Sensors and Actuators A 92 (2001) 242±248
Electroplating and characterization of cobalt±nickel±iron and
nickel±iron for magnetic microsystems applications
F.E. Rasmussena,*, J.T. Ravnkildea, P.T. Tangb, O. Hansena, S. Bouwstraa
a
Mikroelektronik Centret, Technical University of Denmark, Oersteds Plads, Bldg. 345 East, DK-2800 Kgs. Lyngby, Denmark
b
Department of Manufacturing Engineering, Technical University of Denmark, Bldg. 204, DK-2800 Kgs. Lyngby, Denmark
Accepted 3 January 2001
Abstract
The magnetic properties of pulse reverse (PR) electroplated CoNiFe and DC electroplated NiFe are presented. CoNiFe is a very
promising material for magnetic microsystems due to the possibility of achieving a high saturation ¯ux density (Bs ) and a low coercivity
(Hc ). A new bath formulation has been developed, which by means of PR electroplating makes it possible to deposit high Bs CoNiFe with a
low residual stress level. The magnetic properties have been determined using a new simple measurement setup that allows for wafer level
characterization. The results have been validated by comparison to measurements performed with a vibrating sample magnetometer (VSM).
# 2001 Elsevier Science B.V. All rights reserved.
Keywords: PR electroplating; CoNiFe; NiFe; Magnetic properties
1. Introduction
2. Fabrication
Soft magnetic materials, such as PermAlloy (79 at.% Ni
and 21 at.% Fe) are extensively used in electromagnetic
MEMS devices, such as m-relays, m-switches, m-pumps and
m-motors [1±3]. In electromagnetic devices, the attainable
energy density is limited by the saturation ¯ux density (Bs )
of the soft magnetic material used. In order to further
miniaturize and/or improve performance of electromagnetic
devices, development of new soft magnetic high Bs materials
are needed (see also [4]). A strong candidate for replacement
of NiFe in many MEMS applications is the ternary alloy
CoNiFe capable of providing an extremely high saturation
¯ux density combined with reasonably low coercive ®eld
strength [5±7]. However, controlling the electrochemical
deposition of a ternary alloy is a complex matter, since
composition, magnetic and mechanical properties depend on
the various plating parameters. Utilizing the pulse reverse
(PR) electroplating technique, where the current is periodically reversed, the residual stress and composition can
be optimized towards optimum magnetic and mechanical
properties.
The soft magnetic materials NiFe and CoNiFe are electroplated on 4 inch Si-wafers containing a thin layer of
insulating thermal SiO2 , and an evaporated Ti/Au plating
Ê ). In order to characterize the magnetic
base (200/2000 A
properties of the materials with the presented characterization setup, special test structures are fabricated using standard UV lithography and wet etching. The resulting test
structures are illustrated in Fig. 1. The electroplated test
structures have a width of 9.75 mm, a length of 32 mm and
an average thickness ranging from 3 to 4 mm (varying from
experiment to experiment).
*
Corresponding author. Tel.: ‡4526823493/45-25-57-00;
fax: ‡45-45-88-77-62.
E-mail address: fra@mic.dtu.dk (F.E. Rasmussen).
2.1. PR electroplating of CoNiFe
A new formulation of a CoNiFe bath has been developed
especially for PR electroplating (based on [5]). The bath is a
chloride Ni bath, with the addition of Co and Fe as well as
complexing agents. Several variations of the electrolyte have
been investigated in the effort to optimize the CoNiFe
plating process, and to obtain optimum magnetic properties.
The composition of an electrolyte yielding high saturation
¯ux density is shown in Table 1. The CoNiFe electrolyte is
designed to reduce residual stress without using stress reducing sulphur containing additives (SCAs), such as saccharin,
which are known to deteriorate the magnetic properties. The
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 5 5 6 - 8
F.E. Rasmussen et al. / Sensors and Actuators A 92 (2001) 242±248
243
25 liter setup utilizing a ®xed horizontal anode/cathode
con®guration, and a moving paddle for agitation.
