19 Reflective Optical Arrays 313 Fig. 19.7. An MOA consisting of an arrangement of 1D strips to give a 2D focus whatever the radial distance. Additionally, in practice, many more channels would be used compared to the number shown in the schematic diagram; typ- ically channels would be ∼10 μm wide, with walls of comparable thickness, over areas of a few square millimetres. Flexing may be carried out either mechanically or by coating piezo material on, for example, the spokes shown in Fig. 19.6a. By controlling each piece of piezo independently, the X-ray beam could be further manipulated, for example to reduce aberrations in an adaptive or active way. The two-dimensional focusing capabilities of such arrays could be simu- lated by making a series of 1D strips, as shown schematically in Fig. 19.7. As well as being technically less challenging to manufacture each strip could also be flexed independently. Other arrangements, designed for specific appli- cations, would also be possible. 19.3.1 Computer Simulations Modelling the performances of MOAs, even in the simplest way, is challeng- ing. It requires both finite element analysis (FEA), to determine the effect of flexing on the channel walls, and ray tracing to characterise the optical performances. More sophisticated analyses will require wavefront propagation and studies of the effects of diffraction. FEA and ray tracing are both complicated for such optics, as the effects of many channels have to be taken into account. For FEA, this means that the number of elements to be analysed is very large, leading to problems with mesh sizes, while ray tracing has to be carried out non-sequentially as at most two optical surfaces out of many hundreds will be encountered by an individual array. So far, only rudimentary FEA studies have been carried out, but many characteristics of the optical performances have been investigated using the optical design software ZEMAX c  . Recently, ray-tracing analysis has been carried out using the much more flexible (and user-friendly) “Q” software developed at the University of Leicester (UK) [19]. As an example, a silicon MOA designed for X-rays of energy 4.5 keV (Ti K α ) has been modelled using ZEMAX c  . This type of optic will be suit- able for irradiating cells, in studies related to cancer research, using an X-ray 314 S. Lagomarsino et al. Table 19.1. Parameters of the prototype MOA for the Gray Cancer Institute microprobe Source size 5 μm Source to optic distance 160 mm Diameter of optic 2 mm Separation of optic components 1 mm Length of channels 200 μm Width of channels 10 μm Bending radius of first component ∞ Bending radius of second component 100 mm Focal distance 73 mm Fig. 19.8. Results of ray tracing the MOA for the Gray Cancer Institute microprobe. Thesource(left) emitted 4.5 × 10 6 keV photons, of which (right) 8,600 were doubly reflected and brought to focus. The scale bars are 2 μm microprobe at the Gray Cancer Institute (UK) [20]. The parameters are shown in Table 19.1. The bending radius of the second component (the first was unbent) was chosen to give a focal spot size of about 2 μm, assuming a 5 μmdiameter source. Although zone plates can give smaller focal spots than this, the inten- tion was, in the first instance, to aim for something experimentally feasible at an early stage, while providing a focal spot size useful for studies using the microprobe. Smaller spot sizes could be achieved by using a smaller bending radius, or by bending both components. Although this would mean that fewer X-rays would pass straight through without reflection, the effects of roughness would be more pronounced and a detailed analysis needs to be carried out to determine the optimum configuration. The ray tracing took into account the efficiency of each reflection, which decreases radially outwards as the grazing incidence angle increases, as well as the channel wall roughness. Results for zero roughness (Fig. 19.8) indicate that the configuration of Table 19.1 results in a focusing efficiency of slightly 19 Reflective Optical Arrays 315 under 1% (primarily since most X-rays pass straight through). However, the focused flux is some two orders of magnitude higher than that which could be achieved by a state-of-the-art zone plate with a diameter ∼100 μm. A channel wall roughness of 10 nm, the effects of which were modelled using (19.1), reduces this gain by a factor of about 3, suggesting that a roughness of a few nanometres is acceptable at energies of a few keV. An additional advantage of MOAs, over zone plates, is that the focal length is independent of energy, so that (unless energy-dependent effects are being studied) the bremsstrahlung as well as the characteristic radiation could be used, enhancing the gain in useful flux. To date, using zone plates, all studies using the microprobe have been concerned with cell death [21], rather than the much more important phenomenon of mutation which occurs at a rate several orders of magnitude lower; hence the need for increased focused flux. 19.3.2 