- Email: [email protected]
Applications of multilayer optics
Nuclear Instruments and Methods in Physics Research A 623 (2010) 786–790
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Applications of multilayer optics Zhanshan Wang , Jingtao Zhu, Baozhong Mu, Zhong Zhang, Fengli Wang, Jing Xu, Wenbin Li, Lingyan Chen Institute of Precision Optical Engineering (IPOE), Physics Department, Tongji University, Shanghai 200092, China
a r t i c l e in f o
a b s t r a c t
Available online 19 February 2010
Recent development of multilayer mirror and its applications in extreme ultraviolet (EUV), soft X-ray ranges in China was reviewed in this paper. Three types of multilayer mirrors were developed with special performance for dense plasma diagnostics, EUV astronomical observation. Firstly, dual-periodic W/B4C multilayer mirror was designed for Kirkpatrick–Baez (K–B) microscopy working at TiKa line (4.75 keV), which is highly reflective both at hard X-ray (CuKa line at 8.05 keV) and soft X-ray (4.75 keV). Using this mirror, the K–B system can be aligned conveniently in air using hard X-ray instead of in vacuum. The second mirror is aperiodic Mg/SiC multilayer, also a bi-functional mirror with high reflectivity for He-II emission line (30.4 nm) but suppressing He-I emission line (58.4 nm) in astronomy observation, which will replace the traditional combination of periodic multilayer and the fragile film filter. This will be more safe in satellite launching. The third mirror is Mo/Si periodic multilayer, depositing on a parabolic substrate with diameter of 230 mm, which is designed for EUV telescope for imaging of solar corona by selecting Fe-XII emission (19.5 nm). The uniformity of lateral layer thickness distribution is within 7 0.3% along the diameter of mirror, measured by X-ray reflectometry. The measured peak reflectivity is 42% at the wavelength of 19.5 nm. All these multilayer mirrors were prepared by using magnetron sputtering system in our group. & 2010 Elsevier B.V. All rights reserved.
Keywords: Multilayer Aperiodic Magnetron sputtering K–B microscope Astronomical observation
1. Introduction In extreme ultraviolet (EUV), soft X-ray and X-ray ranges, multilayer mirrors are widely used as a key reflective element in EUV lithography, astronomical observation, X-ray laser, synchrotron radiation and plasma diagnostics [1–4]. Traditionally, the multilayer is periodic stack structure with layer thickness in nanometer scale, which is convenient to design using Bragg law and fabricate using sputtering deposition method. Recent developments in EUV and X-ray imaging, plasma diagnostic, and synchrotron radiation, require special multilayer mirrors with specific properties, such as broad incident angular range or photon energy band, dual-channel reflective band, bi-function, etc. [5,6]. In these cases, special multilayer mirrors have to be developed, such as supermirror X-ray plasma diagnostic, dualchannel multilayer mirrors for astrophysics. In China, there are some urgent requirements for multilayer mirrors in EUV, soft X-ray and X-ray regions for dense plasma diagnostics, EUV astronomical observations. In this paper, we summarize the recent developments of multilayer mirrors to meet these needs. Firstly, the design, fabrication and characterizations of multilayer mirrors were described. Then, three Corresponding author. Tel.: + 86 21 65984652; fax: + 86 21 65986323.
E-mail address: [email protected] (Z. Wang). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.098
examples of multilayer mirrors and their application were introduced. The first one is a dual-periodic W/B4C multilayer mirror, which is designed for Kirkpatrick–Baez (K–B) microscopy working at TiKa line (4.75 keV). Secondly, aperiodic Mg/SiC multilayer was designed for astronomy observation for He-II emission line (30.4 nm), which will replace the traditional combination of periodic multilayer and the fragile film filter. The third mirror is Mo/Si periodic multilayer, depositing on a parabolic substrate with diameter of 230 mm, which is designed for EUV telescope for imaging of solar corona by selecting Fe-XII emission (19.5 nm).
2. Design and experiments Keys to design multilayer optics with special performance include how to select merit functions, optimized algorithms, and initial structures. The choice of a merit function depends on the requirements of multilayer optics. Different applications need different merit functions. After the selection of the merit function, optimized algorithms need to be chosen [7–10]. There are three algorithms used in our applications, viz., the simulated annealing algorithm, the random search algorithm and the local optimization algorithm. After the local optimization algorithm is selected, the next important step is to decide the initial structures of
Z. Wang et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 786–790
multilayers, which will determine how good the final performance will be. Quarter wave periodic multilayer and depth period stacks are often used during the optimization process. The initial solutions generated by the analytical expression can be used in most cases. The multilayer optics were deposited by magnetron sputtering in vacuum chamber at base pressure below 1 10 4 Pa, using Ar gas with purity of 99.9999%, and a typical sputtering gas pressure of 0.2 Pa. During depositing multilayers, masks or scanning targets mode were used to obtain the precise thickness uniformity. The structures of the periodic and aperiodic multilayer mirrors were characterized using X-ray reflectometry. The EUV and soft X-ray characterization were performed by a reflectometer at synchrotron radiation. The roughness of the substrate and multilayer mirrors were measured by using an atomic force microscope. Experimental details can be found in previous publications [11].
