ION IMPLANTATION - Chapter 8
Basic Concepts
• Ion implantation is the dominant method of doping used today. In spite of
creating enormous lattice damage it is favored because:
• Large range of doses - 1011 to 1016 /cm2
• Extremely accurate dose control
• Essential for MOS VT control
• Buried (retrograde) profiles are possible
• Low temperature process
• Wide choice of masking materials
• There are also some
significant disadvantages:
• Damage to crystal.
• Anomalous transiently
enhanced diffusion (TED).
upon annealing this damage.
• Charging of insulating
layers.
© 2000 by Prentice Hall
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Ran ge
R
A. Implant Profiles
• At its heart ion implantation is a random
process.
• High energy ions (1-1000keV) bombard
the substrate and lose energy through
nuclear collisions and electronic drag forces.
Proje cte d ran ge
RP
Vacu u m
S ilicon
• Profiles can often be described
by a Gaussian distribution, with
a projected range and standard
deviation. (200keV implants
shown.)
Heavy atoms have smaller projected range and
smaller spread = struggle Rp
2 

x

R
 P  
C(x)  CP exp
 2R2 
P 

(1)

Q   Cxdx or Q  2 RP CP (2)



where Q is the dose in ions cm-2 and is measured by the
integrated beam current.
-2 to 1*1016 cm-2 used in MOS ICs
Doses 1*1012 cm
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Phos
Energy
(keV)
10
20
30
40
50
60
70
80
90
100
120
140
160
180
200
Range
(µm )
0.0199
0.0342
0.0473
0.0598
0.0717
0.0833
0.0947
0.105
0.116
0.127
0.148
0.169
0.189
0.210
0.229
As
Std De v
(µm )
0.0064
0.0125
0.0179
0.0229
0.0275
0.0317
0.0356
0.0393
0.0428
0.0461
0.0522
0.0579
0.0630
0.0678
0.0723
Range
(µm )
0.0084
0.0156
0.0226
0.0294
0.0362
0.0429
0.0495
0.0561
0.0626
0.0692
0.0821
0.0950
0.107
0.120
0.133
Sb
Std De v
(µm )
0.0043
0.0075
0.0102
0.0128
0.0152
0.0176
0.0198
0.0220
0.0241
0.0261
0.0301
0.0339
0.0375
0.0411
0.0446
Range
(µm )
0.0121
0.0219
0.0306
0.0385
0.0459
0.0528
0.0594
0.0656
0.0716
0.0773
0.0883
0.0985
0.108
0.117
0.126
Boro n
Std De v
(µm )
0.0058
0.0100
0.0133
0.0162
0.0187
0.0209
0.0229
0.0248
0.0265
0.0280
0.0309
0.0334
0.0357
0.0378
0.0397
Range
(µm )
0.0473
0.0826
0.114
0.143
0.171
0.198
0.223
0.248
0.272
0.296
0.341
0.385
0.428
0.469
0.509
Std De v
(µm )
0.0249
0.0384
0.0483
0.0562
0.0628
0.0685
0.0736
0.0780
0.0821
0.0857
0.0922
0.0978
0.102
0.107
0.110
Energy Dependence
Rp and Rp for dopants in Si.
Ranges and standard deviation ∆Rp of dopants in randomly oriented silicon.
© 2000 by Prentice Hall
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3D Distribution of P Implanted to Si
• Monte Carlo simulations of the
random trajectories of a group of ions
implanted at a spot on the wafer show
the 3-D spatial distribution of the ions.
(1000 phosphorus ions at 35 keV.)
• Side view (below) shows Rp and ∆Rp
while the beam direction view shows
the lateral straggle.
Lateral struggle R|
Rp =50 nm, Rp =20 nm
© 2000 by Prentice Hall
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Lateral Implantation - Consequences for Devices
• The two-dimensional distribution is often assumed to be composed of just the
product of the vertical and lateral distributions.
 y 2 
Cx, y  C vert xexp


2 
 2R 
(3)
• Now consider what happens at a mask edge - if the mask is thick enough to block
the implant, the lateral profile under the mask is determined by the lateral straggle.
(35keV and 120keV As implants at the edge of a poly gate from Alvis et al.)

