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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 15 — Jul. 28, 2014
  • pp: 18440–18453
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Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 1.2 keV

Kahraman Keskinbora, Anna-Lena Robisch, Marcel Mayer, Umut T. Sanli, Corinne Grévent, Christian Wolter, Markus Weigand, Adriana Szeghalmi, Mato Knez, Tim Salditt, and Gisela Schütz  »View Author Affiliations


Optics Express, Vol. 22, Issue 15, pp. 18440-18453 (2014)
http://dx.doi.org/10.1364/OE.22.018440


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Abstract

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.

© 2014 Optical Society of America

1. Introduction

X-ray microscopy brings new insights to materials researchers from different fields by adding either a 3rd spatial [1

1. J. Vila-Comamala, Y. Pan, J. J. Lombardo, W. M. Harris, W. K. S. Chiu, C. David, and Y. Wang, “Zone-doubled Fresnel zone plates for high-resolution hard X-ray full-field transmission microscopy,” J. Synchrotron Radiat. 19(5), 705–709 (2012). [CrossRef] [PubMed]

,2

2. E. Zschech, C. Wyon, C. E. Murray, and G. Schneider, “Devices, materials, and processes for nanoelectronics: characterization with advanced x‐ray techniques using lab‐based and synchrotron radiation sources,” Adv. Eng. Mater. 13(8), 811–836 (2011). [CrossRef]

] or a temporal [3

3. A. Bisig, M. Stärk, M.-A. Mawass, C. Moutafis, J. Rhensius, J. Heidler, F. Büttner, M. Noske, M. Weigand, S. Eisebitt, T. Tyliszczak, B. Van Waeyenberge, H. Stoll, G. Schütz, and M. Kläui, “Correlation between spin structure oscillations and domain wall velocities,” Nat. Commun. 4, 2328 (2013).

,4

4. M. Kammerer, H. Stoll, M. Noske, M. Sproll, M. Weigand, C. Illg, G. Woltersdorf, M. Fähnle, C. Back, and G. Schütz, “Fast spin-wave-mediated magnetic vortex core reversal,” Phys. Rev. B 86(13), 134426 (2012). [CrossRef]

] dimension to materials analysis, both at very high resolution. To carry out either scanning or full-field transmission microscopy with X-rays, one has to focus the radiation in some way; yet focusing of especially hard X-rays (HXR), to nano-sized spots, is no easy task. First viable focusing optics developed for HXR were based on a pair of mirrors with grazing incidence illumination [5

5. J. Kirz and C. Jacobsen, “The history and future of X-ray microscopy,” J. Phys. Conf. Ser. 186, 012001 (2009). [CrossRef]

] and were developed in the late 1940s by Kirkpatrick and Baez [6

6. P. Kirkpatrick and A. V. Baez, “Formation of optical images by X-Rays,” J. Opt. Soc. Am. 38(9), 766–774 (1948). [CrossRef] [PubMed]

]. Recent developments on the Kirkpatrick-Baez Mirrors (KBMs) allow very high resolution [7

7. S. Matsuyama, T. Wakioka, N. Kidani, T. Kimura, H. Mimura, Y. Sano, Y. Nishino, M. Yabashi, K. Tamasaku, T. Ishikawa, and K. Yamauchi, “One-dimensional Wolter optics with a sub-50 nm spatial resolution,” Opt. Lett. 35(21), 3583–3585 (2010). [CrossRef] [PubMed]

9

9. S. Matsuyama, N. Kidani, H. Mimura, Y. Sano, Y. Kohmura, K. Tamasaku, M. Yabashi, T. Ishikawa, and K. Yamauchi, “Hard-X-ray imaging optics based on four aspherical mirrors with 50 nm resolution,” Opt. Express 20(9), 10310–10319 (2012). [CrossRef] [PubMed]

], however, there is still room for improvement until high resolution imaging can be done via KBMs.

On the other hand, one of the most successful ways of focusing soft X-rays has been to utilize diffractive optics called Fresnel Zone Plates (FZPs). A FZP is a set of concentric rings with radially decreasing widths according to the zone plate law,rn=(nλf+n2λ2/4)1/2. These rings constitute the zones of the FZP. The width of the outermost zone, Δr, defines its full-pitch resolution at 1st diffraction order, δRayleigh =1.22Δr according to the Rayleigh criterion [10

10. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation: Principles and Applications (Cambridge University Press, 2000).

]. Another important property of FZPs is their diffraction efficiency, DE, which is a function of the energy dependent complex refractive index and the FZP thickness [11

11. J. Kirz, “Phase zone plates for x rays and the extreme uv,” J. Opt. Soc. Am. 64(3), 301–309 (1974). [CrossRef]

] while in certain cases Δr can also have an important impact on the DE (especially for small Δr and large aspect ratio, Ar) [12

12. J. Maser and G. Schmahl, “Coupled wave description of the diffraction by zone plates with high aspect ratios,” Opt. Commun. 89(2-4), 355–362 (1992). [CrossRef]

,13

13. G. Schneider, S. Rehbein, and S. Werner, “Volume Effects in Zone Plates,” in Modern Developments in X-Ray and Neutron Optics, A. Erko, M. Idir, T. Krist, and A. Michette, eds. (Springer Berlin / Heidelberg, 2008), pp. 137–171.

