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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 10 — May. 20, 2013
  • pp: 11747–11756
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Ion beam lithography for Fresnel zone plates in X-ray microscopy

Kahraman Keskinbora, Corinne Grévent, Michael Bechtel, Markus Weigand, Eberhard Goering, Achim Nadzeyka, Lloyd Peto, Stefan Rehbein, Gerd Schneider, Rolf Follath, Joan Vila-Comamala, Hanfei Yan, and Gisela Schütz  »View Author Affiliations


Optics Express, Vol. 21, Issue 10, pp. 11747-11756 (2013)
http://dx.doi.org/10.1364/OE.21.011747


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Abstract

Fresnel Zone Plates (FZP) are to date very successful focusing optics for X-rays. Established methods of fabrication are rather complex and based on electron beam lithography (EBL). Here, we show that ion beam lithography (IBL) may advantageously simplify their preparation. A FZP operable from the extreme UV to the limit of the hard X-ray was prepared and tested from 450 eV to 1500 eV. The trapezoidal profile of the FZP favorably activates its 2nd order focus. The FZP with an outermost zone width of 100 nm allows the visualization of features down to 61, 31 and 21 nm in the 1st, 2nd and 3rd order focus respectively. Measured efficiencies in the 1st and 2nd order of diffraction reach the theoretical predictions.

© 2013 OSA

1. Introduction

X-ray microscopy successfully combines micro- and nano-scale resolutions with high penetration depths and high chemical sensitivity as well as magnetic imaging capabilities with polarized X-rays. Over the years it has established as one of the preferred methods to characterize the microstructure, the composition or the magnetic structure of a wide variety of entire samples in their natural environment [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

4

4. B. Kaulich, P. Thibault, A. Gianoncelli, and M. Kiskinova, “Transmission and emission x-ray microscopy: operation modes, contrast mechanisms and applications,” J. Phys.-Condes. Matter 23,083002 (2011).

]. Developments in the last decades have been characterized by the introduction of new X-ray sources of high brilliance [5

5. J. Gelb, “Functionality to failure: Materials Reegineering in the 4th dimension,” Adv. Mater. Process. 170, 14–18 (2012).

13

13. K. Bourzac, “Tabletop X-rays light up,” Nature 486(7402), 172 (2012). [CrossRef] [PubMed]

] and by a substantial increase of the achievable spatial resolutions [14

14. W. L. Chao, B. D. Harteneck, J. A. Liddle, E. H. Anderson, and D. T. Attwood, “Soft X-ray microscopy at a spatial resolution better than 15 nm,” Nature 435(7046), 1210–1213 (2005). [CrossRef] [PubMed]

17

17. S. R. Wu, Y. Hwu, and G. Margaritondo, “Hard-X-ray zone plates: Recent progress,” Materials 5(12), 1752–1773 (2012). [CrossRef]

]. These improvements suggest and support an even expanded utilization of X-ray microscopy in the future, both at large facilities [6

6. A. Cho, “Materials science. What shall we do with the x-ray laser?” Science 330(6010), 1470–1471 (2010). [CrossRef] [PubMed]

] and in laboratories [5

5. J. Gelb, “Functionality to failure: Materials Reegineering in the 4th dimension,” Adv. Mater. Process. 170, 14–18 (2012).

, 11

11. F. Nachtrab, T. Ebensperger, B. Schummer, F. Sukowski, and R. Hanke, “Laboratory X-ray microscopy with a nano-focus X-ray source,” J. Instrum . 6, C11017 (2011).

]. Nevertheless, the fabrication of corresponding high resolution focusing optics is still complex and challenging.

Up to now the favored strategy towards high resolution has consisted in fabricating FZPs with small Δr. In practice this is a highly challenging nano-structuring issue where electron beam lithography (EBL) based methods have established as standard [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

]. In the extreme UV and soft X-ray range they allow for FZPs resolving 20 to 15 nm features and even smaller [20

20. 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]

22

22. 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]

]. However, though very successful EBL based fabrications are rather complex and typically involves at least 4 different fabrication steps [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

, 20

20. 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]

22

22. 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]

].

Here, we show that ion beam lithography (IBL) can be used as a simple single-step method to efficiently prepare Fresnel Zone Plates for a wide range of wavelengths ranging from the extreme UV to the limit of hard X-rays. A gold-FZP with a diameter of 100 µm, a thickness of 500 nm and a Δr of 100 nm has been prepared and characterized. The resolution in the first diffraction order reaches the expected Rayleigh resolution and 61 nm features could clearly be observed. As the method inherently leads to the fabrication of FZPs with trapezoidal profiles, the 2nd diffraction order is efficiently activated and following Eq. (1), resolutions twice as high as in the 1st order of diffraction were achieved. Structures of 31 nm in size were resolved with good quality and reproducibility. Moreover the achieved fabrication accuracy is high enough to allow for good imaging properties up to the 3rd diffraction order. Structures down to nearly 21 nm in size (half pitch cutoff) could also be resolved.

2. Ion beam lithography as manufacturing technique for Fresnel Zone Plates

Over the past decades, alternatives to the well known EBL based technologies for the fabrication of FZPs were conceived and tested with the aim of either simplifying the preparation, enhancing the resolutions or producing FZPs for shorter wavelength. Multilayer FZPs [23

23. R. M. Bionta, K. M. Skulina, and J. Weinberg, “Hard x-ray sputtered-sliced phase zone plates,” Appl. Phys. Lett. 64(8), 945–947 (1994). [CrossRef]

, 24

24. D. Rudolph, B. Niemann, and G. Schmahl, “Status of the sputtered sliced zone plates for x-ray microscopy,” Proc. Soc. Photo Opt. Instrum. Eng. 316, 103–105 (1981).

