Electrodeposition of metallic molybdenum films in ZnCl2-NaCl-KCl-MoCl3 systems at 250 °C

Article abstract of DOI:10.1016/j.electacta.2005.10.041

Molybdenum metal film has been electrodeposited in ZnCl₂-NaCl-KCl (0.60:0.20:0.20, in mole fraction) melt containing MoCl₃ at 250 °C. In this melt, a dense film was obtained by potentiostatic electrolysis at 0.15 V versus Zn(II)/Zn for 3 h. However, the film had a thickness of smaller than 0.5 μm and was not adhesive. On the other hand, addition of 4 mol% of KF to the melt led to larger cathodic current in cyclic voltammogram, and gave a dense, adhesive and thicker metal film of ca. 3 μm thickness in the same electrolysis condition as above. The present process is promising as a new method for molybdenum coating at low temperatures.

Full text of DOI:10.1016/j.electacta.2005.10.041

Electrochimica Acta 51 (2006) 3776–3780
Electrodeposition of metallic molybdenum films in
ZnCl2–NaCl–KCl–MoCl3 systems at 250 ◦C
Hironori Nakajima, Toshiyuki Nohira∗, Rika Hagiwara
Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
Received 1 July 2005; received in revised form 25 October 2005; accepted 25 October 2005
Available online 2 December 2005
Abstract
Molybdenum metal film has been electrodeposited in ZnCl2–NaCl–KCl (0.60:0.20:0.20, in mole fraction) melt containing MoCl3 at 250 ◦C. In
this melt, a dense film was obtained by potentiostatic electrolysis at 0.15 V versus Zn(II)/Zn for 3 h. However, the film had a thickness of smaller
than 0.5 ␮m and was not adhesive. On the other hand, addition of 4 mol% of KF to the melt led to larger cathodic current in cyclic voltammogram,
and gave a dense, adhesive and thicker metal film of ca. 3 ␮m thickness in the same electrolysis condition as above. The present process is promising
as a new method for molybdenum coating at low temperatures.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition; Molten salt; ZnCl2–NaCl–KCl; Molybdenum metal film; Molybdenum chloride; Fluoride ion
1. Introduction
Senderoff and Mellors have obtained refractory metals
including molybdenum in alkali metal fluoride melts at
700–850 ◦C [7,8]. Since then, refractory metals have been
obtained in chloride, fluoride and oxide melts above 500 ◦C
[9,10]. Katagiri et al. succeeded in obtaining metallic tungsten at
350–450 ◦C using ZnBr2–NaBr [11,12] and ZnCl2–NaCl melts
[12,13]. However, electrodeposition of smooth and dense refrac-
tory metals becomes more and more difficult as the operation
temperature decreases. To date, such electrodeposition of refrac-
tory metals at temperatures below 350 ◦C have not been reported
except for the case of chromium. However, electrodeposition of
metals must be made at and lower than 250 ◦C for the LIGA
process due to the limit of heat resistance of the resist sheet
made of polymethylmethacrylate. For coating the LIGA parts,
electrodeposition should be carried out at temperatures as low
as possible to avoid the softening of the substrate materials.
Thus, we have been studying the electrodeposition of refrac-
tory metals in molten salts at around 250 ◦C [14]. It should be
noted that metallic molybdenum deposit has not been obtained
below 550 ◦C [10]. We selected a molten ZnCl2–NaCl–KCl
eutectic (ZnCl2:NaCl:KCl = 0.60:0.20:0.20, in mole fraction;
mp: 203 ◦C [15]) as an electrolyte. The melts containing KF
were also examined since fine molybdenum deposits have been
obtained in fluoride melts at high temperature in the previous
works [9,10], which is possibly related to the formation of flu-
oro complex ions.
Electrodeposition of refractory metals such as tungsten,
molybdenum, tantalum at low temperatures has many advan-
tages from an engineering point of view. One of the promis-
ing applications is the LIGA (Lithographie Galvanoformung
Abformung, German abbreviation) process, a micro-fabrication
technique consisting of lithography, electroforming and molding
[1–6]. Although many products have already been commercially
produced with this method, applicable materials are limited to
metals that are able to be electrodeposited in aqueous elec-
trolytes such as copper, gold, nickel and Ni–Fe alloys [1–6].
Refractory metals are highly expected to be used for the elec-
troforming step of LIGA process owing to their high hardness
and high mechanical-strength. It is also very promising to coat
the conventional LIGA parts with refractory metals by elec-
trodeposition. However, electrodeposition of them has not been
achieved except for chromium in aqueous solutions mostly due
to the narrow electrochemical window of water. If the applica-
tion of refractory metals to the LIGA process is realized, it would
much contribute to improve the performance and reliability of
the Micro-Electro-Mechanical Systems (MEMS) devices.
∗
Corresponding author. Tel.: +81 75 753 4817; fax: +81 75 753 5906.
E-mail address: nohira@energy.kyoto-u.ac.jp (T. Nohira).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.10.041