3. Characterization
Fig. 1. Illustration of magnetic measurement setup. The setup consist of an
U-shaped ferrite core, two coils and electronics for signal processing.
Table 1
Characteristics of CoNiFe electrolyte
Compound
Concentration
Cobalt chloride (g/l)
Iron chloride (g/l)
Nickel chloride (g/l)
Nickel sulfate (g/l)
Boric acid (g/l)
Naphtalene trisulfonic acid (g/l)
5-Sulfosalicylic acid (g/l)
Deposition temperature ( C)
pH
57.08
31.81
73.69
26.28
40.17
4.34
10.16
40
3.9±4.2
stress reducing ability is provided through the combination
of a small amount of naphtalene trisulfonic acid (NTS) and
optimized pulse plating parameters [8].
The CoNiFe ®lms are electroplated at current densities
ranging from 1.5 to 2.08 A/dm2 at a temperature of 40 C.
The PR electroplating process is performed in a 25 liter tank
with a computer controlled pulse generator and a moving
vertical anode and cathode con®guration.
2.2. DC electroplating of NiFe
The DC electroplated NiFe has been fabricated for the
purpose of comparison with CoNiFe. The formulation of the
NiFe bath (presented in Table 2) is based on a Ni sulfate bath
(Watts type) [1]. The NiFe is deposited at elevated room
temperature (28 C) at current densities ranging from 0.25 to
1 A/dm2 . The electroplating process is performed in a
The magnetic properties of the electroplated NiFe and
CoNiFe alloys have been investigated using a new setup
allowing for wafer level characterization of electroplated
®lms. The setup exploits basic electromagnetic principles,
and can easily be fabricated with simple and inexpensive
electronics.
The setup consists of an U-shaped ferrite core, two coils
and electronics for ampli®cation and integration of the
signals. A sinusoidal signal is applied to the primary coil
in series with the resistor R1. The voltage across R1 is
ampli®ed and displayed on an oscilloscope in order to obtain
the current through the primary coil, I1 , and thereby the
magnetic ®eld strength in the alloy sample, Ha . The relation
between current and magnetic ®eld strength is given by
Amperes law
I
X
N1 I1 ˆ H dl ˆ
Hi li ˆ Hy ly ‡ Ha la
(1)
i
where N1 is the number of turns on the primary coil and l the
length of the magnetic path in different parts of the closed
magnetic circuit (see Fig 20). The indices y and a denote the
yoke and the alloy sample, respectively. Eq. (1) is valid
under the assumption that no current flows in the secondary
coil, i.e. I2 ˆ 0, and that no air gap is present between the
ferrite core (the yoke) and the alloy sample, i.e. lair ˆ 0. If it,
additionally, is assumed that the flux in the yoke equals the
flux in the alloy sample, i.e. fy ˆ fa ˆ Ay By ˆ Aa Ba ,
where A is the cross-sectional area and B is the magnetic
flux density, and assuming that there is a linear relationship
between B and H (B ˆ m0 mr H), then Hy can be expressed as
a function of Ha . Accordingly, Ha can be obtained by slightly
rearranging Eq. (1)
Ha ˆ
N 1 I1
Aa =Ay † mr;a =mr;y †ly † ‡ la
(2)
Since Aa Ay (and mr;a mr;y ) Eq. (2) can be reduced to
Ha ˆ
N 1 I1
la
(3)
Table 2
Characteristics of NiFe electrolyte
Compound
Concentration
Nickel sulfate (g/l)
Iron sulfate (g/l)
Nickel chloride (g/l)
Boric acid (g/l)
Saccharin (g/l)
Sodium lauryl sulfate 1 cm (g/l)
Deposition temperature ( C)
pH
200
8
5
25
2
0.1
28
3
Fig. 2. Illustration of magnetic path through yoke and alloy sample. It is
assumed, that I2 ˆ 0 and that no airgap is present between ferrite core and
alloy sample.
