Analysis of tungsten film electrodeposited from a ZnCl2-NaCl-KCl melt

Article abstract of DOI:10.1016/j.electacta.2007.01.032

Microparts for micro-electro-mechanical systems (MEMS) are becoming increasingly important, and the Lithographie Galvanoformung Abformung (LIGA) process for producing such parts is attracting attention [1]. However, as this process requires the step of electroplating in aqueous solution, only copper, nickel and their alloys can be used because of the limit of the potential window of water. We have been attempting to apply the electroplating of refractory metals in molten salts to the LIGA process or to the surface coating of conventional LIGA microparts in order to give higher strength and heat resistance for the microparts. Herein we report the characteristics of a tungsten film electrodeposited from a ZnCl₂-NaCl-KCl melt at 250 °C.

Full text of DOI:10.1016/j.electacta.2007.01.032

Electrochimica Acta 53 (2007) 20–23
Analysis of tungsten film electrodeposited from a ZnCl2–NaCl–KCl melt
Koji Nittaa,∗, Shinji Inazawaa, Kazunori Okadaa, Hironori Nakajimab,
Toshiyuki Nohirab,∗∗, Rika Hagiwarab
a Electronics & Materials R&D Labs, Sumitomo Electric Industries, Ltd., 1-1-3, Shimaya, Konohana-ku, Osaka 554-0024, Japan
b Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
Received 1 February 2006; received in revised form 16 January 2007; accepted 16 January 2007
Available online 30 January 2007
Abstract
Microparts for micro-electro-mechanical systems (MEMS) are becoming increasingly important, and the Lithographie Galvanoformung Abfor-
mung (LIGA) process for producing such parts is attracting attention [1]. However, as this process requires the step of electroplating in aqueous
solution, only copper, nickel and their alloys can be used because of the limit of the potential window of water. We have been attempting to
apply the electroplating of refractory metals in molten salts to the LIGA process or to the surface coating of conventional LIGA microparts in
order to give higher strength and heat resistance for the microparts. Herein we report the characteristics of a tungsten film electrodeposited from a
ZnCl2–NaCl–KCl melt at 250 ◦C.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: MEMS; LIGA; Electrodeposition of tungsten; Tungsten film; Electrodeposition in molten salt
1. Introduction
or more). Also, when such high temperature electrodeposition
techniques are applied to the coating of LIGA parts made of elec-
troformed nickel, the heat induces softening and embrittlement
of the substrate nickel. We have therefore been investigating
tungsten electrodeposition in a temperature range in which the
resist deformation and the softening of electroformed nickel
material can be prevented. We have already reported that elec-
trodeposition of tungsten is possible in a ZnCl2–NaCl–KCl melt
(m.p. 203 ◦C), and that the addition of KF to the melt gives bet-
ter deposits [6]. In this paper, we studied the microstructure
and mechanical characteristics of the obtained tungsten film,
and demonstrate the tungsten coating on LIGA parts made of
nickel.
As a process for manufacturing microparts for micro-electro-
mechanical systems (MEMS), the Lithographie Galvanofor-
mung Abformung (LIGA) process is already in practical use for
some cases and holds great promise for the future. The current
LIGA process, however, requires the electroforming in aqueous
solution, and thus the applicable materials are limited to nickel,
copper, and their alloys.
Still, the strength and durability of the material in the micro-
parts are expected to be improved, wherein refractory metals
(especially tungsten) have the greatest potential. However, elec-
trodeposition of tungsten from an aqueous solution is extremely
difficult. Nevertheless, Senderoff and Mellors obtained elec-
trodeposits of refractory metals from an alkali metal fluoride
meltat700–850 ◦C [2,3], and Katagirietal. havebeensuccessful
with electrodeposition of metallic tungsten from ZnBr2–NaBr
[4] and ZnCl2–NaCl [5] at 350–450 ◦C. Yet the resist sheet for
the LIGA process deteriorates at such high temperatures (350 ◦C
2. Experimental
2.1. Materials and apparatus
All the chemicals were anhydrous reagent grade. ZnCl2
(99.9%, Wako Pure Chemical Industries, Ltd.), NaCl and KCl
(99 each, Wako Pure Chemical Industries, Ltd.) were well mixed
into an eutectic composition (ZnCl2:NaCl:KCl = 0.6:0.2:0.2, in
mole fraction) and dried in a furnace under vacuum at 250 ◦C
for 3 days or more. The dried mixture was then melted at
∗
∗∗
Corresponding author. Tel.: +81 6 6466 5637; fax: +81 6 6466 1274.
Corresponding author. Tel.: +81 75 753 4817; fax: +81 75 753 5906.
E-mail addresses: nitta-koji@sei.co.jp (K. Nitta),
nohira@energy.kyoto-u.ac.jp (T. Nohira).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.01.032

