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Selective Electroless Deposition:
Process, Challenges, and Solutions and
its Critical Role in RC Scaling
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► Electroless deposition
▪ Reaction mechanism
▪ Selective deposition
▪ Areas of application
► Electroless capping
► Via prefill
Agenda
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► Electroless plating
▪ Metal and/or metal alloy plating
process where the deposition occurs
on a catalytic surface from a solution
containing complexed metal ions and
reducing agent(s) without the
application of external current
► Distinctive character
▪ Catalytic surface required for plating
to initiate
— Plating is selective
— Plating can proceed on high sheet
resistance substrate
► Other features
▪ Pure metal or metal alloy plating
possible
▪ Deposition rate
— CVD ELD ECP
▪ Full face plating
Electroless Deposition Mechanism
► Mechanism
▪ Reducing agent supplies the electrons to the catalytically
active surface
— RA + Surface* oxidation product + ze-
▪ Metal ions reduced on the catalytically active surface
— [Me]Z+-Lx + Surface* + ze- Me + xL-
▪ No deposition on dielectric surface (Catalytically inactive)
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Requirements for Electroless Reaction
For an electroless plating reaction to proceed the free energy change
(ΔG; see slide 5) for overall reaction must be negative and the sum of
reaction standard potentials must be positive.
Anodic Reaction: HCHO + H2O HCOOH + 2H+ +2e- E0= +0.056V
Cathodic Reaction: Cu2+ + 2e- Cu(0) E0 = +0.34V
E(0) vs NHE (V)Reaction
Electroless Reaction Half Cell Reduction Potential Examples
Overall reaction: Cu2+ + HCHO + H2O Cu(0) + HCOOH + 2H+ E0= +0.396V
ΔG= -zFE
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Reducing agents/Metals/Metal alloys
Electroless Plating by E. Vaskelis, 27-1. Coatings and Technology Handbook,
Third Edition, Ed. By A. A. Tracton, CRC Press (2005).
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► Electroless deposition process usually made up of several consecutive steps
▪ Pre-clean to clean metal/dielectric surface
▪ Deposition
▪ Post-clean for passivation and particle removal
▪ Dry
Components of the Electroless Bath/Process
Electroless bath
components
Subcategories Examples
Metal salt CoCl2, CuSO4, NiSO4, etc.
Reducing agent Dimethyamine borane, hypophosphite
Complexing agent EDTA, citrate, ethylenediamine
pH adjustor Alkali metal-free TMAH, NH4OH
Buffering agent Borate, Acetate, Phosphate
Additives
Stabilizer Thiourea, O2, Pb2+, 2-mercaptobenzothiazole
Wetting agent Non-ionic and anionic surfactants
Accelerator Propionitrile, O-phenanthroline, chloride
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Electroless Deposition of Metals and Alloys – Catalytic Metals
Metal can be deposited by electroless means in pure form
Elements that can be deposited only together with metals in blue
Metals that can be plated in pure form can act as catalytic surface as long as the right
reducing agent is used
Films deposited from electroless deposition solutions can be pure metals or a large variety
of alloys (CoWP, CoWPB, NiReP, NiCuP, etc.)
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► Main reaction (heterogeneous electron transfer on catalytic surface)
▪ Co2+ + 2e- = Co
▪ N2H4 + 4OH- N2 + 4H2O + 4e-
► Side reactions
▪ Homogeneous deposition (nanoparticle formation in bulk solution)
— Co2+(aq) + 2e- = Co
— To minimize/eliminate this side reaction use stabilizer, stronger complexing agent, lower T
deposition, etc.
▪ Oxygen reduction
— O2 + 2H2O + 4e- = 4OH-
► Charge (current) balance at the catalytic surface
Reactions during Electroless Deposition
icathodic = ianodic
icathodic= iCo dep. + ioxygen red.
ianodic= ihydrazine ox.
Catalyst
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Effect of some parameters on initiation time
345 350 355 360
0
10
20
30
40
50
60
70
80
Initi
atio
n tim
e/s
Temperature/oK
8.8 9.0 9.2 9.4 9.6
10
15
20
25
30
35
Initia
tio
n t
ime
/s
pH
0 20 40 60 80 100
0
10
20
30
40
50
60
Initi
atio
n tim
e/s
DMAB concentration/% of BKMTemperature and DMAB concentration have
stronger effect on initiation time than pH
Temperature and DMAB effect can be described by
B
xAit exp
0 80 160 240 320
-1.0
-0.8
-0.6
-0.4
-0.2
72 oC
75 oC
78 oC
E/V
vs.
