1

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!