Chemical Mechanical PolishingChemical Mechanical Polishing ...
Transcript of 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.
Mechanical Engineering and Chemical Engineering
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
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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
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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
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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
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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
Mechanical Engineering and Chemical Engineering
• 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.
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XPS Studies on Niobium oxidation-I
Oxides formed by Nb:
XPS Analysis Reveals Nb-O BondingNb oxide
3d 5/2Oxide Thickness
XPS Analysis Reveals Nb-O BondingNb oxide
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)
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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)
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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
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199204209214Binding Energy (eV)
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
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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.
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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))
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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.
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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
Polishing Time (min)
SiO2 Slurry
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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
Mechanical Engineering and Chemical Engineering
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
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this approach inside cavities should be explored further
Backup Slides
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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
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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
BCP treatment time (min)
0 -10 0 10 20 30 40 50 60 70 80 90 100
BCP treatment time (min)
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XPS Analysis Reveals Nb-O BondingNb oxide
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
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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
Mechanical Engineering and Chemical Engineering
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
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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
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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
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198203208213Binding Energy (eV)
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
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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
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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
Polishing Time (min)
Su
Surface roughness vs polishing timePolishing Time (min)
Pad and wafer rotational speeds: 60 rpmSupply pressure : 1000 g/cm2C i h d li ill i
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Carrier head linear oscillations : onSlurry : Microplanar CMP 1150 by EKC Technology, Inc.Polishing Pad : IC1000 A2, by Rohm and Haas Electronic Materials
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
Polishing Time (min)
Sur
f
Surface roughness vs polishing time
Pad and wafer rotational speeds: 60 rpmSupply pressure : 1000 g/cm2C i h d li ill i
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Carrier head linear oscillations : onSlurry : Microplanar CMP 1150 by EKC Technology, Inc.Polishing Pad : IC1000 A2, by Rohm and Haas Electronic Materials
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
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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= − − + + + =∫ ∫ ∫ ∫
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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
Mechanical Engineering and Chemical Engineering
• Slurry chemistry
• Pad roughness, elasticity