Chapter 12

Chemical Mechanical
Polishing
Hong Xiao, Ph. D.
hxiao89@hotmail.com
www2.austin.cc.tx.us/HongXiao/Book.htm

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 Objectives
• List applications of CMP
• Describe basic structure of a CMP system
• Describe slurries for oxide and metal CMP
• Describe oxide CMP process.
• Describe metal polishing process.
• Explain the post-CMP clean

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 Overview
• Multi layer metal interconnection
• Planarization of dielectric layers
• Depth of focus require flat surface to
achieve high resolution
• The rough dielectric surface can also cause
problems in metallization

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 Wafer Process Flow
Materials IC Fab

Dielectric Test
Metalization CMP
deposition
Wafers

Thermal Etch Packaging

Implant
Processes PR strip PR strip
Masks

Photo- Final Test

lithography

Design

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 Tungsten CMP
• Tungsten has been used to form metal plugs
• CVD tungsten fills contact/via holes and
covers the whole wafer.
• Need to remove the bulk tungsten film from
the surface
• Fluorine based plasma etchback processes
• Tungsten CMP replaced etchback
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 5
CMOS IC Passivation 2 Silicon Nitride

Passivation 1
Al•Cu Alloy Al•Cu USG
CMP USG Metal 4 Ti/TiN
IMD33
IMD USG
TiN ARC
Metal 3 Al•Cu Alloy
CMP USG, W Ti
IMD 2 USG W
Ti/TiN

M2 Al•Cu
CMP USG, W
IMD 1 W USG W TiSi2

M1 Al•Cu Alloy Sidewall

CMP PSG, W
PMD BPSG Spacer, USG
W
CMP USG PMD Barrier
STI n+ n+ USG p+ p+
Nitride
P-Well N-Well
P-Epi
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 6
P-Wafer
 Definition of Planarization
• Planarization is a process that removes the
surface topologies, smoothes and flattens
the surface
• The degree of planarization indicates the
flatness and the smoothness of the surface

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 Definition of Planarization

Completely Conformal Film, No Planarization

Conformal and Smooth, No Planarization

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 Definition of Planarization

Partial Planarization

Global Planarization

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 Degrees of Planarity

Planarity R(µm) θ
Surface Smoothing 0.1 to 2.0 > 30
Local Planarization 2.0 to 100 30 to 0.5
Global Planarization > 100 < 0.5

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 Definition of Planarity

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 Planarization
• Smoothing and local planarization can be
achieved by thermal flow or etchback
• Global planarization is required for the
feature size smaller than 0.35 µm, which
can only be achieved by CMP

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 Other Planarization Methods
• Thermal flow
• Sputtering etchback
• Photoresist etchback,
• Spin-on glass (SOG) etchback

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 Thermal Flow
• Dielectric planarization
• Pre-metal dielectric
• High temperature, ~1000 °C
• PSG or BPSG, become soft and start to flow
due to the surface tension
• Smooth and local planarization

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 As Deposited

BPSG
SiO2
n+ n+ p+ p+
p+ p+
N-well
P-type substrate

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 After Thermal Flow

BPSG
SiO2
n+ n+ p+ p+
p+ p+
N-well
P-type substrate

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 Etch Back
• Reflow temperature is too high for IMD
– can melt aluminum
• Other planarization method is needed for
IMD
• Sputtering etch back and reactive etch back

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 Etch Back
• Argon sputtering etchback chip off dielectric
at corner of the gap and taper the openings
• Subsequent CVD process easily fills the gap
with a reasonable planarized surface
• Reactive ion etchback process with CF4/O2
chemistry further planarizes the surface

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 CVD USG

USG
Al·Cu·Si
BPSG
SiO2
n+ n+ p+ p+
p+ p+
N-well
P-type substrate

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 Sputtering Etch Back of USG

USG
Al·Cu·Si
BPSG
SiO2
n+ n+ p+ p+
p+ p+
N-well
P-type substrate

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 CVD USG

USG
Al·Cu·Si
BPSG
SiO2
n+ n+ p+ p+
p+ p+
N-well
P-type substrate

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 Reactive Etch Back of USG

USG
Al·Cu·Si
BPSG
SiO2
n+ n+ p+ p+
p+ p+
N-well
P-type substrate

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 Photoresist Etchback
• PR spin-coats can baking
• Planarized solid thin film on wafer surface
• Plasma etch process with CF4/O2 chemistry
• Oxide etched by F and PR by O
• Adjusting CF4/O2 flow ratio allows 1:1 of
oxide to PR selectivity.
• Oxide could be planarized after etchback
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 After Oxide Deposited

Oxide

Metal Metal

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 Photoresist Coating and Baking