(Reprinted with permission of J. Vac. Science and Technology.)
• The description of the profile at the mask edge is given by a sum of point
response Gaussian functions, which leads to an error function distribution under
the mask. (See class notes on diffusion for a similar analysis.)
© 2000 by Prentice Hall
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B. Masking Implants
• How thick does a mask have to be?
• For masking,

x m  R*P

C* x m   C*P exp 
*2

2R P


Dose that penetrates the mask

• Calculating the required mask thickness,
xm 
R*P  R*P
2 
  C


B
(4)
Depends on mask
material
C* 
P  R*  mR*
2ln
P
C  P
 B 
(5)
• The dose that penetrates the mask is given by
2


*
x  R* 

Q
x

R
Q
P
P 

 dx  erfc m
Q

exp

P


* 
*
2

 2 RP 
2R*P x m
 2RP 

(6)
Lateral struggle important in small devices
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Masking Layer in Ion Implantation
Photoresist, oxide
mask
Lateral struggle important in small devices
 y 2 
Cx, y   Cvert xexp
2 
 2R 
Dose that penetrates the mask

 

X  R* 
C* X m   C*p exp m * 2 p  CB
  2R 
p

 
To stop ions:

R*p & R*p are in the m ask m aterial (different from Si)
Ex : As 2 1015cm 2 @ 50 keV to 1 1016cm 3 doped channel.
X poly ? to act as a m ask.
CX m  1 1016
 x m  0.105m  105nm
Poly thickness
@ 80keV &X m  105nm  Q p  2.7  1013cm 2

Masking Efficiency
• Mask edges tapered – thickness not large enough
• Tilted implantation (“halo”) – use numerical calculations ( ex. to decrease
short channel effects in small devices)
Shadowing effect  rotate or implant at 0 Deg.
Implantation Followed by Annealing
 Function rediffused
ForR p 

Backscattering of light atoms. C(x) is Gaussian only near the peak.
2Dt profiles are identical.
Annealing requires additional
Dt terms added to C(x) 
Cp, depth , C(x) remains
Gaussian.
C. Profile Evolution During Annealing
• Comparing Eqn. (1) with the Gaussian
profile from the last set of notes, we see
that ∆Rp is equivalent to 2Dt . Thus
Cx ,t  


2 
 x  RP  
exp
 2



2RP2  2Dt 
2 RP  2Dt  


  
Q
(7)
• The only other profile we can calculate
analytically is when the implanted
Gaussian is shallow enough that it can
be treated as a delta function and the
subsequent anneal can be treated as a
one-sided Gaussian. (Recall example in
Chapter 7 notes.)
Cx ,t  



x 2 

exp 
 4Dt 
Dt


Q
(8)
© 2000 by Prentice Hall
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Arbitrary Distribution of Dopants
• Real implanted profiles are more complex.
• Light ions backscatter to skew the profile up.
• Heavy ions scatter deeper.
• 4 moment descriptions of these profiles are
often used (with tabulated values for these
moments).
Range:
1 
RP 
 x Cxdx
Q 
(9)
2
1 
Std. Dev: R P 
 x  R P  Cxdx
Q 

Skewness:

Kurtosis:

3
 x  R P  Cxdx
  

• Real structures may be even more
complicated because mask edges
or implants are not vertical.
(10)
Q R 3P
(11)
4
 x  R P  Cxdx
  
Q R 4P
(12)
Pearson’s model good for amorphous (&fine grain poly-) silicon or for
rotation and tilting that makes Si look like amorphous materials.

© 2000 by Prentice Hall
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Two – Dimensional Distributions
Thin oxide
Near the mask edge 2D distributions
calculated by MC model should be the
best – verification difficult due to
measuring problems.
Poly-Si
Phenomenological description of
processes is insufficient for small
devices.
Atomistic view in scattering
Verification through SIMS
D. Implants in Real Silicon - Channeling
• At least until it is damaged by the
implant, Si is a crystalline material.
• Channeling can produce unexpectedly
deep profiles.
• Screen oxides and tilting/rotating the
wafer can minimize but not eliminate
these effects. (7˚ tilt is common.)
• Sometimes a dual Pearson profile
description is useful.
• Note that the channeling decreases
in the high dose implant (green
curve) because damage blocks the
channels.
© 2000 by Prentice Hall
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Channeling Effect
As two profiles
<100>
c-Si, B
Dual-Pearson model gives the main profile
and the channeled part. Dependence on
dose: damage by higher doses decreases
channeling. No channeling for As @ high
doses
Parameters are tabulated (for simulators).
Include scattering in multiple layers (also
masks’ edges). IMPORTANT in small
devices!
Screen oxide decreases channeling.
But watch for O knock-out.
Channeling
not forward
scattering
Channeling
P implantation at 4- keV and low dose Q<1013cm-2
0°
8°
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Manufacturing Methods and Equipment
Mass Analysis
For low E implant no acceleration
Centrifugal
force
Lorentz force
B  I
Ion
velocity
B++, B+, F+,
BF, BF2+
Mass
Selection
mr
Gives mass separation
AsH3 PH3 BF2
in 15% H2, all
very toxic
Integrate the current to determine the dose
D
1 I
dt
Aq
Neutral ions can be implanted (w/o deflection=center) but will not be measured
in Dose (use trap)
Edep  V  Idt  VQ
Ion beam heating
T increases - keep it below 200 °C
High Energy Implants
Applications in fabrication of:
wells (multiple implants give correct profiles ex. uniform or retrograde),
buried oxides,
buried layers (MeV, large doses)! - replaces highly doped substrate with
epi-layers
CMOS
In latch-up
Thyristor structure
UEB
0.7V
p-n-p
n-p-n
UBE
0.7V
Decrease of Rsub - less latch-up
Future IC fabrication: implantation at high energy becomes more important - reduction
of processing steps
Ultralow Energy Implants
Required by shallow junctions in VLSI circuits (50 eV- B) - ions will land
softly as in MBE
Extraction of ions from a plasma source ~ 30keV
Options:
•Lowering the extraction voltage Vout
•
•
the space charge limited current limits the dose
J  V1/2extd-2
ex. J2keV=1/4•J5keV
Extraction at the final energy used in the newest implantors but not for high doses due to
self limitation due to sputtering at the surface. Now 250 eV available,; 50 eV to come
• Deceleration (decel mode)
nonuniformities)
more neutrals formed and implanted deeper that ions (doping
• Transient Enhanced Diffusion (TED) present in the low energy Ion Implantation and {311} defects.
Modeling of Range Statistics
dE
  NSn  Se 
dx
• The total energy loss during an ion
trajectory is given by the sum of nuclear
and electronic losses (these can be
treated independently).
R
 