Ag
/Ag
Cl
Time/s
OCP vs. time at different temperature
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► ELD process
▪ Co cap selectivity based (Co film thickness on target surface divided
by Co film thickness on non-catalytic surface)
— 1e6 (Cu vs. BDIII) for 250A film
— 3e7 (Cu vs. TEOS) for 250A film
— Electroless film selectivity ratio increases with target deposition
thickness
▪ Number of particles per deposited thickness
— Non-target surface gets ionic contamination due to contact
with plating solution but no growth
▪ Ionic contamination ~1-5e13 atoms/cm2
▪ Particles usually fall on particles on non-target surface
which can act as nucleation sites for growth
► Main challenges for ELD
▪ Mobile ions on dielectric surface
▪ Particles on wafer surface (dielectric + metal)
Selectivity
Target surface
Non-target surface
t/s
Thic
kness
/nm
Ionic contamination
Film thickness:
0.7micrometer
Film thickness:
1.8micrometer
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Initiation Process and Diffusion Effect (ELD of CoB)
Linear diffusion
Non-linear diffusion
J.W.M. Jacobs, J. M. G. Rikken in Electroless Deposition of Metals and Alloys Ed. by M. Paunovic, I.
Ohno, Proceedings Volume 88-12, The Electrochemical Society
5.0
04
t
DFCj
r
D
t
DFCj
44
5.0
0
► First stage: break-up of DMAB to dimethylamine and borane,
oxidation of borane and reduction of oxygen
► Second stage: oxygen reduction becomes strongly diffusion controlled (linear vs. non-
linear diffusion), electrode potential shift to more negative value (pattern dependence)
► Third stage: cobalt reduction starts, formation of cobalt islands on the surface
(passivation: metastable nucleation)
► Fourth stage: film growth
0 50 100 150 200
-1.0
-0.8
-0.6
-0.4
E/V
vs.
Ag
/Ag
Cl
Time/s
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► Similar situation for via prefill
Linear vs. non-linear Diffusion for Via Prefill Case
Linear diffusion within
the vias in both cases
(however the flux will
be determined by the
diffusion type at the via
opening)
Non-linear diffusion
outside the isolated vias
Linear diffusion
outside the via chain
due to merging
diffusion layers
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Simplified Mixed Potential Model for Initiation
(Effect of Oxygen Concentration)
E/V
i/A
t0
ti
ti: initiation time
At all time the mixed
potential is determined by:
ianodic = icathodic
At t0: ihydrazine,ox = ioxygen,red
At ti: ihydrazine,ox = ioxygen, red + iCo dep
Co ion reduction
Oxygen
reduction
N2H4 oxidation
5.0
t
DnFACi
Diffusion
limited
range
Not diffusion
limited
0 50 100 150 200
-1.0
-0.8
-0.6
-0.4
E/V
vs.
Ag
/Ag
Cl
Time/s
ti
► At low reducing agent concentration (and low oxygen concentration)
▪ The initiation time will be determined by the oxygen current decrease
▪ The higher the oxygen concentration in the solution the longer it takes to initiate
deposition
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Dielectric
O- O- O- O- O- O-
-
-
---
-
- -
► Root cause
▪ Homogeneous nanoparticle formation
— Solution stability
— Contamination
▪ Surface contamination
► Critical steps
▪ Nanoparticle formation
— Cu+ released from Cu oxide can cause nanoparticle formation
▪ Nanoparticle attachment
— Nanoparticles formed through Cu+ injection or homogeneous particle
formation attach to the wafer surface
▪ Particle removal
— Removal of attached particles by (mostly) mechanical means
Particle Formation/Reduction
DielectricCu
Au
nanoparticle
adsorption of
functionalized
silica
T. Zhu, X. Fu, T. Mu, J. Wang, and Z. Liu:
Langmuir 15 (1999) 5197.
Cu
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Where Does ELD Fit In?