K. Nitta et al. / Electrochimica Acta 53 (2007) 20–23
21
250 ◦C in a Pyrex glass beaker installed in a four-necked sep-
arable flask on a heating plate. WCl4 (97%, Sigma–Aldrich
Co.) was added in the melt as a tungsten ion source. KF (99%,
Wako Pure Chemical Industries, Ltd.) was added as a fluoride
ion source. The working electrode was made of 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 elec-
trochemically polished in a sulfuric acid and then immersed in an
oxide remover solution (Kokeisan B, Kizai Corp.). The counter
electrode was a glassy carbon rod (Ø 3 mm, GC-20, Tokai Car-
bon Co. Ltd.). A zinc wire (99.99%, Ø 0.5 mm, Nilaco Corp.)
immersed in the melt was used for the reference electrode. A
nickel micro contact probe electroformed on a stainless sub-
strate was used as the working electrode in the examination of
the surface coating of the micro parts. All the electrochemical
measurements were carried out in a glove box filled with argon.
A chromel–alumel thermocouple was used for the temperature
measurement.
2.2. Electrodeposition and characterization
Potentiostatic electrolysis was performed using electrochem-
ical measurement systems, Hokuto Denko Co. Ltd., HZ-5000.
After the electrolysis, the electrodes were immersed in ace-
tone, then rinsed with distilled water to remove the salts. The
deposits were characterized by scanning electron microscopy
(SEM, Hitachi, S-800), energy dispersive X-ray analysis (EDX,
edax, S-UTW), scanning ion microscopy (SIM) with focused
ion beam (FIB, FEI, FIB200), transmission electron microscopy
(TEM, Hitachi, H-9000UHR) with FIB and energy dispersive
X-ray analysis (EDX, Noran Instruments Inc., TN-5500). The
hardness of the film was measured by nanoindentation (MTS
System Corp., Nanoindenter XP).
Fig. 1. The surface SEM image of the electrodeposited film.
3. Results and discussion
3.1. Deposition of metallic tungsten from a
ZnCl2–NaCl–KCl–KF–WCl4 melt
A potentiostatic electrolysis was performed at 20 mV ver-
sus Zn(II)/Zn in ZnCl2–NaCl–KCl-KF (4 mol% added)–WCl4
(0.05 mol kg−1 added) melt at 250 ◦C. A metallic gray film was
obtained after the potentiostatic electrolysis. Fig. 1 shows the
surface SEM image of the film, which exhibited a smooth sur-
face. Fig. 2 shows the EDX spectrum of the film, in which nickel,
tungsten and zinc peaks are observed. This result is consistent
with our previous study that a metallic tungsten layer formed on
a nickel–zinc alloy layer in the same electrolysis condition [6].
Fig. 2. The EDX spectrum of the electrodeposited film.
3.2. Analysis of the cross-section of the film
Fig. 3 shows a cross-sectional SIM image after FIB cross-
sectioning. A fine layer of 0.65 ␮m on a course layer of 0.87 ␮m
is observed on the nickel substrate. According to our previous
result [6], the fine layer should be metallic tungsten, which is
confirmed in the next section, and the course layer should be
nickel–zinc alloy. formation of nickel–zinc alloy easily occurs
Fig. 3. The cross-sectional SIM image of the electrodeposited film.

