A comprehensive model of radiation-induced carbon contamination of extreme ultraviolet (EUV) optics is presented. The mathematical model describes the key processes that contribute to the deposition of a carbon film on a multilayer optic when the optic is exposed to EUV radiation in the presence of residual hydrocarbons. These processes include the transport of residual hydrocarbons to the irradiated area, molecular diffusion across the optic surface, and the subsequent dissociation or “cracking” of the hydrocarbon by both direct EUV ionization and secondary electron excitation. Model predictions of carbon growth are compared to measurements taken on optics exposed to EUV in the presence of residual hydrocarbons. Model estimates of hydrocarbon film growth under various conditions of hydrocarbon partial pressures and EUV power demonstrate the sensitivity of film growth to varying operating conditions. Both the model and experimental data indicate that the predominant cause of hydrocarbon dissociation is bond breaking by direct photon absorption, rather than by dissociation processes caused by exposure to secondary electrons. Detailed predictions for carbon deposition for a variety of conditions of EUV power and hydrocarbon pressure are reported. The model successfully predicts that light hydrocarbons (<∼100amu) pose a negligible risk to EUV optics, in general agreement with the experiment. Calculations also predict that modest increases in substrate temperature, on the order of 30°C, will substantially reduce optic contamination by increasing hydrocarbon desorption from the surface. Model investigation of surface diffusion indicates that, while surface diffusion is an important surface phenomenon for light gases, for the heavier hydrocarbons that contribute substantially to contamination, surface diffusion is not an important transport phenomenon.

Articles you may be interested inScreening of ferroelectric domains on BaTiO3(001) surface by ultraviolet photo-induced charge and dissociative water adsorption A mathematical model of extreme ultraviolet ͑EUV͒-induced oxidation of a ruthenium ͑Ru͒-coated EUV optic is presented. The model describes the key processes that contribute to the growth of the oxide within the optic when exposed to EUV radiation in the presence of water vapor. These processes include the adsorption and thermal desorption of water to and from the Ru surface, molecular diffusion of water across the optic surface, and the dissociation of the water by both direct EUV ionization and secondary electron excitation. Oxygen produced by dissociation may associatively desorb from the surface or may diffuse into the Ru subsurface where it can react to form an oxide. The presence of oxygen in the Ru coating, whether as oxide or atomic oxygen, reduces the reflectivity of the optic and the overall throughput of the EUV lithographic system. The model predicts oxide thickness over time, which may later be used to estimate the reflectively loss attributable to the oxide in any given EUV environment. Model predictions for EUV-induced oxide growth provide a good description of the oxide growth observed in available electron-beam experiments. The model is also used to estimate oxygen penetration into the Ru coating under various conditions of water partial pressure, EUV power, and temperature. The model predicts reduced oxidation with higher temperatures and for substrates that bind water less tightly than ruthenium.

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