High-resolution Zone Plates Research Articles

A compound Kinoform/Fresnel zone plate lens with 15 nm resolution and high efficiency in soft x-ray

X-ray microscope as an important nanoprobing tool plays a prevailing role in nano-inspections of materials. Despite the fast advances of high resolution focusing/imaging reported, the efficiency of existing high-resolution zone plates is mostly around 5% in soft x-ray and rapidly goes down to 1%–2% when the resolution approaches 10 nm. It is well known that the rectangular zone shape, beamstop, limited height/width ratios, material absorption of light and structural defects are likely responsible for the limited efficiency. Although zone plates with Kinoform profile are supposed to be efficient, progress for achieving both high resolution (<30 nm) and high efficiency (>5%) have hardly been addressed in soft x-ray. In this work, we propose a compound Kinoform/Fresnel zone plate (CKZP) by combing a dielectric Kinoform zone plate with a 15 nm resolution zone plate. Greyscale electron beam lithography was applied to form the 3D Kinoform zone plate and atomic layer deposition was carried out to form the binary zone plate. Optical characterizations demonstrated 15 nm resolution focusing/imaging with over 7.8% efficiency in soft x-ray. The origin of the efficiency improvement behind the proposed compound lens is theoretically analyzed and discussed.

High-Resolution Binary Zone Plate in Double-Sided Configuration for Terahertz Radiation Focusing

A double-sided phase-reversal binary zone plate is numerically investigated and experimentally characterized at the frequency of 1 THz.The double-sided zone plate has a novel design based on a binary phase-reversal zone plate, in which the total zone thickness is split into two halves located at a distance optimized by means of numerical simulations.The samples are fabricated by a three-axis milling technique on slabs of an ultralow-loss cyclo-olefin polymer.The lenses’ characterization at the frequency of 1 THz is performed by means of a terahertz time-domain spectrometer, in terms of both focal length and spatial resolution.The focusing properties of the double-sided zone plate are compared with those of a conventional binary zone plate.The double-sided zone plate features $2.4\times $ smaller focal spot, $1.25\times $ higher focusing efficiency, and an approximately $10\lambda _{\mathbf {0}}$ longer depth of field than the conventional binary configuration.

High-throughput synthesis of modified Fresnel zone plate arrays via ion beam lithography

Fresnel zone plates (FZP) are diffractive photonic devices used for high-resolution imaging and lithography at short wavelengths. Their fabrication requires nano-machining capabilities with exceptional precision and strict tolerances such as those enabled by modern lithography methods. In particular, ion beam lithography (IBL) is a noteworthy method thanks to its robust direct writing/milling capability. IBL allows for rapid prototyping of high-resolution FZPs that can be used for high-resolution imaging at soft X-ray energies. Here, we discuss improvements in the process enabling us to write zones down to 15 nm in width, achieving an effective outermost zone width of 30 nm. With a 35% reduction in process time and an increase in resolution by 26% compared to our previous results, we were able to resolve 21 nm features of a test sample using the FZP. The new process conditions are then applied for fabrication of large arrays of high-resolution zone plates. Results show that relatively large areas can be decorated with nanostructured devices via IBL by using multipurpose SEM/FIB instruments with potential applications in FEL focusing, extreme UV and soft X-ray lithography and as wavefront sensing devices for beam diagnostics.

Near field stacking of zone plates for reduction of their effective zone period

Here we analyze the potential of a new fabrication method for high resolution zone plates with high aspect ratios based on near field stacking of frequency doubled atomic layer deposited (ALD) zone plates. The proposed method enables reduction of the effective zone period by a factor of four with two zone plate layers compared to the initial e-beam lithography exposed outermost zone period. It also overcomes the problem that very small zone widths with high aspect ratios have to be fabricated for high-resolution hard X-ray microscopy. Using rigorous coupled wave theory, we have analyzed the diffraction behavior of these near field stacked zone plates and investigated strategies to optimize fabrication parameters to compensate for separation of stacked zone plates. The calculations performed for 8 keV photon energy and effective outermost zone widths of 28 nm and 15 nm predict diffraction efficiencies ≥ 20% suggesting that such optics could find widespread practical applications.