3. Results and discussions 3.1. X-ray K–B microscope system by using dual-periodic multilayers K–B microscope system configuration consists of two perpendicular concave spherical mirrors in tandem. Rays from an object point are reflected by the first mirror (tangential mirror here) and form a tangential line while not a point. Similarly, rays via the second mirror (sagittal mirror here) form a sagittal line. Thus the
Fig. 1. Reflectivity of dual-periodic W/B4C multilayer designed for both 4.75 and 8.05 keV.
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perpendicular structure plays an important role to overcome the strong astigmatism. K–B microscope is often used in dense plasma diagnostics [12,13]. There is a strong requirement for developing a K–B microscope working at TiKa line (4.75 keV) to investigate the evolvement of laser produced plasma with time. The performance of K–B microscope is dependent on the quality of concave spherical mirrors which should have accurate figure, low roughness, high reflectivity and accuracy of system alignment. Generally, the K–B microscope should be aligned using the same wavelength as in the imaging experiments, if a very high performance wanted to be obtained. Furthermore, this operation has to be done in vacuum for soft X-ray K–B microscope working at 4.75 keV. Since the absorption of 4.75 keV soft X-ray in air is very strong, this kind of alignment experiments are very complex and expensive if the K–B microscope can only work at 4.75 keV. However, if the K–B microscope works both at 4.75 and 8.05 keV, the alignment process can be done in air by using CuKa line at 8.05 keV, then, the K–B microscope can be use in the vacuum for imaging working at 4.75 keV afterwards. This method will simplify the alignment process and save cost. For the above purpose, a dual-periodic multilayer optics was proposed, which consists of two part W/B4C multilayer stacks with different periodic thicknesses. The top stack reflects the soft X-ray at 4.75 keV for imaging, while the bottom stack reflects hard X-ray at 8.05 keV, for alignment in the air. Therefore, this dual-period multilayer mirror is highly reflective both at hard X-ray (CuKa line at 8.05 keV) and soft X-ray (4.75 keV) at the same grazing angle of 1.21, shown in Fig. 1. Thus we can achieve the alignment of 4.75 keV K–B microscope by imaging experiments at 8.05 keV in air condition. Because the FWHM of 8.05 keV is narrower than that of 4.75 keV, the alignment will be more precise than the requirement in the imaging experiments. An one-dimensional advanced K–B microscope with two spherical mirrors in one direction working at 4.75 keV was set up firstly, shown in Fig. 2. The imaging alignments results at 8.05 keV were shown as Fig. 3(a). Then, we marked the best object point with a small ball (with diameter in 200 mm). In Shenguang laser facility in China, we should just make the target at the same point with the marked ball. Finally, we got the imaging results of 1500 lines/in. golden mesh at 4.75 keV (TiKa) (shown in Fig. 3(b)). In summary, dual-periodic multilayer provides a very practical method for the alignment of soft X-ray K–B microscopy system in vacuum. More detailed experiment can be found in previous publications [14]. After the successful experiment of one-dimensional K–B microscope, we were required to make another two-dimensional K–B microscope working at 4.75 keV to cast a dense plasma backlit image at the detector for diagnosing its evolvement. We adopted the same dual-periodic multilayer mirrors to solve the
Fig. 2. Schematic of one dimensional advanced K–B microscope.
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Fig. 3. Imaging experimental results of X-ray KB microscope: (a) Imaging at 8.05 keV in air condition. (b) Imaging at 4.75 keV in vacuum. The bar shows the spatial scale is 50 mm.
