The key requirements of CaF2 lens blanks are transmission, birefringence and optical homogeneity. While it is obvious that high transmission and resistance to fluence- and time-dependent darkening is critical, the subtleties of single crystals, compared to isotropic glasses, play an important role in the implementation of CaF2 in microlithography steppers. The existence of intrinsic birefringence has caused stepper manufacturers to employ various crystallographic orientations in their lens designs. This fact means that crystal growth and annealing processes must be optimized accordingly. Small-scale defects, known as dislocations and sub-grain mis-orientation, manifest themselves as larger-scale defects known as slip and mosaic. These imperfections may impact birefringence and a critical homogeneity parameter known as residual rms. Residual rms is directly related to the concentration of asymmetric defects in CaF2 crystals, such as those aforementioned. In the present paper, the author will report on progress made at Corning to meet the stringent requirements of both 193nm and 157nm systems.
Borosilicate, potassium and sodium silicate and potassium borate glasses containing small quantities of AsO3 and 5½03 darken from the surface when exposed to H2 in the glass transformation range. The depth of surface darkening is a function of time, temperature, and H2 pressure. Change in absorbance at constant wavelength increases with time but shows neither linear nor square root dependence on time. In addition, the absorbance change (X constant) does not show Arrhenius behavior as a function of treatment temperature, but instead appears to approach saturation. The same phenomena are observed for D2 treatments. Absorbance change at 300 nm from the H2 treated sample under identical temperature and pressure conditions differs by a factor of the square root of 2. Relative permeability of H2 and D2 is given by the square root of their mass ratio, i.e. 12. This finding suggests a direct proportionality between the inducedabsorption and the permeability of the diffusing species. The coloration is attributed to the reduction of the fining agent (As/Sb) and is accompanied by an increase in the hydroxyl concentration in the glass. The effect of base glass permeability, fining agent identity and fming agent concentration on the optical absorption will be discussed. Metallic colloidal scattering is proposed as the mechanism for the coloration. Evidence to support the mechanism is presented and discussed.
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