impact chang tutorial 2008

IMPACT • Plasma• 1

IMPACT Seminar

Title: Simulation of Feature Profile Evolution during Plasma-Assisted Processes Faculty: Jane P. Chang Student: John Hoang Department: Chemical and Biomolecular Engineering University: UCLA


Outline

Motivation

An update on the research progress – STI I: Hoang, J. et al. JVSTB 26(6) 2008 (accepted)– STI II: Hsu, C.-C. et al. JVSTB 26(6) 2008 (accepted)

A follow-up on the round table discussion on LER– Origin and current status of LER – Modeling challenges

Potential model systems to study – LER– TSVE– Gap-fill and Cu deposition


Motivation

Stringent control of feature geometry and dimensions futureLower process and development cost by predicting

Henrik Schumacher, CMOS Chip Structure. www.wikimedia.org

Line Edge Roughness (LER)/ Line Width Roughness (LWR)

Chris A. Mack, Field Guide to Optical Lithography, SPIE Field Guide Series, Vol. FG06, Bellingham,

WA, 2006

Back End of Line (BEOL)Atomic Force Microscopy (AFM)

Goldfarb et. al. JVST B 22(2) 2002

Front end of Line (FEOL)

Lam Research. “Etch Technology” www.lamrc.com

Gate Stack Etch

† Adapted from ITRS 2003 Thermal Films Supplemental

Shallow Trench Isolation (STI)

D1

D2

D3

total Si depth

tx1 (nitride)

tx2 (top Si)

tx3 (bot Si)

nitride SWA

top Si SWA

bottom Si side wall angle (SWA)

D4

SixNy

Si

n+

n+

SVG

p+

DVG

p+

STI

p-Si n+ well

STI

STI


Shallow Trench Isolation (STI)

† Adapted from ITRS 2003 Thermal Films Supplemental

Critical Parameters (SEM Measurements) D1

D2

D3 total Si depth

tx1 (nitride)

tx2 (top Si)

tx3 (bot Si)

nitride SWA

top Si SWA

bottom Si side wall angle (SWA)

D4

SixNy

Si

CMP planarization

Isolation stack Pattern nitride and strip PR

Trench etch

PRnitrideoxide

Si

Sidewall oxidation and deposit trench oxide

Strip nitride and remove pad oxide

Shallow Trench Isolation Process AMAT DPS IICl2

O2N2

O2N2

Pressure

DC Ratio (Iouter/Iinner)

Ws

Wb

Wafer

pumping ports

Cl2

O2N2

O2N2

Pressure

DC Ratio (Iouter/Iinner)

Ws

Wb

Wafer

pumping ports

STI Etch ParametersP = 25/45 mTWs = 350/500 WWb = 150/250 WDC ratio = 11/30Cl2 = 140/180 sccmN2 = 30/60 sccmO2 = 15/25 sccm

No Oxygen in Feed Gas

SEM Image – Top View


Kinetic Model for Si Etch in Cl2 Plasma

0 (1 )( ) ( )

Cl Cl Osg sCl Clζ ζ− −+ ∗ →

( )( ) ( )

cg sCl Clφ+ +∗ →

( )( ) ( ) 4( )4 4c Cls s gSi Cl SiClφ β +

+ → + ∗0

2

2( ) ( ) ( )3 2SiCl

sg s sSiCl Si Cl→+ +∗

0 (1 )( ) ( )

O OClsg sO Oζ ζ− −

+ →∗

( ) ( )SPSiY

s gSi Si→ +∗0

( ) ( )Sis

g sSi Si→+∗

2( ) ( ) ( )Clr

g s gCl Cl Cl→+ +∗

Surface Reaction Surface Reaction (cont)

+

+

Ion incident angle φ (degree from normal)

