Temperature-dependent Urbach tail measurements of lutetium aluminum garnet single crystals

Lutetium aluminum garnet (LuAG) is the most promising candidate for a high-index lens material for use in microlithographic imaging lenses. In the deep ultraviolet spectral range the transmission of high-purity LuAG was measured using monochromatized synchrotron radiation. In the vicinity of the band gap below 7.8 eV, a scaling behavior of the absorption as a function of photon energy was observed. Temperature-dependent measurements allowed us to distinguish different absorption mechanisms which differ by their ability to couple to phonon excitations. Interpreting the Urbach tails measured at different temperatures, it was shown that the temperature independent tail is due to defects in the lattice, whereas the temperature-dependent part originates from the short term localization of exciton modes coupling to lattice distortions. These results allowed us to extrapolate the maximum transmittance which can be obtained with LuAG crystals at the lithographic wavelength of 193.39 nm. Accurate determination of the maximum transmission limit is critical in deciding whether the material can meet industry's specification for 193-nm-based high-index lithography.