H. Nakajima et al. / Electrochimica Acta 51 (2006) 3776–3780
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2. Experimental
Nilaco Corp.) immersed in the melt was used for the refer-
ence electrode. A chromel–alumel thermocouple was used for
the temperature measurement. Cyclic voltammetry with positive
feedback IR compensation and potentiostatic electrolysis were
performedusingelectrochemicalmeasurementsystems(Hokuto
Denko Co., Ltd., HZ-5000). After the electrolysis, the electrodes
were immersed in acetone, then, rinsed with distilled water to
remove adherent salts.
The deposits were characterized by scanning electron
microscopy (SEM, Hitachi, S-2600H), energy dispersive X-ray
analysis (EDX, Horiba Co., Ltd., EMAX ENERGY EX-200),
X-ray diffraction (XRD, Rigaku Industrial Co., Ltd., Multi Flex)
with CuKa radiation and X-ray photoelectron spectroscopy
(XPS, Shimadzu Co., Ltd., ESCA-3200).
A schematic drawing of the experimental apparatus is shown
in Fig. 1. The experiments were conducted in an argon-filled
glove box with a gas circulating purifier (MIWA MFG Co., Ltd.).
All the chemicals were anhydrous reagent grade. The eutectic
mixture of ZnCl2 (99.9%, Wako Pure Chemical Industries, Ltd.),
NaCl and KCl (99.5% each, Wako Pure Chemical Industries,
Ltd.) was prepared (ZnCl2:NaCl:KCl = 0.60:0.20:0.20, in mole
fraction), and then dried in a furnace under vacuum at 130 ◦C
for 3 days or more. After that it was melted at 250 ◦C in a glass
beaker (Shibata Hario Glass Co.) installed in a four-necked sep-
arable flask on a heating plate. A band shape heater was wound
around the beaker to have uniform temperature distribution by
heating. MoCl3 (99.5%, Alfa Acer) was added in the melt as a
molybdenum ion source. KF (99%, Wako Pure Chemical Indus-
tries, Ltd.) was added as a fluoride ion source.
3. Results and discussion
The working electrode was a nickel plate (99.7%,
5 mm × 10 mm × 0.2 mm, Furuuchi Chemical Corp.) that was
fully immersed in the melt. The nickel plate was electrochemi-
cally polished in a sulfuric acid then immersed in an acid cleaner
containing NaHF2 (Kizai Corp., Kokeisan B) to remove surface
oxides. The counter electrode was a glassy carbon rod (3 mm,
Tokai Carbon Co., Ltd., GC-20). A zinc wire (99.99%, 0.5 mm,
3.1. Electrodeposition of metallic molybdenum in
ZnCl2–NaCl–KCl–MoCl3 melt
3.1.1. Cyclic voltammetry
Fig. 2 shows a cyclic voltammogram at the nickel plate elec-
trode in ZnCl2–NaCl–KCl–MoCl3(0.05 mol kg−1 added) melt
at 250 ◦C. A blank voltammogram obtained for the same elec-
Fig. 1. Schematic drawing of experimental apparatus. (A) Thermocouple; (B)
working electrode; (C) reference electrode; (D) counter electrode; (E) separable
flask; (F) Pyrex beaker; (G) band heater; (H) ZnCl2–NaCl–KCl melt; (I) heating
plate.
Fig. 2. (a) Cyclic voltammograms for nickel electrodes in ZnCl2–NaCl–KCl
and ZnCl2–NaCl–KCl–MoCl3(0.05 mol kg−1 added) melts at 250 ◦C. Scan rate:
0.05 V s−1. (b) The same voltammogram with a current density axis of smaller
scale.

