EUV Bragg reflectors with photonic superlattices
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The basic properties in the extreme ultraviolet (EUV) of one-dimensional photonic crystals (Bragg reflectors) with incorporated superlattices are investigated by a numerical study using the multiple scattering method. The superlattice is realized in the “standard” Mo/Si system by periodically replacing certain Mo layers by Si layers. At 13.5 nm–the wavelength of interest for EUV lithography–the superlattice sharpens the reflection peak at normal incidence with only weak reduction of the peak value. Between normal incidence and total reflection at large angles, additional reflection peaks appear at certain angles where the reflection is zero for the “standard” Mo/Si system. By combining different superlattices and depth grading, the range of additional reflection peaks is extended towards all-angle reflection. The effect of interface imperfections is considered for the case of the interdiffusion of Mo and Si. The extension to other frequency ranges is addressed via band structure calculations.