Oliver Sohr

Mechanisms of radiation induced defect generation in fused silica

Excimer laser radiation changes the optical properties of fused silica. These changes include radiation induced absorption and changes of the index of refraction, which in turn determine the expected lifetime of silica lenses used in optical microlithography. A fully automated experimental setup designed for the marathon exposure of samples at low energy densities was employed. Measurements of the induced absorption, of the H2 content using Raman spectroscopy as well as wavefront measurements were performed. A model to predict the aging behavior of silica in optical microlithography systems due to defect generation has been developed for both ArF laser irradiation and KrF laser irradiation. The model includes linear and nonlinear defect generation, relaxation processes and the consumption of hydrogen and describes the radiation induced changes of the index of refraction, the increase as well as the decrease. The model calculations were derived by analytical and numerical methods. A very good agreement in the range of parameters used in the experiments is observed.

Excimer-laser-induced defect generation in Lithosil

Fused silica is used as lens material in DUV microlithography systems. The exposure of fused silica to high-energy excimer laser pulses over long periods of time modifies the material. Marathon experiments were conducted at different energy densities with the KrF- and ArF excimer laser to describe the material parameters under long time irradiation. A model was developed to describe the radiation induced absorption and the change of the index of refraction. The defect generation is associated with the consumption of hydrogen. The dependence of hydrogen consumption on the wavelength of irradiation, the energy density and the initial hydrogen content was investigated in detail. The saturation of H2 consumption in Lithosil was proved by different experiments. The results are in very good agreement with the model calculations.