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Holographic X-ray optical elements: transition between refraction and diffraction
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
Holographic X-ray optical elements: transition between refraction and diffraction I. Snigirevaa,*, A. Snigireva, C. Raua, T. Weitkampa, V. Aristovb, M. Grigorievb, S. Kuznetsovb, L. Shabelnikovb, V. Yunkinb, M. Hoffmannc, E. Vogesc a
European Synchrotron Radiation Facility, B.P. 220 (6 rue Jules Horowitz, Zip 38000), F 38043 Grenoble, France b Institute of Microelectronics Technology RAS, 142432 Chernogolovka, Russia c University of Dortmund, D-44227 Dortmund, Germany
Abstract Planar microelectronics technology, involving photolithography and highly anisotropic plasma etching techniques, was applied to fabricate refractive and diffractive (kinoform) lenses. Focusing properties in terms of focus spot and efficiency in the energy range 8–25 keV for both types of lenses were tested at the European Synchrotron Radiation Facility (ESRF) ID22 beamline. Focal spot of 1.5 mm with a gain of 25 was measured at 15 keV. r 2001 Elsevier Science B.V. All rights reserved. PACS: 41.50.+h; 07.85.m Keywords: X-rays; Focusing; Lens; Parabolic; Microfabrication
Notwithstanding the fact that refractive lenses made from low-Z materials are novel optical components, they show a great potential for focusing high-energy synchrotron X-rays [1,2]. Refractive lenses with parabolic shape made from polycrystalline aluminium by a pressing technique allow realizing microanalysis and full-field microscopy applications [3,4]. Planar microelectronics technology proposed recently opens the real possibility to design and manufacture refractive and diffractive lenses as kinoform with the combination of refractive and diffractive proper*Corresponding author. Tel.: 33-476-88-2360; fax: 33-476-882542. E-mail address: [email protected] (I. Snigireva).
ties [5–7]. The kinoform is a phase hologram in which phase modulation is introduced by a dedicated surface profile. The great advantage of this technique is that it permits to create computer generated holographic optical elements allowing to correct incident wavefront and to generate predetermined exit wavefront. In addition, the microfabrication technique gives the possibility to create lenses with desired number of unit lenses not only to shorten focal length but mainly in an effort to minimize technological inaccuracies and heatload. Microfabrication process consisting of two main steps, i.e. pattern generation in the hard mask on the silicon surface and pattern transfer by deep reactive ion etching into the silicon has been used for the lenses fabrication [8,9]. As an example, a
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 5 6 - 3
I. Snigireva et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
Fig. 1. SEM image of Si planar parabolic lenses. Structures are 100 mm deep and have an aperture of 100 mm. Each lens is of length 710 mm. Bridges between adjacent individual lenses are 5 mm wide. Each lens has for 12.4 keV, a focal length of 60 cm.
set of planar parabolic refractive lenses having uniform focal length (F ¼ 60 cm at E ¼ 12:4 keV) fabricated in Si is shown in Fig. 1. Each lens has a different number of individual parabolic lenses ( p ¼ 1; 2; 4; 6; 8) and therefore different radius of parabola apex. This design enables us to investigate the influence of fabrication inaccuracies on lens performance and to study X-ray attenuation in the bridges. Kinoform lens made by removing passive parts of material where the phase variation is a multiple of 2p is depicted in Fig. 2. The lens with the aperture A ¼ 150 mm consists of p ¼ 5 individual lenses each comprising N ¼ 10 pairs of segments. For energy 17 keV, the lens has a focal distance of 80 cm. This approach enables to reduce significantly the absorption. In the energy range 8–25 keV, the transmission values for kinoform lens are approximately two times greater than for parabolic lens and exceed 90% at energies >15 keV. Experimental tests of lenses were performed at the undulator (ID22) beamline at the ESRF [6,7]. Focusing properties in terms of spot size and efficiency were studied in the energy range 8–25 keV. The intensity distribution in the lens focal plane
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Fig. 2. SEM micrograph of refractive lens with minimized absorption. The lens consists of five individual lenses, each comprising of ten pairs of segments. The structure is 100 mm deep with an aperture of 150 mm. For 17 keV, the lens has a focal distance of 80 cm.
