Two-dimensional polymer refractive micro-lenses for X-ray microscopy.
- Aleksandr BARANNIKOV
- Aleksandr BARANNIKOV (Immanuel Kant Baltic Federal University, Kaliningrad, Russian Federation)
- Ksenia ABRASHITOVA (Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation)
- Vladimir BESSONOV (Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation)
- Aleksandr PETROV (Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation)
- Nataliya KOKAREVA (Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation)
- Kirill SAFRONOV (Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation)
- Petr ERSHOV (Immanuel Kant Baltic Federal University, Kaliningrad, Russian Federation)
- Nataliya KLIMOVA (Immanuel Kant Baltic Federal University, Kaliningrad, Russian Federation)
- Ivan LYATUN (Immanuel Kant Baltic Federal University, Kaliningrad, Russian Federation)
- Dr. Vyacheslav YUNKIN (Institute of Microelectronics Technology RAS, Chernogolovka, Russian Federation)
- Maxim POLIKARPOV (European Molecular Biology Laboratory, Hamburg, Germany)
- Dr. Irina SNIGIREVA (European Synchrotron Radiation Facility, Grenoble, France)
- Prof. Andrey FEDYANIN (Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation)
- Dr. Anatoly SNIGIREV (Immanuel Kant Baltic Federal University, Kaliningrad, Russian Federation)
High-performance of laser-like third generation X-ray synchrotron radiation sources provide incredible brightness and coherence, which triggered the development of compound refractive lenses (CRL). Since the first demonstration in 1996  they have been realized in a wide spectrum of designs - in one and two dimensions with spherical, parabolic or kinoform profile. CRLs are fabricated from materials with high refractive index such as Al, Be, C, Ni or Si, they are able to focus high-energy radiation down to micro- and nanoscales, thus fulfilling almost every task arising in modern synchrotron beamlines.
However, the best achievable resolution of metal-based lenses is in the order of 100 nm , and it is mainly influenced by an internal polycrystalline micro-structure, which introduces parasitic scattering and distortions. Another limit is the diffraction-limited resolution that is determined by the numerical aperture (NA), which needs to be maximal. It is clear that the smaller radius of the parabola allows to achieve a higher resolution, approaching the diffraction limit. As for beryllium lenses, the existing manufacturing technology does not allow to produce lenses with a radius smaller than 50 μm.
In connection with all the limitations discussed above, silicon microfabrication technology was applied to reduce the radius of lenses. Silicon one-dimensional nano-lenses have the smallest radii of several microns and are currently capable to execute focusing down to 50 nm . However, silicon planar lenses have a major drawback - their one-dimensional profile makes it impossible to perform two-dimensional imaging of nano-objects. Even with perfectly aligned cross-geometry lenses, the presence of aberrations is simply inevitable.
Therefore, for fabricating small-radius lenses without parasitic X-ray scattering, we decided to use an alternative approach for lens manufacturing from amorphous polymer materials. It turns out that at the moment the most promising is additive manufacturing or 3-D printing. Two-photon absorption lithography (2PP) possesses an unprecedented geometrical freedom, allowing to print very complex designs, including overhanging and self-intersecting structures, inaccessible by conventional methods. This is a simple, reliable and relatively cheap method of forming structures with sub-100 nm feature size from a wide range of processed materials.
Applying the two-photon absorption lithography we manufactured X-ray micro-lenses from the ORMOCOMP polymer. Radius of curvature of a single parabolic surface was in the order of 5 μm, physical aperture was 24 μm, and the distance between the parabola apexes was 5 μm. The focusing properties of lenses were investigated at the Micro-optics test bench in X-ray optics laboratory of the Emmanuel Kant Baltic Federal University using the Metal Jet (Excillium™) microfocus tube with a liquid-gallium jet as an anode . The detected intensity as a function of wire position obtained by knife-edge scan was differentiated and fitted by a Lorentzian function with the FWHM of 5 μm. The resulting beam profile is depicted by a red line. Taking into account the experimental error caused by the knife stage accuracy (± 1 µm), the size of the focused radiation is 5 ± 1 µm. The imaging properties of the lenses were studied at the ESRF ID13 beamline. X-ray microscopy with polymer CRL was realized at 12.7 keV. The X-ray image of the test object Siemens star was obtained at a distance of 1 m from the lens, where the resolution of 200 nm is clearly visible.
To investigate the stability of CRL to X-ray radiation, test structures (cubes) were irradiated for different periods of time, and then characterized by scanning electron microscopy, Raman spectroscopy, and energy dispersive spectroscopy. The results show that X-ray irradiation leads to oxidation of polymer material with simultaneous shrinkage possibly caused by high energy radiation induced cross-linking.
Based on the obtained experimental results, it follows that the radius of curvature can be reduced to 2-3 microns with the current configuration of our 2PP setup and the existing set of polymer materials. Applying more sophisticated 2PP techniques, such as stimulated emission-depletion lithography or diffusion-assisted direct laser writing, it is possible to reduce this value down to a submicron scale.
The manufacturing of polymer lenses was supported by grant contract No 14.W03.31.0008, design and X-ray tests were performed by support of grant contract Nº 14.Y26.31.0002.
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