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Challenges for Interconnect Cu Metallization
Category Root Causes
Yield Issues
• From Cu seed issues
• PVD Cu seed is non-conformal,
requires thin seed to avoid pinch-
off
• Thin Cu seed prone to
discontinuities
• Cannot nucleate on liner so
incomplete Cu Gapfill
Reliability
• Electromigration
• TDDB
• Weak interface of Cu to dielectric
barrier lead to electromigration
• Large voltages across pinch points
create BTS/TDDB issues
RC
• Resistance
• Capacitance
• Barrier takes up room, 50% of cross
section at 12 nm HP, line resistivity
increases
• ULK Integration Challenges
Barrier consumes area,
resistivity goes up higher
Cross section area (nm2)
Cu r
esi
stiv
ity
Pinch
off
Discontinuity
Low k
damage
EM
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Electroless Capping
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► CoWP
▪ Alloying elements altering crystalline structure of deposited film
— Reducing grain size
— Amorphous structure above 12 at. % P
▪ Grain boundary enrichment of P and W expected
▪ W to improve barrier property and increase recrystallization
temperature
▪ Diffusivity of Cu ~2-3 orders of magnitude lower than in pure Cu
► Composition can be controlled by composition of the
solution
▪ Wide variety of composition can be achieved by adjusting
concentration of alloying elements, reducing agent and complexing
agents
► Plating on Cu surface
▪ Cu not catalytic to hypophosphite oxidation
— Use Pd activation
— Use secondary reducing agent in the bath that can initiate deposition
Electroless Deposition of CoWP Alloy
C.-K. Hu, L. Gignac, R. Rosenberg, E.
Liniger, J. Rubino, C. Sambucetti, A.
Domenicucci, X. Chen, and A. K.
Stamper: Appl. Phys. Lett. 81, 1782
(2002).
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► Dielectric (non-catalytic surface) plays important role in particle attachment
while the catalytic metal has major influence on particle generation
Mechanistic View of Metallic Particle Formation (Co Plating
on Cu)
► Nanoparticles form during electroless deposition
► Nanoparticles stay in the solution and grow
► During DI rinse the surface charge changes on the particles and particles attach to dielectric
► Nanoparticles form during electroless deposition
► Nanoparticles attach to dielectric surface and grow on the surface
Dielectric
Dielectric
Mechanism 1
Mechanism 2
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TEOS LK ULK
Pre-clean1 Pre-clean2 Pre-clean3 Pre-clean1 Pre-clean2 Pre-clean3 Pre-clean1 Pre-clean2 Pre-clean3
► Optimization of pre-clean a very important step on getting selective plating
► Type of dielectric surface mostly determines particle attachment to the
surface (considering that the post-CMP Cu contamination level is the same on
all dielectrics)
Impact of Dielectric and Pre-clean on Defectivity
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► Large E-M benefit, expected for metal cap
► TDDB comparable or better than uncapped controls
CoWP Reliability on 32nm Node Qualification Hardware
Published IBM
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Additional Benefit from Co Capping
► Thicker Co cap can also improve EM through
encapsulating Cu by diffusing Co to the Cu/liner
interface
▪ Enables thinner barrier/liner
From IMEC
CuTaCo
Position (nm)
Co
un
ts
6040200.0
100
80
60
40
20
+
21
Position (nm)
Co
un
ts
6050403020100.0
100
80
60
40
20
21
CuTaCo
Position (nm)
Co
un
ts
403020100.0
150
100
50
21
CuTaCo
+ +
T. Kirimura et. al. MAM 2013 (IMEC)
0
200
400
600
800
1000
0 2 4 6 8 10
Vo
id g
ro
wth
alo
ng
Cu
lin
e (
nm
)
Stress time (hrs)
Thin barrier + SiCN cap
Thin barrier + CoWP cap
Thick barrier + SiCN cap
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Electroless Via Prefill
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► Potential via prefill benefits
▪ Barrier area Via R
▪ Via voiding
▪ Less R variation for unlanded via
▪ Enables
— Tall via C
— Tall trench R
— FAV scheme
