X-ray Fresnel Zone Plates Research Articles

Soft x-ray microscopy with 7 nm resolution

The availability of intense soft x-ray beams with tunable energy and polarization has pushed the development of highly sensitive, element-specific, and noninvasive microscopy techniques to investigate condensed matter with high spatial and temporal resolution. The short wavelengths of soft x-rays promise to reach spatial resolutions in the deep single-digit nanometer regime, providing unprecedented access to magnetic phenomena at fundamental length scales. Despite considerable efforts in soft x-ray microscopy techniques, a two-dimensional resolution of 10 nm has not yet been surpassed in direct imaging. Here, we report on a significant step beyond this long-standing limit by combining newly developed soft x-ray Fresnel zone plate lenses with advanced precision in scanning control and careful optical design. With this approach, we achieve an image resolution of 7 nm. By combining this highly precise microscopy technique with the x-ray magnetic circular dichroism effect, we reveal dimensionality effects in an ensemble of interacting magnetic nanoparticles. Such effects are topical in current nanomagnetism research and highlight the opportunities of high-resolution soft x-ray microscopy in magnetism research and beyond.

Measurement and compensation of misalignment in double-sided hard X-ray Fresnel zone plates.

Double-sided Fresnel zone plates are diffractive lenses used for high-resolution hard X-ray microscopy. The double-sided structures have significantly higher aspect ratios compared with single-sided components and hence enable more efficient imaging. The zone plates discussed in this paper are fabricated on each side of a thin support membrane, and the alignment of the zone plates with respect to each other is critical. Here, a simple and reliable way of quantifying misalignments by recording efficiency maps and measuring the absolute diffraction efficiency of the zone plates as a function of tilting angle in two directions is presented. The measurements are performed in a setup based on a tungsten-anode microfocus X-ray tube, providing an X-ray energy of 8.4 keV through differential measurements with a Cu and an Ni filter. This study investigates the sources of the misalignments and concludes that they can be avoided by decreasing the structure heights on both sides of the membrane andby pre-programming size differences between the front- and back-side zoneplates.

X-ray Fourier transform holography by amplitude-division-type Fresnel zone plate interferometer.

A two-block X-ray Fresnel zone plate system forms two-beams - a plane wave and a spherical wave - which interfere at the focal distance of the virtual source of the spherical wave. An object placed in the path of the plane wave forms an object wave and the spherical wave is the reference wave. The recorded intensity distribution is the Fourier transform hologram of the object. Analytical and numerical calculations show the possibilities of this scheme to record the hologram and reconstruct the object image. Examples of recording holograms of a one-dimensional cosine-like grating and a two-dimensional grid object as well as reconstruction of the images are considered.

Direct-write X-ray lithography using a hard X-ray Fresnel zone plate.

Results are reported of direct-write X-ray lithography using a hard X-ray beam focused by a Fresnel zone plate with an outermost zone width of 40 nm. An X-ray beam at 7.5 keV focused to a nano-spot was employed to write arbitrary patterns on a photoresist thin film with a resolution better than 25 nm. The resulting pattern dimension depended significantly on the kind of underlying substrate, which was attributed to the lateral spread of electrons generated during X-ray irradiation. The proximity effect originated from the diffuse scattering near the focus and electron blur was also observed, which led to an increase in pattern dimension. Since focusing hard X-rays to below a 10 nm spot is currently available, the direct-write hard X-ray lithography developed in this work has the potential to be a promising future lithographic method.

High aspect ratio nanostructuring by high energy electrons and electroplating

Due to the ability of 100keV electrons to penetrate deep into resist with little scattering, we were able to directly write various dense and high aspect ratio nanostructures in up to 1.65μm thick layers of poly(methyl methacrylate) (PMMA) resist. The PMMA molds produced by electron beam lithography were developed using a high contrast developer. The molds were used to transfer the pattern into metallic nanostructures by filling the developed trenches with Au by electroplating. By exposing lines narrower than the target width, we observed improved process latitude and line width control. The obtained aspect ratios of the dense structures are nearly 17 in 1.1μm PMMA layers and >13 for structures electroplated into these PMMA molds. The fabrication method was successfully applied to produce Au diffractive X-ray Fresnel zone plates of exceptionally good quality with 70nm outermost zones using 1.1μm thick PMMA mold. In addition, we also produced regular arrays of high aspect ratio and dense Au nanorods and high aspect ratio split ring resonators with line widths down to 65nm and with heights up to 1.5μm.

