11/14/2005
1
FLCCFLCC – Plasma
Feature Profile Evolution during Shallow Trench Isolation (STI) Etch
in Chlorine-based Plasmas
FLCC Presentation
November 14, 2005Jane P. Chang and John Hoang
Department of Chemical and Biomolecular Engineering University of California, Los Angeles
11/14/2005
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FLCCFLCC – Plasma
Motivation• Feature Scale Modeling
– Combining accurate descriptions of plasma fluxes to quantitatively predict the feature profile evolution during etching/depositionprocesses
– Enabling process development by shorting the experimental time and cost
– Feature scale model can be coupled to tool scale (e.g. Prof. Graves, UCB)
– Feature scale model can be coupled with PIC/MC model (Prof. Lieberman, UCB)
• Shallow Trench Isolation (STI)– An enabling technology over local oxidation of Si (LOCOS) since the
0.18 µm node– A lower temperature process avoiding annealing used for thermal
oxidation – A promising technology for even smaller dimensions with properly
developed lithography, etch, and gap-fill technology
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FLCCFLCC – Plasma
Critical Issues in Feature Scale Models
• Accurate reaction kinetics model
— Systematic beam measurement
— Carefully designed design of experiments
• Robust incorporation of competing surface mechanisms
— Etching vs. deposition
— Specular vs. non-specular scattering
— Elemental balance on etching surfaces
• Realistic etching profiles for validation
— SEM
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FLCC – Plasma
e_
Cl+
Cl
Cl2
SiCl2
+
Competing Mechanisms during Etching Processesbulk plasma
SiO2
E
e-e- e-
e-
e-
e-
}+
SiCl4
+ +
+++ + +
↑→++
44 SiClClSi Cl
−− ++→+ eClSiCleSiCl 224
−− +→+ eCleCl 22−+− +→+ eCleCl 2
• Remove thin film directionally• Profile evolution affected by etching and deposition
sheath
mask
poly-Si
oxide
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FLCC – Plasma
Multiple Beam Apparatus
• Independent variation of ionic and radical fluxes• Controllable flux levels within an order of magnitude of what
typically used in high density plasma processes• Etching Yield = f ( Ion, Etchant, Inhibitor, Eion , φion .... )
Ions (30-100 eV Ar+, Cl+)TC
Mass SpectrometerPoly-Si
or oxidethin film Etchants (Cl, Cl2)
Inhibitors (C, SiCl2)
He-Ne LaserReflectanceTime
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FLCC – Plasma
Energy Dependence ( Cl+ and Cl)
• Cl+ ion-enhanced etching yield with Cl
• Dotted lines are model fits, as detailed later
( )ion thE E∝ −
S iC l +
Etching Yield
75eV Cl+ /Cl
55eV Cl+ /Cl
35eV Cl+/ Cl
ClCl +
Flux Ratio
0
1
2
3
4
0 100 200 300 400
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FLCC – Plasma
Etching Yield by Cl vs. Cl2( 55 eV Cl+ )
• Similar etching yield at sufficient high Cl+ flux
• Much higher etching yield with atomic chlorine at low Cl+ flux ( higher flux ratio ) 0
1
2
3
0 100 200 300 400
55eV Cl+/ Cl
55eV Cl+/ Cl2
Cl ClCl
2+
Flux Ratio
SiCl +
Etching Yield
or
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FLCC – Plasma
Angular Dependence Result ( 35eV Cl+ and Cl fluxes )
0
0.3
0.6
0.9
1.2
1.5
0 100 200 300 400
S iC l +
Etching Yield
C lC l +
Normal
70o off-normal
φ
Cl+
• Lower etching yield at high off-normal ion incident angle
Flux Ratio
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FLCC – Plasma
Angular Dependence Result ( 35eV Cl+/Cl, Flux Ratio = 200)
• Maximum yield at normal ion incident angle
Yield (φ) = c (φ) * Yield (φ = 0o)
φ
Cl+
0 30 60 900.0
0.4
0.8
1.2
SiCl +
Etching Yield
Ion incident angle φ (degree from normal)
polynomial fit
physical sputtering
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FLCC – Plasma
Surface Kinetic Model
s)()( sg ClCl →∗+Chlorination :
Sorption of Chlorine ion:
