Electrochemical alloying of copper substrate with tin using ionic liquid as an electrolyte at medium-low temperatures

Article abstract of DOI:10.1149/1.2776230

Electrochemical alloying of Cu substrates through a reduction-diffusion method was investigated using an ionic liquid, trimethyl- n -hexylammonium bis[(trifluoromethyl)sulfonyl]amide, as a solvent for an electrolytic bath. The use of the ionic liquid made it possible to raise the processing temperature beyond 100°C and form the Cu-Sn layers faster than with an aqueous media. The layers obtained from a Cu thin layer under a potentiostatic condition were silver-gray speculum metal composed of Cu6 Sn5, Cu3 Sn, and Cu10 Sn3 intermetallic phases, while those prepared under a galvanic contact condition involved a Β-Sn phase containing a trace amount of copper. The formation of each Cu-Sn phase is discussed in terms of alloy formation thermodynamics.

Full text of DOI:10.1149/1.2776230

D612
Journal of The Electrochemical Society, 154 ͑11͒ D612-D616 ͑2007͒
0
013-4651/2007/154͑11͒/D612/5/$20.00 © The Electrochemical Society
Electrochemical Alloying of Copper Substrate with Tin Using
Ionic Liquid as an Electrolyte at Medium-Low
Temperatures
,
z
Kuniaki Murase,* Ryoichi Kurosaki, Takuma Katase, Hiroyuki Sugimura,
Tetsuji Hirato, and Yasuhiro Awakura
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
Electrochemical alloying of Cu substrates through a reduction-diffusion method was investigated using an ionic liquid, trimethyl-
n-hexylammonium bis͓͑trifluoromethyl͒sulfonyl͔amide, as a solvent for an electrolytic bath. The use of the ionic liquid made it
possible to raise the processing temperature beyond 100°C and form the Cu–Sn layers faster than with an aqueous media. The
layers obtained from a Cu thin layer under a potentiostatic condition were silver-gray speculum metal composed of Cu Sn ,
6
5
Cu Sn, and Cu Sn intermetallic phases, while those prepared under a galvanic contact condition involved a ␤-Sn phase con-
3
10
3
taining a trace amount of copper. The formation of each Cu–Sn phase is discussed in terms of alloy formation thermodynamics.
2007 The Electrochemical Society. ͓DOI: 10.1149/1.2776230͔ All rights reserved.
©
Manuscript submitted May 22, 2007; revised manuscript received July 5, 2007. Available electronically September 11, 2007.
The electrodeposition of alloys and compounds is a key technol-
hydrophobic ionic liquid, trimethyl-n-hexylammonium bis͓͑trifluo-
ogy for thin-layer processing in the fields of, for example, surface
finishing and electronic materials. Cu–Sn alloy coating with compo-
sition of 40–60 wt % Sn, called “speculum metal,” is silver-gray
and has been investigated as promising alternative to an allergenic
romethyl͒sulfonyl͔amide ͑TMHA–Tf N͒, as the solvent for the gal-
vanic contact formation of the Cu–Sn layer, and it was demon-
2
8
strated that the use of this ionic liquid rendered it possible to raise
the processing temperature up to 150°C, thus forming alloy layers
faster than with aqueous media ͑Fig. 2͒. The growth rate of the alloy
layer increased with increasing bath temperature, and the rate at
140–150°C was more than three times that at 90°C in aqueous or
TMHA–Tf2N media. The data plotted in Arrhenius fashion ͑Fig. 2b͒
gave a single straight line with an apparent activation energy of
1-3
nickel coating, which is used, for example, as an underlayer for
chromium or gold electroplating. The layer also has potential as a
4-6
novel anode material for lithium batteries. In both cases, an elec-
troplating technique will provide a useful route to form the Cu–Sn
layer. Although electrodeposition technology is generally considered
to be a “soft solution process,” the waste bath treatment sometimes
involves cumbersome and complicated procedures. Especially, the
treatment of conventional alloy deposition bath, entropy of which is
high, because the bath contains two or more metal components, is
−
1
58 kJ mol , indicating that, in both aqueous and TMHA–Tf2N me-
dia, the rate-determining step of the growth is the solid-phase inter-
diffusion of Cu and Sn atoms in the growing Cu–Sn layer.
