A major goal in the field of X-ray optics development has always been to concentrate more X-ray photons into a smaller area and thus gain in flux. Of course, this gain in flux, measured in photons per unit time and area, is accompanied by an equivalent increase in beam divergence. Therefore, the gain is limited by the maximum divergence that can be tolerated by an experiment. Fortunately, the X-ray beams generated by a third-generation source such as the ESRF are well collimated in angle, typically a few tens of micro-radians, so that four or more orders of magnitude of flux can be gained without jeopardising the quality of the experiment. Another limit is set by the survival of the sample under such high-flux conditions. Once the experimental parameters have been defined, the practically achievable gain depends on the type and quality of the optical element used. The same holds true for the minimum spot size. Further limits are given by the source size S and the magnification M = q/p, where p is the source-to-optics distance and q is the optics-to-focus (i.e. to the sample) distance. So the working distance (q) and thus the overall length of the beamline must also be considered. For perfect optics the focus size F is then given by MxS. However there is a fundamental limit called the diffraction limit (DL): the DL spot size is proportional to the wavelength and the focal length divided by the beam width. Apart from the maximum gain ­ minimum spot size criteria, other arguments such as availability, simplicity of use and cost influence the final choice of the optics.

In recent years, quite substantial progress has been made in the field of microfocusing as shown in previous ESRF Highlights articles and specialised literature such as various proceedings of SPIE and SRI conferences (see some references below). Here we want to present the most recent achievements made at the ESRF, often in collaboration with other laboratories and institutes. They focus on two techniques: reflective optics based on specular reflection by mirrors and multilayered structures and on refractive optics based on beam deviation by refraction like in an optical lens.

Microfocusing by Curved Mirrors and Multilayer Structures

Focusing hard X-rays with the help of grazing-incidence, reflecting surfaces is probably the method mostly used on synchrotron beamlines. A typical example is a toroidal mirror that gives spots several hundreds of micrometres wide. The goal to decrease the spot size to a micrometre or less has triggered technological developments of a specific reflecting system, the so-called Kirkpatrick-Baez system (KB). Two orthogonal mirrors focus the beam successively in the horizontal and in the vertical planes. The system can be either static, with mirrors polished according to the figure optimised for a given incidence angle and focus. Or it can be dynamic, with actuators bending flat mirrors into the elliptic shapes required by the experiment. Figure 116 shows an example of a complete dynamic system consisting of two 170 mm long mirrors with integrated motors for adjustment of bending and positioning.

Fig. 116: Kirkpatrick-Baez system.

The reflection by a single layer is achromatic: a wide range of X-ray wavelengths or energies are reflected at a given angle, typically a few milli-radians. Therefore, experiments requiring energy tuning can be performed without any readjustment of the optics. The transmission can be as high as 70%, with only a small amount of light scattered outside of the central spot when super-smooth polished surfaces are used. A combination of different metal and multilayer coatings makes it possible to cover the range from 2 keV to 90 keV. Multilayers with a graded layer spacing can have an energy bandwidth as high as 6%. They can accept wider beams, up to several mm, due to their ten times larger angles of reflection.

With eight degrees of freedom to be tuned to sub-micrometre precision, the alignment procedure of a dynamic system is relatively complex. To this end automated sequences based on linear optimisation with the help of CCD position sensors have been developed at the ESRF [1]. The vibration level has to be controlled to within a few micro-radians and the figure accuracy of the elliptical mirrors to within a few nanometres. This is technologically challenging. The reflected beam is deflected with respect to the incoming beam. These constraints can all be managed, but have to be taken into account when selecting the most appropriate microfocusing technology.

The final spot size is determined by a convolution of four parameters: the source size (geometrically de-magnified), the mirror imperfections, the vibrations and ultimately by the diffraction limit set by the X-ray wavelength. If the source is sufficiently small and distant, it is seen by the optical system as a "star", which means that the optical system cannot resolve its angular width [2]. The synchrotron source then behaves like a laser: it is "coherent". This is already the case on long beamlines such as ID19, where we are approaching the diffraction limit: a record spot size close to 100 nanometres and a gain of up to five orders of magnitude were obtained. Non-conventional polishing processes have been developed for ultra-precise figuring, for example ion-beam figuring. Diffraction-limited conditions have also been obtained at Spring-8 in Japan.

To date, the ESRF Optics Group has produced 38 bent mirrors, most of them assembled as KB pairs, and several more are planned for 2003. Three standard dimensions (300 mm, 170 mm, 90 mm) and the multilayer-coating capability allow us to tackle a wide range of applications [3].

Microfocusing by Refractive Optics

Other types of microfocusing devices for hard X-rays are based on refraction. Although they function in the same way as visible light optics, there are some differences. Firstly, the X-ray refractive index of a material is smaller than in vacuum or air and, therefore, an X-ray focusing lens has a double concave shape. Secondly, because the refractive index of all materials is very close to unity for hard X-rays, the deflection is usually very small and many lenses have to be placed in series to achieve reasonably short focal lengths. In order to keep absorption to a minimum, these compound refractive lenses (CRLs) should be made from low-Z materials such as beryllium, carbon, aluminium, silicon, etc. CRLs with parabolic shapes made from polycrystalline aluminium by a pressing technique have proven to be well suited for microanalysis and full field microscopy applications for 20-120 keV X-rays [4]. These lenses for two-dimensional focusing were developed in collaboration with Aachen Technical University and are now extensively used as a standard tool in experiments at ID11, ID15 and ID22/18F. The advantages of CRLs are their small size, robustness, and ease of alignment and operation. Their focal length and size is adjustable by adding or removing individual lenses and the lenses can withstand high heatload.

