Note: Descriptions are shown in the official language in which they were submitted.
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Ti e: SOLUTION FOR THE ELECTROPLATING OF SOFT MAGNETIC Co-
Fe-Ni ALLOYS
FIELD OF THE INVENTION
The present invention relates to an electroplating solution for
electroplating of soft magnetic Co-Fe-Ni alloys, and more particularly,
relates
to an electroplating solution having a citrate-based stabilizer for
electroplating
of soft magnetic Co-Fe-Ni alloys. The present invention also relates to a
method for forming a thin Co-Fe-Ni alloy plated magnetic film with high
saturation magnetization and low coercivity from the stable citrate-based
electroplating solution.
BAGK~ROUND OF THE INVENTION
CoFeNi alloys are one of the most studied soft magnetic materials for
the past several decades due to their superior properties over FeNi alloys as
write head core materials in hard-disk-drives. Electrodeposited permalloy
(Ni~Fe2o) was introduced as the core material of thin film inductive heads by
IBM in 1979. With increasing storage density, the need for recording heads to
write on high-coercivity media at high frequencies has raised new
requirements for the write-head material that cannot be met by Ni$oFe2o. New
soft magnetic materials with higher saturation flux density 8$ such as
electroplated CoFe alloys, CoFeNi alloys, CoFeCu alloys, other CoFe-based
alloys, sputtered FeN films and other Fe-based alloys, have been developed.
Electroplating processes have major significance in the fabrication of
thin-film recording heads with the advantages of simplicity, high cost-
effectiveness and controllable patterning. The major properties of common
plated soft magnetic materials for fabricating recording heads have been
summarized by Andricacos, P.C and Roberson, N. in IBM J. Res. Develop.
(Electrochemical Microfabrication), 1998, 42, 671. Among the major
properties of common plated soft magnetic materials for fabricating recording
heads, CoFeNi and CoFeCu alloys have the highest possible saturation
magnetization. Therefore these two materials, especially CoFeNi alloys, have
attracted the most attention of investigators. CoFeNi alloys can be readily
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plated from solutions whose compositions differ from that of a NiFe plating
bath only by adding a Co2+ salt, usually a sulfate or chloride. Table 1 lists
the
composition of a sulfate bath for plating CoFeNi alloys (Osaka, T.; Takai, M.;
Hayashi, K.; Ohashi, K.; Saito, M.; Yamada, K. Naruro 1998, 392, 796.), which
has a pH as low as 2.5 to 3.0 with the addition of acid.
Conventional CoFeNi plating baths suffer from stability problems, that
is, precipitation occurs rapidly with time, which is a critical issue for
commercialization. The plating cell equipped with a filtered recirculation
system to compensate for bath degeneration has been described by
Tabakovic, I., Inturi, V. and Riemer, S. in J. Electrochem. Soc. 2002, 149,
C18. Precipitates can affect the film properties, uniformity and smoothness.
Furthermore, the low pH employed in conventional baths leads to voids in
deposited films, which degenerate film uniformity and magnetic properties,
and low current density efficiency due to the electroplating of H2. Therefore,
the development of a stable bath with a relatively high pH is beneficial for
commercial fabrication of CoFeNi thin films with optimal soft magnetic
properties.
SUMMARY OF THE INVENTION
A novel electroplating solution which comprises at least one citrate salt,
such as sodium citrate, potassium citrate or ammonium citrate, in an amount
effective to act as a stabilizing agent, has been found to provide increased
stability to the electroplating solution.
This present invention therefore relates to a novel Co-Fe-Ni plating
solution comprising salts of Co, Fe, and Ni and a stabilizing agent, wherein
the stabilizing agent comprises at least one citrate salt in an amount
effective
to act as a stabilizing agent.
The present invention further includes a method for forming a thin Co-
Fe-Ni alloy plated magnetic film comprising:
(a) providing a substrate to be plated;
(b) immersing the substrate in a Co-Fe-Ni plating solution; and
(c) applying a plating current.