] were for instance reported [25

25. 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]

] to achieve higher resolutions at shorter wavelengths and higher throughput. As for the simplification of the preparation ion beam lithography [26

26. A. A. Tseng, “Recent developments in nanofabrication using focused ion beams,” Small 1(10), 924–939 (2005). [CrossRef] [PubMed]

, 27

27. H. D. Wanzenboeck and S. Waid, “Focused ion beam lithography,” in Recent Advances in Nanofabrication Techniques and Applications, P. B. C. (Ed.), ed. (InTech, 2011).

] (IBL) based technologies were also envisaged. They would advantageously permit the fabrication of FZPs in one single-step. This would be interesting in many issues, as a simplification and hence a reduction of the number of required skills for the overall FZP fabrication and a reduction of the typical accumulation of errors characteristic of multi-step processes should in any case allow a faster, easier and precise fabrication. As another advantage, the method requires no additional metallic layers, which are usually employed as plating base in EBL for the further electroplating process and which reduces the overall efficiency of the FZP especially at lower energy. Despite those advantages, IBL had received only little attention to date. First attempt of FZP preparation proved to be difficult and till very recently the imaging properties of the prepared FZPs had not been tested [28

28. P. P. Ilinski, B. Lai, N. J. Bassom, J. Donald, and G. Athas, “X-ray zone plate fabrication using a focused ion beam,” Proceedings of SPIE-The International Society for Optical Engineering 4145, 311–316 (2001). [CrossRef]

30

30. A. Surpi, S. Valizadeh, K. Leifer, and S. Lagomarsino, “Focused ion beam fabrication procedures of x-ray micro Fresnel zone plates,” J. Micromech. Microeng. 17(3), 617–622 (2007). [CrossRef]

]. Nevertheless recently, we could show that FZPs with good imaging properties can indeed be prepared with IBL and features as small as 120 nm could be resolved in the soft X-rays range at 1200 eV [31

31. A. Nadzeyka, L. Peto, S. Bauerdick, M. Mayer, K. Keskinbora, C. Grevent, M. Weigand, M. Hirscher, and G. Schutz, “Ion beam lithography for direct patterning of high accuracy large area X-ray elements in gold on membranes,” Microelectron. Eng. 98, 198–201 (2012). [CrossRef]

]. Other IBL prepared FZPs with resolutions of about 172 nm and their utilization as objective lens in a full field laboratory microscope in the extreme UV range at 13 nm wavelength (95 eV) were also reported a few months later [32

32. J. Overbuschmann, J. Hengster, S. Irsen, and T. Wilhein, “Fabrication of fresnel zone plates by ion beam lithography and application as abjective lenses in EUV microscopy at 13 nm wavelength,” Opt. Lett. in press.

].

3. Fabrication and Design

Two identical FZPs were fabricated and tested. The FZPs were prepared in a polycrystalline 500 nm thick gold layer deposited via ion beam sputtering on a 500 nm thick Silicon Nitride Membrane (Silson Ltd., England). For the fabrication a new IBL system was used (ionLINE®, Raith Germany). This IBL tool is a recent advancement over other conventional focused ion beam (FIB) instruments. It uses an advanced 3D variant patterning software, a laser interferometer stage, up to date ion sources as well as new ion focusing optics providing enhanced emission stabilities and spot shapes. With optimized drift corrections and the enhanced beam stability it allows the milling of very small structures even at large ion beam incident angles which enable the fabrication of FZPs with large diameters. As IBL parameters, we used an ion beam energy of 40 keV, an ion beam current of 50 pA, a radiation dose of 0.35 nC/µm2 and a process time of 15 hours. The precise description of the IBL fabrication process can be found elsewhere [31

31. A. Nadzeyka, L. Peto, S. Bauerdick, M. Mayer, K. Keskinbora, C. Grevent, M. Weigand, M. Hirscher, and G. Schutz, “Ion beam lithography for direct patterning of high accuracy large area X-ray elements in gold on membranes,” Microelectron. Eng. 98, 198–201 (2012). [CrossRef]

]. The placement of the absorbing gold zones follows the standard FZP formula [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

] for a wavelength of 1.03 nm (energy 1200 eV), an outermost zone width of 100 nm and a total diameter of 100 µm. The achieved aspect ratio is thus of 5.

Compared with standard FZPs with a rectangular profile [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

], the present FZPs nevertheless present some particularities. Due to typical redeposition processes in IBL [33

33. L. A. Giannuzzi and F. A. Stevie, eds., Introduction to Focused Ion Beams (Springer, 2005).

] the zones of the FZP present a trapezoidal profile (see Fig. 1
Fig. 1 Representation of the fabricated gold FZP (up and cross-sectional side view): Only a few zones are displayed for sake of simplicity. Positive FZP for which the first zone is filled with gold and the last zone is empty; the zone material is polycrystalline gold; the zone height is 500 nm; the diameter is 100 µm; the outermost zone period Λ is 200 nm (corresponding to an outermost zone width Δr of 100 nm); the number of zones N is 251; α is the angle characteristic of the inclination of the walls as a result of ion beam lithography.
lower part) with slanted walls characterized by angles varying from α = 4° to 6° from the inner part to the outer part of the FZP. The inclination of the walls can be adjusted by varying the ion beam current. The use of lower beam currents potentially allows a reduction of α down to about 2° [28

28. P. P. Ilinski, B. Lai, N. J. Bassom, J. Donald, and G. Athas, “X-ray zone plate fabrication using a focused ion beam,” Proceedings of SPIE-The International Society for Optical Engineering 4145, 311–316 (2001). [CrossRef]

