Electroless Capping and Diffusion Barriers For Copper ... · Electroless Capping and Diffusion...

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1 1 Electroless Capping and Diffusion Barriers For Copper Metallization – Material properties Prof. Yosi Shacham – Diamand ,

Transcript of Electroless Capping and Diffusion Barriers For Copper ... · Electroless Capping and Diffusion...

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Electroless Capping and Diffusion Barriers

For Copper Metallization – Material properties

Prof. Yosi Shacham – Diamand ,

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Co alloy barriersCo alloy barriers

CoReP – V. DUbin, 1993

NiReP – N. Petrov et al., 2001

CoWP – Y. Shacham, 1996

CoWB- T. Osaka et al, 2002

CoMoP – Y. Shacham, 1999

(Approximated date of 1st publication or patent )

33

• Low solubility of Cu in Co and no phase formation, • Cu solubility is about 0.1% at 400C• P - Low solubility in Co -

→ enrichment of grain boundaries?→ Affects microstructure, reducing grain size → Froms amorphous structure at high concentration (> 12%)

• W: Low solubility in Co -→ Stuff the grain boundaries of the Cobalt

Co - P

Electroless Co alloys - Co(1-x-y) WxPy

Negligible solid solubilitysolubility of P in fcc Co is less than 0.47 at. %Ishida and Nishizawa, Bull. Alloy Phase Diag. 11, 555 (1990)

Negligible solid solubilitysolubility of W in fcc and E Co is less than 1 at. %

Co - W

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2. Electroless deposition of conformal ultra-thin Co1-x-yWxPy films

Top FieldBottom Field / Sidewall

Sidewall

Comparison to current industrial technologyIonized Metal Plasma PVD

Case study: Co0.9W0.02P0.08

A. Kohn, M. Eizenberg,

and Y. Shacham-Diamand,

Appl. Surf. Sci., to be published

BF CS TEM micrographs

55

400 600 800 1000 1200 140010-1

100

101

T (oC)

W s

olub

ility

in fc

c C

o (a

t. %

)

Sykes Magneli et al. Larikov et al. Takayama et al. (XRD) Takayama et al. (EPMA)

400 600 800 1000 120010-2

10-1

100

101

T (oC)

Cu

solu

bilit

y (a

t. %

)

Hasebe and Nishizawa Bruni and Christian Old and Haworth Hasebe and Nishizawa

Proposed system for ULSI Cu metallization

• Cu: Low solubility in Co and no phase formation• P, W: Low solubility in Co

→ enrichment of grain boundaries?• P: Affects microstructure, reducing grain size

→ amorphous structure?• W: Proposal

→ introduction of a refractory alloying element may improve barrier efficiency?

Co - P

Co - W

Co alloys - Co(1-x-y) WxPy

Co - Cu

Theoretical

calculation

Negligible solid solubilitysolubility of P in fcc Co is less than 0.47 at. %Ishida and Nishizawa, Bull. Alloy Phase Diag. 11, 555 (1990)

66Nishizawa et al. , Bull. Alloy Phase Diag. 5, 161 (1984)

Co - Cu

The CoWP system for ULSI Cu metallization

Nagender Naidu et al., “Phase Diagrams of Binary Tungsten Alloys”Indian Institute of Metals 60 (1991)

• Cu: Low solubility in Co and no phase formation• P, W: Low solubility in Co

→ enrichment of grain boundaries?• P: Affects microstructure, reducing grain size

→ amorphous structure?• W: Proposal

→ introduction of a refractory alloying element may improve barrier efficiency?

Ishida et al. , Bull. Alloy Phase Diag. 11, 555 (1990)

Co - P

Co - W

Co alloys - Co(1-x-y) WxPy

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hcp Co

fcc Co

Orthorhombic Co2P

0.4 0.6 0.8 1.0 1.2 1.4o

Rad

ial I

nten

sity

(a.u

.)

600oC

400oC

as-dep

s (A-1)

Evolution of microstructure with thermal annealCo0.9W0.02P0.08

Radial intensity of the SAEDas a function of the scattering vector

0 5 10 150

100

200

Num

ber o

f gra

ins

(-)

Grain size (nm)

0 5 10 15 200

100

200

Num

ber o

f gra

ins

(-)

Grain size (nm)

0 40 80 1200

100

N

umbe

r of g

rain

s (-)

Grain size (nm)

Dark field plan view TEM micrographs, SAED, apparent grain size histograms

as-dep

400°C

600°C

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As-deposited structure:

Hexagonal close-packed cobalt nanocrystallites (d ~ 3-5 nm), with a preferred basal plane orientation embedded in an amorphous Co(W,P) matrix.

Evolution of structure with thermal anneal:

T ~ 300°C: hcp Co + amorphous Co(W,P) → hcp Co ; Ea = 1.6 ± 0.1 eV, constant nucleation rate, diffusion limited

T ~ 420°C: hcp Co → hcp Co + orthorhombic Co2P ; Ea = 4.7 ± 0.1 eV

T > 500°C: Delayed hcp Co → fcc Co transformation relative to bulk Co

P bonding shifts to covalent bonding at T > 600°C

Structure during failure of barrier:

At T ~ 450°C, the microstructure is hcp Co nanocrsytallites (d ~ 15 nm, 1 hour anneal), and small amounts of orthorhombic Co2P .

→ Failure mechanism : grain boundaries diffusion

Summary: Evolution of microstructure with thermal anneal

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CoMoPCoMoP and and CoWPCoWP were deposited on sputtered seed were deposited on sputtered seed layers:layers:

Ti/Cu or Ti/Co on Silicon oxide.Ti/Cu or Ti/Co on Silicon oxide.

Ti improves the adhesion to the oxide.Ti improves the adhesion to the oxide.

Cu or Co are the seed layer.Cu or Co are the seed layer.