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It has been found that the addition of citrate effectively improved the
stability of CoFeNi plating baths or solutions of the present invention, and
thus, denser CoFeNi films can be plated out because of the higher solution
pH. The present inventors have found that conventional low pH bath suffers
from stability problems, as well as low current density efficiency and voids
in
deposited films due to the electroplating of hydrogen. Bath stability is
crucial
for commercial fabrication of CoFeNi thin films with ideal properties. The
present inventors have found that citrate can effectively improve the
stability
of CoFeNi plating baths. Denser CoFeNi deposits can be plated out from the
citrate-based bath of the present invention because of higher bath pH.
Other features and advantages of the present invention will become
apparent from the following detailed description. It should be understood,
however, that the detailed description and the specific examples while
indicating preferred embodiments of the invention are given by way of
illustration only, since various changes and modifications within the spirit
and
scope of the invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in relation to the drawings in
which:
Figure 1a is a Pourbaix diagram for CoFeNi alloy plating bath having 0.08M
CoS04, 0.015M FeS04 and 0.3M NiS04. The dashed lines a and b refer to
the equilibrium lines for H+/Hz and (Oz +H20)/OH~, respectively. The
predominant areas of Co species, Fe species, and Ni species are defined by
purple, green, and red lines, respectively.
Figure 1 b is a Pourbaix diagram for CoFeNi alloy plating bath having 0.08M
CoS04, 0.015M FeS04, 0.3M NiSOd and 0.206M K3(C6H507). The dashed
tines a and b refer to the equilibrium lines for H+/Hz and (Oz +H20)/OH-,
respectively. The predominant areas of Co species, Fe species, and Ni
species are defined by purple, green, and red lines, respectively.
Figure 1c is a Pourbaix diagram for CoFeNi alloy plating bath having 0.08M
CoS04, 0.015M FeSOø, 0.3M NiS04 and 0.395M (NH4)3(C6H50~). The
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dashed tines a and b refer to the equilibrium lines for H+/H2 and (02
+H20)IOH-, respectively. The predominant areas of Co species, Fe species,
and Ni species are defined by purple, green, and red lines, respectively.
Figure 2a is a photograph of a CoFeNi film plated from a low pH bath of 2.7
without the addition of citrate in Table 3 and at a current density of i at
6mA/cm2.
Figure 2b is a photograph of a CoFeNi film plated from pH bath of 5.3 at a
citrate concentration of 0.206M in Table 3 and at a current density of i at
6mA/cm2.
Figure 3a is a graph of deposit atomic percentage versus dosage of
ammonium citrate which shows the effect on deposit composition at a plating
current density of i at 6mAlcmz.
Figure 3b is a graph of plating rate versus dosage of ammonium citrate which
shows the effect on plating rate at a plating current density of i at 6mA/cm2.
Figure 4 is a graph of deposit atomic percentage versus solution cobalt
concentration at a plating current density of i at 6mA/cm2.
Figure 5 is a graph of deposit atomic percentage versus solution iron
concentration at a plating current density of i at 6mA/cm2.
Figure 6 is a graph of deposit atomic percentage versus solution nickel
concentration at a plating current density of i at 6mA/cm2.
Figure 7 is a graph of deposit atomic percentage versus current density.
Figure 8 is a graph of deposit atomic percentage versus agitation rate at a
plating current density of i at 6mA/cm2.
Figure 9 is a graph of deposit atomic percentage versus on-time tfl" at a
plating current density of i at 6mA/cm2.
Figure 10 is a thin film X-ray diffraction (XRD) spectrum of CoFeNi film
plated
at an ammonium citrate dosage of 50g/L and at a plating current density of i
at
8mA/cm2 in which the film composition is Cos5Fe24N11~~
Figure 11a is a bright field transmission electron microscopy (TEM) image of
a CoFeNi film plated at ammonium citrate dosage of 100g/L and at a plating
current density of i at 10mA/cm2 in which the film composition is Co~aFe2INi~.
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Figure 11b is a dark field transmission electron microscopy (TEM) image of a
CoFeNi film plated at ammonium citrate dosage of 100g/L and at a plating
current density of i at 10mA/cm2 in which the film composition is Co~2Fe2~Ni~.
