U.S. patent application number 11/079071 was filed with the patent office on 2005-09-29 for solution for the electroplating of soft magnetic co-fe-ni alloys.
This patent application is currently assigned to The Governors of the University of Alberta. Invention is credited to Ivey, Douglas G., Zhang, Yahui.
Application Number | 20050211563 11/079071 |
Document ID | / |
Family ID | 34988475 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050211563 |
Kind Code |
A1 |
Ivey, Douglas G. ; et
al. |
September 29, 2005 |
Solution for the electroplating of soft magnetic Co-Fe-Ni
alloys
Abstract
The present invention provides a Co--Fe--Ni plating solution
comprising salts of Co, Fe and Ni and a stabilizing agent. The
stabilizing agent has at least one citrate salt in an amount
effective to act as a stabilizing agent. The present invention also
provides a method for forming a thin Co--Fe--Ni alloy plated
magnetic film with high saturation magnetization and low coercivity
using the citrate-based Co--Fe--Ni plating solution.
Inventors: |
Ivey, Douglas G.; (Edmonton,
CA) ; Zhang, Yahui; (Edmonton, CA) |
Correspondence
Address: |
BERESKIN AND PARR
40 KING STREET WEST
BOX 401
TORONTO
ON
M5H 3Y2
CA
|
Assignee: |
The Governors of the University of
Alberta
Edmonton
CA
|
Family ID: |
34988475 |
Appl. No.: |
11/079071 |
Filed: |
March 15, 2005 |
Current U.S.
Class: |
205/255 ;
205/259 |
Current CPC
Class: |
C25D 5/18 20130101; C25D
3/562 20130101 |
Class at
Publication: |
205/255 ;
205/259 |
International
Class: |
C25D 003/56 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 15, 2004 |
CA |
2,461,107 |
Claims
We claim:
1. 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.
2. The Co--Fe--Ni plating solution according to claim 1, wherein
the Co--Fe--Ni plating solution has a pH greater than or equal to
about 3.5.
3. The Co--Fe--Ni plating solution according to claim 2, wherein
the pH is between about 3.5 and about 8.
4. The Co--Fe--Ni plating solution according to claim 3, wherein
the pH is about 5.3.
5. The Co--Fe--Ni plating solution according to claim 1, wherein
the salt of Ni has a concentration in the range of about 0.05M to
about 0.4M.
6. The Co--Fe--Ni plating solution according to claim 1, wherein
the salt of Ni is NiSO.sub.4.
7. The Co--Fe--Ni plating solution according to claim 6, wherein
NiSO.sub.4 has a concentration of about 0.3M.
8. The Co--Fe--Ni plating solution according to claim 1, wherein
the salt of Co has a concentration in the range of about 0.01M to
about 0.2M.
9. The Co--Fe--Ni plating solution according to claim 1, wherein
the salt of Co is CoSO.sub.4.
10. The Co--Fe--Ni plating solution according to claim 9, wherein
CoSO.sub.4 has a concentration of about 0.08M.
11. The Co--Fe--Ni plating solution according to claim 1, wherein
the salt of Fe has a concentration in the range of about 0.005M to
about 0.05M.
12. The Co--Fe--Ni plating solution according to claim 1, wherein
the salt of Fe is FeSO.sub.4.
13. The Co--Fe--Ni plating solution according to claim 12, wherein
FeSO.sub.4 has a concentration of about 0.015M.
14. The Co--Fe--Ni plating solution according to claim 1, wherein
the citrate salt has a concentration in the range of about 0.05M to
about 0.4M.
15. The Co--Fe--Ni plating solution according to claim 1, wherein
the citrate salt is sodium citrate, potassium citrate or ammonium
citrate.
16. The Co--Fe--Ni plating solution according to claim 15, wherein
potassium citrate has a concentration of about 0.206M.
17. The Co--Fe--Ni plating solution according to claim 15, wherein
ammonium citrate has a concentration of about 0.395M.
18. The Co--Fe--Ni plating solution according to claim 1, further
comprising a pH buffering agent.
19. The Co--Fe--Ni plating solution according to claim 18, wherein
the pH buffering agent has a concentration in the range of about
0.1M to about 0.4M.
20. The Co--Fe--Ni plating solution according to claim 18, wherein
the pH buffering agent is H.sub.3BO.sub.3.
21. The Co--Fe--Ni plating solution according to claim 20, wherein
H.sub.3BO.sub.3 has a concentration of about 0.4M.
22. The Co--Fe--Ni plating solution according to claim 1, further
comprising a surfactant.