high-reflective (HR) at 30.4 nm and low-reflective (LR) at 58.4 nm, was designed, fabricated and characterized. In designing the Mg/SiC aperiodic multilayer the optical constants of magnesium, silicon carbide, and silicon between 10 and 41.3 nm were from the Center for X-ray Optics [18], while the optical constants between 41.3 and 70 nm were from Refs. [19–21]. The incident angle was 51. The layer number was limited to 60 because of absorption. A calculated reflectivity curve of a designed Mg/SiC aperiodic multilayer and a periodic multilayer are shown in Fig. 5. At an incident angle of 51, the aperiodic multilayer has a reflectivity of 54.1% at 30.4 nm and 0.1% at 58.4 nm, while the periodic multilayer exhibits a reflectivity of 58.7% at 30.4 nm and 2.2% at 58.4 nm. The Mg/SiC aperiodic multilayer and periodic multilayer were prepared by magnetron sputtering method in our group, and their EUV reflectivities were measured at Daresbury Laboratory, UK. The measured results were shown in Fig. 6. At an incident angle of 51, the reflectivity of the aperiodic multilayer at wavelengths of 30.4 and 58.4 nm is 36.7% and 2.0% respectively, while the lowest reflectivity is 0.024% at 63.0 nm. In comparison, a periodic multilayer was also shown. Its reflectivity is 34.4% at 30.4 nm and 1.8% at 58.4 nm.
Fig. 4. Two dimensional imaging experimental results of X-ray K–B microscope working at 4.75 keV.
difficulty of alignment in the vacuum at 4.75 keV. The experimental image of 2000 lines/in. golden mesh was shown in Fig. 4, which indicates our K–B microscopy system is successful using dual-period multilayer mirrors, and now this K–B microscopy system has been used in Shenguang laser facility in China. A series of images have been obtained to perform laser-produced dense plasma diagnostics experiments.
Fig. 5. Calculated reflectivity curve of designed Mg/SiC aperiodic multilayer (solid squares) and periodic multilayer (hollow circles) at an incident angle of 51.
3.2. Extreme ultraviolet Mg/SiC aperiodic multilayer with high reflectivity at 30.4 nm and low reflectivity at 58.4 nm The secondary phase of the Chinese Lunar Exploration Program is scheduled to be launched around 2013, which includes an Extreme Ultraviolet Imager (EUVI). The EUVI is designed to observe the Earth’s plasmasphere by imaging the He + emission (at 30.4 nm wavelength) from the lunar orbit. However, a bright background radiation from the earth’s ionosphere, which is the He emission (at 58.4 nm wavelength), must be blocked [15]. Mg/SiC is a new material pair in the extreme ultraviolet (EUV) region [16], which shows good performance in the 25–35 nm region [17]. A novel Mg/SiC aperiodic multilayer, which exhibited
Fig. 6. Measured reflectivity curve of fabricated Mg/SiC aperiodic multilayer (solid squares) and periodic multilayer (hollow circles) at an incident angle of 51.
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Since the Mg/SiC aperiodic multilayer exhibited the lowest reflectivity at 63.0 nm rather than 58.4 nm, the refractive indices of magnesium and silicon carbide used in the design were assumed to be inaccurate.
3.3. Parabolic periodic multilayer optics with large area working at 19.5 nm In recent years, the development of multilayer technology has enabled the construction of instrumentation and led to a number of successful missions including Solar and Heliospheric Observatory/ Extreme ultraviolet Imaging Telescope (SOHO/EIT) and Transition Region and Coronal Explorer (TRACE) [22–24]. At the wavelengths longer than the Si L-absorption edge near 12.4 nm, Mo/Si was widely used in the wavelength of 13–20 nm wavelength region for its high stability and high reflectivity. The accurate deposition of high reflection and uniformity on ultra-smooth polished parabolic substrates is one of the major challenges for constructing a collimator to test the performance of solar telescope working at 19.5 nm. The Mo/Si multilayer mirror with a size of 230 mm was investigated for wavelength of 19.5 nm (Fe-XII). Fig. 7 shows the X-ray reflective (XRR) curves measured by XRD. Eighteen positions were measured along the diameter direction of the mirror with a spacing of 10 mm. For each measured spot, the period thickness can be calculated from the X-ray reflective peaks according to the amended Bragg formula [25]. The period thicknesses of multilayer can be calculated for each measured position spot. The mean period thickness is 10.30 nm, which is in agreement with the design one. Fig. 8 shows the normalization lateral layer thickness distribution along the diameter of 200 mm. The uniformity (DD/D0) is normalized by the period thickness (D0) at center spot (X¼0 mm). The obtained uniformity is within 70.3% in diameter of 200 mm. Finally, the reflectivity is measured by the reflectometer on beam line U26 at National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Reflectivities were measured at the fixed incident angel of 51 using wavelength-scanning mode. Along the diameter of 200 mm, the reflectivity is measured with spacing of 10 mm. Fig. 9 shows all the measured curves. The consistency of the reflectivity curves also indicates a good uniformity of the period thickness. The measured peak reflectivity at 19.5 nm is (4272)%.
Fig. 8. Normalization lateral layer thickness distribution along the diameter of 200 mm, each data point is calculated from the measured period thickness shown in Fig. 7.