Pinhole diffraction holography for fabrication of high-resolution Fresnel Zone Plates

Fresnel zone plates (FZPs) play an essential role in high spatial resolution x-ray imaging and analysis of materials in many fields. These diffractive lenses are commonly made by serial writing techniques such as electron beam or focused ion beam lithography. Here we show that pinhole diffraction holography has potential to generate FZP patterns that are free from aberrations and imperfections that may be present in alternative fabrication techniques. In this presented method, FZPs are fabricated by recording interference pattern of a spherical wave generated by diffraction through a pinhole, illuminated with coherent plane wave at extreme ultraviolet (EUV) wavelength. Fundamental and practical issues involved in formation and recording of the interference pattern are considered. It is found that resolution of the produced FZP is directly related to the diameter of the pinhole used and the pinhole size cannot be made arbitrarily small as the transmission of EUV or x-ray light through small pinholes diminishes due to poor refractive index contrast found between materials in these spectral ranges. We also find that the practical restrictions on exposure time due to the light intensity available from current sources directly imposes a limit on the number of zones that can be printed with this method. Therefore a trade-off between the resolution and the FZP diameter exists. Overall, we find that this method can be used to fabricate aberration free FZPs down to a resolution of about 10 nm.

Angular spectrum simulation of X-ray focusing by Fresnel zone plates.

A computing simulation routine to model any type of circularly symmetric diffractive X-ray element has been implemented. The wavefield transmitted beyond the diffractive structures is numerically computed by the angular spectrum propagation method to an arbitrary propagation distance. Cylindrical symmetry is exploited to reduce the computation and memory requirements while preserving the accuracy of the numerical calculation through a quasi-discrete Hankel transform algorithm, an approach described by Guizar-Sicairos & Gutierrez-Vega [J. Opt. Soc. Am. A, (2004), 21, 53-58]. In particular, the code has been used to investigate the requirements for the stacking of two high-resolution Fresnel zone plates with an outermost zone width of 20 nm.

High-speed X-ray microscopy by use of high-resolution zone plates and synchrotron radiation

X-ray microscopy based on synchrotron radiation has become a fundamental tool in biology and life sciences to visualize the morphology of a specimen. These studies have particular requirements in terms of radiation damage and the image exposure time, which directly determines the total acquisition speed. To monitor and improve these key parameters, we present a novel X-ray microscopy method using a high-resolution zone plate as the objective and the matching condenser. Numerical simulations based on the scalar wave field theory validate the feasibility of the method and also indicate the performance of X-ray microscopy is optimized most with sub-10-nm-resolution zone plates. The proposed method is compatible with conventional X-ray microscopy techniques, such as computed tomography, and will find wide applications in time-resolved and/or dose-sensitive studies such as living cell imaging.

High-resolution Fresnel zone plate fabrication by achromatic spatial frequency multiplication with extreme ultraviolet radiation

We used an approach based on the self-imaging property of gratings to fabricate high-resolution Fresnel zone plates (FZPs). Under certain conditions, the illumination of a parent ZP with a wideband EUV beam produces a radially oscillating intensity distribution with double the spatial frequency of the ZP. This intensity distribution is observed in a certain distance range, given by the local zone width, the focal length of the ZP, and the spectral bandwidth of the illuminating beam. This phenomenon has been used to lithographically record daughter ZPs that have approximately half the zone width, thus twice the resolution, of the parent ZP. FZPs with zone widths as low as 30 nm have been fabricated in this way. Use of this technique in the extreme ultraviolet (EUV) region has the potential for high throughput production of FZPs and similar high-resolution diffraction optics with variable spatial frequency for the EUV and x-ray regions.

13 nm high-efficiency nickel-germanium soft x-ray zone plates

Zone plates are used as objectives for high-resolution x-ray microscopy. Both high resolution and high diffraction efficiency are crucial parameters for the performance of the lens. In this article, the authors demonstrate the fabrication of high-resolution soft x-ray zone plates with improved diffraction efficiency by combining a nanofabrication process for high resolution with a process for high diffraction efficiency. High-resolution Ni zone plates are fabricated by applying cold development of electron-beam-patterned ZEP 7000 in a trilayer-resist process combined with Ni-electroplating. High-diffraction-efficiency Ni–Ge zone plates are realized by fabricating the Ni zone plate on a Ge film and then using the finished zone plate as etch mask for anisotropic CHF3 reactive ion etching into the underlying Ge, resulting in a Ni–Ge zone plate with improved aspect ratio and zone plate efficiency. Ni–Ge zone plates with 13 nm outermost zone width composed of 35 nm Ni on top of 45 nm Ge were fabricated. For co...

