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

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

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

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

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

6 Dynamical calculation : matrix formalism Continuity conditions at the interfaces

7 Fabrication of gratings. n 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 n Dry argon etching: not element specific – 8 inch etching plant (VEECO) – two small etching guns n Chemical etching: element specific – ex: Ni,Fe: FeCl 3 – fast and cheap even over very large areas n Reactive ion etching: element specific – oxygen, SF 6, CH 4 – no good for multilayers (ex. Ni/Ti) – one vacuum chamber for samples of 50x50

9 Etching examples n Chemical etching (Ni etched by FeCl 3 ) Argon etch of a supermirror Rie (SF 6 ) of glass

10 Deposition / Lithography facilities n Deposition: – Sputtering (3 inches) – MBE (limited areas) + sputtering (2 targets) n or use of standard supermirrors (e.g. Swiss Neutronics, Cilas) n Lithography: – clean room with U.V. lithography facility But limited area: 50x50mm² – electron beam lithography: very precise but limited in size too n 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° Specular reflection line Off-specular diffraction modes

12 Increase of the diffraction efficiencies n Increase of the contrast between the incidence medium and the diffraction grating. n 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. n 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 The diffracted beam is not much wider than the specular reflected beam It is possible to obtain good diffraction efficiencies over a large q z range ( nm -1 )

16 Time of flight reflectivity n Cu (30nm) sur Si qq 5 µs pulse Spatial spread = nm

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

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

19 Detector view Specular reflection Mode mm Mode nm 0.2 nm 1.5 nm 0.2 nm Sample horizon I

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

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

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

23 Conclusion n 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 q z or incidence angles. – This suggests the possibility of building a neutron energy analyser by separating a white neutron beam by diffraction on a grating. n Next steps – Production of gratings over large surfaces – Obtain good diffraction resolutions – Use for the study of the reflectivity on liquids