Fig. 9. The reflectivity curves measured on beamline U26 at NSRL.
4. Summary
Fig. 7. X-ray reflectivity curves along the diameter of 200 mm measured with spacing of 10 mm.
In order to meet the needs of special applications in EUV astronomical observation, dense plasma diagnostics in China, three types of multilayer mirrors were successfully developed with specific spectral properties. Firstly, a dual-periodic W/B4C multilayer mirror was designed for Kirkpatrick–Baez (K–B) microscopy working at TiKa line (4.75 keV), which is highly reflective both at hard X-ray (CuKa line at 8.05 keV) and soft X-ray (4.75 keV). Using this mirror, the K–B system can be aligned conveniently in air using hard X-ray instead of in vacuum chamber, which will significantly save cost and time in alignment. Now, 1-dimensional and 2-dimensional K–B microscopy systems have been equipped in Shenguang laser facility in China to perform laser-produced dense plasma diagnostics experiments. Secondly, aperiodic Mg/SiC multilayer was designed for astronomy observation, which is a bi-functional mirror with high reflectivity for He-II emission line (30.4 nm) but suppressing He-I emission line (58.4 nm). It will replace the traditional combination of periodic multilayer and the fragile film filter. This mirror is expected to be applied to the EUV Imager in the second
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phase of the Chinese Lunar Exploration Program. This will be more safe in satellite launching. Thirdly, for the purpose of testing the performance of EUV telescope used for extreme ultraviolet imaging of solar corona by selecting Fe-XII emission line at wavelength of 19.5 nm, Mo/Si multilayer mirror was deposited on large parabolic fused silica substrate with a diameter of 230 mm, having a fairly good uniformity. The uniformity of lateral layer thickness distribution is within 70.3% in the diameter of 200 mm, measured by X-ray diffractometer. The measured peak reflectivity is 42% at the wavelength of 19.5 nm.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant no. 10825521, 10675091, 10675092, and 10876023), High-tech 863 program (Grant no. 2006AA12Z139) and by the Shanghai Committee of Science and Technology, China (Grant no. 09XD1404000, 07DZ22302, 09ZR1434300, 0952nm06900). The authors thank Professor Alan Michette in King’s College London, Dr. Mike MacDonald and Mark Roper in STFC Daresbury Laboratory, and Dr. Franz Sch¨afers and Andreas Gaupp at BESSY-II for their kindly help in discussion and measurement. References [1] J.-Ph. Champeaux, Ph. Troussel, B. Villier, V. Vidal, T. Khachroum, B. Vidal, M. Krumerey, Nucl. Instr. and Meth. Phys. Res. Sect. A 581 (2007) 687. [2] K. Yamashita, et al., Appl. Opt. 37 (1998) 8067. [3] E. Ziegler, Proc. SPIE 2253 (1994) 248.
[4] V.L. Kantsyrev, R. Bruch, R. Phaneuf, N.G. Publicover, J. X-ray Sci. Technol. 7 (1997) 139. [5] J. Gautieret, F. Delmotte, M.F. Ravet, Ar. Jerome, F. Bridou, F. Varnier, F. Auchere, Opt. Commun. 281 (11) (2008) 3032. [6] J.-Ph. Champeaux, B. Villiera, V. Vidalb, T. Khachroumb, B. Vidalb, M. Krumrey, Nucl. Instr. Meth. Phys. Res. A 581 (2007) 687. [7] I.V. Kozhevnikov, I.N. Bukreeva, E. Ziegler, Nucl. Instr. Meth. Phys. Res. A 460 (2001) 424. [8] X. Cheng, Z. Wang, Z. Zhang, F. Wang, L. Chen, Opt. Comm. 265 (2006) 197. [9] P.D. Binda, F.E. Zocchi, Proc. SPIE 5536 (2004) 97. [10] V.T. Alexander, K.T. Michael, V.P. Vladimir, V.V. Andrei, Proc. SPIE 3738 (1999) 248. [11] Z.S. Wang, F.L. Wang, Z. Zhong, H.C. Wang, W.J. Wu, S.M. Zhang, Y. Xu, L.Y. Chen, Opt. Precis. Eng. 13 (2005) 512. [12] J.L. Bourgade, P. Troussel, A. Casner, G. Huser, T.C. Sangster, G. Pien, F.J. Marshall, J. Fariaud, C. Remond, D. Gontier, C. Chollet, C. Zuber, C. Reverdin, A. Richard, P.A. Jaanimagi, R.L. Keck, R.E. Bahr 2, W.J. Armstrong, J. Dewandel, R. Maroni, F. Aubard, B. Angelier, C.Y. Cote, S. Magnan, Rev. Sci. Instrum. 79 (2008) 10E904. [13] G.R. Bennett, Rev. Sci. Instrum. 