Multilayer Fresnel Zone Plate Research Articles

Numerical analysis of X-ray multilayer Fresnel zone plates with high aspect ratios

Abstract A partition calculation method (PCM) for calculating the diffraction efficiency of multilayer Fresnel zone plate with high aspect ratio is proposed. In contrast to the traditional theory, PCM designs and evaluates Fresnel zone plate (FZP) considering material pairs, all zone widths, thicknesses and X-ray energy more completely. The results obtained through PCM are validated by comparing them with the complex amplitude superposition theory and coupled wave theory numerical results. The PCM satisfies the requirements of the theoretical investigation of FZP with small outermost zone width (drN) and large thickness (t). Combining proper numerical analysis with the experimental conditions will present a great potential to break through the imaging performance of X-ray microscopy.

Fabrication of multilayer Fresnel zone plate for hard X-ray microscopy by atomic layer deposition and focused ion beam milling

Fresnel zone plate (FZP) is a widely used optics in X-ray microscopy. In contrast to soft X-ray, high aspect ratio is essential for FZP to focus hard X-ray efficiently. Multilayer FZP (ML-FZP) is a promising solution to fabricate high-aspect-ratio FZP due to the unique sputter-sliced technique. In this work, atomic layer deposition (ALD) and focused ion beam (FIB) milling were utilized to prepare ML-FZP for hard X-ray microscopy. Al2O3/HfO2 ML-FZP with total zone thickness 10 μm was successfully obtained. The zone thickness is much greater than the reported ML-FZPs fabricated by the same method and of great significance to the increase of FZP active region. The ML-FZP with outermost zone width 40 nm and aspect ratio 157 was also tested in transmission X-ray microscopy (TXM) at 9 keV and achieved a 29 nm half-pitch cut-off resolution.

Focused Ion Beam Micromachining Enables Novel Optics for X-ray Microscopy

X-ray microscopy is a strong analytical tool with a plethora of applications in physics, materials science and life sciences [1]. In many high resolution X-ray microscopes, a focusing optic such as a Fresnel zone plate (FZP) is utilized as a lens to form an image. The FZP is a diffractive optic composed of a series of concentric rings (zones) of radially varying grating period, where the width, Δr, of the outermost zone determines the resolution. Synchrotron radiation facilities are required as bright and coherent X-ray sources as the FZPs are usually quite limited in diffraction efficiency. The diffraction efficiency depends on FZP properties such as geometry, material, structure thickness and on the outermost zone width for small Δr. Conventional FZP profiles are binary and can have up to 10-40 % diffraction efficiency for ideal absorption and phase reversal zone plates, respectively. In practice the efficiencies are lower due to fabrication difficulties and errors. Electron beam lithography (EBL), the usual fabrication method, allows fabrication of binary FZPs with ultra-high resolution via processes of ever increasing complexity. However, due to the scattering of electrons within the resist, it is very difficult to fabricate ultra-high resolution FZPs with high structural thicknesses to achieve high aspect ratios. This, so called, proximity effect limits the aspect ratio, restricting the utilization of high resolution FZPs to softer X-ray energies. One special type of refractive/diffractive X-ray optic is the kinoform lens, with a continuous 3D surface profile. Ideally, it has a theoretical diffraction efficiency of 100 %. While in a real lens absorption would hinder 100 % efficiency, diffraction efficiencies well above 40 % are already demonstrated [2] using these lenses. Nevertheless, in order to fabricate these kinoforms, researchers resorted to step approximations using several consecutive overlay EBL steps, which is complicated and lead to much lower efficiencies than theoretically expected due to the vulnerability of the optics to fabrication errors amplified by error accumulations. To solve the issues that are intrinsic limitations of the EBL-FZPs we introduced novel, precise and mostly direct methods relying on the FIBs. The powerful method potentially allows for higher resolutions than EBL [3] and precise 3D sculpting capability enables the realization of highly efficient X-ray optics with high fidelity [4]. We have used a standard multi-purpose FIB instrument (Nova 600 NanoLab, DualBeam, FEI) to fabricate binary FZPs, kinoform lenses and multilayer FZPs (Figure 1).

Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 1.2 keV.