The samples were cleaned prior to the depositionThe samples were cleaned prior to the deposition

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Basic properties of Co(Mo,P)

30 30 –– 60 60 µΩµΩ.cm.cm60 60 –– 180 180 µΩµΩ.cm.cmResistivityResistivity

CoWPCoWPCoMoPCoMoP

1. The resistivity depends on the composition, thickness and seed type

2. Under similar conditions, e.g. same thickness, composition and seed type, the CoMoP layers has higher resistivity than CoWP

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Effect of Effect of CoWPCoWP and and CoMoPCoMoPcapping layers on Cu capping layers on Cu oxidation preventionoxidation prevention

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CoCo--MoMo--P P PourbaixPourbaix diagramdiagram

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CoCo--WW--P P PourbaixPourbaix diagramdiagram

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CoWPCoWP and and CoMoPCoMoP basic deposition Solutionbasic deposition Solution

“Ingredient” “Role” “Concentration”

CoSO4·7H2O Cobalt source 23 gr/l

HB3O3 Buffer 31 gr/l

3Na-citrate Cobalt complexing 130 gr/l

NaH2PO2 Reducing agent and Phosphor source 21 gr/l

RE610 Surfactant gr/l 0.05

KOH pH set 8.9-9

Na2MoO4 Mo source gr/l 0.1

Na2WO4 W source gr/l 10

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CoMoPCoMoP and and CoWPCoWP basic propertiesbasic properties

Properties CoMoP CoWP Mixed

potential -789 mV –702mV

Resistively 60-180 µΩ⋅cm 48 µΩ⋅cm

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XRD resultsXRD results

CoMoPCoMoP CoWPCoWP

Alfa-Co (111)

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CoMoPCoMoP and and CoWPCoWP as as oxidation protection oxidation protection

layerlayer

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Experiment procedureExperiment procedure

““SandwichSandwich”” samples of Cu between barrier layers samples of Cu between barrier layers

on SiOon SiO22 were made and subjected to oxidation were made and subjected to oxidation

condition by heating in open air furnace.condition by heating in open air furnace.

Surface resistance measurement were taken by Surface resistance measurement were taken by

4pp during the process.4pp during the process.

XPS profiling and spectrum was used to see the XPS profiling and spectrum was used to see the

the changes in profile and Cu oxidation state.the changes in profile and Cu oxidation state.

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Heat treatmentHeat treatment

For #1,#6 and 7#:For #1,#6 and 7#:300 min at 200C.300 min at 200C.170 min at 300C.170 min at 300C.200 min at 350C.200 min at 350C.

For #8:For #8:300 min at 200C.300 min at 200C.60 min at 300C.60 min at 300C.

2020

Surface resistance measurementSurface resistance measurement

R/Ro Vs Oxidation time

0

0.5

1

1.5

2

2.5

3

3.5

0 200 400 600 800

Time (min)

R/R

o

CoMoP/Cu/CoMoPon Co seadCoWP/Cu/CoWP onCo seadCoWP/Cu/CoWP onCu seadCoMoP/Cu on Cusead

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1# 1# --XPS profile before treatmentXPS profile before treatment

2222

1# 1# --XPS profile after 110 min at XPS profile after 110 min at 350C350C

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1#1# -- Cu spectrum after 110 min at Cu spectrum after 110 min at 350C350C

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1# 1# --XPS profile end of experimentXPS profile end of experiment

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1# 1# -- Cu spectrum at the end of Cu spectrum at the end of experimentexperiment

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1# 1# -- Cu spectrum at the end of Cu spectrum at the end of experimentexperiment

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DiscussionDiscussionThe The CoWPCoWP and and CoMoPCoMoP oxidizing protection properties are oxidizing protection properties are close, with small advantage for close, with small advantage for CoMoPCoMoP..It seems that Co diffusion to the surface controls the barrier It seems that Co diffusion to the surface controls the barrier layer oxidizing leading to barrier splitting to oxidized Co and layer oxidizing leading to barrier splitting to oxidized Co and Co depleted layers. Co depleted layers. Until the oxidizing of the barrier is well advanced, the barrierUntil the oxidizing of the barrier is well advanced, the barriercontinues to defend the Cu from oxidizing.continues to defend the Cu from oxidizing.Barrier advanced oxidizing leads to itBarrier advanced oxidizing leads to it’’s failure as barrier and s failure as barrier and Cu diffuse to the surface.Cu diffuse to the surface.

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CoMoPCoMoP and and CoWPCoWP -- Cu Cu diffusion barrierdiffusion barrier

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Experiment procedureExperiment procedure

““SandwichSandwich”” samples of Cu between samples of Cu between

barrier layers on SiObarrier layers on SiO22 were made and were made and

subjected to different thermal stress by subjected to different thermal stress by

annealing in a vacuum furnace .annealing in a vacuum furnace .

XPS profiling was used to see the the XPS profiling was used to see the the

changes in profile and Cu penetration.changes in profile and Cu penetration.

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#7 (#7 (CoWPCoWP) after thermal stress) after thermal stress

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Normalized profile for Cu penetration and Normalized profile for Cu penetration and the effective diffusion coefficient the effective diffusion coefficient

CoWP(innerCoWP(inner) ) CoWP(outerCoWP(outer) ) CoMoPCoMoP~0.26~0.26nmnm22/sec /sec ~ 0.21~ 0.21nmnm22/sec/sec ~ 0.1~ 0.1nmnm22/sec/sec

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SummerySummery

CoMoPCoMoP -- a novel electroless deposited film barrier option was presenteda novel electroless deposited film barrier option was presented

and compared to and compared to CoWPCoWP..

Both films shows similar barrier behavior with slight advantage Both films shows similar barrier behavior with slight advantage for for CoMoPCoMoP, ,

which has however higher electrical resistance.which has however higher electrical resistance.

