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X-ray microscopy is advantageous over conventional optical microscopy because of its high resolution and capability to study the inner structure of materials opaque to visible light. Furthermore, this method does not require metallization and vacuum and therefore it can be used to visualize fragile biological samples that cannot be studied by scanning electron microscopy. Focusing X-ray optics may be roughly divided into three groups based on the physical principle of focusing: reflection, diffraction and refraction. The reflection optics includes curved mirrors, multilayers and capillaries; the diffractive optics includes Fresnel zone plates. Refractive optics comprises X-ray compound refractive lenses (CRLs) that are widely used nowadays because of their compactness and ease of fabrication. Focusing performance of the CRL is determined by the refractive index, absorption, the inner structure of the CRL material and the geometry of the lens. The optimal shape for the lens is parabolic with a small radius of curvature, because the smaller radius of the parabola leads to shorter focal distance and therefore allows to achieve higher resolution. The common choice of the CRL material is beryllium. However the resolution of Be lenses is far below theoretically predicted limits because of the parasitic scattering introduced by the grains in the material. Moreover the existing manufacturing technologies do not allow to achieve radius of curvature less than 50 μm. Polymer materials are also popular for the CRL microfabrication because of their amorphous nature, ease of structuring and low price. Among the advanced lithographic techniques the two-photon polymerization lithography (2PP) holds a special place. It is based on polymer solidification by means of two-photon absorption. Nonlinear character of two-photon absorption leads to the transparency of the out-of focus material, while presence of polymerization threshold reduces resolution far below diffraction limit. Therefore 2PP can be used for fabrication 3D structures of almost arbitrary shape including overhanging and self-intersecting structures.
In this work we introduce the 3D X-ray CRL fabricated by 2PP from the commercially available photoresist ORMOCOMP. Hundred double concave individual lenses formed a CRL with the 60 μm distance between adjacent lenses. Radius of curvature of a single parabolic surface was 3 μm that is comparable to radius of 2D silicon nano-lens made by conventional lithography and much less than achievable radius of 3D Be lens. Physical aperture was 28 μm. The optimal processing parameters (power, incident on the sample, and velocity of the laser beam waist movement) were determined. The fabricated CRL was studied by scanning electron microscopy. It was shown that surface of the lens is smooth and the geometrical parameters do not deviate significantly from that of the model.
Focusing performance of lenses was studied by the knife-edge technique. It was obtained that the focal distance is not larger than 2 cm at the energy of 9.25 keV. The radiation resistance of the CRL was tested at the synchrotron DESY: PETRA-III. The CRL was exposed at the non-focused X-ray radiation with the standard power and the energy of 12 keV for more than 10 hours without visible degradation.
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