Feature-level Compensation & Control

CMPSeptember 15, 2005

A UC Discovery Project

FLCC

Chemical Mechanical Planarization - Faculty Team

Mechanical Phenomena

Chemical Phenomena

Interfacial and Colloid

Phenomena

Including ‘slurryless’ planarization - E-CMP

Jan B. TalbotChemical EngineeringUCSB

David A. DornfeldMechanical EngineeringUCB

Fiona M. DoyleMaterials Science and EngineeringUCB

FLCC

Chemical Mechanical Planarization - Student Team

Mechanical Phenomena

Chemical Phenomena

Interfacial and Colloid

Phenomena

Sunghoon Lee ME-UCB

Alex DeFeo ME-UCB

Ling WangMSE UCB

Jihong Choi ME-UCB

Robin IhnfeldtChem Eng UCSB

9/15/05FLCC - CMP

4

FLCC

CMP ResearchDescription: The major objective of this work continues to be to

establish an effective linkage between capable process models for CMP and its consumables to be applied to process recipe generation and process optimization and linked to device design and other critical processes surrounding CMP. Specific issues include dishing, erosion and overpolishing in metal polishing, which have an impact on circuit performance — all pattern dependent effects at the chip level —wetability effects in polishing, and novel consumable design (pads and abrasives) for optimized performance. We develop integrated feature-level process models which drive process optimization to minimize feature, chip and wafer-level defects.

Goals: The final goals remain reliable, verifiable process control in the face of decreasing feature sizes, more complex patterns and morechallenging materials, including heterogeneous structures and process models linked to CAD tools for realizing “CMP compatible chip design.”

9/15/05FLCC - CMP

5

FLCC

Year 2 Highlights• Developed capability in integrated model for determining and

assessing pattern sensitivity.• The effects of various common additives on zeta potential for

alumina slurries used in copper CMP were determined.• The influence of common slurry additives on the colloidal behavior of

alumina suspensions used for copper CMP were characterized.• Design rules for SMART pad design and fabrication are developed

and simulation verification of pad slurry flow characteristics done with initial performance validation tests run.

• Experiments to provide crucial data for prediction of the galvanic damage sometimes seen during CMP were conducted including measurement of relative reactivity of copper and barrier materials in different slurries.

9/15/05FLCC - CMP

6

FLCC

Year 3 Plan• Mechanisms for coupling of chemical and mechanical phenomena in CMP (M22 YII.7)Use kinetic data for weight changes during passivation by peroxide to develop dynamic model for

response after abrasion of surface layers and creation of new copper surface.• Wetting and adhesion studies on two phase or multiphase surfaces (M7 [carried

forward, with modifications, from year 1] YII.8)Study the effects of wetting and adhesional behavior of metals, low-k dielectrics and other phases on

their polishing behavior, using isolated materials and for standard feature set from the cooperative photomask activity. Explore modification of the wetting and adhesional behavior through judicious selection of pad material, and optimized use of surfactants and other solvents.

• Further develop basic understanding of agglomeration/dispersion effects (M23 YII.9)Relate colloidal chemistry to surface charge and particle size distribution changes.• Develop SMART prototyping methodology (M24 YII.10)Determine best manufacturing processes for prototyping pads for use in validation testing based on

common photomask design.• Integrate SMART pad design criteria into comprehensive model (M25 YII.11)Develop capability in integrated model for determining process-based or device design-based criteria for

SMART-pad and other commercial surface and property specifications, specially for assessing pattern sensitivity.

• Basic material removal model development (Milestone Added, YII.12)Continue development of process model with attention to low down force applications/non-Prestonian

material removal as well as subsurface damage effects.