was measured with high resolution two-dimensional X-ray CCD camera (with pixel size of 0.33 mm) and normalized to the primary beam intensity. For planar parabolic lenses, a focal spot (FWHM) of 1.5 mm with a gain of about 25 was measured at 17 keV. Fabricated lenses show low sidewall roughness and random deviations from the parabolic profile. For kinoform lens, a gain of 19 and FWHM ¼ 1:8 mm was obtained at 18 keV. Taking into account the 1.2 mm (FWHM) detector point spread function deconvoluted spot size is lesser than 1 mm, is in good agreement with the expected result. A focal depth close to 7 cm was observed. For the kinoform lens, additional gain maxima to the main one corresponding to the design energy was observed [6,7]. It gives us a hope that such lenses can work as a multi-purpose spectral device. Planar parabolic lenses can withstand high heatload from a synchrotron beam. Simple numerical simulations for temperature peak values at X-ray energy E ¼ 20 keV shows that the incident flux P0 ¼ 40 W/mm2 sufficed to heat the lens up to 3001C. Planar parabolic lens made of diamond allows to increase the incident flux up to 300 W/ mm2 heating the lens up to 3001C [7].
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I. Snigireva et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
Fig. 3. Photo of two planar lenses made of silicon. Each lens consists of five individual lenses with 1.8 mm aperture. Focal distance at 20 keV is 4 m. Structures are 200 mm deep.
Planar technology allows manufacturing lenses with large apertures and quite small radius of curvature at the parabola apex. We realized lenses for extreme applications for energy range from 8 to 100 keV. The optimal design is supposed to be a set of individual lenses formed from pairs of adjacent parabolic lenses where openings of parabolas face each other. In order to equalize the width of the features we have to find a compromise between smoothness of the refractive profile at coincident points of parabolas and the radius of curvature at the apex of each parabola (Fig. 3). A refractive lens consisting of individual lenses with real kinoform profile (in-line segments) has minimum total length. However, such a design is very complicated due to the extremely wide range of the feature width. To narrow this range we proposed an alternative design with ‘‘fern-like’’ profile where even (or odd) segments are inverted (Fig. 4). These lenses will be tested in the near future. In conclusion, we state that the advanced silicon-based microfabrication technique allows to fabricate high-quality hard X-ray lenses providing submicron focal spot, which may be used in hard X-ray microprobe and microscopy applications. The proposed lens technology enables to
Fig. 4. SEM image of refractive lens with ‘‘fern-like’’ profile. The lens consists of eight individual lenses and each individual lens has 45 zones. The outermost zone width is 2.8 mm. Aperture is 500 mm. At 18 keV, the lens has a focal distance of 50 cm.
manufacture holographic optical elements resulting in erasing the distinction between refraction and diffraction. This approach allows to get rid of
I. Snigireva et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
the drawbacks of pure diffractive or refractive lenses and combine their advantages like high transmission and efficiency, absence of zero order and binary noise in the far field, etc. The ability to manipulate the local amplitude and phase of the incoming wave opens the real perspective to make a new class of beamshaping X-ray optics for coherent synchrotron radiation. These silicon lenses are mechanically robust and can withstand the high heatload of the white X-ray beam. The proposed lens technology can be applied to materials like diamond. It is known that diamond has low X-ray absorption, low thermal expansion and high heat conductivity. Therefore in view of the future X-ray free electron lasers, diamond is an ideal material for microoptics.
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Acknowledgements The work was supported by INTAS project no. 99-0469.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
A. Snigirev, et al., Nature 384 (1996) 49. A. Snigirev, et al., Appl. Opt. 37 (1998) 653. B. Lengeler, et al., Appl. Phys. Lett. 74 (1999) 3924. B. Lengeler, et al., J. Synchrotron Rad. 6 (1999) 1153. V.V. Aristov, et al., Opt. Commun. 177 (2000) 33. V. Aristov, et al., Appl. Phys. Lett. 77 (25) (2000) 1. V. Aristov, et al., SPIE 4145 (2001) 285. V.A. Yunkin, et al., Microelectron. Eng. 23 (1994) 373. A.M. Hynes, et al., Sensors and Actuators A 74 (1999) 13.