Nickel-germanium soft x-ray zone plates

This article presents a fabrication process for soft x-ray zone plates in which nickel and germanium are combined to achieve high diffraction efficiency. A nickel zone plate is first fabricated on a germanium film and then used as a hardmask for a CHF3-plasma etch into the germanium. Zone plates with 50–60 nm nickel and 110–150 nm of germanium are presented. The measured diffraction efficiencies were 10%–11% at λ=2.88 nm, which shows that high efficiency is possible even with thin nickel. Thus, the method has a potential for improving the efficiency of high-resolution zone plates for which the high-aspect-ratio structuring of nickel is difficult.

X-ray computed tomography in Zernike phase contrast mode at 8 keV with 50-nm resolution using Cu rotating anode X-ray source

High-resolution X-ray computed tomography (XCT) enables nondestructive 3D imaging of complex structures, regardless of their state of crystallinity. This work describes a sub-50 nm resolution XCT system operating at 8 keV in absorption and Zernike phase contrast modes based on a commercially available Cu rotating anode laboratory X-ray source. The system utilizes a high efficiency reflective capillary condenser lens and high-resolution Fresnel zone plates with an outermost zone width of 35 nm and 700 nm structure height resulting in a spatial resolution better than 50 nm currently. Imaging a fragment of the solid oxide fuel cells (SOFC) with 50 nm resolution is presented as an application example of the XCT technique in materials science and nanotechnology.

Development of chemical-mechanical polished high-resolution zone plates

State-of-the-art zone plates for soft and hard x rays are commonly fabricated in nickel or gold by electroplating. The most critical fabrication step is the controlled filling of the plating mold, which directly affects the performance of the diffractive optics. One problem is that the electroplating rate depends on the actual zone width resulting in an inhomogeneous height profile across the optics. Another problem is the measurement of the actual zone height during the electroplating process to fill exactly the plating mold. In practice, underplating the mold results in a low diffraction efficiency of the zone plate. Overplating yields in unemployable optics. In this article, the authors apply a chemical-mechanical polishing (CMP) process to overcome the described problems. In the new processing step, the zone plate is planarized after overplating. The authors demonstrate for the first time that nickel zone plates with an outermost zone width down to 25 nm can be polished by applying a CMP process. This new step leads to a much better reproducibility in zone plate fabrication and their performance. In addition, to overcome the technical limit of the current aspect ratios of zone plates the authors propose to superimpose polished zone plate layers on top of each other. The authors assume that the introduced CMP process paves the way toward the development of future volume zone plates with ultrahigh aspect ratios for soft and hard x-ray applications.

High-resolution differential interference contrast X-ray zone plates: Design and fabrication

Differential interference contrast is a potentially powerful technique for contrast enhancement in soft X-ray microscopy. We describe the design and fabrication of single-element diffractive optical elements suitable as objectives for high-resolution differential interference contrast microscopy in the water-window spectral range. A one-dimensional pattern calculation followed by an extension to two dimensions results in a pattern resolution of 1 nm, which is well below fabrication accuracy. The same fabrication process as for normal zone plates is applicable, but special care must be taken when converting the calculated pattern to a code for e-beam lithography.

Orientation dependence of linewidth variation in sub-50-nm Gaussian e-beam lithography and its correction

The width of tilted line patterns, such as are needed when drawing circular structures, is found to vary with the oblique angle when it falls into the sub-50-nm scale in Gaussian e-beam lithography. The authors’ analysis shows that this orientation dependence of linewidth variation originates from the nonuniformity of discrete primitive filling in Cartesian coordinates. Two correction schemes based on pattern segmentation are proposed. Test exposures of high resolution zone plate patterns show that both two schemes work well; a double-insert scheme is superior in terms of dose distribution uniformity.

