Transcript Document 7290874

Section 6: Ion Implantation
Jaeger Chapter 5
EE143 – Ali Javey
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. 20-30% for high
temperature pre-deposition processes)
– Low Temperature Process
• Expensive Systems
• Vacuum System
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Force on charged particle
Equipment
Magnetic Field

Fq vxB
B
2mV
qr 2
T
Implanted Dose
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
1
Q
I t dt

mqA 0
Ion Implantation
C(x)
+
y
x
as-implant
depth profile
Blocking mask
Si
Equal-Concentration
contours
Depth x
Reminder: During implantation, temperature is ambient. However,
post-implant annealing step (>900oC) is required to anneal out defects.
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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
n+
n+
p-Si
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SiO2
Ion Implantation Energy Loss Mechanisms
Nuclear
stopping
Si
+
Si
+
Crystalline Si substrate damaged by collision
e
Electronic
stopping
+
Si
e
+
Electronic excitation creates heat
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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+
Electronic stopping
dominates
B+
Electronic stopping
dominates
As+
Nuclear stopping
dominates
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Stopping Mechanisms
B into Si
P into Si
As into Si
E1(keV)
3
17
73
E2(keV)
17
140
800
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Electronic / Nuclear Stopping: Damage
Sn  dE/dx|n
Se  dE/dx|e
Depth x
Surface
E~0
E=Eo
Substrate
A+
Se
Se
Sn
Sn
More damage at
end of range Sn > Se
Less damage
Se > Sn
x ~ Rp
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Eo =
incident
kinetic
energy
Simulation of 50keV Boron implanted into Si
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Model for blanket implantation
Gaussian Profile
 x  R p 2 
N x   N p exp 
2 
 2R p 
R p  Projected Range
R p  Straggle

Dose
Q =  N x dx  2 N p R p
0
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Projected Range and Straggle
Rp and Rp values are given in tables or charts
e.g. see pp. 113 of Jaeger
Note: this means 0.02 m.
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Selective Implantation
N  x, y   N  x F  y 
 ya 
 y  a 
1



F  y   erfc
 erfc


2 
2

R
2

R
 
 



R  transverse straggle
Nx  is one - dimensiona l solution
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Transverse (or Lateral) Straggle (Rt or  R)
Rt
Rp >1
Rt
Rp
Rt
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Feature Enlargement due to lateral straggle
y
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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Definitions of Profile Parameters

(1) Dose   0 C x dx

x  C x dx


1
(3) Longitudinal Straggle: R    x  R   C x dx
(2) Projected Range: R p 
1
0

2

p
(4) Skewness: M 3 
1

0
(x - Rp ) C(x )dx,


3
0
2
p
M 3 > 0 or < 0
-describes asymmetry between left side and right side
(5) Kurtosis: 
 x  R  C x dx

0
4
p
C(x)
Kurtosis characterizes the
contributions of the “tail” regions
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Rp
x
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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Transmission Factor of Implantation Mask
Mask material (e.g. photoresist)
C(x)
What fraction of
dose gets into
Si substrate?
Si substrate
x=0
C(x)
x=d
Mask material with
d=
x=0
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x=d
Transmitted Fraction

T  0 Cx dx  
x dx
d
C
0
Rp , Rp
 d  Rp 
1
 erfc 

2
 2Rp 
erfcx   1 
2


x y2
e dy
0
are values of
for ions into
the masking material
Rule of thumb : Good masking thickness
d  Rp  4.3Rp
Cx  d 
4
~ 10
Cx  Rp 
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Junction Depth
The junction depth is
calculated from the point at
which the implant profile
concentration = bulk
concentration:
N x j   N B
 x j  R p 2 
N p exp 
  NB
2
2R p 

x j  R p  R p
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 Np 

2 ln 
 NB 
Sheet Resistance RS of Implanted Layers
Example:
n
n-type dopants
implanted
into p-type substrate
RS 
x =0
x =xj
p-sub (CB)
x
1

xj
0
q   x  Cx   CB  dx

C(x) log scale
n
CB
p
1017
1019
Total
doping conc
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xj
x
Approximate Value for RS
If C(x) >>CB for most depth x of interest
and use approximation: (x) ~ constant
 Rs 
Rs 
1
q  C x  dx
xj
0

1
q
This expression assumes ALL
implanted dopants are 100%
electrically activated
1
q
 R   ohm
s
use the  for the highest
doping region which carries
most of the current
or ohm/square
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Example Calculations
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 m , Rp=0.0775m
(a) Find peak concentration
Cp = (0.4 x 1013)/(0.0775 x10-4) = 5.2 x1017/cm3
(b) Find junction depths
CBB with xj in m
(b) Cp exp[ -( xj-0.254)2/ 2 Rp2]= N
Rp
 ( xj - 0.254)2 = 2 (0.0775)2 ln [ 5.2 1017/1016]
or xj = 0.254 ± 0.22 m ; xj1 = 0.032 m and xj2 = 0.474 m
x
x
j1
Phosphorus
Implant
j2
p-substrate (1E16 /cm3)
(c) Find sheet resistance
From the mobility curve for electrons (using peak conc as impurity conc), n= 350 cm2 /V-sec
1
1
Rs =
=
 1780 /square.
qn 1.610-19 350 1013
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Channeling
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Use of tilt to reduce channeling
Random component
C(x)
channeled
component
Lucky ions fall into
channel despite tilt
x
Random
Planar Channeling
Axial Channeling
o
To minimize channeling, EE143
we tilt– Ali
wafer
Javeyby 7 with respect to ion beam.
Prevention of Channeling by Pre-amorphization
Step 1
+
Si
High dose Si+
implantation to covert
surface layer into
2
1
E15/cm
amorphous Si
Step 2
Implantation of
desired dopant
into amorphous
surface layer
Si crystal
Amorphous Si
Si crystal
B+
Disadvantage : Needs an additional high-dose implantation step
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Kinetic Energy of Multiply Charged Ions
With Accelerating Voltage = x kV
Singly
charged
Doubly
charged
Triply
charged
B+
P+
As+
Kinetic Energy = x · keV
B++
Kinetic Energy = 2x · keV
B+++
Kinetic Energy = 3x · keV
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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Molecular Ion Implantation
Kinetic Energy = x keV
B
F
F
BF2+
+
accelerating voltage
= x kV
Molecular ion will
dissociate immediately
into atomic components
after entering a solid.
All atomic components
will have same velocity
after dissociation.
Velocity
Solid
Surface
B has 11 amu
F has 19 amu
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vB  vF  vF
1
2
mB  vB
2
1
2
K .E. of F  m F  v B
2
K .E. of B
11

 20%

11  19  19
K .E. of BF2
K .E. of B 
Implantation Damage
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Amount and type of Crystalline Damage
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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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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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Shallow Implantation
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Rapid Thermal Annealing
•Rapid Heating
•950-1050o C
•>50o C/sec
•Very Low Dt
(b)
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Dose-Energy Application Space
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