Holographic X-ray optical elements: transition between refraction and diffraction I. Snigirevaa,*, A. Snigireva, C. Raua, T. Weitkampa, V. Aristovb, M. Grigorievb, S. Kuznetsovb, L. Shabelnikovb, V. Yunkinb, M. Hoffmannc, E. Vogesc a
European Synchrotron Radiation Facility, B.P. 220 (6 rue Jules Horowitz, Zip 38000), F 38043 Grenoble, France b Institute of Microelectronics Technology RAS, 142432 Chernogolovka, Russia c University of Dortmund, D-44227 Dortmund, Germany
Abstract Planar microelectronics technology, involving photolithography and highly anisotropic plasma etching techniques, was applied to fabricate refractive and diffractive (kinoform) lenses. Focusing properties in terms of focus spot and efficiency in the energy range 8–25 keV for both types of lenses were tested at the European Synchrotron Radiation Facility (ESRF) ID22 beamline. Focal spot of 1.5 mm with a gain of 25 was measured at 15 keV. r 2001 Elsevier Science B.V. All rights reserved. PACS: 41.50.+h; 07.85.m Keywords: X-rays; Focusing; Lens; Parabolic; Microfabrication
Notwithstanding the fact that refractive lenses made from low-Z materials are novel optical components, they show a great potential for focusing high-energy synchrotron X-rays [1,2]. Refractive lenses with parabolic shape made from polycrystalline aluminium by a pressing technique allow realizing microanalysis and full-field microscopy applications [3,4]. Planar microelectronics technology proposed recently opens the real possibility to design and manufacture refractive and diffractive lenses as kinoform with the combination of refractive and diffractive proper*Corresponding author. Tel.: 33-476-88-2360; fax: 33-476-882542. E-mail address: [email protected] (I. Snigireva).
ties [5–7]. The kinoform is a phase hologram in which phase modulation is introduced by a dedicated surface profile. The great advantage of this technique is that it permits to create computer generated holographic optical elements allowing to correct incident wavefront and to generate predetermined exit wavefront. In addition, the microfabrication technique gives the possibility to create lenses with desired number of unit lenses not only to shorten focal length but mainly in an effort to minimize technological inaccuracies and heatload. Microfabrication process consisting of two main steps, i.e. pattern generation in the hard mask on the silicon surface and pattern transfer by deep reactive ion etching into the silicon has been used for the lenses fabrication [8,9]. As an example, a
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 5 6 - 3
I. Snigireva et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
Fig. 1. SEM image of Si planar parabolic lenses. Structures are 100 mm deep and have an aperture of 100 mm. Each lens is of length 710 mm. Bridges between adjacent individual lenses are 5 mm wide. Each lens has for 12.4 keV, a focal length of 60 cm.
set of planar parabolic refractive lenses having uniform focal length (F ¼ 60 cm at E ¼ 12:4 keV) fabricated in Si is shown in Fig. 1. Each lens has a different number of individual parabolic lenses ( p ¼ 1; 2; 4; 6; 8) and therefore different radius of parabola apex. This design enables us to investigate the influence of fabrication inaccuracies on lens performance and to study X-ray attenuation in the bridges. Kinoform lens made by removing passive parts of material where the phase variation is a multiple of 2p is depicted in Fig. 2. The lens with the aperture A ¼ 150 mm consists of p ¼ 5 individual lenses each comprising N ¼ 10 pairs of segments. For energy 17 keV, the lens has a focal distance of 80 cm. This approach enables to reduce significantly the absorption. In the energy range 8–25 keV, the transmission values for kinoform lens are approximately two times greater than for parabolic lens and exceed 90% at energies >15 keV. Experimental tests of lenses were performed at the undulator (ID22) beamline at the ESRF [6,7]. Focusing properties in terms of spot size and efficiency were studied in the energy range 8–25 keV. The intensity distribution in the lens focal plane
983
Fig. 2. SEM micrograph of refractive lens with minimized absorption. The lens consists of five individual lenses, each comprising of ten pairs of segments. The structure is 100 mm deep with an aperture of 150 mm. For 17 keV, the lens has a focal distance of 80 cm.
