Transcript Document
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 University
Department 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
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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
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PENETRATION OF PLASMA INTO WAFER-FOCUS RING GAP
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.
Orientation of electric field and ion trajectories, energy and angular
distributions depend on details of the geometry and materials.
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INVESTIGATION OF IEADs INTO WAFER-FOCUS RING GAP
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?
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nonPDPSIM: BASIC EQUATIONS
( q j N j )
Poisson equation: Electric potential
Transport of charged species j
N j
t
j
S
q j S
t j
material
Surface charge balance
Full momentum for ion fluxes
j
qjN jE
1
j v j
Pj
t
Mj
Mj
N j ij v j vi
i
Neutral transport: Navier-Stokes equations.
Improvements to include Monte Carlo simulation of Ion Energy and
Angular distributions (IEADs).
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MESHING TO RESOLVE FOCUS RING GAP
2-dimensional model using an
unstructured mesh to resolve waferfocus ring gaps of < 1 mm.
Numbering indicates materials and
locations on which IEADs are obtained.
Ar, 10 MHz, 100 mTorr, 300 V, 300 sccm
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POTENTIAL, ELECTRIC FIELD, IONS
Potential
Off-axis maximum in
[Ar+] is due to
electric field
enhancement near
focus ring and is
uncorrelated to gap.
E/N
Ar, 10 MHz, 100
mTorr, 300 V
[Ar+]
MIN
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MAX
Gap: 1 mm
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POTENTIAL AND CHARGES (RF CYCLE)
1.0 mm Gap
Surface Charges
Cycle averaged potential
-1.1 x 1011 cm-3
Powered Electrode
Powered Electrode
Highly conductive wafer with small capacitance charges and
discharges rapidly.
Focus ring acquires larger negative surface charges.
Large potential drop in focus ring.
Animation Slide
MIN
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MAX
Log scale
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ION FLUX VECTORS (RF CYCLE)
1.0 mm Gap
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.
Animation Slide
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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
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MAX
Log scale
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ION FLUXES AT DIFFERENT PHASE OF RF CYCLE
1.0 mm Gap
Cathodic rf cycle
Cathodic cycle:
High energy ions at
grazing incident on
side wall.
5
9
Near vertical to
bevel.
Anodic rf cycle:
Anodic rf cycle
Low energy ions
near vertical on
side wall.
5
MIN
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9
MAX
Log scale
High energy angles
a large angle to
bevel.
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CAPACITANCE OF FOCUS RING: ION DENSITY AND CHARGES
Wafer charges
quickly (almost
anti-phase with
focus ring).
1.0 mm Gap
-7.8 x 1010 cm-3
-1.2 x 1011 cm-3
Powered electrode
Powered electrode
Ar+
Powered electrode
More surface
charges collected
on focus ring with
larger capacitance.
Ar+
Ions penetrate into
gap throughout rf
cycle with larger
capacitance.
Powered electrode
Animation Slide
/o= 4
0.5 mm Gap
MIN
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/o= 20
MAX
Log scale
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CAPACITANCE OF FOCUS
RING: IEAD
/o= 4
/o= 20
Penetration
of potential
into focus
ring with low
capacitance
produces
lateral Efield.
IEAD on
substrate is
asymmetric.
MIN
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MAX
Log scale
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FOCUS RING HEIGHT: ION DENSITY AND FLUX
1.0 mm Gap
Powered Electrode
Ions do not fully
penetrate into the
gap with high
focus ring.
Powered Electrode
Ion focusing on
edges.
Substantial
penetration of ion
flux under bevel
with low focus
ring.
Powered Electrode
Powered Electrode
MIN
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MAX
Log scale
Animation Slide
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FOCUS RING HEIGHT: IEAD
1.0 mm Gap
0.25 mm Gap
“Open” edge produces
skewed IEADs
MIN
MAX
Log scale
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DESIGN TO CONTROL IEADs
Configuration of wafer-focus ring gap can be used to control
IEADS.
Example: Extension of biased substrate under dielectric focus ring
of differing conductivity.
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EXTENDED ELECTRODE : CHARGE, E-FIELD AND ION FLUX
Same
conductivity
wafer and FR.
Powered Electrode
Powered Electrode
More uniform
and symmetric
sheath and
plasma
parameters.
0.1 Ohm-1 cm-1
Powered Electrode
MIN
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MAX
Log scale
Animation Slide
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Wafer: 0.1 Ohm-1 cm-1
Ring: 10-8 Ohm-1 cm-1
EXTENDED
ELECTRODE: IEAD
Wafer and Ring:
0.1 Ohm-1 cm-1
On all surfaces
more narrow and
symmetric IEAD
with uniform
electrical boundary
condition.
MIN
MAX
Log scale
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BROADENING OF IEAD ON TOP BEVEL: EFFECT OF FR
/o= 4
/o= 20
High FR
Low FR
FR Conductivity
Always broad and asymmetric
IEAD on tilted surface.
MIN
MAX
Log scale
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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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