8
In the previous study, we employed a bulk Cu sheet ͑thickness,
7
0
.2 mm͒ as a Cu substrate for alloying. In this case, only the surface
energy-consuming in some cases. In 2003, Fujiwara demonstrated
of the sheet was converted to Cu–Sn alloy and the interior of the
sheet remained unalloyed. To develop the processes in Fig. 1b and c,
for example, the complete alloying of a thin copper layer should be
developed so as to ensure the long-term thermodynamic stability of
the resulting Cu–Sn layer. In the present study, we tried using a thin
Cu layer ͑thickness, 0.5 ␮m͒, electrodeposited from an aqueous so-
lution, as a substrate for Cu–Sn alloying and discuss the thermody-
namics of alloy formation.
that a Cu–Sn speculum metal layer can be electrochemically formed
on a Cu substrate by a reduction-diffusion ͑RD͒ method using an
electrochemical contact immersion, or galvanic contact, process,
where the substrate is immersed into an aqueous citrate bath con-
taining only Sn͑II͒ ions and is short circuited to an auxiliary metallic
Sn electrode dipped in the same bath. Here, the Cu substrate and Sn
electrode constitute a kind of short-circuited galvanic cell, where the
former acts as a cathode and the latter as an anode. By applying this
technique, it is thought that a Cu–Sn alloy layer may be obtained on
any conductive or nonconductive substrate through electrodeposi-
tion or electroless deposition of a Cu layer from appropriate baths
containing only Cu͑II͒ ions followed by the RD process to give a
Cu–Sn alloy. Figure 1, for example, compares the scheme of con-
ventional decorative chromium electroplating process using nickel
underlayer to that using Cu–Sn underlayer prepared through the RD
alloying on both conductive and nonconductive substrates. Although
this requires two steps to obtain the Cu–Sn alloy layer, the consecu-
tive use of baths, each containing a single-metal component, makes
the waste bath treatment simple and easy.
Experimental
The ionic liquid, TMHA–Tf N, was prepared by the metathesis
2
of trimethyl-n-hexylammonium bromide ͑TMHABr; Tokyo Kasei
Kogyo Co., Ltd., 98%͒ with lithium bis͓͑trifluoromethyl͒sulfonyl͔
amide ͑LiNTf ; Fluka, 99%͒ as
2
n-C H ͑CH ͒ NBr + LiN͑SO CF ͒
6
13
3 3
2
3 2
+
−
→
n-C H ͑CH ͒ N N ͑SO CF ͒ + LiBr
͓1͔
6
13
3 3
2
3 2
After drying the resulting TMHA–Tf2N in vacuo for more than 2 h
at 120°C in a flask connected to a liquid N2 trap, the residual water
contents, checked by coulometric Karl-Fischer method, were
Ͻ50 ppm. Tin͑II͒ salt, Sn͑Tf2N͒2, was prepared under ambient at-
mospheric conditions using the acid-base reaction of SnO ͑Nacalai
Tesque Inc.͒ with bis͓͑trifluoromethyl͒sulfonyl͔amine ͑HNTf2,
Fluka, 99%͒ as
For the continuous growth of Cu–Sn alloy by the RD process,
rapid diffusion of deposited Sn atoms into the Cu substrate to form
the alloy phase is essential in order to keep the activity of tin, a , in
Sn
the alloy less than unity ͑i.e., aSn Ͻ 1͒. In other words, the diffusion
of Sn atoms is a rate-determining step in this process and, hence, a
7
relatively high process temperature is required. As long as aqueous
or volatile organic baths, if any, are employed, however, a process
temperature higher than 100°C is technically difficult in the ambient
atmosphere without a special vessel, such as an autoclave. In con-
trast, more recently, we tried using a nonvolatile and nonflammable
SnO + 2HNTf → Sn͑Tf N͒ + H O
͓2͔
2
2
2
2
9
The detailed procedure was described in our previous paper. Note
that the resulting Sn͑Tf2N͒2 salt may contain residual water and/or
HNTf . In the present RD process, however, these residues, if any,
2
2
+
have an insignificant effect on the reduction of Sn to Sn, since the
2
+
potential of Sn /Sn redox couple ͑ESn2+/Sn͒ is not that negative. It is
*
Electrochemical Society Active Member.