Fabrication of high-resolution zone plates with wideband extreme-ultraviolet holography

We report an achromatic holographic method to fabricate high-resolution x-ray optics using coherent extreme-ultraviolet radiation from an undulator source. The interference pattern between two spherical beams, which are created using Fresnel zone plates, is recorded to produce a higher-resolution zone plate. Analytical and simulation results showing the formation of the zone plate pattern was confirmed experimentally with the production and testing of a lens with 60-nm outermost zone width. The combination of extreme-ultraviolet light, which exposes photoresists with practically no proximity effect, and holography, which guarantees the accurate placement of zones, addresses the main difficulties faced in the improvement of the resolution of x-ray lenses. Holography with extreme-ultraviolet light has the potential to produce lenses with sub-10-nm resolution.

Simulation of high-resolution x-ray zone plates.

A full-wave approach to quantitative characterization of x-ray zone plate lenses is proposed. Distributed focusing efficiency eta(z) of a multifocus optical element is defined as the energy flux through the Airy disk of a reference perfect lens with variable focal length z. Maxima of this function characterize diffraction efficiencies and spatial resolution of the zone plate foci. The parabolic wave equation is used to take into account diffraction effects inside the optical element. Rough and fuzzy interface models are introduced to describe realistic zone profiles. Numerical simulation reveals the limited capability of zone width reduction to improve the zone plate imaging performance. The possibilities of second-order focus enhancement by optimization of the zone plate thickness, line-to-space ratio, and zone tilt are studied numerically.

Application of X-ray mask fabrication technologies to high resolution, large diameter Ta Fresnel zone plates

High-resolution and large diameter condenser Fresnel zone plates were fabricated in a 0.35-μm-thick Ta layer on an SiN membrane. A newly developed conversion algorithm was applied where sub-field length and position are randamized in order to supress the evolution of small figures at pattern divided parts such as subfield boundaries in the conventional conversion algorithm. A single layer resist system was adopted to write large diameter, ultra-fine, dense ring patterns. FZPs with the outer-most linewidth of 50 nm was successfully fabricated by applying the subtractive x-ray mask fabrication process.

Fabrication of High-Resolution Fresnel Zone Plates by a Single Layer Resist Process

In this work, fabrication of a high-resolution Fresnel zone plate (ZP) by a single layer electron beam (EB) resist process combined with a gold electroplating process is described. The ZP designed has a diameter of 80 µm and is composed of 250 rings (499 zones) of which the width of the outermost ring is nominally 40 nm. In order to fabricate a 150-nm-thick gold layer which acts as a phase-shifter for X-rays, a 200-nm-thick polymethylmethacrylate (PMMA) is used as a single layer resist. A 50 kV EB lithography system with a probe diameter of 3 nm is employed for high-resolution patterning. To prevent resist patterns with an aspect ratio of 5 from collapsing, we reinforce the ZP patterns by combining neighboring rings with ribs. It is demonstrated that a modified ZP pattern with a 40-nm-wide outermost ring which will not collapse can be fabricated whereas the original ZP pattern collapses at a high aspect ratio. Although control of the width of the gold phase shifter should be improved, the outermost ring with the nominal width of 40 nm can be electroplated.

Diffraction optics for X-ray imaging

Diffraction optics for X-ray imaging are being used, for example, for X-ray microscopy experiments. In this paper the fabrication of high resolution zone plates and condenser zone plates is discussed. The fabrication techniques described include electron beam lithography, UV laser lithography, and pattern transfer by reactive ion etching. Zone plates with outermost zone widths down to 19 nm have been built.

Cross-linked polymers for nanofabrication of high-resolution zone plates in nickel and germanium

A high resolution cross-linked PMMA resist has been synthesized and optimized for the generation of zone plate patterns down to 19 nm linewidth with e-beam lithography. This resist shows an increased resolution compared to PMMA for generating periodic structures with a line to space ratio of 1:1. Furthermore, we developed a cross-linked copolymer based on styrene and divinylbenzene, which is used in a new trilevel reactive ion etching (RIE) process. In this process a resist pattern of low aspect ratio can be transferred into a copolymer galvanoform with high aspect ratios for the electrodeposition of nickel. The copolymer has also been used as a highly selective etching mask for zone plate pattern transfer into germanium by RIE.