DETAILED DESCRIPTION OF THE INVENTION
This present application relates to a novel Co-Fe-Ni plating solution
and a method for forming a thin Co-Fe-Ni alloy plated magnetic film.
The present invention therefore includes a Co-Fe-Ni plating solution
comprising salts of Co, Fe and Ni and a stabilizing agent, wherein the
stabilizing agent comprises at least one citrate salt in an amount effective
to
act as a stabilizing agent. The term "amount effective to act as a stabilizing
agent" as used herein is that amount sufficient to achieve beneficial or
desired
results. In the context of an amount effective to act as a stabilizing agent,
this
would be an amount sufficient to achieve a stabilizing effect on the Co-Fe-Ni
solution as compared to the condition obtained without the addition of the
stabilizing agent. The term "stabilizing effect" as used herein refers, for
example, to reduction or prevention of the precipitation of the metal
hydroxides in the plating solution, the metal being Co, Fe or Ni, as well as
to a
pH sufficiently high to retard the electroplating of H2. In accordance with
the
present invention, the stabilizing agent comprises an effective amount of at
least one citrate salt.
In embodiments of the invention, the Co-Fe-Ni plating solution has a
pH greater than or equal to about 3.5. In further embodiments of the
invention, the pH is between about 3.5 and about 8. In still further
embodiments of the invention, the pH is about 5.3.
In embodiments of the invention, the salt of Ni has a concentration in
the range of about 0.05M to about 0.4M. In more particular embodiments of
the invention, the salt of Ni is NiS04. In still further embodiments of the
invention, NiSOa has a concentration of about 0.3M.
In embodiments of the invention, the salt of Co has a concentration in
the range of about 0.01 M to about 0.2M. In further embodiments of the
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invention, the salt of Co is CoS04. In still further embodiments of the
invention, CoSO4 has a concentration of about 0.08M.
In embodiments of the invention, the salt of Fe has a concentration in
the range of about 0.005M to about 0.05M. In further embodiments of the
invention, the salt of Fe is FeS04. In still further embodiments of the
invention, FeS04 has a concentration of about 0.015M.
In embodiments of the invention, the citrate salt has a concentration in
the range of about 0.01 M to about 0.4M. In further embodiments of the
invention, the citrate salt is sodium citrate, potassium citrate or ammonium
citrate, specifically potassium citrate or ammonium citrate. In one
embodiment of the invention, potassium citrate has a concentration of about
0.206M. In another embodiment of the invention, ammonium citrate has a
concentration of about 0.395M.
Moreover, in embodiments of the invention, the Co-Fe-Ni plating
solution further comprises a pH buffering agent. In embodiments of the
invention, the pH buffering agent has a concentration in the range of about
0.1 M to about 0.4M. In more particular embodiments of the invention, the pH
buffering agent is H3BO3. Further, in specific embodiments of the invention,
H3B03 has a concentration of about 0.4M.
In yet another embodiment of the invention, the Co-Fe-Ni plating
solution further comprises a surfactant. In embodiments of the invention, the
surfactant has a concentration in the range of about 0.01 glL to about
0.05g/L.
In more particular embodiments of the invention, the surfactant is sodium
lauryl sulfate. Further in specific embodiments of the invention, sodium
lauryl
sulfate has a concentration of about 0.01 gIL.
The term "about" as used herein means within experimental error.
Unless otherwise indicated, the concentrations provided herein are
expressed as the concentration of the species in the fiinal product or
solution.
The plating solution of the present invention may also contain other
compounds that are common to electroplating solutions or baths, for example
conducting salts such as potassium chloride, sodium chloride and/or
ammonium chloride.
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The present invention further relates to a method for forming a thin Co-
Fe-Ni alloy plated magnetic film comprising:
(a) providing a substrate to be plated;
(b) immersing the substrate in a Co-Fe-Ni plating solution of the
present invention; and
(c) applying a plating current.
In embodiments of the invention, the substrate is Si wafer coated with
Ti/Au blanket metallizations, and the substrate has Au as a seed layer for
plating.