Direct e-beam writing of dense and high aspect ratio nanostructures in thick layers ofPMMA for electroplating

Due to the ability of 100 keV electrons to penetrate deep into resist with little scattering, we wereable to directly write various dense and high aspect ratio nanostructures in 540 nm and 1.1 µm thick layers of poly(methyl methacrylate) (PMMA) resist. The PMMA molds produced byelectron beam lithography were developed using a high contrast developer. Themolds were used to transfer the pattern into metallic nanostructures by fillingthe developed trenches with Au by electroplating. By exposing lines narrowerthan the target width, we observed improved process latitude and line widthcontrol. The obtained aspect ratios of the dense structures are nearly 20 in 1.1 µm PMMAlayers and > 16 for structures electroplated into this PMMA mold. The fabrication method wassuccessfully applied to produce Au diffractive x-ray Fresnel zone plates ofexceptionally good quality with 50 and 70 nm outermost zones using 540 nm and 1.1 µm thick PMMA molds. In addition, we also produced regular arrays of high aspect ratio anddense Au nanorods with periods down to 100 nm and high aspect ratio split-ring resonators.

Focused ion beam fabrication procedures of x-ray micro Fresnel zone plates

In this paper the use of focused ion beam (FIB) mask-less lithography is presented as a novel and simple way to fabricate x-ray Fresnel zone plates (FZPs), and a prototype of a FIB-made 100 nm resolution FZP with 38 zones is described. Considerations for future developments—the maximum aspect ratio achievable by FIB lithography in nickel, a way to produce zones with a parabolic transverse profile, as theoretically required for the highest efficiency in focusing—are also reported.

Application of the parabolic wave equation to X-ray diffraction optics

A new computational approach is suggested to the analysis of imaging properties of realistic X-ray Fresnel zone plates with high aspect ratio. Its mathematical basis is the parabolic wave equation (PWE) describing diffraction inside the zone plate body as well as wave propagation and focusing throughout the optical system. A finite-difference method is used for solving PWE in a truncated free-space domain with a perfectly transparent artificial boundary. The results of global wave field calculation are visualized by means of color graphics. Numerical estimates of resolution, diffraction efficiency and field of view enables one to compare imaging performance of realistic X-ray zone plates with an idealized plane Fresnel diffraction lens.

Fresnel and refractive lenses for X-rays

We present a Gaussian beam analysis of X-ray refractive and Fresnel lenses. The X-ray refractive lens is featured by an intrinsic soft (Gaussian) aperture due to strong absorption of X-rays by materials. We defined a parameter N 0, the critical Fresnel number (CFN), to describe this optical property. The values of N 0 for all practical materials are below 1000 for photon energies exceeding 30 eV, still lower for high- Z materials. The maximum effective Fresnel number of a lens is determined by its material to be 2 N 0 and its maximum enhancement of X-ray intensity is limited to (2 πN 0) 2, independent of its shape. We found that the refractive lens is likely to be useful for manipulating nearly diffraction limited beam in the hard X-ray region and its application is severely restricted by available fabrication capabilities today. X-ray Fresnel lenses, both in cylindrical and linear forms, are proposed as superior focusing elements for hard X-rays. Their high efficiency, up to 100% in optimal construction, will enable us to manipulate beams with multiple lenses and obtain higher performance optics. Their design and fabrication are discussed in reference to those of X-ray Fresnel zone plates and micro Fresnel lenses for optoelectronics.

Characteristics of optical components for soft x-ray microscopy and x-ray holography using an undulator radiation optical system (abstract)

X-ray Fresnel zone plates and transmission gratings, have been fabricated using fine-focused electron-beam lithography and Ta-on-SiN x-ray mask fabrication technique. The optical components for x-ray microscopy and x-ray holography were evaluated in an undulator radiation system. Nanometer lithography using fine-focused electron-beam writing machines is now regarded as an important technology to create new devices on very small structures such as quantum wires or boxes and new scientific probe elements such as x-ray optical components. A new fabrication process of Ta-on-SiN x-ray mask1 for x-ray optical components has been established. Tantalum is used for the x-ray absorber, because of having approximately the same absorption coefficient as that of gold. A thin SiN film, which has a high transparency for soft x rays, acts as a membrane supporting the Ta-absorbing patterns. The fabricated zone plates have an outermost zone width of 0.25 μm with up to 2 mm diameter. The pitch width of the transmission grating is 0.4 μm with up to 1.0 mm2 area. Adopting a highly brilliant and highly coherent undulator2 synchrotron radiation, we have been developing its optical system to apply to x-ray microscopy and holography. An x-ray microscope system3 consists of two kinds of zone plates with different dimensions for imaging and focusing undulator radiation. The resolving power of this microscope system was evaluated by using resolution test charts, which have the same structure as x-ray optical components. A resolution limit of 0.3 μm was obtained, which is very close to Rayleigh’s limit of 0.25 μm for the outer-most zone width. We constructed a divided wave-front interferometer with x-ray transmission gratings, and estimated the dispersion of undulator radiation to be 1/20. The authors thank S. Aoki of Tsukuba University for his useful discussion on x-ray optics. The authors are also grateful for the help from T. Tamamura and H. Yoshihara, NTT for fabrication of x-ray components, and from Y. Toyoshima of the Photon Factory in undulator experiments, respectively.