1. 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 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.

2. • At its heart ion implantation is a random process.
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) or
(2)
where Q is the dose in ions cm-2 and is measured by the integrated beam current.
Doses 1*1012 cm-2 to 1*1016 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

5. 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.)

6. B. Masking Implants (4) (5) (6) • 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

7. Masking Layer in Ion Implantation
Photoresist, oxide mask
Lateral struggle important in small devices
Dose that penetrates the mask
To stop ions:
Poly thickness

8. 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.

9. 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.

10. 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)

11. 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)
Pearson’s model good for amorphous (&fine grain poly-) silicon or for rotation and tilting that makes Si look like amorphous materials.

12. 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

13. 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.

14. 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 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.

15. P implantation at 4- keV and low dose Q<1013cm-2
Channeling
P implantation at 4- keV and low dose Q<1013cm-2 0° 8°

16. 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 deflection=center) but will not be measured in Dose (use trap)
Ion beam heating T increases - keep it below 200 °C

17. 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

18. 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 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) more neutrals formed and implanted deeper that ions (doping nonuniformities)
Transient Enhanced Diffusion (TED) present in the low energy Ion Implantation and {311} defects.

19. 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
Head-on collision (max energy transferred)
Z2, m Elastic collisions
• 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
(16)

20. 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)

21. B. Non-Local and Local Electronic Stopping
• Drag force caused by charged ion in
"sea" of electrons (non-local electronic
stopping).
• Collisions with electrons around
atoms transfers momentum and
results in local electronic stopping.
• To first order,
where
Inelastic Collisions with electrons  momentum transfer, small change of the trajectory.
(17)
C. Total Stopping Power
• 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

22. 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.
n= Number of displaced Si atoms
Si  Si
 Dose – large damage!

23. 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
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) lower concentrations due to local recombiination
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

24. Damage in Implantation Including Amorphization
Damage is mainly due to nuclear energy losses : for Rp. As – everywhere in the Dopant profile.
- Si large doses and spread wider with the increasing Q.
- Si low T of II (LN2) 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 -> 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.

25. 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).

26. Damage Annealing (more)
Formation of End-of-Range (EOR) a/c interface in Si  large damage after 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
{311}&loops
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 4*1014 cm-2 RT- 5*1014 cm-2 produces a-Si
Heating by I-beam - defects harder to be remove
@ RT , 100 nm depth  =25 nm, °C/15 min
@ LN2 NO EOR!

27. 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 = 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
( ˚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)

28. 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
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 defects=no diffusion enhancements

29. 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.

30. 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  ° C.
Full activation
Secondary defects from
Amorphization improves low T leading to high T
Note: very high doses may result in low activation (25%)
High initial activation, full activation is fast @ 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