Chemical Mechanical PolishingChemical Mechanical Polishing

Chemical Mechanical PolishingChemical Mechanical Polishing for Manufacturing of Smooth Nb Surfaces

George Calota1, Natalia Maximova2, Katherine Ziemer2 and Sinan Muftu1,3

1Department of Mechanical Engineering2Department of Chemical Engineering

3NSF-NSEC-Center for High-rate Nanomanufacturing

Northeastern UniversityBoston, MA 02115

[email protected], (617) 373-4743

Acknowledgement:• H.C. Starck, Inc, Newton, MA

• NSF-Center for High-rate Nanomanufacturing (Award # NSF-0425826)

Mechanical Engineering and Chemical Engineering

g g ( )

CMP in Semiconductor Manufacturing

Chemical Mechanical Polishing (CMP) is a g ( )critical step in integrated circuit (IC) manufacturing, typically used for l i iplanarizing:

• Dielectric materials: SiO2C d (C ) d (W)• Conductors: copper (Cu) and tungsten (W)

• Diffusion barrier: Tantalum (Ta)In between processing steps.

Evans, D.R. “Metal Polishing Process,” in Chemical-Mechanical Planarization of Semiconductor Materials, ed. M.R. Oliver, Springer, 2003, p 41.


Planarizing a SiO2 wafer using CMP

Experimental results:Experimental results:Our polishing experiments on SiO2 show:

– Large wavelength roughness is reduced to 1 μm level– Short wavelength roughness is reduced to 1 nm level.– Sub nanometer roughness is typical for Si wafers


Goal and Outline

Th l f thi t ti i t d t t th t ti l f CMPThe goal of this presentation is to demonstrate the potential of CMP as an alternative method to manufacture very (nearly atomically) smooth Niobium surfaces.

Outline• Brief description of chemical mechanical polishing (CMP)

• Studies of Oxidation of Niobium

• Proposed two-step process for polishing of Niobium

• Initial results

• Summary and conclusions


CMP Process

Carrier

Process description:• A wafer is pushed against a polymeric polishing pad

Rotating platen

Polishing pad Wafer

• (Pa = 1-10 psi)

• Pad and wafer rotate independently (~60 rpm).

Sl t i i idi i h i l d b i ti l ig p

Schematic of CMP operation

• Slurry, containing oxidizing chemicals and abrasive particles is supplied into the interface.

• Material removal occurs due to particle abrasion of the h i ll i t d f fchemically passivated wafer surface.

Wafer Slurry

Designed to create (low hardness) oxidesDesigned to create (low-hardness) oxides.

Chemicals: • Oxidizers• Buffers • Surfactants

Particles:• Material:

Silica (SiO2), alumina (Al2O3, Ceria (CeO2)

• Size: 50-150 nm


Surfactants• Shape: Spherical

Contact at wafer-pad interface

Polishing pad (IC1000) Polyurethane Pad wafer interfacial contactwafer

pad particles

waferwafer

The mechanical component of material removal is primarily dominated by particle-wafer contact. Abrasion of the particle

Chemically passivated layerParticle

contact

Direct contact

pad

Abrasion of the particle

Chemically passivated layerParticle

contact

Direct contact

pad

y pBut, particle-to-wafer contact forces depend on many variables:

• Applied pressure• Particle size• Particle concentration• Pad elasticity• Pad thickness• Pad roughness


Pad roughness

Material removal rate

Material Removal:• Material removal is governed by an abrasive removal process:

(') . slidingF LV k

H= k: removal rate constant

F Normal force

L : Sliding distance

Archard’s Law

LS: Sliding distance

H: Hardness (material property)

V(‘): Worn volume

MRR dV kPV= =

In CMP literature material removal rate (MRR) is used:

k: removal rate constant

P A li d h dPreston’s LawMRR ckPVdt

= = Pc: Applied push down pressure

V: Sliding speed

Preston s Law

Removal rate constant k represents the effects of:Removal rate constant k represents the effects of:• Abrasive particle size, concentration, hardness, morphology

• Wafer hardness, surface roughness

Sl h i t


• Slurry chemistry

• Pad roughness, elasticity

XPS Studies on Niobium oxidation

In general a passivated metal oxide is softer and easier to remove mechanicallyIn general a passivated metal-oxide is softer and easier to remove mechanically.