z
E-mail: kuniaki.murase@a0017299.mbox.media.kyoto-u.ac.jp
also expected that most of these volatile residues spontaneously va-
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Journal of The Electrochemical Society, 154 ͑11͒ D612-D616 ͑2007͒
D613
Figure 1. Chromium electroplating using
a͒ Ni underlayer ͑conventional process͒
and ͑b, c͒ Cu–Sn underlayer ͑alternative
process͒ where copper layers for RD al-
loying are ͑b͒ electrodeposited on a con-
ductive substrate or ͑c͒ electroless depos-
͑
ited on
substrate.
a nonconductive, e.g., resin,
porize away when the bath for alloying is operated at a medium-low
temperatures, e.g., 130°C.
used for the same set of Cu substrate ͑cathode͒, Sn sheet ͑anode͒,
and Sn rod ͑reference͒. For both electrochemical setups, the bath
was thermostated at 130°C ͑403 K͒ and agitated at ϳ500 rpm using
a magnetic stirring unit. The reaction time for the alloying was kept
at 72 h, if not otherwise specified.
The surface morphology of the resulting Cu–Sn layers was ex-
amined by scanning electron microscope ͑Keyence VE-7800͒. The
average compositions of the Cu–Sn layers were determined by elec-
tron probe microanalysis ͑EPMA͒ using Hitachi S-3500-H spec-
trometer. The X-ray diffraction ͑XRD͒ measurements of the layer
were carried out with a scan mode of 2␪-␪ using a Rigaku RINT
2200 X-ray diffractometer fitted with a copper X-ray tube.
In order to prepare the Sn͑Tf N͒ –TMHA–Tf N solution, that is,
2
2
2
the bath for alloying, the metal salt Sn͑Tf N͒ was weighed in a
2
2
glove box and dissolved into the TMHA–Tf N to yield a metal ion
2
−
3
concentration of 0.05 mol dm . The Cu–Sn alloying was
carried out under atmospheric conditions ͑not in the glove box͒.
To avoid the oxidation of Sn͑II͒ ions to Sn͑IV͒ state, the
Sn͑Tf N͒ –TMHA–Tf N bath was deaerated with nitrogen gas. A
2
2
2
thin copper layer ͑thickness, 0.5 ␮m͒ electrodeposited galvanostati-
cally at 60 mA cm−2 from a conventional room-temperature aque-
−
3
ous sulfate plating bath containing 0.8 mol dm
CuSO4 and
−
3
0
.5 mol dm H SO on a glassy carbon sheet ͑10 mm ϫ 20 mm;
2
4
Results and Discussion
thickness 0.5 mm͒ was used as the Cu substrate for the RD alloying.
In the present study, we employed two types of electrochemical
setups for the alloying: one was galvanic contact and the other used
a potentiostatic method. In the former case using the galvanic con-
tact, the Cu substrate, which acted as the cathode, was short cir-
cuited with a Sn sheet ͑10 mm ϫ 100 mm; thickness 1.5 mm͒, i.e.,
the anode, using a zero-shunt ammeter ͑Hokuto Denko, HM-104͒.
Here, a Sn rod ͑2 mm ϫ 1.5 mm ϫ 90 mm͒ immersed in the same
solution was used as a quasi-reference electrode to monitor the cath-
ode potential. For potentiostatic alloying, a conventional potentiostat
Galvanic contact alloying.— Although the galvanic contact al-
loying of the bulk Cu sheet yielded a uniform silver-gray Cu–Sn
8
layer with metallic luster, the alloying of a thin Cu layer under the
same condition gave a dark gray layer, with spongelike deposits at
the edges ͑Fig. 3͒. The potential of the substrate during the alloying
2+
was spontaneously kept between +2 mV to 0 mV vs Sn /Sn ͑Sn
rod͒, which was almost the same value for the galvanic contact
8
alloying of bulk Cu substrate at 130°C. Figure 4 shows the XRD
patterns of the resulting layers obtained by alloying for 72 h and
͑
Hokuto Denko, HA-151͒ with a three-electrode configuration was
2
40 h. Each pattern demonstrated that the layers were crystalline,
composed of Cu Sn , Cu Sn, and Cu Sn intermetallic phases like
6
5
3
10
3
8
the case of the bulk Cu sheet ͑note that three broad peaks, or halos,
at 2␪ of 20–30°, 40–60°, and 75–90° were due to a glassy carbon
substrate underneath͒. There was no diffraction of face-centered cu-
bic ͑fcc͒-Cu phase, i.e., the elemental Cu phase, indicating that all
Cu thin layers were converted to Cu–Sn alloys. Also of interest here
is that some amounts of the ␤-Sn phase were recognized at the
cathode. The emergence of spongelike morphology may well be
attributed to the deposition of the ␤-Sn phase. This, however,
seemed strange because, under the galvanic contact condition using
a pure ␤-Sn anode ͑aSn = 1͒, deposition of the pure ␤-Sn phase on
Cu substrate, i.e., the cathode, is thermodynamically impossible.
