X-ray refractive lenses made by ultra-deep LED lithography
X-ray refractive objective lens and condenser lens were developed for hard X ray transmission microscopy with synchrotron beam at photon energies 15-35 keV. The lenses are rows with large number of separate bi-concave parabolic elements with periodic spaces in order to cross perpendicularly 2 linear rows forming point focusing. The maximal raw length is 10 cm. The material of the X-ray refractive lenses is lithographic SU-8 polymer. It is a multi-element composition based on the monomer of diglycidyl ether of bisphenol-A novolack with small amounts of elements of a photo-acid generator, for example: C : O : H : Sb : F : S = 72.3 : 18.2 : 6.9 : 0.9 : 1.2 : 0.6. At 25 keV photon energy, the real and imaginary parts of the refractive index decrement of the SU-8 polymer are equal to 4.32E-7 and 2.61E-10, respectively. We control the current element compositions of the material by means of X-ray fluorescence analysis using INCA X-act system with the HITACHI S 3400N tip II E-beam microscope. The effective apertures are up to 1 mm for the designed X-ray condensers and they are up to 100 microns for the X-ray objectives. The focal distances of the objective lenses are within 30-60 mm and 1-3 m for the condenser lenses. The calculated focal spot (full width at half maximum) of the X-ray objective lenses with ideal parabolic profiles is less than 50 nm. In order to achieve the ideal parabolicity the technological design of the photomask lens elements included size deviations of SU-8 microstructures during photo- and X-ray lithography processes. Photolithography enables to produce X-ray mask for next step of X-ray lithography. The possibility of fabrication of planar X-ray refractive lenses focusing in two directions was demonstrated in previous works with the use of X-ray lithography . In this work, we studied ultra-deep photolithography with the use of 400 nm light-emitting diode (LED) as a point light source, to make linear X-ray lenses on planar surfaces and on butt ends of CuZn substrates employing one lithography process. The exposure area of the compact photolithography setup was within 8-15 cm in diameter. SU-8 resist layer is photoactive for the LED wavelength showing the specific absorbance of 1.7 1/cm . The material transparency results to ratio closed to unity for doses of the absorbed LED radiation at the bottom and at the top of the SU-8 layers of 1 mm thicknesses. The photolithographic dose ratios are comparable with X-ray lithography dose conditions and provide small diffraction distortions with high aspect ratios in the resist layer depth. In the paper, we describe the fabrication process and properties of the X-ray refractive linear lenses with vertical sidewalls of their elements made by the ultra-deep LED lithography as well as the crossed X-ray refractive lenses, which satisfy the X-ray microscope application purpose.
 E. Reznikova, T. Weitkamp, V. Nazmov, A. Last, M. Simon, V. Saile. Investigation of phase contrast hard X-ray microscopy using planar sets of refractive crossed linear parabolic lenses made from SU-8 polymer. Phys. Stat. Sol. 204 (a) 2811–2816 (2007).  E. Reznikova, J. Mohr, H. Hein. Deep photolithography characterization of SU-8 resist layers. Microsys. Technol. 11 (4-5) 282-291 (2005)