Fabrication of MEMS-based Micro-fluxgate Sensor with Runway-shaped Co-based Amorphous Alloy Core

High-precision magnetic micro-sensor is an interdisciplinary subject of magnetic field measurement techniques and micro-electromechanical systems (MEMS) technology. A micro-fluxgate magnetic sensor based MEMS technology was designed and fabricated in this paper. This device is a micro-magnetic sensor with a symmetric construction, closed magnetic circuits and differential form. A 25μm thick Fluxgate core of runway model, made by Co-based amorphous alloy, was etched by laser and pasted on the substrate accurately. Excitation coil and sensing coil of 3D solenoid structure were prepared by RF magnetron sputtering and UV-lithography. The minimum line width of the coil is 50 μm. The experimental result shows that micro-fluxgate devices with the size of 5.7mm×7.1mm×60μm had a stable structure. 1.Introduction Fluxgate magnetic sensor is a measurement device which are widely used in various fields. It has the features of high resolution (up to 10-11T), wide range of measureing weak magnetic fields (in the 10-8T or less), low power consumption, being able to measure the magnetic field component directly and suitable for using in high speed motion systems [1]. Micro-FluxGate is an interdisciplinary subject of magnetic field measurement techniques and micro-electromechanical MEMS technology. With the development of fluxgate sensor, micro-fluxgate sensor produced by micro-mechanical technologies such as anisotropic etching and LIGA technology, has become an inevitable development trend of fluxgate sensor 1 Sponsored by National Natural Science Foundation under the project (60871018) 1st International Symposium on Spintronic Devices and Commercialization (ISSDC2010) IOP Publishing Journal of Physics: Conference Series 263 (2011) 012012 doi:10.1088/1742-6596/263/1/012012 Published under licence by IOP Publishing Ltd 1 [2]. Because of the characteristic of micro-fluxgate, such as light weight, small size, high sensitivity, high resolution and good temperature stability and other remarkable features, it was used to be weak magnetic field measurement [3]. Micro-FluxGate sensor has important application in aerospace field, particularly in satellite technology. Fluxgate sensor is composed with the probe and interface circuits. Probe typically includes magnetic core, excitation coil and sensing coil. Magnetic Core is generally made of soft magnetic materials with high permeability and low coercivity. Excitation coils and sensing coils are in the form of solenoid around the core, the coil material is copper [4]. For the micro-fluxgate, the core thickness and width of copper coils was tens of microns. It is difficult to electroplate the Co-based amorphous alloy on the silicon substrate, therefore the key technology is to produce solenoid coils around the thick Co-based amorphous alloy, particularly the preparation of a micro-vias column. 2.Design of Micro-fluxgate sensor Becaused of the high permeability of the Ni-Fe permalloy, early fluxgate core commonly used it as the magnetic core. While iron-nickel can also get a high saturation magnetic flux density, and its resistance is low [5]. When the Co-based amorphous alloy soft magnetic material appears, because of its high permeability, low coercivity, low hysteresis loss, excellent wear resistance and corrosion resistance, good temperature and aging stability, it become the ideal substitute of permalloy and widely used in production of the fluxgate sensor. For those reasons, cobalt-based amorphous alloy, brand name VITVAC 6025Z, was used as the magnetic core. The relative permeability, magnetic loss, magnetostriction, electrical resistivity, squareness ratio of VITVAC 6025Z is better than Ni-Fe permalloy, and its magnetic parameters as shown below. Table 1. magnetic parameters of permalloy and Co-based amorphous alloy material Relative permeability μr (1kHz) (×103) magnetic lossPcv (kW/m3) squareness ratio Br / Bs (%) magnetostriction coefficient λs(×10-6) Resistivity ρ (μΩ•cm) 80% nickel permalloy 50.