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Fabrication of nested elliptical KB mirrors using profile coating for synchrotron radiation X-ray focusing
Applied Surface Science 258 (2012) 2182–2186
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of nested elliptical KB mirrors using profile coating for synchrotron radiation X-ray focusing Chian Liu a,∗ , G.E. Ice b , W. Liu a , L. Assoufid a , J. Qian a , B. Shi a , R. Khachatryan a , M. Wieczorek a , P. Zschack a , J.Z. Tischler b a b
X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
a r t i c l e
i n f o
Article history: Available online 23 February 2011 PACS: 81.15.Cd 41.50.+h 42.82.Cr Keywords: Profile coating X-ray optics KB mirrors Sputter deposition
a b s t r a c t This paper describes fabrication methods used to demonstrate the advantages of nested or Montel optics for micro/nanofocusing of synchrotron X-ray beams. A standard Kirkpatrick-Baez (KB) mirror system uses two separated elliptical mirrors at glancing angles to the X-ray beam and sequentially arranged at 90◦ to each other to focus X-rays successively in the vertical and horizontal directions. A nested KB mirror system has the two mirrors positioned perpendicular and side-by-side to each other. Compared to a standard KB mirror system, Montel optics can focus a larger divergence and the mirrors can have a shorter focal length. As a result, nested mirrors can be fabricated with improved demagnification factor and ultimately smaller focal spot, than with a standard KB arrangement. The nested system is also more compact with an increased working distance, and is more stable, with reduced complexity of mirror stages. However, although Montel optics is commercially available for laboratory X-ray sources, due to technical difficulties they have not been used to microfocus synchrotron radiation X-rays, where ultra-precise mirror surfaces are essential. The main challenge in adapting nested optics for synchrotron microfocusing is to fabricate mirrors with a precise elliptical surface profile at the very edge where the two mirrors meet and where Xrays scatter. For example, in our application to achieve a sub-micron focus with high efficiency, a surface figure root-mean-square (rms) error on the order of 1 nm is required in the useable area along the X-ray footprint with a ∼0.1 mm-diameter cross section. In this paper we describe promising ways to fabricate precise nested KB mirrors using our profile coating technique and inexpensive flat Si substrates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In 1948 Kirkpatrick and Baez [1] provided a practical solution to image X-rays with a pair of cylindrical mirrors at glancing angles to the X-ray beam and arranged sequentially at 90◦ to each other so that the X-rays can be focused to a point. Nowadays elliptical KB mirrors are widely used in the X-ray community. The elliptical shape of modern mirrors avoids spherical aberrations and with near ideal mirrors provides diffraction-limited focusing. There are a number of ways to achieve elliptical mirrors. Sophisticated bending techniques have been developed to bend uniformly coated Si trapezoidal plates to achieve the desired elliptical shape for microfocusing. Submicron X-ray beams have been achieved by using benders for KB mirrors [2–4]. KB mirrors with benders are flexible in adjusting the focal length, but the benders are bulky and it is hard to achieve diffraction-limited focusing. Monolithic KB mir-
∗ Corresponding author. Tel.: +1630 252 9985; fax: +1630 252 9303. E-mail address: [email protected] (C. Liu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.079
rors are much easier to use if the desired elliptical surface profile can be fabricated. Surface figure errors cause focal aberrations and remain an important barrier for fabrication of synchrotron X-ray KB mirrors. The required precision increases as the X-ray wavelength decreases. Aspherical surfaces are difficult to make and precision elliptical mirrors for hard X-rays are very expensive. So far computer controlled optical surfacing and elastic emission machining techniques are used to produce ultra precise non-spherical surfaces [5,6]. These techniques require many cycles of measuring and polishing to achieve a smooth and accurate surface profile. We have developed a profile coating technique to convert cylindrical and flat inexpensive substrates into precise elliptical mirrors [7,8]. Elliptical KB mirrors with sub-nm figure errors can now be produced with only two runs of profile coatings. Sub-100 nm focusing has been demonstrated in a synchrotron X-ray beamline [9]. In addition to total reflective KB mirrors, laterally graded multilayer mirrors are also wildly used [10,11]. The Bragg angle of a multilayer can be several times larger than the critical angle totalexternal-reflection mirrors, leading to a larger numerical aperture. A higher numerical aperture corresponds to a smaller diffraction limited focal spot. However, in many applications, such as in Laue
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more stable and robust instrument. It also simplifies the alignment process since the system is pre-aligned and only two small angular adjustments are needed. 2.2. The profile coating technique
Fig. 1. Illumination of a Montel KB mirror system for synchrotron X-rays.
