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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