Abstract
The fabrication and design of hard magnetic materials for micro-electro-mechanical system applications by electrochemical deposition has to consider not only the intrinsic material properties but also the shape anisotropy of the micro-devices. Within the scope of the present work, an as-plated process for hard magnetic Co-based materials was developed, with the products intended to be used as magnetic scales in a positioning system with a resolution within the nanometer range. First, the process–material correlations are investigated in a laboratory-scale process. The CoP and CoNiP show a maximum coercivity of HC = 28 and 45 kA/m, respectively, as well as maximum remanence polarizations of JR = 0.65 and 0.40 T, respectively. The CoP process is transferred to a specially developed 20 L plating cell with paddle convection capabilities and a passive bezel to deposit 50 µm wide scales with different thicknesses of up to 55 µm in an integrated process. The in-plane magnetization of the scale bars shows higher remanence polarization than for the out-of-plane direction. Magnetic field-assisted electrochemical deposition promotes the vertical magnetization component resulting in a remanence polarization of 205 mT (out-of-plane) for a scale thickness of 25 µm.
Highlights
Hard magnetic materials used in magnetic MEMS devices require an IC/CMOS compatible fabrication process to achieve an economical edge over the common hard magnetic materials, e.g., sintered NbFeB
Among the manufacturing processes for thick magnetic layers described in the literature, agglomeration of powder magnets by atomic layer deposition [2] can be used to produce thick, integrated micromagnets, with the drawback that the lower packing density requires the use of rare earth elements to achieve sufficiently high magnetic field strengths at a given distance
CoP was picked as the material of choice for the scale bars due to its higher magnetic strength values, which are desirable for their application to ensure a strong enough tunnel magneto-resistive (TMR) sensor signal for working distances ranging from 20 to 90 μm
Summary
Hard magnetic materials used in magnetic MEMS (micro-electro-mechanical systems) devices require an IC/CMOS compatible fabrication process to achieve an economical edge over the common hard magnetic materials, e.g., sintered NbFeB. Only relatively thin layers can be provided through these processes, which means that the effective magnetic field strength is usually too low at working distances that are typical for their respective applications. Among the manufacturing processes for thick magnetic layers described in the literature, agglomeration of powder magnets by atomic layer deposition [2] can be used to produce thick, integrated micromagnets, with the drawback that the lower packing density requires the use of rare earth elements to achieve sufficiently high magnetic field strengths at a given distance. Electrochemical deposition of alloys on the other hand, enables the cost-effective production of thick, integrated Co-based micromagnets with a high degree of geometric freedom and precision. A limitation of the application of this process is the small number of materials for which manufacturing processes exist
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