microdiffraction, an achromatic beam from total reflection KB mirrors is essential [12]. Montel [13] or nested KB mirrors, with a higher demagnification and potentially smaller focusing than standard KB mirrors, are a desirable choice. Montel optics is commercially available for laboratory X-ray sources [14,15]. Because of the limited distance between the source and the focal point (between two foci of an ellipse), available Montel optics use multilayers on prefigured substrates to focus the X-ray beam. There is no commercial Montel optics for synchrotrons because of challenges in both figuring and polishing in the corner area where synchrotron X-rays are reflected. Indeed, because the undulator synchrotron beam used at the beamline is very narrow, ∼100 m, only the mirror surface within 100 m of the corner where the mirrors touch is used. This makes the fabrication of Montel KB mirrors very challenging. The source to sample distance is however, quite large for synchrotrons, in tens of meters or longer, so that total-external-reflection mirrors can be used. This paper describes techniques to fabricate precision nested KB mirrors using the profile coating technique and flat Si substrates. To our knowledge, this is the first successful demonstration of Montel mirror at a synchrotron radiation facility. 2. Experiment 2.1. The mirror design Fig. 1 illuminates the construction of a prototype Montel KB mirror design [12]. Two profile-coated elliptical mirrors are mounted on a precision roll adjustment mirror holder with the two elliptical surfaces focusing the X-ray beam. The two mirrors have the same design parameters, with the source to mirror distance, S1 = 36 m, focal length, S2 = 60 mm, and mirror angle, = 3 mrad. Both mirrors have a surface area of 9 mm × 40 mm and a height (thickness) of 20 mm. X-rays reflected from one mirror and reflected from the other are doubly focused at the focal point. Since the nested KB mirrors are positioned side by side, the back side of the vertically deflecting mirror must be figured or polished to allow close contact with the surface of the other mirror. Otherwise X-rays will miss the mirror surface at the gap between the mirrors. For example, the gap between the two mirror surfaces needs to be small enough so that the mirrors can collect ∼90% of a 120 m × 120 m incident X-ray beam. The two mirrors are pre-aligned perpendicularly with an optical laser. Once the mirrors are aligned relative to each other, the assembled mirror pair is rotated and translated to optimize the beam intensity and focus. This approach eliminates the two translation stages needed in a traditional KB system and allows for a
The profile strategy [7] utilizes a contoured aperture mask in a DC magnetron sputtering system with a simple linear motion of substrates to coat the design profile. The mirror substrates are translated perpendicular to their nominal scattering plane. The coating thickness at each position on the substrate is proportional to the opening width of the mask. The opening width of the contour is calculated every 0.1 mm according to the desired coating profile and the film thickness mapping at the substrate level. The coating profile is the difference between the ideal and the substrate surface profile. A designed mask is then made by electro-discharge machining using a thin invar plate. Au and Pt have been successfully used as the main coating material, with a 50 nm-thick Cr layer pre-coated on the Si substrate to enhance the adhesion between the Si substrate and the noble metal. A test deposition is carried out on a flat Si strip using the mask placed above the sputter gun and ∼0.5 mm away from the substrate. The coating profile of the test run is measured using an ellipsometer. The mirror substrates are then aligned on a sample holder at the exact position as determined by the test run and coated to the right amount as calibrated in the test run measurement. Very precise elliptical KB mirrors with sub-nm rms height errors can be obtained with one primary profile coating followed by a corrective profile coating. A typical primary coating lasts only a few hours for a 10 m-high elliptical profile. Pt was chosen as the main profile-coating material for the nested KB mirrors. Pt has a smaller thermal expansion coefficient than Au and thus a smaller thermal stress for films grown on Si, which has an even smaller thermal expansion. The film stress may undergo a relaxation process due to a diffusive relief of compressive stresses initially generated during deposition [16]. The film stress relaxation may also cause a KB mirror deformation under extended X-ray radiation. A film with a lower stress is expected to have a lower stress relaxation. It has been observed that the Pt-coated KB mirrors are more stable than the Au-coated ones [17]. The depositions were carried out in a small deposition system with a 9.5 inch OD, 4 ft-long vacuum chamber turbo-pumped to a base pressure of low 10−7 Torr. The depositions were carried out at ambient temperatures and at an Ar pressure of 2.3 mTorr with substrates moving linearly over two 3 in. diameter planar sputter guns at a constant speed. To overcome the buildup of film stress in thick films, the profile coatings were carried out in stages, with resting periods of 15 min each for every ∼1 m-height of the desired elliptical profile [8]. We have prepared two sets of nested KB mirrors. In the first set, two super-polished flat Si substrates with dimensions of 9 mm (W) × 40 mm (L) × 20 mm (H) were used. One of them had the side polished. These two substrates were mounted together and coated at the same time. The edge surface of the side-polished mirror and the center area of the other one were measured with metrology to provide data for corrective coatings and final acceptance. It turned out that even though the two substrates were mounted together, the coated profiles were slightly different. The side-polished one needed an additional corrective coating. Apparently, an edge effect was present during coating, resulting in a small coating difference between the surfaces near the edge and at the center. Also, the side-polished mirror had quite a few chips at the edge. In the second set, another super-polished flat Si substrate with dimensions of 25 mm (W) × 50 mm (L) × 4.5 mm (H) was used. The center area was measured with metrology after coating. This time, only one primary plus a corrective profile coating were needed to obtain the desired elliptical profile with