CMP strives to find a delicate balance between oxide formation and mechanical abrasion

The first step of our investigation was to understand oxidation of Niobium for conditions relevant to CMP

Ch t i ti i X Ph t l t S t (XPS) d iCharacterization using X-ray Photoelectron Spectroscopy (XPS) under various processing conditions carried out

1. Characteristics of Niobium Oxide

2. Effect of base and acid on oxidation

3. Effect of buffered chemical polish (BCP) on Niobium

4. Hardness test on Niobium oxide underway.


XPS Studies on Niobium oxidation-I

Oxides formed by Nb:

XPS Analysis Reveals Nb-O BondingNb oxide

3d 5/2Oxide Thickness


3d 5/2Oxide Thickness Oxides formed by Nb:

Conditions

• Exposed to air as received y U

nits

Nb metal3d 5/Nb metal

Nb oxide3d 3/2

~4.5 nm

y U

nits

Nb metal3d 5/Nb metal

Nb oxide3d 3/2

~4.5 nm

Exposed to air, as received

• After CMP using Cu and SiO2 slurry(small amount of material removal)

Arb

itrar

y 3d 5/2Nb metal3d 3/2

Arb

itrar

y 3d 5/2Nb metal3d 3/2

Results

The XPS study shows that 198203208213

Binding Energy (eV) Δ eV is Characteristic

198203208213

Binding Energy (eV) Δ eV is Characteristic

• The oxide thickness is ~ 4.5 nm

• The majority of the oxide is Nb2O5

Δ eV ~ 5 eV of Nb2O5 Sampling Depth: ~ 7 nm Δ eV ~ 5 eV of Nb2O5 Sampling Depth: ~ 7 nm

• The oxide layer thickness self limiting type

(long term exposure to ambient or to an oxidizer doesn’t change oxide depth)


XPS Studies on Niobium oxidation-II

Eff t f B (H2O2) d A id (HF)Effect of Base (H2O2) and Acid (HF)

Conditions• Acid = 5 ml HF, 17 ml Nitric, 51 ml Methanol

Nb 3dNb 3d, ,

• Base = 5 ml ammonium hydroxide, 10 ml H2O2, 50 ml DI water

Results

Active Oxidizer: H2O2

Metal PeaksActive

Oxidizer: H2O2

Metal Peaks

Results

The XPS study shows that

• Base (H2O2) forms oxide Active Etchant: HF

Cannot remove all of the oxide –

idi i i dActive Etchant:

HF

Cannot remove all of the oxide –

idi i i d( )

• Acid (HF) removes oxide199204209214

HF

Binding Energy (eV)

oxidizes in air and seems limited to ~4.5 nm

199204209214

HF

Binding Energy (eV)

oxidizes in air and seems limited to ~4.5 nm

Binding Energy (eV)Binding Energy (eV)


XPS Studies on Niobium oxidation-III

Buffered Chemical Polish (BCP) 0.35

0.4

12

14

um

Material Removal (g) PV (um)

Buffered Chemical Polish (BCP)

BCP Formula: • 10 mL HF (49%), 0.2

0.25

0.3

al R

emov

al (g

6

8

10

ughn

ess,

PV

(u

• 10 mL HNO3 (65%),• 20 mL H3PO4 (85%)

0.05

0.1

0.15

Mat

eria

2

4

6

Surf

ace

rou

Results• 18 min of BCP removed up to 200 um• Surface roughness changed from 8 um 10 um (PV) Nb 3dNb 3d

0-5 0 5 10 15 20

BCP treatment time (min)

0

• Surface roughness changed from 8 um-10 um (PV)