FLCC

Bulk Cu CMP Barrier polishing W CMP Oxide CMP Poly-Si CMP

Physical models of material removal mechanism in abrasive scale

Chemical reactions

Bulk Cu slurry Barrier slurry W slurry Oxide slurry Poly-Si slurry

Mechanical material removal mechanism in abrasive scale

Abrasive type, size and concentration

[oxidizer], [complexing agent], [corrosion inhibitor],

pH …

Pad asperity density/shape

Pad mechanical propertiesin abrasive scale

Pad properties in die scale

Slurry supply/ flow patternin wafer scale

Wafer scale pressure NU Models of WIWNU

Models ofWIDNU

Topography

Wafer scale velocity profile

Wafer bending with zone pressures

Better control of WIWNU

Reducing ‘Fang’

Small dishing & erosion

Ultra low-k integration

Smaller WIDNU

Reducing slurry usageUniform pad performance

thru it’s lifetimeLonger pad life time

Reducing scratch defects

Better planarization efficiency

E-CMPPad groove

Pad design

Fabrication

Test

Fabrication technique

Slurry supply/ flow pattern in die scale

Cu CMP

modeldesign goalPad development

PatternMIT model

Dornfeld modelDoyle

An overview of CMP research in FLCC

Talbot

9/15/05FLCC - CMP

8

FLCC

Today’s Presentation- see the posters for details -

SMART Pad • Model-based pad design• Initial experimental results

Corrosion studies/wettability• galvanic corrosion of Ta-Cu couple• Wetting studies on multi-phase surfaces

Comprehensive model• Linkage with model beyond end-point (dishing/erosion)• Expansion to copper CMP model• Pad development for low down force CMP / E-CMP• Study on non-uniform break thru in a die scale in copper Ecmp

9/15/05FLCC - CMP

9

FLCC

SMART Pad - Pad Characterization (1)

• Ra = 12.5µm

• Rz = 96.7µm

• Pore diameter : 30~50 µm

• Peak to Peak : 200~300µm

100µm

45µm

-45µm 100µm 300µm 500µm

(SEM, x150)

200~300µm

(White light Interferometer, x200)

9/15/05FLCC - CMP

10

FLCC

Pad Characterization (2)

Asperity: Real contact area10~50 µmPores

40~60µm

Simplified Pad Model

Peak to Peak200~300 µm

1. Reaction Region (10~15 µm)

2. Transition Region

3. Reservoir Region

Reaction region

Reservoir region

Transition region

9/15/05FLCC - CMP

11

FLCC

Defects from Conventional Pads

wafer

Pressure

Position

Pad asperity

Nominal pressure

Avg. contact pressure

Cu-CMP defects

wafer

ErosionDishingFang

wafer

50 µm

Large asperity

wafer

ILD

Rounding

10 µmSmall asperity

wafer

ILD

Over polishing

ILD-CMP defects Stress concentration

9/15/05FLCC - CMP

12

FLCC

Design Rules for a New Pad

• Compatible features to abrasive • Constant re-generation of nano

scale surface roughness

• Constant contact area

(width:10-50um)• The ratio of real contact area

(10-15%)

• Conditioning-less CMP

• High slurry efficiency

• Stacked layer

(Hard/soft)

• Slurry channel

Nano scaleMicro scaleMacro scale

Design rules for a pad

Soft Layer(i.e. low stiffness)

Hard Layer(i.e. high stiffness)

Channel Nano scale features

50-70µm

50-200µm

9/15/05FLCC - CMP

13

FLCC

Fabrication Process

1. Master 2. PDMS Casting 3. PDMS Mold 4. Hard LayerCasting

5. Soft LayerCasting

6. Demolding

New pad

9/15/05FLCC - CMP

14

FLCC

Simulation of Slurry FlowType B – With slurry guidanceType A – Without slurry guidance

• Area : 4.294^-10 m2

• Flow rate : 3.24^-10 kg/sec• Area : 4.3^-10 m2

• Flow rate : 3.93^-11 kg/sec8 times more flow rate

On contact area

9/15/05FLCC - CMP

15

FLCC

Results

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1000 1100 1200 1300 1400 1500

New In 10min In 20min-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1000 1100 1200 1300 1400 1500

New in 10mins in 40mins

Rel

ativ

e st

ep h

eigh

t (µm

)

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1000 1100 1200 1300 1400 1500

New in 3mins in 7mins in 10mins

wafer

SiO2

0.77µm

1.7µm

IC1000/SUBA400 (overpolishing:2500Å)

Type A (squares) (Not planrized) Type B (V shape) (overpolishing:800Å)