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<1 nm rms height error. The coated mirror was cut to obtain two pieces with dimensions of 9 mm (W) × 40 mm (L) × 4.5 mm (H) from the central portion. The side of one mirror that needs side-polishing was determined according to the elliptical profile direction relative to the incoming X-ray in the nested KB mirror assembly. After side-polishing, the mirrors were mounted to 9 mm (W) × 40 mm (L) × 15.5 mm (H) aluminium blocks. 2.3. Substrate/mirror side-polishing The mirror substrate to be side-polished before coating had the side surface ground to about 89◦ to the mirror surface. This <90◦ angle makes it possible to bring the two finished KB mirrors in close contact during the final assembling process. The mirror substrate was mounted on a polishing holder surrounded by four 20 mmhigh Si pieces. The mirror substrate and the surrounding pieces were mechanically ground with a 1◦ wedge. Polishing proceeded from loose abrasively grinding using 9 m sized aluminum oxide, to polishing using 1 m diamond slurry. During the final stages of polishing, the sharpness of the edge near to the mirror surface was closely monitored under a microscope. The polishing process was continued until a very sharp straight line appeared. A chemical mechanical polish was used as a final step. For the coated mirrors, an important issue is to protect the coating surface during cutting and polishing. Pure optical wax was used to glue the coated surface to another Si wafer for cutting to the desired dimensions using a diamond-dicing saw. Then the piece that needed side-polishing was glued onto the coated face to the smooth face of another Si piece with the same dimension for side polishing. The same polishing procedure was carried out without angle-grinding. To make a <90◦ angle between the side-polish face and the mirror surface, a special polishing holder with a ∼1◦ tilt is needed. For the present study, the polishing holder had no tilt surfaces. After polishing, the mirror was removed from the holder together with the other mounting Si using a gentle heating. It was then separated from the mounting Si using trichloroethylene and cleaned in acetone and methanol. 2.4. Metrology measurements All metrology measurements were carried out using a stitching interferometer, model MicroXAM RTS [18]. The interferometer was operated in a phase-shifting mode, using filtered light centered at 551 nm wavelength with 25 nm bandwidth. An integrated high-resolution closed-loop translation stage was used to scan the mirror’s surface in both x and y directions to automatically acquire measurement maps at predetermined areas. An objective lens of 2.5× magnification was used, with each individual measurement covering a field of view of approximately 4.9 mm × 4.9 mm. Multiple images along the longitudinal direction were taken with a 30% overlap to obtain the whole image of the mirror. The manufacturer’s proprietary stitching algorithm was used to precisely overlap neighboring measurements and to reconstruct the surface profile. A linear profile can be extracted from any position of the reconstructed surface along the longitudinal direction. Usually it is extracted from the center of the mirror surface and compared with the designed ellipse. The difference between the measured and the design ellipse is the corrective coating profile for the next round of coating process. After a successful corrective profile coating, the measured data is fit with a best-fit-ellipse to obtain the residual height error. For nested KB mirrors, one of them has one side polished to make good contact with the other mirror. Metrology measurements were used to make sure that the very edge of the mirror surface at the corner has the required profile without chips from polishing. The
Fig. 2. Image of the polished edge of a Pt-profile-coated KB mirror sitting on top of a bare Si mounting support. A sharp straight edge is evident in the picture.
edge of the mirror was carefully aligned with the translation stage and parallel to the traveling direction during the measurement. Line profiles at 10 m intervals starting from the edge were extracted from the measured surface profile. For the other mirror that does not need side-polishing, the center area was measured. 3. Result Excellent metrology results have been obtained for the second set of profile-coated nested KB mirrors after cutting and sidepolishing. The coated surface had a very precise elliptical profile with a height error of 0.73 nm rms after one primary and one corrective profile coating. After cutting into two pieces, one piece was side polished and the other piece was cleaned for final assembly. Fig. 2 is an image of the polished edge of the coated mirror glued on top of a Si mounting support with optical wax after the final polishing. The picture was taken using a camera on a stylus profiler with a 260× magnification [19]. The polished side produced a sharp straight edge along the bottom of the coated mirror, as seen in the picture. The coated elliptical surface is perpendicular to the polished side and not visible in the image. After the KB mirror was separated from the mounting Si piece, the corner area of the elliptical surface near the polished side was measured with metrology. Fig. 3 shows the measured profile for the KB mirror after a corrective profile coating before cutting (Fig. 3a) and that for the corner area, at 30 m from the edge after cutting and polishing (Fig. 3b). The measured data and the best-fitellipse (left Y axis) overlap and are not distinguishable in the figure. The residual profile (right Y axis) shows the difference. The fitting parameters (S1, S2, and ) and the rms height errors are presented in the figures. Comparing Fig. 3a and b one can see that the best-fit parameters are very close in these two cases. The height error is slightly larger after polishing, 1.05 nm vs. 0.73 nm rms. This error increases closer to the edge. At 20 m from the edge it becomes 1.32 nm and at 10 m from the edge it is 1.82 nm rms. The increased rms numbers are due to chips at the edge, shown in the metrology measurement as sharp spikes. A few such spikes are seen in Fig. 3b. The overall shape of the residual profile is remarkably similar, as shown in Fig. 3a and b. For the first set of nested KB mirrors, coatings were carried out on a pair of shaped substrates with the right dimensions. One of them had the side that needed side-polishing fine polished. The pair was then profile-coated. The side-polished mirror showed a lot of spikes in the metrology data, leading to a poor height-error of 3.0 nm rms at 40 m from the edge and 8.0 nm rms at 10 m from
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4. Discussions and summary
Fig. 3. Measured Pt profile for the KB mirror after a corrective profile coating, (a) before cutting and (b) at 30 m from the edge after cutting and polishing.
the edge. The other mirror however had excellent surface figure with a 0.76 nm rms height error along the center-line area. The first set of nested KB mirrors has been tested at the Advanced Photon Source 34-ID beamline. Even though the side-polished mirror had many defects at the edge, the test was very successful. Final test results and details will be presented elsewhere [20]. Both mirrors were used to focus up to a 120 m by 120 m incident X-ray beam with broad-bandpass of energies up to ∼30 keV. The side-polished mirror was used to focus in the vertical direction and the other mirror focused in the horizontal direction. The experimental station is located 64 m from the synchrotron undulator source. The size of the source is ∼40 m-high in the vertical plane and ∼700 m-wide in the horizontal plane. To reduce the horizontal size, a ∼100 m-wide slit was placed at 28 m from the synchrotron [9]. The side-polished KB mirror was successfully brought into contact to the companion mirror with a small gap of 8–9 m. For the second set of nested KB mirrors, the side-polished mirror could not be brought to a close contact with the other mirror. The gap could not be made smaller than ∼30 m while keeping the two mirrors perpendicular. Since the mirror was polished without using a tilt polishing holder, the round-off at the edge was severe enough to prevent a smaller gap when the two mirrors were at 90◦ . As perpendicularity of the two KB mirrors is critical for best focusing [9,21], this set of nested KB mirrors was not used in the initial beamline test.
