electrochemical processes for microelectronics …– poor adhesion to dielectrics – rapid...

1Y. Shacham-Diamand, Sept. 04

Electrochemical processes for Electrochemical processes for Microelectronics ApplicationsMicroelectronics Applications

Prof. Yosi Shacham-Diamand


Middle east


Why the middle east is relevant to Cu ULSI technology ?

The origin of the Damascene technology, now used for ULSI metallization, is in the middle east.The name “Damascene” means “from Damascus”


Eastern Roman Empire, ~ 1500 years ago


Damascene: From Damascus


Zichron-Yaakov – my hometown


Tel-Aviv university – Faculty of engineering


List of content

IntroductionThe chip wiring problemCu wiring - the current solution

Cu electroplating – basics and resultsElectroless plating:

Electroless Cu technologyBarrier and capping layers

Summary


Introduction

The chip wiring problemCu wiring - the current solution


Cross section of ULSI interconnect scheme (ITRS 2001)

Hierarchical Scaling


Typical Very Large Scale Integration (VLSI) Chip includes many layers of metallization, however most of them are made by sputtering. For example, multi-level aluminum interconnect (DEC Alpha AXP 21164)


Another example, Intel technology

Intel 2000


- Those examples were all with aluminum technology.- Aluminum metallization dominated VLSI integrated circuits manufacturing for more than 20 years.


Not anymore….Plating is finding its place again as a useful technology for modern VLSI technology…..


Subtractive vs. additive patterning


Copper multi-level metallization: 0.25 µm electroplating technology made by IBM


IBM CMOS 7S process


IBM PowerPC 750


If Aluminum technology is so good why switching to copper?

Al reliability problems:ElectromigrationStress voidingContact electromigration

Al resistivity becomes too high Al structure is actually TiN/Al/TiN/Ti with W plugs…..it’s complicated and expensive.


Moore’ s law“ The number of transistors in a chip doubles every 18 -24 months”

Doubling time of fitted line is 2.3 years


Feature size reduction


Wafer size growth


Gate and interconnect delay versus feature size. Interconnect delay versus feature size. Interconnect delay is shown for repeater spacing (L) of 3000 and 5000

Havermann, 2001


Local vs. global wiring

Havermann, 2001


Interconnect capacitance

The effective are of capacitors is larger than their projected area Capacitance is not scaled down linearly !!!


Interconnect capacitance: line to line vs. line to ground capacitance


Microprocessor speed as function of feature size and technology (Haverman, 2001)


The “long line” problem


The ultimate limits…until further notice

Davis, 2001


The Cu solution to the ULSI metallization scaling problem.


Why copper?Why copper?– Copper resistivity (1.7µΩ-cm) much lower thanaluminum (2.6 µΩ-cm)– Electrical reliability >10x greater than aluminum(electromigration + stress voiding)

What are the issues?What are the issues?– Difficult to Etch– Poor Adhesion to dielectrics– Rapid Diffusion through SiO2– Forms mid-gap energy level in Si


Electroplating Based Process Sequence

Simple, Low-cost, Hybrid, Robust Fill Solution

Pre-clean IMP barrier + Copper Electroplating CMP

25 nm 10-20 nm + 100-200 nm


Electroplating solutions

• Cu ions - Cu sulfate

• Acid - H2 SO4 for pH adjustment

• HCl - Affects Cu surface adsorption; Halide ad-layer drives Cu growth. It also acts as a surfactant and stabilizes grain growth. Cu deposition is driven by the desorption of the halides.


Additives

Wetting agents - penetration into deep feature

Suppressors (Inhibitors)Carriers - Poly Ethylene Glycol (PEG) , MW ~ 2000-5000. Forms thin film that inhibits charge transfer, diffuses slowly.

Levelers - Adsorbed at corners and edges and slow surface diffusion & growth, improve step coverage.

