Reflection on multilayer mirrors: beam profile and coherence properties

Abstract

The main advantage of Bragg reflection from a multilayer mirror as a monochromator for hard X-rays, is the higher photon flux density because of the larger spectral bandpass compared with crystal lattice reflection. The main disadvantage lies in the strong modulations of the reflected beam profile. This is a major issue for micro-imaging applications, where multilayer-based monochromators are frequently employed to deliver high photon flux density. A subject of particular interest is the origin of the beam profile modifications, namely the irregular stripe patterns, induced by the reflection on a multilayer. For multilayer coatings in general it is known that the substrate and its surface quality significantly influence the performance of mirrors, as the coating reproduces to a certain degree the roughness and shape of the substrate. This proceedings article reviews recent experiments that indicate potential options for producing wave front-preserving multilayer mirrors, as well as new details on the particular mirrors our group has extensively studied in the past.

Beam profile and coherence properties of synchrotron beams after reflection on modified multilayer mirrors

“…Similar to the previously published study, both mirrors preserve the coherence of the incoming beam in a similar manner: indicated by the identical stripe pattern of the boron fiber. Still, the modulation of the beam profile reflected by the 5 μbar stripe is significantly weaker than for the 1 μbar stripe (peak-to-valley) [15]. …”

“…Modified multilayer mirrors were produced in terms of increased interlayer roughness: in a first step a superpolished Si substrate was partially coated with a W/B 4 C stripe (120 bi-layers, 2 nm d-spacing) at the ESRF multilayer deposition facility according to published parameters [13,15]. Next to this stripe (in the middle of the substrate) a second multilayer stripe was deposited, using exactly the same parameters but with the Ar pressure in the coating facility being increased from 1 μbar to 5 μbar (DC magnetron sputtering, power: P(B 4 C) = 500 W, P(W) = 50 W, growth rate R = 0.1 -0.2 nm/s).…”

“…The first two stripes were characterized using the above mentioned protocol at beamline ID19 of the ESRF [11]. It was found that both multilayer stripes preserve the coherence properties of the incoming beam in a similar manner while the stripe produced at 5 μbar depicted a significantly more homogeneous wavefront [15].…”

“…Further studies were carried out with coatings deposited by the multilayer laboratory of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) [14]. The results indicated that varying parameters during the deposition such as the Ar pressure in the coating chamber can significantly influence the wavefront preservation performance of a mirror in a positive manner [15].…”

Fast Multi-scale imaging using the Beamline I13L at the Diamond Light Source

“…The multilayer consists of a system of alternating high and low refractive index materials. In general a striped structure is added to the beamprofile [7,8]: difficult to correct when the monochromator is not stabl. The central bandpass energy can be easily selected by tuning the Bragg angle.…”