Micro-imaging performance of multilayers used as monochromators for coherent hard X-ray synchrotron...

Micro-imaging performance of multilayers used as monochromators for coherent hard X-ray synchrotron radiation

A. Rack*a, T. Weitkampa, M. Riotteb, T. Rackb, R. Dietschc, T. Holzc, M. Krämerc, F. Siewertd,

M. Medunae, Ch. Morawea, P. Cloetensa, E. Zieglera

aEuropean Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France; bKarlsruhe Institute of Technology - ANKA, Pf. 3640, 76021 Karlsruhe, Germany;

cAXO Dresden GmbH, Siegfried-Rädel-Str. 31, 01809 Heidenau, Germany; dHelmholtz Zentrum Berlin / BESSY-II, Albert-Einstein-Str. 1, 12489 Berlin, Germany;

eMasaryk University, Kotlářská 267/2, 61137 Brno, Czech Republic

ABSTRACT

We present a systematic study in which multilayers of different composition (W/Si, Mo/Si, Pd/B4C), periodicity (from 2.5 to 5.5 nm), and numbers of layers have been characterised. Particularly, we investigated the intrinsic quality (roughness and reflectivity) as well as the performance (flatness and coherence of the outgoing beam) as a monochromator for synchrotron radiation hard X-ray micro-imaging. The results indicate that the material composition is the dominating factor for the performance. This is of high importance for synchrotron-based hard X-ray imaging which has become a widely applied tool for probing the microstructure of bulk samples. The high spatial resolution and different contrast modalities available here strongly depend on using coherent beams from highly brilliant sources. In order to satisfy the demand for a high flux of quasi-monochromatic photons, multilayer-coated mirrors are commonly used as monochromators. Their properties present a good tradeoff between spectral bandwidth and photon flux density. Since the photon flux density at the sample position is higher than with standard crystal monochromators, better spatial resolution can be reached. This comes at the cost of reduced energy resolution and stronger non-uniformities in the incoming beam profile. By helping scientists and engineers specify the design parameters of multilayer monochromators, our results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal-based devices; i.e., larger spectral bandwidth and high photon flux density for X-ray imaging.

Keywords: Multilayer mirrors, X-ray imaging, coherence, synchrotron radiation, X-ray monochromators, X-ray phase contrast, X-ray optics

1. INTRODUCTION Hard X-ray micro-imaging in two and three dimensions using synchrotron light sources is an established analytical technique, applied to a broad area of scientific fields, ranging from materials research to life sciences and paleontology 1,2,3,4. The performance of such an imaging station is directly connected to the coherence properties as well as the monochromatic photon flux density available. Using monochromators in X-ray imaging increases the contrast while reducing artefacts 5,6,7. Employing coherent and monochromatic radiation gives access to more sophisticated contrast modalities like X-ray inline phase contrast or holotomography 8,9.

An established approach in order to increase the available photon flux density while preserving most of the coherence properties of the beam is the use of multilayer monochromators instead of crystal monochromators. Due to the bandwidth of a multilayer which is typically one or two orders of magnitude larger than that of a crystal and the fact that common synchrotron sources emit a similar or even larger energy band, a gain in the photon flux density with the same order of magnitude can be achieved 10. The drawback of employing multilayer mirrors as monochromators are the inhomogeneities introduced in the beam profile, typical examples are displayed in Figure 1. These stripe patterns are not always removable by a flat-field correction, either due to beam instabilities and/or the finite point-spread function of X-ray imaging detectors, leading to artefacts in the tomographic reconstruction, cf. Figure 2 11. Furthermore, they prevent exploiting the full dynamic range of the detector which can lead to, e. g., different signal-to-noise ratios within the image.

*[email protected]; phone +33 476 88-1781; fax +33 476 88-2785; www.esrf.eu

Advances in X-Ray/EUV Optics and Components V, edited by Ali M. Khounsary, Christian Morawe, Shunji Goto, Proc. of SPIE Vol. 7802, 78020M · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.858355

Proc. of SPIE Vol. 7802 78020M-1

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In this article, results of a study will be shown which highlight that the material composition is the dominating factor when trying to reduce the intensity of the stripe pattern caused by the multilayer reflection. While no explanation can be given for the effects seen, these results can help scientists specify the design parameters of future multilayer monochromators and therefore lead to a better exploitation of their advances. Consequently, this will extend their potential for X-ray imaging applications 12,13.

