Electrodeposition of metallic tungsten films in ZnCl2-NaCl-KCl-KF-WO3 melt at 250 °C

Article abstract of DOI:10.1016/j.electacta.2007.04.082

WO₃ hardly dissolved in ZnCl₂-NaCl-KCl (0.60:0.20:0.20, in mole fraction) melt at 250 °C. The solubility of WO₃ increased by the addition of KF to the melt, which enabled electrodeposition of metallic tungsten. A smooth and fine film of metallic tungsten was obtained on a nickel substrate by the potentiostatic electrolysis at 0.06 V versus Zn(II)/Zn. The thickness reached ca. 2.5 μm after 6-h electrolysis. The surface of the nickel contact probe pin manufactured by the conventional LIGA (Lithographie-Galvanoformung-Abformung, German abbreviation) process was successfully coated with a smooth and adhesive tungsten film by using this melt.

Full text of DOI:10.1016/j.electacta.2007.04.082

Electrochimica Acta 53 (2007) 24–27
Electrodeposition of metallic tungsten films in
◦
ZnCl –NaCl–KCl–KF–WO melt at 250 C
2
3
a
a,∗,1
a
Hironori Nakajima , Toshiyuki Nohira
, Rika Hagiwara ,
b
b
b
Koji Nitta , Shinji Inazawa , Kazunori Okada
Department of Fundamental Energy Science, Graduate School of Energy Science,
Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
Electronics & Materials R&D Laboratories, Sumitomo Electric Industries Ltd.,
-1-3 Shimaya, Konohana-ku, Osaka 554-0024, Japan
a
b
1
Received 3 September 2006; received in revised form 17 April 2007; accepted 18 April 2007
Available online 27 April 2007
Abstract
WO hardly dissolved in ZnCl
◦
3
2
–NaCl–KCl (0.60:0.20:0.20, in mole fraction) melt at 250 C. The solubility of WO
3
increased by the addition
of KF to the melt, which enabled electrodeposition of metallic tungsten. A smooth and fine film of metallic tungsten was obtained on a nickel
substrate by the potentiostatic electrolysis at 0.06 V versus Zn(II)/Zn. The thickness reached ca. 2.5 ␮m after 6-h electrolysis. The surface of the
nickel contact probe pin manufactured by the conventional LIGA (Lithographie-Galvanoformung-Abformung, German abbreviation) process was
successfully coated with a smooth and adhesive tungsten film by using this melt.
©
2007 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition; Molten salt; ZnCl2–NaCl–KCl; Tungsten metal film; Tungsten oxide; Fluoride ion
1
. Introduction
be as low as possible to avoid the softning and embrittlement
of the substrate nickel. We have accordingly been studying the
electrodeposition of refractory metals in molten salts around
Electrodeposition of refractory metals such as tungsten,
◦
molybdenum at low temperatures has many advantages from the
engineering point of view. One of the most promising applica-
tions is the LIGA (Lithographie-Galvanoformung-Abformung,
German abbreviation) process, which is a microfabrication tech-
nique consisting of lithography, electroforming and molding
250 C. We have succeeded in obtaining metallic tungsten [7]
◦
and metallic molybdenum [8,9] at 250 C using a molten
ZnCl2–NaCl–KCl eutectic (ZnCl2:NaCl:KCl = 0.60:0.20:0.20,
◦
inmolefraction, m.p. 203 C[10])asanelectrolyte. Inthesepre-
vious studies, WCl4, MoCl3 and MoCl were used as tungsten
5
[
1–6]. The importance of applying refractory metals to the LIGA
and molybdenum ion sources.
◦
process, which includes coating the conventional LIGA parts
with refractory metals, for the Micro-Electro-Mechanical Sys-
tems (MEMS) has already been stated in our previous papers
In the present study, electrodeposition of tungsten at 250 C
was further investigated using WO3 as a tungsten ion source. As
described later, metallic tungsten was not obtained in the melts
containing only WO3. KF was then added to the melt since
fine deposits have been obtained in the cases of WCl4, MoCl3
[
7,8].
For the LIGA process, electrodeposition of metals must be
◦
carried out at temperatures lower than 250 C due to the limit
of heat resistance of the resist sheet. In the case of coating the
conventional LIGA parts, electrodeposition temperature should
and MoCl [7–9]. There is an advantage in using WO3instead
5
of WCl4 although tungsten metal can be electrodeposited from
WCl4. It is difficult to use the WCl4 containing melt for a long
time because of the decomposition of WCl4. On the other hand,
WO3 is much stable in the melt compared with WCl4. The elec-
trochemical behavior was investigated by cyclic voltammetry.
Tungsten films were prepared by potentiostatic electrolysis and
the obtained films were analyzed by XPS, SEM and EDX. In
∗
Corresponding author. Tel.: +81 75 753 4817; fax: +81 75 753 5906.
E-mail address: nohira@energy.kyoto-u.ac.jp (T. Nohira).
ISE member.
1
0
013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.04.082