Poly

Oxide

0

1

2

3

4

0 30 60 900

0.1

0.2Ion Angular Dependence

0

0.4

0.8

1.2

0 10 20 30

Cl/Cl+ = 120 with SiCl2

Cl+ alone with SiCl2

S iC l +

S iC lC l

2+

Flux Ratio

S iC l SiC l C le4 2 2

−

→ +

Etching Yield

0

0.4

0.8

1.2

0 10 20 30

Cl/Cl+ = 120 with SiCl2

Cl+ alone with SiCl2

S iC l +

S iC lC l

2+

Flux Ratio

S iC l SiC l C le4 2 2

−

→ +

Etching Yield

SiCl2 flux

+ArCl

Flux Ratio

Yield

Poly

Oxide

0

1

2

3

4

0 50 100 150 2000.0

0.1

0.2

0.3

0.4Selectivity+

+

0

1

2

3

4

0 100 200 300 400

S iC l +

Etching Yield

75eV Cl+/Cl

55eV Cl+/Cl

35eV Cl+/Cl

ClCl +

Flux Ratio

ClCl +

Flux Ratio

80 eV(Lam TCP)

n/I+ ratio and Eion +

+

Cho, H.S. et al. Mat. Sci. in Semi. Process. 8 (2005) 239Ulal, S.J et al. J. Vac. Sci. Technol. A 20(2) 2002

Rigorous incorporation of physics and chemistry for deposition/etching


Monte Carlo Feature Scale Model

Vegh et al., IMPACT Spring Workshop 2008

Kinetics validation (MD)

Species flux ratio (Reactor Scale)

Mask (SixNy)

Substrate(Si)

Source plane

Vacuum

+

85º GrazingIon scattering angle and energy (MD)

Abrams et al., JVST A 16(5), 3006 (1998)

IAD (analytical)

Mizutani et al. JVST B 19(4) 2001

Cou

nts

Neutral Angle (Degrees)

Cou

nts

Ion angle (degrees)

Wu et al. JAP 101 (2007)

IED (PIC, analytical)

Cou

nts

Ion Energy (eV)

Semi-empirical with plasma parameter validation using simulation or experiment

+

n

+

+

+

n

+


Surface Representation

Accurate surface representation is important to include the correct fluxes and angular dependent yields

0 - 82

82 - 218

218 - 348

0 - 82

82 - 218

218 - 348

Silicon

Mask

Direct representation

0 50 100 150 200 250 300

Ion Flux

Position Along Interface

Rel

ativ

e Fl

uxN

orm

al

Neutral Flux

Rel

ativ

e Fl

ux

SurfaceNormal

Cell Centered RepresentationSegment Representation

Mahorowala, A. P. et al. JVST B 20 (2002)Hoang, J. et al. JVSTB 26(6) 2008


Reactor Scale Model

0.00 0.05 0.10 0.151.0x1020

1.5x1020

2.0x1020

2.5x1020

3.0x1020

DC=5

DC=1

Ion

Flux

(m-2

)

Radial Position (m)

DC=0.1

Axisymmetric flowRecombination Cl on walls constant while kinetically dependent on waferN2 feed gas assumed to be inert and act like Ar

Cho, H. S. et al. Mater. Sci. Semicond. Process 8, 239 (2005)

Boundary Condition for Cl recombination (γCl)γCl on Si : etch based (SiCl2 ↑)γCl on SS : constant†

Silicon

+

Stainless steel

ClC

l Cl

+

Cl2/O2/ArCl2/ArSample profiles

0.00 0.05 0.10 0.150.20.30.40.50.60.70.80.91.01.11.2

Cl

Cl+

Nor

mal

ized

Den

sity

Radial Position (m)

SiCl2

0.00 0.05 0.10 0.150.20.30.40.50.60.70.80.91.01.11.2

Cl

SiCl2

Nor

mal

ized

Den

sity

Radial Position (m)

Cl+

† Adapted from Shen, M. et. al. SEMICON China 2004

Qualitative Agreement

0.00 0.05 0.10 0.152.0x1017

4.0x1017

6.0x1017

8.0x1017

1.0x1018

1.2x1018

DC = infinity

DC = 1

DC = 0

Plas

ma

Den

sity

(#/m

3 )

