Electrodeposition of Refractory Metalsfrom Molten Salts at 150-250ºC
T. Nohira, H. Nakajima, K. Kitagawa, R. Hagiwara,K. Nitta*, M. Majima* and S. Inazawa*
Kyoto University, Japan*Sumitomo Electric Industries, Ltd., Japan
The 4th Workshop on Reactive Metal ProcessingMarch 14-15, 2008, MIT, Cambridge, U.S.A.
Contents1. Introduction1-1. Refractory metals and LIGA process
1-2. Aims of this study
2. Results and Discussion2-1. W and Mo from NaCl-KCl-ZnCl2 melt at 250ºC
2-2. W and Mo from EMPyrCl-ZnCl2 melt at 150ºC
3. Conclusion
Refractory metals
Table. Refractory metals.
WTaHf
MoNbZr
CrVTi
IV V VI
4
5
6
•High hardness•High mechanical strength•High corrosion resistanceFrom RT to HT
•Surface coating•LIGA process
Refractory metals
3. Electrodeposition
Fig. A flow diagram of LIGA process.
Metal Substrate Resist film
1. Exposure
Deposition
X-ray or UV rayMask
4. Micro-parts 4’. Micro-mold
2. Etching
LIGA process(Lithographie Galvanoformung Abformung)
Current status of LIGA process
• Available metalsCu, Au, Ni, Ni-Fe alloys (Metals electrodeposited from aqueous solutions)
• Highly expected for MEMS(Micro Electro Mechanical Systems) applicationsEx.) Contact probe pin for semiconductor testing, Printer head,
Sensors for automobiles, etc.
If refractory metals can be applied to the LIGA process
Improvement of performance & Expansion of application fields
Ex.) Contact probe pin, micro-mold, micro-parts (gears, turbine, etc.), micro-reactor, etc.
Contact probe pin for semiconductor testing Length: 1.6 mmThickness: 60 μm
Necessity to develop new molten salts
~300 ºCPolyimide~250 ºCEpoxy~150 ºCMethacrylate (ex. PMMA)
Durable temperatureResist material
Durable temperatures of resist materials are not high enough.
Electrodeposition of refractory metals• Impossible to conduct in aqueous solutions (except Cr)• Many studies using high temperature molten salts
• In general, alkali fluoride or alkali chloride-fluoride melts at high temperature (600 – 800 ºC) give better deposits.• Katagiri et. al (1988-)
W from molten NaCl-ZnCl2 and NaBr-ZnBr2, 350 – 450 ºCApplication to LIGA process
Necessary to develop low temperature molten salts.250 ºC → useful 150 ºC → most desirable
Electrodeposition of refractory metals from molten saltsElement Molten salt Temp. / K Ion sources
Ti LiF-NaF-KFLiCl-KClNaCl-KCl
823-1023773-823973-1213
K2TiF6TiCl2, TiCl3, TiCl4, K2TiCl6
TiCl4, K2TiCl6, K2TiF6Zr LiF-NaF-KF
LiCl-KCl1023
723-823ZrF4
Anodic dissolution of Zr
Hf LiF-NaF-KFNaCl-KCl
10231023-1173
HfF4HfCl4
V LiF-NaF-KFLiCl-KClNaCl-KCl
1023893998
VF4Anodic dissolution of V
NaF(7 mol%)+K2NaVF6(5 mol%)Nb LiF-NaF-KF
LiF-NaFNaCl-KCl
973-1098823-1323
1023
K2NbF7K2NbF7K2NbF7
Ta LiF-NaF-KFLiF-NaF
NaCl-KCl
1063823-1323
1023
K2TaF7K2TaF7K2TaF7
Cr LiF-NaF-KFLiCl-KCl
823-1173723
CrF2, CrF3, CrF4CrCl2, CrCl3
Mo LiF-NaF-KFLiCl-KCl
1073723-873
MoF6+MoMoCl3, K3MoCl6
W LiF-NaF-KFLiCl-KCl
ZnCl2-NaCl
873-1073673-773623-723
WF6+W, WO3WCl6, KWCl6, K2WCl6
WCl6
Aims of this study
1. Electrodeposition of W and Mo at 250ºCMelt: NaCl-KCl-ZnCl2 (20:20:60 mol%, m.p. 180ºC)Refractory metal ion sources: WCl4, WO3, MoCl3Additive: F- ion for better deposit
2. Electrodeposition of W and Mo at 150 ºCMelt: EMPyrCl-ZnCl2 (50:50 mol%, m.p. 45ºC)Refractory metal ion sources: WCl6, MoCl5Additive: F- ion for better deposit
Properties of ZnCl2-NaCl-KCl
ZnCl2:NaCl:KCl=60:20:20 mol%, m.p.: 203ºC(Ref.*)
DSC Viscosity
50 100 150 200 250 300 350Temperature / ℃
melting point 180 ºC
Endo
ther
mic
0
50
100
150
200
250
300
180 200 220 240 260 280 300
Temprateure / ℃V
isco
sity
/ c
P
Experiment at 250 ºC
*I. N. Nikonova, S. P. Pavlenko, and A. G. Bergman, Bull. acad. sci. U. R. S. S.,Classe sci. chim., 391 (1941).
