1. Section 6: Ion Implantation
Jaeger Chapter 5
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2. Ion Implantation - Overview
Wafer is Target in High Energy Accelerator
Impurities “Shot” into Wafer
Preferred Method of Adding Impurities to Wafers
Wide Range of Impurity Species (Almost Anything)
Tight Dose Control (A few % vs % for high temperature pre-deposition processes)
Low Temperature Process
Expensive Systems
Vacuum System
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3. Equipment
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4. Ion Implantation as-implant C(x) + depth profile y Blocking mask Si
Equal-Concentration
contours
Depth x x
Reminder: During implantation, temperature is ambient. However,
post-implant annealing step (>900oC) is required to anneal out defects.
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5. Advantages of Ion Implantation
Precise control of dose and depth profile
Low-temp. process (can use photoresist as mask)
Wide selection of masking materials
e.g. photoresist, oxide, poly-Si, metal
Less sensitive to surface cleaning procedures
Excellent lateral uniformity (< 1% variation across 12” wafer)
Application example: self-aligned MOSFET source/drain regions
As+
As+
As+
Poly Si Gate
SiO2 n+ n+
p-Si
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6. Ion Implantation Energy Loss MechanismsSi
Nuclear
stopping Si + +
Crystalline Si substrate damaged by collision e e + +
Electronic
stopping Si
Electronic excitation creates heat
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7. Ion Energy Loss Characteristics
Light ions/at higher energy more electronic stopping
Heavier ions/at lower energy more nuclear stopping
EXAMPLES
Implanting into Si: H+ B+
As+
Electronic stopping
dominates
Nuclear stopping
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8. Stopping Mechanisms
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9. Electronic / Nuclear Stopping: Damage
Sn  dE/dx|n
Se  dE/dx|e
Depth x
Surface
E ~ 0
E=Eo
Substrate A+ Se Se
Eo =
incident
kinetic
energy Sn Sn
More damage at end of range Sn > Se
Less damage
Se > Sn
x ~ Rp
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10. Simulation of 50keV Boron implanted into Si
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11. Model for blanket implantation
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12. Projected Range and Straggle
Rp and DRp values are given in tables or charts
e.g. see pp. 113 of Jaeger
Note: this means 0.02 m.
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13. Selective Implantation
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14. Transverse (or Lateral) Straggle (Rt or  R)
>1
Rp
Rt
Rp
Rt
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15. Feature Enlargement due to lateral straggley
Mask x
x = Rp
Lower
concentration
Higher concentration
Implanted species
has lateral distribution,
larger than mask opening
C(y) at x=Rp y
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16. Definitions of Profile Parameters
(1) Dose
(2) Projected Range:
(3) Longitudinal Straggle: 1
(4) Skewness: ( ) 3 ( ) ò M x - Rp C x dx , M
> or
< 3 3 f
-describes asymmetry between left side and right side
(5) Kurtosis:
C(x)
Kurtosis characterizes the
contributions of the “tail” regions x Rp
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17. Selective Implantation – Mask thickness
Desire Implanted Impurity Level to be Much Less Than Wafer Doping
N(X0) << NB or
N(X0) < NB/10
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18. Transmission Factor of Implantation Mask
Mask material (e.g. photoresist)
C(x)
What fraction of
dose gets into
Si substrate?
Si substrate
x=0
x=d
C(x)
Mask material with d= -
x=0
x=d
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19. ( ) ( ) ( ) ( ) ( ) ò ò ò Transmitted Fraction T = C x dx - C x dx ì 1( ) d ( ) T = ò C x dx - ò C x dx ì 1 d - R ü = p
erfc
are values of
for ions into
the masking material í ý 2 2 D R î þ p 2 ( ) x - y 2
erfc x = 1 - ò e dy p
Rule of
thumb
Good
masking
thickness : ( ) C x = d d R p = + 4 3 D . - 4 ~ 10 ( ) C x = R p
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20. Junction Depth
The junction depth is calculated from the point at which the implant profile concentration = bulk concentration:
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21. Sheet Resistance RS of Implanted Layers
x =0
Example:
n-type dopants
implanted
into p-type substrate
p-sub (CB) n
x =xj x x
C(x) log scale xj CB 
1017
1019 n
Total
doping conc p
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22. Approximate Value for RS
If C(x) >>CB for most depth x of interest
and use approximation: (x) ~ constant
This expression assumes ALL
implanted dopants are 100%
electrically activated
use the m for the highest
doping region which carries
most of the current
or ohm/square
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23. 200 keV Phosphorus is implanted into a p-Si ( CB= 1016/cm3) with a dose of 1013/cm2 .
From graphs or tables , Rp =0.254 mm , Rp=0.0775mm
(a) Find peak concentration
Cp = (0.4 x 1013)/( x10-4) = 5.2 x1017/cm3
(b) Find junction depths
(c) Find sheet resistance
Example Calculations CB
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24. Channeling
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25. Use of tilt to reduce channeling
C(x)
Random component
Lucky ions fall into channel despite tilt
channeled
component x
Axial Channeling
Planar Channeling
Random
To minimize channeling, we tilt wafer by 7o with respect to ion beam.
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26. Prevention of Channeling by Pre-amorphization
Si+
1 E15/cm2
Si crystal
Step 1
High dose Si+
implantation to covert
surface layer into
amorphous Si B+
Si crystal
Amorphous Si
Step 2
Implantation of
desired dopant
into amorphous
surface layer
Disadvantage : Needs an additional high-dose implantation step
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27. Kinetic Energy of Multiply Charged Ions
With Accelerating Voltage = x kV B+ P+
As+
Kinetic Energy = x · keV
B+++
B++
Kinetic Energy = 2x · keV
Kinetic Energy = 3x · keV
Singly
charged
Doubly
charged
Triply
charged
Note: Kinetic energy is expressed in eV . An electronic charge q experiencing a voltage drop of 1 Volt will gain a kinetic energy of 1 eV
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28. Molecular Ion Implantation
Kinetic Energy = x keV
Molecular ion will
dissociate immediately
into atomic components
after entering a solid.
All atomic components
will have same velocity
after dissociation. B F
BF2+
accelerating voltage
= x kV + -
Solid
Surface
B has 11 amu
F has 19 amu
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29. Implantation Damage
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30. Amount and type of Crystalline Damage
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31. Post-Implantation Annealing Summary
After implantation, we need an annealing step.
A typical anneal will:
(1) Restore Si crystallinity.
(2) Place dopants into Si substitutional sites
for electrical activation
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32. Deviation from Gaussian Theory
Curves deviate from Gaussian for deeper implants (> 200 keV)
Curves Pearson Type-IV Distribution Functions (~sum of 4 Gaussians)
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33. Shallow Implantation
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34. Rapid Thermal Annealing
Rapid Heating
o C
>50o C/sec
Very Low Dt
(b)
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35. Dose-Energy Application Space
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