Both films seems to prevent Cu oxidation to temperature of 300C,Both films seems to prevent Cu oxidation to temperature of 300C, and to and to

prevent Cu diffusion to temperature of 500C (materials profilingprevent Cu diffusion to temperature of 500C (materials profiling).).

The two films are interesting options for future capping layer fThe two films are interesting options for future capping layer for Cu or Cu

metallization. metallization.

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0 200 400 600 800

3.0

2.0

#2 – 225 nm

1.0

Time [min]

R(t)/R(0)

#4 - 115 nm

#3 - 210 nm

#1 – 215 nm

200C 300C 350C

Capping layer integrity – annealing in air & measuring resistivity insitu

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Effect of Effect of CoWPCoWP liner on liner on the reliability of Cu Dual the reliability of Cu Dual

damascene interconnectsdamascene interconnects

3535

CoWPCoWP capping layer (IBM) (1)capping layer (IBM) (1)

Cross section views of electromigration test structures:

3 level metal

2 level metal

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CoWPCoWP capping layer (IBM) (2)capping layer (IBM) (2)

TEM cross-sectional image of a Cu interconnect coated with CoWP

2 level metal

3737

CoWPCoWP capping layer (IBM) (3)capping layer (IBM) (3)

Elements concentration (by EDS). The electron probe moved from the top surface of a Cu damascene line, through the CoWP and amorphous SiCxHy coating layers and ended in the SiLK dielectric.

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CoWPCoWP capping layer (IBM) (4)capping layer (IBM) (4)

The resistance of a damascene Cu conductor, with and without a thin metal film on the top surface, vs time.

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CoWPCoWP capping capping layer(IBMlayer(IBM) (5)) (5)

FIB images of EM tested lines for uncoated and CoWP coated samples with current density of 3.6x106 A/cm2.

280 °C for 2.8 h

280 °C for 1100h

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CoWPCoWP capping layer (IBM) (6)capping layer (IBM) (6)

Conclusions

The results of this testing further support the hypothesis that the uncoated surfaces, or interfaces of Cu with dielectric, are the major sources of electromigration and thus reliability degradation.

In summary an investigation of Cu electromigration in Cu damascene interconnections with and without thin CoWP, CoSnP, and Pd coatings showed that electromigration failure lifetimes can be drastically changed. The migration of Cu at the top surface of a Cu damascene line was greatly reduced in the samples with 10–20 nm thick caps so that the Cu electromigration lifetime was markedly improved.

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Characterization and monitoring of:

Barriers

Capping layers and

Liners

4242

Barrier Analysis & monitoringBarrier Analysis & monitoring

Materials science techniques:Materials science techniques:AES, SIMS, RBS, SEMAES, SIMS, RBS, SEM

Electrical characterization:Electrical characterization:II--VVCC--V & CV & C--tt

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Example: Example:

Testing of Testing of CoWPCoWP barrier layersbarrier layers

- AES - Auger Electron Spectroscopy- The Transient Capacitance Method

4444

Copper profiles as measured by AES. The Copper profiles as measured by AES. The Example:Example:sputtering rate was: 12A/min for sputtering rate was: 12A/min for Co(W,PCo(W,P) on Cu, 25 ) on Cu, 25

A/min for Cu, 10A/min for A/min for Cu, 10A/min for Co(W,PCo(W,P) on Co, 8A/min for ) on Co, 8A/min for the sputtered Co.the sputtered Co.

0 10 20 30 40 50 60 70 80 90 100 110 120 130 1400

5

10

15

20

25

30

35

40

Con

cent

ratio

n (A

rb.)

Sputtering Time (min)

Co(W,P) Cu Co(W,P) Co SiO2 Si

600C, 4hr.

As deposited

520C, 2hr

4545

400 600 800 1000 1200 140010-1

100

101

T (oC)

W s

olub

ility

in fc

c C

o (a

t. %

)

Sykes Magneli et al. Larikov et al. Takayama et al. (XRD) Takayama et al. (EPMA)

Proposed system for ULSI Cu metallization

• Cu: Low solubility in Co, no phase formation

• P: Affects microstructure, reducing grain size → amorphous structure?

• P, W: Low solubility in Co → enrichment of grain boundaries?

• W: Proposal → refractory alloying element may improve barrier efficiency?

Co - P Co - W

Co alloys - Co(1-x-y) WxPy

Negligible solid solubilitysolubility of P in fcc Co is less than 0.47 at. %Ishida and Nishizawa, Bull. Alloy Phase Diag. 11, 555 (1990)

4646

Aqueous solution, pH: 8.0 – 9.0, T = 85° - 95°C Alkali ions can be replaced by (NH4)+

Component Aim

CoSO4⋅7H2O Metal ion source

Na2WO4⋅2H2O Metal ion source (induced co-deposition)

NaH2PO2⋅2H2O Reducing agent

C6H5Na3O7⋅2H2O Complexing agent – reducing the electrochemical potential difference

H3BO3 Buffer – fixing the pH

KOH pH adjustment: electrochemical potential, rate, and mechanism

Surfactant RE-610 Reducing surface tension, extracting H2

Reducing agent: Borane – dimethylamine complex C2H10BN Buffer: NH4OH + CH3COOH

Electroless deposition of Co alloys

Co(1-x-y)WxPy

Co(1-y)Py

Co(1-x)Wx

BufferNH4OH + CH3COOH pH~10.5CoSO4 – 13.2 gr/lNa2WO4 – 3.3 gr/l

Reducing agent DMAB – 4 gr/l

4747

1. Obtain electroless Co alloyswith largest possible content of P and / or W

Determined by electron probe micro-analysisCompared to Rutherford backscattering spectroscopy, Auger electron spectroscopy