Ion-enhanced etching: ∗+ →+ 44 )(4)()( gss SiClClSi
c(φ)
c(φ)β Cl+
)()( sg ClCl →∗++
Rsc
cRscY
ccRscRs
cYIcER
Cl
Cl
ClCl
14)(1
)(4
)(11
)(4)()(
)()(
⋅+⋅
≈
+⋅+
⋅=
⋅⋅++⋅+⋅
=
⋅⋅=⋅⋅⋅= +
βφ
φβφ
βφφφθ
θβφθβφ
ionE
0
1
2
3
4
5
0 3 6 9 12 150
0.3
0.6
0.9
1.2
1.5
sβ
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FLCC – Plasma
0
1
2
3
4
0 100 200 300 400
Energy Dependence and Effect of Redeposition( Cl+ and Cl)
• Good agreement with the etching yield measurement in Lam TCP• Significant reduction in etching yield due to redeposition
S iC l +
Etching Yield
75eV Cl+/Cl
55eV Cl+/Cl
35eV Cl+/Cl
ClCl +
Flux Ratio
80 eV(Lam TCP)
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
SiCl SiCl Cle4 2 2
−
→ +
Etching Yield
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FLCC – Plasma
φPoly-Si vs. Oxide(100 eV Ar+ and Cl)
PRpolyoxide
Ion incident angle φ (degree from normal)
• Etching selectivity ~ 30• Distinct angular dependencies between etching of poly and oxide
Selectivity Angular Dependency
Poly
Oxide
+ArCl
Flux Ratio
Poly
Oxide
0
1
2
3
4
0 50 100 150 2000.0
0.1
0.2
0.3
0.4
0
1
2
3
4
0 30 60 900
0.1
0.2
Yield
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FLCC – Plasma
Ar+/Cl/Si Ar+/Cl/SiO2
Si
SiSiSi
SiSiSiSiSiSi
Cl
Cl
Ar+
ClO
O
O
O
O
O
O
O OOO
O
• Cl incorporation caused by knock-on and spontaneous reaction with Si
• Etching scales with surface energy deposition
Si Si Si Si
Si Si Si Si Si Si Si SiSi Si Si Si Si Si Si
/ \ / \ / \ / \ / \
/ \ / \ / \ / \ / \ / \ / \ / \ /
Cl Cl Cl\ / \ /
Cl
Cl Cl
SiCl4
Cl Cl
Ar+Cl Cl
• Cl more confined to the top surface due to the lack of spontaneous reaction with SiO2
• Etching scales with sputtering of Si and O
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FLCC – Plasma
Etching of Patterned Polysilicon Wafers
• Line width: 0.5 µm and 0.35 µm• Photo-resist: Apex-E ( Deep-UV resist )• Orientation w.r.t. Cl+ and Cl beams:
“ Un-shadowed ” “ Shadowed ”
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FLCC – Plasma
Feature Profile etched with 35eV Cl+/Cl( Cl/Cl+ flux ratio = 200 )
PR
poly-Si
SiO2
PR
poly-Si
SiO2
“ Un-shadowed ” “ Shadowed ”
Cl+ Cl+Cl Cl
• Unique etching structure suitable for profile modeling confirmation
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FLCC – Plasma
Monte Carlo Simulation of Surface Evolution
PR
poly-Si
SiO2
Source plane
ClCl+
• Simulation domain: — 20-50 A grid size (~ mixing length)
• Transport of species in grid-length steps
• Surface Reaction:— Reaction probability based on model — Elemental balance— Multiple interaction possible
• Surface advancement— Grid cells removal and/or addition
• Rigorous incorporation of all the physics and chemistry
• Computationally robust and straight-forward
o
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FLCC – Plasma
Monte Carlo Simulation of Surface Interactions
Surface Reactions:
∗+→+
→∗+
→∗+
+
+
44 )(4)()(
)()(
)()(
gCl
ss
sg
sg
SiClClSi
ClCl
ClClCl+
Si
ClCl
SiCl4
Scattering: Ions: specular scatteringNeutrals: diffusive scattering
• Surface composition determined by elemental species balance• Surface reactions occur with measured probabilities
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FLCC – Plasma
Etching of Patterned Polysilicon Wafers( 35eV Cl+ and Cl, Flux Ratio = 200)
• 0.5 recombination probability of Cl on mask, shadowed orientation• Unique etching structure modeled by simulator
1.0 0.0 0.5
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FLCC – Plasma
Effect of Ion Scattering
oxide
poly-Si
oxide
• Non-directional ion distribution (10o FWHM)• Trenching formation due to ion scattering
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FLCC – Plasma
Effect of Deposition/Redeposition
photoresist
poly-Si
oxide
C
SiCl2
• Non-directional ion distribution (10o FWHM)• Trenching formation reduced due to deposition/redeposition of