The XPS study shows that

• BCP treatment exposes a highly ordered Nb6-minute BCP dip6-minute BCP dipBCP treatment exposes a highly ordered Nb

as-receivedas-received


199204209214Binding Energy (eV)


Niobium CMP using SiO2 slurry

PV RMS RaExpon. (PV) Expon. (RMS) Expon. (Ra)

10

icro

ns)

1

Rou

ghne

ss (m

i

0.10 5 10 15 20

Polishing Time (min)

R

Surface roughness vs polishing time

Pad and wafer rotational speeds: 60 rpmSupply pressure : 500 g/cm2C i h d li ill i


Carrier head linear oscillations : onSlurry : Microplanar CMP 1150 by EKC Technology, Inc.Polishing Pad : IC1000 A2, by Rohm and Haas Electronic Materials

XPS Studies on Niobium oxidation - Conclusion

Characterization of Niobium oxide formation under various processing conditions:Characterization of Niobium oxide formation under various processing conditions:

1. Nb2O5, a stable oxide, is the dominant form when Nb is exposed to oxidants

2. This oxide is ~4.5 nm thick and its thickness is self-limiting.2. This oxide is 4.5 nm thick and its thickness is self limiting.

3. a. It does not appear that passivation, in the CMP sense, will be helpful in polishing a relatively rough surface.

b. Passivation may be useful in planarazing a smooth Niobium surface.


Two step polishing process

Two step process :Two step process :

Step-1: Use large, hard abrasive particles to remove large PV roughness

• Slurry-1: 0 5 um Alumina (Al2O3) 11 weight percent dispersed in H20Slurry 1: 0.5 um Alumina (Al2O3) 11 weight percent, dispersed in H20 (calcinated)

• Slurry-2: 1.0 um Alumina (Al2O3) 11 weight percent, dispersed in H20 ( l t lli )(polycrystalline)

Pace Technologies, Tucson, AZ.

Step 2: Use CMP approach to planarize the surface using available slurriesStep-2: Use CMP approach to planarize the surface using available slurries

Pourbaix diagrams give guidance in the selection.

• W slurry• W-slurry

• Cu-slurry

• SiO2 (particle size O(50 - 100 nm))


SiO2 (particle size O(50 - 100 nm))

Abrasive wear

Two body abrasive mode: arises when a h d h f lid i t fthard rough surface slides against a softer surface, digs into it and plows a series of grooves.

Three body abrasive mode: arises when abrasive particles are introduced between lidi fsliding surfaces.

3-body wear produces lower wear rates, and more randomized wear marks.

Stachowiak, G.W., Batchelor A.W. Engineering Tribology, 2nd edition, BH Publishing, 2001.


Two step process

PV RMS Ra5 per Mov Avg (PV) 5 per Mov Avg (RMS) 5 per Mov Avg (Ra)

10

5 per. Mov. Avg. (PV) 5 per. Mov. Avg. (RMS) 5 per. Mov. Avg. (Ra)

1

ss (m

icro

ns)

0.1rfac

e R

ough

nes

Surf

Diluted 0.5 micron alumina polish 0.5 micron

alumina polish1 micron alumina polish

0.5 micron alumina polish

SiO Slurry0.01

0 10 20 30 40 50 60 70


SiO2 Slurry


Evolution of a smooth Nb surface by 2-Step Process

t = 0, PV=7.2 um t = 17 min, PV = 4.6 um t = 28 min, PV = 3.3 um

t = 42 min, PV=1.8 um t = 52 min, PV = 1.5 um t = 58 min, PV = 0.4 um

t = 60 min, PV = 0.5 um t = 67 min, PV = 0.3 um t = 71 min, PV =0.2 um


Summary and conclusions

Chemical mechanical polishing of Niobium is investigatedChemical mechanical polishing of Niobium is investigated

• Niobium forms a stable Ni2O5 oxide, ~ 4.5 nm thick, and self limiting.

• BCP treatment exposes ordered Niobium metal, but prolonged treatment does not improve surface roughness.

A two step procedure is proposed to first polish and then to planarize.

