MODELING OF SEASONING OF REACTORS: EFFECTS OF ION … · 2006-11-18 · ANKUR_AVS06SS_02...

MODELING OF SEASONING OF REACTORS: EFFECTS OF ION ENERGY DISTRIBUTIONS TO

CHAMBER WALLS*

Ankur Agarwala) and Mark J. Kushnerb)

a)Department of Chemical and Biomolecular EngineeringUniversity of Illinois, Urbana, IL 61801, USA

[email protected])Department of Electrical and Computer Engineering

Iowa State University, Ames, IA 50011, [email protected]

http://uigelz.ece.iastate.edu

53rd AVS Symposium, November 2006

*Work supported by the SRC and NSF

Iowa State UniversityOptical and Discharge Physics

AGENDA

• Seasoning of plasma reactors

• Approach and Methodology

• Hybrid Plasma Equipment Model

• Surface Chemistry Model

• Si etching in Ar/Cl2• Effect of seasoning reactor walls on etch rates

• Sputtering of quartz window

• Concluding Remarks

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SEASONING OF PLASMA REACTORS

• Deposition on reactor walls during a process changes surface reactivity (e.g., seasoning)

• Seasoning changes reactive fluxes to substrate. To control wafer-to-wafer variability:

• Clean the seasoned chamber following each wafer.

• Season the chamber prior to process step.

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Ref: E.S. Aydil et al., JES 150, G418 (2003)

Ref: G. Cunge et al., PSST 14, S42 (2005)


SEASONING: CHALLENGES

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• Understanding the role that chamber condition plays in manufacturing variability is important.

• Modeling Challenges:

• Energetic Etch products; products not thermalized• Ion energy distributions are different on different

surfaces of reactor.• Reactivity of surface changes during a process as well

as run-to-run.

• In this talk, seasoning of reactor will be computationally investigated while accounting for variation of IEDs and reactivity on all surfaces.

• Electromagnetics Module: Antenna generated electric and magnetic fields

• Electron Energy Transport Module: Beam and bulk generated sources and transport coefficients.

• Fluid Kinetics Module: Electron and Heavy Particle Transport, Poisson’s equation


HYBRID PLASMA EQUIPMENT MODEL (HPEM)

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• Plasma Chemistry MC Module: IEADs to surfaces

• Surface Chemistry Module: Surface coverage and reactive sticking coefficients.


SURFACE CHEMISTRY MODULE (SCM)

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• Surface Chemistry Module (SCM) is an integrated module in HPEM to address surface reactions on substrate and walls of the reactor.

• The SCM:

• Uses the reactant fluxes from the plasma modules.• Applies a user defined reaction mechanism• Updates surface sticking and product reflection coefficients

for the plasma species.• Calculates surface coverages and etch rates.


SURFACE REACTION PROBABILITIES: IMPLEMENTATION

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• Surface reaction probabilities are a function ion energies.

• Processes with threshold energies are extremely sensitive to details of IEADs: εave < εth but rate could still be finite.

• Compute IEADs to all surfaces in reactor continuously during time evolution of process.

• Obtain surface probabilities vs position from convolution with p(x,ε)

∫∫=

εε

εεε

dxf

dpxfxp

ions

ions

),(

)(),()(

Si ETCHING IN Ar/Cl2: WAFER SURFACE MECHANISM

• Cl adsorbs on forming SiClx passivation layer.

Cl(g) + Si(s) → SiCl(s), p=0.99

Cl(g) + SiCln(s) → SiCln+1(s) p=0.2

• Ions etch passivation (for 200eV).

Cl+(g) + SiCl(s) → SiCl2(g), p=0.3

Cl+(g) + SiCl3(s) → SiCl4(g), p=0.6

M+(g) + SiClx(s) → SiClx(g) p=0.6

• Etch products further passivates, creating etch blocks.