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H. Nakajima et al. / Electrochimica Acta 51 (2006) 3776–3780
trode in the melt without MoCl3 is shown for comparison. These
CVs were recorded after several cycles. In both the plots, a
cathodic current shoulder is observed at around 0.03 V as well
as an anodic current peak at around 0.12 V. They are attributed
to the formation and dissolution of Ni–Zn alloy, respectively
[14]. Cathodic and anodic currents around 0 V in both the
plots are attributed to the deposition and dissolution of metallic
zinc, respectively. In the ZnCl2–NaCl–KCl–MoCl3 melt, a new
cathodic wave, C1, is also observed in the potential range of
0.04–0.42 V, suggesting the electrodeposition of molybdenum.
The formations of Zn–Mo and/or Ni–Mo alloys are also taken
account from the phase diagrams [16].
3.1.2. Potentiostatic electrolysis and characterization of the
deposit
Since electrodeposition of molybdenum was suggested to
occur in the potential region of lower than 0.42 V, potentiostatic
electrolysis was performed at 0.15 V for 3 h, where deposition of
molybdenum was expected whereas that of metallic zinc or the
formation of Ni–Zn alloy was not. After the potentiostatic elec-
trolysis, a black film was deposited. However, only a small area
of the substrate was covered due to bad adhesion. Fig. 3 shows a
spectrum of XPS of Mo 3d for the film. Mo 3d3/2 and Mo 3d5/2
peaks corresponding to metallic molybdenum are observed at
231 and 228 eV, respectively [17]. The film is, thus, identified
as metallic molybdenum. It is also confirmed that the Zn–Mo
and/or Ni–Mo alloys are not formed. EDX results showed that
chloride content in the film was lower than 2 at.%, indicating that
the melt inclusion is not significant. On the other hand, an XRD
pattern did not show any distinctive peaks corresponding to a
metallic molybdenum crystal [18]. This is likely to be caused by
the too small thickness and/or low crystallinity of the film.
Fig. 4 shows a surfaceSEMimageofthefilm, wherespherical
particles and microcracks are observed. The spherical particles
are observed as light gray because the oxidation by rinsing with
the water may have proceeded larger extent compared with the
Fig. 4. A surface SEM image of a deposit obtained after potentiostatic electrol-
ysis at 0.15 V vs. Zn(II)/Zn for 3 h in ZnCl2–NaCl–KCl–MoCl3 (0.05 mol kg−1
added) melt at 250 ◦C.
flat part. The microcracks indicate that tensile stress is gener-
ated on the 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. The CTE mismatching is excluded since CTE of nickel is
larger than that of molybdenum, which gives rise to compressive
stress. Thus, the microcracks would be produced by the deposi-
tion induced tensile stress. The observation of the flakes of the
film showed that it was dense but had a thickness lower than
0.5 ␮m. The metallic molybdenum is also obtained at 0.40 V,
which was confirmed by XPS analysis. Hence, it is confirmed
that cathodic currents at potentials lower than 0.42 V result from
the electrodeposition of molybdenum.
3.2. Electrodeposition of metallic molybdenum in
ZnCl2–NaCl–KCl–MoCl3–KF melt
3.2.1. Cyclic voltammetry
The melts containing KF were examined since fine molyb-
denum deposits have been obtained in fluoride melts at
high temperatures [9,10]. Fig. 5 shows a cyclic voltammo-
gram at a nickel electrode in ZnCl2–NaCl–KCl–KF(4 mol%
added)–MoCl3(0.05 mol kg−1 added) melt at 250 ◦C. A curve
for the melt added only KF to the blank melt is also shown
for comparison. They were recorded after several cycles. Addi-
tion of KF to the blank melt causes no distinctive change
in the potential region examined in the present study. In the
ZnCl2–NaCl–KCl–KF–MoCl3 melt, a cathodic wave, C2, is
observed in the potential region of 0.04–0.45 V. This current
is possibly due to the electrodeposition of metallic molybde-
num, which is approximately three times larger than those in
the melt without KF. The shape change in the cyclic voltamme-
try in the potential region of 0–0.03 V shows the suppression
of both the deposition of metallic zinc and the formation of the
Ni–Zn alloy, which coincides with the relatively large amount
of molybdenum deposition at the electrode. In these potential
Fig. 3. An Mo 3d XPS spectrum of a deposit obtained after potentio-
static electrolysis at 0.15 V vs. Zn(II)/Zn for 3 h in ZnCl2–NaCl–KCl–MoCl3
(0.05 mol kg−1 added) melt at 250 ◦C. Argon ion etching time: 5000 s.