Photoresist

Oxide

Metal Metal

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 Photoresist Etchback

Oxide

Metal Metal

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 Photoresist Etchback

Oxide

Metal Metal

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 Photoresist Etchback

Oxide

Metal Metal

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 Photoresist Etchback
• When F etch oxide, O will be released
• Higher PR etch rate due to extra oxygen
• PR etchback can’t planarize very well
• After the PR etchback, dielectric film
surface is flatter than it is just deposited.
• In some cases, more than one PR etchback
is needed to achieve required flatness
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 29
 SOG Etchback
• SOG replaces PR
• Advantage: some SOG can stay on the wafer
surface to fill the narrow gaps
• PECVD USG liner and cap layer
• USG/SOG/USG gap fill and surface
planarization
• Sometimes, two SOG coat, cure and etchback
processes are used
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 SOG Etchback

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 Necessity of CMP
• Photolithography resolution R = K1λ/NA
• To improve resolution, NA ↑ or λ ↓
• DOF = K2λ/2(NA)2, both approaches to
improve resolution reduce DOF
• DOF is about 2,083 Å for 0.25 µm and
1,500 Å for 0.18 µm resolution.
• Here we assumed K1=K2, λ=248 nm
(DUV), and NA=0.6
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 32
 Necessity of CMP
• 0.25 µm pattern require roughness < 2000 Å
• Only CMP can achieve this planarization
• When feature size > 0.35 µm, other methods
can be used

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 Advantages of CMP
• Planarized surface allows higher resolution of
photolithography process
• The planarized surface eliminates sidewall
thinning because of poor PVD step coverage

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 Metal Line Thinning Due to the
Dielectric Step

Sidewall Thinning Metal 2

IMD 1

Metal 1
PMD
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 Planarized Dielectric Surface, no
Metal Line Thinning Effect

Metal 2
IMD 1
Metal 1
PMD
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 Advantages of CMP
• Eliminate the requirement of excessive
exposure and development to clear the thicker
photoresist regions due to the dielectric steps
– This improves the resolution of via hole and
metal line pattering processes
• Uniform thin film deposition
– Reduce required over etch time
– Reduce chance of undercut or substrate loss
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 37
 Over Exposure and Over
Development
Possible CD loss due to more
exposure and development
Needs more exposure
PR and development PR

PR Metal 2

Metal 2

IMD 1

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 Rough Surface, Long Over Etch

PR Need a long over PR

etch to remove
Metal 2

Metal 2

IMD 1

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 Flat Surface, Short Over Etch

PR Very litter over PR

etch is required
Metal 2 Metal 2

IMD 1

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 Advantages of CMP
• CMP reduce defect density, improve yield
– Reducing the process problems in thin film
deposition, photolithography, and etch.
• CMP also widens IC chip design parameters
• CMP can introduce defects of its own
• Need appropriate post-CMP cleaning

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 Applications of CMP
• STI formation
• Dielectric layer planarization
– PMD and IMD
• Tungsten plug formation
• Deep trench capacitor

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 Applications of CMP

CMP USG

CMP USG

CMP W

CMP USG CMP W

CMP PSG, W
CMP PSG, W
CMP USG
STI

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 Deep Trench Capacitor

Heavily
doped Si Pad
Oxide

Silicon
Substrate

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 Deep Trench Capacitor
Nitride
Heavily
doped Si Pad
Oxide

Silicon
Substrate

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 Deep Trench Capacitor
Nitride
Heavily
doped Si Pad
Oxide

Silicon
Substrate

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 Deep Trench Capacitor
Nitride
Heavily
doped Si Pad
Oxide

Silicon
Substrate

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 Deep Trench Capacitor
Nitride
Heavily
doped Si Pad
Oxide

Silicon
Substrate

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 Deep Trench Capacitor
Nitride
Heavily
doped Si Pad
Oxide

Silicon
Substrate

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 Deep Trench Capacitor

Dielectric Nitride
Layer
Pad
Oxide
Heavily
doped Si

Silicon
Substrate

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 Deep Trench Capacitor

Dielectric Nitride
Layer
Pad
Oxide
Heavily
doped Si

Silicon
Substrate

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 Deep Trench Capacitor
Polysilicon
Dielectric Nitride
Layer
Pad
Oxide
Heavily
doped Si

Silicon
Substrate

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 Deep Trench Capacitor
Polysilicon
Dielectric Nitride
Layer
Pad
Oxide
Heavily
doped Si

Silicon
Substrate

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 Deep Trench Capacitor
Polysilicon
Dielectric Nitride
Layer
Pad
Oxide
Heavily
doped Si

Silicon
Substrate

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 Deep Trench Capacitor
Polysilicon
Dielectric
Layer
Pad
Oxide
Heavily
doped Si