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F.E. Rasmussen et al. / Sensors and Actuators A 92 (2001) 242±248
Thus, yielding a simple relation between primary current
and magnetic field strength in the alloy sample.
The magnetic ¯ux density in the alloy sample Ba can be
obtained by measuring the induced voltage in the secondary
coil. According to the law of induction, the induced voltage
E is related to the magnetic ¯ux f2 through the secondary
coil as
E t† ˆ
df2
dt
(4)
Using f2 ˆ N2 Ay By together with the assumption fy ˆ fa
ˆ Ay By ˆ Aa Ba and an inverting amplifier, the magnetic
flux density in the alloy sample can be written as
Z
1
E t† dt
(5)
Ba ˆ
N2 Aa
The full hysteresis curve is obtained by plotting Ba as a
function of Ha .
The electroplated NiFe and CoNiFe ®lms were characterized with respect to composition by quantitative energy
dispersive X-ray (EDX) analysis, with an estimated accuracy of 0.5 at.% (error bars not visible on given curves).
The residual stress levels of the electroplated ®lms were
determined at room temperature (22 C) using the wafer
curvature method. Cooling the wafers from the deposition
temperature to room temperature results in a thermal stress
contribution. As a general rule of thumb, the thermal
induced stress equals 3 MPa (tensile) for each degree the
deposition temperature is elevated above room temperature.
Thus, the reported stress levels in this work include a thermal
contribution of approximately 18 MPa (tensile) for NiFe
alloys (deposition temperature 28 C) and 54 MPa (tensile)
for CoNiFe alloys (deposition temperature 40 C).
4. Results and discussion
4.1. The DC electroplating of NiFe
The electroplated NiFe alloys exhibit the well known
anomalous codeposition phenomenon, where preferential
deposition of Fe results in a larger Fe content in the deposit,
than directly given by the content of Ni and Fe in the
electrolyte (see Table 2). The Ni and Fe content as a function
of current density is presented in Fig 3.
The residual stress in the electroplated NiFe ®lms was
found to decrease with increasing current density. Typically,
a stress level of 160±170 MPa was obtained for ®lms
deposited at 1 A/dm2 . The current ef®ciency of the NiFe
electroplating process varied from 62 to 80%, which is rather
low, but in agreement with existing results published on
electroplating of NiFe [9,10]. The current ef®ciency was
found to increase with increasing current density in the
investigated region 0.25±1 A/dm2 . The decreasing stress
as a function of current density is believed to be related
to the increasing current ef®ciency. At lower current
Fig. 3. Composition of DC electroplated NiFe vs. plating current density.
The composition is measured using quantitative energy dispersive X-ray
analysis.
ef®ciencies secondary cathode reactions, such as evolution
of hydrogen may contribute signi®cantly to the formation of
stress due to diffusion of hydrogen into the electroplated
alloy [11].
4.2. PR electroplating of CoNiFe
Like the electroplating process of NiFe, electroplating of
CoNiFe also exhibits anomalous codeposition behavior.
However, the mutual relation between deposition of Co,
Ni and Fe in electroplating of the ternary CoNiFe alloy is
more complicated than in electroplating of binary alloys,
such as NiFe.
The composition of the CoNiFe alloy exhibits a strong
dependence on current density as shown in Fig 4. The
average current density can be regarded as an effective
cathodic plating current density. The illustrated dependence
of the current density is general for the observations done in
this work. The Co content is almost constant as a function of
current density, whereas the Fe content increases Ð with a
corresponding decrease in Ni content Ð for increasing current densities. These observations indicate an intensi®ed
Fig. 4. Composition of PR electroplated CoNiFe vs. average plating
current density. The composition is measured using quantitative energy
dispersive X-ray analysis.
