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Réalisation de micro-aimants par assemblage capillaire de nanobâtonnets de cobalt assisté par magnétophorèse - Application aux MEMS à transduction électromagnétique

Pierre Moritz 1
1 LAAS-MEMS - Équipe Microsystèmes électromécaniques
LAAS - Laboratoire d'analyse et d'architecture des systèmes
Abstract : Permanent magnets are hard ferromagnetic materials delivering a spontaneous magnetization and storing magnetic energy which is entirely released outside. Some microelectromechanical systems (MEMS) take benefits of this energy source to implement electromagnetic transduction where mechanical movements are converted to electrical signals and reciprocally. The purpose of this thesis is to fabricate rare-earth free permanent magnets at a sub-millimeter scale offering high performances and being compatible with MEMS fabrication processes. A directed assembly of cobalt nanorods (NRs) under magnetic field is studied and optimized to fabricate anisotropic materials. Firstly, the optimization of the NRs synthesis leads to high-quality magnetic layers assembled onto silicon substrates by capillary forces upon solvent evaporation. These layers exhibit a high coercive field, reaching 490 kA.m-1, indicating the strong magnetic anisotropy of the NRs, and a ratio between the remnant magnetization and the saturation magnetization equals to 0.93, demonstrating a very good NRs alignment inside the magnet. The texture analysis by X-ray diffraction provides the quantitative measurement of the in plane and out of plane angular distributions of the NRs, respectively being 12° and 16°. The mapping of the magnetic induction generated by the magnet is then realized using a Hall effect micro-probe. The volume fraction of metallic cobalt is estimated to be 36 %, providing a maximum energy product of 50 kJ.m-3. Finally, a first proof of concept of MEMS electromagnetic actuation is carried out using the Lorentz force by approaching t! hese nanostructured pe! rmanent magnets close to microfabricated silicon cantilevers and measuring the cantilever vibration via integrated piezoresistances to obtain well defined resonance peaks. In a second step, the capillary assembly of cobalt NRs assisted by magnetophoresis is implemented using ferromagnetic structures to obtain patterned magnets presenting controlled shapes and dimensions. Self-standing millimeter magnets are firstly produced by means of iron rods. Then, magnets are fabricated onto silicon substrates where electrodeposited nickel blocks create magnetic field gradients guiding the cobalt NRs during the assembly. These magnets present lateral dimensions between 100 μm and 500 μm and thicknesses in the range of 20 μm - 150 μm. Magnetic and structural characterizations of the magnets reveal that the patterning method does not affect the intrinsic quality of the NRs alignment. The magnetic induction profiles evidence better performances when the nickel blocks are removed. The fabrication process is improved by optimizing the number of cobalt NRs drop deposits, and the amount of material required is minimized combining surface functionalization an! d ! the elaboration of a resist mask around the nickel blocks. Since all of these steps are compatible with the microfabrication techniques conventionally used in clean room, this work therefore opens numerous perspectives for the full integration of permanent magnets in MEMS fabrication processes.
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Pierre Moritz. Réalisation de micro-aimants par assemblage capillaire de nanobâtonnets de cobalt assisté par magnétophorèse - Application aux MEMS à transduction électromagnétique. Micro et nanotechnologies/Microélectronique. Institut national des sciences appliquées de Toulouse, 2019. Français. ⟨tel-02440847⟩

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