Ion Implantation

1
Ion Implantation

• M.H.Nemati
• Sabanci University

2
Ion Implantation

• Introduction
• Safety
• Hardware
• Processes
• Summary

3
Introduction

• Dope semiconductor
• Two way to dope
• Diffusion
• Ion implantation
• Other application of ion implantation

4
Dope Semiconductor Diffusion

• Isotropic process
• Cant independently control dopant profile and
  dopant concentration
• Replaced by ion implantation after its
  introduction in mid-1970s.

5
Dope Semiconductor Ion Implantation

• Used for atomic and nuclear research
• Early idea introduced in 1950s
• Introduced to semiconductor manufacturing in
  mid-1970s.

6
Dope Semiconductor Ion Implantation

• Independently control dopant profile (ion energy)
  and dopant concentration (ion current times
  implantation time)
• Anisotropic dopant profile
• Easy to achieve high concentration dope of heavy
  dopant atom such as phosphorus and arsenic.

7
Ion Implantation, Phosphorus
P
Poly Si
SiO2
n
n
P-type Silicon
8
Ion Implantation Control

• Beam current and implantation time control dopant
  concentration
• Ion energy controls junction depth
• Dopant profile is anisotropic

9
Stopping Mechanism

• Ions penetrate into substrate
• Collide with lattice atoms
• Gradually lose their energy and stop
• Two stop mechanisms

10
Two Stopping Mechanism

• Nuclear stopping
• Collision with nuclei of the lattice atoms
• Scattered significantly
• Causes crystal structure damage.
• electronic stopping
• Collision with electrons of the lattice atoms
• Incident ion path is almost unchanged
• Energy transfer is very small
• Crystal structure damage is negligible

11
Implantation Processes Channeling

• If the incident angle is right, ion can travel
  long distance without collision with lattice
  atoms
• It causes uncontrollable dopant profile

Lots of collisions
Very few collisions
12
Channeling Effect
Lattice Atoms
Channeling Ion
Collisional Ion
q
Wafer Surface
13
Implantation Processes Channeling

• Ways to avoid channeling effect
• Tilt wafer, 7 is most commonly used
• Screen oxide
• Pre-amorphous implantation, Germanium
• Shadowing effect
• Ion blocked by structures
• Rotate wafer and post-implantation diffusion

14
Implantation Processes Damage

• Ion collides with lattice atoms and knock them
  out of lattice grid
• Implant area on substrate becomes amorphous
  structure

Before Implantation
After Implantation
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Implantation Processes Anneal

• Dopant atom must in single crystal structure and
  bond with four silicon atoms to be activated as
  donor (N-type) or acceptor (P-type)
• Thermal energy from high temperature helps
  amorphous atoms to recover single crystal
  structure.

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Thermal Annealing
Dopant Atom
Lattice Atoms
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Thermal Annealing
Dopant Atom
Lattice Atoms
18
Thermal Annealing
Dopant Atom
Lattice Atoms
19
Thermal Annealing
Dopant Atom
Lattice Atoms
20
Thermal Annealing
Dopant Atom
Lattice Atoms
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Thermal Annealing
Dopant Atom
Lattice Atoms
22
Thermal Annealing
Dopant Atom
Lattice Atoms
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Thermal Annealing
Dopant Atoms
Lattice Atoms
24
Implantation Processes Annealing
After Annealing
Before Annealing
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Ion Implantation Hardware

• Gas system
• Electrical system
• Vacuum system
• Ion beamline

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Implantation Process
Gases and Vapors P, B, BF3, PH3, and AsH3
Next Step
Implanter
Select Ion B, P, As
Select Ion Energy
Select Beam Current
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Ion Implanter
Electrical System
Gas Cabin
Analyzer Magnet
Vacuum Pump
Ion Source
Beam Line
Electrical System
Vacuum Pump
Wafers
Plasma Flooding System
End Analyzer
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Ion Implantation Gas System

• Special gas deliver system to handle hazardous
  gases
• Special training needed to change gases bottles
• Argon is used for purge and beam calibration

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Ion Implantation Electrical System

• High voltage system
• Determine ion energy that controls junction depth
• High voltage system
• Determine ion energy that controls junction depth
• RF system
• Some ion sources use RF to generate ions

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Ion Implantation Vacuum System

• Need high vacuum to accelerate ions and reduce
  collision
• MFP gtgt beamline length
• 10-5 to 10-7 Torr
• Turbo pump and Cryo pump
• Exhaust system

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Ion Implantation Control System

• Ion energy, beam current, and ion species.
• Mechanical parts for loading and unloading
• Wafer movement to get uniform beam scan
• CPU board control boards
• Control boards collect data from the systems,
  send it to CPU board to process,
• CPU sends instructions back to the systems
  through the control board.

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Ion Implantation Beamline

• Ion source
• Extraction electrode
• Analyzer magnet
• Post acceleration
• Plasma flooding system
• End analyzer

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Ion Beam Line
Suppression Electrode
Analyzer Magnet
Vacuum Pump
Ion Source
Beam Line
Extraction Electrode
Post Acceleration Electrode
Vacuum Pump
Plasma Flooding System
Wafers
End Analyzer
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Ion implanter Ion Source

• Hot tungsten filament emits thermal electron
• Electrons collide with source gas molecules to
  dissociate and ionize
• Ions are extracted out of source chamber and
  accelerated to the beamline
• RF and microwave power can also be used to ionize
  source gas

35
Ion Implantation Extraction

• Extraction electrode accelerates ions up to 50
  keV
• High energy is required for analyzer magnet to
  select right ion species.

36
Ion Implantation Analyzer Magnet

• Gyro radius of charge particle in magnetic field
  relate with B-field and mass/charge ratio
• Used for isotope separation to get enriched U235
• Only ions with right mass/charge ratio can go
  through the slit
• Purified the implanting ion beam

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Analyzer
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Ion Implantation The Process

• CMOS applications
• CMOS ion implantation requirements
• Implantation process evaluations

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Implantation Process Well Implantation

• High energy (to MeV), low current (1013/cm2)

P
Photoresist
N-Well
P-Epi
P-Wafer
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Implantation Process VT Adjust Implantation
Low Energy , Low Current
B
Photoresist
USG
STI
P-Well
N-Well
P-Epi
P-Wafer
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Lightly Doped Drain (LDD) Implantation

• Low energy (10 keV), low current (1013/cm2)

P
Photoresist
USG
STI
P-Well
N-Well
P-Epi
P-Wafer
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Implantation Process S/D Implantation

• Low energy (20 keV), high current (gt1015/cm2)

P
Photoresist
n
n
STI
USG
P-Well
N-Well
P-Epi
P-Wafer
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Process Issues

• Wafer charging
• Particle contamination
• Elemental contamination
• Process evaluation

44
Ion Implantation Safety

• One of most hazardous process tools in
  semiconductor industry
• Chemical
• Electro-magnetic
• Mechanical

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Summary of Ion Implantation

• Dope semiconductor
• Better doping method than diffusion
• Easy to control junction depth (by ion energy)
  and dopant concentration ( by ion current and
  implantation time).
• Anisotropic dopant profile.

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Summary of Ion Implantation

• Ion source
• Extraction
• Analyzer magnets
• Post acceleration
• Charge neutralization system
• Beam stop

47
Summary of Ion Implantation

• Well High energy, low current
• Source/Drain Low energy, high current
• Vt Adjust Low energy, low current
• LDD Low energy, low current