]. The relatively easy handling of FZPs also contributed to their success as it is a single optic which simplifies the alignment procedure to a great extent. Despite the success of FZPs in soft X-ray (SXR) microscopy which was facilitated by developments in the conventional fabrication technique of e-beam lithography (EBL), the development of FZPs for HXR remained challenging due to the need for very high aspect ratios which are usually out of limits of EBL (Ar of up to about 25 is achievable with increasingly complex techniques) [14

14. S. Werner, S. Rehbein, P. Guttmann, and G. Schneider, “3-D structured on-chip stacked zone plates for nanoscale X-ray imaging with high efficiency,” Nano Res. 7, 1–8 (2014). [CrossRef]

,15

15. K. Keskinbora, A.-L. Robisch, M. Mayer, C. Grévent, A. V. Szeghalmi, M. Knez, M. Weigand, I. Snigireva, A. Snigirev, T. Salditt, and G. Schütz, “Recent advances in use of atomic layer deposition and focused ion beams for fabrication of Fresnel zone plates for hard x-rays,” Proc. SPIE 8851, 885119 (2013). [CrossRef]

]. Recently, there have been improvements in focusing of HXR via various diffractive optics such as, multilayer Laue lenses, lithographic and multilayer FZPs achieving high resolutions and efficiencies [1

1. J. Vila-Comamala, Y. Pan, J. J. Lombardo, W. M. Harris, W. K. S. Chiu, C. David, and Y. Wang, “Zone-doubled Fresnel zone plates for high-resolution hard X-ray full-field transmission microscopy,” J. Synchrotron Radiat. 19(5), 705–709 (2012). [CrossRef] [PubMed]

,16

16. Y.-T. Chen, T.-N. Lo, Y. S. Chu, J. Yi, C.-J. Liu, J.-Y. Wang, C.-L. Wang, C.-W. Chiu, T.-E. Hua, Y. Hwu, Q. Shen, G.-C. Yin, K. S. Liang, H.-M. Lin, J. H. Je, and G. Margaritondo, “Full-field hard x-ray microscopy below 30 nm: a challenging nanofabrication achievement,” Nanotechnology 19(39), 395302 (2008). [CrossRef] [PubMed]

23

23. X. Huang, H. Yan, E. Nazaretski, R. Conley, N. Bouet, J. Zhou, K. Lauer, L. Li, D. Eom, D. Legnini, R. Harder, I. K. Robinson, and Y. S. Chu, “11 nm hard X-ray focus from a large-aperture multilayer Laue lens,” Sci. Rep . 3, 3562 (2013).

]. FZPs based on multilayer fabrication techniques (ML-FZPs) where a fiber core is coated with a multilayer and sliced to deliver the ML-FZP, overcome the problem of achieving a high aspect ratio by the nature of the fabrication method [19

19. M. Osterhoff, M. Bartels, F. Döring, C. Eberl, T. Hoinkes, S. Hoffmann, T. Liese, V. Radisch, A. Rauschenbeutel, A.-L. Robisch, A. Ruhlandt, F. Schlenkrich, T. Salditt, and H.-U. Krebs, “Two-dimensional sub-5-nm hard x-ray focusing with MZP,” Proc. SPIE 8848, 884802 (2013). [CrossRef]

,24

24. M. Mayer, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, S. Rehbein, G. Schneider, B. Baretzky, and G. Schütz, “Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39nm structures,” Ultramicroscopy 111(12), 1706–1711 (2011). [CrossRef] [PubMed]

32

32. T. Koyama, T. Tsuji, H. Takano, Y. Kagoshima, S. Ichimaru, T. Ohchi, and H. Takenaka, “Development of Multilayer Laue Lenses; (2) Circular Type,” AIP Conf. Proc. 1365, 100–103 (2011).

]. During the ML-FZP fabrication, one can advantageously vary the ML-FZP thickness freely, enabling the fabrication of ML-FZPs with very high aspect ratios, i.e. high DE, required for HXR. This would be extremely beneficial for materials analysis via HXR nanoprobe beamlines, for instance, in micro-nano-diffraction studies [33

33. Z. Cai, Y. Xiao, I. Dragomir-Cernatescu, R. Snyder, Z. Wang, and B. Lai, “Direct Observation of Strain Segregation in ZnO Nanorings Using X-ray Diffraction,” (Argonne National Laboratory (ANL), 2006).

,34

34. R. P. Winarski, M. V. Holt, V. Rose, P. Fuesz, D. Carbaugh, C. Benson, D. Shu, D. Kline, G. B. Stephenson, I. McNulty, and J. Maser, “A hard X-ray nanoprobe beamline for nanoscale microscopy,” J. Synchrotron Radiat. 19(6), 1056–1060 (2012). [CrossRef] [PubMed]

] to determine the crystal structure with high locality, in addition to high resolution imaging and chemical analysis via spectroscopic techniques, as multi-modal X-ray imaging is getting more and more important [35

35. G. E. Ice, J. D. Budai, and J. W. L. Pang, “The race to x-ray microbeam and nanobeam science,” Science 334(6060), 1234–1239 (2011). [CrossRef] [PubMed]

].

