M.F. Ravet

EUVI: the STEREO-SECCHI extreme ultraviolet imager

The Extreme Ultraviolet Imager (EUVI) is part of the SECCHI instrument suite currently being developed for the NASA STEREO mission. Identical EUVI telescopes on the two STEREO spacecraft will study the structure and evolution of the solar corona in three dimensions, and specifically focus on the initiation and early evolution of coronal mass ejections (CMEs). The EUVI telescope is being developed at the Lockheed Martin Solar and Astrophysics Lab. The SECCHI investigation is led by the Naval Research Lab. The EUVI’s 2048 x 2048 pixel detectors have a field of view out to 1.7 solar radii, and observe in four spectral channels that span the 0.1 to 20 MK temperature range. In addition to its view from two vantage points, the EUVI will provide a substantial improvement in image resolution and image cadence over its predecessor SOHO-EIT, while complying with the more restricted mass, power, and volume allocations on the STEREO mission.

Study of normal incidence of three-component multilayer mirrors in the range 20-40 nm.

We study theoretically and experimentally the increase of normal incidence reflectivity generated by addition of a third material in the period of a standard periodic multilayer, for wavelengths in the range 20 to 40 nm. The nature and thickness of the three materials has been optimized to provide the best enhancement of reflectivity. Theoretical reflectivity of an optimized B4C/Mo/Si multilayer reaches 42% at 32 nm. B4C/Mo/Si multilayers have been deposited with a magnetron sputtering system and a reflectivity of 34% at 32 nm has been measured on a synchrotron radiation source.

Surface smoothing of diamond membranes by reactive ion etching process

Abstract This paper describes a procedure based on reactive ion etching developed to reduce the surface roughness of CVD diamond thin films. The technique involves etching a bilayer made up of diamond and a planarizing SiO 2 layer in an SF 6 O 2 plasma mixture. Etching conditions have been determined which yield equal rates for both diamond and the SiO 2 coverlayer and favour the removal of diamond peaks. Smoothed surfaces have been characterized by SEM, AFM and XPS: the results exhibit a significant decrease in roughness. This novel technique offers promising prospects for polishing thin diamond membranes for X-ray lithography applications without the removal of significant amounts of diamond material. The optical transmittance of diamond membranes, mainly affected by light scattering due to surface roughness, is significantly improved using this method of planarization.

Diamond deposition in a bell-jar reactor: influence of the plasma and substrate parameters on the microstructure and growth rate

Abstract This paper describes diamond films that have been grown in a bell-jar (10 cm diameter) microwave plasma reactor. The microstructure, purity of the films and growth rate are controlled by the initial nucleation density, substrate temperature and plasma parameters. The diamond membranes produced have qualities and microstructures which are compatible with X-ray lithographic mask applications.

Thermal evolution of X/C multilayers (with X=W, Ni, or SiWSi): A systematic study

The thermal behavior of X/C multilayers (nanometer‐thick layers made of tungsten, nickel, or SiWSi alternating with carbide or pure carbon) was studied. Two types of annealing were performed: the pulsed laser annealing in air and the classical thermal annealing in a vacuum furnace. Depending on the composition and the structure of the layered materials, thermal stability or diffusion mechanisms were observed and further analyzed by small‐angle x‐ray scattering, transmission electron microscopy, and Auger electron spectroscopy. The results show that the period expansion and the reflectivity evolution, that were observed in some cases after treatment, are caused both by structural changes into the layers and by exchange of matter between layers. These changes always induce a partial graphitization of the amorphous carbon and, in the case of W/C multilayers, the formation of a W2C compound.

MAGNETIZATION REVERSAL IN PATTERNED CO(0001) ULTRATHIN FILMS WITH PERPENDICULAR MAGNETIC ANISOTROPY

Using x‐ray lithography we have patterned dot arrays in Au/Co/Au(111) sandwiches based on ultrathin Co layers with perpendicular anisotropy. Large area arrays of dots with diameters of 1 and 2 μm have been obtained, keeping mostly undamaged the ultrathin Co layer. Hysteresis loops of the arrays depend drastically on the dot diameter. Magneto‐optical domain visualization experiments confirm a magnetization reversal mechanism based on a large distribution of nucleation fields in the film, with complete reversal of the magnetization of one dot through the domain wall propagation after a local nucleation process. This could give information on the magnetization reversal processes in Au/Co/Au(111) continuous films.