In other embodiments of the invention, the method of applying the
plating current is selected from the group consisting of direct current,
pulsed
current, pulsed reversed current, pulsed conditioned current and combinations
thereof. fn particular embodiments of the invention, the plating current is
pulsed current. In still more particular embodiments of the invention, the
pulsed current has a duty cycle of 10ms with 0.3ms of on-time (to") and 9.7ms
of off time.
The present inventors have performed research on the development of
a stable citrate-based bath for the electroplating of CoFeNi films. It has
been
found that the addition of citrate effectively improved the stability of
CoFeNi
plating baths, and thus, denser CoFeNi films can be plated out because of the
higher bath pH, which is greater than 5.
The present inventors have found that conventional low pH baths
suffer from stability problems, as well as low current density efficiency and
voids in deposited films due to the electroplating of hydrogen. Bath stability
is
crucial for commercial fabrication of CoFeNi thin films with ideal properties.
The present inventors have found that citrate can effectively improve the
stability of CoFeNi plating baths. Denser CoFeldi deposits can be plated out
from the citrate-based bath of the present invention because of higher bath
pH. The calculated Pourbaix diagrams (see Figures 1 a-1 c) demonstrate that
citrate has the strongest complexing effect on Fe ions, then on Ni+2 ion, and
the weakest complexing effect on Co+2 ion.
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Generally, metal content in deposited films increases with the metal
concentration in the plating bath. The anomalous behavior of Ni plating was
also observed during the plating with the citrate-based bath of the present
invention. However, the effects of plating conditions on deposited CoFeNi film
composition are not as prominent as that of bath composition.
CoFeNi thin films with preferred composition, mixed face centered
cubic-body centered cubic (fcc-bcc) phases, and 10-20nm grain sizes, which
are necessary for achieving ideal soft magnetic properties, can be plated out
from the new citrate-based bath of the present invention. The saturation flux
density 8S of films plated from the citrate-based bath of the present
invention
exceeds 2 Tesla. The coercivities are slightly larger than the best reported
values (Osaka, T.; Takai, M.; Hayashi, K.; Ohashi, K.; Saito, M.; Yamada, K.
Nature 1998, 392, 796.), but better than those of prior art CoFe films
obtained
with vacuum techniques for recording head fabrication. ( Liao, S. H.; Tolman,
C. H. US Patent 1988, no.4,756,816 and Yu, W.; Bain, J. A.; Peng, Y.;
Laughlin, D. E. IEEE Trans. Magn. 2002, 38, 3030.)
The following non-limiting examples are illustrative of the present
invention:
EXAMPLES
Materials and Methods
Si wafers coated with TilAu blanket metallizations were used as
cathodes, with Au acting as a seed layer for plating. Platinum foil was used
as
the anode. The composition of citrate-based plating bath is listed in Table 2,
below, unless specified otherwise. As used herein, the term "natural" refers
to
the pH of the bath without the addition of any acid or base. All plating,
unless
otherwise indicated, was done using pulsed current (PC) with a duty cycle of
10ms - 0.3ms of on-time (to") and 9.7ms of off time. Agitation was introduced
at a speed of 600 rpm, unless specified otherwise. Plating time was set by the
product of plating time and current density at around 300 minutes'~mAlcm2. All
plating experiments were conducted under ambient temperature and pressure
conditions.
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Stability diagrams (Pourbaix diagrams) were calculated with OLI
Analyzer Version 1.3 software purchased from OLI systems, Inc. The
compositions and microstructures of CoFeNi deposits were characterized
using a Hitachi S-2700 scanning electron microscope (SEM) equipped with an
ultra thin window (UTVIn x-ray detector. A Rigaku rotating anode XRD system,
with a thin film camera attachment, was employed to identify specific CoFeNi
phases. A Cu anode operating at 40kV and 100mA was used, with an incident
angle of 28 = 2°. A JEOL 2010 TEM, also equipped with a UTW x-ray
detector, was used to observe the crystallization process and grain size, and
to obtain diffraction patterns. A Superconducting Quantum Interference
Device (SQUID) magnetometer (Quantum Design) was applied to measure
the magnetic properties of CoFeNi thin films.
