Transcript Document

WAFER EDGE EFFECTS CONSIDERING ION
INERTIA IN CAPACITIVELY COUPLED
DISCHARGES*
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
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
    ( q j N j   )
j
Transport equations for conservation of the charged species j
N j
t
•

   S
Surface charge balance


 
  q j     S     
t  j
 material

•
Full momentum for ion fluxes of species j

x , j
q j N j Ex
1
  x , j V j  
Px , j 
  N j j (Vx , j Vx ,i )
t
Mj
Mj
i
y , j
•






q j N j Ey
1
  y , j V j  
Py , j 
  N j j (Vy , j Vy ,i )
t
Mj
Mj
i
Equations are simultaneously solved using a Newton’s iterations.
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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
 Unstructured mesh
with multiple
refinement zones
was used to
resolve waferfocus ring gap.
 Gaps of < 1 mm
were investigated.
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ELECTRON DENSITY NEAR THE GAPS
 0.9 mm Gap
 0.3 mm Gap
106 –108 cm-3
106 –108 cm-3
 Electron penetration into
the gaps is nominal due to
surface charging and
sheath formation.
Electrons (106 – 3 x109 cm-3)
 Ar, 90 mTorr, 10 MHz, 300
sccm, 500 V
Animation slide
MIN
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MAX
Log scale
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EDGE REGION: NEGATIVE CHARGE
 0.9 mm Gap
 0.3 mm Gap
 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
MIN
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MAX
Log scale
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EDGE REGION: IONS
 0.9 mm Gap
 0.3 mm Gap
106 – 3x108 cm-3
108 –3 x108 cm-3
 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
MIN
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MAX
Log scale
Animation slide
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EDGE REGION: ELECTRON TEMPERATURE
 0.9 mm Gap
 0.3 mm Gap
 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
MIN
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MAX
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Ar/Cl2 DISCHARGE
[e]
[Cl2+]
[Ar+]
[Cl-]
 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
MIN
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MAX
Log scale
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EDGE REGION: Ar+ AND Cl2+ FLUXES
 0.9 mm Gap
 0.9 mm Gap
 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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EDGE REGION: Ar+ AND Cl2+ FLUXES
 0.3 mm Gap
 0.3 mm Gap
 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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EDGE REGION: Ar+ FLUX STREAMTRACES
 0.3 mm Gap
 0.3 mm Gap
 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 slide
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EDGE REGION: Cl2+ FLUX STREAMTRACES
 0.9 mm Gap
 0.3 mm Gap
 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 slide
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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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