23. The Co--Fe--Ni plating solution according to claim 22, wherein
the surfactant has a concentration in the range of about 0.01 g/L
to about 0.05 g/L.
24. The Co--Fe--Ni plating solution according to claim 23, wherein
the surfactant is sodium lauryl sulfate.
25. The Co--Fe--Ni plating solution according to claim 24, wherein
sodium lauryl sulfate has a concentration of about 0.01 g/L.
26. 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 according
to claim 1; and (c) applying a plating current.
27. The method according to claim 26, wherein the substrate is a Si
wafer coated with Ti/Au blanket metallizations, and wherein the
substrate has Au as a seed layer for plating.
28. The method according to claim 26, wherein the plating current
is applied using a method selected from one or more of direct
current, pulsed current, pulsed reversed current and pulsed
conditioned current.
29. The method according to claim 28, wherein the plating current
is pulsed current.
30. The method according to claim 29, wherein the pulsed current
has a duty cycle of 10 ms with 0.3 ms of on-time (t.sub.on) and 9.7
ms of off time.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35 USC
.sctn.119(b) from Canadian patent application S.N. 2,461,107, filed
Mar. 15, 2004.
FIELD OF THE INVENTION
[0002] 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.
BACKGROUND OF THE INVENTION
[0003] 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.sub.80Fe.sub.20)
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.sub.80Fe.sub.20. New soft magnetic
materials with higher saturation flux density B.sub.s such as
electroplated CoFe alloys, CoFeNi alloys, CoFeCu alloys, other
CoFe-based alloys, sputtered FeN films and other Fe-based alloys,
have been developed.
[0004] 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 plated from solutions whose
compositions differ from that of a NiFe plating bath only by adding
a Co.sup.2+ 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. Nature
1998, 392, 796.), which has a pH as low as 2.5 to 3.0 with the
addition of acid.
[0005] 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 H.sub.2. 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
[0006] 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.
[0007] 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.
[0008] The present invention further includes a method for forming
a thin Co--Fe--Ni alloy plated magnetic film comprising:
[0009] (a) providing a substrate to be plated;
[0010] (b) immersing the substrate in a Co--Fe--Ni plating
solution; and
[0011] (c) applying a plating current.
[0012] 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.
[0013] 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
[0014] The invention will now be described in relation to the
drawings in which:
[0015] FIG. 1a is a Pourbaix diagram for CoFeNi alloy plating bath
having 0.08M CoSO.sub.4, 0.015M FeSO.sub.4 and 0.3M NiSO.sub.4. The
dashed lines a and b refer to the equilibrium lines for
H.sup.+/H.sub.2 and (O.sub.2+H.sub.2O)/OH.sup.-, respectively. The
predominant areas of Co species, Fe species, and Ni species are
defined by purple, green, and red lines, respectively.
[0016] FIG. 1b is a Pourbaix diagram for CoFeNi alloy plating bath
having 0.08M CoSO.sub.4, 0.015M FeSO.sub.4, 0.3M NiSO.sub.4 and
0.206M K.sub.3(C.sub.6H.sub.5O.sub.7). The dashed lines a and b
refer to the equilibrium lines for H.sup.+/H.sub.2 and
(O.sub.2+H.sub.2O)/OH.sup.-, respectively.
[0017] The predominant areas of Co species, Fe species, and Ni
species are defined by purple, green, and red lines,
respectively.
[0018] FIG. 1c is a Pourbaix diagram for CoFeNi alloy plating bath
having 0.08M CoSO.sub.4, 0.015M FeSO.sub.4, 0.3M NiSO.sub.4 and
0.395M (NH.sub.4).sub.3(C.sub.6H.sub.5O.sub.7). The dashed lines a
and b refer to the equilibrium lines for H.sup.+/H.sub.2 and
(O.sub.2+H.sub.2O)/OH.su- p.-, respectively. The predominant areas
of Co species, Fe species, and Ni species are defined by purple,
green, and red lines, respectively.
[0019] FIG. 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 6 mA/cm.sup.2.
[0020] FIG. 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 6 mA/cm.sup.2.
[0021] FIG. 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 6 mA/cm.sup.2.
[0022] FIG. 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 6 mA/cm.sup.2.
[0023] FIG. 4 is a graph of deposit atomic percentage versus
solution cobalt concentration at a plating current density of i at
6 mA/cm.sup.2.
[0024] FIG. 5 is a graph of deposit atomic percentage versus
solution iron concentration at a plating current density of i at 6
mA/cm.sup.2.
[0025] FIG. 6 is a graph of deposit atomic percentage versus
solution nickel concentration at a plating current density of i at
6 mA/cm.sup.2.