The initial test of nested KB mirrors demonstrates that Montel optics can be fabricated for synchrotron beamlines and the profile coating technique is capable of producing high quality Montel KB mirrors using flat Si substrates. Comparing the two fabrication methods used in the present study, the method in which the side is polished first and the mirrors are coated together, can produce thicker mirrors. However, additional efforts must be made to prevent chipping at the edge and to improve the accuracy of coated elliptical profile. The second method, to coat a wider mirror first, cut it into two pieces, and then polish the side of one of them, can produce better edges. Multiple mirrors with the same design parameters can be coated in the same coating run. The polishing holder however, must be redesigned to polish the side of one of the KB pair to a <90◦ angle to the elliptical mirror surface. At present the mirror thickness is limited by our diamond dicing saw to 4.5 mm. The dicer has thin blades with sintered small diamonds of 3–6 m in size rotating at very high speed of 20,000 rpm. It provides smooth cuts not achievable from other cutting machines. The thickness of the KB mirrors may need to be larger than 4.5 mm if the mirrors are going to be operated for extended radiation conditions [17]. The focal size of the present tested mirror set can be speculated using the diffraction-limit and the geometrical source demagnification. In the vertical plane, the X-ray source size is ∼40 m, and the demagnification factor is ∼1067. The focal size ideally should reach the diffraction limit of sub-50 nm in the vertical direction [9]. The side-polished mirror was positioned to deflect the vertical beam and ideally should achieve a sub-50 nm focus. The non-ideal fabrication result of imperfections at the edge area however, was expected to affect the performance of this mirror. In the horizontal plane, the source size is redefined by a 100 m-wide slit placed at ∼36 m from the mirror and the demagnification factor becomes ∼600. This small demagnification limits the focal size to ∼160 nm. The mirror positioned to deflect the horizontal beam had a very precise elliptical profile that would help to achieve this limit. The initial test result has confirmed these expectations [20]. In summary, we have successfully fabricated and tested a firstever synchrotron hard X-ray Montel KB mirror system. The profile coating technique combined with fine mechanical polishing provide a simple and economic solution to the challenging problem of fabricating KB mirrors with a very precise elliptical surface profile at the small corner area for synchrotron hard X-rays. This method is quite straightforward and effective in converting inexpensive flat Si substrates into precise elliptical KB mirrors. The success of the initial test and the significant advantages of Montel/nested optics are expected to stimulate more applications of Montel optics in the synchrotron community. Acknowledgements We thank A. Khounsary for ordering the Si substrates, D. Shu for the mirror stage design, and J. Attig for technical assistance, all from Argonne National Laboratory. This work is supported by the UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under contract no. DE-AC02-06CH11357. G.E.I and J.Z.T. are supported by the Center for Defect Physics an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCS99. References [1] P. Kirkpatrick, V. Baez, Formation of optical images by X-rays, J. Opt. Soc. Am. 38 (9) (1948) 766–774.
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[2] H.A. Padmore, M.R. Howells, S.C. Irick, T. Renner, R. Sandler, Y.M. Koo, New schemes for producing high-accuracy elliptical X-ray mirrors by elastic bending, Proc. SPIE 2856 (1996) 145–156. [3] P.J. Eng, M. Newville, M.L. Rivers, S. Sutton, Dynamically figured Kirkpatrick Baez X-ray micro-focusing optics, Proc. SPIE 3449 (1998) 145–156. [4] O. Hignette, et al., Submicron focusing of hard X-rays with reflecting surfaces at the ESRF, Proc. SPIE 4499 (2001) 105–116. [5] Computer Controlled Optical Surfacing, Tinsley Laboratories Inc., 4040 Lakeside Dr. Richmond, CA 94806 USA, www.asphere.com. [6] K. Yamamura, et al., Fabrication of elliptical mirror at nanometer-level accuracy for hard X-ray focusing by numerically controlled plasma chemical vaporization machining, Rev. Sci. Instrum. 74 (2003) 4549–4553. [7] C. Liu, L. Assoufid, A.T. Macrander, G.E. Ice, J.Z. Tischler, Profile coating of KB mirrors at the advanced photon source, SPIE 4782 (2002) 104–112. [8] C. Liu, R. Conley, L. Assoufid, Z. Cai, J. Qian, A.T. Macrander, From flat substrate to elliptical KB mirror by profile coating, AIP Conf. Proc. 705 (2004) 704–707. [9] W. Liu, G.E. Ice, J.Z. Tischler, A. Khounsary, C. Liu, L. Assoufid, A.T. Macrander, Short focal length Kirkpatrick-Baez mirrors for a hard X-ray nanoprobe, Rev. Sci. Instrum. 76 (2005) 113701. [10] C. Morawe, P. Pecci, J.C. Peffern, E. Ziegler, Design and performance of graded multilayers as focusing elements for X-ray optics, Rev. Sci. Instrum. 70 (1999) 3227–3232. [11] H. Mimura, S. Handa, T. Kimura, H. Yumoto, D. Yamakawa, H. Yokoyama, S. Matsuyama, K. Inagaki, K. Yamamura, S. Yasuhisa, K. Tamasaku, Y. Nishino, Y.