]. The FZP was fabricated without center stop. Additionally, it is a positive FZP, which means that the first and the last zones are open [34

34. A. Michette, Optical Systems for Soft X-Rays (Plenum Press, 1986).

] while in our case the first zone was left full. [Fig. 1]

The ability of a FZP to focus a large range of wavelengths is essentially determined by its thickness and to a lesser extent by chromaticity issues. High FZP thicknesses are crucial because they are necessary to the efficient focusing of high energy radiation. Nevertheless, they are difficult to realize with typical lithography processes, for which the achievement of high aspect ratio is difficult. Here the achieved gold thickness of 500 nm allows good theoretical efficiencies between 7.3% and 21.9% for energies between 30 eV and 8000 eV (Table 1

Table 1. Calculated 1st order diffraction efficiencies for a trapezoidal FZP made out of a 500 nm thick gold layer, Δr = 100 nm and α = 6° within the frame work of the thin grating approximation [19] which is valid as long as the aspect ratio is not too high [35].

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). These efficiencies, were calculated within the framework of the thin grating approximation following the approach proposed by Kirz [19

19. J. Kirz, “Phase zone plates for X-rays and extreme UV,” JOSA 64, 301–309 (1974); Note that there is an error (typo) in the proposed solution of Eq. (12) in this article (Eq. (13) and (14))).

] for trapezoidal FZPs by considering a placement of the zones following the standard design [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

] and an angle α [Fig. 1] characterizing the trapezoid of 6°.

As for chromaticity, FZPs are chromatic optical devices and the placement of their zones is energy dependent [1

1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).

]. Nevertheless, the energy dependence is small. In practice the design of a FZP with 100 nm outermost zone width and 100 µm diameter remains essentially unchanged from 100 eV to 8000 eV with difference in zone placement below 1.5 Å. Such small differences in zone placements can be neglected [36

36. Y. Vladimirsky and H. W. P. Koops, “Moire method and zone plate pattern inaccuracies,” J. Vac. Sci. Technol. B 6(6), 2142–2146 (1988). [CrossRef]

], the present FZPs can be employed to focus radiations from 100 eV to 8000 eV. Below 100 eV differences in zone placements are not negligible anymore and specific FZPs are to be prepared following the same preparation process.

In the case of the present FZP, the trapezoidal profile leads to the activation of a 2nd order of diffraction. The theoretical resolutions and diffraction efficiencies were estimated within the framework of the Takagi-Taupin dynamical diffraction theory [40

40. H. Yan, J. Maser, A. Macrander, Q. Shen, S. Vogt, G. B. Stephenson, and H. C. Kang, “Takagi-Taupin description of x-ray dynamical diffraction from diffractive optics with large numerical aperture,” Phys. Rev. B 76(11), 115438 (2007). [CrossRef]

] [Fig. 2
Fig. 2 Local diffraction efficiency as function of the FZP radius (left) and normalized intensity in the 1st and 2nd order focal plane, where the position of the first minimum is the resolution according to the Rayleigh criterion (right) for a trapezoidal FZP with α = 6°. Calculation based on the Takagi-Taupin dynamical diffraction theory.
]. The model takes possible diffraction phenomena within the volume of the zone into account as they may occur at the edges of the trapezoid. Nevertheless, results showed that for this structure dynamical effects are very weak. At 900 eV in the 1st order focus of the FZP with a trapezoidal profile (α = 6°) the model predicts an overall efficiency of 7.8% which is very similar to the 8.1% predicted by the thin grating approximation (Table 1). In the 2nd order the Takagi-Taupin model foresee an efficiency of 2.05% for α = 6.0° and 2.24% for α = 6.5° at 900 eV.

The utilization of the present FZP which is a positive FZP with a number of zones of 251 instead of negative FZPs which are usual should have no impact on the resolution as for a number of zones higher than 200 the resolution of both positive and negative FZPs approaches the resolution of a simple lens [41

41. D. J. Stiglian, R. Mittra, and R. G. Semonin, “Resolving power of a zone plate,” JOSA 57(5), 610–613 (1967). [CrossRef]

] and are given by Eq. (1).

4. Results

a. Characterization of the FZP

The morphology of the FZP has been investigated by scanning electron microscopy (SEM) [Figs. 3(a)
Fig. 3 Images of the FZP (a) SEM image recorded with an Everhart Thornley Detector (ETD): overview of the FZP manufactured according to the described procedure and further employed to perform imaging (b) SEM image recorded with a Through Lens Detector (TLD): closed view of the outermost zones, showing the 200 nm period (Λ) of the structure. (c) Ion Beam Image recorded at 45° on a cross section of a FZP manufactured under the same condition than the FZP in (a) and (b) and showing the trapezoidal from of the zones (prior to the cross-sectioning the zones are over coated with Pt to ensure their protection).
and 3(b)] and focused ion beam microscopy on a cross section [Fig. 3(c)].