[0026] FIG. 7 is a graph of deposit atomic percentage versus
current density.
[0027] FIG. 8 is a graph of deposit atomic percentage versus
agitation rate at a plating current density of i at 6
mA/cm.sup.2.
[0028] FIG. 9 is a graph of deposit atomic percentage versus
on-time t.sub.on at a plating current density of i at 6
mA/cm.sup.2.
[0029] FIG. 10 is a thin film X-ray diffraction (XRD) spectrum of
CoFeNi film plated at an ammonium citrate dosage of 50 g/L and at a
plating current density of i at 8 mA/cm.sup.2 in which the film
composition is CO.sub.65Fe.sub.24Ni.sub.11.
[0030] FIG. 11a is a bright field transmission electron microscopy
(TEM) image of a CoFeNi film plated at ammonium citrate dosage of
100 g/L and at a plating current density of i at 10 mA/cm.sup.2 in
which the film composition is CO.sub.72Fe.sub.21Ni.sub.7.
[0031] FIG. 11b is a dark field transmission electron microscopy
(TEM) image of a CoFeNi film plated at ammonium citrate dosage of
100 g/L and at a plating current density of i at 10 mA/cm.sup.2 in
which the film composition is CO.sub.72Fe.sub.21Ni.sub.7.
DETAILED DESCRIPTION OF THE INVENTION
[0032] 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.
[0033] 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 H.sub.2. In accordance with the present invention, the
stabilizing agent comprises an effective amount of at least one
citrate salt.
[0034] 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.
[0035] 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
NiSO.sub.4. In still further embodiments of the invention,
NiSO.sub.4 has a concentration of about 0.3M.
[0036] In embodiments of the invention, the salt of Co has a
concentration in the range of about 0.01M to about 0.2M. In further
embodiments of the invention, the salt of Co is CoSO.sub.4. In
still further embodiments of the invention, CoSO.sub.4 has a
concentration of about 0.08M.
[0037] 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 FeSO.sub.4.
In still further embodiments of the invention, FeSO.sub.4 has a
concentration of about 0.015M.
[0038] In embodiments of the invention, the citrate salt has a
concentration in the range of about 0.01M 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.
[0039] 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 0.1M to about 0.4M. In more
particular embodiments of the invention, the pH buffering agent is
H.sub.3BO.sub.3. Further, in specific embodiments of the invention,
H.sub.3BO.sub.3 has a concentration of about 0.4M.
[0040] 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 g/L to about 0.05 g/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 g/L.
[0041] The term "about" as used herein means within experimental
error.
[0042] Unless otherwise indicated, the concentrations provided
herein are expressed as the concentration of the species in the
final product or solution.
[0043] 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.
[0044] The present invention further relates to a method for
forming a thin Co--Fe--Ni alloy plated magnetic film
comprising:
[0045] (a) providing a substrate to be plated;
[0046] (b) immersing the substrate in a Co--Fe--Ni plating solution
of the present invention; and
[0047] (c) applying a plating current.
[0048] 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;
[0049] 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. In 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 10 ms with 0.3 ms of on-time
(t.sub.on) and 9.7 ms of off time.
[0050] 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.
[0051] 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 CoFeNi
deposits can be plated out from the citrate-based bath of the
present invention because of higher bath pH. The calculated
Pourbaix diagrams (see FIGS. 1a-1c) demonstrate that citrate has
the strongest complexing effect on Fe ions, then on Ni.sup.+2 ion,
and the weakest complexing effect on Co.sup.+2 ion.
[0052] 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.
[0053] CoFeNi thin films with preferred composition, mixed face
centered cubic-body centered cubic (fcc-bcc) phases, and 10-20 nm
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 B.sub.s 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, U.S. Pat. No. 4,756,816 and Yu, W.; Bain, J. A.; Peng, Y.;
Laughlin, D. E. IEEE Trans. Magn. 2002, 38, 3030.)
[0054] The following non-limiting examples are illustrative of the
present invention:
EXAMPLES
[0055] Materials and Methods
[0056] Si wafers coated with Ti/Au 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 10 ms-0.3 ms of on-time (t.sub.on) and 9.7 ms 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*mA/cm.sup.2. All
plating experiments were conducted under ambient temperature and
pressure conditions.
[0057] 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 (UTW) 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 40 kV and 100 mA was used, with an incident
angle of 20=2.degree.. 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
[0058] (i) Pourbaix Diagrams Calculations: The stability of the
plating bath can be studied through stability diagrams. With
reference to FIGS. 1a, 1b and 1c, the Pourbaix diagrams for CoFeNi
alloy plating baths with no citrate addition, 0.206M potassium
citrate (K.sub.3(C.sub.6H.sub.5O.su- b.7)), and 0.395M ammonium
citrate ((NH.sub.4).sub.3(C.sub.6H.sub.5O.sub.7- )), 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).