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Yabashi, T. Ishikawa, K. Yamauchi, Breaking the 10 nm barrier in hard-X-ray focusing, Nat. Phys. 6 (2) (2010) 57–60. G. Ice, et al., At the limit of polychromatic microdiffraction, Mater. Sci. Eng.: A 524 (2009) 3–9. M. Montel, X-ray Microscopy with Catamegonic Roof Mirrors, in X-ray Microscopy and Microradiography, Academic Press, New York, 1957, p. 177185. X-ray products, Osmic confocal Max-Flux, Rigaku, (http://www.rigaku.com/optics/confocal.html). Incoatec, Montel optics, (http://www.incoatec.de/products/montel-optics). D.J. Quinn, B. Wardle, S.M. Spearing, Residual stress and microstructure of asdeposited and annealed, sputtered yttria-stabilized zirconia thin films, J. Mater. Res. 23 (2008) 609–618. C. Liu, R. Conley, J. Qian, C.M. Kewish, W. Liu, L. Assoufid, A.T. Macrander, G.E. Ice, J. Z. Tischler, Recent developments in profile-coated elliptical KB mirrors, conference paper, (2008), unpublished. ADE Phase Shift Inc., 3470 East Universal Way, Tucson, AZ. 85706, a subsidiary of KLA-Tencor Corp., www.KLA-Tencor.com. Dektak 8 programmable surface profiler, manufactured by Veeco Metrology LLC, 112 Robin Hill Road, Santa Barbara, CA 93117, USA. W. Liu, G. Ice et al., Achromatic nested Kirkpatrick-Baez mirror optics for hard X-ray nanofocusing, to be published. S. Matsuyama, et al., Diffraction-limited two-dimensional hard-X-ray focusing at the 100 nm level using kirkpatrick-baez mirror arrangement, Rev. Sci. Instrum. 76 (8) (2005), 083114.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of nested elliptical KB mirrors using profile coating for synchrotron radiation X-ray focusing Chian Liu a,∗ , G.E. Ice b , W. Liu a , L. Assoufid a , J. Qian a , B. Shi a , R. Khachatryan a , M. Wieczorek a , P. Zschack a , J.Z. Tischler b a b
X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
a r t i c l e
i n f o
Article history: Available online 23 February 2011 PACS: 81.15.Cd 41.50.+h 42.82.Cr Keywords: Profile coating X-ray optics KB mirrors Sputter deposition
a b s t r a c t This paper describes fabrication methods used to demonstrate the advantages of nested or Montel optics for micro/nanofocusing of synchrotron X-ray beams. A standard Kirkpatrick-Baez (KB) mirror system uses two separated elliptical mirrors at glancing angles to the X-ray beam and sequentially arranged at 90◦ to each other to focus X-rays successively in the vertical and horizontal directions. A nested KB mirror system has the two mirrors positioned perpendicular and side-by-side to each other. Compared to a standard KB mirror system, Montel optics can focus a larger divergence and the mirrors can have a shorter focal length. As a result, nested mirrors can be fabricated with improved demagnification factor and ultimately smaller focal spot, than with a standard KB arrangement. The nested system is also more compact with an increased working distance, and is more stable, with reduced complexity of mirror stages. However, although Montel optics is commercially available for laboratory X-ray sources, due to technical difficulties they have not been used to microfocus synchrotron radiation X-rays, where ultra-precise mirror surfaces are essential. The main challenge in adapting nested optics for synchrotron microfocusing is to fabricate mirrors with a precise elliptical surface profile at the very edge where the two mirrors meet and where Xrays scatter. For example, in our application to achieve a sub-micron focus with high efficiency, a surface figure root-mean-square (rms) error on the order of 1 nm is required in the useable area along the X-ray footprint with a ∼0.1 mm-diameter cross section. In this paper we describe promising ways to fabricate precise nested KB mirrors using our profile coating technique and inexpensive flat Si substrates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In 1948 Kirkpatrick and Baez [1] provided a practical solution to image X-rays with a pair of cylindrical mirrors at glancing angles to the X-ray beam and arranged sequentially at 90◦ to each other so that the X-rays can be focused to a point. Nowadays elliptical KB mirrors are widely used in the X-ray community. The elliptical shape of modern mirrors avoids spherical aberrations and with near ideal mirrors provides diffraction-limited focusing. There are a number of ways to achieve elliptical mirrors. Sophisticated bending techniques have been developed to bend uniformly coated Si trapezoidal plates to achieve the desired elliptical shape for microfocusing. Submicron X-ray beams have been achieved by using benders for KB mirrors [2–4]. KB mirrors with benders are flexible in adjusting the focal length, but the benders are bulky and it is hard to achieve diffraction-limited focusing. Monolithic KB mir-
∗ Corresponding author. Tel.: +1630 252 9985; fax: +1630 252 9303. E-mail address: [email protected] (C. Liu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.079
rors are much easier to use if the desired elliptical surface profile can be fabricated. Surface figure errors cause focal aberrations and remain an important barrier for fabrication of synchrotron X-ray KB mirrors. The required precision increases as the X-ray wavelength decreases. Aspherical surfaces are difficult to make and precision elliptical mirrors for hard X-rays are very expensive. So far computer controlled optical surfacing and elastic emission machining techniques are used to produce ultra precise non-spherical surfaces [5,6]. These techniques require many cycles of measuring and polishing to achieve a smooth and accurate surface profile. We have developed a profile coating technique to convert cylindrical and flat inexpensive substrates into precise elliptical mirrors [7,8]. Elliptical KB mirrors with sub-nm figure errors can now be produced with only two runs of profile coatings. Sub-100 nm focusing has been demonstrated in a synchrotron X-ray beamline [9]. In addition to total reflective KB mirrors, laterally graded multilayer mirrors are also wildly used [10,11]. The Bragg angle of a multilayer can be several times larger than the critical angle totalexternal-reflection mirrors, leading to a larger numerical aperture. A higher numerical aperture corresponds to a smaller diffraction limited focal spot. However, in many applications, such as in Laue
C. Liu et al. / Applied Surface Science 258 (2012) 2182–2186
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more stable and robust instrument. It also simplifies the alignment process since the system is pre-aligned and only two small angular adjustments are needed. 2.2. The profile coating technique
Fig. 1. Illumination of a Montel KB mirror system for synchrotron X-rays.