Figure 3(a) is an overview image of the FZP. A few randomly distributed gold particles in the spaces of the FZP are detected [Fig. 3(a)]. They are a result of the polycrystalline nature of the gold layer as grains with specific crystallographic orientations are more resistant to ion beam milling than others. As randomly distributed, the presence of the grains should slightly decrease the efficiency of the FZP but should not affect its resolution. Figure 3(b) confirms the overall period Λ of 200 nm for the outermost zones structures. Note that the gold lines in Fig. 3(b) appear larger than the spaces due to electron scattering on the trapezoidal form of the zones. Figure 3(c) shows a cross section of the outermost zones. The trapezoidal form of the FZP is characterized by a top width of about 50 nm and a bottom width of about 150 nm and 50 nm empty spaces which corresponds to an inclination of the walls of α = 6°. A cross section in the center of the FZP (not shown) demonstrates a slightly lower inclination of the walls of the innermost zones with α = 4 to 5°. The roundness of the FZP was measured in a EBL tool equipped with an interferometer stage and dedicated metrology functionalities (RAITH150TWO) [31

31. A. Nadzeyka, L. Peto, S. Bauerdick, M. Mayer, K. Keskinbora, C. Grevent, M. Weigand, M. Hirscher, and G. Schutz, “Ion beam lithography for direct patterning of high accuracy large area X-ray elements in gold on membranes,” Microelectron. Eng. 98, 198–201 (2012). [CrossRef]

]. The difference in FZP diameter in horizontal and vertical directions has been found to be below 25 nm which is low enough for astigmatism free imaging up to the 3rd order of diffraction (tolerable diameter difference 1.4Δr/mcalculated similarly to [25

25. 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]

, 36

36. Y. Vladimirsky and H. W. P. Koops, “Moire method and zone plate pattern inaccuracies,” J. Vac. Sci. Technol. B 6(6), 2142–2146 (1988). [CrossRef]

]).

b. Imaging performance of the FZP

The focusing performances of the FZP were tested several times at several months intervals on two identical FZPs prepared according to the same procedures at the scanning transmission X-ray microscope (STXM) MAXYMUS located at the undulator UE46 PGM2 at BESSYII, HZB in Berlin [42

42. R. Follath, J. S. Schmidt, M. Weigand, and K. Fauth, “The X-ray microscopy beamline UE46-PGM2 at BESSY,” in Sri 2009: The 10th International Conference on Synchrotron Radiation Instrumentation, R. Garrett, I. Gentle, K. Nugent, and S. Wilkins, eds. (Amer Inst Physics, Melville, 2010), pp. 323–326. [CrossRef]

]. The Beamline was operated with a 600l/mm grating, delivering high photon flux at a reasonable energy resolution. The employed fixfocuskonstante of the monochromator was c = 1.8. The FZP has been tested at various energies ranging from 450 eV to 1500 eV. At energies above 500 eV the undulator was operated in the 3rd harmonic with a linear horizontal polarization while at lower energies below 500 eV the 1st order harmonic and a circular polarization was used. This was necessary to avoid higher harmonic light contaminations at low energies. The amount of higher energy contamination was determined by measuring the absorption of Helium gas between 400 and 900 eV (absorption edge at about 867 eV). After correct alignment and configuration of the whole beamline contamination ratios with light of twice energy were measured to be of about 4%. In case of an improper alignment or configuration, as for instance an improper undulator-gap to monochromator alignment, contaminations up to 20% could be detected. As the FZP is mounted on a 500 nm Silicon Nitride membrane acting as a low energy filter, such contaminations may have dramatic impacts on the results. The full illumination of the FZP was realized by a divergent beam emerging from a 15 x 15 µm2 (HxV) slit situated at the exit of the beamline 3 m upstream from the FZP. An order selecting aperture (OSA) of 35 µm diameter was placed between the FZP and the sample to eliminate higher diffraction orders and a part of the zero order diffraction radiation. Images of a commercially available test object, a Siemens Star (X-30-30-1, Xradia USA) with smallest feature size 30 nm were acquired at 1500, 1200, 900, 600 and 450 eV in the first, second and third order focus of the zone plate. Images of a second test object prepared by mean of a focused ion beam as a 500 nm cross section from a commercial BAM certified multilayer (L200) were recorded at 1200 eV with a beam slit size of 10 x 10 µm2 (HxV) at the 1st, 2nd and 3rd order focus of the FZP to determine its ultimate resolution. The acquired images as well as the employed dwell times and the cutoff half-pitch resolutions determined from the power spectra of the respective images according to the procedure described in [43

43. D. C. Joy, “SMART - a program to measure SEM resolution and imaging performance,” J. Microsc.-Oxf. 208, 24–34 (2002).

] are presented in Fig. 4
Fig. 4 (a) SEM image of a Siemens-star test object (b) (c) (d) Scanning X-ray microscopy of same test object at the 1st 2nd and 3rd order of diffraction acquired at 900 eV with a pixel size 10 nm and a dwell time of 10ms. The half pitch resolution was determined from the power spectrum of these images and are collected in Table 2
, Fig. 5
Fig. 5 Scanning X-ray microscopy image of a certified commercial test sample (BAM L200) recorded at 1200 eV (a) image acquired in the 2nd order focus of the FZP with a pixel size of 5 nm and a dwell time of 10 ms (b) image acquired in the 3rd order focus of the FZP with a pixel size of 5 nm and a dwell time of 15 ms (c) schematic representation of the certified test object (d) width of the features (half pitch)
and Table 2

Table 2. Spatial resolution performances obtained with the FZP in the 1st, 2nd and 3rd order focus. Where η is the efficiency δRay/2is the half-pitch (half-period) Rayleigh resolution and δcutoff/2 is the half-pitch cutoff resolution determined from the power spectra of the images [Fig. 4(a) to (c)] and visually from the images [Fig. 5].

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.

(a) Calculated according to Eq. (1) (b) the half pitch cutoff resolution (δcutoff/2) is defined as the feature size below which no modulation can be detected, it is comparable to the half pitch Sparrow resolution limit [44

44. H. Gross, H. Zügge, M. Peschka, and F. Blechinger, Handbook of Optical Systems, Vol 3 (Wiley-VCH Verlag GmbH & Co. 2007).

] which is equal to δSpa/2=0.475Δr it is hence possible to detect features smaller than the Rayleigh resolutions (c) determined from the power spectra corresponding to the images in Fig. 4 (d) visually determined from the images and from the known certified values in Fig. 5.

