Electrostatic Effects on Particle Mobility
Mark E. Hogsett
Technology Group
Simco-Ion
mhogsett@simco-ion.com
Presentation Outline
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Basic particle charging
Particle electrostatic attraction (ESA)
Basic charged particle entrainment
Particle/wafer charge remediation (ionization)
Neutral particle entrainment
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Basic Particle Charging
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We don’t know what the actual charge levels for
individual particles are.
The SEMI E78-0309 model assumes a Fuchs charge
distribution, which is minimal and very conservative.
Particles of different materials and mechanical or
chemical sources will charge to different levels.
Due to a wide range of possible charge variation, this
adds further complexity to electrostatic particle attraction.
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Charged Particle/Surface Bonding
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If an electrostatically charged particle has 100 extra
electrons (negative charge):
A 100nm diameter particle is subjected to 425 psi
A 10nm particle is subjected to 4.25 million psi
Particles will remain in place until electrostatic forces are
removed.
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Impact of Electrostatic Charge on Sub-22nm
Particle Mobility
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Smaller particles have
higher mobility and are
more easily attracted by
lower charge levels
Increased attraction causes
higher impact velocities
This causes particle-wafer
surface embedding, which
can defeat wafer cleaning
processes.
Deposition Velocity vs. Particle Diameter
1e+0
1
1e-1
10-1
Deposition Velocity (cm/sec)
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Electrostatic
(at 500 volts/in)
-2
1e-2
10
-3
1e-3
10
Diffusion
1e-4
10-4
Gravitational
-5
1e-5
10
0.01
data from the original publication
was traced and replotted
0.1
1
Particle Diameter ( �m)
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300mm Wafer Tolerable Field Boundary for
Particle Attraction
Technology Node
(nm)
*Tolerable Field E0
(V/cm)
10
15
20
21
30
26
50
34
100
47
200
67
300
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Surface voltages greater
than the tolerable field level
lead to electrostatic particle
attraction as the dominant
attraction model (over
gravity and diffusion
models).
Higher particle charge
reduces tolerable field level.
*Assumes very conservative Fuchs charge for particles.
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Tolerable Field for 10nm Charged Particle
Surface V/cm = 45V
Tolerable Field = 15V/cm
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Charged Particle Attraction to Charged Surface
Charged particle
accelerates to high
terminal velocity
under electric field
attraction as
surface approach
distance decreases
and electric field
gradient increases.
Particle – Surface Dipole Attraction
Basic charged particle,
charged surface model
showing dipole field
formation.
Particle impacts charged
surface at high velocity,
often embedding.
Particle – Surface Repulsion
When surface and
particle charges
are the same
polarity, a field
repulsion effect
occurs.
Particles are
rejected from the
charged surface.
Electric Fields for a Multivalent Charged Surface
x
When insulative
surfaces have areas
of different charge
polarity, attracted
particles are forced
to navigate a
complex
attraction/repulsion
field structure.
Particle Attraction From a Neutral Surface
Charged particles
with only mirror
surface image
charges can be
transferred from
near-proximity
charged surfaces
via strong electric
field attraction.
Particle Attraction for Differentially Charged
Surfaces
When surfaces have
charges of the same
polarity but different
magnitude, particles
can be attracted
from one surface to
another.
FOUP backside
“particle rain” onto
the topside of the
wafer below can
occur.
Airborne Charged Particle Ion Bonding
Air ions bind to
oppositely charged
airborne particles
creating very
compact dipoles.
This reduces the
external radiating
electric field which
reduces the particle
attraction to charged
surfaces to effectively
zero.
Stagnant Flow Point Contamination Models
Vertical laminar
flow models
become stagnantpoint flow models
at the 15cm
tolerable field
boundary.
Electrostatically
attracted particles
are pulled out of
the active flow
region.
Charged Particle Horizontal Airflow Model
Turbulence is at a
minimum, there is
no stagnation
region, and
particles move
with entraining
airflow.
However, ESA can
still pull particles
out of the airflow
and onto the wafer
surface.
“Extended” ISO Class 1 Defined
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Simco-Ion utilizes an in-house standard that extrapolates ISO 14644-1 down
to 0.01 micron (10 nm) particles, measured with a condensation nucleus
counter (CNC). We define this as “Extended” ISO Class 1
Extended ISO Class 1 Limit for number of ≥10nm particles is 34.0
Simco-Ion is the only ionizer supplier that develops and qualifies
product cleanliness at ≥10nm particle size
50.0
Particles per Cubic Foot of Air,≤10 nm
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40.0
Extrapolated ISO Class 1 line for 10 nm particles = 34.0
30.0
20.0
10.0
0.0
0
50
100
150
200
250
300
350
400
Hours
16.3 days
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Nitrogen In-line Ionizers
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Model 4214 designed for in-tool point-of-use ionization
Used primarily for wafer and reticle discharge applications
Single crystal silicon emitter point for cleanliness and process compatibility
4214 ionizes pure nitrogen (0.99999)
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Precision balance, extremely low swing voltage and outstanding cleanliness
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Model 4214 Meets “Extended” ISO Class 1
Cleanliness
Average particles per cubic foot of Ionized nitrogen
≥ 10 nm, Measured 6” below a Model 4214. Results
far exceed Extended ISO Class 1 (at 10 nm) limit
Average Particles ≥ 10 nm per Cubic
Foot of Ionized Gas
Figure 4214-10nm.
340
Extended ISO Class 2 (at 10 nm) limit
= 340
50
40
Extended ISO Class 1 (at 10 nm) limit = 34
30
Particles per cubic foot
averaged 4.7.
20
10
0
0
20
40
60
80
Sequential 1-Cubic-Foot Sample
21.75 hours
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Model 5635 AeroBar MP
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Only corona ionizing bar
on the market to
– Meet Extended ISO Class
1 Cleanliness
– Modulated Pulse
Technology provides
� Fast discharge
� Ultraclean ionization with
long maintenance cycles
� Superior balance and
stability
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Ionizing Bars In Tools Need to Meet “Extended”
ISO Class 1 Cleanliness Levels
Extended ISO Class 1 limit for ≥10nm particles = <34
Particles per Cubic Foot of Air ≥ 10 nm
particles/ft³
100
Model 5635 Mean = 0.52 particles/ft³
10
1
0.1
0.01
0
340
Sequential 5-Hour Average over 5 Months
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Neutral Particle Entrainment
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When there is no electrostatic attraction between particle
and surface….
And particles are entrained in an airflow (vertical or
horizontal)….
Particles are deflected off the stagnation or wafer
surface boundary layer and carried away.
Ionizers discharge both particles and surfaces
simultaneously.
Low particle ionizers have a very low probability of
contributing to the wafer particle burden.
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Neutral Particle Entrainment Model
Particles tend to
follow the flow
vector in the
absence of
attractive forces.
(Particle diffusion
velocity for 10nm is
0.03cm/s, which
limits wafer
exposure)
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Thank you.
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