Details regarding the deposition process:

A. Kohn, M. Eizenberg, Y. Shacham-Diamand, and Y. Sverdlov,

Mater. Sci. Eng. A 302, 18 (2001)

Alloy P concentration (at. %)

W concentration(at. %)

Co1-x-yWxPy 8±2 2±1

Co1-yPy 10±2 -

Co1-xWx - 4±1

4848

2. Electroless deposition of conformal ultra-thin Co1-x-yWxPy filmsCase study: Co0.9W0.02P0.08

A. Kohn, M. Eizenberg,

and Y. Shacham-Diamand,

Appl. Surf. Sci., to be published

Bright field, cross-sectional TEM micrographs

Comparison to current industrial technologyIonized Metal Plasma PVD

4949

2. Electroless deposition of conformal ultra-thin Co1-x-yWxPy films

Bottom Field / Sidewall

A. Kohn, M. Eizenberg,

and Y. Shacham-Diamand,

Appl. Surf. Sci., to be published

Case study: Co0.9W0.02P0.08Comparison to current industrial technologyIonized Metal Plasma PVD

Bright field, cross-sectional TEM micrographs

5050

2. Electroless deposition of conformal ultra-thin Co1-x-yWxPy films

Top FieldBottom Field / Sidewall

Comparison to current industrial technologyIonized Metal Plasma PVD

Case study: Co0.9W0.02P0.08

A. Kohn, M. Eizenberg,

and Y. Shacham-Diamand,

Appl. Surf. Sci., to be published

Sidewall

Co

0.9 W0.02 P

0.08

Co0.9 W

0.02 P0.08

Bright field, cross-sectional TEM micrographs

5151

1 – Evaluated by C-t2 – Evaluated by C-V

Evaluation of diffusion barrier quality Results

A. Kohn, M. Eizenberg, Y. Shacham-Diamand,

B. Israel, and Y. Sverdlov

Microelectronic Eng. 55, 297 (2001)

Electroless Co0.9P0.1 , 30 nm thick, are stable barriers at 450°C during approximately 10 hours

Current allowed total thermal budget: Thermal cycles equivalent to 400°C, 10 – 60 minutes

Barrier

30 nm thick

Thermal Stress1 Bias and Thermal Stress2

1 MV/cm at 300ºC / 30 min

Co0.96W0.04 Fails after 450ºC / 1hr Fails

Co0.9P0.1 Fails after 450ºC / 10 hr Stable

Co0.9W0.02P0.08 400ºC/30 min: Stable

500ºC/30 min: Fails

N/A

5252

4. Investigate the structure of electroless Co alloys and its evolution as a result of heat treatments

Questions:1. What is the as-deposited structure?

2. How does the structure change with thermal anneal?

3. What is the structure during failure of the diffusion barrier?

Case study: Co0.9W0.02P0.08Electroless Co0.9W0.02P0.08 10 – 100 nm

Sputtered Co or Cu 2 – 20 nm

(Sputtered Ti) (5 nm)

SiO2 100 nm

Si wafer

5353

30 35 40 45 50 55

Siλ/2 (004)

εCo(01·0)

εCo(01·1) εCo

(00·2)

Difference

Co seed

Co0.9W0.02P0.08 as-dep.

Inte

nsity

(arb

. uni

ts)

2θ (°)

As-deposited structure: ?300°C: hcp Co400°C: hcp Co (main) + orthorhombic Co2P (minor)600°C-700°C: hcp → fcc transition (delayed)Same results on Co and Cu seed layer

Structure of Co0.9W0.02P0.08 and its evolution with thermal treatments

30 35 40 45 50 55

TiP2?

Co2P (201)

εCo(01·0)

αCo(111) εCo

(01·1)

αCo(200)

εCo(00·2)

Siλ/2 (004)

2θ (°)

Inte

nsity

(arb

. uni

ts)

700°C

600°C

500°C400°C300°C

as-dep.150°C

Cobalt seed

Powder XRD, Bragg-Brentano geometry Thermal anneal: 1 hour, Vacuum ≤ 10-6 Torr

5454

Radial intensity of the selected area electron diffraction (SAED)as a function of the scattering vector

hcp Co

fcc Co

0.3 0.4 0.5 0.6

_Si [216]

Si [103]

Si [100]

Si [011]

Rad

ial I

nten

sity

(a.u

.)o

s (A-1)

(01⋅0)(00⋅2)

(01⋅1)

(111)

(002)

DC s =∆ ; C ~ 1 → D ~ 3 – 5 nm

0.4 0.5 0.6o

Rad

ial I

nten

sity

(a.u

.)

s (A-1)

Gaussian fit

• Hexagonal close-packed cobalt• Grain size: several nm• Preferred basal plane orientation

)sin2 s(λ

θ=

As-deposited structureAnalysis of selected area electron diffractions

5555

hcp Co

fcc Co

Orthorhombic Co2P

0.4 0.6 0.8 1.0 1.2 1.4o

Rad

ial I

nten

sity

(a.u

.)