Here, the potential of the copper substrate is automatically kept
more positive than the potential for pure ␤-Sn deposition, and there-
fore, the activity of tin aSn deposited at the cathode should be less
than unity ͑aSn Ͻ 1͒.
To ascertain the cause of the deposition of the ␤-Sn phase, the
spongelike portion was carefully collected and detailed analysis of
the deposits was further carried out. Although the XRD pattern ͑Fig.
5͒ indicated that the spongelike portion was almost pure ␤-Sn with-
out any additional phase, the inductively coupled plasma atomic
emission spectrometry of the diluted H SO aqueous solution, in
which a fraction of the spongelike deposit was dissolved, revealed
that the ␤-Sn phase contained a trace amount of copper. In other
words, it is expected that the aSn of the ␤-Sn phase that formed on
the cathode was slightly less than unity, making the deposition ther-
modynamically feasible. The driving force behind the galvanic con-
Figure 2. Temperature dependence of the thickness of Cu–Sn layers plotted
a͒ on a linear scale and ͑b͒ in Arrhenius fashion; the layers were formed on
a bulk copper sheet by galvanic contact RD alloying for 72 h using
͑
2
4
−
3
0
.05 mol dm Sn͑Tf N͒–TMHA–Tf N solution ͑open circles; Ref. 8͒. The
2
2
thickness was determined from scanning electron microscope ͑SEM͒ images
of the cross section of the layers. The thickness of the layer from aqueous
solution ͑Ref. 7͒ is also indicated by filled circles; the data of Ref. 7 ͑tem-
perature 70°C, 80°C, and 90°C; alloying time 16 h͒ was recalculated for the
1
/2
alloying time of 72 h, assuming the parabolic low, i.e., thickness x = kt
where k is a constant.
,
tact alloying is the difference in a , or difference in potential
Sn
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D614
Journal of The Electrochemical Society, 154 ͑11͒ D612-D616 ͑2007͒
Figure 4. XRD patterns of Cu–Sn layers obtained by galvanic contact RD
0.05 mol dm−
3
alloying for ͑a͒ 72 h and ͑b͒ 240 h using
Sn͑Tf N͒ –TMHA–Tf N bath at 130°C.
a
2
2
2
Figure 3. Appearance of the Cu thin layer ͑a͒ before and ͑b͒ after Cu–Sn
−
3
alloying by galvanic contact for 72 h using
a
0.05 mol dm
Figure 6 depicts the appearance and surface SEM photos of the
resulting Cu–Sn layers. Each of the layers obtained in the potential
Sn͑Tf N͒ –TMHA–Tf N bath at 130°C. ͑c, d͒ Surface SEM images of the
2
2
2
spongelike portion of the layer ͑b͒. ͑d͒ is an enlargement of ͑c͒.
2
+
range +1 mV to +40 mV vs Sn /Sn grew uniformly and appeared
as silver-gray layers without the codeposition of a spongelike phase.
