Calorimetric measurements of optical materials for 193 nm lithography

We have developed a new calorimetric method to accurately determine the absorption coefficient at 193 nm of optical materials such as fused silica and calcium fluoride. The amount of absorbed radiation is the power consumed by a thermoelectric cooler that maintains the sample at room temperature during exposure with a 193 nm laser. We report initial results from five types of fused silica which indicate that absorption coefficients are still a factor of 3 higher than the target of 0.001 cm(-1) and that batch to batch variations remain common. The absorption coefficient for one grade of calcium fluoride has also been obtained. 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 previously reported. Measurements have been performed under a variety of vacuum conditions yielding anomalous losses in transmission. (C) 1996 American Vacuum Society.