P li i i t h b t ti l i t i f fi i h• Preliminary experiments show substantial improvements in surface finish.

• Peak-to-Valley roughness reduced from ~7 um to 0.2 um

Process parameters need to be optimizedProcess parameters need to be optimized• These include pressure, particle size and polishing time, final CMP slurry type

Implementation inside the cavities are not considered in this short term pinvestigation

Considering the potential surface quality obtainable implementation of this approach inside cavities should be explored further


this approach inside cavities should be explored further

Backup Slides


Model predictions of material removal

Uniformity of material removal depends primarily on local contact pressure, but also affected by slurry-chemistry, abrasive size, wafer speed, pad properties/roughness

Polishing uniformity is important at three scales:

• Wafer (affects wafer bow)

• Die (affects die scale bow)

Wafer scale

• Die (affects die-scale bow)

• Feature (affects nano-wire flatness)

Modeling used to uncover fundamentals of the mechanisms Die scale Feature scale

genabling macro- and nano scale material removal.

Non-uniform contact pressureon the wafer will causeon the wafer will cause non-uniform material removaland wafer bow at wafer-scale

Contact pres

Slurry press

More material removal predicted on wafer’s edges

ssure

sure

Slurry pressure Contact pressure


y pdistribution under wafer

Contact pressure distribution on the wafer

BCP treatment

1.4 16Material Removal (g) PV (um)

Removed Material (mm) PV (um)

1

1.2

1.4

val (

g 12

14

16

, PV

(um

0.5

0.6

al (g 12

14

16

PV (u

m

Removed Material (mm) PV (um)

0.6

0.8

eria

l Rem

ov

6

8

10

roug

hnes

s

0 2

0.3

0.4

teria

l Rem

ova

6

8

10

e ro

ughn

ess,

0.2

0.4Mat

e

2

4

Surf

ace

0

0.1

0.2

Mat

0

2

4

Surf

ace

0-10 0 10 20 30 40 50 60 70 80 90 100


0 -10 0 10 20 30 40 50 60 70 80 90 100




3d 5/2Oxide Thickness ~4.5 nm

nits

Nb t l

Nb oxide3d 3/2

trar

y U

n Nb metal3d 5/2Nb metal

3d 3/2

Arb

it

198203208213

Binding Energy (eV) Δ eV is C


Δ eV ~ 5 eV Characteristic

of Nb2O5 Sampling Depth: ~ 7 nm

Nb 3d

Active Oxidizer: H2O2

Metal Peaks

H2O2

Active Etchant: HF

Cannot remove all of the oxide –oxidizes in air andoxidizes in air and seems limited to ~4.5 nm


199204209214

Binding Energy (eV)

BCP Dip Study; Step 1 of 2-Step Process

Buffered Chemical Polishing (BCP) Formula: 10 L HF (49%) 10 L HNO3 (65%)10 mL HF (49%), 10 mL HNO3 (65%), and 20 mL H3PO4 (85%)

0 4Nb 3d

0 2

0.3

0.35

0.4

mov

ed

Mass Removal Rate

6-minute

0.15

0.2

0.25

ram

s re

m BCP dip

0

0.05

0.1Gr

as-received


0 5 10 15 20

Time in minutes199204209214

Binding Energy (eV)

BCP Dip Study; Step 1 of 2-Step Process

0 4Nb 3d

0 2

0.3

0.35

0.4

mov

ed

Mass Removal Rate

6-minute

0.15

0.2

0.25

ram

s re

m BCP dip

0

0.05

0.1Gr

as-received


0 5 10 15 20

Time in minutes199204209214

Binding Energy (eV)

1 i t BCP d d i ifi tl li idth ( d d

Impact of 1-minute BCP Dip1 minute BCP produced significantly narrower linewidth (more ordered matrix) with potentially more surface removal than Cu Slurry alone…..

Nb 3d

FWHM = 1.3 eV

1 minute BCP + Cu Slurry CMP

FWHM = 1.7 eVCu Slurry CMP



Pourbaix Diagram for Nb-H2O System

A li E Ah d T M d Alf t i A “C i f i bi i l h i d h d hl i


Asselin, E., Ahmed, T.M., and Alfantazi, A., “Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 DegC” Corrosion Science, 49(2): p. 694-710, 2007.