A set of XRD patterns showed that the layers were only composed
of intermetallic phases, Cu Sn , Cu Sn, and Cu Sn , and the ␤-Sn
6
5
3
10
3
ESn2+/Sn, between the growing Cu–Sn layer and auxiliary Sn anode
aSn = 1͒. Under such a condition, the system moves automatically
phase was not found ͑Fig. 7͒. As in the galvanic contact method, the
fcc-Cu phase was not detected either, indicating that the Cu layers
on the glassy carbon substrate were thoroughly converted to inter-
͑
in such a way that the aSn of the deposit on cathode is as close to 1
as possible within the range aSn Ͻ 1. On shortcircuiting, the Cu
layer is alloyed from the surface and, here, it is expected that the Sn
atoms are first dissolved into the fcc-Cu phase, where aSn lies at the
lowest possible level. Then, intermetallic phases, i.e., Cu Sn
1
0
3
͑
36 wt % Sn͒, Cu Sn ͑38 wt % Sn͒, and Cu Sn ͑61 wt % Sn͒, with
3 6 5
higher aSn values start to grow. It is important that, even after the
conversion of the whole layer to these intermetallic phases, there
remains some potential difference between the cathode and anode,
and the next phase with the highest possible aSn value, that is the
␤
-Sn phase containing Cu atoms, can be grown on the surface. In
8
the case of galvanic contact alloying of the bulk Cu sheet, we have
never experienced formation of the ␤-Sn phase on the cathode,
probably because the thick Cu sheet was not completely alloyed
within 72 h.
Potentiostatic alloying.— It turned out that the RD alloying of
thin Cu layer is possible using the galvanic contact method, but the
resulting layer involved a spongelike ␤-Sn phase. As described
above, the cathode potential of the Cu substrate during the galvanic
2
+
contact condition lies between +2 mV and 0 mV vs Sn /Sn, and
such a knife-edge potential might be a cause for the ␤-Sn deposi-
tion. To avoid deposition of the ␤-Sn phase, therefore, we tried
alloying using a potentiostatic condition at more positive potentials.
Figure 5. XRD pattern of spongelike portion formed at the edge of the
Cu–Sn layers obtained by galvanic contact for 72 h using a 0.05 mol dm−
Sn͑Tf N͒ –TMHA–Tf N bath at 130°C.
3
2
2
2
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Journal of The Electrochemical Society, 154 ͑11͒ D612-D616 ͑2007͒
D615
Figure 6. Appearance of the thin Cu layer
a͒ before and ͑b,c͒ after Cu–Sn alloying
by the potentiostatic method for 72 h us-
ing a 0.05 mol dm− Sn͑Tf N͒ –TMHA
͑
3
2
2
–
Tf N bath at 130°C. Surface SEM im-
2
ages of the layer ͑d͒ before and ͑e–g͒ after
the alloying are also indicated. The poten-
tials for alloying were ͑b,e͒ +5 mV, ͑c,f͒
2
+
+
10 mV, and ͑g͒ +20 mV vs Sn /Sn.
Lower SEM images are enlargements of
the upper ones.
metallic compounds. It should be noted that resulting Cu–Sn inter-
metallic phases depended on the applied potential for alloying. The
abundance of the Sn-rich phase, i.e., Cu Sn , whose tin activity a
is the highest of the three, increased with decreasing potential for
alloying. In other words, a higher aSn phase͑s͒ was deposited at
relatively lower potentials, while a lower aSn phase͑s͒ was deposited
at higher potentials. When assessed from the XRD patterns, the
galvanic contact and potentiostatic alloying at 130°C. The main
point of interest here is that Cu–Sn alloying under the potentiostatic
condition was self-terminated and currents were automatically re-
duced to zero within 72 h, while that under the galvanic contact
method did not stop even after 72 h of reaction. In the case of
potentiostatic alloying, as mentioned above, each potential corre-
sponds to the activity aSn of the phase to be deposited and, therefore,
6
5
Sn
main phase was Cu Sn5 at +5 mV, while the layers obtained at
the current stops on completion of the phase with given a . In
6
Sn
+
10 mV and +20 mV were composed mainly of the Cu Sn and
contrast, galvanic contact alloying of both bulk Cu sheet and thin Cu
layer did not stop within 72 h. The former was due to the presence
of an unalloyed fcc-Cu phase to which Sn atoms can further diffuse.
The latter is probably because the formation of ␤-Sn phase was not
completed within this time period. The higher current density ob-
tained during the alloy formation while thin Cu layer was used is
also attributed to the formation of spongelike ␤-Sn phase with a
larger surface area.
3
Cu Sn phases. This behavior is also supported by a set of average
1
0
3
composition data obtained by EPMA for each of the layers formed
at five different potentials between +1 mV and +40 mV ͑Fig. 8͒.
The average compositions of the layers obtained at +1 mV and
+
5 mV were close to the Cu Sn ͑45.5 atom % Sn͒ phase, while
6 5
those at 10–40 mV were close to the Cu Sn ͑25.0 atom % Sn͒ or
Cu Sn ͑23.1 atom % Sn͒ phase.
