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Ion ImplantationM.H.Nemati

Sabanci University

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Ion Implantation

• Introduction

• Safety

• Hardware

• Processes

• Summary

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Introduction

• Dope semiconductor

• Two way to dope– Diffusion– Ion implantation

• Other application of ion implantation

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Dope Semiconductor: Diffusion

• Isotropic process

• Can’t independently control dopant profile and dopant concentration

• Replaced by ion implantation after its introduction in mid-1970s.

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Dope Semiconductor: Ion Implantation

• Used for atomic and nuclear research

• Early idea introduced in 1950’s

• Introduced to semiconductor manufacturing in mid-1970s.

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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.

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Ion Implantation, Phosphorus

Poly Si

n+

P-type Silicon

n+

SiO2P+

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Ion Implantation Control

• Beam current and implantation time control dopant concentration

• Ion energy controls junction depth

• Dopant profile is anisotropic

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Stopping Mechanism

• Ions penetrate into substrate

• Collide with lattice atoms

• Gradually lose their energy and stop

• Two stop mechanisms

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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

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Implantation Processes: Channeling

• If the incident angle is right, ion can travel long distance without collision with lattice atoms

• It causes uncontrollable dopant profile

Very few collisions

Lots of collisions

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Channeling Effect

Channeling Ion

Collisional Ion

Lattice Atoms

Wafer Surface

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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

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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 AtomLattice Atoms

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Thermal Annealing

Dopant AtomLattice Atoms

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Thermal Annealing

Dopant AtomLattice Atoms

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Thermal Annealing

Dopant AtomLattice Atoms

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Thermal Annealing

Dopant AtomLattice Atoms

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Thermal Annealing

Dopant AtomLattice Atoms

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Thermal Annealing

Dopant AtomLattice Atoms

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Thermal Annealing

Dopant AtomsLattice Atoms

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Implantation Processes: Annealing

Before Annealing After Annealing

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Ion Implantation: Hardware

• Gas system

• Electrical system

• Vacuum system

• Ion beamline

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Implantation ProcessGases and Vapors:

P, B, BF3, PH3, and AsH3

Select Ion: B, P, As

Select Ion Energy

Select Beam Current

Next StepImplanter

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Ion Implanter

Gas Cabin

Ion Source

Vacuum Pump

Vacuum Pump

Electrical System

Electrical System

Analyzer Magnet

Beam Line

End Analyzer

WafersPlasma Flooding System

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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 >> 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

Ion Source

Vacuum Pump

Vacuum Pump

Analyzer Magnet

Beam Line

End Analyzer

Wafers

Plasma Flooding System

Post Acceleration Electrode

Extraction Electrode

Suppression Electrode

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• 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

Ion implanter: Ion Source

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Ion Implantation: Extraction

• Extraction electrode accelerates ions up to 50 keV

• High energy is required for analyzer magnet to select right ion species.

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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

Ion Beam

Smaller m/q Ratio

Larger m/q Ratio

Right m/q Ratio

Magnetic Field (Point Outward)

Flight Tube

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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-EpiP-Wafer

Photoresist

N-Well

P+

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PhotoresistB+

P-EpiP-Wafer

N-WellP-WellSTI USG

Implantation Process: VT Adjust Implantation

Low Energy , Low Current

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Photoresist

P+

P-EpiP-Wafer

N-WellP-WellSTI USG

Lightly Doped Drain (LDD) Implantation

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

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Implantation Process: S/D Implantation

• Low energy (20 keV), high current (>1015/cm2)

P-EpiP-Wafer

N-WellP-Well

Photoresist

P+

STI USGn+n+

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Process Issues

• Wafer charging

• Particle contamination

• Elemental contamination

• Process evaluation

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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

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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