B. Kaulich

High-efficiency multilevel zone plates for keV X-rays

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 principles: total reflection (as used in optical elements involving mirrors or capillaries), refraction (such as in refractive lenses) 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 microscopy, micro-fluorescence and micro-diffraction, which require medium resolution (500–100 nm) and high flux at fixed energies.

High-resolution lenses for sub-100 nm x-ray fluorescence microscopy

We report on the design, fabrication, and testing of Fresnel zone plates for high-resolution x-ray fluorescence microscopy using the scanning x-ray microscope at the European Synchrotron Radiation Source. The germanium lenses were optimized for operation near the sulphur absorption edge at 2472 eV photon energy. The high measured diffraction efficiencies of up to 9.6% and the good match to the spatial coherence of the undulator beam resulted in a photon flux of about 4×108 photons per second within the bandwidth of a silicon 〈111〉 monochromator. Using a test object consisting of zinc sulphide nanostructures, we were able to image features in sulphur x-ray fluorescence mode with lateral dimensions down to below 100 nm.

In situ x-ray microscopic observation of the electromigration in passivated Cu interconnects

X-ray imaging of electromigration in a passivated Cu interconnect was performed with 100-nm spatial resolution. A time sequence of 200 images, recorded with the European Synchrotron Radiation Facility x-ray microscope in 2.2 h at 4 keV photon energy, visualizes the mass flow of Cu at current densities up to 2×107 A/cm2. Due to the high penetration power through matter and the element specific image contrast, x-ray microscopy is a unique tool for time-resolved, quantitative mass transport measurements in interconnects. Model calculations predict that failures in operating microprocessors are detectable with 30 nm resolution by nanotomography.

Two zone plate interference contrast microscopy at 4 keV photon energy

Abstract The setup and first successful tests of an interference contrast microscope operating at a photon energy of 4 keV ( λ = 0.31 nm) is described. The interference contrast microscope is based on the full-field X-ray microscope operating at the ID21 beamline at the European Synchrotran Radiation Facility with the difference that two zone plates are used for the image generation instead of one. One of the zone plate generates the X-ray image, both together accomplish the “common path” beam splitting. With a suited configuration, the interference pattern generated by the first orders of the two zone plates is superposed by the X-ray image. Polymer test objects were used to detect phase shifts superposed to X-ray microscopy images employing the interference contrast mode.

Feasibility of transmission x-ray microscopy at 4 keV with spatial resolutions below 150 nm

The feasibility of a transmission x-ray microscope operating in the 3-7 keV photon energy range using phase zone plates (ZPs) has been tested. It has been demonstrated that, at a photon energy of 4 keV, structures smaller than 150 nm can be resolved with good contrast using exposure times in the second to minute range. A large diameter gold ZP was used as condenser and a tungsten ZP with an outermost zone width of 128 nm was used as a high spatial resolution imaging objective lens. Images with a field size of 10×10 μm2 were acquired using a charge coupled device camera optically coupled to a phosphor screen. The corresponding pixel size in the object plane was 70 nm.

The X-ray microscopy facility at the ESRF: A status report

ID21 is a beamline dedicated to X-ray imaging and spectro-microscopy in the 0.2–7 keV energy range. Initiated four years ago, the beamline construction is almost completed and the beamline is now entering into the first operational phase. The beamline is installed on a low beta straight section which is equipped with three undulators and serves two independent end-stations on two separate branch-lines. The scanning X-ray Microscope, served by the “direct” branch-line, equipped with two fixed-exit monochromators and the full-field imaging transmission X-ray microscope, served by the side-branch and optimised for imaging techniques in the 3–7 keV range. Both microscopes use zone-plates as focussing lenses. This paper describes the beamline architecture and provides some figures on the current performance of the beamline.

Current status of the Scanning X-ray Microscope at the ESRF

A short description of the Scanning X-ray Microscope of the ESRF ID21 X-ray microscopy beamline is given and the consequences of the relatively wide operating energy range discussed. The current capabilities of the instrument are demonstrated through images and spectra recorded from a variety of pilot experiments, including X-ray fluorescence imaging, microdiffraction and XANES measurements.

Nano-optical elements fabricated by e-beam and X-ray lithography

In this paper we report results obtained in the design and fabrication of diffractive optical elements (DOEs) with minimum feature size down to tens of nanometers by the use of e-beam and x-ray lithography. The DOEs are patterned using e-beam lithography and replicated by x-ray lithography. Since in our days there is an increased interest for extreme ultraviolet and x-ray microscopy our work has been focused toward the fabrication of DOEs mainly for these applications. Different types of zone plates (ZPs) were fabricated for x-ray beam focusing: high resolution ZPs for high resolution beam focusing, multilevel phase ZPs to increase the diffraction efficiency in the desired order and high aspect ratio ZPs for hard x-rays. Recently we have extended the concept of the ZPs to a more general category of DOEs which beside simple focusing can perform new optical functions in the range of x-rays. In particular, the intensity of the beam after the DOE can be distributed with almost complete freedom. We have designed and fabricated DOEs that focus the beam in an array of spots disposed either in plane or along the optical axis. This type of DOEs has been tested successfully in x-ray differential interference contrast microscopy. The possibility to introduce a specified phase shift between the generated spots is demonstrated in this paper by preliminary results obtained from computer simulations and experiments performed in visible light.

Scanning microscopy end station at the ESRF x-ray microscopy beamline

The ID21 x-ray microscopy beamline at the ESRF has two branchlines: one is dedicated to scanning microscopy techniques and the second to full-field imaging microscopy. The scanning x-ray microscope end station is designed for use over a relatively wide spectral range ranging from 0.2 to 8keV giving access to absorption edges from a wide range of elements of interest in both the biological and materials sciences. The microscope is operating initially with Fresnel zone plate optics and, apart from conventional absorption contrast imaging, is designed to accept a variety of complementary imaging modes. In particular considerable effort has been made to optimize the design for spectromicroscopy using both fluorescence imaging and scanning of the primary x-ray probe energy for XANES imaging. A brief overview of the beamline design is given. This is followed by a discussion of the implications of both the source characteristics and the required wide spectral range upon the optical design of the microscope and, leading from this, the technological choices which have been made. Preliminary results obtained with the microscope are presented.

Phase zone plates for hard x-ray microscopy

Phase zone plates are commonly used in x-ray microscopy techniques when high spatial resolution and high photon flux in the focal spot are desired. Extending the fabrication techniques of phase zone plates for use in the soft x-ray region to higher photon energies becomes difficult due to the high aspect ratios of the zone structures which have to be created. As an alternative, the sputtered sliced zone plate method has been applied. In this technology, a thin microwire is deposited alternately with two different materials. Zone plates are generated from the rod by slicing it perpendicular to its axis and thinning the slices down the required zone plate thickness. Theoretical investigations on the demands on generating these zone plates as well as the generation process are discussed. Recent measurements will be presented.