Christine Borel

The ID23-2 structural biology microfocus beamline at the ESRF

Beamline ID23-2, the first dedicated and highly automated high-throughput monochromatic macromolecular crystallography microfocus beamline, is described.

Nanofocusing at ESRF Using Graded Multilayer Mirrors

Dynamically bent graded multilayer mirrors have been developed at ESRF for their large energy bandwidth acceptance, energy tunability and large numerical aperture for limited mirror size. Kirkpatrick Baez systems with spot sizes below 100 nanometers have been installed on two beamlines. A diffraction limited line width of 41 nanometers FWHM was obtained at 24 keV on ID19. This experiment directly confirms that the penetration depth of the X‐rays in the multilayer coating does not limit the obtainable focal spot size. The resolution limits of such a nanofocusing device are discussed as well as figure accuracy and vibration level issues.

In situ study of multilayer reflectivity upon heat treatment under synchrotron radiation

Periodic multilayers deposited by Distributed Electron Cyclotron Resonance (DECR) sputtering were studied with synchrotron radiation at the ESRF bending magnet beam line BM5. In situ reflectivity measurements at a photon energy of 20keV have been carried out on these samples during a specific heat treatment. A dedicated furnace has been developed to heat the multilayers under vacuum from room temperature up to 550°C. [Ru/B4C]70 and [W/B4C]40 samples with repetition periods of about 4nm were chosen. Simulations of reflectivity measurements were performed to understand the evolution of layer thicknesses and interface widths. Additional ex-situ reflectivity measurements were done at 8keV before and after the annealing experiments to investigate irreversible effects. We will discuss the heat impact on the layered structure and in which way multilayer optics could be thermally pre-treated before their installation on synchrotron beam lines.

Small-d-spacing WSi2/Si narrow bandpass multilayers

To develop narrow-bandpass multilayer monochromators, we have studied small d-spacing WSi2/Si multilayers. We found that WSi2/Si is an excellent multilayer system for achieving both the desired spectral resolution and peak reflectivity. Compared to other traditional multilayer systems such as W/Si, WSi2/Si not only has a lower density and lower absorption, but also is a chemically more stable system, since WSi2 is already a silicide. One thus expects better thermal stability and sharper interfaces for WSi2/Si multilayers. There are two approaches to achieve high-resolution multilayers: either decrease the d spacing or use low absorption materials. By using WSi2/Si, we can utilize both approaches in the same system to achieve good energy resolution and peak reflectivity. Another advantage of this system is that the sputtering rate for Si is much higher than other traditional low-Z materials. Several WSi2/Si multilayers have been fabricated at the Advanced Photon Source (APS) deposition lab using dc magnetron sputtering with constant currents of 0.5 A in Ar at a pressure of 2.3 mTorr. A test sample of [9.65Å-WSi2/10.05Å-Si] × 300 was studied at four institutions: using laboratory x-ray diffractometers with Cu Kα (8.048 keV) wavelength at the APS x-ray lab and at European Synchrotron Radiation Facility (ESRF), and using synchrotron undulator x-rays at 10 keV at MHATT-CAT and at 25 keV at ChemMatCARS-CAT of the APS. The measured first-order reflectivity was 54% with a bandpass of 0.46% at 10 keV and 66% reflectivity with a bandpass of 0.67% at 25 keV of undulator x-rays. Similar results were obtained from Cu Kα x-rays. This result is very attractive for the design of a multilayer monochromator for the ChemMatCARS-CAT to be used in the 20 to 25 keV range. Other small d-spacing multilayers are being studied. Comparison between WSi2/Si and W/Si multilayers will be discussed.

Application of double-gradient multilayers for focusing

We present the theoretical design, the fabrication, and the performance of double gradient multilayers to be installed on a Kirkpatrick-Baez focusing system for the ESRF bending magnet beam line BM5. The lateral and the depth gradient of the two coatings were chosen in such a way as to obtain a flat reflectivity response of about 25% after two reflections over an energy range from 12keV to 14keV and at an angle of incidence of 0.5deg at the mirror center. Both mirrors were coated with a non-periodic Ru/B4C structure containing 71 individual layers. The overall depth gradient was identical for both multilayers and optimized at the mirror center while the lateral gradient was adapted to the different focal lengths of each of the two KB elements.