H. Nakajima et al. / Electrochimica Acta 53 (2007) 24–27
25
addition, the electrodeposition of tungsten was applied to coat-
ing a contact probe pin manufactured by the conventional LIGA
process.
2
. Experimental
Experiments were conducted in an argon glove box with a
gas circulating purifier (MIWA MFG Co., Ltd.). ZnCl2 (99.9%,
Wako Pure Chemical Industries, Ltd.), NaCl and KCl (99.5%
each, WakoPureChemicalIndustries, Ltd.)werewellmixedinto
an eutectic composition (ZnCl2:NaCl:KCl = 0.60:0.20:0.20, in
◦
mole fraction) and dried in a furnace under vacuum at 130
C for 3 days or more. After that, it was melted at 250
◦
C
in a Pyrex beaker installed in a four-necked separable flask
on a heating plate. WO3 (99.5%, Wako Pure Chemical Indus-
tries, Ltd.) and KF (99%, Wako Pure Chemical Industries, Ltd.)
were added as tungsten and fluoride ion sources, respectively.
A schematic drawing of the experimental apparatus has been
shown elsewhere [7]. The working electrode was a nickel plate
Fig. 1. Cyclic voltammograms at nickel electrodes in ZnCl2–NaCl–KCl–KF
(
4 mol% added) melt (gray curve) and ZnCl2–NaCl–KCl–KF (4 mol%
◦
added)–WO3(0.54 mol% added) melt (black curve) at 250 C. Scan rate :
0
−
1
.05 V s
.
(
99.7%, 5 mm × 10 mm × 0.2 mm, Furuuchi Chemical Corp.)
that was fully immersed in the melt. Prior to the experiment, the
nickel plate was electrochemically polished in a sulfuric acid,
and then immersed in an acid cleaner (Kizai Corp., Kokeisan
B) containing NaHF2 to remove surface oxides. A zinc wire
deposits have been obtained by the addition of KF in our pre-
vious studies, in which WCl4, MoCl3 and MoCl were used
5
as refractory metal ion sources [7–9]. Before investigating the
(
KF + WO3) added melt, cyclic voltammetry was carried out
(
99.99%, 0.5 mm, Nilaco Corp.) immersed in the melt was
for the only KF added melt. A gray curve in Fig. 1 shows a
cyclic voltammogram obtained at the nickel plate electrode in
ZnCl2–NaCl–KCl–KF (4 mol% added) melt. The shape of the
cyclic voltammogram was identical to that obtained in the orig-
inal ZnCl2–NaCl–KCl melt. Cathodic and anodic current peaks
around 0 V versus Zn(II)/Zn are attributed to the deposition and
dissolution of metallic zinc, respectively. Meanwhile, a cathodic
current peak around 0.03 V and an anodic current peak around
used as the reference electrode. The counter electrode was a
glassy carbon rod (Ø3 mm, GC-20, Tokai Carbon Co., Ltd.).
A chromel–alumel thermocouple was used for the temperature
measurement. Cyclic voltammetry with positive feedback IR