etching products
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FLCCFLCC – Plasma
STI Etching Process • ITRS dictates stringent conditions for optimal trench isolation as minimum feature size decreases
• Positive trench tapering angles desired to avoid sharp recesses leading to “poly wrap-around”
• Smooth sidewalls needed for less physical and electrical damage
• Round bottom corners to minimize stress and avoid voids in gapfill
Desired Properties:
D4 > D2/2
θnitride = 90º – arctan[(D1-D2)/2/tx1]
θtop Si = 90º – arctan[(D2-D3)/2/tx2]
θbot Si = 90º – arctan[(D3-D4)/2/tx3]
Recess < 0.1×D2
Curvature: rNitride top = rSi bottom = 0.1×D2
Shallow Trench Isolation (STI)
Isolation stack Pattern nitride and strip PR
Trench etch
PRnitride
oxide
Silicon
Sidewall oxidation and deposit trench
oxide
Strip nitride and remove pad oxide
CMP planarizationSEM Measured Parameters
D1
D2
D3
Total SiDepth
tx1(nitride
)tx2(top Si)
tx3(bot Si)
Nitride SWA
top Si SWA
bot Si SWA
D4
SWA: sidewall angle† Adapted from ITRS 2003 Thermal Films Supplemental
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FLCCFLCC – Plasma
AMAT DP SII Reactor Setup
• Parameters examined for STI etch
• Chamber Pressure (mTorr)
• Source Power (Ws)
• Wafer bias (Wbias)
• DC ratio = Iouter/Iinner
• Cl2 flowrate (sccm)
• N2 flowrate (sccm)
• O2 flowrate (sccm)
Cl2N2O2
Ws
Ws
Wbias
Coil Power
Substrate Bias
Iouter Iinner
Pressure
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FLCCFLCC – Plasma
Fractional Factorial DOE• SixNy etch DOE • Si etch DOE
ID1 - - - - - - -2 - + - + - + -3 - + - - + - +4 + - + - + - +5 + + - - + + -6 + + - + - - +7 - - + + - - +8 + - - + + - -9 + - + + - + -10 + - - - - + +11 - - + - + + -12 - + + + + - -13 - - - + + + +14 - + + - - + +15 + + + - - - -16 + + + + + + +
Pres
sure
(mT)
W s(W
) W b
(W)
DC ratio
Cl 2
(sccm
) N 2
(sccm
) O 2
(sccm
)
ID1 0 0 0 0 0 0 02 - + - - + - +3 - - + - + + -4 - - + + - - +5 + + + + + + +6 - + + + + - -7 + - - - - + +8 - + - + - + -9 - - - + + + +10 + - - + + - -11 + + - - + + -12 + + + - - - -13 - - - - - - -14 + - + + - + -15 - + + - - + +16 + - + - + - +17 + + - + - - +
Pres
sure
(mT)
W s(W
) W b
(W)
BS He (
sccm
)
CF 4(sc
cm)
Ar (sc
cm)
CHF 3(sc
cm)
• 7 factors, 2 levels, and 16 experiments performed for both etch targets
• Nitride etch: pressure was determined to be the statistically significant effect
• Si etch: pressure and DC ratio had statistically significant effects
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FLCCFLCC – Plasma
Monte Carlo SimulationCl+
Cl
Si or SiCl4• Ions specularly scatter
• Neutrals diffusively scatter
• Surface composition determined by elemental balance
• Surface reactions occur with measured probabilities
•Simulation domain:• 10-50 Å grid size • Transport of species in grid lengths
• Surface reaction:• reaction probability based on kinetic models• elemental balances• multiple interactions possible
• Surface advancement:• grid cells removed and/or added
• Rigorous incorporation of all physics and chemistry• Computationally robust and straight-forward
Mask (SiNx)
Source plane
Periodic boundary conditions
n+
Silicon
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FLCCFLCC – Plasma
Surface Reaction Kinetics ModelSi Etching
0 (1 )( ) ( )
Cl Cl Osg sCl Clζ ζ− −+ ∗ →
( )( ) ( )
cg sCl Clφ++∗ →
( )( ) ( ) 4( )4 4c Cls s gSi Cl SiClφ β +
+ → + ∗0
2
2( ) ( ) ( )3 2SiCls
g s sSiCl Si Cl→+ +∗0(1 )
( ) ( )O OCls
g sO Oζ ζ− −+ →∗
( ) ( )SPSiY
s gSi Si→ +∗0
( ) ( )Sis
g sSi Si→+∗
( ) ( ) ( )2Clrg s gCl Cl Cl→+ +∗
(implemented)(implemented)(implemented)(in development)(in development)(in development)(in development)(implemented)
Chlorination:Sorption of Chlorine ion:Ion-enhanced etching:SiCl2 Deposition:Oxygenation:Sputtering:Sorption of sputtered Si:Recombination of chlorine:
