Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1 Elsevier Science Journal logo Applications of Multilayer Optics Zhanshan Wang*, Jingtao Zhu, Baozhong Mu, Zhong Zhang, Fengli Wang, Jing Xu, and Lingyan Chen Institute of Precision Optical Engineering (IPOE), Physics Department, Tongji University, Shanghai 200092, China Elsevier use only: Received date here; revised date here; accepted date here Abstract Recent Chinese applications of multilayer optics in plasma diagnostics were reviewed in this paper. For some applications such as plasma diagnostics, Extreme ultraviolet (EUV) astronomical observations in China, we developed a series of multilayer optics with special performance. Double periodic Kirkpatrick-Baez (K-B) mirrors with high reflectivity at 8keV and 4.75keV, double bi-functional mirror with high reflectivity at 30.4nm and very low reflectivity at 58.4nm and large parabolic periodic multilayer optics working at 19.5nm for testing the performance of an EUV telescope were successfully designed, fabricated and characterized by using different multilayer stack structures. © 2001 Elsevier Science. All rights reserved Keywords: multilayer optics; reflectivity; magnetron sputtering; EUV; K-B microscope 1. Introduction Multilayer optics are widely used as key reflective elements in EUV lithography, astronomical observations, X-ray lasers, applications at synchrotron radiation facilities and plasma diagnostics, in Extreme ultraviolet (EUV), soft X-ray and X-ray ranges for more than thirty years[1-4]. Except normal periodic multilayer optics, there are some special requirements which are not meet by using periodic multilayer coatings in some of applications, such as bi-function optic, etc. special multilayer optics have been developed in these cases. Progress of thin-film deposition techniques made it possible to fabricate multilayers with lateral dspacing variations of the high precision necessary to modify the performance of these mirrors significantly. Combining the special multilayer optics with figured substrates, EUV, soft X-ray and X-ray optical elements with beam-tailoring properties became possible. Today, multilayer optics with special property is gradually used with a further increasing tendency. In China, there are some special requirements for using multilayer coatings in EUV, soft X-ray and Xray region. This paper summarizes our recent developments of periodic multilayer optics in applications of dense plasma diagnostics and EUV ——— * Corresponding author. Tel.: +86-21-65984652; fax: +86-21--65986323; e-mail: [email protected]. 2 Elsevier Science astronomical observations. In the first section we introduce the design, fabrication and characterization of multilayer optics. In the second section, some examples of multilayer optics with special performance for applications in EUV, soft X-ray and X-ray range were described. 2. Design and Experimental Keys to design of multilayer optics with special performance include how to select merit functions, optimized algorithms, and initial structures. The choice of merit function depends on the requirements of multilayer optics. Different applications need different merit functions. After selection of merit function, optimized algorithms need to choose [5-8]. There are three algorithms we used. They are simulated annealing algorithm, random search algorithm and local optimization algorithm. When the local optimization algorithm is selected, initial structures of multilayers is very important and decide to how good the results obtained. Quarter wave periodic multilayer and multilayer stacks with a variety of periods are often used during the optimization. The initial solutions generated by analytical expression can be used in most of time. The multilayer optics were deposited by magnetron sputtering in vacuum systems at base pressure below 110-4 Pa, using Ar gas with purity 99.9999%, and typical sputtering gas pressure of 0.2 Pa. For depositing multilayers required high thickness uniformity, masks and varying rotation speed methods were used. The structures of the periodic and non-periodic multilayer coatings were characterized using X-ray reflectometry. The EUV and soft X-ray characterization was performed at the synchrotron reflectometer. The roughness of substrate and multilayer optics was measured by using an atomic force microscope. Experimental details can be found in previous publications [9]. 3. Results and Discussions 3.1. X-ray K-B microscope system by using double periodic multilayers K-B configuration consists of two perpendicular concave spherical mirrors in tandem. Rays from 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 perpendicular structure plays an important role to overcome the strong astigmatism. K-B microscope is often used in dense plasma diagnostics [10]. There is a strong requirement for developing a K-B microscope working at 4.75keV 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 and high reflectivity and accuracy of system alignment. Alignment of K-B microscope is generally based on repeating X-ray at wavelength imaging experiments if the very high performance wanted to be obtained. This operation has to be done in vacuum for soft Xray K-B microscope working at 4.75keV because of