Manufacture of Microstructured Optical Arrays Because of the necessity for high aspect ratios, techniques such as the Bosch process [22] of deep etching in silicon are required to manufacture MOAs. The Bosch process utilises successive etch/passivate stages to create the channels while preventing side-wall etching. Until recently, the applications of such manufacture did not require tight tolerances on wall roughness, and so values of the order of micrometres were acceptable. MOAs require improvements of around three orders of magnitude over this, and so new procedures have had to be devised. By shortening the etch/passivate cycle time, the Scottish Micro- electronics Centre at the University of Edinburgh has shown that channel wall roughnesses of less than 20 nm are possible. Subsequent coating with 100 nm of silicon dioxide improved this further to less than about 10 nm [23], which sug- gests that the ultimate goal of roughnesses of a few nanometres is achievable. 19.4 Conclusions The nested and array systems presented here show promising capabilities as future generation X-ray optics. Some technological challenges, including roughness, appear close to being overcome, while others, e.g., control of surface shapes for adaptive systems, must still be addressed in detail. Recent experiments involving coating of nested mirror systems with silica sol-gel showed very promising wall roughness reduction, and 50% reflectivity at Cu line from SU8 walls coated with sol-gel. Acknowledgments In addition to the European Science Foundation support, the work on Micro- structured Optical Arrays is part of that being carried out by the UK Smart X-Ray Optics (SXO) consortium funded by the Basic Technology Programme 316 S. Lagomarsino et al. of Research Councils UK (grant code EP/D04880X/1). The members of the SXO consortium are University College London (including the Mullard Space Science Laboratory), King’s College London, the Gray Cancer Institute, the Scottish Microelectronics Centre at the University of Edinburgh, the Uni- versity of Birmingham, the University of Leicester and STFC Daresbury Laboratory. Silson Ltd is an associate member. IFN acknowledges partial financial support from National project SPARX. References 1. O. Hignette, G. Rostaing, P. Cloetens, A. Rommeveaux, V. Ludwig, A. Freund, Proc. SPIE 4499, 105 (2001) 2. K. Yamamura, K. Yamauchi, H. Mimura, Y. Sano, A. Saito, K. Endo, A. Souvorov, M. Yabashi, K. Tamasaku, T. Ishikawa, Y. Mori, Rev. Sci. Instrum. 74, 4549 (2003) 3. Y. Mori, K. Yamauchi, K. Yamamura, H. Mimura, Y. Sano, A. Saito, K. Ueno, K.Endo,A.Souvorov,M.Yabashi,K.Tamasaku,T.Ishikawa,Proc.SPIE 4782, 58 (2002) 4. I.N. Bukreeva, S.B. Dabagov, S. Lagomarsino, Appl. Opt. 43, 6270 (2004) 5. C.G. Cheng, R.K. Heilmann, P.T. Konkola, O. 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New types of grazing incidence X-ray mirror systems based on single reflections have been studied, including modelling of optical performance, effects of surface figure errors and micro-roughness, actual performances and astronomical and laboratory applications. Ray-tracing simulations of multi-foil reflective optics for focusing radiation from a gas puff plasma source have been studied in detail for soft X-rays in the wavelength range 3–20 nm. Such sources are debris free because of the use of noble gases as the working medium. The ray-tracing was performed for both point and extended sources. The optics consist of two orthogonal stacks of ellipsoidal mirrors with gold reflecting surfaces forming a double focusing device. Unlike multilayer optics, grazing incidence optics are efficient at focusing soft X-rays over a wide wavelength range. Optics designed for collecting solid angle of 0.1sr were manufactured and tested in the visible and EUV regions. It has been demonstrated that multi-foil optics are a good candidate for concentrators of EUV radiation in applications such as lithography. High resolution imaging screens and detector systems with thin YAG:Ce and other monocrystal scintillator screens have been designed and tested. Camera sys- tems based on monocrystal scintillator, optics and CCD detector were built and used for testing of scintillators. Screens with thicknesses down to 5 μm and a fast, high resolution, cooled 16 bit CCD camera have been used to achieve resolutions of 1 μm and comparative studies of sensitivity (for YAG:Ce and fine grain Gadox screen) were carried out. Scintillator screens and systems with sub-micrometre resolution have also been studied. 