Recently, microelectronics planar fabrication technology has been applied to obtain silicon-based devices. One-dimensionally focusing, parabolic refractive lenses have been manufactured in collaboration with the Institute of Microelectronics Technology (Chernogolovka, Russia) and Dortmund University using lithography and highly anisotropic plasma etching techniques. This type of planar lenses is well suited for high-resolution diffraction experiments including those involving the standing-wave technique [5]. It is possible to make a composite lens consisting of a set of parallel parabolas with different focal distances. To change the focal distance or the desirable working energy, one can switch from one array to another by moving the composite lens. Figure 117 shows planar parabolic lenses made of silicon, fabricated by RWTH in Aachen [6]. They have a focal distance in the range of a few millimetres at hard X-ray energies. At ID22, two lenses were used in a crossed geometry to generate a microbeam with a lateral size of 380 nm by 210 nm at 25 keV. The planar technology is being transferred to materials like diamond that have low X-ray absorption, low thermal expansion and high heat conductivity [7]. These lenses are mechanically robust and can withstand the high heatload of the white beam produced by ESRF in-vacuum undulators.

 


Fig. 117: Scanning electron micrograph of an array of parabolic refractive X-ray lenses made of silicon. (a) The shaded areas (i) and (ii) delimit an individual and a compound nanofocusing lens, respectively. The optical axis of the NFL is shown as a white dashed line. (b) Vertical scan of a gold knife-edge through the microbeam.

Recently, holographic or kinoform optical elements with the combination of refractive and diffractive properties have been manufactured. With this method we eliminate drawbacks of purely diffractive or refractive elements and combine advantages like high transmission, absence of zero-order, high efficiency, etc. The ability to manipulate the local amplitude and phase of the incoming wave opens the perspective to make a new class of beamshaping X-ray optics for coherent synchrotron radiation. 300 µm-thick, identically focusing elements with a kinoform profile were made of nickel by the Institute for Micro-Technology (Karlsruhe) using a lithographic process (LIGA) [8]. In these refractive lenses, passive parts of the material that cause multiples of 2p in phase shift are removed thereby reducing absorption (Figure 118). At ID15A, these planar kinoform lenses focused 212 keV X-rays to a focal line 5 micrometres-wide with a 10-fold gain.

 


Fig. 118: Scanning electron micrograph of a Ni kinoform lens with 140 single elements, each 300 µm high and 1500 µm wide. The lens is designed for 4.5 m focal distance at 212 keV X-ray energy. The insert (upper right) shows a 5 µm wide focus line measured at ID15A using monochromatic 212 keV X-rays.

Among the various applications of microfocused X-ray beams we can mention:

  • Projection microscopy (see Figure 119): a magnified image of an object placed close to the focus plane is projected onto a fast CCD detector with a resolution corresponding to the spot size. The efficiency of the set up permits us to obtain a 2000 x 2000 pixel image in a fraction of a second and to perform fast tomography at the sub-micrometre level.
  • Microfluorescence: the information of the chemical content of an object is obtained by scanning the sample in front of a fluorescence detector. Results have been obtained in materials science and cell biology.
  • Microdiffraction: this is needed at the sub-micrometre level and microfocus systems are widely used for high pressure and surface science.

Fig. 119: Two projection images of a 2 x 2 µm dot phase structure at 20.5 keV, with different contrast conditions, obtained with a KB system [9].

References
[1] O. Hignette, G. Rostaing, P. Cloetens, W. Ludwig and A.K. Freund, SPIE Proceedings 4499, 105-116 (2001).
[2] O. Hignette, P. Cloetens, W. Ludwig and G. Rostaing, XRM 2002 Proceedings, Grenoble, 2002, in press.
[3] Y. Dabin, G. Rostaing, A. Rommeveaux and A.K. Freund, SPIE Proceedings 4782, 235-245 (2002).
[4] C.G. Schroer, J. Meyer, M. Kuhlmann, B. Benner, T.F. Günzler, B. Lengeler, C. Rau, T. Weitkamp, A. Snigirev, I. Snigireva, Appl. Phys. Lett. 81 (8), 1527-1529 (2002).
[5] M. Drakopoulos, J. Zegenhagen, A. Snigirev, I. Snigireva, M. Hauser, K. Eberl, V. Aristov, L. Shabelnikov, and V. Yunkin, Appl. Phys. Lett. 81, 2279 (2002).
[6] C.G. Schroer, M. Kuhlmann, U. Hunger, T.F. Günzler, O. Kurapova, S. Feste, F. Frehse, B. Lengeler, M. Drakopoulos, A. Somogyi, A. Simionovici, A. Snigirev, I. Snigireva, C. Schug, A. Schroeder, Appl. Phys. Lett. (2003), to be published.
[7] A. Snigirev, V. Yunkin, I. Snigireva, M. Di Michiel, M. Drakopoulos, S. Kouznetsov, L. Shabelnikov, M. Grigoriev, V. Ralchenko, I. Sychov, M. Hoffmann, E. Voges, SPIE Proceedings, 4783, 1-9, (2002).
[8] A. Snigirev, I. Snigireva, M. Di Michiel, V. Honkimiaki, V. Nazmov, E. Reznikova, J. Mohr, V. Saile, M. Grigoriev, L. Shabelnikov, to be published.
[9] P. Cloetens et al., to be published.

Authors
O. Hignette, A. Snigirev, A. Freund.
ESRF