70 (1999) 608. [14] B.Z. Mu, Z.S. Wang, S.Z. Yi, J.T. Zhu, X. Wang, L. Jiang, Q.S. Huang, Y.H. Bai, Proc. SPIE 7448 (2009) 74480K. [15] D.D. Allred, R.S. Turley, M.B. Squires, Proc. SPIE 3767 (1999) 280. [16] G. Murakami, K. Yoshioka, I. Yoshikawa, Proc. SPIE 6317 (2006) 631714-1. [17] T. Ejima, A. Yamazaki, T. Banse, et al., Appl. Opt. 44 (2005) 5446. [18] /http://www.cxro.lbl.gov/optical_constants/getdb2.htmlS. [19] E.D. Palik, Handbook of Optical Constants of Solids III, Academic Press, San Diego, 1998 (p. 238). [20] E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, Orlando, 1985 (p. 559). [21] B. Kortright, D.L. Windt, Appl. Opt. 27 (1988) 2841. [22] M.F. Ravet, F. Bridou, X. Zhang-Song, A. Jerome, F. Delmotte, R. Mercier, ¨ M. Bougnet, P. Bouyries, J.P. DelaboudiniEre, Proc. SPIE 5250 (2004) 99. [23] L. Gardner, J. Kohl, S. Cranmer, S. Fineschi, L. Golub, J. Raymond, P.L. Smith, L. Strachan, Proc. SPIE 3764 (1999) 134. [24] M. Grigonis, E.J. Knystautas, Appl. Opt. 36 (1997) 2839. [25] W.J. Wu, J.T. Zhu, Z.S. Wang, Z. Zhang, F.L. Wang, H.C. Wang, S.M. Zhang, Y. Xu, Chin. Phys. Lett. 23 (2006) 2534.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Applications of multilayer optics Zhanshan Wang , Jingtao Zhu, Baozhong Mu, Zhong Zhang, Fengli Wang, Jing Xu, Wenbin Li, Lingyan Chen Institute of Precision Optical Engineering (IPOE), Physics Department, Tongji University, Shanghai 200092, China
a r t i c l e in f o
a b s t r a c t
Available online 19 February 2010
Recent development of multilayer mirror and its applications in extreme ultraviolet (EUV), soft X-ray ranges in China was reviewed in this paper. Three types of multilayer mirrors were developed with special performance for dense plasma diagnostics, EUV astronomical observation. Firstly, dual-periodic W/B4C multilayer mirror was designed for Kirkpatrick–Baez (K–B) microscopy working at TiKa line (4.75 keV), which is highly reflective both at hard X-ray (CuKa line at 8.05 keV) and soft X-ray (4.75 keV). Using this mirror, the K–B system can be aligned conveniently in air using hard X-ray instead of in vacuum. The second mirror is aperiodic Mg/SiC multilayer, also a bi-functional mirror with high reflectivity for He-II emission line (30.4 nm) but suppressing He-I emission line (58.4 nm) in astronomy observation, which will replace the traditional combination of periodic multilayer and the fragile film filter. This will be more safe in satellite launching. The third mirror is Mo/Si periodic multilayer, depositing on a parabolic substrate with diameter of 230 mm, which is designed for EUV telescope for imaging of solar corona by selecting Fe-XII emission (19.5 nm). The uniformity of lateral layer thickness distribution is within 7 0.3% along the diameter of mirror, measured by X-ray reflectometry. The measured peak reflectivity is 42% at the wavelength of 19.5 nm. All these multilayer mirrors were prepared by using magnetron sputtering system in our group. & 2010 Elsevier B.V. All rights reserved.
Keywords: Multilayer Aperiodic Magnetron sputtering K–B microscope Astronomical observation
1. Introduction In extreme ultraviolet (EUV), soft X-ray and X-ray ranges, multilayer mirrors are widely used as a key reflective element in EUV lithography, astronomical observation, X-ray laser, synchrotron radiation and plasma diagnostics [1–4]. Traditionally, the multilayer is periodic stack structure with layer thickness in nanometer scale, which is convenient to design using Bragg law and fabricate using sputtering deposition method. Recent developments in EUV and X-ray imaging, plasma diagnostic, and synchrotron radiation, require special multilayer mirrors with specific properties, such as broad incident angular range or photon energy band, dual-channel reflective band, bi-function, etc. [5,6]. In these cases, special multilayer mirrors have to be developed, such as supermirror X-ray plasma diagnostic, dualchannel multilayer mirrors for astrophysics. In China, there are some urgent requirements for multilayer mirrors in EUV, soft X-ray and X-ray regions for dense plasma diagnostics, EUV astronomical observations. In this paper, we summarize the recent developments of multilayer mirrors to meet these needs. Firstly, the design, fabrication and characterizations of multilayer mirrors were described. Then, three Corresponding author. Tel.: + 86 21 65984652; fax: + 86 21 65986323.