20.1 Introduction Recent progress in high-intensity microfocused EUV beam generation is pre- sented in this chapter. Ellipsoidal thin glass foils were used in multifoil optical systems for focusing the radiation in a 50 to 150 eV energy band from a gas– puff laser plasma source. A multifoil optical (MFO) condenser was designed and tested for applications with an Xe laser plasma gas–puff source. A high intensity EUV beam focal spot was recorded, analyzed, and compared with 320 L. Pina theoretical results from computer ray tracing. Direct EUV lithography using radiation-induced decomposition and ablation of TEFLON was studied. EUV sources are considered as the sources for lithography working with the wavelength of 13.5 nm, i.e., 92 eV. Two working media as a laser target, Xe and Sn, were used in our particular case to obtain a high efficiency of laser energy conversion into the radiation in this wavelength range. The EUV sources based on such media emit radiation in relatively wide wavelength range. The appli- cations of such sources include proximity X-ray lithography, soft X-ray contact microscopy, or micromachining of polymers by direct photoetching. Grazing incidence X-ray optics can be used to collect the radiation in a wide wavelength range. However, X-rays are reflected only at grazing inci- dence angles and, thus, only a relatively small collecting angle can be used. Different solutions can be applied to enlarge the collecting angle such as a polycapillary [1], nested Wolter type optics [2], or multifoil optics (MFO) [3]. While grazing incidence optics are commonly used in space X-ray telescopes, they can be also successfully used for laboratory imaging as well as for collect- ing X-rays from the laboratory sources. The critical angle is relatively large in the case of EUV radiation. It can be up to 15 ◦ for gold coated mirrors with surface microroughness below 1 nm and radiation wavelength around 10 nm. The design, ray-tracing X-ray tests and recently obtained results for the mul- tifoil condenser for the laser plasma EUV source presented in this chapter resulted from the cooperation of three laboratories. The gas-puff laser plasma EUV source [4] is operated at the WAT Institute of Optoelectronics, War- saw, where EUV experiments were done. Ray-tracing calculations were done at the Czech Technical University and design was done at Reflex, in Prague, where the multifoil optics technology was developed. The condenser collects photons from the source located in the source chamber and directs them onto a plane in the experimental chamber used for experiments on the interaction of high-intensity EUV radiation with matter and for lithography. There were several requirements for the condenser: • Working energy range E = 80–120 eV • Focal length f = 440 mm to fit into the existing vacuum chamber • Source diameter = 100–500 μm • Focal spot diameter = 500–1,000 μm • Restriction for the front area in order to fit into the existing vacuum chamber, aperture size 80 mm. There are two contradictory requirements. First, there is a large field of view (FOV) and a large solid angle of collection to obtain the best possible effectivity of the condensing system. Second, there is restriction on the focal spot size because it is more difficult to control the distortions and imperfec- tions for larger optics. Wolter type optics and Lobster Eye in the Schmidt [5] arrangement can be considered. The Wolter optical system is the proven tech- nology, and it is successfully used in variety of applications. Wolter optics con- sists of a number of nested axially symmetric hyperbolic/parabolic/ellipsoidal 20 Reflective Optical Structures and Imaging Detector Systems 321 mirrors. The actual shape depends on the specific application. However, manufacturing mirrors with the accuracy required and nesting them precisely into the optical system is rather difficult and expensive. In the case of a con- denser, imaging quality is not so important and a Lobster Eye optics can be sufficient. 20.2 Design The first concept was based on the use of a Lobster Eye (LE) in the Schmidt arrangement. The design consists of two orthogonal sets of reflecting mirrors. However, the simple Schmidt design was shown to be impractical after the first calculations and simulations, because very short reflecting surfaces had to be used. Otherwise, it was not possible to meet the required focal spot size with a given source diameter and to simultaneously utilize the incoming radiation optimally. Initial calculations suggest using curved mirrors in order to increase the focusing power of the Lobster Eye. As long as the problem cov- ers, only the single-point-to-single-point focusing system, the effect of severe image distortions for off-axis sources can be neglected, because the source will always effectively be on the optical axis. The system for focusing from point to point is needed and thus the mirror shape should be elliptical. To opti- mally cover the FOV of the system in this case, each mirror not only has to have a different curvature but also has to be in the different distance from its neighbor, i.e., the mirrors cannot be equally spaced. Several iterations have been done. The mirror length was changed to have the mirrors as long as possible in order to reduce the number of them. These iterations ended up with the final design consisting of 4 cm long, 8 cm wide, and 300 μm thick ellipsoidal mirrors. Half of the profile is shown in Fig. 20.1. Note the changing distances between the mirrors, which were calculated to allow illumination of the entire