3
1
0
3
As evidenced by Fig. 8, it was found that Cu–Sn “speculum
metal” layers, i.e., 40–60 wt % Sn, could easily be prepared by
controlling the alloying potentials between +1 mV and +5 mV vs
Thermodynamics of the Cu–Sn alloying.— To gain thermody-
namic insight into the alloying behaviors, a sampled-current,
2
+
Sn /Sn. Figure 9 compares a set of current density data during the
Figure 8. Average composition of Cu–Sn layers obtained by potentiostatic
RD alloying for 72 h at five different potentials using a 0.05 mol dm−
Sn͑Tf N͒ –TMHA–Tf N bath at 130°C.
3
2
2
2
Figure 9. Change in current density during RD alloying by ͑a͒ galvanic
contact method and ͑b͒ potentiostatic conditions at +5 mV, +10 mV, and
+20 mV vs Sn /Sn using a 0.05 mol dm Sn͑Tf2N͒2–TMHA–Tf2N bath at
130°C.
Figure 7. XRD patterns of Cu–Sn layers obtained by potentiostatic RD
2
+
2+
−3
alloying for 72 h at +5 mV, +10 mV, and +20 mV vs Sn /Sn using a
−
3
0
.05 mol dm Sn͑Tf N͒ –TMHA–Tf N bath at 130°C.
2
2
2
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D616
Journal of The Electrochemical Society, 154 ͑11͒ D612-D616 ͑2007͒
Conclusion
In this study, we tried using the ionic liquid TMHA–Tf N as a
2
nonvolatile and nonflammable solvent for the reduction-diffusion
͑
RD͒ formation of Cu–Sn alloy layers at elevated temperatures be-
yond 100°C. When a thin Cu layer was alloyed by the galvanic
contact method, Cu–Sn layers composed of intermetallic phases,
Cu Sn , Cu Sn, and Cu Sn were formed, but the ␤-Sn phase with
6
5
3
10
3
a trace amount of copper was codeposited. In contrast, uniform
Cu–Sn layers were prepared through potentiostatic alloying at
2
+
+
1 mV to +40 mV vs Sn /Sn. The intermetallic phases formed
were dependent on the alloying potential applied, which roughly
agreed with the value estimated from a set of thermochemical data.
This demonstrated that a Cu–Sn alloy layer can easily be prepared
through successive electrochemical processes using two baths, each
2
+
2+
containing different metal ions, Cu and Sn : the electrodeposition
2
+
of a Cu layer from an aqueous Cu bath followed by potentiostatic
Figure 10. Sampled-current cathodic voltammogram for 0.05 mol dm−3
Sn͑Tf N͒ –TMHA–Tf N bath at 130°C. The increment potential and step
2+
alloying using an ionic liquid-based Sn bath. Recently, we have
2
2
2
been investigating the transport properties of several metal ions in
period were 1 mV and 20 s, respectively. Averaged currents during the final
s of each potential step were sampled.
2
+
TMHA–Tf N and found the diffusion coefficient of Sn ions to be
2
5
7
2
−1
12
3
.5 ϫ 10− cm s ͑50°C͒, which is two orders of magnitude less
than that for aqueous media. Such a slow ionic diffusion in TMHA–
Tf N is attributed to the high viscosity of the liquid and can become
i.e., staircase, cathodic voltammogram for the 0.05 mol dm−3
2
an obstacle when applying the liquid to electrochemical deposition
Sn͑Tf N͒ –TMHA–Tf N bath was measured using a Cu sheet as a
2
2
2
2+
technology. In fact, however, the transport of Sn ions in TMHA–
working electrode ͑Fig. 10͒. In this experiment, current density data
8
Tf N did not control the growth rate of Cu–Sn in the present study.
2
were collected by stepping down the potential at 1 mV intervals
This is because the viscosity of TMHA–Tf N is improved, or low-
from +140 mV to 0 mV vs Sn2+/Sn. During each step, the potential
2
ered, at elevated temperatures. In other words, the present alloying
process effectively utilizes the advantages of the ionic liquid, i.e.,
nonvolatility and nonflammability, without the drawback regarding
ion transport, paving the way for a novel type of electrochemical
deposition using ionic liquids.
was kept constant for 20 s and averaged current density data during
the final 5 s of each step were sampled. The cathodic current started
2
+
to increase from +123 mV, suggesting that the reduction of Sn
ions to Sn atoms and concomitant dissolution of the atoms into the
fcc-Cu phase start from this potential. The cathode current plateaued
between +105 mV to +60 mV and further increased stepwise with
decreasing potential toward 0 mV due to the formation of interme-
tallic phases. Considering the results of potentiostatic alloying ͑Fig.