Pourbaix Diagram for Nb-H2O System

CuCuSlurry

+H OH2O2

CuSlurry

Si

A li E Ah d T M d Alf t i A “C i f i bi i l h i d h d hl i

Slurry


Asselin, E., Ahmed, T.M., and Alfantazi, A., “Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 DegC” Corrosion Science, 49(2): p. 694-710, 2007.

CMP using High-Pressure & Copper slurry

PV RMS Ra

10

ons)

Linear (PV) Linear (RMS) Linear (Ra)

1

urfa

ce R

ough

ness

(mic

ro

0.10 2 4 6 8 10 12 14 16 18


Su

Surface roughness vs polishing timePolishing Time (min)




CMP using High-Pressure & Copper slurry on BCP treated Nb

PV RMS RaLi (PV) Li (RMS) Li (R )

10

(mic

rons

)

Linear (PV) Linear (RMS) Linear (Ra)

1

rface

Rou

ghne

ss

0.10 5 10 15 20


Sur

f

Surface roughness vs polishing time




CMP-Physics-I: Continuum effects

y

τ

θr

z

th hp

wω

Liquid slurry lubrication: Reynolds eqn.θ

x wω

z

z

x y pc, p

xτ α

( ) ( ) ( )( )31 2 1 2

1. 12 .2

Tp sT

hh p V V h V Vt

φ μ σ φ⎡ ⎤⎛ ⎞⎢ ⎥⎜ ⎟

⎝ ⎠⎢ ⎥⎣ ⎦

∂∇ ∇ = + ∇ + − ∇ +∂

r r r r r

y x pc, p yτ

β Wafer

Holder Soft-pad

( ) ( )3 21 24p NE R z d z dzφ∞

= −∫/* /

Multiasperity Contact: Greenwood et al. (1966, 1967)

( ) ( )3c s

d

p NE R z d z dzφ∫

Pad deflections: Elasticity

( ) ( )( ) ( )

2

2 2

', ' ' '1,' '

p x y dx dyw x y

E x x y y

νπ Ω

−=− + −

∫∫Williams, J.A. Engineering Tribology, Oxford, 2000.

1 2Th h wδ δ= + + +Pad wafer clearance


CMP Physics-II: Force Balance

y z wω

τ

θ

x wω

z

r

z

x y

th hp

pc, p xτ α

Forces acting on the pad need to be in balance:

• Slurry pressure, p

y x pc, p yτ

β Wafer

Holder Soft-pad

• Normal and tangential contact tractions, µpc, pc

0z

f c

extF c z

A A

R pdA p dA F= + − =∫ ∫f

( ) 0x

f c f c

extM x h c h c xx

A A A A

R t dA p t dA pxdA p xdA Mτ μ= − − − − + =∫ ∫ ∫ ∫

( ) 0y

f c f c

extM y h c h c yy

A A A A

R t dA p t dA pydA p ydA Mτ μ= − − + + + =∫ ∫ ∫ ∫


CMP-Physics:-III

Material Removal:• Material removal is governed by an abrasive removal process:

(') . slidingF LV k

H= k: removal rate constant

F Normal force

L : Sliding distance

Archard’s Law

LS: Sliding distance

H: Hardness (material property)

V(‘): Worn volume

MRR dV kPV= =

In CMP literature material removal rate (MRR) is used:

k: removal rate constant

P A li d h dPreston’s LawMRR ckPVdt

= = Pc: Applied push down pressure

V: Sliding speed

Preston s Law

Removal rate constant k represents the effects of:Removal rate constant k represents the effects of:• Abrasive particle size, concentration, hardness, morphology

• Wafer hardness, surface roughness

Sl h i t


• Slurry chemistry

• Pad roughness, elasticity

Documents

Chemical Mechanical PolishingChemical Mechanical Polishing