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An ion-beam deposition system has been used to fabricate Mo-Si multilayer coatings for masks and imaging optics to be used for extreme-ultraviolet lithography. In addition to high reflectivity and excellent profile control, ion-beam deposition has the capability to smooth rough substrates. For example, we achieved reflectivity of 66.8% on a substrate with 0.39-nm roughness. Smoothing can be further enhanced with a second ion source directed at the multilayer coating. The smoothing capabilities relax the requirement on the finish of the mirror and the mask substrates and could dramatically reduce the cost of these components. Thickness profile control is in the +/-0.01% range, and the figure error added to the mirror substrate by errors in the multilayer thickness is less than 0.1 nm. Peak reflectivities obtained on smooth substrates are 67.5-68.6%.
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Spiller et al. (2003) studied this question.
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