Title: Method for electroplating of tantalum
Abstract: The present invention provides a method of electrolytically forming a tantalum film by using a molten-salt electrolytic bath having a low temperature. The electrolytic bath includes a molten salt containing tantalum pentachloride, alkylimidazolium chloride, and an alkali metal or alkali earth metal fluoride such as lithium fluoride.
Patent Number: 6,936,155 Issued on 08/30/2005 to Morimitsu, et al.
| Inventors:
|
Morimitsu; Masatsugu (Kitakyushu, JP);
Matsunaga; Morio (Kitakyushu, JP)
|
| Assignee:
|
Japan Science and Technology Agency (Kawaguchi, JP)
|
| Appl. No.:
|
239836 |
| Filed:
|
November 8, 2000 |
| PCT Filed:
|
November 8, 2000
|
| PCT NO:
|
PCT/JP00/07835
|
| 371 Date:
|
May 30, 2003
|
| 102(e) Date:
|
May 30, 2003
|
| PCT PUB.NO.:
|
WO01/75193 |
| PCT PUB. Date:
|
October 11, 2001 |
Foreign Application Priority Data
| Mar 30, 2000[JP] | 2000-97861 |
| Current U.S. Class: |
205/230 |
| Intern'l Class: |
C25D 003/66 |
| Field of Search: |
205/230,234
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
Patent Abstracts of Japan, Publication No. 07-118888, dated May 9, 1995.
Patent Abstracts of Japan, Publication No. 06-057479, dated Mar. 1, 1994.
F. Lantelme et al.; Electrodeposition of Tantalum in NaCl-KCl-K2TaF7
Melts, J. Electrochem Soc., vol. 139, No. 5, pp. 1249-1255, May 1992.
Morio Matsunaga et al.; Proceedings of the seventh China-Japan Bilateral Conference
on Molten Salt Chemistry and Technology, pp. 209-213, Oct. 26-30, 1998, Xian, China.
M. Morimitsu et al.; Electrochemical Behaviors of TaCl5-EMIC Molten
Salts, The Electrochemical Society of Japan, p. 233, 1998.
T. Matsuo et al; Electrochemical Reduction of Ta(V) in TaCl5-EMIC
Low Temperature Molten Salts (II), The 31st . Symposium on Molten Salt
Chemistry, pp. 35-52.
|
Primary Examiner: King; Roy
Assistant Examiner: Leader; William T.
Attorney, Agent or Firm: Westerman, Hattori, Daniels & Adrian, LLP
Claims
1. A method for electrolytically forming a tantalum film, comprising the steps of:
providing molten-salt a electrolytic bath containing tantalum pentachloride,
alkylimidazolium chloride, and an alkali metal or alkali earth metal fluoride;
providing an anode and a cathode in said electrolytic bath; and
supplying electric current between said anode and said cathode.
2. A method as defined in claim 1, wherein the alkylimidazolium chloride is 1-ethyl-3-methylimidazolium chloride.
3. A method as defined in claim 1, wherein the alkali metal or alkali earth metal
fluoride is lithium fluoride.
4. A molten-salt electrolytic bath for forming a tantalum film, comprising:
tantalum pentachloride;
alkylimidazolium chloride; and
an alkali metal or alkali earth metal fluoride.
5. The molten-salt electrolytic bath as defined in claim 4, wherein the alkylimidazolium
chloride is 1-ethyl-3-methylimidazolium chloride.
6. The molten-salt electrolytic bath as defined in claim 4, wherein the alkali
metal or alkali earth metal fluoride is lithium fluoride.
Description
TECHNICAL FIELD
The present invention relates to a method of electrolytically forming a tantalum
film onto metal, alloy, conductive ceramics, semiconductive ceramics or the like
by using a molten-salt electrolytic bath.
BACKGROUND ART
Tantalum is used in a wide range of fields such as electrolytic capacitors,
materials of electronic products and materials of chemical devices, by taking advantage
of its properties such as a high melting point, a sufficient ductility and malleability
and an excellent corrosion resistance. Depending on an intended purpose, tantalum
is directly used as a material of such products or devices in some cases, and otherwise
used in the form of a tantalum film on a base material, e.g. a tantalum thin-film
formed as a barrier film on a copper wiring of a LSI.
Various physical and chemical vapor deposition methods such as a vacuum deposition
method and a spattering method have been used for forming the tantalum film. In
terms of commonly-used methods of forming a film, it is contemplable to use a plating
method instead of the above dry type methods. However, it is practically impossible
to form a tantalum film through a plating method using an aqueous solution, and
a plating method using a molten-salt bath has been known as the only way to form
a tantalum film.
