Novel Fabrication Process for the Integration of MEMS Devices with Thick Amorphous Soft Magnetic Field Concentrators
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Novel Fabrication Process for the Integration of MEMS Devices with Thick Amorphous Soft Magnetic Field Concentrators Simon Brugger1, Wilhelm Pfleging2, and Oliver Paul1 1 Department of Microsystems Engineering (IMTEK), University of Freiburg, Freiburg, 79110, Germany 2 Institute for Materials Research I, Forschungszentrum Karlsruhe GmbH, Karlsruhe, 76021, Germany ABSTRACT This paper reports a novel fabrication process enabling the integration of mechanical MEMS devices with thick amorphous soft magnetic field concentrators. The integration process combines silicon on insulator technology for the MEMS device fabrication and epoxy-resinbased attachment of 18-µm-thick amorphous soft magnetic ribbons followed by a wet chemical structuring process. The fabrication process is reported on the basis of a field-concentrator-based resonant magnetic sensor combining an electrostatically driven micromechanical resonator and a planar magnetic field concentrator with two narrow gaps. For realization of the concentrator gaps, the integration process is extended by micro-patterning of the soft magnetic ribbons via UV-laser ablation using an excimer laser system. The characterization of the fabricated resonant magnetic sensor using a stroboscopic video microscope for in-plane motion measurement shows a high sensitivity of 390 kHz/T at a magnetic flux density of 158 µT.
INTRODUCTION Over the past years, several CMOS magnetic sensors were combined with planar magnetic concentrators to improve their magnetic sensing properties. For example, it has been shown that the use of magnetic concentrators considerably improves the resolution of CMOS magnetotransistors [1] and CMOS Hall devices [2,3]. Further, the integration of planer magnetic concentrators enables standard Hall sensors to measure in-plane magnetic fields and therefore facilitates the design and fabrication of three-axis magnetic sensor systems [4]. Since most concentrator-based magnetic sensing applications require magnetic concentrators with a thickness of 15 µm and more, high demands are placed on the wafer-level deposition process. Vacuum deposition processes for soft-magnetic materials, e.g. sputtering, enable soft-magnetic layers with a thickness of only a few microns. Thus, the electroplating of permalloy (NiFe) [1,5] and the attachment of macroscopically fabricated amorphous soft-magnetic ribbons [3,6] were previously explored for the realization of thick magnetic concentrators. To enable magnetic sensors with high resolution, the coercivity of the used soft magnetic material is required to be as low as possible. In contrast to crystalline soft magnetic materials, amorphous soft magnetic materials have a much lower coercivity and are particularly qualified for realizing planar magnetic concentrators. Since the coercivity of macroscopically fabricated cobalt-based amorphous ribbons is at least 10 times lower than that of thick electroplated amorphous layers [7], the attachment of amorphous ribbons is the most widely used technology for the integration
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