Radial Distance (m)


Linking Hybrid Model to SEM

Steady state: link feature scale model to experimentIon flux limited: link feature scale model to reactor scale modelWell planned experiments (one parameter change at a time)

1. Steady state 2. Ion flux limited

Assumptions

Particle count correlated with

measured etched area

0 50 100 150 200 250 300 350 4000123456789

10

235 eV195 eV155 eV

115 eV

75 eV55 eV

Si E

tch

Yie

ld (S

i/Cl+ )

Neutral to Ion Ratio (Cl/Cl+)

35 eV

Allows use of species densities from

qualitatively verified reactor model

Controlled Experiments


Hybrid Model Results for STIE

0 1 14 15

0.9

1.0

Nor

mal

ized

Si E

tch

Dep

th

(w.r.

t. ce

nter

)

Radial Position (cm)10 15 20 25 30

-2

0

2

4

6

SWA

Var

iatio

n (%

)

DC Ratio0.0 0.5 14.0 14.5

80

82

84

86

88

Side

wal

l Ang

le (D

eg.)

Radial Position (cm)

0.00 0.05 0.10 0.15 0.20

0.0

0.2

0.4

0.6

0.8

1.0 Cl+

SiCl2

O

Nor

mal

ized

Den

sitie

s

Radial Position (m)

0.00 0.05 0.10 0.150.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Cl S

urfa

ce C

over

age

Radial Position (m)

0.00 0.05 0.10 0.15 0.200

1x1019

2x1019

3x1019

4x1019

5x1019

6x1019

Cl D

ensi

ty (m

-3)

Radial Position (m)

Wafer

Center 10 mm 5 mmEdge


• Hybrid model captures center-edge variation in etch depth and SWA


Hybrid Model Results for STIE

• SWA and depth variation captured using normalized species densities

0.00 0.05 0.10 0.150.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

SiCl2

Den

sity

x 1

019 (m

-3)

Radial Position (m)

Cl

0.00 0.05 0.10 0.150.0

2.0

4.0

6.0

8.0

Den

sity

x 1

019 (m

-3)

Radial Position (m)

Cl

SiCl2

0.00 0.05 0.10 0.150.0

2.0

4.0

6.0

8.0

Den

sity

x 1

019 (m

-3)

Radial Position (m)

Cl

SiCl2


0.00 0.05 0.10 0.150.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Cl+

Flux

x 1

019 (m

-2s-1

)Radial Position (m)

0.00 0.05 0.10 0.150.0

2.0

4.0

6.0

8.0

Flux

x 1

019 (m

-2s-1

)

Radial Position (m)

Cl+

0.00 0.05 0.10 0.150.0

2.0

4.0

6.0

8.0

Flux

x 1

019 (m

-2s-1

)

Radial Position (m)

Cl+

Ws = 350 W, Wb = 150 W, P = 25 mT, DC = 11, Ar/O2/Cl2 : 30/15/140




Outline

Motivation

An update on the research progress – STIE Part I: Hoang, J. et al. JVSTB 26(6) 2008 (accepted)– STIE Part II: Hsu, C.-C. et al. JVSTB 26(6) 2008 (accepted)

A follow-up on the round table discussion on LER– Origin and current status of LER – Modeling challenges

Potential model systems to study – LER– TSVE– Gap-fill and Cu deposition


Line Edge Roughness Transfer

Line edge roughness (LER) is a major concern with the use of 193 nm (ArF) resistsLER degrades device performance, yield and reliabilityCountermeasures include litho/etch process optimization and the addition of pattern transfer layer

A. Asenov, S. Kaya, and A. R. Brown, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 5, MAY 2003

• IMPACT round-table discussion in the summer, led by C. Gabriel, stimulated lots of discussions, signifying the importance to understand and model LER


Line Edge Roughness Transfer

Line edge roughness dictated by ion bombardmentAt near grazing incidence angles, ion enhanced etch yield is comparable to the sputter yield