Figure 1. Two examples of stripe modulations in the background of an image after reflection by multilayer mirrors (not the multilayer samples studied in this proceedings article). The left image is taken at the bending magnet (BM) beamline B16 of the Diamond light source (UK) at 15 keV, with the multilayer coating consisting of Ru/B4C (170 bi-layers, d-spacing = 2.8 nm) 14. The right image was taken at the wavelength-shifter (WLS) insertion device beamline BAMline of the BESSY-II light source (Germany) at 18 keV with the multilayer coating consisting of 150 bi-layers W/Si (d-spacing = 2.88 nm) 15. Both experimental stations employ a double multilayer monochromator (DMM).

2. MOTIVATION As stated above, the irregular stripe patterns introduced in the beam profile by reflection on a multilayer mirror have a negative effect on the performance of an X-ray imaging station. This is mainly related to the fact that the stripe patterns often to a certain degree remain after a flat-field correction, cf. Figure 2 (top). In case that a significant amount of radiographic projections is affected by these stripes during a tomographic scan, then the stripes are reproduced by the tomographic reconstruction. Hence, the acquired volume images are modulated with the stripe pattern as well (cf. Figure 2, bottom). This leads to all kinds of further artefacts in the horizontal slices.

In order to specify a double-multilayer monochromator for hard X-ray imaging applications at the bending-magnet beamline TopoTomo of the ANKA synchrotron light source (Karlsruhe, Germany), a study was performed. The aim was to figure out optimal parameters of a multilayer mirror in order to tune its performance with respect to i) its integral reflectivity in the available X-ray energy range, ii) the stripe pattern in the images' background, and iii) coherence properties of the reflected beam 16,17,18,19.

Despite the fact that multilayer monochromators are more and more employed at synchrotron light sources around the globe (e. g., the TOMCAT beamline at the Swiss Light Source installed a DMM with two stripes (Ru/C and W/Si), at the Advanced Photon Source (APS) a DMM is installed at the micro-imaging station 2-BM (W/B4C), at the ANKA storage ring one more DMM besides the already mentioned is installed at the FLUO beamline (W/Si), the Pohang Light Source (PLS) at the 1B2/microprobe beamline uses a DMM (W/Si), and a multilayer monochromator is installed at the tomography beamline of the CAMD synchrotron with a W/B4C coating), there is no formalism relating the performance of multilayer mirrors to their structural quality in a quantitative and rigorous manner. Hence, a rather broad parameter range was chosen: three material compositions (Mo/Si, W/Si, Pd/B4C) with different d-spacings (nominal 2.5 nm, 4.0 nm and 5.5 nm) and number of layers (from 30 to 220 bi-layers) 16.

0.5 mm0.5 mm

BM DMM EXP 24 m 23 m

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WLS DMM EXP 20 m 17 m



Figure 2. Examples of artefacts due to non-perfect flat-field correction of the stripe pattern in the background of the

images caused by the use of a multilayer monochromator. The top image shows a radiography of an almost homogeneous tooth tissue, the bottom image a sagittal cut through a tomographic reconstruction of a similar specimen (samples kindly provided by the RWTH Aachen, images taken at 25 keV photon energy and 1.6 µm effective pixel size at the BAMline of the BESSY-II light source 19).

3. MULTILAYERS The multilayer coatings were deposited on commercial one-sided superpolished silicon single crystals (General Optics, Gooch & Housego), each 25.40 mm in diameter and 6.35 mm thick. Characterization of the substrate surfaces prior to the coating was carried out at the metrology laboratory of the BESSY-II light source, Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Germany 20,21,22. All substrates were found to have a homogeneous high level of surface quality. AXO Dresden GmbH deposited the multilayer structures by means of magnetron sputtering 23. The surfaces of the multilayers were characterized, again at the metrology laboratory of BESSY-II. Exemplary results of these measurements are depicted in Figure 3.

The samples were further analysed using specular and non-specular X-ray reflectivity, full-field imaging as well as fractional Talbot imaging. Here, Mo/Si (2.48 nm d-spacing, 200 bi-layers, Γ = 0.5), Pd/B4C (2.47 nm d-spacing, 220 bi-layers, Γ = 0.5) and W/Si (2.53 nm d-spacing, 120 bi-layers, Γ = 0.5) showed similar surface and interlayer roughness. The results presented in this proceedings article are focused on these specific samples. Further results have been published elsewhere 16. Full-field imaging performed at ANKA (TopoTomo beamline 18,19) with a beam reflected by one of the three mirrors showed that the maximum spatial resolution reachable does not depend on the multilayer composition employed.