E-mail address: [email protected] (Z. Wang). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.098
examples of multilayer mirrors and their application were introduced. The first one is a dual-periodic W/B4C multilayer mirror, which is designed for Kirkpatrick–Baez (K–B) microscopy working at TiKa line (4.75 keV). Secondly, aperiodic Mg/SiC multilayer was designed for astronomy observation for He-II emission line (30.4 nm), which will replace the traditional combination of periodic multilayer and the fragile film filter. The third mirror is Mo/Si periodic multilayer, depositing on a parabolic substrate with diameter of 230 mm, which is designed for EUV telescope for imaging of solar corona by selecting Fe-XII emission (19.5 nm).
2. Design and experiments Keys to design multilayer optics with special performance include how to select merit functions, optimized algorithms, and initial structures. The choice of a merit function depends on the requirements of multilayer optics. Different applications need different merit functions. After the selection of the merit function, optimized algorithms need to be chosen [7–10]. There are three algorithms used in our applications, viz., the simulated annealing algorithm, the random search algorithm and the local optimization algorithm. After the local optimization algorithm is selected, the next important step is to decide the initial structures of
Z. Wang et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 786–790
multilayers, which will determine how good the final performance will be. Quarter wave periodic multilayer and depth period stacks are often used during the optimization process. The initial solutions generated by the analytical expression can be used in most cases. The multilayer optics were deposited by magnetron sputtering in vacuum chamber at base pressure below 1 10 4 Pa, using Ar gas with purity of 99.9999%, and a typical sputtering gas pressure of 0.2 Pa. During depositing multilayers, masks or scanning targets mode were used to obtain the precise thickness uniformity. The structures of the periodic and aperiodic multilayer mirrors were characterized using X-ray reflectometry. The EUV and soft X-ray characterization were performed by a reflectometer at synchrotron radiation. The roughness of the substrate and multilayer mirrors were measured by using an atomic force microscope. Experimental details can be found in previous publications [11].
3. Results and discussions 3.1. X-ray K–B microscope system by using dual-periodic multilayers K–B microscope system configuration consists of two perpendicular concave spherical mirrors in tandem. Rays from an object point are reflected by the first mirror (tangential mirror here) and form a tangential line while not a point. Similarly, rays via the second mirror (sagittal mirror here) form a sagittal line. Thus the
Fig. 1. Reflectivity of dual-periodic W/B4C multilayer designed for both 4.75 and 8.05 keV.
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perpendicular structure plays an important role to overcome the strong astigmatism. K–B microscope is often used in dense plasma diagnostics [12,13]. There is a strong requirement for developing a K–B microscope working at TiKa line (4.75 keV) to investigate the evolvement of laser produced plasma with time. The performance of K–B microscope is dependent on the quality of concave spherical mirrors which should have accurate figure, low roughness, high reflectivity and accuracy of system alignment. Generally, the K–B microscope should be aligned using the same wavelength as in the imaging experiments, if a very high performance wanted to be obtained. Furthermore, this operation has to be done in vacuum for soft X-ray K–B microscope working at 4.75 keV. Since the absorption of 4.75 keV soft X-ray in air is very strong, this kind of alignment experiments are very complex and expensive if the K–B microscope can only work at 4.75 keV. However, if the K–B microscope works both at 4.75 and 8.05 keV, the alignment process can be done in air by using CuKa line at 8.05 keV, then, the K–B microscope can be use in the vacuum for imaging working at 4.75 keV afterwards. This method will simplify the alignment process and save cost. For the above purpose, a dual-periodic multilayer optics was proposed, which consists of two part W/B4C multilayer stacks with different periodic thicknesses. The top stack reflects the soft X-ray at 4.75 keV for imaging, while the bottom stack reflects hard X-ray at 8.05 keV, for alignment in the air. Therefore, this dual-period multilayer mirror is highly reflective both at hard X-ray (CuKa line at 8.05 keV) and soft X-ray (4.75 keV) at the same grazing angle of 1.21, shown in Fig. 1. Thus we can achieve the alignment of 4.75 keV K–B microscope by imaging experiments at 8.05 keV in air condition. Because the FWHM of 8.05 keV is narrower than that of 4.75 keV, the alignment will be more precise than the requirement in the imaging experiments. An one-dimensional advanced K–B microscope with two spherical mirrors in one direction working at 4.75 keV was set up firstly, shown in Fig. 2. The imaging alignments results at 8.05 keV were shown as Fig. 3(a). Then, we marked the best object point with a small ball (with diameter in 200 mm). In Shenguang laser facility in China, we should just make the target at the same point with the marked ball. Finally, we got the imaging results of 1500 lines/in. golden mesh at 4.75 keV (TiKa) (shown in Fig. 3(b)). In summary, dual-periodic multilayer provides a very practical method for the alignment of soft X-ray K–B microscopy system in vacuum. More detailed experiment can be found in previous publications [14]. After the successful experiment of one-dimensional K–B microscope, we were required to make another two-dimensional K–B microscope working at 4.75 keV to cast a dense plasma backlit image at the detector for diagnosing its evolvement. We adopted the same dual-periodic multilayer mirrors to solve the
Fig. 2. Schematic of one dimensional advanced K–B microscope.
