| 摘要 |
PROBLEM TO BE SOLVED: To enhance the exposure throughput of a device by using a calcium fluoride crystal having a prescribed purity value as an optical material, in a photolithographic device using an ArF excimer laser as its light source. SOLUTION: In this device, the calcium fluoride crystal used is required to contain a <=1×10<18> atoms/cm<3> of the total amount of alkaline earth metals as impurities. When this crystal is irradiated with an ArF laser, color centers due to impurities in the crystal are generated. Although an absorption band(s) due to the color centers appears in the wavelength region(s) other than 193nm, since the absorption band(s) is not sharp or steep, the color centers also affect the transmissivity at 193nm of the crystal. Since the concn. of color centers correlates with the concn. of impurities represented by alkaline earth metals and the energy density of the ArF laser used, selection of fluorite having appropriate impurity concn. according to the design and in particular use of fluorite contg. a restricted amount of Sr are important. Thus, the reduction in transmissivity of the fluorite crystal is decreased and the exposure throughput of the device is enhanced to a level ten times as high as the conventionally attainable throughput. |
