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Cyanide Free Acidic ThioureaElectrolyte for Gold

ElectrodepositionM. Rea, L. Vonella, A. VicenzoM. Bestetti and P.L. Cavallotti

Dipartimento Chimica, Materiali e Ingegneria Chimica

Politecnico di Milano

Joint Meeting of the Physical Chemistry and Electrochemistry

Divisions of the Italian Chemical Society

XXXII Italian Meeting of Physical Chemistry

G.E.I. 2002

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Introduction

Gold in Electronics

ECD films

Bonding wire

Sputter targets

Gold PropertiesContact ResistanceResistivityCorrosion Resistance

Wear Resistance

contacts, connectors, solderable films on PCB

conductor tracks and contact pads on ICsUses:

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Gold Electrodeposition

Au(I) COMPLEX [Au(CN)2]- [Au(SO3)2]

3- [Au(CS(NH2)2]2+

K 1039 1010 1025

pH 311 59 < 1

[Au+] / g l-1 450 320 1 0 2 0

T / C 3060 4060 2 5 5 0

ddc / A dm-2 0.11.5 0.25 0.5 0 . 1 1 .5

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Acidic Thiourea Electrolyte

Formamidine Disulphide (FDS)

2(H2

N)2

CS (HN)2

(H2

N)2

C2

S2

+2H+ +2e- E0

= 0.42 V

Strong Adsorption: TU-Au, G0ad ~ -43 kJ mol-1

Chemical and/or Anodic Oxidation

Thiourea (TU)

Formation of higher oxidation products possible

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Au(I) - CS(NH2)2 Electrolyte

Au[CS(NH2)2]2+ + e > Au + 2 CS(NH2)2 E0=0.35 V

TU 10-1; Au+ 10-2

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Au(I) - CS(NH2)2 - H2SO4

Electrochemical Behaviour

Characterisation of ECD gold films

PURE GOLD PLATING PROCESS DEVELOPMENT

Electro-synthesis and Characterisation

of Binary and Ternary Gold Alloys

Au-Ag ; Au-Sn

Ag-Au-Sn ; Ag-Au-Pd

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[Au(TU)2+] Reduction Potential

Au[CS(NH2)2]2+

+ e

Au + 2 CS(NH2)2

H2SO4 1 M, Au(I) 5 mM, 25 C / Au 99.9%

Stability Conditions: pH < 1 [Tu] / [Au+] > 5 25 < T < 50 C

Formal Potential

E0 = 130 mV

vs Ag/AgCl (KCl 3M)

0.01 0.1 1-50

0

50

100

150

200

Evs

Ag/AgCl/m

V

log[TU / mol l

-1

]

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Cyclic Polarisation

0.01 0.1 1 10 100-800

-600

-400

-200

0

200

EvsAg/

AgCl/m

V

i / mA cm-2

Au 99.9% - H2SO4 1 M, Au+ 0.1 M, TU 1 M - 40C - 20 mV s-1

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Current Efficiency

CE and deposition potential for ECD Au on Si\Ta\Au(50nm)from Au+ 0.1 M, TU 1 M, H2SO4 1 M - 4 mA cm2 and 40C

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1E-3 0.01 0.1 1 10 100

-600

-400

-200

0

200

400

600

[TU] / M

No

0.01

0.1

1

E

vsAg/AgCl/m

V

i / mA cm

-2

1E-3 0.01 0.1 1 10 100

-600

-400

-200

0

200

400

600

i / mA cm-2

H2SO41 M, FDS 10 mM - 5 mV s-1 - 25C

RSSR + 2 H++ 2 e- 2 TuE= E + 0,0295log([FDS][H+]2/[TU]2)

E0 190 mV vs Ag/AgCl (3 M)

FDS Electroreduction

Pt Au

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Cathodic Passivation Related to FDS Electroreduction

Electrodeposition cd restricted to 2 mA cm-2

Weak Influence of Stirring and Temperature

Search for & Selection of ADDITIVES

Halides Chloride and Bromide: relatively weak effects

dependent on concentration

Best Iodide

Acidic Thiourea Solution

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Au(I) - CS(NH2)2 - KI

Au 99.9% - Au+ 0.1 M, H2SO4 1 M, TU 1 M - 25C 20 mV s-1

0.01 0.1 1 10 100

-600

-400

-200

0

200

KI

No

3 mM

EvsAg

/AgCl/m

V

i / mA cm-2

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0 2 4 6 8 10 12 14 16

40

50

60

70

80

90

100

KI / mM

No

0.63

6

30

/

%

i / mA cm-2

H2SO4 1 M, TU 1 M, Au+ 0.1M - Si\Au, 40C

Current Efficiency

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0 2 4 6 8 10 12 14 16 18

0.0

0.2

0.4

0.6

0.8

1.0

30 mM KI 3 mM

(111)

(200)

(220)

(311)

M

hkl

cd / mA cm-2

Crystal Structure

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Crystal Structure

1 10 1000.0

0.2

0.4

0.6

0.8

(111)

(100)

(110)

(311)

M

hkl

[KI] / mM

H2SO4 1 M, TU 1 M, Au+ 0.1M - Si\Au, 5 mA cm-2 - 40C

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Mechanical Properties

160-190110-18070-10060-90HV -Hardness20 g

SulphiteAcid

CyanideHGNeutral

Cyanide

Alkaline

Cyanide

0 5 10 1550

100

150

200

i / mA cm-2

0 5 10 15

50

75

100

125

150

3 mA cm-2 900s 40C KI 5g/l 900s 40C

100 100050

100

150

200

HV

10mN

[KI] / ppm

50

75

100

125

150100 1000

E/GPa

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Conclusion

Gold Electrodeposition from simple acidicthiourea solution is characterised by cathodic

passivation

Addition of halides (in particular iodide)reduce interface inhibition and greatly extends

the range of deposition cd of practical interest

ECD Au from Au(I)-TU electrolytes is a viableplating process

Thiourea based electrolytes are promising asbase solutions for Au alloys electrodeposition