SiCl2(g) + Si(s) → Si2Cl2(s) p=0.3

SiCl2(g) + SiCln(s) → Si2Cln+2 p=0.1-0.2

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Si ETCHING IN Ar/Cl2: WALL SURFACE MECHANISM

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• On chamber walls

SiCl2(g) + W(s) → SiCl2(s) p=0.2

SiCl2(g) + SiCl2(s) → (no reaction)

M+(g) + SiCl2(s) → SiCl(s) + Cl(g)p=0.6

M+(g) + SiCl2(s) → SiCl2(g) + W(s) p=0.6

• Passivated walls effect reactivityof Cl.

Cl(g) + W(s) → Cl(s) p=0.1

Cl(g) + Cl(a) → Cl2(g) + W(s) p=0.1

Cl(g) + SiCl2(s) → (no reaction)

Si ETCHING IN Ar/Cl2

• Seasoning investigated for Si etch products in Ar/Cl2.

• Base case conditions:

• Ar/Cl2 = 90/10, 100 sccm• 15 mTorr, 300 W• 50 V bias at 5 MHz

• Silicon etching by chlorine is the source SiClx.

• Transport of SiClx results in deposition (and further sputter/etch) on other surfaces.

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Si ETCHING IN Ar/Cl2:REACTANT FLUXES

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• Dominant ions are Ar+ and Cl+due to dissociation of Cl2.

• Dominant neutrals are Cl, SiCl2and SiCl4.

• SiCl2 is potentially reactive with surfaces; SiCl4 is not.

• Ar/Cl2=90/10, 100 sccm, 15 mTorr, 300 W, 50 V at 5 MHz.

Si ETCH: ION ENERGY ANGULAR DISTRIBUTIONS

• Ion energies on wafer are 50-75 eV, elevated above applied voltage due to dc bias.

• Ion energies on other reactor walls peak at time averaged floating plasma potential (40 V).

• Reactivity of wafer and walls differ due to differences in threshold energies and IEDs

• Ar/Cl2 = 90/10, 100 sccm, 15 mTorr, 300 W, 50 V at 5 MHz

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SURFACE COVERAGES: WAFER

• Surface site coverages depend on bias voltage.

• Surface coverage of SiCl2 and SiCl3decreases with increasing bias.

• More rapid removal of higher chlorinated sites.

• SiCl fraction increases as removal rate of higher chlorinated sites uncovers native Si.

• Ar/Cl2=90/10, 100 sccm, 15 mTorr, 300 W

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• Surface coverages after 1 min.

SURFACE COVERAGES: REACTOR WALLS

• Surface coverage of walls depend on wafer surface kinetics.

• Low biases: Etch products not significant in the bulk plasma. Sites dominated by Cl(s).

• High biases: Large production of etch products produce higher SiCland SiCl2 surface coverage.

• Etch products’ surface coverage ultimately saturate with time at about 0.25-0.30.

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• Ar/Cl2=90/10, 100 sccm, 15 mTorr, 300 W

• Surface coverages after 1 min.

SEASONING EFFECT: ETCH RATE

• Si etch for 1 min for each wafer.

• Etch rate in seasoned chamber decreases.

• Passivation of walls by SiCl2decreases further reactivity of SiCl2increasing density in plasma.

• SiCl2 passivates wafer SiClx sites forming Si2Cly etch blocks.

SiCl2(g) + SiCln(s) → Si2Cln+2(s)

• Ions removes Si2Cly with no net contribution to etch rate.

• Rate of change of etch rate decreases with number of wafers; chamber wall conditions stabilize.


• Ar/Cl2=90/10, 100 sccm, 10 mTorr, 300 W, 75 V at 5 MHz

Wafer

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SEASONED CHAMBER ETCH RATE: VOLTAGE• Si etch rates decrease in seasoned

chamber.

• With additional wafers, etch rates stabilize as chamber reaches final state.

• Etch rate stabilizes sooner at higher voltages.

• Higher etch rates and more etch products season chamber faster.

• Larger ion energies remove overlying Si2Cln more rapidly.

• In spite of lower reactivity of Cl on walls (and larger Cl in plasma), etch rates decrease due to site blockage.

