An alternate multilayer (AML) grating has been prepared by coating an ion etched lamellar grating with a multilayer (ML) having a layer thickness close to the groove depth. Such a structure behaves as a 2D synthetic crystal and can reach very high efficiencies when the Bragg condition is satisfied. This AML coated grating has been characterized at the SOLEIL Metrology and Tests Beamline between 0.7 and 1.7 keV and at the four-crystal monochromator beamline of Physikalisch-Technische Bundesanstalt (PTB) at BESSY II between 1.75 and 3.4 keV. A peak diffraction efficiency of nearly 27% was measured at 2.2 keV. The measured efficiencies are well reproduced by numerical simulations made with the electromagnetic propagation code CARPEM. Such AML gratings, paired with a matched ML mirror, constitute efficient monochromators for intermediate energy photons. They will extend the accessible energy for many applications as x-ray absorption spectroscopy or x-ray magnetic circular dichroism experiments.
© 2014 Optical Society of America
The domain of synchrotron radiation features a transition in beamline technology between hard and soft x-rays. In the hard x-ray range, the energy selection is performed by Bragg reflection on single crystals. The accessible energy range is limited toward low energies by the unit cell size of available crystal and their stability under radiation ( for Si, for InSb) [1,2]. For soft x-rays, reflective gratings with lamellar, or more often, triangular (blazed) groove profiles are used under grazing incidence. Despite innovations in grating fabrication processes [3,4], the low efficiency of gratings still limits their use of energy above approximately 2.2 keV. Nevertheless, high-efficiency diffraction gratings are of prime importance for some applications in the energy range up to 3.5 keV. For example, x-ray absorption spectroscopy and x-ray magnetic circular dichroism experiments at -edges of the rare earth elements and at the -edge of sulfur would benefit from alternate multilayer (AML) gratings.
At lower energy ranges, in the domain of extreme ultraviolet, multilayer (ML)-coated lamellar gratings have been proposed as a way to enhance the efficiency [5,6]. Recently, ML-coated blazed gratings have been distinguished [7–9] with a reported diffraction efficiency approximately 44% at 13.1 nm (). In the soft x-rays domain, lamellar ML amplitude gratings have been described, but the reported efficiency was only around 8% at 1.5 keV [10,11]. Ways to improve the efficiency and selectivity of blazed grating in this spectral range were also shown [12,13]. However, producing a low blaze angle 1 deg or less as required is a difficult task. Additionally, maintaining the groove profile geometry on the large surface needed for implementation in a beamline monochromator is even more challenging. An alternative would be using an AML grating . As shown in Fig. 1, the AML grating presents a double periodicity at nanometer scale. In the plane of the surface, the period is the pitch of the laminar grating while it is the spacing of the ML in the vertical direction. Therefore, an AML grating has properties similar to a crystal with the advantage of the freedom of choosing the periodicities.
In the ideal case depicted in Fig. 1, a perfect laminar profile is coated with a perfect ML and all the layers have a thickness equal to the grating groove depth. In the cross-section of the grating, the two materials of the ML alternate are in a checkerboard structure and the Bragg planes are defined by their angle with respect to the grating plane:1) and on the optical constants of the involved materials . Thus, the performance of AML gratings relies on an agreement between the thicknesses of each stacked layer and the depth of etching, as well as on the refractive index of each material.
In a previous study of Mo/Si AML, simulations and measurements were performed and results showed an efficiency amounting to about 50% of the expectations . The difference was attributed to the rather strong diffusion of Mo into Si [17,18], affecting the symmetry of the checkerboard structure that is critical for the high performance of the AML grating. The solution, which is widely discussed in ML literature and consists of using a layer as a barrier between Mo and Si [17,18], cannot be used due to the required symmetry.