Silicon
Substrate

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 Deep Trench Capacitor
Polysilicon
Dielectric
Layer

Heavily
doped Si

Silicon
Substrate

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 Applications of CMP
• Copper interconnection.
• Copper is very difficult to dry etch,
• Dual damascene: process of choice
• Tungsten plug is a damascene process

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 Applications of CMP
• It uses two dielectric etch processes,
– one via etch and one trench etch
• Metal layers are deposition into via holes
and trenches.
• A metal CMP process removes copper and
tantalum barrier layer
• Leave copper lines and plugs imbedded
inside the dielectric layer
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 PECVD Nitride

Nitride

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 PECVD USG

Nitride

USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 PECVD Etch Stop Nitride

Nitride

USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 PECVD USG

USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Photoresist Coating
Photoresist
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Via 1 Mask

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Via 1 Mask Exposure and Development
Photoresist
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Etch USG, Stop on Nitride
Photoresist
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Strip Photoresist

USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Photoresist Coating
Photoresist
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Metal 1 Mask

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 Metal 1 Mask Exposure and
Development
Photoresist
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Etch USG and Nitride
Photoresist
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Strip Photoresist

USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Deposit Tantalum Barrier Layer

USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 Deposit Copper
Copper
USG
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 CMP Copper and Tantalum

M1 USG Cu
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 PECVD Seal Nitride

M1 USG Cu
USG

PSG W

STI n+ n+ USG p+ p+
P-Well N-Well
P-Epi
P-Wafer
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 CMP Hardware
• Polishing pad
• Wafer carrier
• Slurry dispenser

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 Chemical Mechanical Polishing
Slurry Dispenser
Pressure
Membrane
Wafer Holder
Wafer
Retaining Ring Slurry

Polishing Pad

Platen

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 Linear Polishing System
Pressure Slurry Dispenser
Membrane
Wafer Carrier Pad
Wafer
Retaining Ring Slurry Conditioner

Support Fluid Bearing

Belt and Polishing Pad

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 Orbital Polishing
ωc Down Force
Carrier
Film

Wafer
Polish
Pad

Orbital Motion, ωp
Slurry

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 Polishing Pad
• Porous, flexible polymer material
– cast, sliced polyurethane or urethane coated
polyester felt
• Pad directly affects quality of CMP process
• Pad materials: durable, reproducible,
compressible at process temperature
• Process requirement: high topography
selectivity to achieve surface planarization
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 Polishing Pad Hardness
• Harder polishing pad: higher removal rate
and better within die (WID) uniformity
• Softer pad: better within wafer (WIW)
uniformity.
• Hard pads easier to cause scratches.
• The hardness is controlled by pad chemical
compositions or by cellular structure.

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 Polishing Pad
• Cells absorb polishing slurry
• Filler improve mechanical properties
• Polishing pad surface roughness determines
the conformality range.
– Smoother pad has poorer topographical
selectivity less planarization effect.
– Rougher pad has longer conformality range and
better planarization polishing result
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 Hard Rough Pad

Wafer

Film

Polishing Pad Pad Movement

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 Soft Smooth Pad

Wafer

Film

Polishing Pad
Pad Movement

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 Pad Conditioning
• Pad becomes smoother due to the polishing
• Need to recreate rough pad surface
• In-situ pad conditioner for each pad
• The conditioner resurfaces the pad
• Removes the used slurry
• Supplies the surface with fresh slurry

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Polishing Pad and Pad Conditioner

Slurry
Wafer Dispenser
Carrier

Polishing
Pad Pad
Conditioner

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 Polishing Head
• Polishing head is also called wafer carrier
• It consists of a polishing head body
• Retaining ring
• Carrier membrane
• Down force driving system

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 Polishing Head
Retaining Ring

Carrier
Membrane

Polishing Head Body

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 Schematic of Polishing Head
Vacuum Chuck
Downforce
Pressure

Carrier Restraining Ring

Chamber Positioning

Restraining
Ring Membrane Restraining
Ring
Wafer
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 Pad Conditioner
• Sweeps across the pad to increase surface
roughness required by planarization and
removes the used slurry
• Conditioner is a stainless steel plate coated
with nickel-plated diamond grits
• Diabond CMP conditioner: stainless steel
plate coated with CVD diamond film plated
diamond grids
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 Surface of CMP Conditioners

Diamond Grits Diamond Grits (~ 20 µm)

Nickel Film Diamond Film

Silicon Substrate
Stainless Steel Plate Stainless Steel Plate

Conventional Diabond

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 CMP Slurries
• Chemicals in the slurry react with surface
materials, form chemical compounds that
can be removed by abrasive particles
• Particulate in slurry mechanically abrade
the wafer surface and remove materials
• Additives in CMP slurries help to achieve
desired polishing results

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 CMP Slurries
• CMP slurries work just like toothpaste
• Chemicals kill gems, remove tartar, and
form protection layer on the teeth
• Particles abrade away unwanted coating
from tooth surface during tooth brushing

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 CMP Slurries
• Water-based chemicals with abrasive particles and
chemical additives
• Different polishing processes require different
slurries
• Slurry can impact removal rate, selectivity,
planarity and uniformity
• Slurries always are engineered and formulated for
a specific application.

