FABRICATION AND EVALUATION OF NDFEB …cap.ee.ic.ac.uk/~pdm97/powermems/2009/pdfs/papers/149_0098.pdf · FABRICATION AND EVALUATION OF NDFEB MICROSTRUCTURES FOR ELECTROMAGNETIC ENERGY
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FABRICATION AND EVALUATION OF NDFEB MICROSTRUCTURES FOR
ELECTROMAGNETIC ENERGY HARVESTING DEVICES
Yonggang Jiang1, Takayuki Fujita
1,2, Minoru Uehara
3, Kensuke Kanda
1,
Tomohiko Toyonaga2, Keisuke Nakade
2, Kohei Higuchi
1 and Kazusuke Maenaka
1,2
1Maenaka Human-sensing Fusion Project, Japan Science and Technology Agency, Japan
2Gradulate School of Engineering, University of Hyogo, Japan
3Magnetic Materials Research Laboratory, NEOMAX Co., Ltd., Osaka, Japan
Abstract: A novel electromagnetic energy harvester is proposed using micro-fabricated NdFeB permanent
magnet. The simulation results show that the miniaturization of NdFeB structures can achieve a magnetic field
gradient as high as 3000 T/m at 16 μm away from the micro-magnetic array. The generated power is then
calculated using the results of the magnetic field distribution, the geometry of coils, and the vibration conditions.
NdFeB microstructures as thick as 12 μm are successfully fabricated using magnetron sputtering and silicon
molding technologies. Magnetic force microscopy (MFM) method is used to characterize the magnetic field
generated by the NdFeB microstructures.
Keywords: energy harvesting, magnetic array, sputtered magnetic film, MFM.
INTRODUCTION Over the past decades, the electronic devices and
wireless sensors have shrunk in size and energy
consumption to unprecedented levels. Vibration-
driven energy harvesters become very attractive as the
power source to take the place of the micro-batteries
in the field of wireless sensor network and heath
monitoring system [1]. The transduction mechanisms
varying from electromagnetic, electrostatic,
piezoelectric have been demonstrated for vibration-
driven energy harvesters [2-4]. Electromagnetic
energy harvesters are widely studied due to the
established theories and progress in integration of
permanent magnets with MEMS devices. In addition,
electromagnetic devices usually have a long lifetime,
while the piezoelectric and electrostatic devices suffer
from degradation in piezoelectric properties and
charge leakage effect, respectively. The technological
difficulty encountered at smaller size is to achieve
high magnetic flux gradients [5]. High magnetic flux
gradients can be generated by microfabricated
magnetic arrays with narrow spacing and high
magnetic flux density. The technological difficulty
becomes fabricating magnetic microstructures with
excellent magnetic properties.
Nano-patterning techniques for magnetic
materials have been widely used in the field of high-
density magnetic recording media, magnetic quantum
devices, and micro-magnetic sensors [6]. For the
applications such as electromagnetic actuators and
energy harvesters, micro-scale patterning of magnetic
films as thick as tens of microns is required to
generate sufficient force or power. To achieve this,
there are two challenges to be faced. The first
challenge is to preparation of thick magnetic films
with high uniformity in thickness and magnetic
property over relatively large surface areas. According
to the work of one of our authors, NdFeB films
deposited by magnetron sputtering have shown
magnetic properties that can catch up with that of
commercial sintered NdFeB magnets [7]. NdFeB
films as thick as 20 μm are achievable with a
deposition rate of 90 nm/min. The second challenge is
the structuring of the films at micro-scale size. Both
wet chemical etching and reactive ion etching (RIE)
methods have been used to fabricate magnetic
microstructures [8, 9]. However, fine patterning
cannot be achieved by wet etching method due to the
large under-etch effect. It is also very difficult to
fabricate thick NdFeB microstructures by RIE due to
its relatively low etching rate and selectivity to mask
materials. In our work, in order to develop an
electromagnetic energy harvester as shown in Fig. 1, a
silicon molding technique is used to fabricate high
aspect ratio NdFeB magnetic microstructures. The
details of fabrication and characterization results will
be described in the following sections.
DEVICE MODELLING As shown in Fig. 1, the electromagnetic energy
harvester comprises of a bi-drectional micro-magnetic
array and serially connected microcoils. The electrical
power is generated due to the relative motion between
the magnetic array and microcoils. The voltage output