the absorption of 4.75keV soft X-ray in air, so the experiments are very complex and expensive. The alignment can be done in air by using 8keV Cu K line and use in the vacuum for imaging working at 4.75keV if the K-B microscope works at 4.75keV and 8keV at the same time in order to simplify the alignment and save money. A double-periodic multilayer optics was proposed for above purpose, which consists of two part stacks with different periodic thicknesses. The top stack reflects the soft X-rays at 4.75keV for imaging, while the bottom stack is designed at X-rays at 8keV, Cu K line for alignment in the air. Therefore, it’s available for Xrays of two different energies to be reflected at the same grazing angle. It is possible to design the top and bottom stacks individually and then put them together. We developed the double period multilayer which includes W/B4C top layers and W/B4C bottom layers. Shown as Fig. 1, this multilayer has high reflectivity at grazing angle of 1.2° both for 4.75keV and 8keV rays. Thus we can realize the alignment of 3 Elsevier Science 4.75keV K-B microscope by imaging experiments at 8keV in air condition. Because the FWHM of 8 keV is narrower than that of 4.75keV, the alignment accuracy will be adequate enough. A one dimensional advanced K-B microscope with two spherical mirrors in one direction working at 4.75keV was set up firstly, shown as Fig. 2, whose imaging performance was better than that who has one spherical mirror in one direction because the spherical and coma aberration were partly corrected. The imaging alignments results at 8keV were shown as Fig 3 (a). Then, we marked the best object point with a small ball. In Shenguang laser facility, we should just make the target at the same point with the marked ball. Finally, we got the imaging results of 1500l/inch mesh at 4.75keV (Ti Kα) (shown in figure 3 (b)). In summary, double periodic multilayer provides a very practical method for the alignment of soft X-ray grazing system in vacuum. Fig.1. Reflectivity of double periodic W/B4C multilayer designed both for 4.75keV and 8keV periodic multilayer optics to solve the difficulty of alignment in the vacuum at 4.75keV. The experimental image of 2000lines/inch golden mesh was shown in Figure 4 and a series of images of a dense plasma were gotten which produced by hitting eight laser beams with 300J pulse energy and 2ns time width along four directions perpendicular to each other to a cylindrical target with 260 micron in diameter and thickness of chamber of 20 micron. The laser beam to produce the Ti plasma has width of 100ps and 800J energy per pulse. The different time dense plasma imaging during its evolvement could be obtained by changing the backlit laser time relative to the laser starting time who produced the dense plasma. (a) (b) Fig. 3. Imaging experimental results of X-ray KB microscope. (a) Imaging at 8keV in air condition. (b) Imaging at 4.75keV in vacuum. Fig. 2. Schematic of one dimensional advanced K-B microscope After the successful fabrication of one dimensional advanced K-B microscope, we were asked to make another two dimensional K-B microscope working at 4.75keV to cast a dense plasma backlit image at the detector for diagnostics of its evolvement. We adopted the same double Fig. 4. Two dimensional imaging experimental results of Xray KB microscope working at 4.75keV 4 Elsevier Science 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+ 30.4 nm emission from the lunar orbit. However, a bright background radiation from the earth’s ionosphere, which is a He 58.4 nm emission, must be blocked [11]. Mg/SiC is a new material pair in the extreme ultraviolet (EUV) region [12], which shows good performance in the 25-35 nm region [13]. A novel Mg/SiC nonperiodic multilayer, which exhibited HR at 30.4 nm and LR at 58.4 nm, was designed, fabricated and characterized. The Mg/SiC nonperiodic multilayer was designed. Substrates were pieces taken from 100 orientation polished single-crystal silicon wafers. The optical constants of magnesium, silicon carbide, and silicon between 10 nm and 41.3 nm were from the Center for X-ray Optics [14], while the optical constants between 41.3 nm and 70 nm were from Ref. [15,16,17]. The incident angle was 5°. The layer number was limited to 60 because of absorption. periodic multilayer exhibits a reflectivity of 58.7% at 30.4 nm and 2.2% at 58.4 nm. In Figure 6, at an incident angle of 5°, the reflectivity of the nonperiodic multilayer at wavelengths of 30.4 nm 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 fabricated and measured, whose reflectivity is 34.4% at 30.4 nm and 1.8% at 58.4 nm. Since the Mg/SiC nonperiodic 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. Fig. 6. Measured reflectivity curve of fabricated Mg/SiC aperiodic multilayer (solid squares) and periodic multilayer (hollow circles) at an incident angle of 5° 3.3. Parabolic periodic multilayer optics with large area working at 19.5nm Fig. 5. Calculated reflectivity curve of designed Mg/SiC nonperiodic multilayer (solid squares) and periodic multilayer (hollow circles) at an incident angle of 5° A calculated reflectivity curve of a designed Mg/SiC nonperiodic multilayer and a periodic multilayer is shown in Figure 5. At an