surface of each of the mirrors. The necessary number of mirrors is optimized. The final design can be described as a multifoil, bifacial Kirkpatrick–Baez system. Term “bifacial” is used to stress that the reflecting mirrors are on Fig. 20.1. One half of the multifoil (MFO) EUV bifacial Kirkpatrick–Baez condenser 322 L. Pina Fig. 20.2. Front view of the multifoil (MFO) EUV bifacial Kirkpatrick–Baez con- denser (left). The ray-tracing simulation of a 0.5 mm size EUV source focus (right). Vertical and horizontal intensity profiles give a peak FWHM of 0.45 and 0.65 mm, respectively both sides of the optical axis unlike the case of the classic KB system, where the optics are asymmetric. A number of ray-tracing simulations of the selected design, as well as of each of the particular designs during the iterative design process were per- formed. The ray-tracing simulation of the final system design for a flat circular photon source with diameter of 0.5 mm is plotted in Fig. 20.2 together with the front view of the condenser. Mirrors with varying distances between them are clearly depicted. The intermirror distance increases for larger mirror off- axis distances. The gray central cross is part of the optics holding structure. It additionally shields the central part of the optics, where no reflection is possible, against the direct beam. The optics has different magnifications in two perpendicular directions. This is because the reflection occurs at different distances from the source for different directions and an asymmetry is thus introduced. If the mirrors could penetrate each other to form the channels, i.e., if all the mirrors were at the same distance from the source, no differences in magnification would be visible. Simulated peak widths strongly indicated that the proposed condenser is feasible. Distortions due to the design itself are acceptable. Decay of the intensity with the distance from the optical axis was also theoretically studied. An extremely extended uniform source has been simulated in this case. It can be seen that the FWHM FOV (i.e., the intensity falls to 1/2 at the edges of the FOV) is 5 ×5cm 2 , i.e., 13 ×13 deg 2 . This means that although not originally designed to be an imaging device, this MFO still have some imaging power in relatively large focal area. The ratio of the number of photons gathered inside the central peak to the number of photons blocked by the central support structure characterizes the MFO compared to the LE design. The ratio of all the photons in the entire focal cross without the central peak to the photons inside the central peak is 20 Reflective Optical Structures and Imaging Detector Systems 323 0.2%. The length of the cross bar is about 10 mm. This means that the cross structure is strongly reduced if compared to standard Schmidt LE because of the curved and optimized mirrors. While still present, it plays a negligible role in the experiments described. The solid angle from which the photons are collected in case of the MFO condenser and the maximal solid angle which can be reached with an axially symmetric condenser have also been studied in order to compare the efficiency of the systems. The simulation indicates that the solid angle which is covered by the MFO condenser is about 0.09 sr. This number includes correction on the gaps between the mirrors and shows how many photons are reflected at least once. The solid angle which can be reached by the nested shells of a corresponding axially symmetric condenser is about 0.18 sr at maximum. In fact, this number should be smaller because only a finite length of the mirror can be used, or, equivalently, only a limited number of shells are feasible. Therefore, the ratio between the MFO condenser and the ideal condenser with the same outer dimensions is about 0.5. The realistic ratio including various technical aspects is somewhere between 0.5 and 0.9. 20.3 MFO The exact parameters of each of the reflecting mirrors were calculated. Some problems were encountered during the manufacturing, however (unclear). All the previously created devices of the comparable size and complexity employed only flat mirrors, which was not the case of the proposed condenser. Hence, the technology of shaping the mirrors with the desired quality has had to be developed. In the framework of alternative proposals, thermal shaping of glass mirrors was studied. Thermally shaped mirrors keep their proper shape after the pro- cess, and the internal stress is minimized. Therefore, the probability of glass damage and/or cracks is substantially reduced even under extreme conditions, such as strong vibrations and temperature changes. Although the technology has promising results, its economy is a great disadvantage for the condenser. As mentioned earlier, each mirror of the con- denser has a different shape. If the thermal forming technology is used, a number of different forms would be required, one for each mirror. A different approach was developed to avoid this problem. The key features are as follows: • A modular concept of multifoil optics with a large number of mirrors is used to facilitate assembling them into complex optical systems • Relatively low-cost methods to create many differently shaped mirrors in one module are used • The accuracy of shaping is lower compared to the thermal shaping, however, there is still room for further development [...]