5 July 2000 Dynamic change of transmission of CaF2 single crystals by irradiating with ArF excimer laser light
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Abstract
The laser induced absorption of CaF2 caused by ArF excimer laser light has been observed at energy densities of F equals 2-30 mJ/cm2 per pulse and a repetition rate of R equals 50 Hz. The experiments show that the transmission of CaF2 samples depends on the pulse energy density. The change of the absorption coefficient with the time of irradiation can be described by an exponential model. Different experiments were performed where the energy density was increased and decreased stepwise. They prove that color centers not only are formed but also are annihilated by irradiation. Laser induced decrease of absorption was observed in all samples as soon as the energy density was decreased. Coloring and bleaching of the samples are completely reversible processes. The level of transmission depends on the energy density of the laser light and the quality of the material but not on the history of irradiation. The damage resistance of the material can be adjusted by the appropriate choice of the raw material and the process parameters. The reversibility of the laser induced absorption can be explained by a reaction equilibrium. This leads to a model where the concentration of absorbing defects depends on the current irradiation conditions. Using these equations the reversibility and the observed exponential dependence of the change of transmission with time can be explained. Assuming different dependencies of the reaction constants of coloring and bleaching on the energy density, the change of the absorption coefficient with pulse energy density can be calculated.
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Jochen Alkemper, Joerg Kandler, Lorenz Strenge, Ewald Moersen, Christian Muehlig, Wolfgang Triebel, "Dynamic change of transmission of CaF2 single crystals by irradiating with ArF excimer laser light", Proc. SPIE 4000, Optical Microlithography XIII, (5 July 2000); doi: 10.1117/12.389004; https://doi.org/10.1117/12.389004
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