The choice of and as materials for the AML grating was widely discussed and justified in our previous work . Actually, the and couple offers a high contrast of refractive index. In addition, compared with other systems such as and Mo/Si, the ML presents less interdiffusion and is well modeled by a symmetric structure without interfacial layers. Figure 2 presents a comparison of the efficiency versus the photon energy between three different configurations of AML gratings: a supposedly perfect system, a realistic tri-layer coating where IL is the interfacial layer (1.2 nm of thickness) , and an alternative system. The simulations were performed for a perfect rectangular profile (, , duty cycle ), with our electromagnetic propagation code CARPEM (CARPEM, Calcul de Réseaux par Propagation ElectroMagnétique) . All curves in Fig. 2 show efficiencies higher than previously published ML grating structures [10,11]. Two configurations offer efficiencies close to the ideal perfect system: and AML systems. However, the presence of an IL formed at the expense of the deposited material makes the layer thicknesses difficult to control. A small variation of layer thicknesses affects the checkerboard symmetry of the unit cell. Thus, the ML appears as a better choice to achieve the AML grating because it is without IL with a more simple fabrication process.
An AML grating was fabricated by coating a ML on a grating surface having a laminar groove profile. HORIBA Jobin Yvon fabricated this grating by ion-etching silicon substrate. The fabrication process does not affect the micro-roughness of the high polished substrate. Thanks to an absolute calibration given by a calibration transfer from the standard AFM of the Laboratoire National de Métrologie et d’Essais  to the atomic force microscopy (AFM) of Synchrotron SOLEIL, the accuracy of the measurements of the height of the grating was better than . Table 1 summarizes the features of the laminar grating substrate. It shows that the laminar grating actually presents a trapeze shaped profile which, due the very shallow height, is characterized by a slope around 4 deg, which actually corresponds to an inclined area of only 17%.
A magnetron sputtering deposition system described in [19,22] was used to deposit the ML on the silicon laminar grating. Before the sputtering process, the chamber of preparation was pumped down to a base pressure of . The sputtering was performed at an argon gas pressure of . The targets size was . The radio-frequency power was 150 W for the target and the direct current for the target was 0.07 A. The deposition rate was carefully calibrated using grazing incidence reflectometry. It gives a good control on the achieved period with respect to the target value. In our case of the grating, the groove depth was measured to be 2.51 nm and the target period was fixed to . The duty cycle was measured over the surface to vary between 0.44 and 0.48, slightly below the optimum value of 0.5 (for more details see Table 1). After the fabrication, a grazing incidence reflectometry was performed with the incidence plane parallel to the grating lines to avoid diffraction. We obtained 2.72 and 2.64 nm for the respective and thicknesses, hence a period of 5.36 nm.
The diffraction efficiency of this AML grating was measured at the low energy branch of the Metrology and Tests Beamline at Synchrotron SOLEIL  in the energy range from 0.7 to 1.7 keV, and at the four-crystal monochromateur (FCM) beamline of Physikalisch-Technische Bundesanstalt (PTB) at BESSY II in the energy range from 1.75 to 3.4 keV . The results are depicted in Figs. 3 and 4. In Fig. 3, the experimental efficiency of the first diffraction order versus the photon energy confirms the ability to obtain a high-efficiency () throughout a wide energy range (from 1 to 3.4 keV) by keeping a constant grating rotation angle . The angle is defined as the grating rotation from specular reflection and is related to the grazing incidence and exit angles and by . The fixed value is an easy condition to drive a monochromateur using an AML grating very close to the Bragg condition. Figure 3 also shows a high-efficiency around 27% at 2.2 keV. The abrupt drop of the efficiency curve at 2.5 keV is due to the absorption edge of Mo. In addition, the agreement between the behavior of the experiment curves and the simulation result of CARPEM can be noted.
Figure 4 shows a detector scan at a fixed energy of 2.2 keV and at a fixed incidence angle of 7.8 deg. It illustrates the preferential coupling of almost all the incident flux into the first diffraction order when the Bragg condition is satisfied, which is a signature of 2D diffraction and explains why AML gratings can have very large efficiencies.