600oC

400oC

as-dep

s (A-1)

Evolution of microstructure with thermal annealCo0.9W0.02P0.08

Radial intensity of the SAEDas a function of the scattering vector

0 5 10 150

100

200

Num

ber o

f gra

ins

(-)

Grain size (nm)

0 5 10 15 200

100

200

Num

ber o

f gra

ins

(-)

Grain size (nm)

0 40 80 1200

100

N

umbe

r of g

rain

s (-)

Grain size (nm)

Dark field plan view TEM micrographs, SAED, apparent grain size histograms

as-dep

400°C

600°C

5656

Co0.9W0.02P0.08

0 200 400 600

60

80

100

120

hcp Co + Co2Phcp Cohcp Co + amorph. Co

Cooling 1st Run

1st Run

2nd Run

Res

istiv

ity (µ

Ω·c

m)

Temperature (oC)

Heating rate ~ 2.8°C/min ; Vacuum ~ 5 × 10-6 Torr

Tracking structural changesIn-situ resistivity as a function of temperature

After anneal:Significant temperature independent

contribution to electron scattering

(imperfections including impurities)Co BulkFilm dT

d dTd ρ

BulkFilm ρ>ρ

575713.6 14.0 14.4

-14

-13

-12

-11

-10 460 440 420

104/Tc (K-1)

ln[(d

T/dt

)/Tc2 ] (

-)

Tc (oC)

Ea – apparent activation energydT/dt – heating rateTc ≡ dρ / dT is minimal

(maximum rate of crystallization)

Constant kTE

T)dt

dT( ln

C

a2

C

+−=

240 280 320 36090

100

110

120

(e)(c)(d)(b)(a)

Res

istiv

ity (µ

Ωcm

)

Temperature (oC)

(a) 1.4 oC/min(b) 2.8 oC/min(c) 5.6 oC/min(d) 11.2 oC/min(e) 16.8 oC/min

Crystallization and Co2P phase formation processes

16.4 16.8 17.2 17.6 18.0-13

-12

-11

-10

340 320 300Tc (oC)

ln[(d

T/dt

)/Tc2 ] (

-)

104/Tc (K-1)

Ea = 1.6 ± 0.1 eV Ea = 4.7 ± 0.1 eV

hcp Co + amorphous Co(W,P) → hcp Co hcp Co → hcp Co + orthorhombic Co2P

Kissinger analysis:

5858

[ ] )t-(tk(T)- exp - 1 )t(f n0⋅=

)0(tf(t)-1

)(tf(t) 1

=ρ+

∞→ρ=

ρ

n = a + b⋅ca = 1 ; Constant nucleation rate (N ~ t )b = 3 ; 3D growthc = 0.5 ; Diffusion controlled growth

0 10 20 30

100

110

120n2 n3n1

Res

istiv

ity (µ

Ω·c

m)

T ~ 260oC

Time (X103 sec)

f(t) – fraction crystallized

k (T) – apparent rate coefficientt0 – incubation timen – J-M-A index

Fraction crystallized as a function of time at a constant temperature:

Crystallization processJ-M-A analysis:

103 104

-4

-2

0

2

n1 = 1.5 ± 0.2

n2 = 2.6 ± 0.2

n3 = 1.3 ± 0.2

ln(-l

n(1-

f)) (-

)

Time (sec)

5959

Chemical binding statesCo, W – no observable change in chemical binding (XPS, EELS)P – significant changes

X-ray photoelecton spectroscopy:The 2p core level can not be fitted by a single doublet p3/2, p1/2

Fitting is obtained by two doublets marked as P1 and P2

As-deposited: amorphous matrix 600°C: covalent binding

as-dep Data Fitting

P2

P2

P1P1

400oC

P1 P1P2

P2

P1 P1P2

P2

600oC

132 130 128 126

P1P1P2

P2

Co2P

Binding Energy (eV)

Inte

nsity

(arb

. uni

ts)

Sample

Area ratio P1/P2 (-)

±10%

Co0.9W0.02P0.08 as-dep 1:1.1 Co0.9W0.02P0.08 400oC 1:2 Co0.9W0.02P0.08 600oC 1:4.9 Co2P Reference 1:5

6060

As-deposited structure: • hcp Co nanocrystallites (d ~ 3-5 nm), preferred basal plane orientation• amorphous CoWP matrix.

Evolution of structure with thermal anneal: • T ~ 300°C: hcp Co + amorphous Co(W,P) → hcp Co ; Ea = 1.6 ± 0.1 eV,

constant nucleation rate, diffusion limited• T ~ 420°C: hcp Co → hcp Co + orthorhombic Co2P ; Ea = 4.7 ± 0.1 eV• T > 500°C: Delayed hcp Co → fcc Co transformation relative to bulk Co• P Chemical binding shifts to covalent at T > 400°C

Structure during failure of barrier: At T ~ 450°C, the microstructure is hcp Co nanocrystallites

and small amounts of orthorhombic Co2P

→ Failure mechanism : grain boundaries diffusion

Summary: Evolution of microstructure with thermal anneal

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5. Kinetics of Cu diffusion via electroless Co alloysand a comparison to PVD Co

Questions:1. What is the type of kinetics of Cu diffusion through the electroless Co alloy films?

2. What are the kinetic parameters of this diffusion?

3. Why are Co1-x-yWxPy alloy thin films, predominately Co (1 – x – y ≈ 0.9),

effective barriers, while polycrystalline pure Co films are ineffective?

6262

Special case of ultrafine-grained polycrystals

For Co0.9W0.02P0.08 :

d – nanometer scale

s >>1 - low solubility of Cu in Co

d << sδ/2

d – grain size

s – segregation coefficient

δ – grain boundary width

Cu lattice diffusion in Co:Ea ≈ 2.86 eV ; D0 ≈ 1 cm2/sec

C' Kinetics T/Tm ~ 0.3

L = 2(Dlt)1/2 << d

A0 Kinetics T/Tm ~ 0.5 or long anneal times

L = 2(Dlt)1/2 ≥ d

Landolt-Börnstein Numerical Data and Fundamental Relationships in Science and Technology,edited by O.Madelung and H.Mehrer(Springer,Berlin,1999),Group III,Vol.26,p.52.

6363

Analysis of type-C Cu grain boundary diffusivity

What are the kinetic parameters of the grain boundary diffusion?

