Temperature-stable NdFeB micromagnets with high-energy density compatible with CMOS back end of line technology
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Temperature-stable NdFeB micromagnets with high-energy density compatible with CMOS back end of line technology Tim Reimer1,2, Fabian Lofink1, Thomas Lisec1, Claas Thede3, Steffen Chemnitz1,2, Bernhard Wagner1,2 1 Fraunhofer Institute for Silicon Technology, Fraunhoferstraße 1, 25524 Itzehoe, Germany 2 Institute for Electrical Engineering, University of Kiel, Kaiserstr. 2, 24143 Kiel, Germany 3 Institute for Materials Science, University of Kiel, Kaiserstr. 2, 24143 Kiel, Germany
ABSTRACT The performance of a novel type of NdFeB micromagnets fabricated by agglomeration of magnetic powder by atomic layer deposition is investigated. The ALD-bonded micromagnets can withstand standard BEOL (back-end of line) processing and heat treatments at temperatures of up to 400 °C in air and vacuum without any significant impact on the demagnetization curves. By optimized packing density a remanence of 660 mT is realized for the micromagnets. The coercivity μ0Hc = 890 mT remains constant for all samples and corresponds to the powder value. A comparison of the demagnetizing behavior of micromagnets with theory of solid body magnets prove that the influence of particle shape and hollow spaces on demagnetizing field is low. Hence, a similar impact of shape on stray field and forces as for solid body magnets can be assumed when integrating NdFeB ALD-bonded micromagnets in applications. INTRODUCTION High energy density micromagnets in MEMS applications are in the very focus of recent research, e.g., energy harvesters [1], magnetic actuators [2], or microspeakers [3]. Since the magnetic forces scale with the volume of the magnet [4, 5], deposition techniques as widely used in microfabrication are not suitable to provide magnets of required size, especially in terms of thickness. An alternative approach is to build high-volume magnetic structures (up to ~ 1 mm3) from magnetic particles [6]. Different techniques are known to produce powder-magnets bonded with polymer binders or wax [3] or by sealing of cavities filled with the loose powder with parylene [1, 4]. However, such structures show poor post-processing capabilities and poor corrosion resistance due to low thermal and chemical stability of the organic polymer matrix as well as insufficient protection of the magnetic material against environmental influences. In this paper, we present NdFeB micromagnets integrated on 8" silicon substrates which are compatible with standard back-end of line (BEOL) technology. The structures are fabricated using a novel technique based on the agglomeration of powder by atomic layer deposition. EXPERIMENTAL To create the micromagnets cavities are etched in 8" silicon wafers using deep reactive ion etching (DRIE). NdFeB powder with various particle size from Molycorp (D50% 4.5-6.5 μm and
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35-55 μm), as well as a mixtures consisting of 30% wt. fine particles and 70% wt. coarse particles are filled into the cavities by squeegeeing using both dry material and water-based slurries. Atomic layer deposition (ALD) is conducted with methods as described
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