Microfabrication of Magnetostrictive Beams Based on NiFe Film Doped with B and Mo for Integrated Sensor Systems

Microfabrication of Magnetostrictive Beams Based on NiFe Film Doped with B and Mo for Integrated Sensor Systems
Microfabrication of magnetostrictive beams based on NiFe film doped with
B and Mo for integrated sensor systems
A. Alfadhel, Y. Gianchandani, and J. Kosel
Citation: J. Appl. Phys. 111, 07E515 (2012); doi: 10.1063/1.3679016
View online: http://dx.doi.org/10.1063/1.3679016
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i7
Published by the American Institute of Physics.
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Microfabrication of magnetostrictive beams based on NiFe film doped
with B and Mo for integrated sensor systems
A. Alfadhel,1,a) Y. Gianchandani,2 and J. Kosel1
Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology,
Thuwal 23955, Saudi Arabia
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor,
Michigan 48109, USA
(Presented 1 November 2011; received 11 October 2011; accepted 30 November 2011; published
online 9 March 2012)
This paper reports the development of integrated micro-sensors consisting of 1 -lm-thick
magnetostrictive cantilevers or bridges with 500 lm in length and conducting interrogation
elements. The thin films are fabricated by sputter deposition of NiFe doped with B and Mo, and
the magnetic properties are enhanced by field annealing, resulting in a coercivity of 2.4 Oe. In
operation, an alternating current applied to the interrogation elements magnetizes the
magnetostrictive structures. The longitudinal resonant frequency is detected as an impedance
change of the interrogation elements. The magnetostrictive micro-beams provide high resonant
frequencies—2.95 MHz for the cantilever and 5.46 MHz for the bridge—which can be exploited to
C 2012 American Institute of Physics. [doi:10.1063/1.3679016]
develop sensors of high sensitivity. V
High-performance resonating sensors have attracted
interest for many MEMS applications, especially in the field
of biological species detection.1 For such applications, the
surface of the sensor is functionalized to specifically bind to
a target analyte. Binding of the target causes a change in the
resonant frequency, which, in turn, enables analyte detection
and quantification.2 Recently, devices that utilize magnetostrictive materials for sensing have been investigated.
Compared to other options, such as silicon cantilevers, these
vibrate in longitudinal direction instead of transverse direction, leading to higher resonant frequencies. This offers the
potential advantage of higher sensitivity.
Previous researches showed that magnetostrictive sensors in the form of beams have a high signal quality factor in
air, with reasonable signal damping in higher viscosity fluids.2,8 Such sensors have been successfully used for physical,
chemical, and biological sensing.3–6 The materials typically
used for magnetostrictive sensor applications are amorphous
magnetostrictive ribbons of about 25 lm thickness fabricated
by melt spinning. So far, these sensors have been operated
by external coils for actuation and sensing. This approach
does not easily allow integration using a standard microfabrication process, and their potential for miniaturization is
limited. Further miniaturization, though, can be expected to
yield an increase in sensitivity, since the resonant frequency
scales with the inverse of the sensor size. The main challenges for the development of magnetostrictive sensor systems in the micro-scale regime are the fabrication of soft
magnetic thin films with high magnetostriction and obtaining
a high signal-to-noise ratio. The latter can only be achieved
if the interrogation elements are in close proximity with
the magnetostrictive material. In this paper, we report the
fabrication of magnetostrictive fixed-free (cantilever) and
fixed-fixed (bridge) beams and their integration with
microstructures for interrogation on a common chip. The
fabricated magnetostrictive material is similar to the commercially available Metglas 2826MB that is characterized by
a very small coercivity, which makes it easy to magnetize,
and large magnetomechanical coupling, which provides a
large mechanical response upon application of a magnetic
field. The developed sensor system enables, for the first time,
the detection of the resonant frequency of magnetostrictive
micro-beams on an integrated device.
A. Sensor design
The sensor system consists of a magnetostrictive cantilever or bridge of length 500 lm, width 100 lm, and thickness
1 lm fabricated 2 lm above the interrogation elements, which
are 23 conducting lines of 10 0.5 lm2 arranged parallel to
each other with a pitch of 10 lm (Fig. 1). The magnetostrictive beams are actuated by the magnetic field produced by the
current IAC applied to the interrogation elements. With IAC 20
mA, a magnetic field of 0.5 Oe per conducting line is produced 2 lm above the interrogation elements. The beam
response depends, besides others, on the actuation frequency
and will be highest at the resonant frequency.
The longitudinal resonant frequency for the fixed-fixed
beams (Eq. (1))7 and fixed-free beams (Eq. (2))7 can be
evaluated as follows:
f ¼
2L qð1 Þ
f ¼
Electronic mail: [email protected]
111, 07E515-1
qð1 Þ
C 2012 American Institute of Physics
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Alfadhel, Gianchandani, and Kosel
J. Appl. Phys. 111, 07E515 (2012)
FIG. 1. (Color online) Illustration of the sensor system consisting of a cantilever beam above interrogation elements. An AC current provides actuation
of the cantilever and a DC magnetic field is applied in the longitudinal direction of the beam to set the working point.
where L is the length, E is Young’s modulus, q is the density,
and is Poisson’s ratio. The Young’s modulus for the magnetostrictive material is 100-110 GPa, and the density is
7:54 g=cm3 . Loading the beam with a mass, for example, a biological target analyte, will change the effective value of q. Due
to the inverse magnetostrictive effect, the vibration of the beam
causes a change of the permeability, which is sensed by the
interrogation elements as a change in impedance. Therefore,
the impedance of the interrogation elements is used to determine the resonant frequency of the beams, which contains the
sensing information.