microdiffraction, an achromatic beam from total reflection KB mirrors is essential [12]. Montel [13] or nested KB mirrors, with a higher demagnification and potentially smaller focusing than standard KB mirrors, are a desirable choice. Montel optics is commercially available for laboratory X-ray sources [14,15]. Because of the limited distance between the source and the focal point (between two foci of an ellipse), available Montel optics use multilayers on prefigured substrates to focus the X-ray beam. There is no commercial Montel optics for synchrotrons because of challenges in both figuring and polishing in the corner area where synchrotron X-rays are reflected. Indeed, because the undulator synchrotron beam used at the beamline is very narrow, ∼100 m, only the mirror surface within 100 m of the corner where the mirrors touch is used. This makes the fabrication of Montel KB mirrors very challenging. The source to sample distance is however, quite large for synchrotrons, in tens of meters or longer, so that total-external-reflection mirrors can be used. This paper describes techniques to fabricate precision nested KB mirrors using the profile coating technique and flat Si substrates. To our knowledge, this is the first successful demonstration of Montel mirror at a synchrotron radiation facility. 2. Experiment 2.1. The mirror design Fig. 1 illuminates the construction of a prototype Montel KB mirror design [12]. Two profile-coated elliptical mirrors are mounted on a precision roll adjustment mirror holder with the two elliptical surfaces focusing the X-ray beam. The two mirrors have the same design parameters, with the source to mirror distance, S1 = 36 m, focal length, S2 = 60 mm, and mirror angle, = 3 mrad. Both mirrors have a surface area of 9 mm × 40 mm and a height (thickness) of 20 mm. X-rays reflected from one mirror and reflected from the other are doubly focused at the focal point. Since the nested KB mirrors are positioned side by side, the back side of the vertically deflecting mirror must be figured or polished to allow close contact with the surface of the other mirror. Otherwise X-rays will miss the mirror surface at the gap between the mirrors. For example, the gap between the two mirror surfaces needs to be small enough so that the mirrors can collect ∼90% of a 120 m × 120 m incident X-ray beam. The two mirrors are pre-aligned perpendicularly with an optical laser. Once the mirrors are aligned relative to each other, the assembled mirror pair is rotated and translated to optimize the beam intensity and focus. This approach eliminates the two translation stages needed in a traditional KB system and allows for a
The profile strategy [7] utilizes a contoured aperture mask in a DC magnetron sputtering system with a simple linear motion of substrates to coat the design profile. The mirror substrates are translated perpendicular to their nominal scattering plane. The coating thickness at each position on the substrate is proportional to the opening width of the mask. The opening width of the contour is calculated every 0.1 mm according to the desired coating profile and the film thickness mapping at the substrate level. The coating profile is the difference between the ideal and the substrate surface profile. A designed mask is then made by electro-discharge machining using a thin invar plate. Au and Pt have been successfully used as the main coating material, with a 50 nm-thick Cr layer pre-coated on the Si substrate to enhance the adhesion between the Si substrate and the noble metal. A test deposition is carried out on a flat Si strip using the mask placed above the sputter gun and ∼0.5 mm away from the substrate. The coating profile of the test run is measured using an ellipsometer. The mirror substrates are then aligned on a sample holder at the exact position as determined by the test run and coated to the right amount as calibrated in the test run measurement. Very precise elliptical KB mirrors with sub-nm rms height errors can be obtained with one primary profile coating followed by a corrective profile coating. A typical primary coating lasts only a few hours for a 10 m-high elliptical profile. Pt was chosen as the main profile-coating material for the nested KB mirrors. Pt has a smaller thermal expansion coefficient than Au and thus a smaller thermal stress for films grown on Si, which has an even smaller thermal expansion. The film stress may undergo a relaxation process due to a diffusive relief of compressive stresses initially generated during deposition [16]. The film stress relaxation may also cause a KB mirror deformation under extended X-ray radiation. A film with a lower stress is expected to have a lower stress relaxation. It has been observed that the Pt-coated KB mirrors are more stable than the Au-coated ones [17]. The depositions were carried out in a small deposition system with a 9.5 inch OD, 4 ft-long vacuum chamber turbo-pumped to a base pressure of low 10−7 Torr. The depositions were carried out at ambient temperatures and at an Ar pressure of 2.3 mTorr with substrates moving linearly over two 3 in. diameter planar sputter guns at a constant speed. To overcome the buildup of film stress in thick films, the profile coatings