Etude des contraintes et des déformations au cours de la croissance de couches métalliques; analyse de la structure et du profil de composition aux interfaces par diffusion des rayons X

During the growth of nanometric metallic films, considerable stress can be generated. The mechanisms responsible for stress generation and relaxation are not well known. In particular, segregation of substrate atoms towards the surface may occur during deposition, leading to an interfacial concentration gradient. This work reports on the development of an apparatus allowing the determination of stress via in situ sample curvature measurements, and on the deposition of Ag on Si(111) in order to initiate the growth of metallic multilayers. The stress evolution during the deposition of silver, which cannot be interpreted through simple misfit accommodation, is confronted to structural analyses of the film. Complementing the determination of stress with RHEED measurements during growth, the stress and strain evolution in Cu-Ni(100) multilayers was studied. In spite of the weak misfit between Ni and Cu, partial relaxation takes place from the beginning of the growth. Moreover, the results reveal an interfacial mixing occurring when depositing Ni onto Cu. In the case of Au-Ni(111) multilayers, which are more highly mismatched, dynamical segregation occurs when depositing Ni onto Au, though Au and Ni are immiscible at room temperature. The comparison between x-ray anomalous diffraction spectra and numerical simulations reveal a composition gradient (associated with an interplanar distance gradient) extending on about six atomic planes around the Ni/Au interfaces, while the Au/Ni interfaces are abrupt. The local environment of atoms, and more particularly the first neighbour distance distribution, has been analyzed in these multilayers by x-ray absorption spectroscopy (EXAFS).Lors de la croissance de films métalliques d'épaisseur nanométrique, des contraintes importantes peuvent être générées. Les mécanismes responsables de l'établissement de ces contraintes et de leur relaxation sont encore mal connus. Notamment, la ségrégation des atomes du substrat vers la surface peut se produire au cours du dépôt, donnant lieu à un gradient de concentration à l'interface. Ce travail présente le développement d'un appareillage permettant de déterminer l'évolution des contraintes en mesurant la courbure de l'échantillon in situ; ainsi que la mise au point du dépôt d'une couche d'argent en épitaxie sur Si(111) permettant d'amorcer la croissance de multicouches métalliques. L'évolution des contraintes lors du dépôt d'argent, qui ne peut s'interpréter simplement en termes d'accommodation des paramètres de maille, est confrontée aux analyses structurales du film. En complétant la détermination de courbure par des mesures de RHEED en cours de croissance, l'évolution des contraintes et des déformations dans des multicouches Cu-Ni(100) a été étudiée. Malgré le faible désaccord de maille, une relaxation partielle se produit dès le début du dépôt. De plus, un mélange a été mis en évidence lors du dépôt de Ni sur Cu. Dans le cas de multicouches Au-Ni(111), qui présentent un plus grand désaccord de maille, une ségrégation dynamique se produit également lors du dépôt de Ni sur Au, malgré le caractère immiscible de Au et Ni à température ambiante. La comparaison des spectres de diffraction anomale des rayons X et de simulations numériques montre qu'un gradient de composition (accompagné d'un gradient de distances interplanaires) s'étend sur 6 plans atomiques environ autour des interfaces Ni/Au, tandis que les interfaces Au/Ni sont abruptes. L'environnement local des atomes, et notamment la distribution des distances entre premiers voisins, a été analysé dans ces multicouches par spectroscopie d'absorption X (EXAFS)