AcceleratorsBrightners - Organo-sulfides, diffuses rapidly, Catalyzes charge transfer.


Single vs. dual Damascene


Damascene: Damascus Scenery

It is an ancient art from the Byzantine empire (East Roman)Damascus, now in Syria, was a big metallurgical center 2000 years agoThey invented the embedded gold art, now you can find it near Toledo in Spain. (Spain was under Arabic invasion ~ 1200 years ago)

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/advanced/t5_1_1.html


Damascene Technique in Metal Working

Metal was inlaid with silver or gold


Damascene technology

The "steel" part - Iron rich in Phosphorous and not necessarily Carbon The forge welding - folding over the same basic material which, however, may have been quite inhomogeneous. Lots of folding and forge welding created a homogeneous looking material - this is the Japanese way.The welding technique was not only continued (and somewhat irregular) folding and hammering, but a more complicated technique, called "pattern welding". Was used.


Part of a very old damascene (= pattern welded) sword blade. )from the Internet article "Blade Patterns Intrinsic to Steel Edged

Weapons ")from Lee .A .Jones


In Japan, around 600 A.D., smelting technology was introduced from China and Korea. The Japanese specialty was the mass production of (impure) steel, which was folded so many times and forge welded again that all the impurities were driven out of the steel and the carbon became as evenly distributed as modern steels we have today


Cu electroplating basics


Tafel plot for Cu electrodepositing from aqueous solution CuSO4 (0.15 N) and H2SO4 (1.0N)


Anodic and cathodic currents of Cu electrode

In aqueous solution CuSO4 (0.15 N) and H2SO4 (1.0N)


Plating regions of operation


Deposition systems types

Fountain type – horizontal waferImmersion type – vertical wafer


Fountain type


Electroplating solutions – Acid bath


• Acid - H2 SO4 for pH adjustment

• HCl - Affects Cu surface adsorption; Halide ad-layer drives Cu growth. It also acts as a surfactant and stabilizes grain growth. Cu deposition is driven by the desorption of the halides.


Electroplating Based Process Sequence

Simple, Low-cost, Hybrid, Robust Fill Solution

Pre-clean IMP barrier + Copper Electroplating CMP

25 nm 10-20 nm + 100-200 nm


Electroplating solutions – Basic bath


• Pyrophosphate (P2O7)4-

• Nitrate, NO3-

• Ammonia

• Orthophosphate (HPO4)2-

• Additives

pH ~8.0-8.8

T=50-60C


Additives

Wetting agents - penetration into deep feature

Suppressors (Inhibitors)Carriers - Poly Ethylene Glycol (PEG) , MW ~ 2000-5000. Forms thin film that inhibits charge transfer, diffuses slowly.

Levelers - Adsorbs at Corners and slows surface diffusion & growth, improve step coverage, Amidofunctional groups.

AcceleratorsBrightners - Organo-sulfides, diffuses rapidly, Catalyzes charge transfer.

5959

Applications for plating

On-chip wiring3D integrationPackaging


Dual Damascene integration scheme


Problems in a Damascene process


Dual Damascene with trench first


Cu+low-K technologyFully planar - better lithographyFaster - lower resistivity and capacitanceMore reliable lower power - P=1/2 CV2fLower cost - Dual damascene process has fewer steps and its capital and tool investment is ~40% less than Al process.Electroplating is compatible with Electroplating is compatible with lowlow--K technologyK technology


Low-K materials for Cu interconnects


Sub-100 nm interconnects (a)

Problems:Materials properties – texture, morphologyProcess – coverage, uniformity.Integration – compatibility, integrabilityTools – new processes require new toolsResistivity – dimension effects: Narrow Line

Resistivity Increase (NLRI) effectReliability: voiding, delamination, electromigration, corrosionHigh complexity, large chips with high lead counts packaging

All this lead to long learning time, high NRE, low yield and eventually, to cost problems


Sub-100 nm interconnects (b)

Solutions:Better deposition methods – ALD, seedless electrolessClever integration methods - Integrated barrierAlloying –.