F.E. Rasmussen et al. / Sensors and Actuators A 92 (2001) 242±248
Fig. 5. Composition of CoNiFe as a function of current density for
different PR electroplating baths (I±IV). The arrows indicate the direction
of increased plating current density. Regions A and B corresponds to
CoNiFe compositions potentially having a coercivity of less than 160 A/m
[5]. The dashed line defines a region of CoNiFe compositions potentially
having a saturation flux density above 1.8 T [5].
anomalous codeposition behavior of Ni and Fe at higher
current densities.
A three element plot showing the composition of CoNiFe
®lms deposited from different PR electroplating baths (I±IV)
is shown in Fig. 5. Bath II has an increased content of Fe
compared to bath I. Bath III corresponds to bath II with
increased pH. Bath IV has an increased Fe and Co content,
and a decreased Ni content compared to bath III (shown in
Table 1). The arrows indicate the directions of increased
current density.
Regions A and B in Fig. 5 correspond to CoNiFe compositions potentially having a coercivity of less than 160 A/
m [5]. The dashed line de®nes the compositional region
reported to have a saturation ¯ux density (Bs ) above 1.8 T
[5]. Generally it can be stated, that increased Fe content and
decreased Ni content enhance Bs , when the Co content
retains a constant value lying between 40 and 65 at.%
[5]. The development process of the CoNiFe electrolytes
in this work has, therefore, been focused on impeding Ni
deposition and increasing Fe deposition, while maintaining a
high Co content. The Fe content in the deposited alloy is
increased at the expense of Ni by increasing the Fe content in
the electrolyte. Correspondingly, the Co content can be
increased in the alloy by increasing the Co content in the
electrolyte. The pH value of the electrolyte also in¯uences
the composition of the CoNiFe alloy. Thus, an increase in Fe
content in the alloy was observed at increased pH (II ! III).
The resulting stress levels in the PR electroplated CoNiFe
®lms were low with values ranging from 69 to 101 MPa
including a thermal contribution of 54 MPa. The variation of
the stress level with current density is shown in Fig. 6.
As evident the residual stress increases as a function of
current density. Furthermore, an increase in the residual
stress level was observed for increasing Fe content (see
Fig. 7). The relation between Fe content in the CoNiFe alloy
and residual stress level indicates, that a compositional
245
Fig. 6. Residual stress level of PR electroplated CoNiFe films as a
function of average plating current density. The stress level has been
determined from the wafer curvature method.
Fig. 7. Residual stress level of PR electroplated CoNiFe as a function of
Fe content. The stress level has been determined from the wafer curvature
method.
optimum yielding optimum magnetic properties and minimum residual stress can be obtained.
The current ef®ciency of the PR electroplating process
has been optimized, since this parameter is of great importance for industrial application of the process. The current
ef®ciency exhibited a strong dependence on the pH value of
the electrolyte (see Fig. 8).
Fig. 8. Current efficiency of CoNiFe PR electroplating process at two
different plating temperatures as a function of pH value.
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F.E. Rasmussen et al. / Sensors and Actuators A 92 (2001) 242±248
Increasing the pH value from 1.7 to 2.3 improved the
current ef®ciency from 66 to 86%. The current ef®ciency
was further improved to 96% by decreasing the temperature
of the electrolyte (from 43 to 40 C). The current ef®ciency
remained stable (96±97%) for current densities in the investigated region 1.5±2.08 A/dm2 . This level of ef®ciency is
high compared to the current ef®ciency of CoNiFe electroplating processes reported in literature (e.g. [7]).
The pH value was observed to increase as a function of
plating time. This is attributed to evolution of hydrogen
occurring despite the high current ef®ciency. A constant
increase in pH of 0.04 was observed in plating experiments
converting a charge of 900 C. As a consequence the pH of
the electrolyte has to be continuously monitored and
adjusted. In this work, the pH was allowed to vary from
3.9 to 4.2 before adjustment was performed.
4.3. Magnetic properties
The magnetic properties of main interest are the saturation
¯ux density Bs and the coercivity Hc. The magnetic properties of CoNiFe and NiFe ®lms with the most promising
compositions were characterized using a vibrating sample
magnetometer (VSM). The results of the VSM measurements are shown in Fig. 9.
