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Incorporation of fluorine into dry silica is known to extend the transmittance of the glass to shorter wavelengths relative to fluorine-free silica. Fluorine-doped silica has been shown to have greater than 80%/cm internal transmittance at 157-nm. Its high transmittance, low thermal expansion, ease of polishing and relative availability make it a perfect candidate for use at 157-nm for photomasks and potentially for other short wavelength optical elements. In the course of investigating composition-property relationships of F-doped, dry silica glasses, the effect of fluorine concentration on color center formation as a function of excimer laser exposure was considered. A series of glasses with fluorine concentrations from 0 to 1.8 wt% was studied. Using the 157-nm exposure wavelength, all glasses developed the silicon E' center, characterized by its absorption at 215-nm. Also noted was the non-bridging oxygen hole center (NBOHC) which appears in the UV at 260-nm. A band at 165-nm, identified as the oxygen deficient center (ODC) was also observed. This band was most pronounced in higher F concentration glasses, with the lowest F and non-F containing glasses showing no 165-nm band evolved. The assignment of the ODC is based on the fluorescence of the center and its reaction with oxygen, which causes diminishment of the band. The evolution of the color center formation (absorption) can limit the lifetime of optics by driving down transmission with use. For the 157-nm wavelength the absorption of importance is the one that appears at the 165-nm, the ODC. The exact atomic structure of the ODC is not important for this discussion; in fact, the defect is described by two different structures, the metal- metal bond and the divalent silicon: (O)3Si-Si(O)3 and O-Si-O(2+) The ODC has significant absorption at 157- nm and so minimizing its formation during exposure would be advantageous for optics life.
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