compensation and potentiostatic electrolysis were performed
using an electrochemical measurement system (Hokuto Denko
Co., Ltd., HZ-5000). The obtained samples were immersed in
acetone, and then rinsed with distilled water to remove adherent
salts.
The deposits were characterized by scanning electron
microscopy (SEM, Hitachi, Ltd., S-2600H), scanning ion
microscopy (SIM) with focused ion beam (FIB, FEI Corp.,
FIB200) cross sectioning, energy dispersive X-ray analysis
0.12 V are attributed to the formation and dissolution of Ni–Zn
alloy [7], respectively.
In the (KF + WO3) added melt, it was observed that WO3
dissolved in the melt and the color of the melt changed from
colorless to gray. The dissolution of WO3 probably proceeded
by the formation of oxo-fluoro tungsten complex ions. A certain
portion of WO3, however, remained undissolved. Thus, the melt
is regarded as saturated with WO3. A black plot in Fig. 1 shows
a cyclic voltammogram measured for the nickel plate electrode
in the ZnCl2–NaCl–KCl–KF (4 mol% added)–WO3 (0.54 mol%
(
EDX, Horiba Co., Ltd., EMAX ENERGY EX-200), X-ray
diffraction (XRD, Rigaku Industrial Co., Ltd., Multi Flex) with
Cu K ␣ radiation and X-ray photoelectron spectroscopy (XPS,
Shimadzu Co., Ltd., ESCA-3200).
◦
added) melt at 250 C. A new cathodic current is observed in the
3
. Results and discussion
potential region of 0.05–0.45 V, suggesting electrodeposition of
metallic tungsten. Neverthless, the current peak which is often
observed for the electrodeposition of metals in high temperature
molten salts [11,12] did not appear in the present case. The shape
change of the voltammogram in the potential region of 0–0.03 V
is explained by the deposition of tungsten, suppressing both the
deposition of pure zinc and the formation of Ni–Zn alloy.
3
.1. Cyclic voltammetry
Cyclic voltammetry was performed using a nickel elec-
trode after the addition of WO3 (0.54 mol% added) in
ZnCl2–NaCl–KCl melt at 250 C. The shape of the voltammo-
gram did not change by the addition of WO3. It was observed
that WO3 hardly dissolved in ZnCl2–NaCl–KCl melt. Accord-
ingly, no tungsten deposit was obtained by electrolysis in the
ZnCl2–NaCl–KCl–WO3 melt.
◦
3.2. Potentiostatic electrolysis and characterization of
deposit
Since fluoride melts generally have higher solubilities of
metal oxides than chloride melts, KF was added as a fluoride
ion source to increase the solubility of WO3. Moreover, fine
Since electrodeposition of tungsten was speculated to occur
in the potential region lower than 0.45 V, potentiostatic elec-