Example 1: Stability of Plating Bath
(i) Pourbaix Diagrams Calculations: The stability of the plating bath can
be studied through stability diagrams. With reference to Figures 1 a, 1 b and
1 c, the Pourbaix diagrams for CoFeNi alloy plating baths with no citrate
addition, 0.206M potassium citrate (K3(C6H50~)), and 0.395M ammonium
citrate ((NH4)3(C6H50~)), respectively have been calculated. As is known to
those skilled in the art, complexing agents are usually employed to stabilize
a
metal or alloy plating bath. The main differences in the bath composition
developed by the present inventors (Table 2) relative to the conventional bath
composition (Table 1 ) are the introduction of citrate as a complexing agent
and a higher pH (3.5 - 8).
As can be best seen in Figure 1 a, thermodynamically, the stability of a
CoFeNi alloy plating bath open to air is dominated by the precipitation of
Fe(OH)3 at a pH~3.1. This result is in line with the selection of bath pH in
the
range of 2.5 to 3.0 by previous researchers (Osaka, T., Takai, M., Hayashi,
K., Ohashi, K., Saito, M. and Yamada, K. Nature 1998, 392, 796; Osaka, T.,
Takai, M., Hayashi, K., Sogawa, Y., Ohashi, K. and Yasue, Y. IEEE Trans.
Magn. 1998, 34, 1432; Liu, X., Zangari, G. and Shamsuzzoha, M. J.
Electrochem. Soc. 2003, 950, C159). The present inventors have found that
in the cases reported by previous researchers, the acids are employed as the
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bath stabilizer. After the addition of 0.206M potassium citrate, the CoFeNi
alloy plating bath is thermodynamically stable up to a pH~4.7 under the given
concentrations of metal ions (Figure 1 b). With the introduction of 0.395M
ammonium citrate, the CoFeNi alloy plating bath is thermodynamically stable
until the precipitation of Fe(OH)3 at pH=5.8 (Figure 1 c), due to the
formation of
stable complexing species, FeCsH507, Co[C6H50~)', and Ni[CgH50~)'. The
adoption of ammonium citrate creates an additional stable region of
Co(NH3)6+3 from pH 6.6 to 10 because of the complexing effect of NH3 on
Co+3 ion. From Figures 1 b and 1 c, it is apparent that citrate has the
strongest
complexing power for Fe ions, followed by Ni+2 ion, and the weakest
complexing effect for Co+2 ion. The main complexing reactions in the above
CoFeNi alloy plating bath with the addition of 0.395 M ammonium citrate can
be summarized as follows:
Fe+z + [CsH50~) 3 = Fe[CsH50~)-
4Fe[C6H50~)' - 4e = 4Fe[C6H5O~), 02 + 2H20 + 4e = 40H-
Co+2 + [C6H50~) 3 = Co[C6H50~)'
Ni+2 + [C6H50~)'3 = Ni[C6H507)'
The calculated stability diagrams demonstrate that, thermodynamically,
citrate can effectively stabilize the CoFeNi alloy plating baths, preventing
the
precipitation of metal hydroxides at higher pH.
(ii) Bath Stability Tests: Bath stability tests on baths with and
without the addition of citrate have been conducted. Table 3 summarizes
these results and demonstrates that citrate can significantly improve the
stability of a CoFeNi alloy plating bath. For citrate-free baths, a low pH
bath is
more stable.
Example 2: Effects of Bath Composition on the Electroplating of CoFeNi
Thin Films
The present inventors have found that besides the stability problem,
traditional low pH baths suffer from low current density efficiency and voids
in
deposited CoFeNi films, which will degenerate the magnetic properties and
uniformity of the films, due to the electroplating of H2 (Figure 2a). As shown
in
Table 4, H+/HZ has a more positive equilibrium potential than the metal
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electrodes, which means hydrogen is more easily plated out than the metals.