SEM Top View

AFM

Goldfarb et al. JVST B 22(2) 2002

PRARC

SiO2

Si

0 30 60 900

1

2

3

4

0.0

0.1

0.2

Yiel

d

Ion Incident Angle φ (from normal)

Poly

Oxide

Oxide etch in CF4-CHF3 plasma afterARC etch using N2/H2 plasma

Si and SiO2 etching in simulatedCl2 plasma using beam experiments


Overview of mechanismsLitho– Nonhomogeneous resist film—“soft” and “hard” patches– Dimensions (resist thickness and CD) are close to the size of

polymer aggregatesEtch– Factors: Ion bombardment, radicals, photons, electrons, heat,

polymer deposition, but also linewidth and spacewidth seem to matter

– Ions and radicals attack soft areas in resist mask, creating rough surface and edges that transfer down into underlying films

– Worse when etching thick films with strong bonds, like SiO2– Film stress, adhesion, and stress relief may also play a role

– Volume expansion from ion implantation into mask– Wafer heating and cooling in plasma– Polymers deposited on mask and feature sidewalls

C. Gabriel, Spansion, IMPACT presentation, May 2008.


Role of photoresist

Both 248 nm and 193 nm resists use chemical amplification to make up for the relatively low intensity of DUV light produced by the KrF and ArF lasersChemically amplified resists form “spongy” walls—photoacid diffusion and catalytic reaction form coiled polymer chains or polymer aggregates, leading to a roughened sidewall when developedThe developed resist is nonhomogeneous and likelyto be further roughened by the physical and chemical action of plasma etching

H. Namatsu et al., J. Vac. Sci. Technol. B 16, 6, 1998, 3315

C. Gabriel, Spansion, IMPACT presentation, May 2008.


Challenges in Modeling

Physics – Effect of ions – Synergistic effect with photons and electrons

Chemistry – Simple chemistry (halogen) – Complex chemistry (fluorocarbon)

Materials – Reaction kinetics


Ion Bombardment Induced Surface Roughness

Begins with nucleation which affects surface advancement and propagates

Nucleation Surface AdvancementInitial Surface

Smooth SiO2

1. Surface composition

3. Surface density2. Ion flux

SiO

FC

CFx+ ion

Caused by variations in: Caused by:

CFx+ ion

1. Angular dependence of etching

3. Scattering and shadowing2. Local curvature dependence

4. Redeposition of etch products


Ion Angular Dependence: Roughness Striation

Transverse roughness at 45~60° off normal incidenceParallel roughness >75° off normal incidence

Ziberi et al. Physical Review B 72, 235310 (2005)

Bradley et al. JVST A 6(4), 2390(1988)

Isotropic roughness

θ = 0°

Transverse roughness

θ = 45-60°

Parallel roughness

θ >75°

HRTEM/AFM Xe+ ion beam eroded Si surface

5° 45° 75°

Ion Angular Dependence on Roughness Striation

1200 eV ions, 1.34x1019 cm-2 fluence

2000 eV ions, 6.7x1018 cm-2 fluence


Ion Flux Dependence: Deposited Energy

Energy of ion bombardment is dissipated in an elliptical symmetry with high at centerEtching caused by ion striking surface at A is highest at BCurvature and ion incident angle affects deposited energy profile

P. Sigmund. J. Materi. Sci. 8, 1545 (1973)

contour of deposited energy

ion

AB

surface

θ

Slower etch rate forNegative curvature

Higher etch rate for positive curvature

Curvature and angle dependence on etch yield


Density Dependence

Films with larger pores roughen at earlier stages during etchingPore filling with polymeric deposits causes surface imhomogeneity

QMS Neutral Spectra

QMS Ion Spectra

C2F6/Ar plasma etch, 5 sccm, beam source pressure ~10 mT, rf source power 250 W

Solid organosilicate glass (OSG)