2
6
H. Nakajima et al. / Electrochimica Acta 53 (2007) 24–27
Fig. 2. A W 4f XPS spectrum of the film obtained by potentiostatic elec-
trolysis at 0.06 V vs. Zn(II)/Zn for 3 h in ZnCl2–NaCl–KCl–KF (4 mol%
◦
added)–WO3(0.54 mol% added) melt at 250 C. Argon ion etching time: 4500 s.
trolysis was performed at 0.06 V for 3 h, where deposition of
tungsten was expected and that of zinc was not. After the poten-
tiostatic electrolysis, a bright brown film was obtained. Fig. 2
shows an XPS spectrum of W 4f for the deposit after argon ion
etching for 4500 s. W 4f5/2 and W 4f7/2 peaks corresponding to
metallic tungsten are observed at 33 and 31 eV, respectively [13].
Since the peaks ascribed to other oxidation state of tungsten are
not observed, deposition of composite coatings of W and undis-
solved WO3 is negligible. On the other hand, the XRD pattern
did not show any distinctive peak corresponding to the metallic
tungsten crystal [14], indicating that the obtained tungsten was
amorphous. EDX results showed that the chlorine content was
less than 1 at%, proving that melt inclusion did not occur during
the electrodeposition.
Fig. 4. A cross-sectional SIM image of the film obtained by potentiostatic
electrolysis at 0.06 V vs. Zn(II)/Zn for 6 h in ZnCl2–NaCl–KCl–KF (4 mol%
added)–WO3 (0.54 mol% added) melt at 250 C.
◦
deposited film. There are two possibilities for the tensile stress.
One is the coefficient of thermal expansion (CTE) mismatching
and the other is the deposition-induced internal stress. Here, the
CTE mismatching is excluded since CTE of nickel is larger than
that of tungsten, which gives rise to compressive stress. Thus,
the microcracks would be produced by the deposition-induced
tensile stress. Fig. 4 shows a cross-sectional SIM image of a
film obtained by potentiostatic electrolysis at 0.06 V for 6 h. To
obtain the cross-sectional image, the FIB cross sectioning was
conducted after platinum was vapor-deposited on the surface of
the deposit. The milling was firstly performed with gallium ion
beam at acceleration voltage of 30 kV and current of 1000 pA for
Fig. 3 shows a surface SEM image of the film. Although there
are some microcracks, the surface is quite smooth on the whole.
The microcracks indicate that tensile stress was generated on the
2
h. Subsequently, the milling at acceleration voltage of 30 kV
and current of 350 pA was carried out for 30 min. The film is
dense and the thickness is ca. 2.5 ␮m. The microcracks of ca.
0.8 ␮mlongarealsoobservedinthesurfaceregion, whichispos-
sibly due to the deposition-induced internal stress as explained
above.
Finally, to confirm the feasibility of the present method as
a new tungsten coating technique, electrodeposition was con-
ducted for a nickel contact probe pin. The contact probe pin is
manufactured by the conventional LIGA process and is practi-
cally used for semiconductor testing. In the present experiment,
theprobepinwascoatedwithrhodiumpriortotheelectrodeposi-
tion to prevent the Ni–Zn alloy formation. The electrodeposition
was carried out at 0.06 V for 2 h in the ZnCl2–NaCl–KCl–KF
◦
(
4 mol% added)–WO3 (0.54 mol% added) melt at 250 C. As
shown in Fig. 5, a smooth and adhesive film desired was obtained
on the probe pin. Since tungsten is potentially superior to nickel
in thermal stability and mechanical strength, the performance
and durability of the probe pin is expected to be improved largely
by this method. Moreover, the present method is promising as a
new method of tungsten coating which can be applied to various
Fig. 3. A surface SEM image of the film obtained by potentiostatic electrolysis
at 0.06 V vs. Zn(II)/Zn for 3 h in ZnCl2–NaCl–KCl–KF (4 mol% added)–WO3
◦
(
0.54 mol% added) melt at 250 C.

H. Nakajima et al. / Electrochimica Acta 53 (2007) 24–27
27
a nickel contact probe pin manufactured by the conventional
LIGA process was successfully coated with tungsten by using
this melt. The present method is promising as a new tungsten
coating method at low temperature.
Acknowledgments
This study was supported by Industrial Technology Research
Grant Program in 2003 from the New Energy and Industrial
Technology Development Organization (NEDO) of Japan.
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WO3 hardly dissolved in ZnCl2–NaCl–KCl (0.60:0.20:0.20,
◦
in mole fraction) melt at 250 C. After the addition of KF to
the melt, however, WO3 dissolves in the melt and a smooth
and dense tungsten film is obtained at 250 C. The surface of
◦