One important advantage of ML-FZPs is that many of them can be cut out from the same multilayer coated fiber with thicknesses tailored for different energies. In the present work we take advantage of such an approach and demonstrate two high resolution ML-FZP optics with 35 nm Δr, each of which is optimized in thickness for either hard or soft X-rays. In the HXR range the full width at half maximum (FWHM) at the focal spot is deduced from an autocorrelation analysis [36

36. A. Ruhlandt, T. Liese, V. Radisch, S. P. Kruger, M. Osterhoff, K. Giewekemeyer, H. U. Krebs, and T. Salditt, “A combined Kirkpatrick-Baez mirror and multilayer lens for sub-10 nm x-ray focusing,” AIP Adv. 2(1), 012175 (2012). [CrossRef]

38

38. J. R. Fienup, “Reconstruction of an object from the modulus of its Fourier transform,” Opt. Lett. 3(1), 27–29 (1978). [CrossRef] [PubMed]

] of the far field diffraction pattern of the ML-FZP, which was illuminated by a KBM pair. This analysis indicates a spot size smaller than 30 nm (FWHM) in two-dimensions, theoretically, indicating a half-pitch resolution of sub-15 nm. Furthermore, in the SXR range, a ML-FZP, with a thickness optimized for that energy region, was tested as focusing optic in a state of the art scanning transmission X-ray microscope (STXM) for direct imaging experiments. The experimentally determined cut-off resolution was 21 nm in feature size. Along with the results of the autocorrelation analysis of HXR experiments, the direct imaging results at the SXR range, confirm the functionality of the fabricated ML-FZP for high energies demonstrating the highest resolution imaging ever achieved by a multilayer type Fresnel zone plate.

2. Experimental methods

2.1 Fabrication of the ML-FZPs

ML-FZPs with 35 nm Δr and 38 µm diameter were fabricated by depositing alternating layers of Al2O3 and Ta2O5 with a total thickness of 4 µm on a 30 µm diameter glass fiber (A2 by SCHOTT AG, Germany) via atomic layer deposition (ALD) as described previously [24

24. M. Mayer, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, S. Rehbein, G. Schneider, B. Baretzky, and G. Schütz, “Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39nm structures,” Ultramicroscopy 111(12), 1706–1711 (2011). [CrossRef] [PubMed]

]. The resulting multilayer coated fibers were cut, via a focused ion beam (FIB) system (Nova NanoLab, FEI, The Netherlands) to thicknesses optimized for either hard (HXR-FZP, 5.9 µm thick) or soft X-rays (SXR-FZP, 1.6 µm thick). The ML-FZPs were then transferred onto molybdenum TEM grids. The final polishing of the FZPs was carried out via the FIB with successively lower currents until a satisfactory surface quality was reached. In case of the SXR-FZP a Pt beamstop of approximately 21 µm diameter and ~1.9 µm thickness was deposited directly on the glass core via focused ion beam induced deposition in the FIB. The diameter of the beamstop was held smaller than the fiber core in order to protect the zones from parasitic Pt deposition but larger than the OSA used in the SXR experiments to block direct light. The parameter overview of the fabricated ML-FZPs is given in Table 1.

Table 1. Summary of the multilayer Fresnel zone plates used as focusing optics at hard and soft X-ray ranges

table-icon
View This Table

2.2 Hard X-ray range experiments

The focusing performance of the 5.9 µm thick HXR-FZP, was tested at the coherence beamline P10 of Petra III (DESY, Germany) using an undulator type insertion device and equipped with a Si(111) monochromator. Incoming X-rays, with the energy of 7.9 keV, were prefocused to illuminate the FZP by a set of two elliptical KBMs. The HXR-FZP was located out of the KBM focus, approximately 4 cm downstream its focal plane in order to ensure complete illumination. The far field diffraction pattern of the HXR-FZP was collected by a 649 x 487 pixels (172 µm pixel size) Pilatus 300K detector with single photon count detection (Detrics Inc., Switzerland) located 5.1 m away from the FZP. In order to protect the camera from the intense direct beam, an external central stop located in the flight-tube and/or a 50 µm thick Molybdenum foil located prior to the KBM acting as an attenuator could be inserted into the beam path.

2.3 Soft X-ray range experiments

Scanning transmission X-ray microscopy (STXM) was carried out using MAXYMUS [39

39. R. Follath, J. S. Schmidt, M. Weigand, K. Fauth, R. Garrett, I. Gentle, K. Nugent, and S. Wilkins, “The X-ray microscopy beamline UE46-PGM2 at BESSY,” AIP Conf. Proc. 1234, 323–326 (2010). [CrossRef]

], a state of the art STXM, located at the UE46-PGM2 beamline of the BESSYII synchrotron radiation facility (Helmholtz Zentrum, Germany). The beamline employs an APPLE II type undulator, a plane grating monochromator and a slit system to coherently illuminate the FZP. The 1.6 µm thick SXR-FZP was mounted as the focusing optic in MAXYMUS where the 1st order focal spot was selected by a 17 µm wide order selecting aperture (OSA) and utilized for microscopy. Prior to the imaging experiments, the alignment of the ML-FZP with respect to the optical axis was carried out via an in-house developed 2-axes tilting stage. The alignment was carried out by tilting the SXR-FZP in pitch and yaw until a full ring was observed in the diffraction pattern recorded on the CCD. This alignment process is similar to that described earlier [40