The efficiency of the FZP in the first diffraction order at various energies and in the 2nd diffraction order at 900 eV was determined experimentally as the ratio of light impinging on the FZP to the light focused by the FZP onto the detector through a 35 µm OSA, corrected for the 0th and higher order radiation according to the procedure described in [25

25. 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 efficiencies at the first order focus are given in Fig. 6
Fig. 6 Measured and theoretical efficiencies for the first order focus of the FZP. The theoretical efficiencies for a trapezoidal FZP were calculated for an angle α = 6° within the frame work of the thin grating theory (TG) and the Takagi-Taupin dynamical diffraction theory (TT-DDT). Note that in the real structure the angle α takes value between 4° and 6° which could explain the differences between theoretical and measured values.
. The efficiency of the second diffraction order focus was measured to be 2.2% at 900 eV, which is in very good agreement with the efficiencies predicted by the Takagi-Taupin dynamical diffraction theory. This is in turn a sign of the high accuracy of the zone placement and of the fabrication. This is equally well illustrated by the high quality and contrast of the images obtained by X-ray scanning microscopy [Fig. 4 and Fig. 5].

5. Conclusion

Acknowledgments

The Authors thank Ulrike Eigenthaler and Dr. Michael Hirscher (Max-Planck-Institute for Intelligent Systems) for their support with the DualBeam, as well as Bernd Ludescher (Max-Planck-Institute for Intelligent Systems) for the gold layer preparation. In addition we want to thank Dr. Hermann Stoll (Max-Planck-Institute for Intelligent Systems), Janos Kirz and Tolek Tyliszczak (Advanced Light Source, Lawrence Berkeley National Laboratory) for the fruitful discussions. H. Yan would like to acknowledge the support by the Department of Energy, Office of Basic Energy Sciences under contract DE-AC-02-98CH10886.

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T. Popmintchev, M. C. Chen, D. Popmintchev, P. Arpin, S. Brown, S. Alisauskas, G. Andriukaitis, T. Balciunas, O. D. Mücke, A. Pugzlys, A. Baltuska, B. Shim, S. E. Schrauth, A. Gaeta, C. Hernández-García, L. Plaja, A. Becker, A. Jaron-Becker, M. M. Murnane, and H. C. Kapteyn, “Bright Coherent Ultrahigh Harmonics in the keV x-ray Regime from Mid-Infrared Femtosecond Lasers,” Science 336(6086), 1287–1291 (2012). [CrossRef] [PubMed]

10.

S. P. D. Mangles, “Compact X-ray sources: X-rays from self-reflection,” Nat. Photonics 6(5), 280–281 (2012). [CrossRef]

11.

F. Nachtrab, T. Ebensperger, B. Schummer, F. Sukowski, and R. Hanke, “Laboratory X-ray microscopy with a nano-focus X-ray source,” J. Instrum . 6, C11017 (2011).

12.

E. L. Ritman, “Current Status of Developments and Applications of Micro-CT,” in Annual Review of Biomedical Engineering, Vol 13, M. L. Yarmush, J. S. Duncan, and M. L. Gray, eds. (2011), pp. 531–552.

13.

K. Bourzac, “Tabletop X-rays light up,” Nature 486(7402), 172 (2012). [CrossRef] [PubMed]

14.

W. L. Chao, B. D. Harteneck, J. A. Liddle, E. H. Anderson, and D. T. Attwood, “Soft X-ray microscopy at a spatial resolution better than 15 nm,” Nature 435(7046), 1210–1213 (2005). [CrossRef] [PubMed]

15.

A. Guilherme, G. Buzanich, and M. L. Carvalho, “Focusing systems for the generation of X-ray micro beam: An overview,” Spectroc. Acta Pt. B-Atom. Spectr. 77, 1–8 (2012).

16.

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]

17.

S. R. Wu, Y. Hwu, and G. Margaritondo, “Hard-X-ray zone plates: Recent progress,” Materials 5(12), 1752–1773 (2012). [CrossRef]

18.

A. Erko, V. Aristov, and B. Vidal, Diffraction X-Ray Optics (Taylor and Francis, 1996).

19.

J. Kirz, “Phase zone plates for X-rays and extreme UV,” JOSA 64, 301–309 (1974); Note that there is an error (typo) in the proposed solution of Eq. (12) in this article (Eq. (13) and (14))).

20.

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]

21.

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]

22.

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]

23.

R. M. Bionta, K. M. Skulina, and J. Weinberg, “Hard x-ray sputtered-sliced phase zone plates,” Appl. Phys. Lett. 64(8), 945–947 (1994). [CrossRef]

24.

D. Rudolph, B. Niemann, and G. Schmahl, “Status of the sputtered sliced zone plates for x-ray microscopy,” Proc. Soc. Photo Opt. Instrum. Eng. 316, 103–105 (1981).

25.

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]

26.

A. A. Tseng, “Recent developments in nanofabrication using focused ion beams,” Small 1(10), 924–939 (2005). [CrossRef] [PubMed]

27.

H. D. Wanzenboeck and S. Waid, “Focused ion beam lithography,” in Recent Advances in Nanofabrication Techniques and Applications, P. B. C. (Ed.), ed. (InTech, 2011).

28.

P. P. Ilinski, B. Lai, N. J. Bassom, J. Donald, and G. Athas, “X-ray zone plate fabrication using a focused ion beam,” Proceedings of SPIE-The International Society for Optical Engineering 4145, 311–316 (2001). [CrossRef]

29.

