29 August 2006 EUV multilayer mirrors with enhanced stability
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Abstract
The application of multilayer optics in EUV lithography requires not only the highest possible normal-incidence reflectivity but also a long-term thermal and radiation stability at operating temperatures. This requirement is most important in the case of the collector mirror of the illumination system close to the EUV source where a short-time decrease in reflectivity is most likely. Mo/Si multilayer mirrors, designed for high normal reflectivity at the wavelength of 13.5 nm and deposited by dc magnetron sputtering, were directly exposed to EUV radiation without mitigation system. They presented a loss of reflectivity of more than 18% after only 8 hours of irradiation by a Xe-discharge source. Another problem of Mo/Si multilayers is the instability of reflectivity and peak wavelength under high heat load. It becomes especially critical at temperatures above 200°C, where interdiffusion between the molybdenum and the silicon layers is observed. The development of high-temperature multilayers was focused on two alternative Si-based systems: MoSi2/Si and interface engineered Mo/C/Si/C multilayer mirrors. The multilayer designs as well as the deposition parameters of all systems were optimized in terms of high peak reflectivity (≥ 60 %) at a wavelength of 13.5 nm and high thermal stability. Small thermally induced changes of the MoSi2/Si multilayer properties were found but they were independent of the annealing time at all temperatures examined. A wavelength shift of -1.7% and a reflectivity drop of 1.0% have been found after annealing at 500°C for 100 hours. The total degradation of optical properties above 650°C can be explained by a recrystallization process of MoSi2 layers.
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Nicolas Benoit, Sergiy Yulin, Torsten Feigl, Norbert Kaiser, "EUV multilayer mirrors with enhanced stability", Proc. SPIE 6317, Advances in X-Ray/EUV Optics, Components, and Applications, 63170K (29 August 2006); doi: 10.1117/12.680719; https://doi.org/10.1117/12.680719
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