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 CMP Slurries
• Oxide slurry: alkaline solution with silica
• Metal slurry: acidic solution with alumina
• Additives control the pH value of slurries
– oxide, pH at 10 to 12
– metal, pH at 6 to 2

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 96

 pH Values

Neutral

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

More Acidic pH More Basic

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 97

 Slurry Delivery
• Slurry components are stored separately
– DI water with particulate
– additives for pH control
– oxidants for metal oxidation
• Flow to a mixer to mix at required ratio

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 98

 Slurry Flow

DI + Suspensions LFC

CMP
pH Adjuster LFC Mixer
Tool

Oxidant LFC

LFC: liquid flow controller

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 99

 Oxide Slurry
• Based on experience of optical industry,
which polish silicate glass to make lenses
and mirrors for a long time
• Oxide slurry is a colloidal suspension of
fine fumed silica (SiO2) particles in water
• KOH is used to adjust the pH at 10 to 12
• NH4OH can also be used

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 100

 Oxide Slurry
• Abrasives: fumed silica particles
• Normally contain ~ 10% solids
• Shelf lifetime of up to 1 year with proper
temperature control

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 101

 Fumed Silica
• Fumed silica particles are formed in a vapor
phase hydrolysis of SiCl4 in a hydrogen-
oxygen flame
2 H2 + O2 → 2 H2O
SiCl4 + 2 H2O → SiO2 + 4HCl ↑
• Overall reaction
SiCl4 + 2 H2 + O2 → SiO2 + 4HCl ↑
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 102
 Fumed Silica Particle Formation
Collection
Cooling System

<1710 °C Agglomerates

Aggregates
>1800 °C

O2 SiCl4
H2
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 103
 Fumed Silica Particles

Courtesy of Fujimi Corporation

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 104
 Metal Polishing Slurry
• Metal CMP process is similar to the metal
wet etch process
– Oxidant reacts with metal to form oxide
– Metal oxide is removed
– Repeat metal oxidation and oxide removal

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 105

 Metal Polishing Slurry
• The metal CMP slurries usually are pH-
adjusted suspensions of alumina (Al2O3)
• The slurry pH controls the two competing
metal removal mechanisms
– metal corrosive wet etching
– metal oxidation passivation

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 106

 Metal Polishing Slurry
• Different metal oxides have different solubility
• If oxide is soluble, wet etch will dominate
– Not favored: isotropic with no topographic selectivity
• If oxide is insoluble, it blocks further oxidation
– Particles mechanically abrade oxide layer
– Repeating metal oxidation and oxide abrasion
– favorable: high surface topographic selectivity
• The pH value controls oxidation process
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 107
 Tungsten Slurry
• Pourbaix diagram
• When pH < 2, tungsten is in passivation regime
• Tungsten can form passivation oxide WO3 with
pH lower than 4 in the presence of an oxidant
– Oxidants: potassium ferricyanid (K 3Fe(CN)6), ferric
nitrade (Fe(NO3)3), and H2O2
• For a higher pH, the soluble W12O4110−, WO42−,
and W12O396− ions can be formed, cause wet etch
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 108
 Pourbaix Diagram for Tungsten
2 W12O396- Stable
Potential (Eh) Volts

1
W12O4110-
WO3
WO42-
0
WO2 Corrosive

-1
W

-2
0 2 4 6 8 10 12 14
pH
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 109
 Tungsten Slurry
• Adjusting slurry pH allows low wet etch rates
and chemical-mechanical polish removal
• Tungsten slurries normally are quite acidic with
pH level from 4 to 2.
• Tungsten slurries have lower solid contents and
much shorter shelf lifetime.
• Tungsten slurries require mechanical agitation
prior to and during delivery to the CMP tools
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 110
 Aluminum Slurry
• Water-based acidic solutions
• H2O2 as oxidant,
• Alumina as abrasives.
• Limited shelf lifetime
• H2O2 molecule is unstable
• Aluminum CMP is not popularly used
– Hard to compete with copper metallization
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 111
 Copper Slurry
• Acidic solutions
• Oxidants: hydrogen peroxide (H2O2),
ethanol (HOC2H5) with nitric acid (HNO4),
ammonium hydroxide (NH4OH) with
potassium ferri- and ferro-cyanide, or nitric
acid with benzotriazole
• Alumina as abrasives

