CALORIMETRIC MEASUREMENTS OF OPTICAL-MATERIALS FOR 193 NM LITHOGRAPHY

We have developed a new calorimetric method to accurately determine th e absorption coefficient at 193 nm of optical materials such as fused silica and calcium fluoride. The amount of absorbed radiation is the p ower consumed by a thermoelectric cooler that maintains the sample at room temperature during exposure with a 193 nm laser. We report initia l results from five types of fused silica which indicate that absorpti on coefficients are still a factor of 3 higher than the target of 0.00 1 cm(-1) and that batch to batch variations remain common. The absorpt ion coefficient for one grade of calcium fluoride has also been obtain ed. On one fused silica sample, an upper bound has been placed on the two-photon absorption coefficient at a factor of 2 lower than those pr eviously reported. Measurements have been performed under a variety of vacuum conditions yielding anomalous losses in transmission. (C) 1996 American Vacuum Society.