CA2461107A1 - Solution for the electroplating of soft magnetic co-fe-ni alloys - Google Patents
Solution for the electroplating of soft magnetic co-fe-ni alloys Download PDFInfo
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- CA2461107A1 CA2461107A1 CA002461107A CA2461107A CA2461107A1 CA 2461107 A1 CA2461107 A1 CA 2461107A1 CA 002461107 A CA002461107 A CA 002461107A CA 2461107 A CA2461107 A CA 2461107A CA 2461107 A1 CA2461107 A1 CA 2461107A1
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/56—Electroplating: Baths therefor from solutions of alloys
- C25D3/562—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of iron or nickel or cobalt
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/18—Electroplating using modulated, pulsed or reversing current
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/60—Electroplating characterised by the structure or texture of the layers
- C25D5/615—Microstructure of the layers, e.g. mixed structure
- C25D5/617—Crystalline layers
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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.
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.
Description
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 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.
, CA 02461107 2004-03-15 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 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~.
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 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.
, CA 02461107 2004-03-15 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 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~.
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 . CA 02461107 2004-03-15 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.
- $ -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.
_g_ 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.
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 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 , CA 02461107 2004-03-15 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).
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.
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 (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 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.
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 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) 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
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.
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)
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 . CA 02461107 2004-03-15 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.
- $ -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.
_g_ 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.
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 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 , CA 02461107 2004-03-15 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).
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.
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 (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 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.
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 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) 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
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.
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)
Claims (30)
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 any one of claims 1-4, 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 any one of claims 1-4, wherein the salt of Ni is NiSO4.
7. The Co-Fe-Ni plating solution according to claim 6, wherein NiSO4 has a concentration of about 0.3M.
8. The Co-Fe-Ni plating solution according to any one of claims 1-8, 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 any one of claims 1-9, wherein the salt of Co is CoSO4.
10. The Co-Fe-Ni plating solution according to claim 9, wherein CoSO4 has a concentration of about 0.08M.
11. The Co-Fe-Ni plating solution according to any one of claims 1-10, 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 any one of claims 1-11, wherein the salt of Fe is FeSO4.
13. The Co-Fe-Ni plating solution according to claim 12, wherein FeSO4 has a concentration of about 0.015M.
14. The Co-Fe-Ni plating solution according to any one of claims 1-13, 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 any one of claims 1-14, 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 any one of claims 1-17, 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.
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 H3BO3.
buffering agent is H3BO3.
21. The Co-Fe-Ni plating solution according to claim 20, wherein H3BO3 has a concentration of about 0.4M.
22. The Co-Fe-Ni plating solution according to any one of claims 1-21, 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 0.01g/L to about 0.05g/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 any one of claims 1 to 25; and (c) applying a plating current.
(a) providing a substrate to be plated;
(b) immersing the substrate in a Co-Fe-Ni plating solution according to any one of claims 1 to 25; 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 10ms with 0.3ms of on-time (t on) and 9.7ms of off time.
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| US11/079,071 US20050211563A1 (en) | 2004-03-15 | 2005-03-15 | Solution for the electroplating of soft magnetic Co-Fe-Ni alloys |
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