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 112

 Pourbaix Diagram for Copper
2
CuO CuO22-

Corrosive
Corrosive
Potential (Eh) Volts

1 Passivation
Cu2+
Cu2O
0 Cu+ Cu

-1 Passivation
regime with Immunity
stable alumina
-2
0 2 4 6 8 10 12 14
pH
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 113
 Copper Slurry
• Need colloidally stable slurry to achieve
consistent polishing process results
• A colloidally stable alumina suspension can
be achieved at pH just below 7.
• Only a small window for copper slurries to
achieve both electrochemical passivation
and colloidally stable suspension of
aqueous alumina particles
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 114
 CMP Basics
• Removal rate
• Uniformity
• Selectivity
• Defects

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 115

 Removal Rate
• Mechanical removal rate R was found by
Preston
• The Preston equation can be expressed as
R = Kp⋅p⋅∆v
• p is the polishing pressure
• Kp is the Preston coefficient
• ∆v is relative velocity of wafer and pad
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 116
 Removal Rate
• Preston equation works very well for the
bulk film polishing processes
• The protruding portions on a rough surface
have higher polishing pressure
• Removal rate of protruding parts is higher
• This helps to remove surface topography
and planarize the surface

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 117

Protruding Parts with Higher Pressure

Wafer No Pressure, No Removal

High Pressure, Fast Removal Film

Polishing Pad

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 118

 Removal Rate
• Thickness difference before and after CMP
divided by CMP time
• Multiple measurement for uniformity
• Test wafer, blanket film
• Daily tool qualification

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 119

 Uniformity
• Usually 49-point, 3σ standard deviation as
the definition of the uniformity for the CMP
process qualifications
• Changes of the film thickness before and
after CMP process is monitored
• For the production wafers, uniformity after
CMP process is monitored
• Normally use 9 or 13 points measurement
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 120
 Uniformity
• Both WIW and WTW uniformity can be affected
by the polish pad condition, down force pressure
distribution, relative speed, restraining ring
positioning, and the shape of the wafers.
• By using harder pad and lower pressure a good
global uniformity can be achieved
• Lower pressure, lower removal rate, affect
throughput
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 121
 Selectivity
• Ratio of removal rates of different materials
• Affect CMP defects, such as erosion or dishing
• The slurry chemistry is the primary factor that
affects removal selectivity of CMP process
• STI oxide CMP require high oxide to nitride
selectivity, from 100:1 to 300:1
• Because only polish oxide, selectivity is not
important in PMD and IMD CMP processes
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 122
 Selectivity
• For tungsten CMP process, selectivity to oxide
and titanium nitride is very important.
• Usually tungsten to TEOS oxide selectivity is
very high, from 50 to 200
• Slurry chemistry, oxidant
• Selectivity is also related to the pattern density
• higher pattern density, lower removal selectivity
– lead to erosion of the tungsten and oxide film
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 123
 Erosion Caused by High Pattern
Density

W Oxide W W

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 124

 IC Layout and Erosion
• IC design layout can directly affect the
erosion problems
• By designing opening area less than 30% of
the chip surface, it can help to solve the
erosion problem

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 125

 Defects
• CMP removes defects and improves yield
• Introduce some new defects
– scratches, residual slurry, particles, erosion, and
dishing.
• Large foreign particles and hard polish pad
can cause scratches
– Tungsten fill the scratches in oxide surface
cause short circuit and reduce the IC yield.
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 126
 Defects
• Improper down force pressure, worn pad,
inadequate pad conditioning, particle
surface attraction, and slurry drying
• Slurry residue on the wafer surface and
cause contamination
• Post-CMP clean is very important to
remove slurry residue and improve process
yield
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 127
 Erosion
• Increases depth of via holes
• Incomplete via hole etch
• Open loop between the different layers in
the next dual damascene interconnection

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 128

Circuit Opening Caused by Erosion
Via Etch Stop Open Caused by Erosion

Cu Oxide Cu Cu

W Oxide W W

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 129

 Dishing Effect
• Usually happens at a larger opening area
– large metal pads
– STI oxide in the trenches.
• More materials are removed from the center
• Cross-section view looks like a dish

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 130

 Dishing Effect

USG Tungsten

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 131

 Dishing/Erosion and Selectivity
• Both dishing and erosion effects are related
to the removal selectivity
• Tungsten CMP process,
– If tungsten to oxide selectivity is too high, more
tungsten removal, cause dishing and recessing
– If the selectivity is not high enough, both oxide
and tungsten will be polished, causes erosion

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 132

 Dishing/Erosion and Selectivity
• Oxide CMP with high selectivity of oxide to
nitride can cause oxide dishing during the
oxide overpolishing step of the oxide CMP
in the STI formation