For example, there has been reported a method using a molten-salt bath comprising
lithium chloride-potassium chloride molten salt having tantalum pentachloride added
thereto to allow a tantalum film to be electrolytically formed at a molten-salt
bath temperature of 450° C., and a method using a molten-salt bath comprising
potassium chloride-sodium chloride molten salt having K
2TaF
7
added thereto to allow a tantalum film to be electrolytically formed at a molten-salt
bath temperature of about 700° C. (J. Electrochem. Soc., Vol. 139, No. 5,
May 1992, pp. 1249-1255).
Further, Japanese Patent Laid-Open Publication No. 06-057479 discloses a
method of plating tantalum onto an object such as iron by using a tantalum plate
as an anode and an electrolytic bath which comprises a molten-salt consisting of
a fluoride eutectic mixture of LiF-NaF-KF (50-30-20 mol %) and K
2TaF
7
additionally dissolved therein and has a temperature of 600 to 900° C., and
periodically reversing the direction of a supply current.
However, the above conventional tantalum plating methods are based on a
high-temperature molten-salt bath, and there has not developed any molten-salt
electrolytic bath capable of providing an electrolytically formed tantalum film
at a low electrolytic bath temperature, e.g. room temperature or about 100° C.
DISCLOSURE OF THE INVENTION
(Problem to be Solved by the Invention)
For the aforementioned reasons, the dry type methods are predominantly used to
form a tantalum film, but undesirably place a limit on the size and/or thickness
of the obtained tantalum film. While the plating method is advantageously free
from such limitations, any tantalum-film forming method using the conventional
molten salts has not been commercially used due to unsatisfactory workability,
safety and/or cost resulting from the high-temperature and highly-reactive molten-salt bath.
It is therefore an object of the present invention to provide a method of electrolytically
forming a tantalum film by using a low-temperature electrolytic bath, so as to
overcome the above disadvantages.
(Means for Solving the Problem)
The inventors have researched electrode reaction of metal ion using various kinds
of molten salts. Through the research, it was found that a molten salt was formed
at a temperature of 100° C. or less by mixing tantalum pentachloride and 1-ethyl-3-methylimidazolium
chloride. This finding has been reported ("Abstracts of technical papers in the
autumn meeting 1998 of the Electrochemical Society of Japan", p. 233, 1998; "Proc.
of the 7th China-Japan Bilateral Conf. on Molten Salt Chem. and Technol." pp. 209-213, 1998).
Further, the inventors found that a tantalum film could be electrolytically
formed even at a low electrolytic bath temperature through the use of a molten
salt prepared by adding an alkali metal or alkali earth metal fluoride into the
molten salt having the above composition. This knowledge opened the way for realistically
achieving a tantalum plating method using a low temperature electrolytic bath,
and the present invention has been finally made.
Specifically, the present invention provides a method of electrolytically
forming a tantalum film by using an electrolytic bath which comprises a molten
salt containing tantalum pentachloride, alkylimidazolium chloride, and an alkali
metal fluoride or alkali earth metal fluoride.
In the present invention, the alkylimidazolium chloride may be 1-ethyl-3-methylimidazolium chloride.
Further, the alkali metal fluoride or alkali earth metal fluoride may be
lithium fluoride.
The alkylimidazolium chloride used in the method of the present invention may
include 1-methyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride,
1-ethyl-3-ethylimidazolium chloride, 1-methyl-3-propylimidazolium chloride, 1-methyl-3-butylimidazolium
chloride, and 1-butyl-3-butylimidazolium chloride, but the alkyl group and combination
thereof are not limited to the above examples.
The alkali metal fluoride or alkali earth metal fluoride used in the method of
the present invention preferably has a relatively strong ionic bonding and a capability
to provide a fluoride ion readily in a molten-salt bath. Specifically, the alkali
metal fluoride or alkali earth metal fluoride may include lithium fluoride, sodium
fluoride, potassium fluoride, beryllium fluoride, magnesium fluoride, and calcium
fluoride. In particular, lithium fluoride has a strong ionic bonding and a capability
to generate a fluoride ion readily in a molten-salt bath and provide a lithium
ion which is a cation having a small ionic radius.
While a detailed mechanism has not been figured out, it can be assumed that
a part or all of chloride ions coordinated at tantalum in the molten salt are substituted
with fluoride ions, which provides a deteriorated symmetry in the electrical structural
of a pentavalent tantalum complex in the molten salt, and the pentavalent tantalum
complex having the resultingly increased reduction property is deoxidize and formed
in zero-valent tantalum allowing a tantalum film to be electrolytically formed.
