In 1999 multilayers have been increasingly requested as optical devices. They provide monochromatic beams with a spectral bandwidth of about 1% and thus 100 times more flux than perfect crystal monochromators. Because the layer materials and spacings can be varied, multilayers can be tailored to specific requirements. More than ten different specimens were made by sputtering and have already been installed on ESRF beamlines, either permanently, or for dedicated experimental setups. The recent quality improvements resulting from both excellent substrates and deposition conditions has permitted multilayers to be used for the first time on the Topography Beamline ID19 where the slightest inhomogeneities in the beam reflected by the optics create artefacts in the images. The 100-fold gain with respect to the standard crystal monochromator allowed the users to perform experiments with much improved time resolution. Currently, the most prominent application is to microfocusing experiments with laterally graded multilayers mounted on dynamically bent substrates (see ESRF Highlights 97/98, p. 102).

Recent developments in our multilayer laboratory have aimed at two important improvements: a better control of the thickness gradient and a versatile design technique that now allows us to cover a wide range of photon energies using only one bent multilayer element. As a result, we have succeeded in reducing the error of the lateral gradient of the bilayer spacing along the mirror length of 300 mm to less than 0.5% (rms). This excellent figure is well below that of commercially available multilayers. It is also smaller than the typical energy resolution of a multilayer and therefore full reflectivity is obtained at each position of the optical element.

Focusing setups may be used for different energies without changing the geometry, for example the focal distance. In certain cases, the whole energy range available on a beamline must be covered and here one layer pair alone cannot optimally reflect the X-ray beams. Therefore, we have developed a technique to deposit two or more parallel multilayer stripes on the same substrate. Their gradients and constituent materials are usually different. However, since the optimum lateral thickness gradient of a focusing multilayer is directly related to the focal distance, the coating parameters have to be correlated precisely.

As an example, we have manufactured a focusing device for ID13 based on two Ru/B4C multilayer stripes [1]. The two stripes were designed so that synchrotron beams of 13 keV and 20 keV can be reflected while keeping both the incident angle and the focal distance constant. Model calculations and experimental data of the resulting gradients show excellent agreement, as illustrated in Figure 126. Thus, it is possible to switch between two energies during an experiment by a simple translation of the mirror. The 170 mm long multilayer was mounted on a standard ESRF two-moment bender. Focusing experiments were conducted on the Optics Beamline BM5 where a focal spot size of 0.7 µm was achieved, mainly limited by the source size. The beam size before the focusing element was 1 mm and the reflectivity close to 80%. Therefore the flux gain with respect to a pinhole of the same size was about three orders of magnitude which represents a new record.

[1] Ch. Morawe, J.-Ch. Peffen, O. Hignette, E. Ziegler, SPIE Proceedings, 3773, 90-99 (1999).

C. Morawe, J.-C. Peffen, E. Ziegler.