FABRICATION AND EVALUATION OF NDFEB MICROSTRUCTURES FOR
ELECTROMAGNETIC ENERGY HARVESTING DEVICES
Yonggang Jiang1, Takayuki Fujita1,2, Minoru Uehara3, Kensuke Kanda1,
Tomohiko Toyonaga2, Keisuke Nakade2, Kohei Higuchi1 and Kazusuke Maenaka1,2
1
Maenaka Human-sensing Fusion Project, Japan Science and Technology Agency, Japan
2
Gradulate School of Engineering, University of Hyogo, Japan
3
Magnetic 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
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.
Over the past decades, the electronic devices and
wireless sensors have shrunk in size and energy
consumption to unprecedented levels. Vibrationdriven 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 vibrationdriven 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 highdensity 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,
0-9743611-5-1/PMEMS2009/$20©2009TRF
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
can be increased by connecting the microcoils in
series.
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PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
By assuming that the vibrating acceleration is 1 G,
the averaging frequency of the vibration source is 10
Hz, the vibration amplitude of the device is 1mm, the
total coil area is 0.5 cm2 with a turn number of 3, the
and the impedance of the load is equal to that of the
microcoils, the calculated power output as a function
of distance between the microcoil layer and the
magnet layer is plotted in Fig. 3. The power output
can be greatly improved by decreasing the distance
between the microcoil layer and the magnetic layer,
and increasing the vibration amplitude in x direction.
As a result, the moving part of the device should be
designed with a low spring constant in x direction to
increase the vibration amplitude and a high spring
constant in the thickness direction to avoid collision
between the magnetic layer and microcoils.
(a)
(b)
Figure
1.
Schematic
structure
of
the
electromagnetic energy harvester (a) and crosssectional view of the device (b). The electrical
power is generated due to the relative motion
between the magnetic array and the serially
connected microcoils.
Using the measured J-H characteristics of NdFeB
thin films [7], magnetic field distribution is simulated
as shown in Fig. 2. Assuming the thickness of
magnetic microstructures is 12 μm and the width is 40
μm, magnetic flux density as high as 0.12 T is
achieved at 16 μm above the magnet surface. The
average magnetic flux gradient in the direction
parallel to the surface of the magnetic layer (x
direction) is correspondingly as high as 3000 T/m.
Figure 3. Generated power as a function of the
distance between the microcoil layer and the
magnet layer, by assuming the acceleration
a=1G, the average frequency of vibration sources
f=10Hz, the total coil area S=0.5 cm2 with a
number of turns N=3, and the load resistance is
equal to the coil resistance.
FABRICATION
ARRAY
OF
MICRO-MAGNETIC
The fabrication process for the bi-directional
micro-magnetic array is shown in Fig. 4. The starting
material is a silicon substrate with a thickness of 525
μm (Fig. 4(a)). Photolithography and deep reactive
ion etching (RIE) are utilized to fabricate the silicon
mold with a depth of 12 μm (Fig. 4(b)). Magnetron
sputtering is used to deposit the NdFeB-Ta mutilayers (Fig.4(c)). The top Tantalum layer is used to
protect the NdFeB layer from oxidation. The NdFeB
microstructures are formed by a polishing process
(Fig. 4(d)). Deep RIE is done again to form the silicon
mold (Fig. 4(e)). Chromium and gold is deposited by
plasma sputtering as the seed layer for the following
Figure 2. Magnetic flux density generated by the
bi-directional magentic array variation with the
distance (d) from the magnet surface.
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electroplating process. The NdFeB magnetic array is
magnetized at a high magnetic field. Nickel is
electroplated to fill the silicon trenches (Fig. 4(f)).
The final structure of the bi-directional magnetic array
is formed after a final polishing process (Fig. 4(g)).
The magnetic direction of the Ni microstructures is
inversely to that of the NdFeB microstructures due to
a natural magnetization effect at the presence of the
magnetic
field
generated
by
the
NdFeB
microstructures.
Figure 5 illustrates a cross-sectional scanning
electron microscope (SEM) micrograph of silicon
trenches which are seamlessly filled by Ta-NdFeB
multi-layers with a thickness of 12 μm. Figure 6(a) is
the cross-sectional SEM micrograph of NdFeB
magnetic array after the polishing process is finished.
Figure 6(b) is the top view micrograph of the polished
magnetic array measured by a confocal microscope.
The NdFeB magnetic array is successfully fabricated
using the silicon molding technique. The next step is
to magnetize the NdFeB microstructures and fabricate
the Ni magnetic structures using the silicon molding
process and electroplating technique.
Figure 5. SEM image of silicon molded NdFeB
microstructure.
Figure 6. Cross-sectional SEM micrograph of the
polished NdFeB magnetic layer (a), and
micrograph of its top surface measured by a
confocal microscope (b).
CHARACTERIZATION AND DISCUSSION
After magnetizing the NdFeB microstructures,
magnetic force microscopy (MFM) is used to
characterize the magnetic field distribution generated
by the NdFeB magnetic array. MFM is carried out
using an atomic force microscope (AFM) and an
AFM probe with a magnetic tip (MFMR-10; Toyo
Corporation, Japan). Figure 7(a) illustrates the atomic
force microscope image of the surface of the magnetic
array, and Fig. 7(b) shows the MFM image, which
indicates the magnetic field distribution at 1μm above
the NdFeB magnetic surface. The magnetic field line
Figure 4. SEM image of silicon molded NdFeB
microstructure.
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of the NdFeB microstructure is narrower than its
corresponding topography line, which is probably due
to the oxidation of the edges of the magnetic
microstucture. After the polishing process shown in
Fig 4(d), a tantalum layer should be deposited again to
protect magnetic microstructures from oxidation.
(a)
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(b)
Figure 7. AFM image of NdFeB microstructures
(a), and magnetic field distribution at 1μm above
the surface of NdFeB microstructures measured
by magnetic force microscopy (b).
CONCLUSION
NdFeB magnetic microstructure was fabricated
using silicon molding techniques. The magnetic field
distribution was measured using a magnetic force
microscope. By combining with a Ni electroplating
technique, a bi-directional magnetic array can be
realized for electromagnetic energy harvesting
application. The bi-directional magnetic array can
generate high magnetic flux gradient according to our
simulation, and will greatly improve the performance
of electromagnetic energy harvesting devices.
ACKNOWLEDGEMENT
The author appreciates the discussions and help in
device fabrication from Mr. Iga and Mr. Hashimoto
from University of Hyogo.
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