... Pollock, Phys Rev 71, 8 29 ( 194 7) 4 B.L Henke, E.M Gullickson, J.C Davis, At Data Nucl Data Tables 54, 181 ( 199 3) http://www-cxro.lbl.gov/optical constants/ 5 S Suehiro, H Miyaji, H Hayashi, Nature 352, 385 ( 199 1) 6 T Tomie, Japanese Patent 6-045288, 18 Feb 199 4; U.S Patent 5, 594 ,773, 199 7; German Patent DE 199 50 195 05433, 199 8 7 A Snigirev, V Kohn, I Snigireva, B Lengeler, Nature 384, 49 ( 199 6) 8 B Lengeler,... passed through aluminum filter and was focused by the Wolter optics and detected by a back illuminated CCD camera The image obtained is shown in Fig 20.5 The image is not corrected for the geometrical inclination or for any possible change of intensity with the inclination angle Second, the profile of the focal spot was mapped for two different gases, Xe and Kr, using a scanned pinhole inside the source... M Sczurek, “Wide band laser-plasma soft X-ray source using a gas puff target for direct photo-etching of polymers,” in Infrared Photoelectronics, ed by A Rogalski, E.L Dereniak, F.F Sizov, Proceedings of the SPIE, Volume 595 8, p 2 79 (2005), K.M Abramski, A Lapucci, E.F Plinski, eds., p 2 79, Sept 2005 5 W.K.H Schmidt, Nucl Instrum Methods 127, 285 ( 197 5) 21 CLESSIDRA: Focusing Hard X-Rays Efficiently with... is used in magnifying configuration with ×6.8 magnification applied only to the vertical direction The pictures cover 0.125 mm in this direction (a) The first two prisms in the two lens halves adjacent the central air gap In (b) and (c) rows 13–17 from the upper lens half and in (d) and (e) the last four rows 26– 29 are shown In (a), (b), and (d) are obtained with the maximum of the crystal rocking curve... to the incident beam In (c) and (e) the crystal was inclined by 0.175 mrad and by 0.35 mrad with respect to (a), respectively The grey scale in (f) has always black for a signal of 0, while white is a signal of 1.2 (scale to the left of the bar) in (a), (c), and (e), and it is 0.2 (scale to the right of the bar) in (b) and (d) Fig 21.8 Radiographs from a clessidra lens well aligned in tilt and roll... instruments for tests and a careful and time consuming production process The interesting point in the large tolerance for the surface errors in X-ray transmission lenses is the fact that these lenses, at these tolerances, do not even require polishing Indeed the tolerance is even within reach of standard workshop tools Consequently, Suehiro et al [5] resume the discussion of X-ray transmission lenses... plotted in Fig 20.6 The pinhole was placed in a particular position and the flux was measured After measurement of sufficient number of distinct pinhole positions, the interpolation was used to create the profile Because of the nature of the probing process, each point in the resulting image is based on a completely different set of shots Thus, an averaging of a large number of shots exactly as in case... plexiglass (PMMA) as a solid line and for silicon as a dashed line the following parameters depending on photon energy: D in (a) is the amount of material that shifts the phase of a passing wave by 2π according to (21.28) 1.4L/D in (b) is the number of 2π phase shifts, which occur in a slab with a transmission of t = 0.5 according to (21.32) (δL/λ)1/2 in (c) is the factor in (21.34) by which the optimum... h According to (21.21) in such a lens Δl needs to increase linearly with distance y from the optical axis This concept is shown in Fig 21.1d in a compacted version of such a lens So while the first Fresnel lens (Fig 21.1b, c) has mostly constant average absorption throughout the lens aperture, in the lens in Fig 21.1d the average absorption increases approximately linearly with increasing y In the present... refraction efficiency integrated over four prism rows was measured by scanning a slit of width 100 μm vertically through the lens aperture upstream of it and registering the intensity in the incoherent image in the best refractive image plane [17] These results, poorer than expected, are also presented in Fig 21.5 as filled circles The refraction in the single rows was investigated by reducing the aperture . Wilkins, Rev. Sci. Instrum. 62, 1542 ( 199 1) 7. M.A. Kumakhov, Proc. SPIE 3444, 424 ( 199 8) 8. S.W. Wilkins, A.W. Stevenson, K.A. Nugent, H. Chapman, S. Steenstrup, Rev. Sci. Instrum. 60, 1026 ( 198 9) 9. . forming a double focusing device. Unlike multilayer optics, grazing incidence optics are efficient at focusing soft X-rays over a wide wavelength range. Optics designed for collecting solid angle of. imaging screens and detector systems with thin YAG:Ce and other monocrystal scintillator screens have been designed and tested. Camera sys- tems based on monocrystal scintillator, optics and