Electrostatic Effects on Particle Mobility

Mark E. HogsettTechnology GroupSimco-Ionmhogsett@simco-ion.com

Presentation Outline

� Basic particle charging

� Particle electrostatic attraction (ESA)

� Basic charged particle entrainment

� Particle/wafer charge remediation (ionization)

� Neutral particle entrainment

s i m c o - i o n c o n f i d e n t i a l 2

Basic Particle Charging

� 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.

s i m c o - i o n c o n f i d e n t i a l 3

Charged Particle/Surface Bonding

� 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.

s i m c o - i o n c o n f i d e n t i a l 4

s i m c o - i o n c o n f i d e n t i a l

Impact of Electrostatic Charge on Sub-22nm Particle Mobility

� 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. Particle Diameter ( �m)

0.01 0.1 1 10

Dep

ositi

on V

eloc

ity (c

m/s

ec)

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

data from the original publication

was traced and replotted10-5

10-4

10-3

10-2

10-1

1

Gravitational

Electrostatic

Diffusion

Deposition Velocity vs. Particle Diameter

(at 500 volts/in)

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 82

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.

6

*Assumes very conservative Fuchs charge for particles.

7

Tolerable Field for 10nm Charged Particle

Surface V/cm = 45VTolerable Field = 15V/cm

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 effectoccurs.

Particles are rejected from the charged surface.

Electric Fields for a Multivalent Charged Surface

x When insulativesurfaces 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 stagnant-point 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� 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

s i m c o - i o n c o n f i d e n t i a l16.3 days

0.0

10.0

20.0

30.0

40.0

50.0

0 50 100 150 200 250 300 350 400

Hours

Par

ticle

s pe

r C

ubic

Foo

t of A

ir, ≤1

0 nm

Extrapolated ISO Class 1 line for 10 nm particles = 34.0

0.0

10.0

20.0

30.0

40.0

50.0

0 50 100 150 200 250 300 350 400

Hours

Par

ticle

s pe

r C

ubic

Foo

t of A

ir,

Extrapolated ISO Class 1 line for 10 nm particles = 34.0

18

Nitrogen In-line Ionizers

� 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)� Precision balance, extremely low swing voltage and outstanding cleanliness

s i m c o - i o n c o n f i d e n t i a l

Model 4214 Meets “Extended” ISO Class 1 Cleanliness

s i m c o - i o n c o n f i d e n t i a l

0

10

20

30

40

50

340

0 20 40 60 80

Ave

rag

e P

art

icle

s ≥

10

nm

pe

r C

ub

ic

Fo

ot

of

Ion

ize

d G

as

Figure 4214-10nm. 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

Sequential 1-Cubic-Foot Sample

21.75 hours

Particles per cubic footaveraged 4.7.

Extended ISO Class 1 (at 10 nm) limit = 34

Extended ISO Class 2 (at 10 nm) limit = 340

20

Model 5635 AeroBar MP

� 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

Ionizing Bars In Tools Need to Meet “Extended” ISO Class 1 Cleanliness Levels

s i m c o - i o n c o n f i d e n t i a l

0.01

0.1

1

10

100

Pa

rtic

les

pe

r C

ub

ic F

oo

t o

f A

ir ≥

10

nm

0 340

Sequential 5-Hour Average over 5 Months

Model 5635 Mean = 0.52 particles/ft³

Extended ISO Class 1 limit for ≥10nm particles = <34

particles/ft³

Neutral Particle Entrainment

� 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.

s i m c o - i o n c o n f i d e n t i a l 22

Neutral Particle Entrainment Model

s i m c o - i o n c o n f i d e n t i a l 23

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)

Thank you.

s i m c o - i o n c o n f i d e n t i a l 24