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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