were carried out in stages, with resting periods of 15 min each for every ∼1 m-height of the desired elliptical profile [8]. We have prepared two sets of nested KB mirrors. In the first set, two super-polished flat Si substrates with dimensions of 9 mm (W) × 40 mm (L) × 20 mm (H) were used. One of them had the side polished. These two substrates were mounted together and coated at the same time. The edge surface of the side-polished mirror and the center area of the other one were measured with metrology to provide data for corrective coatings and final acceptance. It turned out that even though the two substrates were mounted together, the coated profiles were slightly different. The side-polished one needed an additional corrective coating. Apparently, an edge effect was present during coating, resulting in a small coating difference between the surfaces near the edge and at the center. Also, the side-polished mirror had quite a few chips at the edge. In the second set, another super-polished flat Si substrate with dimensions of 25 mm (W) × 50 mm (L) × 4.5 mm (H) was used. The center area was measured with metrology after coating. This time, only one primary plus a corrective profile coating were needed to obtain the desired elliptical profile with
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<1 nm rms height error. The coated mirror was cut to obtain two pieces with dimensions of 9 mm (W) × 40 mm (L) × 4.5 mm (H) from the central portion. The side of one mirror that needs side-polishing was determined according to the elliptical profile direction relative to the incoming X-ray in the nested KB mirror assembly. After side-polishing, the mirrors were mounted to 9 mm (W) × 40 mm (L) × 15.5 mm (H) aluminium blocks. 2.3. Substrate/mirror side-polishing The mirror substrate to be side-polished before coating had the side surface ground to about 89◦ to the mirror surface. This <90◦ angle makes it possible to bring the two finished KB mirrors in close contact during the final assembling process. The mirror substrate was mounted on a polishing holder surrounded by four 20 mmhigh Si pieces. The mirror substrate and the surrounding pieces were mechanically ground with a 1◦ wedge. Polishing proceeded from loose abrasively grinding using 9 m sized aluminum oxide, to polishing using 1 m diamond slurry. During the final stages of polishing, the sharpness of the edge near to the mirror surface was closely monitored under a microscope. The polishing process was continued until a very sharp straight line appeared. A chemical mechanical polish was used as a final step. For the coated mirrors, an important issue is to protect the coating surface during cutting and polishing. Pure optical wax was used to glue the coated surface to another Si wafer for cutting to the desired dimensions using a diamond-dicing saw. Then the piece that needed side-polishing was glued onto the coated face to the smooth face of another Si piece with the same dimension for side polishing. The same polishing procedure was carried out without angle-grinding. To make a <90◦ angle between the side-polish face and the mirror surface, a special polishing holder with a ∼1◦ tilt is needed. For the present study, the polishing holder had no tilt surfaces. After polishing, the mirror was removed from the holder together with the other mounting Si using a gentle heating. It was then separated from the mounting Si using trichloroethylene and cleaned in acetone and methanol. 2.4. Metrology measurements All metrology measurements were carried out using a stitching interferometer, model MicroXAM RTS [18]. The interferometer was operated in a phase-shifting mode, using filtered light centered at 551 nm wavelength with 25 nm bandwidth. An integrated high-resolution closed-loop translation stage was used to scan the mirror’s surface in both x and y directions to automatically acquire measurement maps at predetermined areas. An objective lens of 2.5× magnification was used, with each individual measurement covering a field of view of approximately 4.9 mm × 4.9 mm. Multiple images along the longitudinal direction were taken with a 30% overlap to obtain the whole image of the mirror. The manufacturer’s proprietary stitching algorithm was used to precisely overlap neighboring measurements and to reconstruct the surface profile. A linear profile can be extracted from any position of the reconstructed surface along the longitudinal direction. Usually it is extracted from the center of the mirror surface and compared with the designed ellipse. The difference between the measured and the design ellipse is the corrective coating profile for the next round of coating process. After a successful corrective profile coating, the measured data is fit with a best-fit-ellipse to obtain the residual height error. For nested KB mirrors, one of them has one side polished to make good contact with the other mirror. Metrology measurements were used to make sure that the very edge of the mirror surface at the corner has the required profile without chips from polishing. The
Fig. 2. Image of the polished edge of a Pt-profile-coated KB mirror sitting on top of a bare Si mounting support. A sharp straight edge is evident in the picture.