Porous methylsilsesquioxane (MSQ) low-k film

unetched

rms 0.9 nm

115 nm etch 300 nm etch

rms 3.2 nm rms 9.0 nm

unetched

rms 1.2 nm

97 nm etch 238 nm etch

rms 0.9 nm rms 1.1 nm

Yin, Y, Rasgon, S. and Sawin, H. H. JVST B 24(5) 2006


Cell Composition Averaging of Mixing Layer

Retain concept of translated mixed layer in 3D profile simulator using cell composition averaging and densification

Oxide Etching with Fluorocarbon chemistry( ) ( )

( ) ( )

( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( )

( ) ( )

( ) ( )

( ) ( )

( ) ( ) 2( )

( ) 4( ) 4( )

( ) ( )

2

4

g s

g s

g s

x y g s s

x y g s s

x y g s s

s g

s g

s g

s g

s s g

s s g

s s

F F

F F

F F

C F xC yF

C F xC yF

C F xC yF

Si Si

O O

C C

F F

Si F SiF

Si F SiF

C O C

+

+

→

→

→

→ +

→ +

→ +

→

→

→

→

+ →

+ →

+ → ( )

( ) ( ) 2( )

( ) ( ) 4( )

2

4

g

s s g

s s g

O

C F CF

C F CF

+ →

+ →

212

2 3Si

E SiF FSi C

xr xx x

β=+

F neutral adsorption on Si

F neutral adsorption on CF ion adsorptionCxFy neutral adsorption on oxygenCxFy neutral adsorption on carbon

CxFy ion adsorptionPhysical sputtering of SiPhysical sputtering of OPhysical sputtering of CPhysical sputtering of F

1 _ _ _ _A F on Si F Si for Sir s R x=

2 _ _ _ _A F on C F C for Fr s R x=

3A F Fr s R+ +=

5 _ _ _ _x y x yA C F on O C F O for Cr s R x=

_ _6 x y on C x yA C F C F Cr s R x=

7 7x y

A C Fr s R +=

1S Si Sir Y x=

2S O Or Y x=

3S C Cr Y x=

4S F Fr Y x=

2

2

22

2 3Si

E SiF FSi C

xr xx x

β

= +

33

2 3C

E CO OSi C

xr xx x

β=+

4C

E CF FSi C

xr xx x

β=+

25

CE CF F F

Si C

xr x Rx x

α=+

Ion-induced SiF2 production rxn

Ion-induced SiF4 production rxn

Ion-induced CO production rxn

Ion-induced CF2 production rxn

Fluorocarbon film removal by incoming F fluxTranslated Mixed Layer

SiO2

F Si

V C F V

C F O Si F

O V F V V

Si C F V C

Si O V

F C Si F

V F F O F

F Si F C V

F C V O C

C F F

F C C Si F

F C V O C

C F O Si F

C F O Si F

Si V Si O V

F Si F C V

F C V O C

F C V O C

F Si F C V

C F O Si F

O V F V V

V F F O F

F C V O C

Si C F V C

V F F O F

Si V Si O V

F C C Si F

F Si F C V

SiO2 SiO2

V : Vacancy

CFx+ or F+


Competition between etch and deposition

Neutral-to-ion ratio, rf, pressure and chemistry have major effects on yields

Kwon, O. and Sawin, H. H. JVST A 24(5) 2006

C2F6

C4F8(+Ar)

RF

P RF

P


Fast algorithm to determine yield

Translating mixed layer comparison with experiments and MCKwon, O. and Sawin, H. H. JVST A 24(5) 2006

SiO2 etch with F chemistry

Si etch with Cl chemistry

1 112

( ) ( )

Cl Si Clr s R x x

g sCl Cl = − →

2 2

( ) ( )Cl

r s Rg sCl Cl+=+ →

3( ) ( ) 2( )2 Clr xs s gSi Cl SiClβ=+ →

4( ) ( )

Si Sir Y xs gSi Si=→

5( ) ( )

Cl Clr Y xs gCl Cl=→

1 121

2 2( ) ( )

SiF Si F

Si O

xr s R x x

x xg sF F

= − + →

2 2 2( ) ( )

OF O F

Si O

xr s R x x

x xg sF F

= − + →

4 12

2( ) ( ) 2( )