The H+ concentration in the newly developed citrate-based bath (optimally
pH>5) is hundreds of times lower than that in the conventional bath (pH = 2.5-
3.0) (Osaka, T., Takai, M., Hayashi, K., Ohashi, K., Saito, M. and Yamada, K.
Nature 1998, 392, 796; Osaka, T., Takai, M., Hayashi, K., Sogawa, Y.,
Ohashi, K. and Yasue, Y. IEEE Trans. Magn. 1998, 34, 1432; Liu, X., Zangari,
G. and Shamsuzzoha, M. J. Electrochem. Soc. 2003, 150, C159). Therefore,
more uniform and denser films have been plated out (Figure 2b).
(i~ Effect of Ammonium Citrate: The effect of ammonium citrate on
the electroplating of CoFeNi films has been studied. The effect of ammonium
citrate on the composition of CoFeNi deposits is shown in Figure 3a by a
graph of atomic percentage versus dosage of ammonium citrate at a plating
current density of i at 6mA/cm2. Generally, ammonium citrate has the most
prominent effect on Fe content, followed by Ni content, and only a minor
effect
on Co content. The results agree with the calculated stability diagrams (see
Figures 1 b and 1 c), which demonstrate that citrate has the most powerful
complexing effect on Fe ions, then Ni+2, and finally Co+2. At low citrate
dosage, the Fe content in the deposited films is lowered, while as the citrate
dosage is increased, the Fe content goes up. This is because at low citrate
dosage, only Fe ions are complexed; as citrate dosage increases, the Ni and
Co ions will also be complexed. Metals are more difficult to plate out from
the
complexed metal ions, due to higher activation energies and lower
diffusivities
to the cathode.
At an ammonium citrate dosage of 50g/L (0.206 M), a film with a
composition of Co~Fe24Ni,~ has been plated out. This film is very close in
composition to the film with optimal soft magnetic properties, which has a
composition of Co65Fe23Ni~2 with a high saturation flux density 8$ of 2.1
Tesla
and low coercivity H~ of 1.20 Oe, claimed by Osaka and coworkers (Osaka,
T., Takai, M., Hayashi, K., Ohashi, K., Saito, M. and Yamada, K. Nature 1998,
392, 796 and Osaka, T., Takai, M., Hayashi, K., Sogawa, Y., Ohashi, K. and
Yasue, Y. IEEE Trans. Magn. 1998, 34, 1432).
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The effect of ammonium citrate dosage on plating rate is shown in
Figure 3b by a graph of plating rate versus dosage of ammonium citrate at a
plating current density of i at 6mA/cmz. The ammonium citrate dosage has a
minor effect on plating rate up to a concentration of 50g/L, whereas, the
plating rate drops rapidly at high ammonium citrate dosages.
(ii) Effect of Cobalt Concentration: The effect of cobalt concentration on
the composition of deposited CoFeNi films has been studied. A graph of the
atomic percentage versus cobalt concentration is shown in Figure 4 at a
plating current density of i at 6mAlcm2. The graph shows that Co content in
the deposit increases rapidly, while Fe and Ni contents decrease as the cobalt
concentration increases. This corresponds to the kinetics of plating process.
(iii) Effect of Iron Concentration: The effect of iron concentration on
the composition of deposited CoFeNi films has been studied. A graph of the
atomic percentage versus iron concentration is shown in Figure 5 at a plating
current density of i at 6mA/cm2. The graph demonstrates that deposit iron
content increases, and cobalt content decreases, with increasing iron
concentration in the plating bath. ft is interesting that Ni content is almost
constant as the iron concentration is varied, which may be due to the much
lower solution concentration of iron relative to nickel.
(iv) Effect of Nickel Concentration: The effect of nickel concentration on
the composition of deposited CoFeNi films has been studied. A graph of the
atomic percentage versus nickel concentration is shown in Figure 6 at a
plating current density of i at 6mA/cm2. The deposit Ni content increases,
while Co and Fe contents oscillate, as nickel concentration in the bath goes
up. From the plating bath composition (with reference to Table 2), it is clear
that the metal contents in the deposits are not proportional to the metal
concentrations in the plating bath. By referring to Figures 4 to 6, Ni is the
most
difficult metal to be plated out. However, from Table 4, Ni2+/Ni has the most
positive potential among the three metal electrodes, so it should be the metal
plated out first. This anomalous phenomenon for Ni plating has been reported
previously by several researchers (Zhuang, Y. and Podlaha, E. J. J.
