordechai Rothschild, Stephen T. Palmacci, Nikolay N. Efremow, Jan H. C. Sedlacek, and Andrew Grenville "Accelerated damage to blank and antireflectance-coated CaF2 surfaces under 157-nm laser irradiation", Proc. SPIE 5040, Optical Microlithography XVI, (26 June 2003);

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