ELD Via Pre-Fill: Improved Via Performance for 5nm
Technology Node and Beyond
High
yield via
fill
Easy
trench fill
True Bottom-Up Fill Low impurity, crystalline filmCo
Cu
Cu
Barrier
Fundamentally 100% Selective
Low Resistivity
Metal barrier/liner
0
4
8
12
16
20
0 100 200 300 400 500 600 700
Resistivity/μcm
Thickness/A
PVD Cu
ELD Co1, pH=7
ELD Co1, pH=7.5
Co1a, pH=7.5
Composition (SIMS)/ppm
B C N O Na S P Cl K
160 240 50 370 0.12 <4 <2 1 0.01
As deposited
film
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Selective Electroless Fill Options for BEOL Application
Challenges
• PVD Cu seed is non-conformal, requires
thin seed to avoid pinch-off results in
discontinuities incomplete Cu Gapfill
• Barrier takes up room, 50% of cross section
at 12 nm HP, line resistivity increases RC
delay and IR drop increasing
Solutions
• Electroless Co/Cu via prefill
• Extending Cu metallization for BEOL
• Lower via resistance• 20%-50% lower via resistance at 10nm technology node
w/ Co
• Over 50% via resistance reduction w/ Cu
• Less via resistance variation for unlanded via
Pinch
off
Discontinuity
Low k
damage
Barrier consumes
area, resistivity goes
up higher
Cross section area (nm2)
Cu r
esi
stiv
ity
• Targeting V0 application (HAR vias)
• Targeting lower level BEOL vias
• Testing via prefill w/ and w/o
selective barrier
M1
V0
Co
Co
Cu
Cu M2
V1Cu
Cu via prefill
Co via prefill
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► Via resistance can be lower than baseline even
with Co film
▪ No barrier for Co
▪ TaN barrier moves from via bottom to via top
► High resistivity film will increase via resistance
► Cu can drop via resistance
▪ This scenario is valid if dielectric barrier used
Via Prefill Simulation (Without Barrier in the Via)
► Thin metal barrier and low fill metal resistivity
are required to achieve low via resistance for
via pre-fill approach
CoW Fill
Co Fill
Cu Fill
Reference
CoW
Co
Cu
Simulation Parameters
Std. Barrier Scheme with
fill resistivity of 4 µΩ*cm
Via Dimensions
40 nm Tall
16 nm Top CD
85° Taper
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ELD Cu Prefill TDDB Risk Assessment
Cu diffusion
into dielectric ?
L Zhao et al., Appl. Phys. Lett. 106, 072902 (2015)
TDDB degradation
on SiO2 without a
barrier
No significant
TDDB degradation
on dense low-k
without a barrier
No significant TDDB degradation with dense low-k (OSG) even without a barrier
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Selective Electroless Cu Via Fill
➢ Selective dielectric
barrier development
under evaluation
➢ Alternative to move
alloying elements
through the metallized
via after electroless
plating
Via Chain
(Link 46) Center
Via Chain
(Link 46) EdgeIso
TEM
on Via Chain
Selective barrier ELD Cu via prefill concept
M2
M1
V1
M2
M1
V1
Cu
ELD Cu
Cu
Cu
Cu
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➢ Key Factors:
▪ Optimum X concentration
▪ Adhesion to ULK
▪ Barrier performance
▪ Resistivity performance
Selective Electroless CuX for Via Prefill
Cu
Cu
Cu
CuX via prefill concept
M2
M1
V1
M2
M1
V1
Cu
Cu
CuXX-rich barrier/adhesion layer
ELD Cu + anneal
ELD CuX + ELD Cu 200nm + anneal
Adhesion improvement w/ CuX
Subst
rate
: ThO
x/ 3
nm
Ru
Selective plating w/ CuX
Anneal
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Pre-clean1 Pre-clean2 Pre-clean3 Typical Defect
► Similar to capping process pre-clean optimization is important to reduce
defectivity
Defect Reduction – Electroless Cu Via Prefill
No defect
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► Electroless plating technology provides selective deposition of pure metal or
metal alloys on catalytic metals
► Selectivity primarily determined by bath stability/reactivity (homogeneous
reaction) and/or bath contamination
► Electroless deposition was introduced in production for selective capping
► Selective via prefill under evaluation by multiple companies due to it’s benefit
in extending BEOL metallization and significant improvement in via resistance
Summary