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Fig. 3. Imaging experimental results of X-ray KB microscope: (a) Imaging at 8.05 keV in air condition. (b) Imaging at 4.75 keV in vacuum. The bar shows the spatial scale is 50 mm.
high-reflective (HR) at 30.4 nm and low-reflective (LR) at 58.4 nm, was designed, fabricated and characterized. In designing the Mg/SiC aperiodic multilayer the optical constants of magnesium, silicon carbide, and silicon between 10 and 41.3 nm were from the Center for X-ray Optics [18], while the optical constants between 41.3 and 70 nm were from Refs. [19–21]. The incident angle was 51. The layer number was limited to 60 because of absorption. A calculated reflectivity curve of a designed Mg/SiC aperiodic multilayer and a periodic multilayer are shown in Fig. 5. At an incident angle of 51, the aperiodic multilayer has a reflectivity of 54.1% at 30.4 nm and 0.1% at 58.4 nm, while the periodic multilayer exhibits a reflectivity of 58.7% at 30.4 nm and 2.2% at 58.4 nm. The Mg/SiC aperiodic multilayer and periodic multilayer were prepared by magnetron sputtering method in our group, and their EUV reflectivities were measured at Daresbury Laboratory, UK. The measured results were shown in Fig. 6. At an incident angle of 51, the reflectivity of the aperiodic multilayer at wavelengths of 30.4 and 58.4 nm is 36.7% and 2.0% respectively, while the lowest reflectivity is 0.024% at 63.0 nm. In comparison, a periodic multilayer was also shown. Its reflectivity is 34.4% at 30.4 nm and 1.8% at 58.4 nm.
Fig. 4. Two dimensional imaging experimental results of X-ray K–B microscope working at 4.75 keV.
difficulty of alignment in the vacuum at 4.75 keV. The experimental image of 2000 lines/in. golden mesh was shown in Fig. 4, which indicates our K–B microscopy system is successful using dual-period multilayer mirrors, and now this K–B microscopy system has been used in Shenguang laser facility in China. A series of images have been obtained to perform laser-produced dense plasma diagnostics experiments.
Fig. 5. Calculated reflectivity curve of designed Mg/SiC aperiodic multilayer (solid squares) and periodic multilayer (hollow circles) at an incident angle of 51.
3.2. Extreme ultraviolet Mg/SiC aperiodic multilayer with high reflectivity at 30.4 nm and low reflectivity at 58.4 nm The secondary phase of the Chinese Lunar Exploration Program is scheduled to be launched around 2013, which includes an Extreme Ultraviolet Imager (EUVI). The EUVI is designed to observe the Earth’s plasmasphere by imaging the He + emission (at 30.4 nm wavelength) from the lunar orbit. However, a bright background radiation from the earth’s ionosphere, which is the He emission (at 58.4 nm wavelength), must be blocked [15]. Mg/SiC is a new material pair in the extreme ultraviolet (EUV) region [16], which shows good performance in the 25–35 nm region [17]. A novel Mg/SiC aperiodic multilayer, which exhibited
Fig. 6. Measured reflectivity curve of fabricated Mg/SiC aperiodic multilayer (solid squares) and periodic multilayer (hollow circles) at an incident angle of 51.
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Since the Mg/SiC aperiodic multilayer exhibited the lowest reflectivity at 63.0 nm rather than 58.4 nm, the refractive indices of magnesium and silicon carbide used in the design were assumed to be inaccurate.