ILD pattern

9/15/05FLCC - CMP

16

FLCC

Corrosion - the Problems

Copper tends to corrode in solutions open to air, barrier materials such as Ta have similar corrosion problems

electrolyte

2H+ 2H2OCu2+

2e-

Adsorbed O2

Cu = Cu2+ +2e-(acidic)

O2 +4H+ + 4e- =2H2O

i corr, CuEcorr, Cu

log I

(-) p

oten

tial

(+)

Ecu/Cu2+

EO2/H2O

i0O2/H2O

i0Cu/Cu2+

Polarization curves for corrosion reactions, describing the change in reaction rate as each half cell reaction is polarized away from equilibrium state; both the anodic dissolution of metal and cathodic reduction of an oxidant are polarized to a common potential, the corrosion potential, Ecorr

Cu corrosion in areated solution. The nature of copper species depends on the solution chemistry. Continuous corrosion requires electrical and electrolytic conduction paths and the presence of a potential difference

9/15/05FLCC - CMP

17

FLCC

log I

(-) p

oten

tial

(+)

Ecorr,

N

Ecorr,

Micorr, M

icorr, N

i’ corr, Ni’ corr, N

log I

(-) p

oten

tial

(+)

Ecorr,

N

Ecorr,

Micorr, M

icorr, N

i’ corr, Ni corr, N

i’ corr, Ni’ corr, M

Egalvanic

log I

(-) p

oten

tial

(+)

Ecorr, N

Ecorr, MM= Mm+ + me-

N= Nn+ + ne-

Cathodic reaction on M

Cathodic reaction on N

icorr, M

icorr, N

M

N

M N

Corrosion of more noble metal, M, is suppressed due to cathodic polarization

Corrosion of more active metal, N, accelerates due to anodic polarization

A new common potential, called mixed potential, is reached at the steady state of corrosion

Galvanic effect: accelerates corrosion of the more active metal, N, because of anodic polarization and suppresses corrosion of the more noble metal, M, because of cathodic polarization

Solution chemistry might change the polarization behavior of each material; thereby affecting the preferential galvanic corrosion of the corroding material

Increasing the area of the more noble material accelerates corrosion of N

Galvanic effects lead to more severe corrosion of the more active metal. Damage sensitive to solution chemistry and area ratio effects

9/15/05FLCC - CMP

18

FLCC

The Problems

ANODE CATHODE

Bulk electrolyte

0.1 um1 um

5 um

0.3

0.2

0.1

00 0.5 1.0

Radius, r (cm)Lo

cal c

urre

nt

dens

ity

0.1 um-thick electrolyte

1 um

5 um

Bulk electrolyte

ANODE CATHODE

Radius, r (cm)0 0.5 1.0

Pote

ntial

, E (V

)

1.0

0.5

0.0

Anode Cathode

Anode Cathode

Galvanic corrosion in bulk solution

Galvanic corrosion in thin-layer of solution

expected structures

The characteristic pronounced corrosion at the phase boundary in galvanic corrosion is due to the steep potential gradient and high current density at the boundary between two phases

θ?θ

Hydrophilic surface

θ

θ: contact angle for characterizing the wetting of a surface by a specific solution; zero or small contact angles = good wetting

Simulated potential and current distribution, E. McCafferty, JES 124 (12), 1977

The accessibility and distribution of electrolyte (ions and water) on surfaces may affect galvanic corrosion. The wettability of materials may affect the distribution of the solution on surfaces

Hydrophobic surface

solution

9/15/05FLCC - CMP

19

FLCC

Experimental Study

Cu TaPt

Cu wires for electrode connection

Sample and sample carrier

Slurry film Polishing padRotating platen

Slurry deliverypressurePotentiostat

samp

le2

Ref.

