WAFER EDGE EFFECTS CONSIDERING ION INERTIA IN CAPACITIVELY COUPLED

DISCHARGES*

Natalia Yu. Babaeva and Mark J. Kushner

Iowa State UniversityDepartment of Electrical and Computer Engineering

Ames, IA 50011, USA natalie5@iastate.edu mjk@iastate.edu

http://uigelz.ece.iastate.edu

June 2006

* Work supported by Semiconductor Research Corp. and NSF

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AGENDA

Wafer Edge effects and their origin.

Description of the model:

Improvement of nonPDPSIM to include ion momentum equation

Effect of wafer-focus ring gaps on Ar and Ar/Cl2 CCPs

Plasma penetration

Ion focusing

Concluding remarks

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WAFER EDGE EFFECTS

It is desirable to use wafer area to the edge of the wafer to maximize utilization.

Perturbation of fluxes may occur by method of terminating wafer and matching to tool material

Wafer is beveled at edge with small gap (< 1 mm) between wafer and focus ring.

Penetration of plasma into gap is bad due to formation of particles and deposition of contaminating films.

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ION MOMENTUM EQUATION IN nonPDPSIM

Goal is to computationally investigate edge effects and penetration of plasma into wafer-focus ring gap.

Large dynamic range (> 100) requires unstructured mesh.

Large Knudson number in gap requires accounting for inertia.

nonPDPSIM, a 2-dimensional plasma hydrodynamics model, was improved by adding ion momentum equations on unstructured mesh.

The coupling between the dynamics of charged and neutral transport is through the species resolved collision terms in momenta equations.

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nonPDPSIM CHARGE PARTICLE TRANSPORT• Poisson equation for the electric potential

• Transport equations for conservation of the charged species j

• Surface charge balance

• Full momentum for ion fluxes of species j

• Equations are simultaneously solved using a Newton’s iterations.

)( j

jj Nq

St

N j

materialj

j Sqt

)(1

)(1

,,,,,

,,,,,

iyi

jyjjj

yjjjy

jjjy

jy

ixi

jxjjj

xjjjx

jjjx

jx

VVNM

ENqP

MV

t

VVNM

ENqP

MV

t

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2-D GEOMETRY AND CONDITIONS

Conditions:

Ar, 90 mTorr, 300 sccm, 500 V

Ar/Cl2 = 70/30, 90 mTorr, 300 sccm, 500 V

Biased substrate, grounded housing

Showerhead to wafer distance = 4 cm

Transport of energetic secondary electrons from biased substrate is addressed with a Monte Carlo simulation.

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MESHING TO RESOLVE WAFER-FOCUS RING GAP

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Unstructured mesh with multiple refinement zones was used to resolve wafer-focus ring gap.

Gaps of < 1 mm were investigated.

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Optical and Discharge PhysicsMIN MAX

Log scale

Electron penetration into the gaps is nominal due to surface charging and sheath formation.

Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V

Animation slide

ELECTRON DENSITY NEAR THE GAPS

0.9 mm Gap

106 –108 cm-3 106 –108 cm-3

Electrons (106 – 3 x109 cm-3)

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0.3 mm Gap

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EDGE REGION: NEGATIVE CHARGE

Negative charging of wafer surface (and focus ring) extends beyond edge of bevel in large gap.

Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V

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0.9 mm Gap 0.3 mm Gap

MIN MAX Log scale

EDGE REGION: IONS

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Animation slide

106 – 3x108 cm-3108 –3 x108 cm-3

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MIN MAX Log scale

0.9 mm Gap 0.3 mm Gap

Ions are modulated by 10 MHz e-field variation.

Ions penetrate into the large gap reaching the biased substrate.

Ions do not penetrate into the small gap but do respond to “sentinal” surface charge.

Ar, 90 mTorr, 10 MHz,300 sccm, 500 V

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EDGE REGION: ELECTRON TEMPERATURE

MIN MAX

Te is higher near the small gap due to overlapping os sheaths and higher local electric fields.

Electron temperature (and electron density) is negligibly small inside the gaps.

Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V

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0.9 mm Gap 0.3 mm Gap

Ar/Cl2 DISCHARGE

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Maximum electron density shifts towards the focus ring.

Negative ion density comparable to electron density, though are trapped in the plasma bulk and do not reach the wafer

Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V

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MIN MAX Log scale

[e] [Cl2+]

[Ar+] [Cl-]

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EDGE REGION: Ar+ AND Cl2+ FLUXES

Cl2+ flux is larger and less collisional than Ar+ due to lower rate of

charge exchange. There is some focusing of flux to the corner of the bevel that

could lead to excessive heating and sputtering. Some ion trajectories terminate on the lower bevel. Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V

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0.9 mm Gap 0.9 mm Gap

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EDGE REGION: Ar+ AND Cl2+ FLUXES

Less focusing of ion fluxes to corner of bevel occurs with the smaller gap due to lack of charging of wafer into wafer-focus ring cavity.

Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V

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0.3 mm Gap 0.3 mm Gap

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EDGE REGION: Ar+ FLUX STREAMTRACES

Streamlines penetrate into large gap throughout rf cycle.

In small gap, momentary penetration occurs at peak of cathode cycle. Slightly conductive wafer is able to dissipate that charge.

Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V

Animation slideICOPS2006_Natalie_15

0.3 mm Gap 0.3 mm Gap

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EDGE REGION: Cl2+ FLUX STREAMTRACES

Focusing of ion flux streamlines to edge of wafer is more severe for Cl2

+ than Ar+ due to lower collisionality.

Periodic flux into gap is also larger.

Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V

Animation slideICOPS2006_Natalie_16

0.9 mm Gap 0.3 mm Gap

CONCLUDING REMARKS

Penetration of plasma into narrow wafer-focus ring gap of a capacitively coupled discharge was computationally investigated.

Gap sizes > 0.5 mm allow significant penetration of the plasma.

Charging and ion fluxes may penetrate to bottom side of bevel.

Focusing of ion flux to the corner of the bevel depends on the ion species and collisionality: chemically enhanced sputtering is problematic.

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