Conducting metal - barrier-less metallization, better conductivity for sub-50 nm linesBarriers: Better materials. e.g. Co(W,P) is a better barrier than Co(P), electroless barriers offer high selectivity and save process steps


Narrow Line Resistivity Increase (NLRI) effect is aggravated by the presence of the barrier

Linewidth, nm40 60 80 100 120 140

Res

ista

nce

per u

nit l

engt

h, Ω

m-1

0

1000

2000

3000

4000

5000"Barrierless" Cu "Barrierless" Ag Cu with 10 nm barrierCu with 5 nm barrier

“barrierless” metallization

Conventional Dual Damascene


Cu is better than Al–ReliabilityCu alloys are better than pure Cu


Cu technology main problemsCu technology main problems

Difficult to EtchPoor Adhesion to dielectricsRapid Diffusion through SiO2 and low-K

inter-level-dielectrics (ILD)Forms mid-gap energy level in SiVia/trench fillingPost CMP damageCorrosion


A Cu interconnect process for the 130nm process technology nodeP. Moon, INTEL (AMC, 2001)


Sub-100 nm Cu interconnects - A

Modified metallization process

• 10 nm Ta barrier PVD• 30 nm Cu seed PVD• 700 nm Cu ECD• CMP

100 nm

Extending Copper Metallization Technology for Wiring to End-of-Roadmap Feature SizesM. Engelhardt1, G. Schindler1, K. Mosig1,2, G. Steinlesberger1,W. Steinhögl1, G. Gebara2

1: Infineon Technologies, Corporate Research, CPR NP, Otto-Hahn-Ring 6, D-81730 Munich, Germany2: International Sematech , 2706 Montopolis Drive, Austin, TX 78741-6499, USA

Trenches remain open for ECD


SiO2

Si3N4

25 nm Ta100 nm Cu seed

10 nm Ta30 nm Cu seed

Sub-100 nm Cu interconnects - B


sFull Cu Vertical Interconnect

2.5x2.5 µm2 Through-wafer Via-Chain

P. Ramm, D. Bonfert, R. Ecke *, F. Iberl, A. Klumpp, S. Riedel *, S.E. Schulz *, R. Wieland, M. Zacher *, and T. Gessner *

Fraunhofer Institute Reliability and Microintegration, Munich, Germany * Chemnitz University of Technology, Germany

Challenge:

Via filling of deep and high aspect ratio features


Additive ConcentrationCS

TOP0

Height

CSBOTTOM

CONSUMPTION

INFLOW

Step-coverage and filling modeling –Explain super-filling, voids

Few groups work on modeling, for example -

• T. Moffet et al. (NIST, USA)

•T. Smy (Carlton Univ., Canada)

•N. Kovarsky et al (AMAT)

The models include

• Solution components transport modeling

• Additive effects

• Curvature effects

• Modeling at various scales – tool, trench


attribute Cr TiN TiN/Ti Ti/TiN TiN/Ta TaN β-Ta TaN/α-Ta TaSiN WNx

Cu barrier X a a a a a a a a a

Liner adhesion to ILD a a a a a a X a a a

Cu adhesion to liner a X a X a X a a X ?/XLiner adhesion onto Cu a ? ? a ? a a a a ?Low in-plane resistivity a X a a ? X X a X Xno Cu poisoning a a X a a a a a ? a

CMP - high yield ? X X X X/ a a a a a ?single chamber a a a a X a a a a a

Low via resistance a a X ? a a a a a ? aLow contact-resistance a a a a a a a a ?No Cu corrosion ? X X X X/ a a a a a ?Thermal stability ? a X X a a a a ? a

Low stress, no cracking X a a a a a a a a a

Good step coverage ? a a a a a a a a a

Final X X X X X X X a X X

Evaluation Criteria for Cu Liner,D. Edelstaein et al, IBM, AMC 2001

77Y. Shacham-Diamand, Sept. 04*E. Cooney III, et al., AVS (1999)

PVD TaN/Ta/Cu-seedPVD TaN/Ta/Cu-seed

I-PVD TaN/Ta/Cu-seed, plated-Cu fill*I-PVD TaN/Ta/Cu-seed, plated-Cu fill*

SEMs of > 4:1 Cu dual-Damascene.SEMs of > 4:1 Cu dual-Damascene.