Acknowledgments
This work was supported in part by Japan Science and Technol-
ogy Agency ͑JST͒ in Research for Promoting Technological Seeds, a
Grant-in-Aid for Scientific Research ͑no. 19360343͒ from the Japan
Society for the Promotion of Science ͑JSPS͒, and Kyoto University
21st Century COE Program, “United Approach to New Materials
Science,” from the Ministry of Education, Culture, Sports, Science
and Technology of Japan.
8͒, it is likely that the three onsets found at +57 mV, +45 mV, and
+
15 mV correspond to Cu Sn , Cu Sn, Cu Sn phases in order of
10 3 3 6 5
activity aSn.
10
Using a set of thermochemical data, average activities of tin
aSn of the Cu n and Cu Sn phases were calculated to be 0.677 and
6
5
3
0
.126, respectively, at 130°C. According to the Nernst equation for
2
+
Kyoto University assisted in meeting the publication costs of this article.
the reaction, Sn + 2e = Sn, these aSn values correspond to the
redox potentials +6.8 mV and +36 mV vs Sn /Sn. Here, we omit-
ted the Cu Sn phase, since this phase is metastable at 130°C. In
2
+
References
11
1
0
3
1
. T. Nakamura, Y. Mizutani, T. Nagayama, N. Shinohara, and H. Nawafune, Hyomen
Gijutsu, 55, 484 ͑2004͒.
addition, aSn of the fcc-Cu phase, where Sn atoms fully dissolved
1
1
−5
͑
i.e., 0.7 atom % Sn ͒, was estimated to be 5.1 ϫ 10 , correspond-
2. M. Kume and T. Oki, Hyomen Gijutsu, 46, 829 ͑1995͒.
3. T. Nakamura, Y. Mizutani, T. Nagayama, and N. Shinohara ͑Kyoto City͒, Jpn
Kokai Tokkyo Koho ͑A͒, P2004–10907 ͑Jan 15, 2004͒.
2
+
ing to +172 mV vs Sn /Sn. It is worth noting that, as listed in Table
I, the experimentally observed potentials for the formation of each
phase were in rough agreement with the redox potentials calculated
from the thermochemical data.
4
5
. W. Choi, J. Y. Lee, and H. S. Lim, Electrochem. Commun., 6, 816 ͑2004͒.
. S. D. Beattie and J. R. Dahn, J. Electrochem. Soc., 150, A894 ͑2003͒.
6. Y. Xia, T. Sakai, T. Fujieda, M. Wada, and H. Yoshinaga, J. Electrochem. Soc.,
1
48, A471 ͑2001͒.
7
8
. Y. Fujiwara, Thin Solid Films, 425, 121 ͑2003͒.
. T. Katase, R. Kurosaki, K. Murase, T. Hirato, and Y. Awakura, Electrochem. Solid-
State Lett., 9, C69 ͑2006͒.
Table I. Potentials for formation of each of Cu–Sn phases stable
at 130°C (mV vs Sn /Sn)
2
+
9. K. Murase, K. Nitta, T. Hirato, and Y. Awakura, J. Appl. Electrochem., 31, 1089
2001͒.
0. J.-H. Shim, C.-S. Oh, B.-J. Lee, and D. N. Lee, Z. Metallkd., 87, 205 ͑1996͒.
11. N. Saunders and A. P. Miodownik, in Phase Diagrams of Binary Copper Alloys, P.
R. Subramanian, D. J. Chakrabarti, and D. E. Laughlin, Editors, Monograph Series
on Alloy Phase Diagrams, Vol. 10, p. 412, ASM International, Materials Park, OH
͑1994͒.
͑
1
Phase
Experimental
Calculated
Sn in fcc-Cu
+123
+45
+172
+36
Cu Sn
3
Cu Sn
+15
+6.8
12. Y. Nishizaki, K. Murase, and H. Sugimura, Unpublished data.
6
5
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