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Introduction to Plasma Immersion Ion Implantation
Technologies
Emmanuel Wirth
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Plan of the presentation
Ion beam / plasma ion immersion Generation of ions Reminder of basic plasma parameters The plasma immersion device Zoom into the substrate zone
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Ion implantation
Ion beam: Conventional ion implantation setups extract a stream of ions from an ion source and focuses them into a beam Expensive
Plasma Immersion: the target is placed inside the ion source itselfMuch cheaper
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Generation of a ionelectron ionisation
An incident electron collides one neutral atom If the incident electron has enough energy
Ee > Ei one electron of the valence layer can be ejected
e -Ee > Ei
Atom e -Ion
e -
Ar Ar+
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Definition: PlasmaMain free path Plasma: partially ionised gas which contains
no net space charges The mean free path is the average distance
between collisions in a gas
SCollision
S= cross sectionN= number of particles per unit volume
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Basic parameters of the plasma 3 species: neutral, ions, electrons Temperature
Ti, Te,Tn
Density ne, ni, nn
Debye length Screening distance over which electric field are
effectively excluded
Ti=Te: balanced plasma
Ti≠Te: unbalanced plasma
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V
AnodePlasmaCathode
CathodeSheath
Generation of Ion: Use of Plasma 3 Processes occur:
Self sustained regime Thermoelectronic
emission Field emission
The e- are accelerated in the cathode sheath
1 0-10
1 0-7
1 0-4
1 0-1
Current, A
Vol
tage
A
B C
D E
F
G
Breakdown
Abnorm al glow
Normal glowSaturation
Arc
H
I-V characteristic of gas discharge
Intensive ionization
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Generation of Ion: Use of Plasma 3 Processes occur:
Self sustained regime
Thermoelectronic emission
Field emission
1 0-10
1 0-7
1 0-4
1 0-1
Current, A
Vol
tage
A
B C
D E
F
G
Breakdown
Abnorm al glow
Normal glowSaturation
Arc
H
α : Number of ionisation per 1 e- for 1 m along Eγ : Number of ions necessary to create one 2nd e- at the cathode
I-V characteristic of gas discharge
Cathode surface
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Generation of Ion: Use of Plasma 3 Processes occur:
Self sustained regime Thermoelectronic
emission Electrons of surface receive
sufficient kinetic energy to pass the potential barrierand are emitted from cathode
Field emission
1 0-10
1 0-7
1 0-4
1 0-1
Current, A
Vol
tage
A
B C
D E
F
G
Breakdown
Abnorm al glow
Normal glowSaturation
Arc
H
I-V characteristic of gas discharge
Cathode surface
+-
+-+-
+-+-
+-
+-
+-+-
+-+-
+-
+-
Avalanche
-
Thermoelectronic emission
+
Intensive Ion bombardmentT E kin of e-
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Generation of Ion: Use of Plasma 3 Processes occur:
Self sustained regime Thermoelectronic
emission Field emissionI increase extra ionisationspace charge approaches
the cathode E near cathode Channelling effect
1 0-10
1 0-7
1 0-4
1 0-1
Current, A
Vol
tage
A
B C
D E
F
G
Breakdown
Abnorm al glow
Normal glowSaturation
Arc
H
I-V characteristic of gas discharge
+-
+-+-
+-+-
+-
+-
+-+-
+-+-
+-
+-
Avalanche
-
Field emission
APlasma
C
CathodeSheath
Cathode surface
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Plasma as a source of ion
Generation of a plasma by auxiliary device DC Magnetron
Immersion of the substrate in the plasma
Plasma
Cathode AnodeU < 0
A field to compensate the negative bias take
place: a sheath is created
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Zoom in the sheath zone
Surface of plasma = source of ions
Positive ions are attracted from the plasma
Within the sheath ρ ≠ 0
U < 0
+-
δ+
+
++
+
+
+
+
+
+
+
+
+ +
+ +-
--
--
-- -
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2 cases for the sheath
If δ < λ Ions pass the sheath
like in vacuum
If δ > λ Ions perform
collisions
You have scattering of ions
Ions go directly to substrate
It is very important to have an estimation of the thickness of the sheath δ
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If δ < λ: expression of δ
If gap can be considered as vacuum, you can calculate the thickness of the sheath layer
Start from the Poisson equation
You can estimate x= δ
You measure V, ji mi: mass of the ionqi: charge of the ion
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Conclusion
Plasma is a cheaper way to implant ions Plasma is formed by a gas discharge Negative bias voltage on substrate create a
sheath The vacuum chamber should be larger than δ!