High energy micro-diffraction

Many natural and technologically important interfaces lie deeply buried beneath their exposed surfaces. One needs to access them with a non-destructive, structural, in-situ probe with atomic-scale resolution. Using high energy x-rays the beam can penetrate through centimetres of materials, thereby making a wide variety of buried interfaces accessible for structural studies. Numerous interfaces can now be probed, such as, solid-solid, solid-liquid, and liquid-liquid interfaces. Newly installed at the ID15A hutch during January 2005, the High-Energy Micro-Diffraction (HEMD) end-station built by MPI/Stuttgart brings a unique instrument to the ESRF for the purpose of investigating buried surfaces and interfaces. Combining micro-focussed, high-energy x-rays with a new approach to accessing buried interfaces creates an opportunity to conduct novel experiments not available at any other surface-science beamline. The basic principle relies on the deep penetration of high-energy x-rays so that only the one interface of interest is illuminated with x-rays, as show in Figure 8. This reduces the number of interfaces that contribute to the scattered signal and enables the use of two general techniques, x-ray reflectivity (XRR) and grazing-incidence diffraction (GID), on interfaces that were inaccessible before.

a)

b)

Geometries for a) standard and b) high-energy x-ray reflectivity.

It has turned out that the very small size of the incident beam, as required for the small angles at the high energies, serves to reduce the background. The detector slit opening can be made very small, and the reduction of the background gives an increase to the dynamical range of the instrument, which is typically ten orders of magnitude giving in most cases access to ~1/Å vertical momentum transfer. The incident radiation (typically 71keV) is focused by Aluminium compound refractive lenses installed 4-5m meters before the sample, and the typical beam size is 16(H)x4(V)µm2. Unfortunately, lately the changes in the ESRF storage ring lattice have increased the average electron beam size and in recent experiments it has been impossible to obtain vertical spot sizes smaller than 8µm.

The diffractometer motor stages are constructed on a large cradle mounted on the top of four granite towers (see Figure 9). This cradle is used to define the angle between the surface of the sample and the incoming beam. A high precision is obtained by the use of a high resolution linear translation that pulls or pushes the whole cradle, and the precise value of the diffractometer inclination is obtained by an encoder. This technique allows the control of the gracing incidence within 1°/10000. Several motorised translation and rotation stages are mounted on the cradle for the sample alignment. Each of these stages has been especially designed and constructed by Huber for this instrument. The use of an additional set of monochromator crystals allows investigating liquid surfaces that are not accessible by standard surface diffraction techniques without sample movements.

For highest flexibility in the experiments the diffractometer has to accommodate different sorts of sample environments like ultra-high-vacuum (UHV) chambers which can be very heavy. Therefore, the diffractometer has been designed to take loads of up to 500 kg without loss of accuracy.

The weight of the diffractometer together with the heavy shielding of the experimental hutch acts as effective passive vibration dampers. For most surface experiments no additional active suppression is needed but the diffractometer tower can be equipped with a vibration isolation stage (TS-150 from Table Stable) which isolates against all translational and rotational vibrations in a frequency range from 0.7 to 10Hz.

a)

b)

A schematic a) and photograph b) of the HEMD instrument. The four granite towers hold the cradle which holds the translation and rotation stages for the sample alignment. The detector table is totally decoupled from the diffractometer. Additionally, new x-ray optics for measurements of liquid surfaces or liquid/liquid interfaces has been included into the set-up (left in a) for greater measurement capabilities. This new optics scheme is fully described in reference Honkimäki et al. (2006).

The HEMD setup is designed to work with two kinds of detectors: point counters and 2D detectors. The point detector arm (with motorised detector and collimator slits) can move in vertical and horizontal plane around the sample position. Furthermore, the detector stages follow automatically the change of the scattering geometry when instrument is working in liquid surface/interface operation mode. High energy experiments are especially suited for use of 2D detectors, since the Ewald sphere is almost flat. This makes it possible to map almost flat projections of reciprocal space with a single shot. The six circle mode is used for grazing incidence x-ray diffraction and to measure non-specular crystal truncation rods. It allows scans along directions in reciprocal space that correspond to complicated motor movements by the actual diffractometer and greatly simplifies measurements of diffraction signals with in-plane components.