22
K. Nitta et al. / Electrochimica Acta 53 (2007) 20–23
compared with tungsten electrodeposition for the present con-
dition. Thus, the nickel–zinc alloy formation is thought to
mainly occur in the initial period. However, the nickel–zinc
alloy formation stops after the surface of the electrode is fully
covered with the metallic tungsten layer. Assuming that the
metallic tungsten is electrodeposited from the tetravalent tung-
sten species, the current efficiency is calculated to be 6.9%.
The causes of the low current efficiency are assumed to be,
in addition to the formation of a nickel–zinc alloy, the reduc-
tion of water and other impurities residing in the salts, and the
so-called shuttle reaction. When high valence ion is reduced
to low valence ion at the cathode and the reverse reaction
occurs at the anode, it is called a shuttle reaction. Since tung-
sten exhibits a wide variety of oxidation states, ranging from
+6 to 0, there is a possibility that the shuttle reaction takes
place.
3.3. TEM observation of the tungsten film
Fig. 5. The EDX spectrum of the electrodeposited film.
To observe the microstructure of the film, TEM analysis was
performed. The TEM image and the EDX spectrum of the film
are shown in Figs. 4 and 5. These results show that the film is
extremely compact, and that the melt and other impurities are not
included in the film. Fig. 6 shows an electron diffraction image.
Plain rings can be seen in the diffraction image indicating that
the obtained tungsten is polycrystalline in structure.
d
␤222 = 14.58 nm, respectively. Then, they yield d␤210/d␤200 =
0.912, d␤211/d␤200 = 0.624, d␤211/d␤210 = 0.913 and d␤222
d
d
/
␤210 = 0.707. As a result, rA/rB and rA/rC agree well with
␣200/d␣100 and d␣211/d␣100, respectively, and do not with the
ratios for ␤-W. The obtained film is thus identified as ␣-W.
In addition to general ␣-W structure, ␤-W structure has
been reported for the electrodeposits from molten salts [7]. To
identify the crystal structure of the electrodeposited tungsten
film, the ratios of radii of the rings in the electron diffraction
image are compared with the ratios of plane spacings of the
␣-W and ␤-W crystal lattices. When radii of the rings A, B and
C in Fig. 6 are defined as rA, rB and rC, the ratios are calculated
as rA/rB = 0.696 and rA/rC = 0.571. Separately, we calculated
the ratios of plane spacings of the major lattice planes using
JCPDS cards [8]. For ␣-W, plane spacings of (1 0 0), (2 0 0)
3.4. Hardness measurement of the tungsten film
Nanoindentation was performed to measure the hardness of
the tungsten film. A probe was inserted vertically into the sub-
strate from the surface of the film. Fig. 7 shows relationship
between insertion depth of the probe and hardness at room
temperature. The maximum hardness was 9.1 GPa, however,
bulk tungsten was about 7 GPa. The decrease of the hardness
as the increase in the insertion depth above 120 nm is due to
the effect of the nickel–zinc alloy layer and the nickel substrate.
and (2 1 1) are d␣100 = 22.38 nm, d␣200 = 15.82 nm and d␣211
=
12.92 nm, respectively. They hence yield d␣200/d␣100 = 0.706
and d␣211/d␣100 = 0.577. On the other hand, for ␤-W,
d
␤200 = 25.25 nm,
d
␤210 = 22.58 nm,
d␤211 = 20.62 nm and
Fig. 4. The TEM image of the electrodeposited film.
Fig. 6. The electron diffraction image of the electrodeposited film.

K. Nitta et al. / Electrochimica Acta 53 (2007) 20–23
23
Since the hardness of the glossy nickel electroformed in aque-
ous solution was 5.4 GPa, the obtained tungsten film has superior
hardness.
3.5. Demonstration of tungsten coating on microparts
We demonstrated tungsten coating on microparts manufac-
tured by the LIGA process [9]. Potentiostatic electrolysis was
performed on nickel microparts placed on a stainless substrate
at 20 mV versus Zn(II)/Zn in ZnCl2–NaCl–KCl–KF (4 mol%
added)–WCl4 (0.05 mol kg−1 added) melt at 250 ◦C. Fig. 8
shows an SEM surface image of the microparts after the
electrolysis. It is confirmed that a well-adhering and smooth
tungsten coating on microparts is feasible using the present
method.
4. Conclusion
Potentiostatic electrolysis in
a ZnCl2–NaCl–KCl–KF
(4 mol% added)–WCl4 (0.05 mol kg−1 added) melt at 250 ◦C
gave a smooth tungsten film with no incorporated melts and
impurities. The electron diffraction analysis showed that the
deposited tungsten film has ␣-W structure. Nanoindentation
analysis revealed that the hardness of the film was 9.1 GPa.
Coating on microparts with an adhesive and smooth tungsten
film was successfully demonstrated.
Fig. 7. Thehardnessoftheelectrodepositedfilmdeterminedbynanoindentation.
Acknowledgements
A part of this study was supported by the Industrial Tech-
nology Research Grant Program in 2003 from the New Energy
and Industrial Technology Development Organization (NEDO)
of Japan.
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Fig. 8. The microparts coated with electrodeposited tungsten.