40. M. Mayer, K. Keskinbora, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, A. Snigirev, I. Snigireva, and G. Schütz, “Efficient focusing of 8 keV x-rays with multilayer Fresnel zone plates fabricated by atomic layer deposition and focused ion beam milling,” J. Synchrotron Radiat. 20(3), 433–440 (2013). [CrossRef] [PubMed]

]. To determine the resolution, a Siemens star (X30-30-2 Xradia, USA) with specified smallest features of 30 nm, as well as two 500 and 200 nm thick FIB lamellae sliced from a GaAs/Al0.7Ga0.3As multilayer sample (L200, BAM, Germany) [41

41. J. Vila-Comamala, K. Jefimovs, J. Raabe, T. Pilvi, R. H. Fink, M. Senoner, A. Maassdorf, M. Ritala, and C. David, “Advanced thin film technology for ultrahigh resolution X-ray microscopy,” Ultramicroscopy 109(11), 1360–1364 (2009). [CrossRef] [PubMed]

] with certified layer thicknesses, were positioned at the 1st order focal plane and raster scanned over the focus. Right behind the test object, an avalanche photodiode (APD, S2382 Hamamatsu, Japan) was employed to collect the total transmitted light at each sample position corresponding to the pixel values of the image. The DE of the SXR-FZP was determined using a slightly different setup. In this setup a 4.4 µm pinhole FIB milled in a ~6 µm thick gold film positioned at the 1st order focal plane, raster scanned over a large area covering the focus and an unobstructed reference region. This way, the intensity at the focus and the incoming intensity over the ML-FZPs active area could be determined in a single experiment and the ratio of two gives the DE.

3. Structure of the multilayer Fresnel zone plate

The combination of Al2O3 and Ta2O5 is a robust one for the fabrication of a ML-FZP via the atomic layer deposition (ALD) technique, as already demonstrated [24

24. M. Mayer, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, S. Rehbein, G. Schneider, B. Baretzky, and G. Schütz, “Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39nm structures,” Ultramicroscopy 111(12), 1706–1711 (2011). [CrossRef] [PubMed]

]. Alumina and Tantalum(V)Oxide were deposited via well-established ALD processes and resulted in sharp layer interfaces [24

24. M. Mayer, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, S. Rehbein, G. Schneider, B. Baretzky, and G. Schütz, “Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39nm structures,” Ultramicroscopy 111(12), 1706–1711 (2011). [CrossRef] [PubMed]

] and a smooth coverage around the fiber circumference. Two ML-FZPs were cut from the deposited multilayer with thicknesses optimized for the HXR (labeled as HXR-FZP) and the SXR range (labeled accordingly as SXR-FZP). See Table 1 for an overview of the parameters of both FZPs.

Fig. 1 SEM images of the SXR-FZP prior to beamstop deposition; a) Overview of the SXR-FZP exhibiting the excellent surface quality and homogeneous coverage. Charging in the glass fiber, as a result of the electron bombardment, is responsible for the curvilinear fine structures seen in the core. b-d) higher magnification images from around the FZP. One can see the very well defined layers throughout the multilayer as well as all around the ML-FZP.
Fig. 2 The diffraction efficiency map of the multilayer Fresnel zone plate made out of Al2O3 of 3.95 g/cm3 density and Ta2O5 of 8.2 g/cm3 density (bulk crystalline densities), calculated according to the TGA. Note that the structure gives reasonable diffraction efficiency even down below 1 keV and increases well above 20% at 8 keV. Two important absorption edges, one at ~1555 eV and one at ~1710 eV, are due to Al K-edge and Ta M-edge, respectively.
Figure 1 shows various regions of the multilayer around the fiber and the resulting ML-FZP structure along with an overview [Fig. 1(a)]. As it can be seen from Figs. 1(b), 1(c) and 1(d), the multilayer interfaces are very well defined. In addition, apart from a few artefacts due to the curtaining effect during FIB preparation, the surface of the ML-FZP is scratch free due to the high precision FIB polishing. An important feature of the ML-FZP shown in Fig. 1(a) is that it has a relatively large diameter (D) of 38 µm, along with a 35 nm Δr, resulting in reasonable focal lengths (fDΔr/λ) [10

10. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation: Principles and Applications (Cambridge University Press, 2000).