R. Garg, J. Evertsen, and G. Denbeaux, “Novel method for fabrication of high efficiency optics for short wavelength radiation - art. no. 61100S,” in Micromaching Technology for Micro-optics and Nano-Optics IV, E. G. Johnson, G. P. Nordin, and T. J. Suleski, eds. (Spie-Int Soc Optical Engineering, 2006), pp. S1100–S1100.

30.

A. Surpi, S. Valizadeh, K. Leifer, and S. Lagomarsino, “Focused ion beam fabrication procedures of x-ray micro Fresnel zone plates,” J. Micromech. Microeng. 17(3), 617–622 (2007). [CrossRef]

31.

A. Nadzeyka, L. Peto, S. Bauerdick, M. Mayer, K. Keskinbora, C. Grevent, M. Weigand, M. Hirscher, and G. Schutz, “Ion beam lithography for direct patterning of high accuracy large area X-ray elements in gold on membranes,” Microelectron. Eng. 98, 198–201 (2012). [CrossRef]

32.

J. Overbuschmann, J. Hengster, S. Irsen, and T. Wilhein, “Fabrication of fresnel zone plates by ion beam lithography and application as abjective lenses in EUV microscopy at 13 nm wavelength,” Opt. Lett. in press.

33.

L. A. Giannuzzi and F. A. Stevie, eds., Introduction to Focused Ion Beams (Springer, 2005).

34.

A. Michette, Optical Systems for Soft X-Rays (Plenum Press, 1986).

35.

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]

36.

Y. Vladimirsky and H. W. P. Koops, “Moire method and zone plate pattern inaccuracies,” J. Vac. Sci. Technol. B 6(6), 2142–2146 (1988). [CrossRef]

37.

A. Kuyumchyan, A. A. Isoyan, E. V. Shulakov, V. V. Aristov, M. Kondratenkov, A. A. Snigirev, I. Snigireva, A. Souvorov, K. Tamasaku, M. Yabashi, T. Ishikawa, and K. Trouni, “High-efficiency and low-absorption Fresnel compound zone plates for hard X-ray focusing,” Proc. SPIE 4783, 92–96 (2002).

38.

M. Baciocchi, R. Maggiora, and M. Gentili, “High resolution fresnel zone plates for soft x-rays,” Microelectron. Eng. 23(1-4), 101–104 (1994). [CrossRef]

39.

J. Yi, Y. S. Chu, Y.-T. Chen, T.-Y. Chen, Y. Hwu, and G. Margaritondo, “High-resolution hard-x-ray microscopy using second-order zone-plate diffraction,” J. Phys. D Appl. Phys. 44(23), 232001 (2011). [CrossRef]

40.

H. Yan, J. Maser, A. Macrander, Q. Shen, S. Vogt, G. B. Stephenson, and H. C. Kang, “Takagi-Taupin description of x-ray dynamical diffraction from diffractive optics with large numerical aperture,” Phys. Rev. B 76(11), 115438 (2007). [CrossRef]

41.

D. J. Stiglian, R. Mittra, and R. G. Semonin, “Resolving power of a zone plate,” JOSA 57(5), 610–613 (1967). [CrossRef]

42.

R. Follath, J. S. Schmidt, M. Weigand, and K. Fauth, “The X-ray microscopy beamline UE46-PGM2 at BESSY,” in Sri 2009: The 10th International Conference on Synchrotron Radiation Instrumentation, R. Garrett, I. Gentle, K. Nugent, and S. Wilkins, eds. (Amer Inst Physics, Melville, 2010), pp. 323–326. [CrossRef]

43.

D. C. Joy, “SMART - a program to measure SEM resolution and imaging performance,” J. Microsc.-Oxf. 208, 24–34 (2002).

44.

H. Gross, H. Zügge, M. Peschka, and F. Blechinger, Handbook of Optical Systems, Vol 3 (Wiley-VCH Verlag GmbH & Co. 2007).

45.

J. Gierak, E. Bourhis, M. N. M. Combes, Y. Chriqui, I. Sagnes, D. Mailly, P. Hawkes, R. Jede, L. Bruchhaus, L. Bardotti, B. Prevel, A. Hannour, P. Melinon, A. Perez, J. Ferre, J. P. Jamet, A. Mougin, C. Chappert, and V. Mathet, “Exploration of the ultimate patterning potential achievable with focused ion beams,” Microelectron. Eng. 78–79, 266–278 (2005). [CrossRef]

46.

J. Gierak, E. Bourhis, G. Faini, G. Patriarche, A. Madouri, R. Jede, L. Bruchhaus, S. Bauerdick, B. Schiedt, A. L. Biance, and L. Auvray, “Exploration of the ultimate patterning potential achievable with focused ion beams,” Ultramicroscopy 109(5), 457–462 (2009). [CrossRef] [PubMed]

47.

J. E. E. Baglin, “Ion beam nanoscale fabrication and lithography-A review,” Appl. Surf. Sci. 258(9), 4103–4111 (2012). [CrossRef]

48.

