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