The range
Computers used to find R
R

0
E
dx 
1 0
dE

N 0 Sn(E ) Se (E )
A. Nuclear Stopping

• An incident ion scatters off the core charge on
an atomic nucleus, modeled to first order by a
screened Coulomb scattering potential.
(14)
Scattering potential
Role of electrons in
(15)
screening
Thomas Fermi model
Energy transferred
Z2, m2
(13)
Elastic collisions
Head-on collision (max
energy transferred)
• This potential is integrated along the path of the
ion to calculate the scattering angle. (Look-up
tables are often used in practice.)
• Sn(E) in Eqn. (14) can be approximated as shown
below where Z1, m1 = ion and Z2, m2 = substrate.
Nuclear stopping power
Sn(E )  2.8x1015
Z1 Z2
m1
eV - cm2
1/ 2 m m

1
2
Z12 / 3  Z22 / 3 



(16)
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Models and Simulations
•
Rutherford(1911) - (He) backscattered due to collision with a + nucleus.
•
Bohr- the nuclear energy loss due to + atoms cores and electronic loss due to free electrons decrease
Many contributors.
•
Lindhard, Scharff and Schiott (1963) (LSS)
B. Non-Local and Local Electronic Stopping
Nonlocal
Local
• Drag force caused by charged ion in
"sea" of electrons (non-local electronic
stopping).
• To first order,
Se(E)  cvion  kE1 / 2
• Collisions with electrons around
atoms transfers momentum and
results in local electronic stopping.
where
Inelastic Collisions with electrons  momentum
transfer, small change of the trajectory.


k  0.2x1015 ev1/2 cm2
C. Total Stopping Power
(17)
• The critical energy Ec when the nuclear
and electronic stopping are equal is
B: ≈ 17keV
P: ≈ 150keV
As, Sb : > 500keV
• Thus at high energies, electronic
stopping dominates; at low energy,
nuclear stopping dominates.
Energy loss w/o the trajectory change
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Damage Production
Ed=Displacement energy (for a Frenkel pair)  15eV  large damage induced by Ion Implantation
• Consider a 30keV arsenic ion, which has a range
of 25 nm, traversing roughly 100 atomic planes.
30 keV As  Rp  25mm
E decreases to Ed so that ions stop.
Si  Si
n= Number of displaced Si atoms

En @ E down to 2 E
d : atom s  the
n


2Ed num ber of displaced atom s

n


30,000
 1000 displaced atom s by 1 As io
2 15
25nm
0.25nminterplanar spacing

 Dose – large damage!

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Damage in Implantation
• Molecular dynamics simulation of a 5keV Boron ion
implanted into silicon [de la Rubia, LLNL].
• Note that some of the damage anneals out between 0.5 and 6
psec (point defects recombining).
Time for the ion to stop
t
Rp
E 2m
 1013 sec
1 ion  primary damage: defect
clusters, dopant-defect complexes,
I and V
Damage accumulates in subsequent cascades
and depends on existing N -local defects
Damage evolution
(atomic interaction)
stabilization @
lower concentrations
due to local
recombiination

