Fluorine-doped SiO_2 glasses for F_2 excimer laser optics:?fluorine content and color-center formation

Color-center formation in F-doped, OH-free synthetic ${\mathrm{SiO}}_2$ glasses by irradiation with ${\mathrm{F}}_2$ excimer lasers (157 nm) was examined as a function of the F content. The concentration of photoinduced ${\mathrm{E}}^{\prime }$ centers was reduced to $\sim 1/20$ by $1 \mathrm{mol}.\% {\mathrm{F}}_2$ doping and remained almost constant on further doping to 7.3 mol. %. The absorption edge was considerably shifted to a lower wavelength ($157.4 \mathrm{nm}\to 153 \mathrm{nm}$ for a 5-mm-thick sample) by 1-mol. % doping and decreased only slightly on further doping. The intensities of the Raman bands that are due to three- and four-membered ring structures were significantly reduced by 1-mol. % F doping. These results strongly suggest that elimination of strained Si—O—Si bonds by F doping plays a central role in the improvement of radiation resistance of ${\mathrm{SiO}}_2$ glasses to ${\mathrm{F}}_2$ laser light.