Z

z = L

6464

Assuming a 1D case:2

gb2

gbgb

z)t,z(C

D t

)t,z(C∂

∂=

∂∂

Copper grain boundary diffusivity at processing temperatures : 0.3 – 0.4 Tm

6565

Boundary conditions:

Assuming a 1D case:2

gb2

gbgb

z)t,z(C

D t

)t,z(C∂

∂=

∂∂

0 z

C

Lz

gb =∂

=

(2)

impuritygb C )0t,z(C == ; 0<z≤L(3)

Copper grain boundary diffusivity at processing temperatures : 0.3 – 0.4 Tm

geometricfactormax,gbgb C

dq )t,0z(C ⋅

δ== q = 2 ; d ≥ L

q = 3 – d/L ; d < L(1)

6666

⎩⎨⎧ π+

⋅⎭⎬⎫π+

+−

π=

===

∑∞

= L2z)1n2(cos

L4tD)1n2(

- exp )1n2(

)1(4 - 1 )0t,z(C - )t,0z(C

)0t,z(C - )t,z(C2

gb22

0n

n

gbgb

gbgb

Solution:

Boundary conditions:

Assuming a 1D case:2

gb2

gbgb

z)t,z(C

D t

)t,z(C∂

∂=

∂∂

0 z

C

Lz

gb =∂

=

(2)

impuritygb C)0t,z(C == ; 0<z≤L(3)

2 2

2 2 2n 0

gb(2 1)8 1 1 - exp -(2 1) 4

Dt n tn L

MM

ππ

=∞

⎧ ⎫+⎪= ⎨ ⎬+ ⎪ ⎭⎩∑ L)t,0z(C M gb ⋅==∞;

Copper grain boundary diffusivity at processing temperatures : 0.3 – 0.4 Tm

max,gbgb Cd

q )t,0z(C ⋅δ

==(1)geometricfactor

q = 2 ; d ≥ Lq = 3 – d/L ; d < L

6767

10-6

10-4

10-2

300oC 350oC 400oC 450oC

10-6

10-4

10-2

0 40 80 12010-6

10-4

10-2

Sputter depth (nm)

I Cu/

Co

(-)

Co0.9W0.02P0.08

Co0.9P0.1

Sputtered Co

Sputtered Co

Co0.9W0.02P0.08

SIMS depth profiles(a)300°C 8 hr(b)450°C 8 hrBright field cross sectional TEM micrographs

8 hour anneal

6868

14 15 16 17 1810-18

10-16

10-14

440 400 360 320

This study Pellerin et al.J. Appl. Phys. 75, 5052 (1994)

Dgb

(cm

2 /sec

)

104/T (K-1)

T (oC)

Ea = 1.8 ± 0.2 eVD0 = 10(-1.2 ± 0.8) cm2/secδ = 0.5 nm, q = 2

Cu grain boundary diffusion parameters

Sputtered Co

6969

Ea = 1.8 ± 0.2 eVD0 = 10(-1.2 ± 0.8) cm2/secδ = 0.5 nm, q = 2

Cu grain boundary diffusion parameters

13 14 15 16 17 1810-20

10-18

10-16 480 440 400 360 320

104/T (K-1)

Dgb

(cm

2 /sec

) Electroless Co0.9W0.02P0.08 Electroless Co0.9P0.1

T (oC)

Ea = 1.25 ± 0.20 eVD0 = 10(-8.3 ± 1.0) cm2/secδ = 0.5 nm, q = 3 – d/T

Ea ≈ 1.3 eVD0 ≈ 10-7.2 cm2/secδ = 0.5 nm, q = 3 – d/T

Electroless Co0.9W 0.02P 0.08

Electroless Co0.9P 0.1

Sputtered Co

14 15 16 17 1810-18

10-16

10-14

440 400 360 320

This study Pellerin et al.J. Appl. Phys. 75, 5052 (1994)

Dgb

(cm

2 /sec

)

104/T (K-1)

T (oC)

7070

gb

2

fail D4L t ≡

For a 30 nm thick Electroless Co0.9P 0.1 barrier:

According to Diffusion kinetics measurements:

At 450°C: Dgb ~ 4×10-17 cm2/sec → tfail ~ 16 hours

According to Electrical measurements:

At 450°C: → tfail ~ 8-12 hours

Estimated lifetime for a 10 nm thick Co0.9W0.02P0.08 barrier at 400°C

→ tfail ~ 35 hours

Failure criterion

tfail – time to failure of the diffusion barrierL – thickness of the diffusion barrierDgb – grain boundary diffusivity

7171

A0 Kinetics T/Tm ~ 0.5 or long anneal times

L = 2(Dlt)1/2 ≥ d

Why do we see a significant reductionin the pre-exponential kinetic diffusion parameter?

Analysis of steady-state type-A Cu grain boundary diffusivity

7272

Steady-state type-A Cu grain boundary diffusivityAnneal: ~0.5Tm → A0 kinetics 8 hours → steady statePlateau level is given by:

solC)dq-(1 gbC

dq C ⋅

δ+⋅

δ=

With increase of temperature: 1st term ↓ (d ↑)2nd term ↑ (Csol ↑ ; d ↑)

- plateau concentrationCgb – grain boundary concentrationCsol – solubility of Cu in Coq – geometrical factorδ - grain boundary widthd – grain size

C

10-4

10-3

10-2

10-1

as-dep 550oC 600oC 650oC 700oC

10-4

10-3

10-2

0 40 80 120

10-4

10-3

10-2

Depth (nm)

Co0.9W0.02P0.08

Co0.9P0.1

PVD Co

I Cu/

Co

(-)

SIMS depth profiles

7373

Assumptions:

5. Exponential dependence of the solubilityCsol = Cs0 ⋅exp (-Es/kT)

e.g.: for bulk Co, 900°C-1100°C:ES ~ 0.7 – 0.8 eV ; Cs0 ~ 4.5 ⋅ 103 at. %

4. q δ ⋅ Cgb : weak temperature dependence

2. δ ~ 0.5 nm regardless of the temperatureMishin and Herzig, Mat. Sci. Eng. A 260, 55 (1999)

1. For Co0.9W0.02P0.08 : gbCdq

⋅δ << solC)

dq-(1 ⋅

δ

3. Grain boundaries’ volume fraction:

dqδ

< 0.05

kTE - Cln ) (Iln s0s

Cu/Co α≅

(α ~ 3.3 : Calibration coefficient for SIMS measurements: Csol = α ⋅ ICu/Co)

8 10 12 14

0.1

1

10

1000 800 600 400T (oC)

hcp Cofcc Co

Cu

solu

bilit

y (a

t. %

)

104/T (K-1)

Hasebe et al. Bruni et al. Old et al. Hasebe et al.