SiF

Si O

xr x

x xs s gSi F SiF

β=++ →

5 2 2( ) ( ) ( )

OF

Si O

xr x

x xs s gO F OF

β=++ →

6( ) ( )

Si Sir Y xs gSi Si=→

7( ) ( )

O Or Y xs gO O=→

8( ) ( )

F Fr Y xs gF F=→


Determination of kinetic parameters

Atomic fluorine flux is a major factor that determines the etching behavior. With a chemistry having a small amount of atomic fluorine such as the C4F8 chemistry, etching yield shows stronger dependence on the composition change in the gas flux

Kwon, O. and Sawin, H. H. JVST A 24(5) 2006


Challenges in Modeling

Understand the effect of ions – Effect of relevant yet complex chemistry

Need to deconvolute the effect of dominant ions/radicals – Low ion energy reduces the LWR (R. Gottscho, 2008)– Ion energy can be controlled by frequency mix, pressure, bias

Need to determine the energy deposition on a shaped surfaceUnderstand the effect of photons/electrons – Wavelength and fluence– VUV flux and its ratio to ions

Need to quantify the reaction kinetics Expand the simulator into 3-D


Beam system 193 nm PR roughening S

imul

tane

ous

1x10

18io

ns·c

m-2

&2.

5x10

17ph

oton

s·cm

-21

1.52 2.17 3.5320ºC 40ºC 65ºC

1.32

10 nm

1.23

10 nm

1.02

10 nm

10 nm 10 nm 25 nm

1x10

18io

ns·c

m-2

ions

onl

y

200 nm

Elevated surface roughness on the order of plasmas exposures is observed during simultaneous ion bombardment, VUV radiation, and heatingThe surface roughness formation is photon/ion ratio dependent

VUV breaks C=O and C-O-C bonds (as verified by FTIR analysis)

150 eV Ar147 nm (Xe)

RMS of 0.3 nm for as-prepared and photon-radiated samples

D. Graves, UC Berkeley, IMPACT presentation, October 2008.


Translating Mixed-Layer Representation

• Robust kinetic model able to handle complex chemistries

Kwon, O. et al. JVST A 24(5) 2006

Cl2 Plasma Etch of Si

50100

150200

250

0.0

0.5

1.0

1.5

2.0

2.5

4060

80100

120140

Etc

h Yi

eld

n/I+ R

atioIon Energy (eV)

020

4060

80

0.0

0.5

1.0

1.5

10075

5025

0

Etc

h Yi

eld

n/I+ R

atio (

Total

)

Ion Incident Angle

C4F8 Plasma Etch of SiO2

0.150.20

0.250.30

0.35

0

1

2

3

0.12

0.140.16

0.18

Etch

Yie

ld

s Carbons

Flourine

400800

12001600

2000

-0.50.00.51.01.5

2.0

2.5

3.0

12090

6030

0

Etch

Yie

ld

n/I+ Ratio (T

otal)Ion Energy (eV)

020

4060

80

-2-10123456

040

80120

Etch

Yie

ld

n/I+ Ratio

Ion Incident Angle


Surface Algorithm

-Vacuum Cell

-Solid Reference Cell

-Solid Interface Cell

-Solid Inside cell

Steps:

1. Create “Quick Normal”

2. Locate interface cells around reference cell

3. Connect interface cells with “snaking”algorithm


Particle Intersection and Normal

Steps:

4. With particle trajectory and starting point, find intersection with a plane

5. Calculate surface normal

Reference Cell


Feature Scale Model Extension into 3D

• 3D representation essential in capturing complex surface advancement (such as LER transfer)

Neutral Flux Plots


Potential Directions

• STIE with new chemistry (Spansion) • LER (AMD and Spansion) • TSV etch (Synopsis) • Gap-fill and Cu deposition (Novellus) • Scatterometry (Timbre)


Acknowledgement

UC IMPACTHerb Sawin, MIT (Equipment donation and software)

impact chang tutorial 2008

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