The cation having a small ion radius such as lithium ion in the molten salt would
further deteriorate the electronic symmetry in the structure of the tantalum complex
to facilitate deoxidization of the tantalum complex. From the above reasons, lithium
fluoride is more desirable than other alkali metal fluorides or alkali earth metal fluorides.
FIG. 1 is a cyclic voltammogram obtained by using a platinum electrode in an
electrolytic bath comprising a molten salt which contains tantalum pentachloride,
1-ethyl-3-methylimidazolium chloride and lithium fluoride (with a mixing ratio
of 30:60:10 mol % in order of the description), at an electrolytic bath temperature
of 100° C.
In FIG. 1, four reduction waves A, B, C and D observed during a potential scan
to the cathode direction correspond to reduction of pentavalent tantalum complex,
and show that the reduction from the pentavalent tantalum complex to zero-valent
tantalum is caused in the molten salt under at least four stages.
By contrast, in a cyclic voltammogram obtained when lithium fluoride is excluded
from the above molten salt, only the waves A and B were observed. Thus, it is proved
that the reduction for finally forming the zero-valent tantalum or providing an
electrolytically formed tantalum film is caused at the stages corresponding to
the waves C and D which are caused newly by adding the lithium fluoride.
Preferably, the tantalum pentachloride, alkylimidazolium chloride and
alkali metal or alkali earth metal fluoride are mixed at the following ratio. As
to two components of the tantalum pentachloride and the alkylimidazolium chloride,
the mole ratio of the tantalum pentachloride to the two components is preferable
from 30 mol % to 50 mol %, and the mole ratio of the alkylimidazolium chloride
to the two components is preferably from 50 mol % to 70 mol %. The mol ratio of
the alkali metal or alkali earth metal fluoride to the total mole number of the
tantalum pentachloride and the alkylimidazolium chloride is preferably from 2 mol
% to 13 mol %.
If the tantalum pentachloride is less than 30 mol % or the alkylimidazolium chloride
is greater than 70 mol %, a melting point for forming the molten salt will undesirably
go up, and thereby the molten salt will not be formed at a low temperature of 100°
C. or less. Similarly, if the tantalum pentachloride is greater than 50 mol % or
the alkylimidazolium chloride is less than 50 mol %, the melting point for forming
the molten salt will undesirably go up, and thereby the molten salt will not be
formed at 100° C. or less.
On the other hand, if the alkali metal or alkali earth metal fluoride is less
than 2 mol %, the deoxidizing effect for providing the zero-valent tantalum will
not be obtained due to an insufficient ratio of the fluoride, and thereby it will
be difficult to electrolytically form a tantalum film. If the alkali metal or alkali
earth metal fluoride is greater than 13 mol %, a part of the fluoride cannot be
dissolved and left in the molten salt as a solid substance or unprofitable fluoride
which does not act as the molten salt.
More preferably, as to two components of the tantalum pentachloride and the
alkylimidazolium chloride, the mole ratio of the tantalum pentachloride to the
two components is preferable from 33.3 mol % to 45 mol %, and the mol ratio of
the alkylimidazolium chloride to the two components is preferably from 66.7 mol
% to 55 mol %. The mol ratio of the alkali metal or alkali earth metal fluoride
to the total mole number of the tantalum pentachloride and the alkylimidazolium
chloride is preferably from 5 mol % to 10 mol %.
A representative example of electrolysis conditions will be described below. A
cathode may be made of various materials such as metal, alloy, conductive ceramics
or semiconductive ceramics. For example, the material of the cathode may include,
but not limited to, an iron material, nickel and copper. A thin-film-shaped metal,
alloy or ceramics formed on a different material may also be used as the cathode.
An anode may be formed of, but not limited to, a plate-shaped material made of
tantalum, tungsten, molybdenum, platinum or the like, or a thin-film-shaped material
made of these materials and formed on a different material.
When the anode is made of tantalum, an anodic reaction will be dominated by
dissolution of the tantalum. In contrast, when using other material, the anodic
reaction will be dominated by generation of chlorine. In this case, it is desirable
to use an anode material having a low overvoltage and a high durability for the
reaction of generating chlorine.
A current density may be arranged, for example in the range of 0.01 A/cm
2
to 1 A/cm
2, based on a value used in a conventional electroplating.