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• Ar/Cl2=90/10, 100 sccm, 10 mTorr, 300 W

Si ETCH: QUARTZ SPUTTER

• Capacitive coupling of coils was included to facilitate sputtering of O atoms from the quartz window.

• Base case conditions:

• Ar/Cl2 = 90/10, 100 sccm• 15 mTorr, 300 W• 50 V substrate bias at 5 MHz

• The voltage at the “turns” was fixed at

• 300 V (turn 1)• 400 V (2)• 500 V (3)

to emulate inductive voltage drop across coil.

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QUARTZ SPUTTER: SURFACE MECHANISM

• Ion sputtering of top quartz window is the source of O in the plasma.

M+(g) + Quartz(s) → O(g)

p=1.0 at 200 eV (εth= 60 eV)

• O in bulk plasma may under go electron impact reactions (e.g., ionization).

• Surface reaction mechanism (any occurrence of SixCly)

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O(g) + SixCly(s) → SiOCl(s) p=1.0

M+(g) + SixOCly(s) → SixCly(s) + O(g) + M(g) p=1.0 at 200 eV (εth= 60 eV)

IEADs: CAPACITIVE COUPLING

• Ion induced sputtering of O from quartz through capacitive coupling of coils.

• Total IEADs peaks are near the sputter threshold of quartz.

• O species adsorb on the wafer, forming SixOCly etch blocks.

• Energetic ion bombardment on the wafer may remove the etch block.

• Ar/Cl2=90/10, 100 sccm, 15 mTorr, 300 W, 50 V bias at 5 MHz

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SiOCl PASSIVATION: WAFER and METAL WALL

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• For t < 0, ICP only with capacitive coupling produces O atoms from quartz and SiOCl etch blocks on wafer.

• At t=0, bias is applied, removing SiOCl etch blocks and increasing density of etch products.

• O atoms adsorb on etch product passivated sidewalls as SiOCl.

QUARTZ SPUTTER: IMPACT ON ETCH RATE

• Capacitive coupling (O atom sputtering) reduces etch rates at low voltages.

• O atoms adsorb on wafer forming SiOCl (an etch block) which is not removed.

• Large bias voltages increases ion energies and sputters the etch block restoring higher etch rates.

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• Ar/Cl2=90/10, 100 sccm, 15 mTorr, 300 W

• Pathway to decrease in etch rate

O(g) + SixCly(s) → SiOCl(s) p=1.0

M+(g) + SiOCl(s) → SiCl(s) + O(g) p=1.0 at 200 eV (εth= 60 eV)

CHANGE IN CAPACITIVE COUPLING

• The degree of capacitive coupling from the coils was reduced by moving coils above the dielectric window.

• Reduced capacitive coupling reduces ion energies onto the quartz (less sputtering).

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CAPACITIVE COUPLING: ETCH RATE

• Reduction in capacitive coupling

• Lower ion energies

• Sputter O atoms density lower.

• O atom adsorption on wafer forming SiOCl are less rate-limiting.

• At higher voltages, the etch rate increases due to less energy absorbing O in the plasma.

• Ion flux increases

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• Ar/Cl2=90/10, 100 sccm, 15 mTorr, 300 W


CONCLUDING REMARKS• Chamber seasoning was investigated in Si etch using Ar/Cl2

plasmas.

• IED integrated with Surface Chemistry Model to update wall recombination coefficients; change surface loss rates.

• Etch rates decreased in a seasoned chamber.

• Seasoned reactor increases SiCl2 flux back to wafer.• Feedback of etch products (SiCl2) from the plasma form

SixCly etch blocks.• Removal of SixCly does not contribute to etch rate.

• Sputtering of quartz window is a source of O atoms.

• O atoms adsorb on wafer forming etch block.• Reduction of etch rates at low biases• At large bias, SiOCl removed.

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MODELING OF SEASONING OF REACTORS: EFFECTS OF ION … · 2006-11-18 · ANKUR_AVS06SS_02...

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