Cu Dual-Damascene Metallization (D. Edelstein, AMC 2002)


90±5 nm

500C

400CAs dep.

nm

NixW1-x Cu

SIMS profiles of thin NiW films on Cu before and after 30 min. anneal. Example: Ni60W40

7979

Electroless Plating for ULSI and MEMS Applications


Metal displacement reactionExample: when Zinc is immersed in a Copper Sulfate solution:

V 0.76E 2 020 =+→ −+ eZnZn

V 0.34-E 2 002 =→+ −+ CueCu

1. Anodic oxidation of Zinc

2. Cathodic reduction of Cu

Result:

Cu is deposited on the Zn sample

After all the Zn sample is coated with Cu the deposited stops.

E0 = 0.76 (Oxidation)

E0 = +0.34 (Reduction)


Pourbaix Diagram for Cu


Nernst equation

E0 – the potential versus a standard hydrogen electrode (SHE)

Note that E0 is defined for the standard RedOx reaction

[ ][ ]d

OxnFRTdneOx Re

0 lnEE Re +=→+ −

Therefore E0 is defined for [Ox]=[Red]=1M

E is the RedOx potential; it is related to the free energy change of the reaction as

∆G=-nqE


Electroless plating solution1. Cu salt – Cu is reduced

Cu+2 + 2e- Cu0 E0Cu = 0.34 eV

2. Reducing agent – oxidized at the interface

Red-n Red-n+2 + 2e- Oxidation potential = -Ered

(Red-n+2 + 2e- Red-n , -Ered < Ecu

3. Total reaction – Cu+2 + Red—n Cu0 + Red-n+2

∆E= ECu+ Ered >0 ∆G=-2q∆E < 0,

i.e. spontaneous deposition !!!


Typical Cu electroless deposition solutions


Component Aim Cu deposition

Co(W,P) deposition

Metal salt metal ion source CuSO4•5H2O CoSO4•6H2O Na2WO4•2H2O

Reducing agent

reducing the metal ion HCOH Na2H2PO2

Complexing agent

reducing electrochemical potential difference

EDTA Na3C6H4O7•H2O

pH adjustments

electrochemical potential, rate and mechanism

N(C2H5)4OH,

KOH or TMAH

Stabilizer stabilizing the solution, preventing hydrogen inclusion

N(CH3)4CN 2,2'-dipyridyl

Surfactant reducing surface tension, extracting H2

RE-610 RE-610

Electroless Cu and Co(W,P) for VLSI metallization


I [A]

Electroless solution potential, V

Oxidation of the reducer

ia

Ic Reduction of the metal ions

E0Mz+

E0red

Mixed potential

Emp

Total current

In electroless plating the total current = 0

Therefore, the anodic current balances the cathodic current


Cu dep. Rate vs pH (Cu2+, HCOH solution)


Solution composition for electroless Cu deposition with Glyoxylic acid

Components Concentration

CuSO4·5H2O 0.0305 M

EDTA 0.0351 M

Glyoxylic acid 0.054 – 0.11 M

Surfactant RE-610 0.004 g/l

KOH or TMAH Adjust pH to 12.3-12.7


ab

cSEM pictures of via contacts (a) before

deposition (with seed layer), (b) after 1 min

electroless copper film deposition, and (c) after 2

minutes of ELD Cu.


Electroless deposition of alloys is also possible for VLSI applications…..