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 133

 Dishing Effect of STI USG

Pad Oxide Nitride

USG USG

Silicon Substrate

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 134

 Particles and Defects
• Particles and defects cause irregular
topography on wafer surface
• Scattering incident light
• Monitor particles and defects by detecting
the scattered light

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 135

Particle Detection By Light Scattering
Incident Light

Reflected Light
Scattered light
Photodetector

Scattered light

Particle
or Defect Scattered light

Substrate

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 136

 Particle Measurement
• Intensity of the scattered light is very weak
• Elliptical mirror is used to collect the light
• Elliptical curve has two focuses
• Light from one focus reflects to another focus

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 137

 Particle Measurement
• Laser beam scans wafer surface vertically at
one focus of elliptical mirror and a photo-
detector is placed at another focus
• Moving wafer, and collecting scattered light
to detect tiny particles and defects
• Mapping particle/defect locations on the
wafer surface

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 138

 Laser Scan

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 139

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 140

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 141

 Particle Measurement: Particle 1

Elliptical Mirror

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 142

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 143

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 144

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 145

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 146

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 147

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 148

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 149

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 150

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 151

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 152

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 153

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 154

 Particle Measurement, Particle 2

Elliptical Mirror

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 155

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 156

 Particle Measurement

Elliptical Mirror
Reflected Light

Scanning Laser Beam

Wafer Photodetector

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 157

 CMP Processes
• Oxide removal mechanism
• Metal removal mechanisms
• Endpoint methods

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 158

 Oxide CMP
• Early development in the mid-1980s in IBM
• Combined knowledge and experience of
glass polishing and silicon wafer polishing

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 159

 Oxide CMP
• Hydroxyls on both film and silica surfaces
• Form hydrogen bonds of silica and surface
• Form molecular bonds of silica and surface
• Mechanical removal of the particles bonded
with wafer surface
• Tear away atoms or molecule from film on
wafer surface
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 160
 Oxide CMP

Abrasive Particle
Si
O
H
H H H H H H H H H H H H H H H

O O O O O O O O O O O O O O O
Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si
O O O O O O O O O O O O O O O
Silicon Oxide Surface
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 161
 Oxide CMP, Hydrogen Bond

Abrasive Particle
Si
O
H
H H H H H H H H H H H H H H H

O O O O O O O O O O O O O O O
Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si
O O O O O O O O O O O O O O O
Silicon Oxide Surface
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 162
 Oxide CMP, Molecule Bond

Abrasive Particle O H
H
H H H H H H H H H Si H H H H H

O O O O O O O O O O O O O O O
Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si
O O O O O O O O O O O O O O O
Silicon Oxide Surface
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 163
 Oxide CMP, Removal of Oxide

O H
Abrasive Particle Si
O H
Si

H H H H H H H H H H H H H H

O O O O O O O O O O O O O O
Si Si Si Si Si Si Si Si Si Si Si Si Si Si
O O O O O O O O O O O O O O O
Silicon Oxide Surface
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 164
 Tungsten CMP
• Form plugs to connect metal lines between
different layers
• Tungsten etch back and Tungsten CMP
– Fluorine based tungsten RIE etchback
• In-situ with tungsten CVD process in a cluster tool
• Recessing of the Ti/TiN barrier/adhesion layer due
to the aggressive fluorine chemical etch of Ti/TiN
and affects the chip yield
– Tungsten CMP: winner for higher yield
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 165
Recess of Ti/TiN due to W Etchback

Ti/TiN Barrier & Recess Caused

Adhesion Layer Tungsten by Etchback
Plug

USG

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 166

 Tungsten CMP
• Two completing removal mechanisms
• Wet etch: a pure chemical process
– Unfavorable
• Passivation oxidation and oxide abrading:
chemical and mechanical process
– Favorable
• Controlled by pH value of slurry
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 167
 Tungsten CMP
• Potassium ferricyanide, K3Fe(CN)6, is used as
both etchant and oxidant
• The wet etch chemistry can be expressed
W+6Fe(CN)6-3+4H2O → WO4-2+6Fe(CN)6-4+8H+
• The competing passivation oxidation reaction
W+6Fe(CN)6-3+3H2O → WO3+6Fe(CN)6-3+6H+

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 168

 Tungsten CMP
• Normally tungsten CMP uses two step process
• The first step remove bulk W with slurry pH < 4,
• The second step remove TiN/Ti stacked
barrier/adhesion layer with slurry pH > 9

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 169

 Metal CMP Process

Polishing Pad

Passivation Oxide Slurry

Metal Oxide
Wet Etch of Abrasive
“Soft Oxide” Alumina
Particle

Metal

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 170

 Copper CMP
• Difficult to plasma etch copper
– Lack of volatile inorganic copper compounds
• Copper CMP key process in copper
metallization process
• H2O2, or HNO4 can be used as oxidant
• Alumina particulate is used for abrasion