X rays Reflective Multilayers Optic for Microbeam Radiation Therapy at the European Synchrotron Radiation Facility

We present first experimental results on the fabrication and characterization of x-rays reflective multilayers optic providing micro beams for synchrotron radiation therapy foreseen at ESRF Insertion device medical beamline ID17.

Reflectivity and stress responses of multilayers upon isothermal treatment

Periodic multilayers exposed to a non-destructive annealing sequence have shown reversible and irreversible structural modifications. In-situ x-ray reflectometry experiments at the ESRF bending magnet beam line BM5 demonstrate that the overall periodic structure remains stable during the annealing process. At the same time, initially present asymmetric interdiffusion layers have been reduced, in particular, in Ru/B4C. The controlled thermal treatment of multilayer optics before its installation on synchrotron beam lines can help to avoid alterations during their use as optical elements. An important issue is the reduction of stress introduced during the coating process. The evolution of stress in multilayer test coatings deposited on wafers was worked out from measurements done by optical metrology before and after coating and annealing. The investigation of the influence of a thermal action on their reflectivity response is a real challenge. We will present our experimental approach: deposition technique, multilayer choice, isothermal sequence, reflectivity and stress measurements. We will also discuss compromises made to keep both reflectivity and stress optimized versus thermal treatment. Future studies will have to deal with the impact of radiation on multilayer optics and its distinction from annealing effects.

Kirkpatrick-Baez mirrors used in high resolution X-ray microscopy, tomography and fluorescence analysis

The combination of a very brilliant X-ray source and state-ofthe-art X-ray optics opens the way to nano-focusing applications. Dynamically bent graded multilayers set in the KB-geometry are developed at the ESRF as an extremely efficient device to focus undulator radiation to spots below 100 nm [1]. The smallest 2D focus reached so far with this device is of the order of 80 nm in both directions. This has been achieved both on a long, coherent beamline and on a shorter beamline using the concept of a secondary source. The first mirror of the system, coated with a graded multilayer, plays both the role of vertical focusing device and monochromator, resulting in a very high flux (a few 10 11 photons/s) and medium monochromaticity (ΔE/E∼10p). Two different fields benefit particularly from the combination small focus / high flux: projection microscopy (magnified tomography) and X-ray fluorescence mapping. In projection microscopy the sample is set at a small distance (10-100 mm) downstream or upstream of the focus and a magnified Fresnel diffraction pattern is recorded on a medium resolution detector set at a large distance (several meters) from the focus. This approach allows to overcome the spatial resolution limit imposed by the detector in the parallel beam geometry. It was used to give direct evidence of nanometric invasion-like grain boundary penetration in the Al/Ga system [2]. For tomography, scans are acquired at different focussample distances. This yields variable magnifications, but also different effective propagation distances (∼equal to the focussample distance). The principal difficulties in this approach are related to the mirror imperfections and the pronounced refraction effects at the sample edges when going to sub-100 nm pixel sizes. Satisfactory results were however obtained in local tomography mode on aluminium alloys. A correction for mirror figure error and a phase retrieval method adapted to the imaging setup are incorporated in the tomograhic procedure [3]. In fluorescence imaging the sample is scanned through the focal plane while the spectrum of the emitted fluorescence is recorded with an energy dispersive detector. This rich probe, complementary to transmission imaging, provides element specific information and allows to image and quantify trace elements. The possibility to apply this method to the field of neurochemistry at the sub-cellular level has been demonstrated. Finally, the best one dimensional focus reached so far with reflective multilayer optics is 40 nm. The ultimate limits of this focusing approach will be discussed.

Layer intermixing in design and fabrication of double-gradient multilayers

Non periodic Ru/B4C double gradient multilayers were deposited by Distributed Electron Cyclotron Resonance (DECR) sputtering. These coatings will allow focusing with a constant reflectivity of 50% over an energy range from 12 keV to 14 keV. In this case, a lateral gradient is needed to fulfill the Bragg condition along the multilayer length, and a depth gradient is required to obtain the expected energy bandwidth. Our design approach was based on numerical calculations to define each layer thickness independently. During calibration tests, we had to take into account intermixing between Ru and B4C. We will present the importance of layer intermixing of each material (angular shift and bandwidth of first Bragg peak) and how these parameters can be integrated in the design without affecting the required reflectivity profile. We will also discuss compromises made to keep both lateral and depth gradient optimized in view of the technical limitations of our deposition process.