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Use of gratings in neutron instrumentation

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  1. Use of gratings in neutron instrumentation F. Ott, A. Menelle, P. Humbert and C. Fermon Laboratoire Léon Brillouin CEA/CNRS Saclay

  2. Objective • Study of the neutron diffraction on periodical gratings.(produced by lithographic techniques). • Theoretical calculation of the diffraction intensities: • Born / DWBA approximation (fails for large diffraction intensities) • matrix formalism : full dynamical calculation. • Application of gratings in neutron optics. • Example: energy analyser for time of flight neutron reflectometer

  3. Outline • Theoretical background • Fabrication of gratings • Energy analysis on a reflectometer using gratings

  4. Off-specular diffraction geometry Side view Thin film plane + grating Diffraction condition

  5. Modelisation of the grating Medium - stratified along (Oz) - periodical of period d along (Ox) Þ it is possible to divide the grating in sub-layers in which the optical index varies as a rectangle function of period d

  6. Dynamical calculation : matrix formalism Continuity conditions at the interfaces

  7. Fabrication of gratings. • Fabrication of periodical gratings by optical U.V. lithography. • Periodicities from 2 to 50 µm, resolution of 1 µm • aspect ratio of the order of 1

  8. Etching techniques • Dry argon etching: not element specific • 8 inch etching plant (VEECO) • two small etching guns • Chemical etching: element specific • ex: Ni,Fe: FeCl3 • fast and cheap even over very large areas • Reactive ion etching: element specific • oxygen, SF6, CH4 • no good for multilayers (ex. Ni/Ti) • one vacuum chamber for samples of 50x50

  9. Etching examples Argon etch of a supermirror Chemical etching (Ni etched by FeCl3) Rie (SF6) of glass

  10. Deposition / Lithography facilities • Deposition: • Sputtering (3 inches) • MBE (limited areas) + sputtering (2 targets) • or use of standard supermirrors (e.g. Swiss Neutronics, Cilas) • Lithography: • clean room with U.V. lithography facilityBut limited area: 50x50mm² • electron beam lithography:very precise but limited in size too • Masks price: (ordered outside) from 1-2 k€

  11. Off-specular diffraction on a glass grating (periodicity 10 µm, lines width 7 µm) Measurement on the time of flight reflectometer EROS (LLB) Detector fixed at 1.5°, scan in the reciprocal space obtained by rocking curves around 0.75° Off-specular diffraction modes Specular reflection line

  12. Increase of the diffraction efficiencies • Increase of the contrast between the incidence medium and the diffraction grating. • Three possibilities : • grating made out of a high index material (Nickel) • incidence medium with an index >1 (Titanium) • use of materials with an «high artificial index» : supermirrors. • Results • under some conditions, efficiencies > 20% • increase of the “diffraction bandwidth”:- high efficiency for a wide wavelength spectrum- or for a large range of incidence angles.

  13. Glass grating with and without a Ni coating

  14. Titanium coating(1st order diffraction mode efficiencies)

  15. Supermirror gratings It is possible to obtain good diffraction efficiencies over a large qz range (0.3 - 0.6 nm-1) The diffracted beam is not much wider than the specular reflected beam

  16. Time of flight reflectivity Cu (30nm) sur Si Dq l = 2 - 0.2 nm 5 µs pulse Spatial spread

  17. Application in neutron instrumentation: Energy analysis. The diffraction direction is a function of the wavelength

  18. Application on a time of flight spectrometer for energy analysis.

  19. Detector view l 0.2 nm Mode 1 I 1.5 nm 200 mm Specular reflection 1.5 nm Mode -1 0.2 nm Sample horizon

  20. Intensity gain • Use of a white beam • a reflectivity curve in a single “shot”. • Study of the evolution of materials or liquids on a time scale of a few minutes • Examples: • liquid interfaces • diffusion, sticking, breaking

  21. Example: PS-PPMA di-block copolymers • Study of the ordering

  22. Techni project • Fabrication and tests of small prototypes (20x20mm²) • choice of materials, periodicities, shape of the grating • optimisation in the resolution, useful q range • Comparison with simulations • Extension to large surfaces (100x50mm²) • Integration on the EROS reflectometer for measurements on liquids • Data processing (deconvolution)

  23. Conclusion • Off-specular diffraction on grating: • We have shown that is is possible to produce grating which have diffraction efficiencies as high as 30% • The good diffraction efficiency can be obtained over a rather broad range of qz or incidence angles. • This suggests the possibility of building a neutron energy analyser by separating a white neutron beam by diffraction on a grating. • Next steps • Production of gratings over large surfaces • Obtain good diffraction resolutions • Use for the study of the reflectivity on liquids