] both in the SXR (f=1.29 mm) and the HXR (f=8.58 mm) range (see Table 1). Furthermore, the diameter is small enough to ensure coherent illumination with large flux such that the slit opening which illuminates the ML-FZP can be relatively large. Moreover, the Al2O3/Ta2O5 combination is quite efficient over a wide energy range, letting the utilization of the ML-FZP at various energies, only by optimizing the thickness of the slice. This is shown in Fig. 2 where the diffraction efficiency (DE) of the Al2O3/Ta2O5 material system (using bulk crystal densities) is plotted as a function of photon energy and the zone plate thickness according to the thin grating approximation (TGA) [11

11. J. Kirz, “Phase zone plates for x rays and the extreme uv,” J. Opt. Soc. Am. 64(3), 301–309 (1974). [CrossRef]

,42

42. W. B. Yun, P. J. Viccaro, B. Lai, and J. Chrzas, “Coherent hard x-ray focusing optics and applications,” Rev. Sci. Instrum. 63(1), 582 (1992). [CrossRef]

]. The maximum theoretical DE is about 5% in the vicinity of 1.5 keV and increases to more than 20% as the photon energy increases to 8 keV [Fig. 2]. The theoretical DEs for the SXR- and HXR-FZPs tested in this study are 4% at 1.2 keV and 20.8% at 8 keV, respectively (Table 1). These properties provide the structure its versatility and allow focusing high energy radiation at both hard and soft X-rays with high fidelity.

4. Focusing hard X-rays

Fig. 3 a) Schematic drawing of the experiment. K-B mirrors were used to collect light and illuminate the ML-FZP. A set of absorbers upstream the K-B mirrors (not shown), as well as a beamstop located downstream in the flight-tube can be used in order to protect the detector from direct light. b) Photographic image of the experimental setup showing the zone plate holder. c) Microscopic image of ML-FZP and the TEM holder on which the zone plate is positioned.
ML-FZPs can be fabricated with very high aspect ratios (Ar) that are required for efficient (up to >15% [40

40. M. Mayer, K. Keskinbora, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, A. Snigirev, I. Snigireva, and G. Schütz, “Efficient focusing of 8 keV x-rays with multilayer Fresnel zone plates fabricated by atomic layer deposition and focused ion beam milling,” J. Synchrotron Radiat. 20(3), 433–440 (2013). [CrossRef] [PubMed]

]) focusing of HXR. Despite the proposed improvements in the EBL for increased Ar [14

14. S. Werner, S. Rehbein, P. Guttmann, and G. Schneider, “3-D structured on-chip stacked zone plates for nanoscale X-ray imaging with high efficiency,” Nano Res. 7, 1–8 (2014). [CrossRef]

,43

43. S. Werner, S. Rehbein, P. Guttmann, S. Heim, and G. Schneider, “Towards high diffraction efficiency zone plates for X-ray microscopy,” Microelectron. Eng. 87(5-8), 1557–1560 (2010). [CrossRef]

], aspect ratios in the order of 102 are achievable, so far, only for ML-FZPs [15

15. K. Keskinbora, A.-L. Robisch, M. Mayer, C. Grévent, A. V. Szeghalmi, M. Knez, M. Weigand, I. Snigireva, A. Snigirev, T. Salditt, and G. Schütz, “Recent advances in use of atomic layer deposition and focused ion beams for fabrication of Fresnel zone plates for hard x-rays,” Proc. SPIE 8851, 885119 (2013). [CrossRef]

]. Here, the HXR-FZP has an Ar of approximately 169. Combined with the high efficiency of the structure it provides conditions for robust focusing of HXR. In the experimental configuration shown in Fig. 3, a KBM pair was utilized as a pre-focusing optic to concentrate light onto the ML-FZP [Fig. 3(a) and 3(b)] located on a TEM grid [Fig. 3(c)]. The light is further focused by the ML-FZP into a small focal spot the size of which is determined by the Δr of the HXR-FZP [10

10. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation: Principles and Applications (Cambridge University Press, 2000).

]. Behind the focus of the KBM/ML-FZP system, the beam propagates 5.1 m within an evacuated flight tube until it is registered by a 2D single photon counting detector. The measurement configuration is detailed in the experimental section.

Fig. 4 a) Far field intensity as measured by an area detector with an acquisition time of 100 seconds. One can see the hollow diffraction cones are cut at the detector plane resulting in the 1st, 2nd and 3rd order ring patterns clearly. Acquired while the beamstop was in the beam path and without any attenuator. b) Far field intensity without the beamstop but with a 50 µm thick Mo attenuator prior to the KBM. Scale bars are 100 µm−1. c) 3D representation of the topmost part of the autocorrelation function (ACF) as calculated by an inverse Fourier transform of the far field pattern shown in b. d) The small peak in the center of the autocorrelation function was isolated and fit with a Gaussian along the x- and y- direction. The resulting horizontal and vertical line profiles of the autocorrelation function are plotted here. The FWHM of the Gaussian fits were 59 and 52 nm in the horizontal and vertical direction, respectively.
Figure 4(a), recorded while an external beamstop was present in beampath, shows the diffraction rings covering 1st through 3rd order, possibly up to higher orders. The flux at the 1st order focus was calculated by summing up all pixel values of the 1st order diffraction ring and dividing by the exposure time resulting an estimated photon flux of about 4.5∙107 photons/second. The focal spot size was estimated by an autocorrelation analysis in the following way; the far field diffraction pattern shown in Fig. 4(b), which was recorded while a molybdenum attenuator was inserted in beampath (no beamstop), was used to calculate the autocorrelation function (ACF) at the focal plane which is given in Fig. 4(c). According to the autocorrelation theorem [37