Electrochem. Soc. 2003, 950, C219; Vaes, J., Fransaer, J. and Celis, J. P. J.
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Electrochem. Soc. 2000, 147, 3718 and Golodnitsky, D., Gudin, N. V. and
Volyanuk, G. A. J. Electrochem. Soc. 2000, 147, 4156).
Example 3: Effects of Plating Conditions on the Electroplating of Col=eNi
Thin Films
(i) Effect of Current Density: Tests on the effect of current density
on the electroplating of CoFeNi thin films have been performed. A graph of
atomic percentage versus current density is shown in Figure 7. The graph
demonstrates that at low current densities, the composition of deposited
CoFeNi films varies as the current density increases. At current densities
higher than 6mA/cm2, the deposited metal contents are almost constant.
(ii) Effect of Agitation: Tests on the effect of agitation on the
electroplating on the composition of CoFeNi films have been performed. A
graph of the atomic percentage versus agitation rate is shown in Figure 8 at a
plating current density of i at 6mAlcm2. As can be seen from Figure 8, the
introduction of agitation changes the composition of plated CoFeNi films. This
is because agitation accelerates the diffusion of metal ions to the cathode
and
affects the metal ion ratio near the cathode surface. The Fe and Ni
compositions are more affected, with Little change in Co.
(iii) Effect of to": Tests on the effect of on-time tor, on the composition of
CoFeNi films have been performed. To obtain uniform composition in the
deposited film through the thickness, i.e., to avoid metal content gradients,
pulsed current plating is usually employed for maintaining initial metal ion
concentrations around the cathode. A graph of atomic percentage on toy is
shown in Figure 9 at a plating current density of i at 6mA/cm2. The graph
shows the effect of on-time tor, of the duty cycle on the plating of CoFeNi
alloys. The metal contents in deposits have very little fluctuation with toy
variation. The films have a composition around Co6~Fe~Ni».
Example 4: Studies on Phase Formation and Grain Size in Deposited
Films
Thin film X-ray diffraction (XRD) and transmission electron microscopy (TEM)
methods were employed to analyze the phase formation and grain size in
deposited CoFeNi films. The major XRD peaks for fcc and bcc phases are
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(111) for fcc at 26~ 44.1° and (110) for bcc at 28~ 45.2°,
respectively (Liu, X.,
Zangari, G. and Shamsuzzoha, M. J. Electrochem. Soc. 2003, 150, C159 and
Tabakovic, I., Inturi, V. and Riemer, S. J. Electrochem. Soc. 2002, 149, C18).
A thin film XRD spectrum of CoFeNi film plated at an ammonium citrate
dosage of 50g/L and i at 8mAlcm2 is shown in Figure 10 in which the film
composition is Cos5Fe24Ni". As can be seen in Figure 10, both fcc and bcc
phases can be co-deposited from the newly developed bath.
TEM bright field and dark field images (Figure 11a and 11b) show that
the grains in CoFeNi deposits are 10-20 nm in diameter, which is similar to
the grain sizes in CoFeNi films with the best soft magnetic properties
obtained
by Osaka et al (Osaka, T., Takai, M., Hayashi, K., Ohashi, K., Saito, M. and
Yamada, K. Nature 1998, 392, 796). The dark field image was formed from
part of the fcc (111) and bcc (110) diffraction rings.
Example 5: Studies on Magnetic Properties of Plated CoFeNi Thin Films
The magnetic properties of representative CoFeNi dims plated from
conventional low pH baths and the newly developed citrate-based bath are
listed in Table 5. CoFeNi films with optimal soft magnetic properties (high BS
and low H~) have been plated out from the low pH bath. The results are close
to those reported in the literature (Osaka, T., Takai, M., Hayashi, K.,
Ohashi,
K., Saito, M. and Yamada, K. Nature 1998, 392, 796 and Osaka, T., Takai,
M., Hayashi, K., Sogawa, Y., Ohashi, K. and Yasue, Y. IEEE Trans. Magn.
