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

can be increased by connecting the microcoils in

series.

PowerMEMS 2009, Washington DC, USA, December 1-4, 20090-9743611-5-1/PMEMS2009/$20©2009TRF 582

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.

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.

FABRICATION OF MICRO-MAGNETIC

ARRAY 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 muti-

layers (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.

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.

(a)

(b)

Figure 1. Schematic structure of the

electromagnetic energy harvester (a) and cross-

sectional view of the device (b). The electrical

power is generated due to the relative motion

between the magnetic array and the serially

connected microcoils.

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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.

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.

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).

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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.

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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(a) (b)

Figure 7. AFM image of NdFeB microstructures

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