A project of advanced solid-state neutron polarizer for PF1B instrument at ILL

Among Super-Mirror (SM) polarizers, solid-state devices have many advantages. The most relevant is 5-10 times smaller length compared to air-gap polarizers allowing to apply stronger magnetic fields. An important condition for a good SM polarizer is the matching of the substrate SLD (Scattering Length Density) with the SM coating SLD for spin-down neutrons. For traditional Fe/Si SM on Si substrate, this SLD step is positive when a neutron goes from the substrate to the SM, which leads to a significant loss of the polarizer performance at small Q. Instead, we use single-crystal Sapphire/Quartz substrates. The latter show a negative SLD step for spin-down neutrons at the interface with Fe and, therefore, avoid the total reflection regime at small Q. To optimize the polarizer performance, we formulate the concept of Sapphire V-bender, perform ray-tracing simulations of Sapphire V-bender, compare results with those for traditional C-bender on Si, and study experimentally V-bender prototypes with different substrates. Our results show that the choice of substrate material, polarizer geometry and the strength and quality of magnetizing field have dramatic effect. In particular, we compare the performance of polarizer for the applied magnetic field strength of $50 mT$ and $300 mT$. Only the large field strength provides an excellent agreement between the simulated and measured polarization values. For the double-collision configuration, a record polarization $>0.999$ was obtained in the neutron wavelength band of $0.3-1.2 nm$ with only $1\%$ decrease at $2 nm$. Without any collimation, the polarization averaged over the full outgoing capture spectrum, $0.997$, was found to be equal to the value obtained previously only using a double polarizer in the "crossed" (X-SM) geometry. These results are applied in a full-scale polarizer for the PF1B instrument

A concept of advanced broad-band solid-state supermirror polarizers for cold neutrons

International audienceAn ideal solid-state supermirror (SM) neutron polarizer assumes total reflection of neutrons from the SM coating for one spin-component and total absorption for the other, thus providing a perfectly polarized neutron beam at the exit. However, in practice, the substrate's neutron-nuclei optical potential does not match perfectly that for spin-down neutrons in the SM. For a positive step in the optical potential (as in a Fe/SiNx SM on Si substrate), this mismatch results in spin-independent total reflection for neutrons with small momentum transfer Q , limiting the useful neutron bandwidth in the low- Q region. To overcome this limitation, we propose to replace Si single-crystal substrates by media with higher optical potential than that for spin-down neutrons in the SM ferromagnetic layers. We found single-crystal sapphire and single-crystal quartz as good candidates for solid-state Fe/SiNx SM polarizers. To verify this idea, we coated a thick plate of single-crystal sapphire with a m=2.5 Fe/SiNx SM. At the T3 instrument at the ILL, we measured the spin-up and spin-down reflectivity curves with λ=7.5Å neutrons incident from the substrate to the interface between the substrate and the SM coating. Results of this experimental test are in excellent agreement with our expectations: the bandwidth of high polarizing power extends significantly into the low- Q region. This finding, together with the possibility to apply a strong magnetizing field, opens a new road to produce high-efficiency solid-state SM polarizers with an extended neutron wavelength bandwidth and near-to-perfect polarizing power

Cinétique de dissolution d'un dépôt Fe/Cu

La prise en compte des moteurs de ségrégation superficielle dans un modèle cinétique (KTBIM) permet de rendre compte des cinétiques de dépôt-dissolution observées expérimentalement. Nous nous proposons de mettre en évidence le comportement des premiers plans près de la surface dans le cas d'un dépôt de fer sur un substrat de cuivre : une (ou deux) couche(s) flottante(s) de cuivre apparaissent sur le dépôt de fer avant la dissolution de celui-ci, en très bon accord avec les observations expérimentales. Le concept d'équilibre local nous permet de faire le lien entre les profils de concentration observés lors de la cinétique de dissolution et les profils d'équilibre de la couche mince dite équivalente

High spatial-resolution reflectivity and fluorescence mapping of multilayers using a sub-micrometer focused synchrotron X-ray beam

The quality of X-ray multilayers (roughness, composition, layer registry) is usually derived from specular reflectivity and diffuse scattering measurements, these signals being usually averaged over a beam footprint a few millimeters wide. However, some imaging applications involving multilayer optics, e.g., Extreme Ultra-Violet Lithography (EUVL) or synchrotron phase holo-tomography, require multilayers that provide a uniform response at the $\mu$m-scale or below. Therefore, the détection of defects at this scale, either present in or replicated through the multilayer, is essential to the development of defect-free reflective surfaces. In this paper a method capable of detecting such defects has been investigated. White a x-y piezo-driven stage raster scans the sample under the beam, the reflected signal can be recorded. Results obtained for Ru/Al$_2$O$_3$ and Ru/B$_4$C designed for synchrotron optics and for Mo/Si multilayers for EUVL applications are presented. By choosing an incident energy slightly higher than the absorption edge of the heavy element of the multilayer structure, the fluorescence signals of the various components could be monitored simultaneously during reflectivity mapping