Grain boundaries’ passivation

7474

2. Solubility of Cu in Co

for 550° – 700°C regime:

Es = 0.52 ± 0.15 eV

Cs0 = (6 ± 1) × 102 at. %

Results:1. For Co0.9W0.02P0.08 : Cgb is negligible

Passivation of the grain boundaries

8 10 12 14

0.1

1

10

1000 800 600 400T (oC)

hcp Cofcc Co

Cu

solu

bilit

y (a

t. %

)

104/T (K-1)

Hasebe et al. Bruni et al. Old et al. Hasebe et al. This study

10.0 11.0 12.0

10-3

700 650 600 550 T (oC)

I Cu/

Co (

-)

104/T (K-1)

A. Kohn, M. Eizenberg, and Y. Shacham-Diamand, J. Appl. Phys. 92, 5508 (2002)

Mechanism of improved barrier properties:Grain boundaries’ passivation

kTE - Cln ) (Iln s0s

Cu/Co α≅

7575

1.

2. Tungsten increases the passivation of the grain boundaries

What is the contribution of the tungsten alloying?

d1 ) - C ( Cq ) - I( I CoWP

gbCoPgb

CoWPCu/Co

CoPCu/Co ⋅

αδ

=

% at. 3 C - C CoWPgb

CoPgb ≈

Results: 0.025 0.030 0.035 0.040 0.045

5

10

15

20

25

400 600

20

40

d [n

m]

T [oC]

∆I

Cu/

Co X

10-5 (-

)

d-1 (nm-1)

7676

Classification of Cu transport kinetics via the Co alloy films: T/Tm ~ 0.3 : Predominately via the grain boundaries (type-C)T/Tm ~ 0.5 or long anneal times: Effective diffusivity (type-A)

Kinetic parameters of Cu diffusion:

(δ = 0.5 nm)

Why are electroless Co-alloy films effective barriers? Passivation of the grain boundaries

Summary: Kinetics of Cu diffusion via electroless Co alloysand a comparison to PVD Co

PVD Co 1.8 ± 0.2 eV 10(-1.2 ± 0.8) cm2/secCo0.9W0.02P0.08 1.25 ± 0.20 eV 10(-8.3 ± 1.0) cm2/secCo0.9P0.1 ≈ 1.3 eV ≈ 10-7.2 cm2/sec

7777

SummaryI. Electroless deposition of Co(W,P) alloys

→ Potential process for depositing barriers and encapsulation layers in ULSI Cu metallization

II. Co0.9P0.1, Co0.9W0.02P0.08 effective barriers at 450°C→ Relevant barrier for ULSI Cu metallization

III. Study of the structure of the electroless deposited Co(W,P) films→ Failure mechanism of the barriers is grain boundaries diffusion

IV. Kinetic parameters of Cu diffusion in PVD Co and electroless Co alloys→ Diffusivity is 2-3 orders of magnitude lower

Significant difference in the pre-exponential factor

V. Proposed explanation for effectiveness of electroless Co-alloy barriers→ Passivation of the grain boundaries

7878

30 35 40 45 50 55 60

2

4

6

8

Co seed layeras-deposited

Siλ/2 (004)

Inte

nsity

(a.u

.)

2θ (°)

Co0.9P0.1 Co0.9P0.08W0.02

The influence of W on the as-deposited structure?

7979Nishizawa et al. , Bull. Alloy Phase Diag. 5, 161 (1984)

Co - Cu

Proposed system for ULSI Cu metallization

Nagender Naidu et al., “Phase Diagrams of Binary Tungsten Alloys”Indian Institute of Metals 60 (1991)

• Cu: Low solubility in Co and no phase formation• P, W: Low solubility in Co

→ enrichment of grain boundaries?• P: Affects microstructure, reducing grain size

→ amorphous structure?• W: Proposal

→ introduction of a refractory alloying element may improve barrier efficiency?

Ishida et al. , Bull. Alloy Phase Diag. 11, 555 (1990)

Co - P

Co - W

Co alloys - Co(1-x-y) WxPy

8080

[001]

[010]

[100]

[111]

Space group = 62 Pnmaa = 0.565 nm, b = 0.351 nm, c = 0.66 nm; ρ = 7.56 gr/cm3

Orthorhombic Co2P

Cell viewed in various directions

Blue – Co ; Red – P

8181

Major structural changes occur after a short timecompared to the MOS failure time. Failure can not be attributed to recrystallization and recovery.

35 40 45 500.0

2.0x102

4.0x102

6.0x102

2θ (°)

As dep.

Inte

nsity

(a.u

.)

450oC / 2 hr 450oC / 5 hr 450oC / 10 hr

εCo (01-10)

Co2P (201) εCo

(01-11)

εCo(0002)Si

λ/2 (004)

Phase evolution (XRD)Co0.9P0.1 / Co

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

Co0.9P0.1 Co0.96W0.04

Thermal treatment (450oC) time (hr)

Rs/R

0 (-)

Relative sheet resistance

Failure mechanism of the d.b.

