selective electroless deposition: process, challenges, and ... · confidential - limited access and...

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Selective Electroless Deposition:

Process, Challenges, and Solutions and

its Critical Role in RC Scaling


► Electroless deposition

▪ Reaction mechanism

▪ Selective deposition

▪ Areas of application

► Electroless capping

► Via prefill

Agenda


► 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)


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


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).


► 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


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.)


► 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


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


► 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


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


► 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


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


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


Where Does ELD Fit In?


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


Electroless Capping


► 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).


► 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


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


► Large E-M benefit, expected for metal cap

► TDDB comparable or better than uncapped controls

CoWP Reliability on 32nm Node Qualification Hardware

Published IBM


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


Electroless Via Prefill


► 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


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


► 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


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


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


➢ 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


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


► 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

selective electroless deposition: process, challenges, and ... · confidential - limited access and...

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