Photolithography using 157 nm pulsed fluorine lasers has emerged as the leading candidate technology for the post-193-nm generation. Preliminary data have indicated that at 157 nm there are optical materials transparent enough to enable the fabrication of refractive elements, both in the projection and illumination part of the optical train. However, a number of critical issues still remain. Optical materials must show no appreciable degradation with laser irradiation. The availability of transparent photomask substrates must be ascertained. Optical coatings must be developed and qualified. At this short wavelength, interface effects, subsurface damage, and adsorbate effects become increasingly prominent. We present recent experimental results on the durability tests of calcium fluoride, modified fused silica, and optical coatings for 157 nm applications. Our initial assessment of several grades of modified fused silica demonstrates that at least one grade already meets transparency and durability requirements for reticle substrates for 157 nm applications. For both bulk calcium fluoride and antireflectance coatings our tests to date show no degradation for 300 million pulses at fluences up to 3 mJ/cm 2 /pulse. We do observe degradation of beam steering mirrors in our experimental setup. Detailed damage analysis of these coatings is presented.

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Thermophoresis is considered as a candidate for protection of extreme ultraviolet lithography masks from particle contamination during vacuum exposures. A thermophoretic force is exerted on a particle by surrounding gas molecules within a temperature gradient. Gas molecules on the "warm side" of the particle provide more momentum than on the "cool side," so particles move from the warm to the cool region. In this study, thermophoretic protection of a critical surface from particles injected with known initial speeds into a quiescent gas has been investigated at 100, 50, and 25 mTorr. Initial particle speed was varied from 10 to 31 m / s depending on the gap distances ͑1, 2, and 3 cm͒, particle sizes ͑125 and 220 nm͒, and system pressures. A pinhole plate is used to supply speed-controlled particles with almost no accompanying gas flow. The results demonstrate that the window of protection offered by thermophoresis is very narrow for inertial particles, and that thermophoresis offers the greatest protection for low-velocity particles subject to diffusional motion in the vacuum system. Furthermore, the experimental results verify the results of an analytical model, developed by Asbach et al. ͓Appl. Phys. Lett. 87, 234111 ͑2005͔͒. The analytical model can be used to predict the particle stopping distance under any thermophoretic gradient.

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