electr

ode

samp

le1

Experimental setup and samples

Galvanic currents and potentials will be measured with or without polishing, to investigate the effects of surface modification by chemicals and mechanical force on the polarity and galvanic current; area ratio of Cu:Ta is to be varied using different sample assemblies with different relative areas

sample assembly capable electrochemical control and measurement

Polisher

9/15/05FLCC - CMP

20

FLCC

Results of electrochemical measurements

0 200 400 600 800 1000-2.0x10-6

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

1.2x10-5

1.4x10-5

1.6x10-5

H2O2

Gal

vani

c C

urre

nt,A

Time,s

Ta:Cu=2.6:6.0 (cm2) Ta:Cu=3.9:6.0 (cm2) Ta:Cu=3.9:4.5 (cm2)

in pH=9 carbonate buffer, 0.01M glycine and 0.3%H2O2

0 200 400 600 800 1000-0.300

-0.250

-0.200

-0.150

-0.100

-0.050

0.000

0.050

0.100

0.150

0.200

Pote

ntia

l, V

vs S

CE

Time, s

Ta:Cu=3.9:6.0 (cm2) Ta:Cu=2.6:6.0 (cm2) Ta:Cu=3.9:4.5 (cm2)

Area ratio effect, without polishing

DI H2OpH9

buffer glycine

H2O2

9/15/05FLCC - CMP

21

FLCC

Results of electrochemical measurement

Effects of [H2O2], pH=9, without polishing

0 300 600 900 1200

0.0

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

Gal

vani

c C

urre

nt, A

Time,s

0.3% 0.1% 0.6% 1.0%

0 300 600 900 1200-0.300

-0.200

-0.100

0.000

0.100

0.200H2O2, 30 wt%

stock glycine sltn

carbonate buffer

Mix

ed P

oten

tial,

V vs

SC

E

0.3% 0.1% 0.6% 1.0%

in pH=9 carbonate buffer, 0.01M glycine and varying [H2O2]

9/15/05FLCC - CMP

22

FLCC

Effects of polishing on galvanic currents and potentials

Masako Kodera, etc., J. Electrochem. Soc., 152 (6) G506-G510 (2005)

9/15/05FLCC - CMP

23

FLCC

Contact angle measurement

Sessile drop method: Angle between the baseline of the drop and the tangent at the drop boundary is measured.

Kruss Contact Angle Measuring System (Goniometry approach)The basic elements of a goniometer include a light source, sample stage, lens and image capture. Based on the image of the liquid drop, contact angle can be assessed directly by measuring the angle formed between the solid and the tangent to the drop surface

The contact angles as function of solution chemistry on Ta/TaN, Cu and dielectric material (TEOS or SiO2) will be explored

9/15/05FLCC - CMP

24

FLCC

Comprehensive CMP Model

- Statistical representation of CMP pad surface

- Interaction between wafer topography and pad asperities

- Hertzian contact model for elastic pad deformation

- Linkage with conventional chip-scale pattern density model

RHDyx

dPHDyxEVCyxMRR

wp

yxz

zppad

2/34/1

),( 4/72/3*

),(

)(),,(),(

κρ

δδδε∫ ×⋅=

pattern density effect

pad surface conditionprocess parameters

thin film propertiespad material properties

abrasive size

chemical reactions

wafer topography

9/15/05FLCC - CMP

25

FLCC

Effect of Pad Asperity Height Distribution

smooth padrough pad

σ = 3µm, α = 3µm σ = 6µm, α = 3µm

Time (x10sec)

Oxi

de th

ickn

ess

(µm

)

Time (x10sec)

Oxi

de th

ickn

ess

(µm

)Higher planarization efficiency, Lower removal rate

Lower planarization efficiency, Higher removal rate

9/15/05FLCC - CMP

26

FLCC

Effect of Nominal Down Pressure

low pressure high pressure

9/15/05FLCC - CMP

27

FLCC

Oxide Test Pattern Wafer for Model Calibration and Verification

100%50%

50%

20%

10%

10% 20%

0%

0%

0%

8 mm

8 mm

Line width : 20 µm

4 inch wafer

(11 dies across the diameter)

CVD nitride

Pattern nitride (mask #1)

CVD OxideSiO2

Etch oxide (mask #2)

SiSi3N4

20 nm

2.5 µm

0.5 µm

9/15/05FLCC - CMP

28

FLCC

Model vs. Experiment (Oxide CMP)

highest point in the die (measured)

lowest point in the die (measured)

lowest point in the die (model)

highest point in the die (model)

t=0min t=2min t=4min t=8min

Experiment :

Model :

9/15/05FLCC - CMP

29

FLCC

Comparison of Final Oxide Thickness Variation Over the Test Die

experimentmodel prediction

RMS error = 30.8 nm(100 measurement sites)