incident angle of 5°, the nonperiodic multilayer has a reflectivity of 54.1% at 30.4 nm and 0.1% at 58.4 nm, while the 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) [18,19,20]. At the wavelengths longer than the Si L-absorption edge near 12.4nm, 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 reflective and uniformity on ultra-smooth polished parabolic substrates is one of the major challengers for construct a collimator to test the performance of solar telescope working at 19.5nm. the Mo/Si multilayer Elsevier Science mirror with the size of 230mm was investigated for wavelength of 19.5nm (Fe-XII). Figure 7 shows the X-ray reflective (XRR) curves measured by XRD. 18 positions were measured alone the diameter direction of the mirror with a spacing of 10mm. For each measured spot, the period thickness can be calculated from the X-ray reflective peaks according to the amended Bragg formula [11].The period thicknesses of multilayer can be calculated for each measured position spot. The mean period thickness is 10.30nm, in agreement with the design one. Fig. 7. X-Ray reflectivity curves along the diameter of 200mm measured with spacing of 10mm 5 calculated uniformity is within ±0.3% in diameter of 200mm. 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 5 degree using wavelength-scanning mode. Along the diameter of 200mm, the reflectivity is measured with spacing of 10mm. Figure 9 shows all the measured curves. The consistency of the reflectivity curves also indicates a good uniformity. The measured peak reflectivity at 19.5nm is (42±2)%. Fig. 9. The reflectivity curves measured on beamline U26 at NSRL 4. Summary Fig. 7. Normalization lateral layer thickness distribution along the diameter of 200mm, each data point is calculated from the measured period thickness shown in Figure 7 Figure 8 shows the normalization lateral layer thickness distribution along the diameter of 200mm. The uniformity (ΔD/D0) is normalized by the period thickness (D0) at center spot (X=0mm). The Some multilayer optics for different applications in EUV, soft X-ray and X-ray wavelength region were studied. All the multilayers were deposited by using DC magnetron sputtering and characterized by grazing incidence x-ray reflectometry analysis and synchrotron radiation. A series of multilayer optics were developed to meet the special requirement of some experiments in China, such as dense plasma diagnostics and EUV astronomical observations. The double periodic multilayer mirrors are shown to become essential optical elements in K-B microscope system for dense plasma diagnostics. It will be convenient to align the K-B microscope system in air, not in vacuum chamber. A Mg/SiC nonperiodic multilayer for the EUV with high reflectivity at 30.4 6 Elsevier Science nm and low reflectivity near 58.4 nm has been demonstrated. This kind of nonperiodic multilayer is expected to be applied to the EUV Imager in the second phase of the Chinese Lunar Exploration Program. For the purpose of testing the performance of EUV telescope gotten extreme ultraviolet imaging of solar corona by selecting Fe-XII emission line at wavelength of 19.5nm, Mo/Si multilayer mirror was deposited on large parabolic substrate with diameter of 230mm with a fairly good uniformity. The uniformity of lateral layer thickness distribution is within ±0.3% in diameter of 200mm, measured by Xray diffractometer. The measured peak reflectivity is 42% at wavelength of 19.5nm. 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 Schäfers 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, and M. Krumerey, Nucl. Instrum. Methods 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, and N. G. Publicover, J. X-Ray Sci. Technol 7 (1997) 139. [5] I.V. Kozhevnikov, I.N. Bukreeva, E. Ziegler, Nuci. Instr. Meth. Phys. Res. A 460 (2001) 424. [6] X. Cheng, Z. Wang, Z. Zhang, F. Wang, and L. Chen, Opt. Comm. 265 (2006) 197. [7] P. D. Binda, and F. E. Zocchi, Proc. SPIE 5536 (2004) 97108. [8] V. T. Alexander, K. T. Michael, V. P. Vladimir, and V. V. Andrei, Proc. SPIE 3738 (1999) 248. [9] Z. S. Wang, F. L. Wang, Z. Zhong, H.C. Wang, W. J. Wu, S. M. Zhang, Y. Xu, L.Y. Chen, Optics and Precision Engineering, 13 (2005) 512 [10] 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, and S. Magnan, Rev. Sci. Instrum. 79, (2008) 10E904 [11] D. D. Allred, R. S. Turley, M. B. Squires. Proc. of SPIE, 3767 (1999) 280. [12] G. Murakami, K. Yoshioka, I. Yoshikawa. Proc. of SPIE, 6317 (2006) 631714 [13] T. Ejima, A.Yamazaki, T. Banse et al. Appl. Opt., 44 (2005) 5446. [14] http://www.cxro.lbl.gov/optical_constants/getdb2.html [15] E.D. Palik. Handbook of optical constants of solids Ⅲ. San Diego: Academic Press, 1998, 238 [16] E. D. Palik. Handbook of optical constants of solids. Orlando: Academic Press, 1985, 559 [17] B. Kortright, D. L. Windt. Appl. Opt., 27 (1988) 2841 [18] M F Ravet, F Bridou, X Zhang-Song, A. Jerome, F. Delmotte, R. Mercier, M. Bougnet,P.Bouyries and J.P. DelaboudiniËre. Proc. SPIE, 5250 (2004) 99 [19] L. Gardner, J. Kohl, S. Cranmer, S. Fineschi, L. Golub, J. Raymond, P. L. Smith, L. Strachan. Proc. SPIE, 3764 (1999) 134. [20] M. Grigonis, E J Knystautas. Appl. Opt., 36 (1997) 2839 [21] 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.