The measurements on the CoNiFe alloy reveal a high
saturation ¯ux density, ca. 1.7 T, and a reasonably low
coercivity, ca. 510 A/m. The coercivity is low, but not below
160 A/m as reported in [5]. The low coercivity obtained in
[5] is attributed to a small grain size in the material of ca.
10±20 nm, which is obtainable in the presence of mixed
crystalline phases [6]. The present results of Hc indicate, that
the optimum relation between crystalline phases (fcc and
bcc) is not present in the developed material.
As evident from Fig. 9, the DC electroplated NiFe exhibits
an extremely low coercivity, and a fairly high saturation ¯ux
density. The value of Hc is on the edge of the detection limit
Fig. 9. Hysteresis loops of selected CoNiFe and NiFe samples obtained
with a VSM. The characterized CoNiFe alloy with a composition of 58.5
at.% Co, 18.1 at.% Ni and 23.4 at.% Fe is located in region A in the three
element plot shown in Fig 5.
Fig. 10. Hysteresis loops of selected CoNiFe sample obtained with the
new characterization setup (compensated) and with a VSM. The
characterized CoNiFe alloy with a composition of 65.8 at.% Co, 13.1
at.% Ni and 21.2 at.% Fe is located in region A in the three element plot
shown in Fig. 5.
of 10 A/m of the VSM. The saturation ¯ux density is
approximately 1.1 T, which is high compared to values of
Bs for PermAlloy given in literature (ca. 0.8±1.1 T) [3,10].
The developed CoNiFe and NiFe have a reasonably high
permeability in the range of 2000±3000; roughly corresponding to the permeability of other electroplated high m
materials, such as NiFeMo, given in the literature [4].
In order to validate the functionality of the simple characterization setup illustrated in Fig. 1, CoNiFe and NiFe
samples have been characterized using the new setup and a
VSM. The resulting hysteresis loops for a CoNiFe ®lm with
a composition of Co 65.8 at.%, Ni 13.1 at.% and Fe 21.2
at.% are shown in Fig. 10.
The hysteresis loop obtained with the new characterization setup deviated slightly from the hysteresis loop obtained
with the VSM. The magnitude of the measured magnetic
®eld strength H was in agreement with the values obtained
with the VSM, whereas the measured ¯ux density B was 20±
30% larger than the corresponding VSM values of B. The
deviation of B is primarily attributed to insuf®cient sample
thickness (3±4 mm), which increases the loss of ¯ux to the
ambient environment and direct coupling between primary
and secondary coil. The hysteresis loop shown in Fig. 10 has
been compensated for this deviation, subsequently exhibiting an overall shape in agreement with the VSM result.
In order to verify that the alloy sample thickness is
important for obtaining reliable values of B, a commercially
available amorphous soft magnetic alloy, VitroVac 6025, has
been characterized with the new characterization setup. The
20 mm thick VitroVac 6025 sample is known to have a
saturation ¯ux density Bs of 0.55 T, a permeability mr
approaching 100,000 and an extremely low coercivity.
The hysteresis loop of VitroVac 6025 obtained with the
new characterization setup is shown in Fig. 11.
As evident the saturation ¯ux density is measured to
approximately 0.55 T, and the obtained hysteresis loop reveals
an extremely high permeability as well as an extremely low
F.E. Rasmussen et al. / Sensors and Actuators A 92 (2001) 242±248
Fig. 11. Hysteresis loop of VitroVac 6025 obtained with the new
characterization setup. VitroVac 6025 is known to have a saturation flux
density of 0.55 T, and an extremely low coercivity.
coercivity. Thus, the obtained magnetic properties of VitroVac 6025 is in close agreement with the data stated by the
manufacturer of VitroVac (Vacuumschmeltze). The
increased accuracy of the characterization result is attributed
to the increased sample thickness. Since the magnetic
reluctance of the alloy sample decreases when the sample
thickness increases an increased fraction of the total ¯ux
¯ows through the alloy sample. Thus, the ¯ux density B
inferred from the measured induced voltage E t† in the
secondary coil, becomes an increasingly better estimate
of the actual ¯ux density in the alloy sample.
