The Extreme Ultraviolet reticle is a multilayer system ideally comprised of 40 alternating layers of molybdenum and silicon. The real reticle situation is not ideal, instead MoSi2 regions are found between each Mo and Si layer in the reflector. Reflectance differences arise as the Si, Mo, and SiMo2 proportions change. The material proportions are not necessarily constant throughout the reflector stack, which results in reflectance variations. Small variations in the mask reflectance resulting from material proportion differences can affect CD formation and uniformity on the wafer. The exact impact of a material variation on wafer CD is dependent on many factors. One factor is the depth in the mask stack at which the variation is found. Previous studies have demonstrated that multilayer variations at different multilayer locations produce imaging effects that are dependent on the CD and pitch. This study proposes to quantify the contribution of each multilayer set (1 through 40) to the intensity used to form the wafer pattern. This will be accomplished by computationally placing absorbers and transmitters into the stack, then measuring output intensities as well as final wafer CDs in order to provide a better understanding of material variation impacts in the mask multilayer.
Lawrence S. Melvin, Yudhi Kandel, Artak Isoyan, and Weimin Gao, "Individual multilayer reflectance and near field image formation in an EUV reticle," Proc. SPIE 10450, International Conference on Extreme Ultraviolet Lithography 2017, 104500F (Presented at SPIE Photomask Technology and EUV Lithography: September 13, 2017; Published: 16 October 2017); https://doi.org/10.1117/12.2282008.
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