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8/12/2019 gold plating thiourea electrolyte
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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