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V. M. Kolomiets, S. M. Kravchenko, I. M. Kononenko, A. G. Ponomarev, V. A. Rebrov, and S. V. Kolinko
«Construction of X-Ray Optics Elements with a Proton-Beam Lithography»
645–655 (2018)

PACS numbers: 07.85.Fv, 41.50.+h, 61.05.-, 68.55.J-, 81.15.Cd, 81.16.Nd, 82.80.Yc

X-ray phase-contrast image visualization as a study method of object structure is extremely promising as to its practical application, in particular, in medicine. However, it requires high level of x-ray coherence. The Talbot interferometer method based on x-ray phase diffraction gratings allows operations with polychromatic radiation of an x-ray tube; thus, there is no need in expensive and bulky equipment like synchrotron. Since gold has rather high density and high x-ray absorption coefficient, the gratings based on a silicon single crystal with an electrodeposited gold usually present the model phase gratings. As gold is a very expensive material, bismuth may be applied as an absorption material, because it is cheap and insignificantly deteriorate the physical and technical properties of x-ray phase diffraction gratings. This work proposes a fabrication method for the grating structures based on single-crystalline silicon with bismuth as an absorption material and the proton-beam lithography applied (a nuclear scanning microprobe end-station of an electrostatic accelerator at the Institute of Applied Physics of the N.A.S. of Ukraine, Sumy). Positive resistant material PMMA is coated with a centrifuge method on the thin copper films (of 50 nm) with a Ti sublayer (of 15 nm) preliminary sprayed on a single-crystalline Si substrate with a magnetron sputtering technique. Some areas of the samples with PMMA photoresist underwent the exposure of a proton beam with an experimentally determined irradiation dose (of about 90 nC/mm2) required for the complete etching of the radiated areas. The exposure is necessary for further stages of phase-gratings fabrication, namely, etching of the radiated areas and electroplating. Bismuth electroplating is performed with current density of about 3 mA/cm2. In the course of the experiments, the test samples of the grating structures of 15-m height, 85-m period, and 0.4 off-duty ratio are obtained. Rutherford backscattering of ions is used to study the structure thickness at certain stages; scanning electron microscopy is applied to study the structure and height of the samples at the final stage of fabrication.

Keywords: x-ray phase-contrast imaging, x-ray phase diffraction grating, phase proton-beam lithography, magnetron sputtering, resistant material PMMA, exposure, etching, electroplating

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