X-ray microscopy is a successful technique with applications in several key fields. Fresnel zone plates (FZPs) have been the optical elements driving its success, especially in the soft X-ray range. However, focusing of hard X-rays via FZPs remains a challenge. It is demonstrated here, that two multilayer type FZPs, delivered from the same multilayer deposit, focus both hard and soft X-rays with high fidelity. The results prove that these lenses can achieve at least 21 nm half-pitch resolution at 1.2 keV demonstrated by direct imaging, and sub-30 nm FWHM (full-pitch) resolution at 7.9 keV, deduced from autocorrelation analysis. Reported FZPs had more than 10% diffraction efficiency near 1.5 keV.

Efficient focusing of 8 keV X-rays with multilayer Fresnel zone plates fabricated by atomic layer deposition and focused ion beam milling. Erratum

Corrections to the article by Mayeret al.[J. Synchrotron Rad.(2013),20, 433–440] are given.

Efficient focusing of 8 keV X-rays with multilayer Fresnel zone plates fabricated by atomic layer deposition and focused ion beam milling.

Fresnel zone plates (FZPs) recently showed significant improvement by focusing soft X-rays down to ~10 nm. In contrast to soft X-rays, generally a very high aspect ratio FZP is needed for efficient focusing of hard X-rays. Therefore, FZPs had limited success in the hard X-ray range owing to difficulties of manufacturing high-aspect-ratio zone plates using conventional techniques. Here, employing a method of fabrication based on atomic layer deposition (ALD) and focused ion beam (FIB) milling, FZPs with very high aspect ratios were prepared. Such multilayer FZPs with outermost zone widths of 10 and 35 nm and aspect ratios of up to 243 were tested for their focusing properties at 8 keV and shown to focus hard X-rays efficiently. This success was enabled by the outstanding layer quality thanks to ALD. Via the use of FIB for slicing the multilayer structures, desired aspect ratios could be obtained by precisely controlling the thickness. Experimental diffraction efficiencies of multilayer FZPs fabricated via this combination reached up to 15.58% at 8 keV. In addition, scanning transmission X-ray microscopy experiments at 1.5 keV were carried out using one of the multilayer FZPs and resolved a 60 nm feature size. Finally, the prospective of different material combinations with various outermost zone widths at 8 and 17 keV is discussed in the light of the coupled wave theory and the thin-grating approximation. Al2O3/Ir is outlined as a promising future material candidate for extremely high resolution with a theoretical efficiency of more than 20% for as small an outermost zone width as 10 nm at 17 keV.

Microstructure of Multilayer Fresnel Zone Plate for X-ray Focusing

We have been fabricated the multilayer Fresnel zone plate (FZP) for hard X-ray focusing. The multilayer FZP consists of a series of concentric multilayer of low-Z (Al) zone and high-Z (Cu) zone, alternatively. The spatial resolution of the multilayer FZP is related with the outermost zone width and the smoothness of the multilayer. In case of the concentric multilayer FZP, the undulations were often observed. The smoothness of the multilayer is strongly related with he optical resolution. In this study we reported the microscopic structure of the multilayer FZP with a scanning transmission electron microscope (STEM) observation and discussed the seed of the undulation.

Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39 nm structures

Best resolutions in X-ray focusing are obtained to date by using diffractive lenses called Fresnel zone plates (FZPs). Their further improvement is nevertheless hindered by fundamental limitations in the employed manufacturing techniques. Here, we show a novel method to fabricate FZPs based on multilayer deposition with atomic layer deposition (ALD) and subsequent sectioning with focused ion beam (FIB). For the first time a multilayer FZP working in the soft X-ray range was prepared and could achieve the best resolution obtained so far for multilayer FZPs by resolving features below 39nm in size in a scanning soft X-ray microscope. The new technique presents high potential for high resolution microscopy in both the soft and hard X-ray range.

Quasi-kinoform type multilayer zone plate with high diffraction efficiency for high-energy X-rays

Fresnel zone plate (FZP) with high diffraction efficiency leads to high performance X-ray microscopy with the reduction of the radiation damage to biological specimens. In order to attain high diffraction efficiency in high energy X-ray region, we have developed multilevel-type (6-step) multilayer FZPs with the diameter of 70 micron. The efficiencies of two FZPs were evaluated at the BL20XU beamline of SPring-8. For one FZP, the peak efficiency for the 1st-order diffraction of 51% has been obtained at 70 keV. The efficiencies higher than 40% have been achieved in the wide energy range of 70–90 keV. That for the 2nd-order diffraction of 46% has been obtained at 37.5 keV.