Melting point:180ºCConductivity:85 mS cm-1(250ºC)
Experimental Apparatus
R.E.: Zn wire (Zn(II)/Zn)
Pyrex beaker
Separable flask
W.E.: Ni plate
Thermocouple
Molten Salts
Additives : WCl4, WO3, KF
C.E.: Glassy carbon
In an argon glove box
T=250 ºC
C.E.W.E. R.E.
Heater
Melt: ZnCl2-NaCl-KCl(60:20:20 mol%, m.p. 180 ºC)
Cyclic voltammetry (WCl4)
Potential / V (vs. Zn(II)/Zn)
Cur
rent
den
sity
/ m
A c
m-2 WCl4 added
Blank
Scan rate : 50 mV s-1
20 mVPotentiostatic electrolysis : 3 hours
Ni-Zn alloy formation
Zn deposition-0.08-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Fig. Cyclic voltammograms for Ni electrodes in a molten ZnCl2-NaCl-KCl and a molten ZnCl2-NaCl-KCl-WCl4 (0.54 mol% added) system at 250 °C. Scan rate: 0.05 V s
-1
-0.005
-0.003
-0.001
0.001
0.003
0.005
0 0.1 0.2 0.3 0.4
XPS and SEM (WCl4)
Fig. A cross-sectional SEM image.
25303540
W 4f
Fig. A W4f XPS spectrum of the deposit.20 mV (Zn(II)/Zn) for 3 h.Ar etching : 4500 s
Binding energy / eV
W metal Dense, not smoothThickness 0.5 µm
Metallic gray deposit
(Cl < 2 at%)
0.5 cm
Inte
nsity
/ A
. U.
W(0)4f5/2
W(0)4f7/2
XPS and SEM (WCl4+KF)
Fig. A W4f XPS spectrum of the deposit. 20 mV (Zn(II)/Zn) for 3 h.KF: 4 mol% added, WCl4: 0.54 mol% added.Ar etching: 4500 s.
Binding energy / eV
W metal Dense, smoothThickness 0.5 µm(Cl < 1 at%, F was not detected)
Fig. A cross-sectional SEM image.
Metallic gray deposit
0.5 cm
Inte
nsity
/ A
. U. W 4f
W(0)4f5/2
W(0)4f7/2
25303540
XPS and SEM (WO3+KF)
25303540
W4fIn
tens
ity /
A.U
.
Binding energy / eV
Thickness 0.7 µm
Fig. A cross-sectional SEM image.
W metal Dense, smooth
Fig. A W4f XPS spectrum of the deposit. 60 mV (Zn(II)/Zn) for 3 h.KF: 4 mol% added, WO3: 0.54 mol% added.Ar etching:13000 s
(Cl < 1at%, F was not detected)
W(0)4f5/2
W(0)4f7/2
Cross-sectional TEM
W layerZn-NiAlloy layer
Nisubstrate
A tungsten layer is dense.
0.2 µm
Cross-sectional TEM image
Identification of crystal structure by electron diffraction
The deposit is α-W.
Electron diffraction
Index Index
JCPDS Card Data
Intensityd/d(110)d (Å) d (Å) d/d(200) d/d(210) Radius(a, b, c)Diff. ADiff. BDiff. C
Measured value at the film
Intensity
Hardness of the deposited W
Measurement by Nano-indentor® (MTS systems corp.)