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 171

 Copper CMP
• CuO2 is porous and can’t form a passivation
layer to stop further copper oxidation
• Additive is needed to enhance passivation
• NH3 is one of additives used in slurry
• Other additives such as NH4OH, ethanol or
benzotriazole can also be used as complexing
agents to reduce wet etch effect

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 172

 Copper CMP
• Dual-damascene copper metallization
• Both bulk Cu and barrier Ta layer need to be
removed by the CMP process.
• Cu slurry can’t effectively remove Ta, the
lengthy over polishing step for Ta removal
can cause copper recess and dishing effects

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 173

 Copper Deposition

Copper Tantalum
USG Barrier
Layer

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 174

 Copper CMP

Copper Tantalum
USG Barrier
Layer

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 175

 Over Polish to Remove Tantalun

Copper Tantalum
USG Barrier
Layer

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 176

 Copper Dishing and Recessing

Copper Tantalum
USG Barrier
Layer

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 177

 Copper CMP
• Two-slurry polishing
• The first slurry remove bulk copper layer
• The second slurry remove Ta barrier layer
• The two-slurry CMP process reduces
– Copper recessing and dishing
– Oxide erosion
• Multiple polishing platens greatly simplifies
multi-slurry CMP processing
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 178
 CMP Endpoint Detection
• Monitoring the motor current
• Optical measurement

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 179

 Motor Current CMP Endpoint
• When CMP process closing to end, polish pad
start to contact and polish underneath layer
• Friction force start to change
• Current of the polish head rotary motor will
change to keep constant pad rotation rate
• Monitoring the change of motor current can
find endpoint of the CMP process

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 180

Motor Current During Copper CMP
Oxide Exposed
Sensor Output (Arbitrary Unit)

Cu Ta Exposed

0 60 120 180 240 300 360

Time (sec)

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 181

 Optical Endpoint: Dielectric
• Endpoint by either thickness measurement
• Reflected lights interfere with each other
• Constructive and destructive interference
• Change of the film thickness causes the
periodically changes of interference state
• Dielectric film thickness change can be
monitored by the change of reflection light
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 182
 Endpoint of Dielectric CMP

Substrate
Dielectric Film

Laser
Photodetector

Light
Intensity

Time
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 183
 Optical Endpoint: Metal
• The change of reflectivity can be used for
metal CMP process endpoint
• Usually metal surface has high reflectivity
• Reflectivity significantly reduces when
metal film is removed
• Trigger endpoint

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 184

 Endpoint of Metal CMP
USG USG
Metal Film

Laser Laser

Photodetector Photodetector

Reflective
Intensity

time
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 185
 Post CMP Clean
• Post-CMP cleaning need remove both particles
and other chemical contaminants
• Otherwise, defect generation and low yield
• Mechanical scrubbing cleaners with DI water
• Larger DI water volume, higher brush pressure
high cleaning efficiency
• Three basic steps: clean, rinse, and dry
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 Post CMP Clean
• Usually brush is made of porous polymers,
allows chemicals to penetrate through it and
deliver to wafer surface
• Double-sided scrubbers are used in the post-
CMP clean process

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 Brush System

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 Post CMP Clean
• Slurry particles can chemically bond to atoms
on wafer surface if slurry dried
• Chemical additives, such as NH4OH, HF or
surfactants is needed to remove bonded
particles by weakening or breaking the bonds
• Additives also help particles diffuse away
from the surface

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 Post CMP Clean
• Chemical solution is also used to adjust the
wafer and particle surface charges so that
electrostatic repulsion keeps particles from re-
deposition on the surface
• Acidic solutions can be used to oxidize and
dissolve organic or metal particles

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 Particle Removal Mechanisms

Acidic Solution:
Oxidation and
Dissolution

Alkaline Solution:
Surface Etch and
Electrostatic
Repulsion

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 191

 Post Oxide CMP Clean
• Silica particles adhere to or embedded in
oxide surface
• Usually an alkaline chemical, NH4OH, is
used for post oxide CMP clean
• The alkaline solution charges both silica
particles and oxide surface negatively
• Electrostatic force expels particles from the
surface
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 Post Oxide CMP Clean
• HF is used to remove particles with strong
molecule bonds with surface,
• Breaking the bonds, dissolving silica particles
and some oxide surface
• Megasonics (MHz ultrasound wave) is
commonly used to release shock waves that
help dislodge the particles
• DI water rinse
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 193
 Post Tungsten CMP Clean
• Tungsten slurries are much harder to remove
than oxide slurries.
• DI wafer with NH4OH is commonly used
• Fe(NO3)3 as the oxidant results in high Fe3+
ion concentration in the solution.
• The Fe3+ ion interacts with OH− to form
Fe(OH)3 particulate that grow to 1 micron

