In this review we propose to address the question: for the life-science researcher, what does X-ray microscopy have to offer that is not otherwise easily available?We will see that the answer depends on a combination of resolution, penetrating power, analytical sensitivity, compatibility with wet specimens, and the ease of image interpretation.

Soft X-ray microscopes and their biological applications

The development of high brilliance X-ray sources coupled with advances in manufacturing technologies has led to significant improvements in submicrometre probes for spectroscopy, diffraction and imaging applications. The generation of a small beam spot size is commonly based on three principles1: total reflection (as used in optical elements involving mirrors or capillaries), refraction (such as in refractive lenses2) and diffraction. The latter effect is employed in Bragg–Fresnel or Soret lenses, commonly known as Fresnel zone plate lenses. These lenses currently give the best spatial resolution, but are traditionally limited to rather soft X-rays—at high energies, their use is still limited by their efficiency. Here we report the fabrication of high-efficiency, high-contrast gold and nickel multistep (quaternary) Fresnel zone plates using electron beam lithography. We achieve a maximum efficiency of 55% for the nickel plate at 7 keV. In addition to their high efficiency, the lenses offer the advantages of low background signal and effective reduction of unwanted diffraction orders. We anticipate that these lenses should have a significant impact on techniques such as microscopy3, micro-fluorescence4 and micro-diffraction5, which require medium resolution (500–100 nm) and high flux at fixed energies.

High-efficiency multilevel zone plates for keV X-rays

Focusing of 8 keV x rays to a spot size of 150 and 90 nm full width at half maximum have been demonstrated at the first- and third-order foci, respectively, of a phase zone plate (PZP). The PZP has a numerical aperture of 1.5 mrad and focusing efficiency of 13% for 8 keV x rays. A flux density gain of 121 000 was obtained at the first-order focus. In this article, the fabrication of the PZP and its experimental characterization are presented and some special applications are discussed.

Nanometer focusing of hard x rays by phase zone plates

Principles and Applications of Zone Plate X-Ray Microscopes

We report the fabrication of silicon nanostructures using aromatic hydroxybiphenyl self-assembled monolayers as ultrathin (1.1 nm) negative tone electron-beam resist. The formation of the monolayer and the electron-induced crosslinking have been characterized by x-ray photoelectron spectroscopy. Nanometer size patterns were defined by electron-beam lithography in the molecular layer and transferred into silicon by wet chemical etching with potassium hydroxide. We demonstrate the fabrication of silicon line gratings with a resolution of ∼20 nm and of isolated silicon lines with linewidths down to ∼10 nm.

Nanostructuring of silicon by electron-beam lithography of self-assembled hydroxybiphenyl monolayers

In this work, an e‐beam tool consisting of a commercial pattern generator and a scanning electron microscope is used for the fabrication of high‐resolution zone plates for x‐ray microscopy applications. In order to avoid misplacement of the zone plate structures, which would result in a deterioration of the imaging properties, various possible distortions of the lithography system are investigated. Misscaling and nonorthogonality of the deflection axes are eliminated by a simple alignment routine using contamination marks. Drift is found to be sufficiently small after several hours of thermal stabilization. The nonlinearity error is measured by the exposure of Moire patterns. We were able to reduce the total misplacement of the zone plate structures to less than 3×10−4 of the zone plate’s diameter. The resist patterns can be transferred in germanium and tantalum by reactive ion etching using trilevel processes resulting in structures with linewidths down to 26 nm. The quality of the dry etching process is shown by measurement of the x‐ray diffraction efficiency. Imaging an e‐beam‐generated test object with the fabricated zone plates at λ=2.4 nm proves that the total distortions of the e‐beam tool were sufficiently reduced.

Low‐distortion electron‐beam lithography for fabrication of high‐resolution germanium and tantalum zone plates

The authors report the manufacture of germanium phase zone plates as objective lenses for high resolution X-ray imaging at 2.4 nm wavelength. The phase shifting properties of the germanium can be used to enhance the diffraction efficiency of a zone plate. Zone plate patterns with structure widths down to 30 nm are generated by low distortion electron beam lithography. The structures are transferred into a 280 nm thick layer of germanium by reactive ion etching (RIE) using a highly selective trilevel mask. 100 nm thick boron doped silicon windows made from single crystal wafers serve as high transmission support membranes. Measurements at the BESSY storage ring proved diffraction efficiencies up to 6.2%. X-ray imaging of a test object showed clearly resolved 30 nm features.

R. Medenwaldt

Papers

Low‐distortion electron‐beam lithography for fabrication of high‐resolution germanium and tantalum zone plates

Electron beam generated phase zone plates with 30 nm zonewidth for high resolution X-ray microscopy