edge of the mirror was carefully aligned with the translation stage and parallel to the traveling direction during the measurement. Line profiles at 10 m intervals starting from the edge were extracted from the measured surface profile. For the other mirror that does not need side-polishing, the center area was measured. 3. Result Excellent metrology results have been obtained for the second set of profile-coated nested KB mirrors after cutting and sidepolishing. The coated surface had a very precise elliptical profile with a height error of 0.73 nm rms after one primary and one corrective profile coating. After cutting into two pieces, one piece was side polished and the other piece was cleaned for final assembly. Fig. 2 is an image of the polished edge of the coated mirror glued on top of a Si mounting support with optical wax after the final polishing. The picture was taken using a camera on a stylus profiler with a 260× magnification [19]. The polished side produced a sharp straight edge along the bottom of the coated mirror, as seen in the picture. The coated elliptical surface is perpendicular to the polished side and not visible in the image. After the KB mirror was separated from the mounting Si piece, the corner area of the elliptical surface near the polished side was measured with metrology. Fig. 3 shows the measured profile for the KB mirror after a corrective profile coating before cutting (Fig. 3a) and that for the corner area, at 30 m from the edge after cutting and polishing (Fig. 3b). The measured data and the best-fitellipse (left Y axis) overlap and are not distinguishable in the figure. The residual profile (right Y axis) shows the difference. The fitting parameters (S1, S2, and ) and the rms height errors are presented in the figures. Comparing Fig. 3a and b one can see that the best-fit parameters are very close in these two cases. The height error is slightly larger after polishing, 1.05 nm vs. 0.73 nm rms. This error increases closer to the edge. At 20 m from the edge it becomes 1.32 nm and at 10 m from the edge it is 1.82 nm rms. The increased rms numbers are due to chips at the edge, shown in the metrology measurement as sharp spikes. A few such spikes are seen in Fig. 3b. The overall shape of the residual profile is remarkably similar, as shown in Fig. 3a and b. For the first set of nested KB mirrors, coatings were carried out on a pair of shaped substrates with the right dimensions. One of them had the side that needed side-polishing fine polished. The pair was then profile-coated. The side-polished mirror showed a lot of spikes in the metrology data, leading to a poor height-error of 3.0 nm rms at 40 m from the edge and 8.0 nm rms at 10 m from
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Fig. 3. Measured Pt profile for the KB mirror after a corrective profile coating, (a) before cutting and (b) at 30 m from the edge after cutting and polishing.
the edge. The other mirror however had excellent surface figure with a 0.76 nm rms height error along the center-line area. The first set of nested KB mirrors has been tested at the Advanced Photon Source 34-ID beamline. Even though the side-polished mirror had many defects at the edge, the test was very successful. Final test results and details will be presented elsewhere [20]. Both mirrors were used to focus up to a 120 m by 120 m incident X-ray beam with broad-bandpass of energies up to ∼30 keV. The side-polished mirror was used to focus in the vertical direction and the other mirror focused in the horizontal direction. The experimental station is located 64 m from the synchrotron undulator source. The size of the source is ∼40 m-high in the vertical plane and ∼700 m-wide in the horizontal plane. To reduce the horizontal size, a ∼100 m-wide slit was placed at 28 m from the synchrotron [9]. The side-polished KB mirror was successfully brought into contact to the companion mirror with a small gap of 8–9 m. For the second set of nested KB mirrors, the side-polished mirror could not be brought to a close contact with the other mirror. The gap could not be made smaller than ∼30 m while keeping the two mirrors perpendicular. Since the mirror was polished without using a tilt polishing holder, the round-off at the edge was severe enough to prevent a smaller gap when the two mirrors were at 90◦ . As perpendicularity of the two KB mirrors is critical for best focusing [9,21], this set of nested KB mirrors was not used in the initial beamline test.
The initial test of nested KB mirrors demonstrates that Montel optics can be fabricated for synchrotron beamlines and the profile coating technique is capable of producing high quality Montel KB mirrors using flat Si substrates. Comparing the two fabrication methods used in the present study, the method in which the side is polished first and the mirrors are coated together, can produce thicker mirrors. However, additional efforts must be made to prevent chipping at the edge and to improve the accuracy of coated elliptical profile. The second method, to coat a wider mirror first, cut it into two pieces, and then polish the side of one of them, can produce better edges. Multiple mirrors with the same design parameters can be coated in the same coating run. The polishing holder however, must be redesigned to polish the side of one of the KB pair to a <90◦ angle to the elliptical mirror surface. At present the mirror thickness is limited by our diamond dicing saw to 4.5 mm. The dicer has thin blades with sintered small diamonds of 3–6 m in size rotating at very high speed of 20,000 rpm. It provides smooth cuts not achievable from other cutting machines. The thickness of the KB mirrors may need to be larger than 4.5 mm if the mirrors are going to be operated for extended radiation conditions [17]. The focal size of the present tested mirror set can be speculated using the diffraction-limit and the geometrical source demagnification. In the vertical plane, the X-ray source size is ∼40 m, and the demagnification factor is ∼1067. The focal size ideally should reach the diffraction limit of sub-50 nm in the vertical direction [9]. The side-polished mirror was positioned to deflect the vertical beam and ideally should achieve a sub-50 nm focus. The non-ideal fabrication result of imperfections at the edge area however, was expected to affect the performance of this mirror. In the horizontal plane, the source size is redefined by a 100 m-wide slit placed at ∼36 m from the mirror and the demagnification factor becomes ∼600. This small demagnification limits the focal size to ∼160 nm. The mirror positioned to deflect the horizontal beam had a very precise elliptical profile that would help to achieve this limit. The initial test result has confirmed these expectations [20]. In summary, we have successfully fabricated and tested a firstever synchrotron hard X-ray Montel KB mirror system. The profile coating technique combined with fine mechanical polishing provide a simple and economic solution to the challenging problem of fabricating KB mirrors with a very precise elliptical surface profile at the small corner area for synchrotron hard X-rays. This method is quite straightforward and effective in converting inexpensive flat Si substrates into precise elliptical KB mirrors. The success of the initial test and the significant advantages of Montel/nested optics are expected to stimulate more applications of Montel optics in the synchrotron community. Acknowledgements We thank A. Khounsary for ordering the Si substrates, D. Shu for the mirror stage design, and J. Attig for technical assistance, all from Argonne National Laboratory. This work is supported by the UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under contract no. DE-AC02-06CH11357. G.E.I and J.Z.T. are supported by the Center for Defect Physics an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCS99. References [1] P. Kirkpatrick, V. Baez, Formation of optical images by X-rays, J. Opt. Soc. Am. 38 (9) (1948) 766–774.
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