X-ray Focusing Optics with Multilayer Structure and its Application

The X-ray lens is a key technology for the X-ray microscopy at synchrotron radiation (SR) facility. Various types of the focusing optics (lens) for high-energy X-rays have been developed up to now. A multilayer Fresnel zone plate (FZP), which consists of periodic multilayer structure, is one of promising optics. In this review paper, firstly the recent status of various focusing optics for high-energy SR X-rays is described. And next, some of our representative results of the multilayer FZP with high diffraction efficiency and the X-ray microscopic image are shown.

階段状構造を有するＸ線用多層膜レンズの開発

A multilayer (sputtered-sliced) Fresnel zone plate (FZP) is one of promising focusing optics for high-energy X-rays. In order to attain high diffraction efficiency, two types of multilevel-type multilayer FZPs have been fabricated, and these focusing characteristics have been measured at SPring-8. The FZPs composed of concentric multilayers of alternating high-Z(Cu), low-Z(Al), and several types of composite materials layers by Cu and Al. As a result, (1) no significant difference was observed both the measured beam size and the diffraction efficiency of a FZP during 9 months to the FZP with 4-step structure, (2) the diffraction efficiency of the 1st order focus and the focused beam size measured by the knife-edge scanning were more than 50% and 0.8 μm at 41.3 keV, respectively to the FZP with 6-step structure.

硬Ｘ線用１次元フレネルゾーンプレートの作製

A one-dimensional multilayer Fresnel zone plate (1D-FZP) is capable to be an optical element for the micro X-ray computer tomography imaging with high spatial resolution. We developed the 1D-FZP with sputtered-sliced method. The 1D-FZP composes of high-Z (Cu) layer and low-Z (Al) layer, alternately. In this study, we fabricated the Cu/Al multilayer on a Si substrate at room temperature and 200°C, respectively. The smooth section of both multilayer was observed with scanning electron microscope.

New approaches to fabrication of multilayer Fresnel zone plate for high-energy synchrotron radiation X-rays

A multilayer (sputtered-sliced) Fresnel zone plate (FZP) is one of the promising focusing optics with high spatial resolution for the high-energy X-ray region. This is because a large “aspect ratio” can be realized easily. In addition, it is important that the kinoform FZP (theoretical diffraction efficiency=100%) can be fabricated by the sputtered-sliced method. This paper presents the experimental results of two new approaches for fabrication of a multilayer FZP for X-rays. (1) To achieve higher diffraction efficiency, a multilevel-type (4-step: quasi-kinoform type) FZP was fabricated. This FZP was composed of concentric multilayers of alternating high-Z, low-Z, and composite materials. The composite material layer was deposited by co-sputtering of high-Z and low-Z materials. (2) To achieve smoother zones (multilayer interfaces) at the conventional-type FZP, each target of a sputtering apparatus with two DC-sputtering guns was surrounded by a cover with an aperture, and Ar gas was supplied inside the cover, which led to the deposition at lower Ar gas pressure. As a result, for the former, the efficiency was improved markedly as compared with conventional FZP, and for the latter, the zone roughness was reduced, which has resulted in the improvement of the spatial resolution of the FZP.

Sputtered-sliced fabrication of kinoform zone plate for hard X-ray focusing

We are planning to develop a kinoform zone plate (ZP) for hard X-ray using a sputtered-sliced (SS) method with two materials (Cu/Al). The SS kinoform ZP is composed of a Cu/Al multilayer that is based on radial modulation of the refractive indices of deposition materials. The X-ray through the zones interferes constructively at the focal point. The kinoform ZP, therefore, has an advantage of high focusing efficiency. According to the our theoretical calculation, the focusing efficiency of the Cu/Al kinoform PZP is up to 86% in the 0-30 keV X-ray energy region, while one of the conventional Cu/Al multilayer Fresnel zone plate is up to 38%.

Development of a multilayer Fresnel zone plate for high-energy synchrotron radiation X-rays by DC sputtering deposition.

Hard X-ray microscopy with high spatial resolution (<or=0.1 microm) using a high-energy and high-brilliance X-ray microprobe is expected to be a promising technology for various types of analysis, imaging etc. in materials science, biology and medicine. A multilayer Fresnel zone plate (FZP) could be a promising approach to focusing optics in the high-energy X-ray region (>or=20 keV) because a large thickness (aspect ratio) can be available. Various types of multilayer FZPs have been fabricated by DC sputtering deposition. Their focusing characteristics have been evaluated at the high-brilliance undulator beamline BL47XU of SPring-8. An optical system using a Cu/Al multilayer FZP (with an outermost zone width of 0.25 microm) as the focusing optics fabricated by the optimum deposition condition with precise film (zone) thickness control has attained an almost diffraction-limited microbeam of 0.3-0.35 microm at 8.9 keV. A line-and-space resolution test pattern has been observed: fine structures up to 0.2 microm were clearly observed in the measured image. This FZP has been working since 1995, keeping good focusing characteristics. It can be said from these results that a spatial resolution better than 0.1 microm in the high-energy X-ray region is in prospect by the development of a multilayer FZP with a narrower outermost zone width in the near future.

