Sub‐100 Nanometer Focusing of Hard X‐rays Using SU‐8 Compound Kinoform Focusing Lenses

A novel aberration‐free X‐ray compound refractive kinoform lens design based on the Cartesian oval curve is presented, designated as the OVAL‐OK (OVAL Overlap Kinoform) lens. Material infilling of the kinoform step structure maintains focal spot dimensions while reducing focal intensity and reproducibility of structures. A SU‐8 OVAL‐OK lens fabricated through X‐ray lithography achieved vertical focal sizes of 70.8 nm (knife‐edge scanning) and 56 nm (wavefront propagation analysis) under 15 keV X‐ray illumination, using a 120 μm × 200 μm (horizontal × vertical) aperture and 40.8 mm working distance. The lens exhibits a horizontal structural depth of 170 μm and a minimum feature size of 5 μm. The observed discrepancy between direct knife‐edge measurements and wavefront‐derived values is attributable to the combined effects of geometric, diffraction, coherence, instrumental instability, etc. These results demonstrate the potential for achieving sub‐50 nm 2D focusing in future iterations through enhanced structural depth and expanded aperture dimensions.