In Fig. 5, the measured reflectance of the first diffraction order at 2.2 keV is compared to a CARPEM fit assuming a perfect rectangular profile of the grooves and a perfect replication of this profile by the deposited ML. In the fitting process, the reflectance is most sensitive to the respective thickness of and . Small changes of thicknesses result in a shift of the angle of maximum reflectivity. The obtained thicknesses agree with grazing measurements to better than 0.1%. The width of the profile depends on the number of layer periods. An almost perfect agreement is found for 30 periods. The grating duty cycle affects the peak efficiency. The best agreement was obtained for . The measured peak reflectance amounts to around 85% of the calculated maximum. The comparison confirms the good agreement between the measured efficiency and the best-fit simulations, as shown on Fig. 3. The difference between experiment and simulation can be explained by the rectangular profile assumption in the simulation, while the real profile is trapezoidal. In addition, the other imperfections of the structure, such as the interdiffusion and the interfacial roughness in the range of 0.1 nm, can contribute to reducing the efficiency from the computed 31% with to the measured . Unfortunately, the grating imperfections concerning geometry or roughness cannot be modeled by the CARPEM code in its present development stage.
We showed here the design realization and characterization of a AML grating with a high-efficiency between 1 and 3.4 keV. Good agreement between calculated and measured performance was achieved. This AML grating, paired with a ML mirror of the same deviation, has been successfully installed into the DEIMOS beamline monochromator . We are currently studying other ML material combinations in order to extend the high-efficiency of such monochromator at energy up to 5 keV.
This work is supported by a grant from the RTRA (réseaux thématiques de recherche avancée) program “Triangle de la Physique.” The deposition and the characterization of the multilayers were performed on CEMOX (Centrale d’Elaboration et de Métrologie d’Optique X), a platform of the LUMAT federation (CNRS FR2764). The authors thank Stefanie Marggraf and Levent Cibik for their help during the experiments at FCM beamline of PTB at BESSY II. The authors also thank Pascal Mercere for his help during the experiments at Metrology and Tests Beamline at Synchrotron SOLEIL. The authors acknowledge the remarkable work achieved by Audrey Liard and her team at HORIBA Jobin Yvon in preparing the ion etched grating substrate.
1. T. M. Mooney, T. Toellner, W. Sturhahn, E. E. Alp, and S. D. Shastri, Nucl. Instrum. Methods Phys. Res., Sect. A 347, 348 (1994). [CrossRef]
2. Y. V. Shvyd’ko, S. Stoupin, A. Cunsolo, A. H. Said, and X. Huang, Nat. Phys. 6, 196 (2010). [CrossRef]
3. D. Cocco, A. Bianco, B. Kaulich, F. Schaefers, M. Mertin, G. Reichardt, B. Nelles, and K. F. Heidemann, AIP Conf. Proc. 879, 497 (2007). [CrossRef]
4. B. Lagarde, F. Sirotti, A. Taleb-Ibrahimi, C. Miron, and F. Polack, J. Phys. Conf. Ser. 425, 152022 (2013). [CrossRef]
5. J. F. Seely, M. P. Kowalski, R. G. Cruddace, K. F. Heidemann, U. Heinzmann, U. Kleineberg, K. Osterried, D. Menke, J. C. Rife, and W. R. Hunter, Appl. Opt. 36, 8206 (1997). [CrossRef]
6. M. P. Kowalski, R. G. Cruddace, J. F. Seely, J. C. Rife, K. F. Heidemann, U. Heinzmann, U. Kleineberg, K. Osterried, D. Menke, and W. R. Hunter, Appl. Opt. 22, 834 (1997).
7. D. L. Voronov, M. Ahn, E. H. Anderson, R. Cambie, C. H. Chang, E. M. Gullikson, R. K. Heilmann, F. Salmassi, M. L. Schattenburg, T. Warwick, V. V. Yashchuk, L. Zipp, and H. A. Padmore, Opt. Lett. 2615, 35 (2010).