was measured with high resolution two-dimensional X-ray CCD camera (with pixel size of 0.33 mm) and normalized to the primary beam intensity. For planar parabolic lenses, a focal spot (FWHM) of 1.5 mm with a gain of about 25 was measured at 17 keV. Fabricated lenses show low sidewall roughness and random deviations from the parabolic profile. For kinoform lens, a gain of 19 and FWHM ¼ 1:8 mm was obtained at 18 keV. Taking into account the 1.2 mm (FWHM) detector point spread function deconvoluted spot size is lesser than 1 mm, is in good agreement with the expected result. A focal depth close to 7 cm was observed. For the kinoform lens, additional gain maxima to the main one corresponding to the design energy was observed [6,7]. It gives us a hope that such lenses can work as a multi-purpose spectral device. Planar parabolic lenses can withstand high heatload from a synchrotron beam. Simple numerical simulations for temperature peak values at X-ray energy E ¼ 20 keV shows that the incident flux P0 ¼ 40 W/mm2 sufficed to heat the lens up to 3001C. Planar parabolic lens made of diamond allows to increase the incident flux up to 300 W/ mm2 heating the lens up to 3001C [7].
984
I. Snigireva et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
Fig. 3. Photo of two planar lenses made of silicon. Each lens consists of five individual lenses with 1.8 mm aperture. Focal distance at 20 keV is 4 m. Structures are 200 mm deep.
Planar technology allows manufacturing lenses with large apertures and quite small radius of curvature at the parabola apex. We realized lenses for extreme applications for energy range from 8 to 100 keV. The optimal design is supposed to be a set of individual lenses formed from pairs of adjacent parabolic lenses where openings of parabolas face each other. In order to equalize the width of the features we have to find a compromise between smoothness of the refractive profile at coincident points of parabolas and the radius of curvature at the apex of each parabola (Fig. 3). A refractive lens consisting of individual lenses with real kinoform profile (in-line segments) has minimum total length. However, such a design is very complicated due to the extremely wide range of the feature width. To narrow this range we proposed an alternative design with ‘‘fern-like’’ profile where even (or odd) segments are inverted (Fig. 4). These lenses will be tested in the near future. In conclusion, we state that the advanced silicon-based microfabrication technique allows to fabricate high-quality hard X-ray lenses providing submicron focal spot, which may be used in hard X-ray microprobe and microscopy applications. The proposed lens technology enables to
Fig. 4. SEM image of refractive lens with ‘‘fern-like’’ profile. The lens consists of eight individual lenses and each individual lens has 45 zones. The outermost zone width is 2.8 mm. Aperture is 500 mm. At 18 keV, the lens has a focal distance of 50 cm.
manufacture holographic optical elements resulting in erasing the distinction between refraction and diffraction. This approach allows to get rid of
I. Snigireva et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 982–985
the drawbacks of pure diffractive or refractive lenses and combine their advantages like high transmission and efficiency, absence of zero order and binary noise in the far field, etc. The ability to manipulate the local amplitude and phase of the incoming wave opens the real perspective to make a new class of beamshaping X-ray optics for coherent synchrotron radiation. These silicon lenses are mechanically robust and can withstand the high heatload of the white X-ray beam. The proposed lens technology can be applied to materials like diamond. It is known that diamond has low X-ray absorption, low thermal expansion and high heat conductivity. Therefore in view of the future X-ray free electron lasers, diamond is an ideal material for microoptics.
985
Acknowledgements The work was supported by INTAS project no. 99-0469.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
A. Snigirev, et al., Nature 384 (1996) 49. A. Snigirev, et al., Appl. Opt. 37 (1998) 653. B. Lengeler, et al., Appl. Phys. Lett. 74 (1999) 3924. B. Lengeler, et al., J. Synchrotron Rad. 6 (1999) 1153. V.V. Aristov, et al., Opt. Commun. 177 (2000) 33. V. Aristov, et al., Appl. Phys. Lett. 77 (25) (2000) 1. V. Aristov, et al., SPIE 4145 (2001) 285. V.A. Yunkin, et al., Microelectron. Eng. 23 (1994) 373. A.M. Hynes, et al., Sensors and Actuators A 74 (1999) 13.