Chapter 8 Ion Implantation

1
Chapter 8Ion Implantation
2
Ion Implantation

• Introduction
• Hardware
• Processes
• Summary

3
Wafer Process Flow
IC Fab
4
Introduction

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

5
Dope Semiconductor Diffusion

• Isotropic process
• Cant independently control dopant profile and
  dopant concentration
• Mostly replaced by ion implantation after its
  introduction in mid-1970s.
• Still used for dopant drive-in

6
Dope Semiconductor Ion Implantation

• Used for atomic and nuclear research
• Independently control dopant profile (ion energy)
  and dopant concentration (ion current times
  implantation time)
• Anisotropic dopant profile

7
Ion Implantation, Phosphorus
P
Poly Si
SiO2
n
n
P-type Silicon
8
Misalignment of the Gate
Gate Oxide
Metal Gate
Metal Gate
p S/D
p S/D
n-Si
n-Si
Aligned
Misaligned
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Comparison of Implantation and Diffusion
Doped region
PR
SiO2
Si
Si
Junction depth
Ion implantation
Diffusion
10
Comparison of Implantation and Diffusion
11
Other Applications

• Oxygen implantation for silicon-on-insulator
  (SOI) device
• Pre-amorphous silicon implantation on titanium
  film for better annealing
• Pre-amorphous germanium implantation on silicon
  substrate for profile control

12
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

13
Stopping Mechanism

• The total stopping power
• Stotal Sn Se
• Sn nuclear stopping, Se electronic stopping
• Low E, high A ion implantation mainly nuclear
  stopping
• High E, low A ion implantation, electronic
  stopping mechanism is more important

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Stopping Mechanisms
Ion
Random Collisions (SSnSe)
Channeling (S?Se)
Back Scattering (S?Sn)
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Ion Projection Range
ln (Concentration)
Projected Range
Substrate Surface
Depth from the Surface
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Projected Range in Silicon
1.000
P
B
Projected Range (mm)
0.100
As
Sb
0.010
10
100
1000
Implantation Energy (keV)
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Barrier Thickness to Block 200 keV Ion Beam
1.20
1.00
0.80
B
0.60
P
Mask Thickness (micron)
0.40
As
0.20
Sb
0.00
PR
Si
SiO2
Si3N4
Al
18
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
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Channeling Effect
Lattice Atoms
Channeling Ion
Collisional Ion
q
Wafer Surface
20
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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Shadowing Effect
Ion Beam
Polysilicon
Doped Region
Substrate
Shadowed Region
22
Shadowing Effect
After Annealing and Diffusion
Polysilicon
Doped Region
Substrate
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Lattice Damage With One Ion
Light Ion
Damaged Region
Heavy Ion
Single Crystal Silicon
24
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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Implantation Processes Annealing
After Annealing
Before Annealing
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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 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

30
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
32
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

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Ion Source
Source Gas or Vapor
Arc Power 120 V
Tungsten Filament
Anti-cathode

-
Filament Power, 0-5V, up to 200A
Plasma
Magnetic Field Line
Source Magnet
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RF Ion Source
Dopant Gas
RF Coils

RF
Plasma
-
Extraction Electrode
Ion Beam
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Microwave Ion Source
Microwave
Magnetic Coils
ECR Plasma
Magnetic Field Line
Extraction Electrode
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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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Extraction Assembly
Suppression Electrode
Extraction Electrode
Top View
Ion Source
Plasma
Ion Beam


Slit Extracting Ion Beam
Extraction Power, up to 60 kV
Suppression Power, up to 10 kV
Terminal Chassis


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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
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Post Acceleration
Suppression Electrode
Acceleration Electrode
Ion Beam


Post Accel. Power, up to 60 kV
Suppression Power, up to 10 kV

Terminal Chassis

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Charge Neutralization System

• Implanted ions charge wafer positively
• Cause wafer charging effect
• Expel positive ion, cause beam blowup and result
  non-uniform dopant distribution
• Discharge arcing create defects on wafer
• Breakdown gate oxide, low yield
• Need eliminate or minimize charging effect

42
Charging Effect
Ions trajectory

Wafer
43
Charge Neutralization System

• Need to provide electrons to neutralize ions
• Plasma flooding system
• Electron gun
• Electron shower are used to

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Ion Implantation Plasma Flooding System

• Ions cause wafer charging
• Wafer charging can cause non-uniform doping and
  arcing defects
• Electrons are flooding into ion beam and
  neutralized the charge on the wafer
• Argon plasma generated by thermal electrons emit
  from hot tungsten filament

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Plasma Flooding System
Ar
DC Power
Tungsten Filament
Ion Beam

?
Filament Current
Plasma
Electrons
Wafer
46
Electron Gun
Secondary Electron Target
Secondary Electrons
Electrons
Ion Beam
Electron Gun
Thermal Filament
Wafer
47
Spin Wheel
Wafers
Spin arm
Spin rate to 2400 rpm
Ion beam
Swing period 10 sec
Implanted stripe
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Ion Implantation The Process

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

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CMOS Implantation Requirements
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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
54
Process Evaluation

• Four-point probe
• Thermal wave
• Optical measurement system (OMS)

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Four-Point Probe

• Perform after anneal
• Measure sheet resistance
• Sheet resistant is a function of dopant
  concentration and junction depth
• Commonly used to monitor doping process

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Four-Point Probe Measurement
I
V
P1
P2
P3
P4
S1
S2
S3
Dope Region
Substrate
R r L/Wt Rs r/t (sheet resistance)
57
Thermal Wave System

• Argon pump laser generates thermal pulses on
  wafer surface
• He-Ne probe laser measures DC reflectivity (R)
  and reflectivity modulation induced by the pump
  laser (DR) at the same spot
• Ratio DR/R is called thermal wave (TW) signal,
• TW signal DR/R related to the crystal damage
• crystal damage is a function of the implant dose

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Thermal Wave System
Pump Laser
I
DR
Thermal Waver Signal Detector
R
I
t
t
Probe Laser
DR/R Thermal Wave Signal
Wafer
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Thermal Wave System

• Performed immediately after the implant process
• Four-point probe needs anneal first
• Non-destructive, can measure production wafers
• Four-point probe is only good for test wafers
• Low sensitivity at low dosage
• Drift of the TW signal over time
• needs to be taken as soon as the implantation
  finished
• Dont have very high measurement accuracy
• Laser heating relax crystal damage

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Optical Measurement System (OMS)

• transparent wafer coated a with a thin layer of
  copolymer, which contains energy sensitive dye
• During ion implantation, energetic ions collide
  with dye molecules and break them down
• Makes the copolymer becomes more transparent
• The higher the dosage, the higher the
  transparency
• Photon count change before and after implantation
• Determine dosage of certain ion at certain energy

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Optical Measurement System (OME)
Quartz Halogen Lamp
600 nm Filter
Photo Detector
PDI Count
PDI Count
Before Implantation
After Implantation
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CMOS on SOI Substrate
n source/drain
p source/drain
Gate oxide
Polysilicon
STI
USG
p-Si
n-Si
Buried oxide
Balk Si
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SOI Formation

• Implanted wafers
• Heavy oxygen ion implantation
• High temperature annealing
• Bonded wafers
• Two wafers
• Grow oxide on one wafer
• High temperature bond wafer bonding
• Polish one wafer until thousand Å away from SiO2

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Oxygen Ion Implantation
Silicon with lattice damage
Oxygen rich silicon
Balk Si
65
High Temperature Annealing

Single crystal silicon
Silicon dioxide
Balk Si
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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.

67
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