Why calcium fluoride over fused silica?

Fused silica is the primary optical material used for 248- and 193-nm lithography. Calcium fluoride is used as a companion material to fused silica for color correction at 193 nm and is also used in areas of high flux because of its high laser damage resistance. Fused silica has more than 99%/cm of bulk transmission at 193 nm; at 157 nm, however, its transmission drops to less than 5%/cm.

On the other hand, calcium fluoride has a bulk transmission of greater than 98%/cm at 157 nm. Calcium fluoride is clearly better than fused silica at this wavelength, but still represents a significant relative reduction in transmission from materials at 193 nm, considering that a typical lithographic projection lens might have 60 surfaces and nearly a meter of optical path. More.

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