N
nx   nfrec  1 

N





N  localdefects
N  10 % NSi  amorphization
Fraction of recombined defects (displaced atoms)
Increment in
damage
more recombination for heavy
ions since damage is less
dispersed than for light ions:
B-0.1, P-0.4, As-0.6.
Damage related to dose and energy
Damage in Implantation Including Amorphization
Damage is mainly due to nuclear energy losses : for B @ Rp. As – everywhere in the Dopant profile.
- Si forms @ large doses and spread wider with the increasing Q.
- Si forms @ low T of II (LN2) , @ RT or higher – recombination  (in-situ annealing)
- Si is
buried
• Cross sectional TEM images of amorphous layer formation with increasing
implant dose (300keV Si -> Si) [Rozgonyi]
• Note that a buried amorphous layer forms first and a substantially higher dose is
needed before the amorphous layer extends all the way to the surface.
• These ideas suggest preamorphizing the substrate with a Si (or Ge) implant to
prevent channeling when dopants are later implanted.
Preamorphization
eliminates the channeling
effect
Damage Annealing - Solid Phase Epitaxy
• If the substrate is amorphous,
it can regrow by SPE.
• In the SPE region, all damage
is repaired and dopants are
activated onto substitutional
sites.
• Cross sectional TEM images of
amorphous layer regrowth at
525˚C, from a 200keV, 6e15 cm-2
Sb implant.
• In the tail region, the material is not amorphized.
• Damage beyond the a/c interface can nucleate
stable, secondary defects and cause transient
enhanced diffusion (TED).
© 2000 by Prentice Hall
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Damage Annealing (more)
Formation of End-of-Range (EOR) defects @ a/c interface in Si  large damage after II @ the C-Si side but below
the threshold for amorphization. Loops R= 10 nm grow to 20 nm in 1000 °C
Solid Phase Epitaxy
5 min
{311}&loops
60 min
Furnace
RTP
850 °C
1000 °C
1 sec
60 sec
400 sec  1000 °C gives stable
dislocation loops
1100 °C/60 sec may be enough to remove
the dislocation loops .
960 min
Loops in P-N junctions  leakage
Optimize annealing: Short time, high T to limit
dopant diffusion but remove defects
Optimize I2 : LN2 Ge 4*1014 cm-2 RT- 5*1014
cm-2 produces a-Si
@ RT , EOR @ 100 nm depth  =25 nm, 1010 cm-2 @ 900 °C/15 min
@ LN2
NO EOR!
Heating by I-beam - defects
harder to be remove
Damage Annealing - “+1” Model
Goals:
• Remove primary damage created by the implant and activate the dopants.
• Restore silicon lattice to its perfect crystalline state.
Primary defects start to anneal at 400
• Restore the electron and hole mobility.
°C  all damage must be annealed
• Do this without appreciable dopant redistribution.
with only +1 atom remaining. (+1
model)
Fast
Frenkel pairs
After 10-2s only “I”
• In regions where SPE does not take place (not amorphized),
damage is removed by point defect recombination. Clusters
of I recombine = dissolve @ the surface
• Bulk and surface recombination take place on a short time
scale.
• "+1" I excess remains. These
I coalesce into {311} defects
which are stable for longer
periods.
• {311} defects anneal out in sec
to min at moderate temperatures
(800 - 1000˚C) but eject I TED.
@ 900 °C, 5 sec  1011 cm-2 of {311}; not long=10 nm rods
then dissolve if below critical size or else grow
dislocation loops (stable) = extrinsic e. i. Si I planes on {111}
 = secondary defects. (difficult to remove)
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Solid State Epitaxy
Regrowth from the C-Si acting as a seed (as in crystal growth from melt)
@ 600 deg C, 50 nm/min <100>
20 nm/min <110>
2 nm/min <111>
2.3 eV is for Si-Si bond
breaking
  2.3 
v  A exp

 T 
Regrowth
rate
Regrowth 10x larger for
highly doped regions
Time
Dopants are active
=substitutional position
with very little diffusion.
But high T might be needed
for EOR annealing.
No defects=no diffusion
enhancements
Dopant Activation
• When the substrate is amorphous, SPE provides an ideal way of repairing the
damage and activating dopants (except that EOR damage may remain).
• At lower implant doses, activation is much more complex because stable defects
form.
• Plot (above left) of fractional activation versus anneal temperature for boron.
• Reverse annealing (above right) is thought to occur because of a competition
between the native interstitial point defects and the boron atoms for lattice sites.
© 2000 by Prentice Hall
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Dopant Activation – No Premorphization
Low T Annealing is enough for low doses – low primary damage can be easily annealed.
High doses – damage below amorphization secondary defects = difficult to anneal and requires high T  950-1050 °
C.
Full activation
Secondary defects from
Amorphization improves activation @ low T leading to 100% @ high T
Note: very high doses may result in low activation (25%)
1.
High initial activation, full
activation is fast @ low T,
2.
Low initial activation, traps anneal
out, I compete with B for
substitutional sites, I –B complexes
3.
More damage so activation
decreases with dose maintaining the
same behavior.
(1)
(2)
(3)
Increasing
dose
Doses below amorphization
High doses - high T required which
causes more diffusion - in small devices
unacceptable
Carriers’ mobility increases with
damage anneal