[0059] As can be best seen in FIG. 1a, thermodynamically, the
stability of a CoFeNi alloy plating bath open to air is dominated
by the precipitation of Fe(OH).sub.3 at a pH.about.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, 150, C159). The present inventors
have found that in the cases reported by previous researchers, the
acids are employed as the bath stabilizer. After the addition of
0.206M potassium citrate, the CoFeNi alloy plating bath is
thermodynamically stable up to a pH.about.4.7 under the given
concentrations of metal ions (FIG. 1b). With the introduction of
0.395M ammonium citrate, the CoFeNi alloy plating bath is
thermodynamically stable until the precipitation of Fe(OH).sub.3 at
pH=5.8 (FIG. 1c), due to the formation of stable complexing
species, FeC.sub.6H.sub.5O.sub.7, Co[C.sub.6H.sub.5O.sub.7].sup.-,
and Ni[C.sub.6H.sub.5O.sub.7].sup.-. The adoption of ammonium
citrate creates an additional stable region of
Co(NH.sub.3).sub.6.sup.+3 from pH 6.6 to 10 because of the
complexing effect of NH.sub.3 on Co.sup.+3 ion. From FIGS. 1b and
1c, it is apparent that citrate has the strongest complexing power
for Fe ions, followed by Ni.sup.+2 ion, and the weakest complexing
effect for Co.sup.+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.sup.+2+[C.sub.6H.sub.5O.sub.7].sup.-3.dbd.Fe[C.sub.6H.sub.5O.sub.7].sup-
.-
4Fe[C.sub.6H.sub.5O.sub.7].sup.--4e.dbd.4Fe[C.sub.6H.sub.5O.sub.7],
O.sub.2+2H.sub.2O+4e.dbd.4OH.sup.-
Co.sup.+2+[C.sub.6H.sub.5O.sub.7].sup.-3.dbd.Co[C.sub.6H.sub.5O.sub.7].sup-
.-
Ni.sup.+2+[C.sub.6H.sub.5O.sub.7].sup.-3.dbd.Ni[C.sub.6H.sub.5O.sub.7].sup-
.-
[0060] 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.
[0061] (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
[0062] 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 H.sub.2 (FIG. 2a). As shown in Table 4,
H.sup.+/H.sub.2 has a more positive equilibrium potential than the
metal 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 (FIG.
2b).
[0063] (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 FIG. 3a by a graph of atomic percentage versus dosage of
ammonium citrate at a plating current density of i at 6
mA/cm.sup.2. 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 FIGS. 1b and 1c), which demonstrate that
citrate has the most powerful complexing effect on Fe ions, then
Ni.sup.+2, and finally Co.sup.+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.
[0064] At an ammonium citrate dosage of 50 g/L (0.206 M), a film
with a composition of CO.sub.65Fe.sub.24Ni.sub.11 has been plated
out. This film is very close in composition to the film with
optimal soft magnetic properties, which has a composition of
CO.sub.65Fe.sub.23Ni.sub.12 with a high saturation flux density
B.sub.s of 2.1 Tesla and low coercivity H.sub.c 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. 199.8; 34, 1432).
[0065] The effect of ammonium citrate dosage on plating rate is
shown in FIG. 3b by a graph of plating rate versus dosage of
ammonium citrate at a plating current density of i at 6
mA/cm.sup.2. The ammonium citrate dosage has a minor effect on
plating rate up to a concentration of 50 g/L, whereas, the plating
rate drops rapidly at high ammonium citrate dosages.
[0066] (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 FIG. 4 at a plating current density of i
at 6 mA/cm.sup.2. 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.
[0067] (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 FIG. 5 at a plating current density of i at 6
mA/cm.sup.2. The graph demonstrates that deposit iron content
increases, and cobalt content decreases, with increasing iron
concentration in the plating bath. It 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.
[0068] (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 FIG. 6 at a plating current density of i
at 6 mA/cm.sup.2. 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 FIGS. 4 to 6, Ni is the most difficult metal to be
plated out. However, from Table 4, Ni.sup.2+/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, 150,
C219; Vaes, J., Fransaer, J. and Celis, J. P. J. 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 CoFeNi Thin
Films
[0069] (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 FIG. 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 6
mA/cm.sup.2, the deposited metal contents are almost constant.
[0070] (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 FIG. 8 at a plating current density of i at 6
mA/cm.sup.2. As can be seen from FIG. 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.