3.3. Parabolic periodic multilayer optics with large area working at 19.5 nm In recent years, the development of multilayer technology has enabled the construction of instrumentation and led to a number of successful missions including Solar and Heliospheric Observatory/ Extreme ultraviolet Imaging Telescope (SOHO/EIT) and Transition Region and Coronal Explorer (TRACE) [22–24]. At the wavelengths longer than the Si L-absorption edge near 12.4 nm, Mo/Si was widely used in the wavelength of 13–20 nm wavelength region for its high stability and high reflectivity. The accurate deposition of high reflection and uniformity on ultra-smooth polished parabolic substrates is one of the major challenges for constructing a collimator to test the performance of solar telescope working at 19.5 nm. The Mo/Si multilayer mirror with a size of 230 mm was investigated for wavelength of 19.5 nm (Fe-XII). Fig. 7 shows the X-ray reflective (XRR) curves measured by XRD. Eighteen positions were measured along the diameter direction of the mirror with a spacing of 10 mm. For each measured spot, the period thickness can be calculated from the X-ray reflective peaks according to the amended Bragg formula [25]. The period thicknesses of multilayer can be calculated for each measured position spot. The mean period thickness is 10.30 nm, which is in agreement with the design one. Fig. 8 shows the normalization lateral layer thickness distribution along the diameter of 200 mm. The uniformity (DD/D0) is normalized by the period thickness (D0) at center spot (X¼0 mm). The obtained uniformity is within 70.3% in diameter of 200 mm. Finally, the reflectivity is measured by the reflectometer on beam line U26 at National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Reflectivities were measured at the fixed incident angel of 51 using wavelength-scanning mode. Along the diameter of 200 mm, the reflectivity is measured with spacing of 10 mm. Fig. 9 shows all the measured curves. The consistency of the reflectivity curves also indicates a good uniformity of the period thickness. The measured peak reflectivity at 19.5 nm is (4272)%.
Fig. 8. Normalization lateral layer thickness distribution along the diameter of 200 mm, each data point is calculated from the measured period thickness shown in Fig. 7.
Fig. 9. The reflectivity curves measured on beamline U26 at NSRL.
4. Summary
Fig. 7. X-ray reflectivity curves along the diameter of 200 mm measured with spacing of 10 mm.
In order to meet the needs of special applications in EUV astronomical observation, dense plasma diagnostics in China, three types of multilayer mirrors were successfully developed with specific spectral properties. Firstly, a dual-periodic W/B4C multilayer mirror was designed for Kirkpatrick–Baez (K–B) microscopy working at TiKa line (4.75 keV), which is highly reflective both at hard X-ray (CuKa line at 8.05 keV) and soft X-ray (4.75 keV). Using this mirror, the K–B system can be aligned conveniently in air using hard X-ray instead of in vacuum chamber, which will significantly save cost and time in alignment. Now, 1-dimensional and 2-dimensional K–B microscopy systems have been equipped in Shenguang laser facility in China to perform laser-produced dense plasma diagnostics experiments. Secondly, aperiodic Mg/SiC multilayer was designed for astronomy observation, which is a bi-functional mirror with high reflectivity for He-II emission line (30.4 nm) but suppressing He-I emission line (58.4 nm). It will replace the traditional combination of periodic multilayer and the fragile film filter. This mirror is expected to be applied to the EUV Imager in the second
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phase of the Chinese Lunar Exploration Program. This will be more safe in satellite launching. Thirdly, for the purpose of testing the performance of EUV telescope used for extreme ultraviolet imaging of solar corona by selecting Fe-XII emission line at wavelength of 19.5 nm, Mo/Si multilayer mirror was deposited on large parabolic fused silica substrate with a diameter of 230 mm, having a fairly good uniformity. The uniformity of lateral layer thickness distribution is within 70.3% in the diameter of 200 mm, measured by X-ray diffractometer. The measured peak reflectivity is 42% at the wavelength of 19.5 nm.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant no. 10825521, 10675091, 10675092, and 10876023), High-tech 863 program (Grant no. 2006AA12Z139) and by the Shanghai Committee of Science and Technology, China (Grant no. 09XD1404000, 07DZ22302, 09ZR1434300, 0952nm06900). The authors thank Professor Alan Michette in King’s College London, Dr. Mike MacDonald and Mark Roper in STFC Daresbury Laboratory, and Dr. Franz Sch¨afers and Andreas Gaupp at BESSY-II for their kindly help in discussion and measurement. References [1] J.-Ph. Champeaux, Ph. Troussel, B. Villier, V. Vidal, T. Khachroum, B. Vidal, M. Krumerey, Nucl. Instr. and Meth. Phys. Res. Sect. A 581 (2007) 687. [2] K. Yamashita, et al., Appl. Opt. 37 (1998) 8067. [3] E. Ziegler, Proc. SPIE 2253 (1994) 248.
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