37. J. W. Goodman, Introduction to Fourier optics (Roberts and Company Publishers, 2005).

] the inverse Fourier transform of a real valued intensity distribution of a two-dimensional field, e.g. a far field diffraction pattern, gives the ACF of the source plane, i.e. focal plane of ML-FZP, via the following relationship;

F1{G|fX,fY|2}=g(x,y)g*(x,y)
(1)

FWHM values are smaller than the Rayleigh resolution of the present FZP, δRayleigh = 42.7 nm (FWHM≈36 nm) and reflects the specific resolution properties of FZPs with large central stops, sometimes referred to as “apodized” FZPs [44

44. H. Rarback and J. Kirz, “Optical Performance of Apodized Zone Plates,” Proc. SPIE 0316, 120–127 (1982). [CrossRef]

]. This large central stop, composed of the glass fiber core in this case, modifies the Airy pattern [45

45. M. J. Simpson and A. G. Michette, “Imaging properties of modified Fresnel zone plates,” Optica Acta: International Journal of Optics 31(4), 403–413 (1984). [CrossRef]

] of the optic so that the first minimum of Airy pattern shifts from 42.7 nm to approximately 29.9 nm and the FWHM to approximately 27.9 nm. The experimentally determined focal spot size is, therefore, in good agreement with the theoretical expectation. A sub-30 nm full-pitch (FHWM) resolution corresponding to a theoretical sub-15 nm half-pitch resolution can be expected from the optic.

5. Focusing soft X-rays

Fig. 5 a) Imaging: the SXR-FZP is shown after alignment. The 1st order focus is isolated by an OSA (17 µm in diameter) positioned between the SXR-FZP and the sample. A test object sample was raster scanned over the focus while the total transmitted light was collected by an avalanche photodiode (APD) to construct the STXM image. b) DE measurement: A smaller pinhole (4.4 µm in diameter) positioned close to the 1st order focal plane is raster scanned across ML-FZP, also covering an unobstructed area serving as a reference to determine the incident light falling on the ML-FZP.
A scanning transmission X-ray microscope, MAXYMUS [39

39. R. Follath, J. S. Schmidt, M. Weigand, K. Fauth, R. Garrett, I. Gentle, K. Nugent, and S. Wilkins, “The X-ray microscopy beamline UE46-PGM2 at BESSY,” AIP Conf. Proc. 1234, 323–326 (2010). [CrossRef]

], was utilized for the experiments in the SXR. Overview of experimental setups for imaging and DE measurements are presented in Fig. 5(a) and 5(b), respectively. Figure 5(a) describes the experimental setup which was utilized for the determination of the spatial resolution. Three samples, a Siemens star with 60 nm smallest periodicity (30 nm feature size) and two lamellae (250 and 500 nm thick) of a multilayer with certified feature sizes (BAM L200 test object), were used as test objects to determine the resolution of the SXR-FZP. Figure 5(b) illustrates how the DE was determined experimentally. For further details of the setup, samples, beamline and measurement procedures are given in the experimental section above.

5.1 Imaging using soft X-rays

Last but not least, it is worth noting that a second slice of SXR-FZP, cut-out from a different place on the fiber, with ~1.8 µm thickness and without a beamstop (all other parameters same), was also tested using BAM L200 sample. It was also possible to resolve 21 nm features of the BAM L200 sample, albeit at elevated noise level due to lack of a beamstop. In the light of that experiment we conclude that the results are repeatable and the deposition quality of the multilayer structure is homogenous over large distances.

5.2 Diffraction efficiency at soft X-rays

Fig. 7 a) A line by line image scan of the small pinhole (4.4 µm) showing the projection of the light through the different portions of the SXR-FZP. Taken at 1570 eV with 80 µm x open slit, 500 nm step size, 5 ms dwell time. b) A line scan over the 4.4 µm wide pinhole at various energies, utilized for the demonstration of the rapid increase in efficiency near 1550 eV due to the absorption edge of the lens material, namely the Al in Al2O3, followed by a rapid decrease. c) Theoretical and experimental diffraction efficiencies as a function of energy for different material densities (bulk and amorphous thin films as detailed in the text). A sharp change in the DE near the aluminum absorption edge, following the calculated value, can be seen in the vicinity of 1555 eV.
The diffraction efficiency (DE) of the SXR-FZP was measured at several energies ranging from 1000 eV to 1570 eV using a similar procedure described earlier [48

48. K. Keskinbora, C. Grévent, U. Eigenthaler, M. Weigand, and G. Schütz, “Rapid prototyping of Fresnel zone plates via direct Ga+ ion beam lithography for high-resolution x-ray imaging,” ACS Nano 7(11), 9788–9797 (2013). [CrossRef] [PubMed]

