Abstract
While channel-cut crystals, in which the diffracting surfaces in an asymmetric cut are kept parallel, can provide beam collimation and spectral beam shaping, they can in addition provide beam compression or expansion if the cut is V-shaped. The compression/expansion ratio depends in this case on the total asymmetry factor. If the Ge(220) diffraction planes and a total asymmetry factor in excess of 10 are used, the rocking curves of two diffractors will have a sufficient overlap only if the second diffractor is tuned slightly with respect to the first one. This study compares and analyses several ways of overcoming this mismatch, which is due to refraction, when the Cu Kα1 beam is compressed 21-fold in a V21 monochromator. A more than sixfold intensity increase was obtained if the matching was improved either by a compositional variation or by a thermal deformation. This provided an intensity gain compared with the use of a simple slit in a symmetrical channel-cut monochromator. The first attempt to overcome the mismatch by introducing different types of X-ray prisms for the required beam deflection is described as well. The performance of the V-shaped monochromators is demonstrated in two applications. A narrow collimated monochromatic beam obtained in the beam compressing mode was used for high-resolution grazing-incidence small-angle X-ray scattering measurements of a silicon sample with corrupted surface. In addition, a two-dimensional Bragg magnifier, based on two crossed V15 channel monochromators in beam expansion mode and tuned by means of unequal asymmetries, was successfully applied to high-resolution imaging of test structures in combination with a Medipix detector.
Highlights
With progress in materials science and technology, X-ray sources and beam conditioning optics that can provide high brightness and high resolution in reciprocal and real space are all the more important for the metrology of advanced microand nanostructures
Because of the unequal refraction angle shifts at the channel walls, the overlap of the two rocking curves inside the channel decreases if the asymmetry angles increase
The intensity transmitted through the monochromator decreases significantly for total asymmetry factors greater than 10
Summary
With progress in materials science and technology, X-ray sources and beam conditioning optics that can provide high brightness and high resolution in reciprocal and real space are all the more important for the metrology of advanced microand nanostructures. Standard laboratory high-resolution X-ray diffractometry with a Gobel mirror and Bartels monochromator can be, in many cases, successfully replaced by an X-ray microsource, Montel optics and crystal optics adjusted to required parameters (degree of beam monochromaticity, collimation and beamsize). A special group of Bragg case channel-cut crystals with nonparallel channel walls (V-shaped monochromators) offering beam footprint control (one-dimensional beam expansion or compression) was discussed by Pietsch et al (2004) and dealt with in detail by Korytar et al (2008). Crossed V-channel monochromators were successfully demonstrated recently as Bragg magnifying imaging optics in combination with a FReLoN camera (Vagovicet al., 2011). This paper presents our theoretical and experimental study on how higher asymmetry V-channel monochromators can be tuned. Two practical applications are demonstrated: (1) a beam compressing V-channel monochromator in a grazing-incidence small-angle X-ray scattering (GISAXS) system for use in metrology and (2) an imaging application
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