H. Nakajima et al. / Electrochimica Acta 51 (2006) 3776–3780
3779
Fig. 5. Cyclic voltammograms for nickel electrodes in ZnCl2–NaCl–KCl and
ZnCl2–NaCl–KCl–KF–MoCl3 (0.05 mol kg−1 added) melts at 250 ◦C. Scan
rate: 0.05 V s−1. Starting potential for the ZnCl2–NaCl–KCl–KF–MoCl3 melt:
0.59 V vs. Zn(II)/Zn.
regions, the possibilities of the formations of the Zn–Mo and/or
Ni–Mo alloys cannot be dismissed.
3.2.2. Potentiostatic electrolysis and characterization of the
deposit
Since electrodeposition of molybdenum was suggested in the
potential region lower than 0.4 V, potentiostatic electrolysis was
performed at 0.15 V for 3 h. A black film was deposited and its
adhesion was better than that of the film obtained in the previous
section. Fig. 6 shows an XRD pattern for the film. In this case,
a diffract pattern is ascribed to metallic molybdenum [18]. It
is also confirmed by XPS analysis that no Zn–Mo and Ni–Mo
alloys are formed. EDX results revealed that the chloride con-
tent in the film is lower than 1 at.%, indicating that the melt
inclusion is also minimized in this case. Fig. 7(a) shows a sur-
face SEM image of the film. Although the surface was covered
with spherical particles, the deposited molybdenum layer seems
dense. The formation of the microcracks is also explained by
the deposition-induced internal stress as the case for the melt
without KF. However, the number and gaps of the microcracks
markedly decreased with addition of KF. Fig. 7(b) shows a cross-
Fig. 7. (a) A surface SEM image and (b) a cross-sectional SEM image of a
deposit obtained after potentiostatic electrolysis at 0.15 V vs. Zn(II)/Zn for 3 h
in ZnCl2–NaCl–KCl–KF (4 mol% added)–MoCl3 (0.05 mol kg−1 added) melt
at 250 ◦C.
sectional SEM image of the film. Although the molybdenum
film contains a few fine microcracks, it is almost dense and
has the average thickness of approximately 3 ␮m. The improve-
ment in adhesiveness, thickness and microcracks by addition
of KF should be related to the change of complex structure of
molybdenum species in the melt, such as the formation of fluoro
or chloro–fluoro complex. This effect is now under investiga-
tion by Raman spectroscopy. The microcracks will be reduced
further by the other additives and/or the other electrolysis con-
ditions such as the pulse technique. The electrodeposition rate
observed, 3 ␮m per 3 h, is not large enough to apply this process
directly to the electroforming step of the LIGA process. How-
ever, further improvements on this process is expected because
there are much room for increasing the electrodeposition rate,
for example, by increasing the molybdenum composition in the
melt, introducing the pulse electrodeposition technique, etc. The
present method can be applied with minor improvements for
Fig. 6. An XRD pattern of a deposit obtained after potentiostatic electrolysis at
0.15 V vs. Zn(II)/Zn for 3 h in ZnCl2–NaCl–KCl–MoCl3 (0.05 mol kg−1 added)
melt at 250 ◦C.

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H. Nakajima et al. / Electrochimica Acta 51 (2006) 3776–3780
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the conventional LIGA parts.
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4. Conclusion
Metallic molybdenum film was deposited by potentio-
static electrolysis in ZnCl2–NaCl–KCl eutectic melt containing
MoCl3 at 250 ◦C. In this melt, the film was dense, but was not
adhesive. On the other hand, addition of 4 mol% of KF to the
melt gave a dense, adhesive and thicker metal deposit in the
same condition as above. The present process is promising as a
new method for molybdenum coating at low temperatures such
as the coating on the conventional LIGA parts.
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Acknowledgments
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This study was supported by Industrial Technology Research
Grant Program in 2003 from the New Energy and Industrial
Technology Development Organization (NEDO) of Japan. We
thank Sumitomo Electric Industries, Ltd. for their financial and
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