ION IMPLANTATION Chapter 8

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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.
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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.
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
(1)
(2)
or
where Q is the dose in ions cm-2 and is measured
by the integrated beam current.
Doses 11012 cm-2 to 11016 cm-2 used in MOS ICs
3
Energy Dependence
Rp and ?Rp for dopants in Si.
Ranges and standard deviation ?Rp of dopants in
randomly oriented silicon.
4
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
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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.
(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.)
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B. Masking Implants
How thick does a mask have to be? For
masking,
(4)
Dose that penetrates the mask
Calculating the required mask thickness,
Depends on mask material
(5)
The dose that penetrates the mask is given by
(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
Dose that penetrates the mask
To stop ions
Poly thickness
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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.
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Implantation Followed by Annealing
? Function rediffused
Annealing requires additional Dt terms added to
C(x) ? Cp?, depth ?, C(x) remains Gaussian.
Backscattering of light atoms. C(x) is Gaussian
only near the peak.
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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 . Thus
(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.)
(8)
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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
(9)
Std. Dev
(10)
Skewness
(11)
Kurtosis
Real structures may be even more
complicated because mask edges or implants
are not vertical.
(12)
Pearsons model good for amorphous (fine grain
poly-) silicon or for rotation and tilting that
makes Si look like amorphous materials.
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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.
Phenomenological description of processes is
insufficient for small devices. Atomistic view
in scattering
Poly-Si
Verification through SIMS
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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.
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Channeling Effect
As two profiles
lt100gt
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 _at_ high doses Parameters are tabulated (for
simulators). Include scattering in multiple
layers (also masks edges). IMPORTANT in small
devices!
Channeling not forward scattering
Screen oxide decreases channeling. But watch
for O knock-out.
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P implantation at 4- keV and low dose Qlt1013cm-2
Channeling
0
8
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Manufacturing Methods and Equipment
Mass Analysis
Lorentz force
For low E implant no acceleration
Centrifugal force
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
Neutral ions can be implanted (w/o
deflectioncenter) but will not be measured in
Dose (use trap) Ion beam heating T
increases - keep it below 200 C
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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
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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. J2keV1/4J5keV
• 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) more
  neutrals formed and implanted deeper that ions
  (doping nonuniformities)
• Transient Enhanced Diffusion (TED) present in
  the low energy Ion Implantation and 311 defects.

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Modeling of Range Statistics
The total energy loss during an ion
trajectory is given by the sum of nuclear and
electronic losses (these can be treated
independently).
(13)
The range
(14)
Computers used to find R
A. Nuclear Stopping
Scattering potential Role of electrons in
screening
An incident ion scatters off the core charge on
an atomic nucleus, modeled to first order by
a screened Coulomb scattering potential.
(15)
Thomas Fermi model Energy transferred
Elastic collisions
Head-on collision (max energy transferred)
Z2, m2
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.
(16)
Nuclear stopping power
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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)

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B. Non-Local and Local Electronic Stopping
Nonlocal
Local
Drag force caused by charged ion in "sea"
of electrons (non-local electronic stopping).
Collisions with electrons around atoms
transfer momentum and result in local
electronic stopping.
To first order,
where
(17)
Inelastic Collisions with electrons ? momentum
transfer, small change of the trajectory.
C. Total Stopping Power
The critical energy Ec when the nuclear
and electronic stopping are equal is B
17keV P 150keV As, Sb gt 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
EdDisplacement 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. n Number of displaced Si atoms
?Si ? Si
? Dose large damage!
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Damage in Implantation
Molecular dynamics simulation of a 5keV Boron
ion implanted into silicon de la Rubia. Note
that some of the damage anneals out between 0.5
and 6 psec (point defects recombining).
Time for the ion to stop
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 _at_ lower concentrations due to local
recombination
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
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Damage in Implantation Including Amorphization
Damage is mainly due to nuclear energy losses
for B _at_ Rp. As everywhere in the Dopant
profile. ?- Si forms _at_ large doses and spread
wider with the increasing Q. ?- Si forms _at_ low T
of II (LN2) , _at_ RT or higher recombination ?
(in-situ annealing)
?- Si is buried
Preamorphization eliminates the channeling effect
Cross sectional TEM images of amorphous layer
formation with increasing implant dose
(300keV Si -gt 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.
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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
525C, 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).
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Damage Annealing (more)
Formation of End-of-Range (EOR) defects _at_ a/c
interface in Si ? large damage after II _at_ the
C-Si side but below the threshold for
amorphization. Loops R 10 nm grow to 20 nm in
1000 C
Furnace 850 C
RTP 1000 C
Solid Phase Epitaxy
5 min
1 sec
311loops
60 min
60 sec
400 sec ? 1000 C gives stable dislocation loops
960 min
1100 C/60 sec may be enough to remove the
dislocation loops .
Loops in P-N junctions ? leakage Optimize
annealing Short time, high T to limit dopant
diffusion but remove defects Optimize I2 LN2
Ge 41014 cm-2 RT- 51014 cm-2 produces a-Si
Heating by I-beam - defects harder to be remove
_at_ RT , EOR _at_ 100 nm depth ? ?25 nm, 1010 cm-2 _at_
900 C/15 min _at_ LN2 NO EOR!
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Damage Annealing - 1 Model
Goals Remove primary damage created by the
implant and activate the dopants. Restore
silicon lattice to its perfect crystalline state.
Restore the electron and hole mobility. Do
this without appreciable dopant redistribution.
Primary defects start to anneal at 400 C ? all
damage must be annealed with only 1 atom
remaining. (1 model)
Fast
In regions where SPE does not take place (not
amorphized), damage is removed by point defect
recombination. Clusters of I recombine dissolve
_at_ the surface Bulk and surface recombination
take place on a short time scale.
Frenkel pairs
After 10-2s only I
"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
- 1000C) but eject I ?TED.

• _at_ 900 C, 5 sec ? 1011 cm-2 of 311 not long10
  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)
_at_ 600 deg C, 50 nm/min lt100gt
20 nm/min lt110gt 2 nm/min
lt111gt
2.3 eV is for Si-Si bond breaking
Regrowth rate
Regrowth 10x larger for highly doped regions
Dopants are active substitutional position with
very little diffusion. But high T might be
needed for EOR annealing.
Time
No defectsno diffusion enhancements
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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.
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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 _at_ low T leading
to 100 _at_ high T Note very high doses may result
in low activation (25)

• High initial activation, full activation is fast
  _at_ low T,
• Low initial activation, traps anneal out, I
  compete with B for substitutional sites, I B
  complexes
• More damage so activation decreases with dose
  maintaining the same behavior.

(1)
(2)
(3)
Doses below amorphization High doses - high T
required which causes more diffusion - in small
devices unacceptable
Increasing dose
Carriers mobility increases with damage anneal