482Sintered W675Deposited W
207Substrate NiHvObject
Load resolution: 0.1μgfDepth resolution: 0.01nm
Hardness is calculated from the load/displacement curve.
Diamond tip
0.65μm
0.05μm
W layer
Substrate Ni
5μm
Surface SEM
The deposited W film was smooth.Elements deriving from the melt such as Na, K, Cl were not detected.
10 mm
(substrate Ni)
To obtain a thicker film (ZnCl2-NaCl-KCl-KF-WO3)
Electrolysis potential: 80 mV vs. Zn(II)/ZnTime: 6 hr
Appearance
W
Ni(substrate)
O
Ni(substrate)
W W
EDX
C
To obtain a thicker film (ZnCl2-NaCl-KCl-KF-WO3)
1μm
W layer
Ni substrate
Cross-sectional SIM image(prepared by FIB)
XRD
Thickness: 2.5 μ mCrystallite diameter: 17 Å
D (Crystallite diameter) =K (Scherrer const.) × λ(wave length of X-ray)
β(FWHM) × cosθ
Scherrer Equation
W Ni
Ni
W W
θ-2θ,θ=1 °
Cou
nts
/ S2θ/ deg. (Cu/Kα)
20 30 40 50 60 70
*Ni: substrate Ni
A high quality tungsten film was obtained.
Coating tungsten on a micro-part WO3: 0.54 mol%, KF: 4 mol%0.06 V vs. Zn(II)/Zn for 2 h
・Adhesiveness and coverage are very good.→Remarkable improvement in performance and durability.
Mo from NaCl-KCl-ZnCl2 at 250 ºC
Fig. An XRD pattern of the deposit obtained.150 mV (vs. Zn(II)/Zn) for 3 h.KF: 4 mol% added, MoCl3: 0.54 mol% added.
20 40 60 80 100 120
2 θ / deg. (CuKα)
MoNi (Substrat e)
Mo metal
Fig. A cross-sectional SEM image
Dense, adhesiveThickness 3 µm(Cl < 2 at%, F was not detected)
実験の目的
ZnCl2-EMPyrCl systems (EMPyrCl: N-ethyl-N-methylpyrrolidinium chloride)
m.p. 45 ºC at XZnCl2=50 mol%MoCl5, WCl6 → Mo, W deposition (150-200 ºC )
Development of lower temperature melts(100-200℃)
ZnCl2-TAACl systems(TAACl: tetraalkylammonium chloride)
ZnCl2-TMPeACl: m.p. 80 ºC at XZnCl2=60 mol%TaCl5 → Ta deposition (150-200 ºC )
NR
RR
R+
(R=CnH2n+1)
-Cl
MeEtN+ -Cl
ZnCl2-EMImCl systems (EMImCl: 1-ethyl-3methylimidazolium chloride)m.p. < 40 ºC at XZnCl2=50 mol%Zn, Zn-Fe, Zn-Pt alloy, etc. (50-130 ºC )1) Y.-F. Lin, I.-W. Sun, Electrochim. Acta, 44, 2771 (1999).2) J.-F. Huang, I.-W. Sun, J. Electrochem. Soc., 151, C8 (2004).3) J.-F. Huang, I.-W. Sun, Chem. Mater., 16, 1829 (2004).
MeEtN -ClN+
Nonaromatic cations have higher thermal stabilities.
Pyrrolidinium cations have higher conductivities.
Phase diagram and conductivity of EMPyrCl-ZnCl2
Fig. Phase diagram for the EMPyrCl-ZnCl2 system.
0
50
100
150
200
250
300
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Tem
pera
ture
/ ºC
X(ZnCl2)Fig. Conductivity of EMPyrCl-ZnCl2 systems.
0
5
10
15
20
25
30
35
40
40 60 80 100 120 140 160 180 200
Con
duct
ivity
/ m
S cm
-1
Temperature / oC
• X(ZnCl2) = 0.45
• X(ZnCl2) = 0.5
• X(ZnCl2) = 0.6
X(ZnCl2) = 0.5、m.p.: 45ºC X(ZnCl2) = 0.5、σ = 22.5 mS cm-1 (150℃)
X(ZnCl2) = 0.5 was selected and used at 150ºC.
