M. Gentili

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.

NANOMETER FOCUSING OF HARD X RAYS BY PHASE ZONE PLATES

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.

Hard x-ray phase zone plate fabricated by lithographic techniques

A Fresnel phase zone plate with an unprecedented focusing efficiency of 33% was fabricated using an x‐ray lithographic technique and was tested using synchrotron x rays. Contributions by the zeroth‐order x ray to the focus were minimal. Spatial resolution in the micrometer range was achieved. The measured spot size was dominated by geometric demagnification of the source. It should be possible to obtain submicrometer resolution by aperturing the source. Experimental results of focusing efficiency measurements, intensity distribution at the focal plane, and spatial resolution tests are reported.

Development of zone plates with a blazed profile for hard x-ray applications

A linear and a circular zone plate with a blazed zone profile (ZPBP) have been fabricated and characterized using synchrotron x rays. The ZPBPs have significantly improved performances in terms of focusing efficiency and the background near the focus compared to those of a zone plate with a square profile, of which the transmission function can be characterized by a binary square wave. In many respects and practical cases, an x-ray ZPBP may be used in a way analogous to an optical lens in the visible light region. In this article, the experimental characterization of the ZPBPs is presented and some special applications are discussed.

SYNCHROTRON RADIATION SCANNING PHOTOEMISSION MICROSCOPY: INSTRUMENTATION AND APPLICATION IN SURFACE SCIENCE

X-ray photoelectron spectroscopy has become a true microscopic technique at third generation soft X-ray synchrotron sources, finding applications in many domains of academic and applied research. This paper describes the present status of scanning photoemission microscopy, where by using photon optics the photon beam can be focused to micron or submicron dimensions and imaging or spectroscopy can be performed. It discusses different photon focusing optical elements and describes the major components of the constructed scanning microscopes. The applications of imaging and spectroscopy with high lateral resolution in surface science are illustrated, and some recent results obtained in different laboratories are briefly reviewed.

Analytical derivation of the optimum triplicator

Abstract The analytical derivation of the phase profile of a diffractive optical element that produces three equi-intense replicas of an input beam with the maximum efficiency is presented. Such derivation, based on a functional minimization procedure, leads to a closed form for the phase profile and to an efficiency value slightly lower than the predicted theoretical upper bound.

DEVELOPMENT OF A HARD X-RAY IMAGING MICROSCOPE

A hard x‐ray imaging microscope based on a phase zone plate has been developed and tested. The zone plate, with a 5 cm focal length and a 0.2 μm smallest linewidth, was used to image 8 keV x rays from the samples. The imaging microscope can be used to obtain nearly diffraction‐limited resolution over the entire imaging field, and its resolution is almost independent of source size and source motions. We have tested such an imaging microscope, and a resolution of about 0.4 μm was obtained. The images were obtained with an exposure time of less than 1 min, for a magnification factor of 30 in the x rays. The x rays were then converted into visible light, and another 7 times magnification were obtained by using a lens system coupled to a charge coupled device camera. The results from the imaging microscope, and possible applications, will be discussed.

High-performance multilevel blazed x-ray microscopy Fresnel zone plates: Fabricated using x-ray lithography

Diffractive lenses are becoming the optical elements of choice for many applications. One type of diffractive lens, the binary zone plate, has already demonstrated high‐resolution performance experimentally. However, in order to increase the diffraction efficiency of these zone plates, a blazed grating profile must be used. This can best be approximated by a staircase grating profile, created by multilevel exposures. Using x‐ray lithograph, we fabricated for the first time circular, linear bi‐ and trilevel zone plates, with gold structures 0.75 μm thick (per level), on silicon nitride substrates. The zone plates were designed for use at a wavelength of 1.54 A, and had a theoretical efficiency of 68.5% for bilevel and 81.5% for trilevel zone plates. Due to the large depth of focus and high resolution inherent to x‐ray lithography, the finished zone plate exhibits very steep sidewall profiles, with linewidth resolution down to 0.25 μm. Such vertical sidewalls are essential for achieving high lens efficiency...

Fabrication of 5 nm Resolution Electrodes for Molecular Devices by Means of Electron Beam Lithography

Electron beam lithography is used to fabricate two-metal electrode tip-shaped structures. The distance between the tips is continuously controlled to be between 5 and 70 nm. The electron beam lithography process is robust and the tip separation is well controlled in the sense that the smallest distance between the tips is a consequence of the design and not a consequence of randomly distributed metal spots around the tip area. Interest in these structures is due to the fact that they can be used to fabricate rectifiers, working with single molecule, designed to exhibit semiconductor properties.

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.