]. The experimental DE of the active FZP region (deposited area) was determined to be above 10% between 1550 and 1555 eV while the efficiency of the whole device is 37.7% of this value corresponding to the active area due to the large inactive central obstruction. The change in the intensity in the 1st order focus relative to the incident light at various energies can be seen by analyzing a line scan that was carried out by a 4.4 µm pinhole from 1350 eV to 1675 eV [Figs. 7(a) and 7(b)]. In Fig. 7(b), the recorded intensity shows the diffraction behavior of the SXR-FZP at different energies as well as the amount of light transmitted through the beamstop, the glass fiber core and the 0th order light transmitted through the zones and hence gives a complete picture of the behavior of the optic. Figure 7(b) clearly shows that the intensity at the focal spot (the portion of the image labeled as the 1st order intensity) is especially high near the 1555 eV mark, corresponding to the measured DE behavior plotted as data points in Fig. 7(c). The predicted DE from the calculations based on bulk crystalline densities for the 1.6 µm thick SXR-FZP given in Fig. 2, is overlaid as solid black line in Fig. 7(c). It appears that the measured DE data points lay slightly above this predicted line. The difference is attributed to the amorphous nature of deposited thin films leading to lower densities compared to their bulk counterparts. To account for this variation, DEs were calculated for the same material pair using various theoretical and experimental densities given in the literature for amorphous thin films [49

49. R. Lizárraga, E. Holmström, S. C. Parker, and C. Arrouvel, “Structural characterization of amorphous alumina and its polymorphs from first-principles XPS and NMR calculations,” Phys. Rev. B 83(9), 094201 (2011). [CrossRef]

56

56. C. Chaneliere, J. L. Autran, R. A. B. Devine, and B. Balland, “Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications,” Mater. Sci. Eng. Rep. 22(6), 269–322 (1998). [CrossRef]

]. The mean calculated DE as well as the corresponding standard deviations are shown in Fig. 7(c). The measured values lie within this range and vary from ~3% at 1000 eV to ~10% at 1500 eV, with a maximum of about 12.55% near the Al absorption edge at 1555. These high efficiencies demonstrate, one more time, the high quality zone structure enabled by the ALD process followed by high precision FIB cutting and polishing.

6. Conclusions

Focusing of high energy radiation via utilization of ML-FZPs was demonstrated. The fabrication process is intrinsically capable of delivering FZPs with high efficiency imaging performance at both soft and hard X-rays. The presented ML-FZPs allow for astigmatism free imaging, illustrating both the accuracy of the manufacturing process and the precision of the 2-axes tilt stage specifically developed for the alignment of the optic with respect to the beam and implemented here for the first time. High resolution soft X-ray microscopy with a smallest visible feature size of 21 nm was demonstrated representing the highest direct imaging resolution ever achieved by a ML-FZP to the best of our knowledge. We are convinced that this resolution performance can be extrapolated to the HXR region, as the autocorrelation analysis of the HXR range diffraction experiment supports sub-30 nm full-pitch resolution of the optic. The results show that ML-FZPs are up-and-coming for focusing of hard X-rays to nano-sized focal spots required for direct imaging [1

1. J. Vila-Comamala, Y. Pan, J. J. Lombardo, W. M. Harris, W. K. S. Chiu, C. David, and Y. Wang, “Zone-doubled Fresnel zone plates for high-resolution hard X-ray full-field transmission microscopy,” J. Synchrotron Radiat. 19(5), 705–709 (2012). [CrossRef] [PubMed]

,23

23. X. Huang, H. Yan, E. Nazaretski, R. Conley, N. Bouet, J. Zhou, K. Lauer, L. Li, D. Eom, D. Legnini, R. Harder, I. K. Robinson, and Y. S. Chu, “11 nm hard X-ray focus from a large-aperture multilayer Laue lens,” Sci. Rep . 3, 3562 (2013).

,57

57. S. Gorelick, J. Vila-Comamala, V. A. Guzenko, R. Barrett, M. Salomé, and C. David, “High-efficiency Fresnel zone plates for hard X-rays by 100 keV e-beam lithography and electroplating,” J. Synchrotron Radiat. 18(3), 442–446 (2011). [CrossRef] [PubMed]

,58

58. E. Nazaretski, H. Yan, J. Kim, K. Lauer, K. Gofron, D. Shu, and Y. Chu, “Development of a 10 nm spatial resolution Hard X-ray Microscope for the Nanoprobe beamline at NSLS-II,” Bull. Am. Phys. Soc. 58, J46003 (2013).

], spectroscopy and for nano-diffraction [33

33. Z. Cai, Y. Xiao, I. Dragomir-Cernatescu, R. Snyder, Z. Wang, and B. Lai, “Direct Observation of Strain Segregation in ZnO Nanorings Using X-ray Diffraction,” (Argonne National Laboratory (ANL), 2006).

35

35. G. E. Ice, J. D. Budai, and J. W. L. Pang, “The race to x-ray microbeam and nanobeam science,” Science 334(6060), 1234–1239 (2011). [CrossRef] [PubMed]

,59

59. Z. Cai, B. Lai, W. Yun, P. Ilinski, D. Legnini, J. Maser, and W. Rodrigues, “A hard x-ray scanning microprobe for fluorescence imaging and microdiffraction at the advanced photon source,” AIP Conf. Proc. 507, 472–477 (2000). [CrossRef]

,60

60. Y. Xiao, Z. Cai, Z. L. Wang, B. Lai, and Y. S. Chu, “An X-ray nanodiffraction technique for structural characterization of individual nanomaterials,” J. Synchrotron Radiat. 12(2), 124–128 (2005). [CrossRef] [PubMed]

] by using ML-FZPs.