8282

Electroless deposition of Co alloysProcess:

Co(II)Cit + e- → Co(I)CitadsCo(I)Citads + e- → Co(s) + Cit

Reduction of Co ions (complexed with the citrate)

H2PO2- → HPO2

-ads + Hads

Reducing agent is hypophosphite:H ?extraction? on the catalytic seed layer

Reaction with OH- ions:

HPO2-ads + OH- → H2PO3

- + e-

Reaction of H atoms is dependant on the catalytic seed layer:

Hads + OH- → H2O + e- Catalytic seed layer: Pd, Pt, Rh2Hads → H2 Catalytic seed layer: Cu, Au, Ag

In parallel, a competing reaction of hypophosphitedeposits P:

H2PO2- + 2H+ + e- → P + H2O

W deposition? Induced co-deposition: iron group ion + refractory metalProposed Mechanism: Podlaha et al., J. Electrochem. Soc. 144 (1997) 1672

Induced co-deposition of MoO42- and ion M (Fe2+, Co2+, Ni2+) complexed with a ligand L

MoO42- + M(II)L + 2H2O + 2e- → [M(II)LMoO2]ads + 4OH-

[M(II)LMoO2]ads + 2H2O + 4e- → Mo(s) + M(II)L + 4OH-

8383

Periodicity in P depth profileAES depth profiles

SiSiSiOSiO22

TiTiCoCo

CoCo0.90.9WW0.020.02PP0.080.08

as-deposited

3000C

5000C

7000C

4000C

~12 nm

8484

Chemical binding statesCo, W – no observable change in chemical binding (XPS, EELS)P – significant changes

X-ray photoelecton spectroscopySuggested fitting of 2p3/2 and 2p1/2 P states :P1 and P2 2p binding states1. p3/2 and p1/2 ∆E = 0.84 eV Goodman et al., Phys. Rev B 27, 7440 (1983)2. p3/2 and p1/2 area ratio = 2:13. FWHM, G/L ratio equal for p3/2 and p1/2 for P1, P24. 0.2 ≤ G/L ≤ 0.8Results:1. P1 and P2 – equal B.E., FWHM, G/L ratio for all samples

→ validates assumptions2. Area ratio of P1:P2 → ratio of binding states

as-dep Data Fitting

P2

P2

P1P1

400oC

P1 P1P2

P2

P1 P1P2

P2

600oC

132 130 128 126

P1P1P2

P2

Co2P

Binding Energy (eV)

Inte

nsity

(arb

. uni

ts)

Sample

Area ratio P1/P2 (-)

±10%Co0.9W0.02P0.08 as-dep 1:1.1 Co0.9W0.02P0.08 400oC 1:2 Co0.9W0.02P0.08 600oC 1:4.9 Co2P Reference 1:5

8585

Same results on Co and Cu seed layer

No phase reaction between Cu and the electroless film up to 700°C

Structure of Co0.9W0.02P0.08 and its evolution with thermal treatmentsCu : Influence of seed layer and / or phase interaction?

Powder XRD, Bragg-Brentano geometry

Thermal anneal:1 hourVacuum ≤ 10-6 Torr

Copper seed

30 35 40 45 50 55

5 x 1 0 2

2 x 1 0 3

3 x 1 0 3

4 x 1 0 3

5 x 1 0 3

2θ (°)

as depInte

nsity

(a.u

.)

300oC

500oC

700oC

Cu (111)

αCo(200)

εCo(01·1)

Co2P (201)

Siλ/2 (004) αCo

(111)

εCo (00·2)

εCo (01·0)

8686200 400 600

2.025

2.030

2.035

2.040

2.045

2.050

o

fcc Co (111)

hcp Co (00·2)

d-sp

acin

g (A

)

Annealing temperature (oC)

Pseudo-Voigt, single-line fit of the (00·2) hcp / (111) fcc Co plane Th. H. De Keijser et al., J. Appl. Cryst. 16, 309 (1983)

Domain size, microstrain

Interplanar spacing

Structure of Co0.9W0.02P0.08 and its’evolution with thermal treatments

Calibrated using the λ/2 Si (004) reflection

• As-deposited: fcc?• 300°C: hcp• Above 400°C: transition to fcc

Analysis of XRD data

0 200 400 6000

20

40

60

80

0.0

0.5

1.0

1.5

2.0

2.5

Mic

rost

rain

, ∆d/

d ·1

0-2 (-

)

Dom

ain

size

, D (n

m)

Annealing temperature (oC)

8787

file: “S150n13500KleftisSiO2”

30 35 40 45 50 55

Siλ/2 (004)

εCo(01·0)

εCo(01·1) εCo

(00·2)

Difference

Co seed

Co0.9W0.02P0.08 as-dep.

Inte

nsity

(arb

. uni

ts)

2θ (°)

As-deposited structure?

Powder XRD, Bragg-Brentano geometry

I. Electroless Co0.9W0.02P0.08

II. Sputtered Co (2 nm thick)III. SiO2

Cross sectional phase contrast TEM image – as deposited film

Z.A. [00⋅1]

(10⋅0)

(12⋅0)–5 nm

I.

II.

III.

Fast Fourier transform

8888

0 5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

400oC300oC

l (nm)

α (-

)

8 at. % 10 at. % )

r1)

C1 - 1 (

r1(V

l4 )

rr()

C1 - 1 - (1

1 2

pP2Co

hcp2

p

Co

P

⋅++

⋅=α

α - Fraction coverage of the grain boundaryrCo – Atomic radius of CorP – Covalent radius of PCP – Atomic concentration of P in the filmVhcp – Volume of hcp unit cell

Why does Co2P nucleate at ~ 420°C?

Estimation of grain boundary coverage by Passuming 1 ML of P enveloping hcp Co grains with a side length or diameter, l

Co - PEnrichment of P at the grain boundaries