Synchrotron Laue micro-diffraction: a new beam line project at SOLEIL for phase identification and mechanics of materials

Abstract

Micro-focused x-ray beams from 1 down to less than 0.1 micron in size have been one of the real success stories of 3rd generation synchrotron x-ray machines (SR), thanks to a large panel of focusing devices for hard and soft x-rays [1-5]. There is clearly a rapidly growing need for very small x-ray beams – 10 nm is theoretically possible which allow for non-destructive local scale measurements of structure and chemistry. This need encompasses many different scientific fields: Microelectronics and microsystems, Metallurgy and mechanics, Environmental and earth sciences, Art and archaeology, Life sciences and soft condensed matter. In all these different research fields one would ideally like to get information on a local scale of the structure, the chemical composition and the local atomic environment. This implies performing at the submicron scale: XRD, XRF, EXAFS, XANES. However, the case of XRD technique is specific in the sense that the recorded signal does not only depend on the beam size but also on the ratio of the beam size to the grain size. If this ratio is large enough one is left with powder diffraction when using monochromatic x-ray beam (MB) and the diffraction information is an average over the size of the beam. On the other hand if the beam size over grain size ratio is small one gets single crystals diffraction. It is thus possible to obtain intra grain structural information. Transmission configuration with hard x-rays is done using either MB or white beam (WB) XRD with generally energy dispersive mode. For the proposed beam line, the working geometry is reflection. It is important to realize, however, that recording a significant number of diffraction spots from a single crystal requires either a movement of the sample under the beam (goniometry) or the use of a polychromatic incoming beam (Laue diffraction) combined with a two-dimensional area detector such as CCD type. Since there are no available goniometers with a sphere of confusion (SOC) radius smaller than the micrometer x-ray probe size it is thus necessary to use WB. Details concerning the different applications of the technique may be found for instance at the Advanced Light Source (ALS) web site address (http://xraysweb.lbl.gov/microdif/index.htm), this synchrotron source being close enough to the French facility SOLEIL. This beam line project is unique in Europe since there is almost nothing in European countries in terms of fully dedicated WB μXRD beam line except the BM32 project at ESRF (10 % of the full beam line time dedicated to microfocus WB during 3 years). The Scientific Advisory Committee (SAC) of SOLEIL approved the APS (preliminary beam line project*) in November 2005 and the SOLEIL Council decided in July 2006 to build this additional beam line and to provide 1⁄4 of the total beam line budget which will be completed by external funds (not yet found).