When compared with the conventional FZP manufacturing technique, namely the EBL, ML-FZPs present a number of interesting characteristics. Advantages of EBL-FZPs include currently the highest spatial resolutions among point focusing optics [41

41. J. Vila-Comamala, K. Jefimovs, J. Raabe, T. Pilvi, R. H. Fink, M. Senoner, A. Maassdorf, M. Ritala, and C. David, “Advanced thin film technology for ultrahigh resolution X-ray microscopy,” Ultramicroscopy 109(11), 1360–1364 (2009). [CrossRef] [PubMed]

,61

61. W. Chao, P. Fischer, T. Tyliszczak, S. Rekawa, E. Anderson, and P. Naulleau, “Real space soft x-ray imaging at 10 nm spatial resolution,” Opt. Express 20(9), 9777–9783 (2012). [CrossRef] [PubMed]

,62

62. S. Rehbein, P. Guttmann, S. Werner, and G. Schneider, “Characterization of the resolving power and contrast transfer function of a transmission X-ray microscope with partially coherent illumination,” Opt. Express 20(6), 5830–5839 (2012). [CrossRef] [PubMed]

] and easy access to larger diameters in general. They are also conveniently fabricated, via well developed schemes, on flat membranes which makes them easier to align. On the other hand, increasing their resolution further seems challenging. EBL-FZPs are limited in aspect ratios and are especially efficient in SXR. In contrast, ML-FZPs can be tailored for a wide energy range by cutting them to the appropriate thicknesses and can be prepared with very high aspect ratios for high efficiency at HXR. Practical difficulties arising from the small diameter of the ML-FZPs with limited focal (i.e., working) distances have to be solved. Addressing this issue leads to work on the optimization of the deposition process for higher deposition rates towards larger diameters. Although the ALD fabricated ML-FZPs are still in their infancy, interestingly, the resolution achieved here in the soft X-ray range is already quite close to that of commercially available EBL-FZPs (usually Δr = 25 nm). Last but not least, ML-FZPs can potentially reach resolutions beyond current lithographic capabilities as thinner layers of 10 nm and even well below that with high conformality are routinely achievable via ALD. This places the technique among the serious potential candidates for ultrahigh resolution (<10 nm) FZP fabrication methods for both soft and hard X-rays. Future perspectives include the optimization of the material selection [40

40. M. Mayer, K. Keskinbora, C. Grévent, A. Szeghalmi, M. Knez, M. Weigand, A. Snigirev, I. Snigireva, and G. Schütz, “Efficient focusing of 8 keV x-rays with multilayer Fresnel zone plates fabricated by atomic layer deposition and focused ion beam milling,” J. Synchrotron Radiat. 20(3), 433–440 (2013). [CrossRef] [PubMed]

], increased active region and the reduction of the Δr. Demonstration of the best HXR and SXR resolutions, ideally by direct imaging in order to unleash the full potential of these optics remains the ultimate goal.

Acknowledgments

Authors would like to thank to Ulrike Eigenthaler for her help with focused ion beam and to Michael Bechtel for his help with the operation of MAXYMUS and fruitful discussions on the successful deployment of the tilt-stage therein. Finally, feedbacks and inputs from Dr. Stefan Rehbein, Dr. Stefan Werner, Dr. Gerd Schneider from Helmholtz Zentrum Berlin, Dr. Michael Hirscher, Dr. Eberhard Goering and Dr. Herrmann Stoll are gratefully acknowledged.

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E. Nazaretski, H. Yan, J. Kim, K. Lauer, K. Gofron, D. Shu, and Y. Chu, “Development of a 10 nm spatial resolution Hard X-ray Microscope for the Nanoprobe beamline at NSLS-II,” Bull. Am. Phys. Soc. 58, J46003 (2013).

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62.

S. Rehbein, P. Guttmann, S. Werner, and G. Schneider, “Characterization of the resolving power and contrast transfer function of a transmission X-ray microscope with partially coherent illumination,” Opt. Express 20(6), 5830–5839 (2012). [CrossRef] [PubMed]

OCIS Codes
(340.0340) X-ray optics : X-ray optics
(340.6720) X-ray optics : Synchrotron radiation
(340.7460) X-ray optics : X-ray microscopy
(340.7480) X-ray optics : X-rays, soft x-rays, extreme ultraviolet (EUV)
(050.1965) Diffraction and gratings : Diffractive lenses
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
X-ray Optics

History
Original Manuscript: March 17, 2014
Revised Manuscript: June 18, 2014
Manuscript Accepted: June 19, 2014
Published: July 23, 2014

Virtual Issues
Vol. 9, Iss. 9 Virtual Journal for Biomedical Optics

Citation
Kahraman Keskinbora, Anna-Lena Robisch, Marcel Mayer, Umut T. Sanli, Corinne Grévent, Christian Wolter, Markus Weigand, Adriana Szeghalmi, Mato Knez, Tim Salditt, and Gisela Schütz, "Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 1.2 keV," Opt. Express 22, 18440-18453 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-18440


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