In order to compare our results with a multilayer mirror of larger dimensions we characterised a Ru/B4C mirror (3.91 nm d-spacing, 65 bi-layers, Γ = 0.5, 10 mm width, 300 mm length) installed at the beamline ID19 of the European

0.5 mm



Synchrotron Radiation Facility (ESRF) by means of full-field and fractional Talbot imaging 27,28. The coating was deposited at the ESRF multilayer laboratory 24,25.

Figure 3. The micro-roughness (high spatial frequency) of multilayer coatings consisting of Mo/Si (left), Pd/B4C (middle)

and W/Si (right), measured with a Micromap Promap 520 interference microscope (20x magnification) at the metrology laboratory of the BESSY-II light source 16,20,21,22.

4. HARD X-RAY MICRO-IMAGING PERFORMANCE The results of the full-field and the fractional Talbot imaging analysis are displayed in Figure 4. For the three test samples, namely Mo/Si (2.48 nm d-spacing, 200 bi-layers, Γ = 0.5), Pd/B4C (2.47 nm d-spacing, 220 bi-layers, Γ = 0.5) and W/Si (2.53 nm d-spacing, 120 bi-layers, Γ = 0.5) one can note strong differences between the irregular stripe pattern in the beam profile, cf. Figure 4 (left column). While for the Mo/Si sample the stripes are strongly pronounced, they are much weaker for W/Si and almost disappear for Pd/B4C. As the surface roughness of the three substrates employed was similar and as well the layer and interlayer roughness showed comparable values for the three samples under study, these results indicate that the material composition is the dominating factor which influences the beam profile after reflection. The Ru/B4C on its longer substrate (ESRF multilayer laboratory) shows a stripe pattern which seems to be on the same level as for Mo/Si.

The coherence properties of the reflected beam were investigated by means of Talbot imaging, see as well Figure 4 (right column). Mo/Si, which shows the most pronounced stripe pattern in the beam profile, affects the coherence properties of the beam less than Pd/B4C and much less than W/Si. It is important to note that the coherence properties of the reflected beam cannot be ranked in the same order as the effect of the stripe pattern: while Pd/B4C shows the smoothest beam profile of all samples under study, it conserves the coherence of the beam better than W/Si. The Ru/B4C multilayer shows a well pronounced stripe pattern and preserves the coherence of the beam, similar to the Mo/Si multilayer.

5. SUMMARY A study of the influence of different multilayer compositions on the properties of the reflected hard X-ray synchrotron beam has been performed. Selected results have been shown as part of this proceedings article. It was briefly indicated that the material composition is the dominating factor in order to acquire a smooth beam profile in combination with a coherent beam. Both characteristics are important in order to extend the potential of multilayers in use at monochromators for synchrotron-based radiography and tomography.

For the future it is planned to test further material compositions in a similar manner as described above, which is mainly full-field imaging and Talbot imaging. It is worth to mention that Pd/B4C, which has not been reported before to have been used as monochromator for hard X-ray imaging applications is now employed as part of a double-multilayer monochromoator at the bending-magnet beamline TopoTomo of the ANKA light source (Karlsruhe, Germany) 17.



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Figure 4. The left images show stripe patterns due to reflection on one of the multilayer mirrors under study, from top to bottom: Mo/Si (2.48 nm d-spacing, 200 bilayers, Γ = 0.5), Pd/B4C (2.47 nm d-spacing, 220 bilayers, Γ = 0.5), W/Si (2.53 nm d-spacing, 120 bilayers, Γ = 0.5, all acquired at BM05 of the ESRF), and Ru/B4C (3.91 nm d-spacing, 65 bilayers, Γ = 0.5, acquired at ID19 of the ESRF). The right column shows the corresponding measurements of the coherence properties of the reflected beam by means of Talbot imaging (performed at ID19, ESRF) 16,26,27,28.

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ACKNOWLEDGMENTS

We acknowledge Kawal Sawhney (Diamond Light Source, UK) for providing the flat-field image taken downstream of the B16 double-multilayer monochromator as well as Hans-Georg Gräber (RWTH Aachen, Germany) for the tooth specimens.

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