L. Bruchhaus, S. Bauerdick, L. Peto, U. Barth, A. Rudzinski, J. Mussmann, J. Klingfus, J. Gierak, and H. Hovel, “High resolution and high density ion beam lithography employing HSQ resist,” Microelectron. Eng. 97, 48–50 (2012). [CrossRef]

OCIS Codes
(180.5810) Microscopy : Scanning microscopy
(180.7460) Microscopy : X-ray microscopy
(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

ToC Category:
Microscopy

History
Original Manuscript: January 29, 2013
Revised Manuscript: April 15, 2013
Manuscript Accepted: April 17, 2013
Published: May 7, 2013

Virtual Issues
Vol. 8, Iss. 6 Virtual Journal for Biomedical Optics

Citation
Kahraman Keskinbora, Corinne Grévent, Michael Bechtel, Markus Weigand, Eberhard Goering, Achim Nadzeyka, Lloyd Peto, Stefan Rehbein, Gerd Schneider, Rolf Follath, Joan Vila-Comamala, Hanfei Yan, and Gisela Schütz, "Ion beam lithography for Fresnel zone plates in X-ray microscopy," Opt. Express 21, 11747-11756 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-10-11747


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References

  1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge University Press, 1999).
  2. A. Sakdinawat and D. Attwood, “Nanoscale X-ray imaging,” Nat. Photonics4(12), 840–848 (2010). [CrossRef]
  3. R. Falcone, C. Jacobsen, J. Kirz, S. Marchesini, D. Shapiro, and J. Spence, “New directions in X-ray microscopy,” Contemp. Phys.52(4), 293–318 (2011). [CrossRef]
  4. B. Kaulich, P. Thibault, A. Gianoncelli, and M. Kiskinova, “Transmission and emission x-ray microscopy: operation modes, contrast mechanisms and applications,” J. Phys.-Condes. Matter23,083002 (2011).
  5. J. Gelb, “Functionality to failure: Materials Reegineering in the 4th dimension,” Adv. Mater. Process.170, 14–18 (2012).
  6. A. Cho, “Materials science. What shall we do with the x-ray laser?” Science330(6010), 1470–1471 (2010). [CrossRef] [PubMed]
  7. 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]
  8. M. Fuchs, R. Weingartner, A. Popp, Z. Major, S. Becker, J. Osterhoff, I. Cortrie, B. Zeitler, R. Horlein, G. D. Tsakiris, U. Schramm, T. P. Rowlands-Rees, S. M. Hooker, D. Habs, F. Krausz, S. Karsch, and F. Gruner, “Laser-driven soft-X-ray undulator source,” Nat. Phys.5(11), 826–829 (2009). [CrossRef]
  9. T. Popmintchev, M. C. Chen, D. Popmintchev, P. Arpin, S. Brown, S. Alisauskas, G. Andriukaitis, T. Balciunas, O. D. Mücke, A. Pugzlys, A. Baltuska, B. Shim, S. E. Schrauth, A. Gaeta, C. Hernández-García, L. Plaja, A. Becker, A. Jaron-Becker, M. M. Murnane, and H. C. Kapteyn, “Bright Coherent Ultrahigh Harmonics in the keV x-ray Regime from Mid-Infrared Femtosecond Lasers,” Science336(6086), 1287–1291 (2012). [CrossRef] [PubMed]
  10. S. P. D. Mangles, “Compact X-ray sources: X-rays from self-reflection,” Nat. Photonics6(5), 280–281 (2012). [CrossRef]
  11. F. Nachtrab, T. Ebensperger, B. Schummer, F. Sukowski, and R. Hanke, “Laboratory X-ray microscopy with a nano-focus X-ray source,” J. Instrum. 6, C11017 (2011).
  12. E. L. Ritman, “Current Status of Developments and Applications of Micro-CT,” in Annual Review of Biomedical Engineering, Vol 13, M. L. Yarmush, J. S. Duncan, and M. L. Gray, eds. (2011), pp. 531–552.
  13. K. Bourzac, “Tabletop X-rays light up,” Nature486(7402), 172 (2012). [CrossRef] [PubMed]
  14. W. L. Chao, B. D. Harteneck, J. A. Liddle, E. H. Anderson, and D. T. Attwood, “Soft X-ray microscopy at a spatial resolution better than 15 nm,” Nature435(7046), 1210–1213 (2005). [CrossRef] [PubMed]
  15. A. Guilherme, G. Buzanich, and M. L. Carvalho, “Focusing systems for the generation of X-ray micro beam: An overview,” Spectroc. Acta Pt. B-Atom. Spectr.77, 1–8 (2012).
  16. G. E. Ice, J. D. Budai, and J. W. L. Pang, “The race to X-ray microbeam and nanobeam science,” Science334(6060), 1234–1239 (2011). [CrossRef] [PubMed]
  17. S. R. Wu, Y. Hwu, and G. Margaritondo, “Hard-X-ray zone plates: Recent progress,” Materials5(12), 1752–1773 (2012). [CrossRef]
  18. A. Erko, V. Aristov, and B. Vidal, Diffraction X-Ray Optics (Taylor and Francis, 1996).
  19. J. Kirz, “Phase zone plates for X-rays and extreme UV,” JOSA 64, 301–309 (1974); Note that there is an error (typo) in the proposed solution of Eq. (12) in this article (Eq. (13) and (14))).
  20. 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,” Ultramicroscopy109(11), 1360–1364 (2009). [CrossRef] [PubMed]
  21. 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. Express20(9), 9777–9783 (2012). [CrossRef] [PubMed]
  22. 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. Express20(6), 5830–5839 (2012). [CrossRef] [PubMed]
  23. R. M. Bionta, K. M. Skulina, and J. Weinberg, “Hard x-ray sputtered-sliced phase zone plates,” Appl. Phys. Lett.64(8), 945–947 (1994). [CrossRef]
  24. D. Rudolph, B. Niemann, and G. Schmahl, “Status of the sputtered sliced zone plates for x-ray microscopy,” Proc. Soc. Photo Opt. Instrum. Eng.316, 103–105 (1981).
