Planar lenses for high energy x–ray micro- and nano-focusing

Driven by the requirements of new nano-beam applications Si nano-lenses were designed and fabricated. These lenses have a short focal distance in the energy range of 20 and 100keV. The optical test of the new planar lenses was performed at ID15. The resolution about 200-300nm was measured in the energy region of 30-80keV. The best resolution of 150nm was achieved at 50keV energy. Si parabolic refractive lenses have been manufactured in collaboration with the Institute of Microelectronics Technology (Chernogolovka, Russia) and the Dortmund University.

SEM image of the Si planar refractive lens. Insert shows the 2 μm web size.

Knife-edge scan profile of the focused beam at 50 keV. Achieved FWHM is 150nm.

The applicability of Si or Al lenses at energies above 100keV is limited by the physical size of the lens assembly because the number of individual lenses required to produce a reasonable focal distance grows quickly with energy. Using denser lens materials, such as nickel, the number of lenses, which is needed, can be drastically reduced. While the absorption in nickel is still tolerable, its density and thus its refraction are higher compared to the low Z materials used. Nickel is the most promising since it is radiation and corrosion stable and what is more important it is the one of the best material for electroplating. A LIGA technology including deep x-ray lithography and electroplating was used to manufacture Ni planar refractive lenses for x-ray energies from 40 to 220keV. Sub-micrometer focusing was measured in the energy range from 40 to 150keV.

Recently, holographic or kinoform optical elements with the combination of refractive and diffractive properties were manufactured. In these refractive lenses passive parts of the material that cause multiples of 2π in phase shift are removed thereby reducing absorption. With this method drawbacks of purely diffractive or refractive elements are eliminated and advantages like high transmission, absence of zero-order, high efficiency are combined. Ni kinoform lenses made by LIGA focused 212keV X-rays to a focal line 5microns wide with a 10-fold gain. The ability to manipulate the local amplitude and phase of the incoming wave opens the perspective to make a new class of beam-shaping x-ray optics for coherent synchrotron radiation. Ni planar and kinoform lenses have been manufactured in collaboration with the Institute of Microelectronics Technology (Chernogolovka, Russia) and the Institute of Microstructures Technology (Karlsruhe, Germany).

SEM image of Ni kinoform lens.

Intensity distribution across the focal line (image recorded with high resolution film).

Nazmov V., Reznikova E., Snigirev A., Snigireva I., DiMichiel M., Grigoriev M., Mohr J., Matthis B., Saile V. - LIGA fabrication of X-ray nickel lenses - Microsystem Technologies 11, 292-297 (2005).
Nazmov V., Reznikova E., Boerner M., Mohr J., Saile V., Snigirev A., Snigireva I., DiMichiel M., Drakopoulos M., Simon R., Grigoriev M. - Refractive lenses fabricated by deep SR lithography and LIGA technology for X-ray energies from 1 keV to 1 MeV - AIP Conference Proceedings 705, 752-755 (2004).