Single point diamond nanomachining allows to finish surfaces of some hard-to-machine brittle materials as for example germanium with high shape accuracy, low surface roughness and low subsurface damage. Quality of these surfaces is sufficient when infrared or visible light wavelengths are used. However, using hard X-ray wavelengths, additional polishing of the nanomachined surface is necessary. We are developing polishing methods of germanium surfaces produced by nanomachining, while optimizing nanomachining conditions such as the cutting speed, cutting depth or tool geometry in nanomachining centre. Micro Raman spectroscopy and atomic force microscopy (AFM) were used to study the effect of the cutting speed on the mode of the single point diamond machining (flycutting) of Ge(110) surface. Selected samples were also studied by the rocking curve imaging (RCI) at BM05 beam-line (ESRF), which serves as a test and development station for X-ray optical elements. The surface study indicates that the amorphous layer which originates by nanomachining in the sub-surface area of Ge (110) surface can be delaminated together with all structural imperfections within the chip using appropriate cutting conditions. Results of the study also show a dislocation-free single crystal lattice beneath the Ge (110) nanomachined surface, which demonstrates the potential of the applied method for the preparation of high quality surfaces for hard X-ray crystal optics.
We present the numerical optimization and the technological development progress of x-ray optics based on asymmetric germanium crystals. We show the results of several basic calculations of diffraction properties of germanium x-ray crystal monochromators and of an analyzer-based imaging method for various asymmetry factors using an x-ray energy range from 8 to 20 keV. The important parameter of highly asymmetric monochromators as image magnifiers or compressors is the crystal surface quality. We have applied several crystal surface finishing methods, including advanced nanomachining using single-point diamond turning (SPDT), conventional mechanical lapping, chemical polishing, and chemomechanical polishing, and we have evaluated these methods by means of atomic force microscopy, diffractometry, reciprocal space mapping, and others. Our goal is to exclude the chemical etching methods as the final processing technique because it causes surface undulations. The aim is to implement very precise deterministic methods with a control of surface roughness down to 0.1 nm. The smallest roughness (∼0.3 nm), best planarity, and absence of the subsurface damage were observed for the sample which was machined using an SPDT with a feed rate of 1 mm/min and was consequently polished using a fine polishing 15-min process with a solution containing SiO2 nanoparticles (20 nm).
Advanced X-ray imaging techniques of weakly absorbing structures require an increase of the sensitivity to small
refractive angles considering that they are based more on coherent X-ray phase contrast than on X-ray absorption one.
Simulations of diffraction properties of germanium (Ge) X-ray crystal monochromators and of analyzer based imaging
(ABI) method were performed for various asymmetry factors and several lattice plane orientations using an X-ray energy
range from 8 keV to 20 keV. Using an appropriate phase/amplitude retrieval method one can recover the phase
information from the ABI image, which is directly proportional to the projected electron density. We are using
germanium based optics for X-ray imaging or image magnification. The use of Ge crystals offers several advantages
over silicon crystals. The integrated reflectivity of Ge crystals is two to three times larger than that of Si crystals. The
spatial resolution of Ge magnifiers is typically two times better than the spatial resolution of Si magnifiers. We used high
asymmetry diffractions to increase effectively the propagation distance and decrease the effective pixel size of the
detector, to achieve a sufficient magnification of the sample and to improve coherence and increase output intensity. The
most important parameter of a highly asymmetric monochromators as image magnifiers is the crystal surface quality. We
have applied several crystal surface finishing methods including conventional mechanical lapping, chemical polishing,
chemo-mechanical polishing and advanced nano-machining using single point diamond turning (SPDT), and we have
evaluated these methods by means of AFM, diffractometry, reciprocal space mapping and others.
Beam inhomogeneities of asymmetric Ge(220)-based V-shaped and single bounce monochromators have been studied
both in metrological and imaging applications for photon energies around 8 keV. Presence of growth striations in graded
GeSi, grains in single Cu crystal, and strains in thermally tuned V-channel monochromators observed in X-ray
topographs excludes these materials from imaging applications. As for stochastic surface processing, chemomechanical
polishing (CMP) produces better surface homogeneity than chemical polish. However, CMP is more difficult to be
applied in V-channels, where chemical polishing is prefered. For comparison, measurements on surfaces processed by a
deterministic mechanical method of single point diamond turning (SPDT) have shown SPDT to be a perspective
technology. Again, to prepare deep grooves with this technique is also a challenge, mainly for tool makers. Some process
induced features are observed as wavefield distortions in interference fringes.
In the present work the lattice plane curvature of a nearly dislocation free S:doped InP and a semi-insulating GaAs wafer crystals has been investigated using the method of X-ray rocking curve imaging based on the FRELON CCD area detector with a pixel resolution from 10 to 40 μm at the ID19 ESRF beamline. The geometry of the experiment is based on a vertical Si (111) monochromator and a horizontal sample scattering planes in the Bragg geometry (σ-π geometry). To determine the local lattice inclination, the effect of such dispersive setup on the measured local diffraction peak position has been accurately determined and the equations to determine the lattice plane curvature of the crystals under the condition of isotropic distribution of dislocation Burgers vectors are obtained. The analysis of the data showed that the shift of the Bragg condition is almost completely due to the lattice tilt rather than to the lattice parameter variation. Lattice displacements from the ideal lattice as large as 200 μm are found at the edges of the InP crystal. Non random distributions of dislocation Burgers vectors are observed in both samples.
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