Shigeru Sakuma

Influence of sodium impurities on ArF excimer-laser-induced absorption in CaF 2 crystals

The formation of color centers induced by irradiation with ArF excimer lasers in CaF(2) crystals was found to depend strongly on the sodium impurity concentration. Sodium-related color centers were generated by two-photon absorption because the slope of the induced absorption coefficient just after irradiation started was proportional to the square of the laser fluence. The saturation absorption also depended on laser fluence, and a photobleaching induced absorption phenomenon was observed. We concluded that the saturation absorption level was determined by the equilibrium between two-photon excitation and one-photon reverse reaction.

Nikon F2 exposure tool development

Present status of development of F2 (157nm) exposure tool in Nikon is described. Key points of F2 exposure tool are reported; low aberration projection optics, CaF2 quality, coating durability and gas purging of the pellicle space. We also report the measurement of refractive index inhomogeneity inside CaF2 crystals, which is suspected as the cause of local flare. Characteristics of high NA optics over 0.9 are investigated by imaging simulations for both 193nm and 157nm wavelengths, which are compared NA=0.85 imaging.