8. D. L. Voronov, E. H. Anderson, R. Cambie, S. Cabrini, S. D. Dhuey, L. I. Goray, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H. A. Padmore, Opt. Express 19, 6320 (2011). [CrossRef]
9. D. L. Voronov, E. H. Anderson, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H. A. Padmore, Opt. Lett. 37, 1628 (2012). [CrossRef]
10. R. Benbalagh, J.-M. André, R. Barchewitz, Ph. Jonnard, G. Julié, L. Mollard, G. Rolland, C. Remond, Ph. Troussel, R. Marmoret, and E. O. Filatova, Nucl. Instrum. Methods Phys. Res. A 541, 590 (2005). [CrossRef]
11. R. Benbalagh, J.-M. André, R. Barchewitz, M.-F. Ravet, A. Raynal, F. Delmotte, F. Bridou, G. Julié, A. Bosseboeuf, and Ph. Troussel, Nucl. Instrum. Methods Phys. Res. A 458, 650 (2001). [CrossRef]
12. A. Bianco, G. Sostero, B. Nelles, K. F. Heidemann, and D. Cocco, Proc. SPIE 5918, 591810 (2005). [CrossRef]
13. D. L. Voronov, S. Diez, P. Lum, S. A. Hidalgo, T. Warwick, N. A. Artemiev, and H. A. Padmore, Proc. SPIE 8848, 88480Q (2013). [CrossRef]
14. F. Polack, M. Idir, E. Jourdain, and A. Liard-Cloup, “Two-dimensional diffraction network with alternating multilayer stacks and method for the production thereof and spectroscopic devices comprising said networks,” European patent application EP1700141 (July 17, 2005).
15. F. Polack, B. Lagarde, M. Idir, A. Liard Cloup, and E. Jourdain, AIP Conf. Proc. 879, 639 (2007). [CrossRef]
16. F. Polack, B. Lagarde, M. Idir, A. Liard-Cloup, E. Jourdain, and M. Roulliay, AIP Conf. Proc. 879, 489 (2007). [CrossRef]
17. A. Patteli, J. Ravagnana, V. Rigatoa, G. Salmasoa, D. Silvestrinia, E. Bontempic, and L. E. Deperoc, Appl. Surf. Sci. 238, 262 (2004). [CrossRef]
18. H. Maury, P. Jonnard, J.-M. André, J. Gautier, M. Roulliay, F. Bridou, F. Delmotte, M.-F. Ravet-Krill, A. Jérome, and P. Holliger, Thin Solid Films 514, 278 (2006). [CrossRef]
19. F. Choueikani, F. Bridou, B. Lagarde, E. Meltchakov, F. Polack, P. Mercere, and F. Delmotte, Appl. Phys. A 111, 191 (2013). [CrossRef]
20. A. Mirone, E. Delcamp, M. Idir, G. Cauchon, F. Polack, P. Dhez, and C. Bizeuil, Appl. Opt. 37, 5816 (1998). [CrossRef]
21. S. Ducourtieux and B. Poyet, Meas. Sci. Technol. 22, 094010 (2011). [CrossRef]
22. J. Gautier, F. Delmotte, F. Bridou, M. F. Ravet, F. Varniere, M. Rouilliay, A. Jerome, and I. Vickridge, Appl. Phys. A 88, 719 (2007). [CrossRef]
23. M. Idir, P. Mercere, T. Moreno, A. Delmotte, P. Da Silva, and M. H. Modi, AIP Conf. Proc. 1234, 485 (2009).
24. M. Krumrey and G. Ulm, Nucl. Instrum. Methods Phys. Res. A 467-468, 1175 (2001). [CrossRef]
25. P. Ohresser, E. Otero, F. Choueikani, K. Chen, S. Stanescu, F. Deschamps, T. Moreno, F. Polack, B. Lagarde, J.-P. Daguerre, F. Marteau, F. Scheurer, L. Joly, J. P. Kappler, and B. Mulle, Rev. Sci. Instrum. 85, 013106 (2014). [CrossRef]