93Y. Shacham-Diamand, Sept. 04Source: J.A. Cunningham, SI, April 2000

9494

, Zn, Co & Ni, Zn, Co & NiSnSnSolutions a for Cu alloys with Solutions a for Cu alloys with

• CuSO4 and Cu(NO3) 2 based solutions,

• EDTA or Rochelle salt (sodium-potassium tartrate) as complexing agents

• SnCl2.2H2O, ZnCl2 CoSO4 or NiCl2 as a source of alloy-metal ions.

• Formaldehyde or sodium borohydride (NaBH4).

• pH ~ 12-13

• T = RT to 90C


Thickness of electroless deposited thin films (A)

0 200 400 600 800 1000 1200 1400

Elec

trol

ess

depo

site

d th

in fi

lms

resi

stiv

ity, Ω

.cm

1e-5

1e-4

1e-3

CuCo/Co CuNi/Co

Resistivity of electroless Cu-Co & Cu-Ni as a function of film thickness


Molar ratio [Cu++]/[Ni++] in bath

0.00 .05 .10 .15 .20 .25 .30 .35 .40

Res

istiv

ity o

f ele

ctro

less

dep

osite

dth

in C

u-N

i film

s, Ω

.cm

0e+0

1e-6

2e-6

3e-6

4e-6

5e-6

6e-6

7e-6

8e-6

9e-6

1e-5on Cu as depositedon Co as depositedon Cu(2500C) on Co(2500C)

Resistivity of Cu-Ni films as a function of The molar ratio [Ni++]/[Cu++]


Electroless Co and Ni alloys


Advantage of Electroless barriers

ConformalLow costGood quality - low ρ, low stress can be integrated with electroless copper

Barrier

Cu

ILD


Requirements from barrier layers

• Step coverage on high aspect ratio holes and trenches

• Low thin film resistivity

• Adhesion to the ILD

• Adhesion to Cu

• Stable at all process temperatures

• Process compatible to the ILD

• Process compatible to CMP

• Act as a good barrier

101101

Conformal deposition of electroless Co0.9W0.02P0.08

Deposition on periodic structure of 0.35 µm wide trenches with

0.7 µm pitch(a) several trenches and (b) close up trench(Shacham-Diamand et. al. Microelectronic Engineering 37/38 (1997) 77-88)


CoWB capping, (Itabashi 2002)


Dewetting of Cu from the underlying barrier material during high temperature and short time heat stress, which is typically observed after post-ECP annealing and post dielectric deposition/coating curing processes [T. Oshirnaet al, IEDM 2000].


CoWP capping layer (IBM) slow down electrmigration: The resistance of a damascene Cu conductor, with and without a thin metal film on the top surface, vs time.


Bright Field cross sectional

TEM micrographs of IMPTa/TaNx, IMP Cu andelectroless Co0.9W0.02P0.08

deposited into a damascene

trench structure.

overview

sidewall of a

trench

bottom and sidewall of a trench

top of a

trench

A. Kohn¤ and M. Eizenberg Y. Shacham-Diamand , To be published, 2002


Summary

•• Electroplating and electroless plating are viable Electroplating and electroless plating are viable technology for metal deposition for microelectronicstechnology for metal deposition for microelectronics

•• It can be used also for MEMS and MEOMS It can be used also for MEMS and MEOMS applicationsapplications

•• Electroplating: typically acid bath, low pH, current Electroplating: typically acid bath, low pH, current dependent, superdependent, super--filling is possiblefilling is possible

•• Cu Basic bath can complement the acid bathCu Basic bath can complement the acid bath

••Electroless plating: Basic bath, high pHElectroless plating: Basic bath, high pH

•• Electroless plating: conformal, surface reaction rate Electroless plating: conformal, surface reaction rate dependentdependent

electrochemical processes for microelectronics …– poor adhesion to dielectrics – rapid...

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