Applying a DC bias field HDC in the longitudinal direction enhances the response of the magnetostrictive beams,
which is typically highest around the knee of the magnetization curve. When applying HDC to a magnetostrictive material, the Young’s modulus changes8 such that
EðHDC Þ ¼ Eo þ uðHDC Þ;
FIG. 2. SEM image of a 500 -lm-long bridge with interrogation structure
has been optimized in several steps toward low coercivity and
is deposited through co-sputtering of Ni50Fe50, boron (B), and
molybdenum (Mo) targets. DC power is used for Ni50Fe50
(200 W) and Mo (55 W) targets, and RF power is used for B
(110 W). To open the gold contact pads of the interrogation
elements, a patterning and etching step through amorphous Si
and the Si3N4 film is performed using reactive ion etching.
Finally, the beams are released by etching the amorphous Si
sacrificial film using XeF2 vapor phase etching. Scanning
electron microscopy (SEM), x-ray photoelectron spectroscopy
(XPS), and vibrating sample magnetometry (VSM) are used
to characterize the properties of the deposited material.
A. Fabrication results
where HDC is the applied DC magnetic field, E(HDC) is the
varying Young’s modulus, Eo is the zero field Young’s modulus, and u is a nonlinear function that relates HDC to the
stiffness change. This effect yields a dependence of the
resonant frequency on HDC, and it is, therefore, important for
HDC to be homogeneous and constant throughout the
Fig. 2 shows a 500-lm-long bridge fabricated with the
developed process. Using XPS, the composition of the deposited magnetic film is found to be Fe45Ni31Mo10B13. Some
topography of the interrogation elements can be observed at
the beam surface. The conductive stack deposited into the Si
trenches was 90 nm less than the trench depth and, e.g., mechanical polishing would be required before the Si3N4
B. Microfabrication process
B. Magnetic properties
The developed fabrication process is completed on standard silicon wafers. Patterning of the structures is done through
photolithography. First, the interrogating microstructure is
patterned. Reactive ion etching is used to etch the Si wafer,
and, then the trenches are filled with 500 nm of gold through
sputter deposition. The reason for having the interrogating
microstructures integrated in the substrate is to reduce the conformal growth for the following layers. A silicon nitride
(Si3N4) film is deposited to provide electrical isolation
between the interrogating elements and the beams. It also
serves as an etch stop layer when releasing the beams. In the
next step, 2 lm of amorphous silicon are deposited with
PECVD to be used as the sacrificial layer. In order to create
the anchors, the amorphous Si is etched down to the Si3N4
film through the use of reactive ion etching. The beams are
patterned through bi-layer lift-off lithography to allow easy
lift-off with clean edges. The magnetostrictive film fabrication
Fig. 3 and Fig. 4 show the magnetization curves in longitudinal and transverse direction, respectively, of a
FIG. 3. (Color online) Magnetization curves of a 500 -lm-long cantilever in
longitudinal direction before and after annealing.
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Alfadhel, Gianchandani, and Kosel
J. Appl. Phys. 111, 07E515 (2012)
FIG. 4. (Color online) Magnetization curves of a 500 -lm-long cantilever in
transverse direction before and after annealing.
FIG. 6. (Color online) Impedance as a function of the frequency measured
for a 500-lm-long bridge.
cantilever before and after field annealing. The as-prepared
films are almost isotropic, with a coercivity around 8.6 Oe.
After field annealing in the longitudinal direction at 350 C
and 1000 Oe, an induced anisotropy was observed and the
coercivity decreased to 2.4 Oe in the longitudinal direction
and 4.3 Oe in the transverse direction. The remanence in longitudinal direction became lower after annealing, while the
remanence in the transverse direction increased.
found experimentally. This is due to the effect of HDC that
increases the Young’s modulus (see Eq. (3)), which, in turn,
yields an increase of the resonant frequency. The resonant
frequencies of the cantilever and the bridge are not exactly
multiple of each other, which could be due to differences in
the effective lengths of the beams. Further studies are conducted to investigate the effect of other vibration modes on
the beam longitudinal resonant frequency.
C. Resonant frequency measurement
The resonant frequency of the beams is detected by
measuring the AC impedance of the interrogation elements.
The impedance is measured with an impedance analyzer,
applying an AC current IAC of amplitude 20 mA to the interrogation element while sweeping the frequency. An external
DC magnetic field HDC of 20 Oe is applied in the longitudinal direction using Helmholtz coils to bias the magnetostrictive sensor to its point of highest sensitivity. The impedances
measured for the cantilever and the bridge are shown in
Fig. 5 and Fig. 6, respectively. At the resonant frequencies,
the impedances exhibit an abrupt change of about 0.1 X. The
resonant frequencies are found to be 2.95 MHz for the cantilever and 5.46 MHz for the bridge. From Eq. (1) and Eq. (2),
the theoretical values of the resonant frequencies are about
4.6 MHz in the case of the bridge and about 2.3 MHz in the
case of the cantilever. These values are smaller than the ones
A new sensor system using magnetostrictive microbeams and integrated interrogation elements was reported.
A thin film process for fabricating soft magnetic, magnetostrictive sensor materials has been developed, including
co-sputtering of FeNi, Mo, and B and field annealing to
release stress and induce magnetic anisotropy. The integration of 500 -lm-long magnetostrictive cantilevers and
bridges with conducting interrogation elements is described.
The impedance of the interrogation elements clearly showed
the resonant frequencies of the magnetostrictive beams,
which could be utilized for sensor applications.
This work was supported by the KAUST Global Collaborative Research program. The authors acknowledge the
support from the KAUST advanced nanofabrication facility
(KANF), especially Ahad Sayed and Dr. Zhihong Wang.
FIG. 5. (Color online) Impedance as a function of the frequency measured
for a 500-lm-long cantilever.
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