1998, 34, 1432). For the films plated from the citrate-based bath, the
saturation flux density B$ exceeds 2 Tesla, which is desired. However, the
coercivities of the films are slightly larger than those of the films plated
from
low pH bath. The coercivities of CoFeNi films plated from the newly developed
bath are lower than those for CoFe films obtained with vacuum techniques for
recording head fabrication, which are around 20 to 60 Oe (Liao, S. H, and
Tolman, C. H. US Patent 1988, no.4,756,816 and Yu, W., Bain, J. A., Peng,
Y. and Laughlin, D. E. IEEE Trans. Magn. 2002, 38, 3030)
While the present invention has been described with reference to what
are presently considered to be the preferred examples, it is to be understood
that the invention is not limited to the disclosed examples. To the contrary,
the
CA 02461107 2004-03-15
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invention is intended to cover various modifications and equivalent
arrangements included within the spirit and scope of the appended claims.
All publications, patents and patent applications are herein
incorporated by reference in their entirety to the same extent as if each
individual publication, patent or patent application was specifically and
individually indicated to be incorporated by reference in its entirety. Where
a
term in the present application is found to be defined differently in a
document
incorporated herein by reference, the definition provided herein is to serve
as
the definition for the term.
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Table 1. Composition of bath for electroplating CoFeNi alloys
Chemical Concentration Chemical Concentration
CoS04 0.03-0.0875 H3B03 0.4 M
M
FeS04 0.005-0.045 Sodium lauryl0.01 g/L
M
sulfate
NiS04 0.2 M NH4CI 0.28 M
Bath nH = 2.5-3.0
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Table 2.
Composition of citrate-based bath for electroplating CoFeNi thin films
Chemical ConcentrationChemical Concentration
CoS04 0.08 M H3B03 0.4 M
FeS04 0.015 M Sodium lauryl 0.01 g/L
sulfate
NiS04 0.3 M Ammonium 0.206 M
citrate
Bath pH = 5.3 (natural)
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Table 3. Bath stability tests on baths with and without addition of citrate
Bath composition ~ pH Stability
0.08M CoS04
0.015M FeS04 Plated bath was transparent
0.3M NiS04 5.3 fter more than one
month.
0.4M H3B03 natural) Plating results were
0.01 g/L sodium lauryl repeatable after 6
sulfate days.
0.206M (NH4)3(CsHsO~)
0.08M CoSO
0.015M FeS04
0.3M NiS04 5.3 Precipitate appeared
in bath
0.4M H3B03 natural) ithin 2 hours during
plating.
0.01 g/L sodium lauryl
sulfate
0.28M NH4Ci
0.08M CoS04
0.015M FeS04
0.3M MS04,
Precipitate appeared
in
pH adjusted fated bath after less
wit than 2
'
0.4M HaB03
flute H2S04) ays.
0.01 g/L sodium lauryl
sulfate
0.28M NH4CI
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Table 4.
Equilibrium Potentials of Selected Electrochemical Electrodes
Electrochemical electrode Equilibrium potential (V)
H''/H2 0
Ni2+/Ni -0.23
Co2+/Co -0.28
Fe2+/Fe -0.44
*Andricacos, P. C. and Robertson, N. IBM J. Res. Develop. (Electrochemical
Microfabrication), 1988, 42, 671.
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Table 5. Magnetic properties of representative CoFeNi films plated from
a low pH bath and the newly developed bath
Plating bath Film Coercivity Saturation flux
composition He (Oe) density B$
(Tesla)
Low pH bath Co~,Fe24Ni,2 1.5 2.01
(pH 2.7) COg5Fe24N~11 5.5 1.91
Cos~Fe29Ni~~ 18 1.84
Newly CossFe~zNi,o 11 2.03
developed bath Cos~FezsNi,o 15 2.10
(pH 5.3)