247
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Biographies
5. Conclusion
A new bath formulation has been developed for PR
electroplating of CoNiFe. A current ef®ciency of 96%
and a residual stress of 70 MPa have been obtained. The
magnetic properties of CoNiFe exhibit a high Bs of 1.7 T and
reasonably low Hc of 510 A/m. In comparison, the electroplated NiFe shows an extremely low Hc of less than 10 A/m
and a Bs of 1.1 T. The results show that PR electroplated
CoNiFe could become a serious alternative to NiFe. Magnetic measurements using a new characterization setup were
validated with a VSM with a reasonable result.
Acknowledgements
The authors would like to acknowledge Dr. J.C. Lodder
and T. Bolhuis, Information Storage Technology Group
(ISTG), MESA‡ Research Institute, Twente University,
The Netherlands, for performing the VSM measurements.
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Frank Engel Rasmussen was born in 1975 in Copenhagen, Denmark. He
received his MSc in Electrical Engineering (Master's thesis title:
Development of Soft Magnetic Materials for Microsystems Applications)
from Mikroelektronik Centret (MIC), the Technical University of
Denmark, in 2000. He is currently working towards an industrial PhD
degree within the field of advanced packaging at the Danish company
Oticon A/S.
Jan Tue Ravnkilde received his MSc in Electrical Engineering from the
Technical University of Denmark in 1997 and is currently working towards
his PhD degree. He is developing electroplated microelectromechanical
systems for fabrication on preprocessed integrated circuits.
Peter T. Tang received his MSc in Chemical Engineering from the
Technical University of Denmark in 1991, and his PhD in 1998 based on a
thesis entitled ``Fabrication of Micro Components by Electrochemical
Deposition''. He is presently employed as assistant research professor by
the Department of Manufacturing Engineering at the Technical University of
Denmark. He has worked on various projects within the field of
electrochemistry and electrochemical deposition and holds two patents.
Current research topics are fabrication of tools for microinjection moulding,
as well as electrodeposition of alloys and pure metals for microcomponents.
Ole Hansen received his MSc in Electrical Engineering from the
Semiconductor Laboratory, the Technical University of Denmark in
1977. Since 1977, he has done research first at the Semiconductor
Laboratory, the Technical University of Denmark, now at Mikroelektronik
Centret (MIC), the Technical University of Denmark, in the fields of
Bipolar Technology, CMOS Technology, Micromachining of Silicon,
Microsystems Technology, Microdevices by Metal Plating, Scanning
Probe Technology, and Biochemistry in Microsystems. Since 1994, he has
248
F.E. Rasmussen et al. / Sensors and Actuators A 92 (2001) 242±248
been an associate professor at MIC. He is presently teaching two lecture
courses: Semiconductor Technology, and Semiconductor Devices.
Siebe Bouwstra received his MSc in Mechanical Engineering in March
1984 from the University of Twente, The Netherlands. In April 1984, he
joined the Micromechanics research group at the University of Twente as
an Associate Researcher, where he was active in public±private
collaboration projects with Oce Copiers B.V. and ASM International
B.V., respectively. He received his Doctorate degree in March 1990 based
on his thesis entitled ``Resonating Microbridge Mass Flow Sensor''. After
this he was awarded a Research Fellowship of the Royal Dutch Academy
of Sciences, with which he collaborated with the Center for Integrated
Sensors and Circuits of the University of Michigan, USA. In November
1992, he was appointed Associate Professor at Mikroelektronik Centret at
the Technical University of Denmark, where he has been responsible for
the Micro-Electro-Mechanical Systems research and education programme. Since January 2000, he has joined the private company
Microcosm Technologies B.V. in The Netherlands, the European technical
office of Microcosm Technologies Inc., where he is responsible for the
company's Technical Services in Europe.