ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL FEATURES IN PLASMA ETCHING REACTORS: THE WAFER-...

ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL FEATURES IN PLASMA ETCHING

REACTORS:THE WAFER- FOCUS RING GAP*

Natalia Yu. Babaeva and Mark J. Kushner

Iowa State UniversityDepartment of Electrical and Computer Engineering

Ames, IA 50011, USA [email protected] [email protected]

http://uigelz.ece.iastate.edu

AVS 54th International Symposium

October 2007

* Work supported by Semiconductor Research Corp., Applied Materials and NSF

AVS2007_Natalie_01

Iowa State UniversityOptical and Discharge Physics

AGENDA

Wafer edge effects

Description of the model

Ion energy and angular distribution on different surfaces in wafer-focus ring gap for focus ring:

Capacitance Height Conductivity

Concluding remarks

AVS2007_Natalie_02


Orientation of electric field and ion trajectories, energy and angular distributions depend on details of the geometry and materials.

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Gap (< 1 mm) between wafer and focus ring in plasma tools for mechanical clearance.

Beveled wafers allow for “under wafer” plasma-surface processes.

Penetration of plasma into gap can deposit of contaminating films.

PENETRATION OF PLASMA INTO WAFER-FOCUS RING GAP

Iowa State UniversityOptical and Discharge PhysicsAVS2007_Natalie_03

The ion energy and angular distributions (IEADs) into the wafer-focus ring gap are important;

Angular distribution determines erosion (e.g., maximum sputtering at 60o.

Time between replacement of consumable parts depends on erosion.

Spacing, materials (e.g., dielectric constant, conductivity) determine electric field in gap and so IEADS.

In this presentation, results from a computational investigation of IEADs onto surfaces in wafer-focus ring gap will be discussed.

Model: nonPDPSIM using unstructured meshes.

Goal: How does one control the IEADs?

INVESTIGATION OF IEADs INTO WAFER-FOCUS RING GAP

nonPDPSIM: BASIC EQUATIONS

Poisson equation: Electric potential

Transport of charged species j

Surface charge balance

Full momentum for ion fluxes

Neutral transport: Navier-Stokes equations.

Improvements to include Monte Carlo simulation of Ion Energy and Angular distributions (IEADs).

)( j

jj Nq

St

N j

materialj

j Sqt

iji

ijjj

jjj

jjj

j vvNM

ENqP

Mv

t

1


MESHING TO RESOLVE FOCUS RING GAP


2-dimensional model using an unstructured mesh to resolve wafer-focus ring gaps of < 1 mm.

Numbering indicates materials and locations on which IEADs are obtained.

Ar, 10 MHz, 100 mTorr, 300 V, 300 sccm

POTENTIAL, ELECTRIC FIELD, IONS

Iowa State UniversityOptical and Discharge PhysicsMIN MAX

E/N

Potential

[Ar+]

AVS2007_Natalie_06

Off-axis maximum in [Ar+] is due to electric field enhancement near focus ring and is uncorrelated to gap.

Ar, 10 MHz, 100 mTorr, 300 V

Gap: 1 mm


POTENTIAL AND CHARGES (RF CYCLE)

1.0 mm Gap

Animation Slide

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-1.1 x 1011 cm-3

Highly conductive wafer with small capacitance charges and discharges rapidly.

Focus ring acquires larger negative surface charges. Large potential drop in focus ring.

MIN MAX Log scale

Surface Charges Cycle averaged potential

Powered Electrode Powered Electrode


Directions of electric fields near surfaces evolve slowly during rf

cycle due to slowly changing surface charge.

Direction of ion fluxes changes during rf cycle from nearly vertical to perpendicular to surface with transients in electric field.

ION FLUX VECTORS (RF CYCLE)

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1.0 mm Gap

Animation Slide

Powered Electrode


ION ENERGY AND ANGULAR DISTRIBUTIONS

Broad IEAD on top bevel due to ions arriving during positive and negative parts of rf cycle.

Grazing angles for ions striking vertical surfaces.

MIN MAX Log scale


Cathodic cycle:

High energy ions at grazing incident on side wall.

Near vertical to bevel.

Anodic rf cycle:

Low energy ions near vertical on side wall.

High energy angles a large angle to bevel.

ION FLUXES AT DIFFERENT PHASE OF RF CYCLE

AVS2007_Natalie_10

1.0 mm Gap

9

9

MIN MAX Log scale

Cathodic rf cycle

5

5

Anodic rf cycle


Log scale

Wafer charges quickly (almost anti-phase with focus ring).

More surface charges collected on focus ring with larger capacitance.

Ions penetrate into gap throughout rf cycle with larger capacitance.

AVS2007_Natalie_11

1.0 mm Gap

Ar+

0.5 mm GapAnimation Slide

-7.8 x 1010 cm-3

Ar+

/o= 4 /o= 20

CAPACITANCE OF FOCUS RING: ION DENSITY AND CHARGES

Powered electrode Powered electrode

Powered electrode Powered electrode

-1.2 x 1011 cm-3


MIN MAX Log scale

AVS2007_Natalie_12

/o= 4

/o= 20

CAPACITANCE OF FOCUS RING: IEAD

Penetration of potential into focus ring with low capacitance produces lateral E-field.

IEAD on substrate is asymmetric.


Log scale

Ions do not fully penetrate into the gap with high focus ring.

Ion focusing on edges.

Substantial penetration of ion flux under bevel with low focus ring.

FOCUS RING HEIGHT: ION DENSITY AND FLUX

1.0 mm Gap

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Animation Slide




1.0 mm Gap 0.25 mm Gap

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“Open” edge produces skewed IEADsMIN MAX

Log scale

FOCUS RING HEIGHT: IEAD


Configuration of wafer-focus ring gap can be used to control IEADS.

Example: Extension of biased substrate under dielectric focus ring of differing conductivity.

DESIGN TO CONTROL IEADs

AVS2007_Natalie_15


Same conductivity wafer and FR.

More uniform and symmetric sheath and plasma parameters.

0.1 Ohm-1 cm-1

MIN MAX Log scale

EXTENDED ELECTRODE : CHARGE, E-FIELD AND ION FLUX

Animation Slide

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Powered Electrode

Powered ElectrodePowered Electrode


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On all surfaces more narrow and symmetric IEAD with uniform electrical boundary condition.

Wafer: 0.1 Ohm-1 cm-1

Ring: 10-8 Ohm-1 cm-1

Wafer and Ring: 0.1 Ohm-1 cm-1

EXTENDED ELECTRODE: IEAD

MIN MAX Log scale


MIN MAX Log scale

Always broad and asymmetric IEAD on tilted surface.

FR Conductivity

BROADENING OF IEAD ON TOP BEVEL: EFFECT OF FR

/o= 4 /o= 20 High FR Low FR

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CONCLUDING REMARKS

Ion energy and angular distributions were investigated on surfaces inside wafer-focus ring gap.

Different regions of the IEADs are generated during different parts of the rf cycle. Even vertical surfaces receive some normal angle ion flux.

Narrow IEAD are obtained with

High focus ring

High focus ring capacitance

High focus ring conductivity.

Uniform electrical boundary conditions leads to more symmetric sheath over the gap and narrows IEADs.

On tilted surfaces broad and asymmetric IEADs are obtained for most conditions.


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ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL FEATURES IN PLASMA ETCHING REACTORS: THE WAFER-...

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