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 Post Tungsten CMP Clean
• The Fe(OH)3 particles can cause high surface
defect density and contaminates the brush
• Commonly called brush loading
• The defect caused by Fe(OH)3 particles can
be reduced by using 100:1 HF clean
• DI water rinse

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 195

 Wafer Drying
• Residue-free drying process
• Physically removal, without water evaporation
– Evaporation drying cause contamination by leaving
dissolved chemicals in DI water behind
• Most commonly used technique: spin-drying
– Centrifugal force drives water out the wafer
• Ultra-clean dry air or nitrogen flow remove
water from wafer center
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 196
 Wafer Drying
• Vapor drying
– Ultra-pure solvent with high vapor pressure
– Most commonly used: isopropyl alcohol (C3H8O,
IPA)
– Displace water from the wafer surface

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 Dry-in Dry-out CMP
• Integrated CMP and post-CMP clean systems
• Allow so-called “dry-in dry-out” process
• CMP, post-CMP clean, and wafer drying
processes in one sequence
• Improve process through put and yield

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 198

 Process Issues
• CMP process is a relatively new process
• Very limited process details are available
• The main concerns for CMP processes
– Polishing rate, planarization capability, within die
uniformity, within wafer uniformity, wafer to wafer
uniformity, removal selectivity, defects and
contamination control

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 Process Issues: Polish Rate
• Polish rate affected by
– Downforce pressure
– Pad hardness
– Pad condition
– Applied slurry
– Rotation speed

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 Process Issues: Planarization
• Planarization capability is mainly determined
by the stiffness and surface condition of the
polish pad.

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 Process Issues: Uniformity
• Uniformity affected by
– Polish pad condition,
– Down force pressure,
– Relative speed of the wafer to the polish pad,
– Curvature of wafers, which is related to film stress
• Downforce pressure distribution is the most
important knob to control the CMP uniformity

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 202

Process Issues: Removal Selectivity
• Mainly controlled by the slurry chemistry
• Also related to the pattern density
– determined by the design layout.

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 203

 Process Issues: Defects
• There are many different kinds of defects,
which relate with many different process
parameters

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 Process Issues: Contamination
Control
• Contamination Control:
– Isolate CMP bay from other processing areas
– Restrict movement between CMP bay and other
area
• Dedicate copper CMP tools
– Avoid copper contamination of the silicon wafer
– Copper contamination can cause unstable
performance of MOSFETs and ruin the IC chips
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 205
 Process Issues: Contamination
Control
• IF slurry has spilled, it is very important to
immediately wash and clean it thoroughly
• Dried slurry leaves huge amount of tiny
particles, which is easy to airborne can become
a source of particle contamination.

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 206

 Future Trends
• More widely used copper CMP
• Copper and low-κ dielectric interconnection
– low-κ dielectric CMP
– Copper and barrier layer CMP processes with
high selectivity to low-κ dielectric
• DRAM applications: CMP processes involve
with polysilicon and high-κ dielectric

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 207

 Summary
• Main applications of CMP are dielectric
planarization and bulk film removal
– STI, PMD and IMD planarization, tungsten plugs,
and dual damascene copper interconnections.
• Need CMP for <0.25 µm features patterning due
to depth-of-focus requirement
• Advantages of CMP: high-resolution patterning,
higher yield, lower defect density
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 208
 Summary
• A CMP system usually consists of wafer carrier,
a polishing pad on a rotating platen, a pad
conditioner, and a slurry delivery system
• Oxide slurries: alkaline solutions at 10< pH < 12
with colloidal suspension silica abrasives
• Tungsten slurries are acidic solutions at 4< pH <
7 with alumina abrasives
• Copper slurries: acidic with alumina abrasives
Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 209
 Summary
• The important factors of CMP processes:
– Polish rate, planarization capability, selectivity,
uniformities, defects and contamination controls
• Polish rate affects by: downforce pressure, pad
stiffness, pad surface condition, relative speed
between pad and wafer, and slurry type.
• CMP uniformity affects by down force pressure
distribution, pad stiffness, and pad condition

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 210

 Summary
• The removal selectivity is mainly determined by
the slurry chemistry
• Oxide CMP process: silica particles form
chemical bonds with surface atoms and abrade
removal of materials from the surface
• Two metal removal mechanisms in metal CMP
process: wet etch and passivation/abrade

Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm 211

 Summary
• Endpoint detections
– Optical
• Thickness measurement for dielectric film
• Reflectivity measurement for metal film
– Motor current
• Post-CMP clean reduce defects and improve
yield

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