Crystal Laboratory

Activities

The Crystal Laboratory is in charge of manufacturing perfect crystal monochromators for all ESRF beamlines. Most crystals are made from top purity silicon float zone ingots 100 mm in diameter and up to 1 m long that can be obtained from industry. As seen in the photograph (fig. 1) there is a great variety of monochromators, ranging from simple rectangular plates to complex Bragg-Laue crystals and nested channel-cut devices for high spectral and angular resolution. The task is then to transform these ingots into objects of various sizes and shapes as defined by the beamline scientists.

 

Fig. 1

The preparation of the crystals is done in several steps. First, the raw ingots must be oriented. This is performed on an X-ray single crystal diffractometer (fig. 2). The outer shape of the ingot shows growth features corresponding to basic crystallographic orientations and a flat surface is ground corresponding roughly to one of these low-index lattice planes. Then the crystal is held against a flat vertical surface serving as a reference in the center of the two circle diffractometer. The beam (CuKα₁-radiation) emitted horizontally by a sealed X-ray tube is transmitted through a window in the center of the reference plate, diffracted by the crystal and received by the detector set at twice the Bragg angle when Bragg's condition is fulfilled. Then the crystal is turned about an axis normal to its surface. If the Bragg peak remains constant, the surface is parallel to the lattice planes. If not, the miscut angle is read on the diffractometer and the surface orientation is corrected by grinding. After one or several iterations the right orientation is obtained. An accuracy of 1/1000 degree can be obtained, but usually 1/100 degree is requested.

Fig. 2

Once two perpendicular faces of the ingot have been prepared, the crystal is glued on a glass plate with a mixture of beeswax and collophane that is glued on a steel plate. The whole is clamped on the magnetic base of the support in a circular diamond saw. Four degrees of freedom (three high-precision translations and one rotation) are available and simple shapes can be produced with this saw (fig. 3). The longest cut is 500 mm; the deepest 120 mm. For generating more complicated shapes we have a milling machine that can be equipped with different diamond tools. Holes (for cooling purposes) are obtained with a special core-drilling machine.

Fig. 3

The further processing consists in fine grinding and lapping of the surfaces exposed to the X-ray beam, either on grinding wheels or by hand (e.g. inside channel-cut crystals), followed by chemical etching to remove the surface damage (fig. 4). According to the wishes, the surface can be polished. This is necessary for applications that require coherence preservations, e.g. imaging techniques where surface structure must be avoided. Mechano-chemical polishing (Syton process) is the last step that follows polishing of the etched surface using very fine diamond powder. The best surface finish presently obtained is about 1Å (rms) while the best figure (flatness) for a thick specimen is around 1 µrad (rms).

Fig. 4

About one ton of silicon single crystal material has been processed and many different types of flat or bent, also directly cooled objects have been produced. The demand and workload has not decreased, because X-ray crystal optics like beamline concepts constantly evolve following the general evolution of instrumentation for synchrotron X-rays to meet new scientific goals. Critical issues are precise strain-free mounting and bonding of the crystals on supports for cooling and bending. Here the traditional art and skill of handcrafting meet with the modern tools of mechanical and thermal engineering such as finite element analysis. The last not least ingredient for the production of adequate crystal optics is the detailed understanding of the diffraction processes in perfect, curved or otherwise deformed crystals that allow us to predict their effect on X-ray beam propagation and on the final performance of optical schemes.