Multilayer Fresnel zone plate for high-energy X-ray by DC sputtering deposition

A hard X-ray microbeam is a key technology for third-generation high-brilliance synchrotron radiation sources such as SPring-8. A Fresnel zone plate (FZP) is one of the promising focusing elements for X-rays. A multilayer (sputtered–sliced) FZP is suitable for use in hard X-ray region, because large thickness can be available. Zone (multilayer interface) roughness is inevitable to the multilayer FZP, which results in the inferior-focusing characteristic. Some Cu/Al concentric multilayers were fabricated by DC sputtering deposition. In order to reduce the oblique incidence of sputtered atoms on the wire substrate, a cylindrical slit was placed between the target and the wire substrate. As a result, the interface roughness was improved with very good repeatability.

Suppression of Corrugated Boundaries in Multilayer Fresnel Zone Plate for Hard X-Ray Synchrotron Radiation Using Cylindrical Slit

Corrugated zone roughness of a circular Cu/Al multilayer Fresnel zone plate (FZP) was successfully suppressed. Such a zone corrugation was often observed in the concentric multilayers prepared using a conventional sputtering apparatus. The zone corrugation is a consequence of the oblique component of the deposition flux (i.e. shadowing effect). We therefore set a cylindrical slit (a linear slit on the surface of a stainless steel cylindrical shield) between the target and the substrate in order to eliminate the oblique flux. As a result, the zone corrugation was not observed using a scanning ion microscope (SIM).

Computer-Controlled Fabrication of Concentric Multilayer Fresnel Zone Plate for Synchrotron Radiation Hard X-Ray

AbstractA Fresnel zone plate (FZP) is a micro-focusing optical element of synchrotron radiation hard X-ray. Our developed multilayer FZP produced a sub-micron size spot at the hard X-ray energy and the focusing efficiency is about 25%. However the higher efficiency FZP is required to utilize the sub-micron probe in microscopy, diffraction and imaging applications. A kinoform FZP composed of gradient refractive index phase zones has the advantage of a high focusing efficiency compared to the conventional multilayer FZP. In order to fabricate the kinoform FZP, we developed a computer-control system. The developed computer-control system automatically operates whole combinatorial deposition processes of the kinoform FZP by inputting the optical parameters of the FZP (focal length, X-ray wavelength, number of zones, zone materials).

Focusing efficiency of a multilayer Fresnel zone plate for hard X-ray fabricated by DC sputtering deposition

A hard X-ray microbeam with submicrometer spot size from third-generation high brilliance synchrotron radiation (SR) sources is expected to be a powerful tool for various fields of research. A Fresnel zone plate (FZP) is one of the promising, focusing elements for X-rays. Focusing efficiency is one of the important features of the FZP. In order to fabricate an FZP with a high focusing efficiency, it is necessary to investigate the relation between the experimental values of the focusing efficiency and the theoretical ones. We have fabricated Ag/C and Cu/Al multilayer (sputtered-sliced) FZPs for use in hard X-ray region, and have compared the measured focusing efficiency data with the calculated ones. The experimental data have been in good agreement with the theoretical values.

Synchrotron Radiation Hard X-Ray Microprobe by Multilayer Fresnel Zone Plate

ABSTRACTA hard X-ray microbeam with submicrometer spot size from synchrotron radiation (SR) sources is expected to add a new dimension to various X-ray analysis methods. A Fresnel zone plate (FZP) is one of the promising focusing elements for X-rays. In order to develop high performance multilayer FZP for use in the hard X-ray region, Cu/Al concentric multilayers were fabricated by use of a DC sputtering deposition process. Lower Ar gas pressure or higher rotating speed of a wire substrate has been effective in forming smoother multilayer interfaces. From a focusing test of the Cu/Al FZP (100-zones) by the SR (λ= 0.154nm), microbeams of 1.5 μm φ and 0.8 μm φ have been achieved for the first- and third-order focal beams, respectively.