Measurements by DSC and direct observation
Electrochemical window of EMPyrCl-ZnCl2(50:50)
-15
-10
-5
0
5
10
-0.2 0 0.2 0.4 0.6 0.8 1
Cur
rent
den
sity
/mA
cm
-2
Potential /V (vs. Zn(II)/Zn)
-10
0
10
20
30
40
50
0.5 1 1.5 2 2.5 3
Potential / V vs. Zn(II)/Zn
Cur
rent
den
sity
/ m
A c
m-2
Fig. Cyclic voltammogram for a Mo electrode in EMPyrCl-ZnCl2 (50:50) melt at 150 ゚C. Scan rate: 10 mV s-1 .
Fig. Cyclic voltammogram for a GC electrode in EMPyrCl-ZnCl2 (50:50) melt at 150 ºC. Scan rate: 10 mV s-1 .
Electrochemical window is ca. 2 V.Cathodic limit is Zn deposition.Anodic limit is decomposition of EMPyr+ ion.
-4
-3
-2
-1
0
1
2
3
-0.2 0 0.2 0.4 0.6 0.8 1
Potential / V(vs. Zn(II)/Zn)
Cur
rent
den
sity
/ m
A c
m-2
CV in ZnCl2-EMPyrCl at 150 ºC
Fig. Cyclic voltammograms of Mo electrode in EMPyrCl-ZnCl2 and EMPyrCl-ZnCl2-KF(3 mol% added)-MoCl5(0.9 mol% added) melts at 150 ºC. Scan rate: 0.01 V s-1.
BlankMoCl5 added
Potentiostatic electrolysis
New cathodic currents at more negative than 0.6 V (vs. Zn(II)/Zn)Electrodeposition at 0.01 V for 3 h
-0.5
0
0.5
1
1.5
2
2.5
3
0.5 1 1.5 2 2.5
Cur
rent
den
sity
/ m
A c
m-2
Potential / V(vs. Zn(II)/Zn)
Fig. Cyclic voltammogram of Mo electrode in EMPyrCl-ZnCl2 melt at 150ºC. Scan rate: 0.05 V s-1.
Anodic dissolution of Mo
Surface SEM and cross-sectional SIM (MoCl5: 0.9 mol%, KF: 3 mol%)
10μm
Fig. Surface SEM image of the deposit obtained at 0.01 V vs. Zn(II)/Zn for 3h in a EMPyrCl-ZnCl2-KF-MoCl5(0.9 mol% added) melt at 150 ºC.
Fig. Cross-section SIM image of the deposit obtained at 0.01 V vs. Zn(II)/Zn for 3h in a EMPyrCl-ZnCl2-KF-MoCl5(0.9 mol% added) melt at 150 ºC.
A smooth and dense film.Thickness: 0.2 μm
XPS (MoCl5: 0.9 mol%, KF: 3 mol%)
Fig. Mo 3d XPS spectrum of the deposit obtained at 0.01 V for 3 h in a EMPyrCl-ZnCl2-KF-MoCl5 melt at 150 ºC.
・Binding energies indicate a metallic Mo.
・Zn and Cl were not detected.
・The deposit was Mo metal.
・No inclusion of the melt.
・No codepositon of Zn.
Inte
nsity
/ a.
u.
216220224228232236Binding energy / eV
Mo0 3d3/2 Mo0 3d5/2
After 10s etching
・Stirring the melt and/or pulse electrolysis → Thickness: 1μm・WCl4, WCl6 → Depositions of W at 150ºC were confirmed by SEM and XPS.
Conclusion1. NaCl-KCl-ZnCl2 melt at 250ºC
• Melting point: 180ºC at 20:20:60 mol%• WCl4 → Metallic W, not smooth• WCl4 + KF → Better deposit• WO3 + KF → Best deposit (dense and smooth)
→ High hardness and small crystallite diameter→ Coating a contact probe pin
• MoCl3 + KF → Metallic Mo, dense and smooth
2. EMPyrCl-ZnCl2 melt at 150ºC• Melting point: 45ºC at 50:50 mol%• MoCl5 + KF → Metallic Mo• WCl6 + KF → Metallic W