  25. 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,” Ultramicroscopy111(12), 1706–1711 (2011). [CrossRef] [PubMed]
  26. A. A. Tseng, “Recent developments in nanofabrication using focused ion beams,” Small1(10), 924–939 (2005). [CrossRef] [PubMed]
  27. H. D. Wanzenboeck and S. Waid, “Focused ion beam lithography,” in Recent Advances in Nanofabrication Techniques and Applications, P. B. C. (Ed.), ed. (InTech, 2011).
  28. P. P. Ilinski, B. Lai, N. J. Bassom, J. Donald, and G. Athas, “X-ray zone plate fabrication using a focused ion beam,” Proceedings of SPIE-The International Society for Optical Engineering4145, 311–316 (2001). [CrossRef]
  29. R. Garg, J. Evertsen, and G. Denbeaux, “Novel method for fabrication of high efficiency optics for short wavelength radiation - art. no. 61100S,” in Micromaching Technology for Micro-optics and Nano-Optics IV, E. G. Johnson, G. P. Nordin, and T. J. Suleski, eds. (Spie-Int Soc Optical Engineering, 2006), pp. S1100–S1100.
  30. A. Surpi, S. Valizadeh, K. Leifer, and S. Lagomarsino, “Focused ion beam fabrication procedures of x-ray micro Fresnel zone plates,” J. Micromech. Microeng.17(3), 617–622 (2007). [CrossRef]
  31. A. Nadzeyka, L. Peto, S. Bauerdick, M. Mayer, K. Keskinbora, C. Grevent, M. Weigand, M. Hirscher, and G. Schutz, “Ion beam lithography for direct patterning of high accuracy large area X-ray elements in gold on membranes,” Microelectron. Eng.98, 198–201 (2012). [CrossRef]
  32. J. Overbuschmann, J. Hengster, S. Irsen, and T. Wilhein, “Fabrication of fresnel zone plates by ion beam lithography and application as abjective lenses in EUV microscopy at 13 nm wavelength,” Opt. Lett.in press.
  33. L. A. Giannuzzi and F. A. Stevie, eds., Introduction to Focused Ion Beams (Springer, 2005).
  34. A. Michette, Optical Systems for Soft X-Rays (Plenum Press, 1986).
  35. 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]
  36. Y. Vladimirsky and H. W. P. Koops, “Moire method and zone plate pattern inaccuracies,” J. Vac. Sci. Technol. B6(6), 2142–2146 (1988). [CrossRef]
  37. A. Kuyumchyan, A. A. Isoyan, E. V. Shulakov, V. V. Aristov, M. Kondratenkov, A. A. Snigirev, I. Snigireva, A. Souvorov, K. Tamasaku, M. Yabashi, T. Ishikawa, and K. Trouni, “High-efficiency and low-absorption Fresnel compound zone plates for hard X-ray focusing,” Proc. SPIE4783, 92–96 (2002).
  38. M. Baciocchi, R. Maggiora, and M. Gentili, “High resolution fresnel zone plates for soft x-rays,” Microelectron. Eng.23(1-4), 101–104 (1994). [CrossRef]
  39. J. Yi, Y. S. Chu, Y.-T. Chen, T.-Y. Chen, Y. Hwu, and G. Margaritondo, “High-resolution hard-x-ray microscopy using second-order zone-plate diffraction,” J. Phys. D Appl. Phys.44(23), 232001 (2011). [CrossRef]
  40. H. Yan, J. Maser, A. Macrander, Q. Shen, S. Vogt, G. B. Stephenson, and H. C. Kang, “Takagi-Taupin description of x-ray dynamical diffraction from diffractive optics with large numerical aperture,” Phys. Rev. B76(11), 115438 (2007). [CrossRef]
  41. D. J. Stiglian, R. Mittra, and R. G. Semonin, “Resolving power of a zone plate,” JOSA57(5), 610–613 (1967). [CrossRef]
  42. R. Follath, J. S. Schmidt, M. Weigand, and K. Fauth, “The X-ray microscopy beamline UE46-PGM2 at BESSY,” in Sri 2009: The 10th International Conference on Synchrotron Radiation Instrumentation, R. Garrett, I. Gentle, K. Nugent, and S. Wilkins, eds. (Amer Inst Physics, Melville, 2010), pp. 323–326. [CrossRef]
  43. D. C. Joy, “SMART - a program to measure SEM resolution and imaging performance,” J. Microsc.-Oxf.208, 24–34 (2002).
  44. H. Gross, H. Zügge, M. Peschka, and F. Blechinger, Handbook of Optical Systems, Vol 3 (Wiley-VCH Verlag GmbH & Co. 2007).
  45. J. Gierak, E. Bourhis, M. N. M. Combes, Y. Chriqui, I. Sagnes, D. Mailly, P. Hawkes, R. Jede, L. Bruchhaus, L. Bardotti, B. Prevel, A. Hannour, P. Melinon, A. Perez, J. Ferre, J. P. Jamet, A. Mougin, C. Chappert, and V. Mathet, “Exploration of the ultimate patterning potential achievable with focused ion beams,” Microelectron. Eng.78–79, 266–278 (2005). [CrossRef]
  46. J. Gierak, E. Bourhis, G. Faini, G. Patriarche, A. Madouri, R. Jede, L. Bruchhaus, S. Bauerdick, B. Schiedt, A. L. Biance, and L. Auvray, “Exploration of the ultimate patterning potential achievable with focused ion beams,” Ultramicroscopy109(5), 457–462 (2009). [CrossRef] [PubMed]
  47. J. E. E. Baglin, “Ion beam nanoscale fabrication and lithography-A review,” Appl. Surf. Sci.258(9), 4103–4111 (2012). [CrossRef]
  48. L. Bruchhaus, S. Bauerdick, L. Peto, U. Barth, A. Rudzinski, J. Mussmann, J. Klingfus, J. Gierak, and H. Hovel, “High resolution and high density ion beam lithography employing HSQ resist,” Microelectron. Eng.97, 48–50 (2012). [CrossRef]

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