Chemical and morphological modifications of silicon wafers treated by ultrasonic impacts of powders: consequences on diamond nucleation

Abstract The mechanical enhancement of diamond nucleation on silicon wafers by the use of ultrasonic impacts of different abrasive powders in various liquid media is investigated via surface-enhanced Raman spectroscopy (SERS), atomic-force microscopy (AFM) and scanning electron microscopy (SEM). The growth of diamond nuclei is carried out in a bell-jar microwave-assisted plasma set-up. The nucleation densities obtained are compared to those resulting from a highly oriented pyrolitic graphite (HOPG) substrate, a virgin silicon wafer and a silicon wafer previously in-situ etched by a hydrogen plasma. The results are analyzed in terms of free carbon formation, surface morphology and particle implantation. The role of the morphological aspect is seen to be negligible compared with the chemical role. When non-diamond powders are used, the nucleation density is seen to depend on the amount of amorphous carbon phases formed in the physical sites (defects, stresses), at the silicon surface. When diamond powder is used, nucleation density up to 1.5 × 10 10 cm −2 is obtained. It is attributed to an implantation of the diamond particles occurring during the ultrasonic impacts. On the HOPG substrate, a low and non-uniform crystal density, related to the distribution of scratches on the surface, is obtained. This highlights the role played by carbon phases, located at highly defective regions, in promoting diamond nucleation.

Ion-beam-deposited Mo/Si multilayers for EUV imaging applications in astrophysics

Imaging of the solar corona by selecting Fe IX (λ=17.1nm,), Fe XII (λ=19.5nm), Fe XV (λ=28.4nm) and He II (λ=30.4nm) emission lines with a Ritchey-Chretien telescope requires to coat the optics with multilayers having a high accuracy in their layer thicknesses, a high reflectivity and an optimal bandpass. Multilayers were simulated in order to determine the most adequate formula for each wavelength channel. Mo/Si coatings were deposited by using the ion beam sputtering technique in a high vacuum chamber equipped with a micro balance and an in-situ reflectometer. The multilayers were studied by grazing angle reflectometry at 0.1541nm, and their reflectances around the operating wavelengths were measured on the SA62 IAS/LURE beam line of the SuperACO synchrotron facility located at Orsay. In addition, aging versus time and behavior of the multilayers under a rapid thermal annealing were investigated. Performances of the ion-beam deposited multilayers have been improved compared to the Mo/Si coatings obtained in the past by the e-beam evaporation technique for the SOHO mission Extreme UV Imaging Telescope (EIT). The EUVI telescopes for the STEREO mission are being proceduced by depositing these new generation of multilayers onto primary and secondary mirrors. The reflectivity measurements on a telescope are presented.

Planarization of diamond thin film surfaces by ion beam etching at grazing incidence angle

Abstract Ion beam etching at grazing incidence angle has been used to reduce the surface roughness of polycrystalline diamond thin films, 1 ± 0.2 μm thick, deposited by microwave plasma chemical vapor deposition (MWPCVD). Different reactive etching conditions have been tested for a beam energy of 1 keV with various ion beam and gas atmosphere compositions: pure Ar + , mixture of Ar + + N 2 + and Ar + under a controlled oxygen partial pressure. Their influence on the etching kinetics, roughness and morphology evolutions of the film, crystalline quality have been studied using SEM, AFM and Raman spectroscopy. The optical transmittance has also been measured. The ion beam processes studied result in a drastic reduction of the film surface roughness with a slightly better efficiency for the Ar + + oxygen gas mixture. The roughness decrease leads to an increase in the optical transmittance without any change in the crystalline quality of the film. This planarizing method should be particularly well-adapted for thicker films taking into account the balance between the initial values of thickness and roughness on the one hand, and the material loss on the other. Furthermore, it could be usefully applied to produce large-area membranes.

X-ray–ultraviolet beam splitters for the Michelson interferometer

With the aim of realizing a Michelson interferometer working at 13.9 nm, we have developed a symmetrical beam splitter with multilayers deposited on the front and back sides of a silicon nitride membrane. On the basis of the experimental optical properties of the membrane, simulations have been performed to define the multilayer structure that provides the highest reflectivity-transmission product. Optimized Mo-Si multilayers have been successfully deposited on both sides of t he membrane by use of the ion-beam sputtering technique, with a thickness-period reproducibility of 0.1 nm. Measurements by means of synchrotron radiation at 13.9 nm and at an angle of 45 degrees provide a reflectivity of 14.2% and a transmission of 15.2% for a 60% s-polarized light, close to the simulated values. Such a beam splitter has been used for x-ray laser Michelson interferometry at 13.9 nm. The first interferogram is discussed.