Nitride Etching• Implemented only etch selectivity and angular dependence due to limited kinetic measurements during the etching of nitride in CF4/Ar/CHF3 plasma
• Experimentally determined reaction kinetics model enables predictable feature profile evolution
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FLCCFLCC – Plasma
Elemental Balance in Cells
• Si provides available sites • Etchant such as Cl leads to the formation of
volatile products • Reactant such as O leads to the oxidation thus
changing the etching characteristics • Deposition of SiCl2 and Si will add sites
SiNOCl
Source plane
Periodic boundary conditions
n+
Mask (SiNx)
Silicon
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FLCCFLCC – Plasma
Major Enhancements in Simulation
0 50 100 150 200 250 300 350 4000.01.02.03.04.05.06.07.08.09.0
10.0
Etch
ing
Yie
ld (S
i/Cl+
)
Flux Ratio (Cl/Cl+)
235eV Cl+ 195eV Cl+ 155eV Cl+
115eV Cl+
75eV Cl+
35eV Cl+
55eV Cl+
• Implemented sloped mask
• Determined surface normals using least squares regression fit to center of cells considered – effective rounding of corners
• Implemented ion energy distribution function (to be enhanced with real experimental or plasma simulation results) Collaboration with Graves and Lieberman
• Implemented ion etching yield dependence as a function of ion energy
“Least squares method” normals
0 50 100 150 200 250 300 3500
100
200
300
400
500
num
ber o
f ion
s
Ion energy (eV)Gas cell
Solid interface cell
Solid non-interface cell
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FLCCFLCC – Plasma
Simulation DetailsParameters affecting profile evolution:
• plasma chemistry (Cl2, HBr, O2, …)• plasma composition (Cl, Cl2
+, Cl+, O, …) • electron temperature and distribution
(Te and EEDF ni, nn, …)• substrate bias (Ws Eion)• substrate temperature (Tsub)
Baseline Conditions:• initial aspect ratio: 0.55• ion angular distribution (IAD) FWHM: 5.3º• ion energy distribution (IED) FWHM: 23.5 eV• ion energy: 200 eV• neutral to ion ratio: 100• selectivity of nitride to Si: 33.3
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FLCCFLCC – Plasma
Simulator CapabilitiesDeposition Spontaneous Etching Micro-Trenches
Bowing Effect of Mask AngleEffect of Selectivity
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FLCCFLCC – Plasma
Comparison of Simulation with Experiments
• Similar plasma densities• Substrate bias governs the etch depth
DOE 205-06 DOE 205-07pressure (mTorr) 45 25
Ws (W) 500 350Wb (W) 150 250DC ratio 30 30
Cl2 (sccm) 140 140N2 (sccm) 30 30O2 (sccm) 25 25
DOE 205-10 DOE 205-14pressure (mTorr) 45 25
Ws (W) 350 500Wb (W) 150 250DC ratio 11 11
Cl2 (sccm) 140 140N2 (sccm) 60 60O2 (sccm) 25 25
Simulation on-going
(significantly different sidewall slope could be due to a change in plasma composition)
• High density versus low density plasmas • Plasma composition controls profile evolution
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FLCCFLCC – Plasma
Future Goals• Determine plasma gas phase chemistry experimentally • Validate the simulation results with specially planned additional experiments • Enhance current simulator by including feature charging and simultaneous deposition and etching• Correlate plasma operating parameters to simulation input profiles to allow a more direction comparison of the simulation results to experiment outcomes
Special Acknowledgements: Helena Stadniychuk and Andrey Zagrebelny at Cypress
• Acknowledgment• Funded by Advanced Micro Devices, Applied Materials,
ASML, Atmel, Cadence, Canon, Cymer, Cypress, DuPont, Ebara, Hitachi Global Storage Technologies, Intel, KLA-Tencor, Mentor Graphics, Nikon Research, Novellus Systems, Panoramic Technologies, Photronics, Synopsys, Tokyo Electron, and the UC Discovery Grant.