However, the current density is not limited to the above range, and should be selectively
arranged depending on whether the electrolytic bath is used in a static or flowing
state, or other factors such as a molten-salt bath temperature or a current waveform,
because an adequate value of the current density is varied in response to such factors.
In addition to a current control method, a potential control method may be used
to supply current during plating. In the current control method, various kinds
of current waveforms such as a constant current and a pulse current may be used,
or a reverse current may be periodically applied. Similarly, in the potential control
method, various kinds of voltage waveforms such as a constant voltage and a pulse
voltage may be used, or a reverse voltage may be periodically applied.
The molten-salt bath temperature is preferably 150° C. or less, more preferably
100° C. or less. The temperature of greater than 150° C. causes decomposition
of the alkylimidazolium chloride, resulting in undesirably accelerated deterioration
of the molten salt. According to the plating method of the present invention under
the above electrolysis conditions, a tantalum film having a thickness of up to
about 100 μm can be electrolytically formed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cyclic voltammogram obtained by using an electrolytic bath comprising
a molten salt containing tantalum pentachloride and 1-ethyl-3-methylimidazolium
chloride which have a mole ratio of 1:2, and lithium fluoride added thereto.
FIG. 2 is a graph showing an X-ray diffraction pattern of a tantalum film electrolytic
formed through the method of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
(Examples 1 to 4), (Comparative Examples 1 to 2)
Examples of the present invention (inventive examples) will now be described
in detail in contrast to comparative examples. In a glove box under inert gas atmosphere,
tantalum pentachloride (TaCl
6), 1-ethyl-3-methylimidazolium chloride
(EMIC) and lithium fluoride (LiF) were scaled for each of the inventive examples
according to respective given amounts as shown in Table 1. A set of the materials
for each of the inventive examples was put in a glass tube, and the respective
glass tubes were depressurized and sealed. Then, each of the glass tubes was heated
up to 100° C. to form a molten salt.
| |
TABLE 1 |
| |
| |
Composition of Molten Salt |
Presence of |
| |
|
1-ethyl-3- |
|
formed tantalum film |
| |
Tantalum |
methylimidazolium |
|
(after 4 hours from |
| |
pentachloride |
chloride |
lithium fluoride |
current supply) |
| |
| Inventive |
30 mol % |
60 mol % |
10 mol % |
Formed |
| Example 1 |
| Inventive |
45 mol % |
45 mol % |
10 mol % |
Formed |
| Example 2 |
| Inventive |
33 mol % |
65 mol % |
2 mol % |
Formed |
| Example 3 |
| Inventive |
49 mol % |
49 mol % |
2 mol % |
Formed |
| Example 4 |
| Comparative |
33.3 mol % |
66.7 mol% |
W/O |
None |
| Example 1 |
| Comparative |
50 mol % |
50 mol % |
W/O |
None |
| Example 2 |
In the glove box, each of the molten salts was transferred into a glass cell
having
a pair of platinum electrodes, and the respective glass cells were depressurized
and sealed. Each of the glass cells was placed in an electric furnace, and an electrolytic
process was performed by supplying a constant current from a power supply at a
molten salt temperature of 100° C. After continuing the electrolytic process
by supplying the current for four hours, each surface of the platinum electrodes
used as a cathode was rinsed out, and then analyzed by an X-ray diffractometer
to check whether a tantalum film was electrolytically formed. FIG. 2 is a graph
showing an X-ray diffraction pattern of one of the formed tantalum film. FIG. 2
shows diffraction peaks of the platinum used as the cathode and diffraction peaks
of tantalum. This proves that a tantalum film is electrolytically formed on the cathode.
On the other hand, molten salts each having a composition as shown in Table 1
or composed of a mixture of tantalum pentachloride and 1-ethyl-3-methylimidazolium
chloride were prepared as comparative examples, and the same operations as those
in the inventive examples were performed. The presence of an electrolytically formed
tantalum film in each of the inventive and comparative examples is shown in Table 1.
As can be seen from the results of Table 1, the plating method of the present
invention allows a tantalum film to be electrolytically formed even at a low molten-salt
bath temperature of 100° C. at which the conventional plating method has not
been able to form a tantalum film. Further, it was verified that the inventive
examples 1 and 3 each having a small mole ratio of tantalum pentachloride could
electrolytically form a tantalum film through an electrolytic process at room temperature.
INDUSTRIAL APPLICABILITY
According to the present invention, a tantalum film can be electrolytically
formed in a molten-salt electrolytic bath even at a low bath temperature of 100°
C. or less. Thus, the present invention can provide a tantalum-film forming method
significantly advantageous to workability, safety and/or cost.
*