[0071] (iii) Effect of t.sub.on: Tests on the effect of on-time
t.sub.on 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
t.sub.on is shown in FIG. 9 at a plating current density of i at 6
mA/cm.sup.2. The graph shows the effect of on-time t.sub.on of the
duty cycle on the plating of CoFeNi alloys. The metal contents in
deposits have very little fluctuation with t.sub.on variation. The
films have a composition around CO.sub.67Fe.sub.22Ni.sub.1- 1.
Example 4
Studies on Phase Formation and Grain Size in Deposited Films
[0072] 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 (111) for fcc at
2.theta..about.44.1.degree. and (110) for bcc at
2.theta.-45.2.degree., 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 50 g/L and i at 8 mA/cm.sup.2 is shown in FIG. 10
in which the film composition is CO.sub.65Fe.sub.24Ni.sub.11. As
can be seen in FIG. 10, both fcc and bcc phases can be co-deposited
from the newly developed bath.
[0073] TEM bright field and dark field images (FIG. 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
[0074] The magnetic properties of representative CoFeNi films
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.sub.c) 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 Bs 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, U.S. Pat. No.
4,756,816 and Yu, W., Bain, J. A., Peng, Y. and Laughlin, D. E.
IEEE Trans. Magn. 2002, 38, 3030).
[0075] 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 invention is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
[0076] 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.
1TABLE 1 Composition of bath for electroplating CoFeNi alloys
Chemical Concentration Chemical Concentration CoSO.sub.4
0.03-0.0875 M H.sub.3BO.sub.3 0.4 M FeSO.sub.4 0.005-0.045 M Sodium
lauryl 0.01 g/L sulfate NiSO.sub.4 0.2 M NH.sub.4Cl 0.28 M Bath pH
= 2.5-3.0
[0077]
2TABLE 2 Composition of citrate-based bath for electroplating
CoFeNi thin films Chemical Concentration Chemical Concentration
CoSO.sub.4 0.08 M H.sub.3BO.sub.3 0.4 M FeSO.sub.4 0.015 M Sodium
lauryl 0.01 g/L sulfate NiSO.sub.4 0.3 M Ammonium citrate 0.206 M
Bath pH = 5.3 (natural)
[0078]
3TABLE 3 Bath stability tests on baths with and without addition of
citrate Bath composition pH Stability 0.08 M CoSO.sub.4 5.3 Plated
bath was transparent 0.015 M FeSO.sub.4 (natural) after more than
one month. 0.3 M NiSO.sub.4 Plating results were 0.4 M
H.sub.3BO.sub.3 repeatable after 6 days. 0.01 g/L sodium lauryl
sulfate 0.206 M (NH.sub.4).sub.3(C.sub.6H.sub.5O.sub.7) 0.08 M
CoSO.sub.4 5.3 Precipitate appeared in bath 0.015 M FeSO.sub.4
(natural) within 2 hours during plating. 0.3 M NiSO.sub.4 0.4 M
H.sub.3BO.sub.3 0.01 g/L sodium lauryl sulfate 0.28 M NH.sub.4Cl
0.08 M CoSO.sub.4 2.7 Precipitate appeared in plated 0.015 M
FeSO.sub.4 (pH adjusted bath after less than 2 days. 0.3 M
MSO.sub.4, with 0.4 M H.sub.3BO.sub.3 dilute H.sub.2SO.sub.4) 0.01
g/L sodium lauryl sulfate 0.28 M NH.sub.4Cl
[0079]
4TABLE 4 Equilibrium Potentials of Selected Electrochemical
Electrodes* Electrochemical electrode Equilibrium potential (V)
H.sup.+/H.sub.2 0 Ni.sup.2+/Ni -0.23 Co.sup.2+/Co -0.28
Fe.sup.2+/Fe -0.44 *Andricacos, P. C. and Robertson, N. IBM J. Res.
Develop. (Electrochemical Microfabrication), 1998, 42, 671.
[0080]
5TABLE 5 Magnetic properties of representative CoFeNi films plated
from a low pH bath and the newly developed bath Saturation flux
Film Coercivity density B.sub.s Plating bath composition Hc (Oe)
(Tesla) Low pH bath Co.sub.64Fe.sub.24Ni.sub.12 1.5 2.01 (pH 2.7)
Co.sub.65Fe.sub.24Ni.sub.11 5.5 1.91 Co.sub.60Fe.sub.29Ni.sub.11 18
1.84 Newly Co.sub.68Fe.sub.22Ni.sub.10 11 2.03 developed bath
Co.sub.64Fe.sub.26Ni.sub.10 15 2.10 (pH 5.3)
* * * * *