Ion Implantation
Issues to Address…
 Ion Implantation
MOSFET
MOSFET
Gate Oxide
VS
VG
Metal or polysilicon
Source
(n+)
p-type
VD
Drain
(n+)
Si Substrate
VB
Methods of Doping:
 Diffusion
 Ion Implantation
Diffusion
Disadvantages of Diffusion Process
– Isotropic process, lateral diffusion of dopants.
– High temperature process.
– Difficult to control doping profile (peak concentration always on the
surface).
– Doping is limited by solid solubility.
– In MOSFET fabrication, misalignment of gate may occur.
VG
SiO2
SiO2
Si substrate
Doping profile “diffusion +
drive in”
VS
Source
(n+)
p-type
VG
VD
Drain
(n+)
VS
Source
(n+)
p-type
VD
Drain
(n+)
Si Substrate
Si Substrate
VB
VB
Gate aligned MOSFET
Gate-misaligned MOSFET
Ion Implantation
What is Ion Implantation ?
– In this process, energetic dopant ions are implanted into the
semiconductor by means of an ion beam of a particular type (ion
mass) and energy.
– Profile of dopant distribution is mainly determined by ion mass and
the implanted-ion energy.
– Doping profile has peak concentration inside semiconductor.
Example:
Energetic Ion for
implantation
Crystal before Ion implantation
Modified surface
Crystal after Ion implantation w/o drive-in
Ion Implantation
Unique features of Ion Implantation:
– Precise dose (# implanted-ions into 1 cm-2 surface) control (~ 1%).
– Wide range of dose 1010 to 1018 cm-2.
– Large area dose uniformity (300 mm wafer < 1%) from wafer to
wafer.
– Precise depth profile (peak depth and spread range) by varying
energy & dose.
– Anisotropic dopant profile.
– Less contamination and very clean process.
– Low temperature process.
– Not limited by thermodynamics (solid solubility limit).
SiO2
SiO2
ln(C)
Si substrate
PR
PR
Si substrate
x
Doping profile “diffusion + drive in”
ln(C)
x
Ion Implantation Doping Profile
Ion Implantation
Ion Implantation Applications:
–
–
–
–
–
Implantation for n-well or p-well formation.
Self aligned process.
Threshold voltage adjustment.
Polysilicon doping.
Silicon-On-Insulator (SOI) wafer preparation:


SIMOX (Separation by IMplantation of OXygen)
Hydrogen implantation for Smart-cut.
 Self aligned Process
SiO2
P-type
Si Substrate
Phosphorous
Poly
Si
Poly
Si
P-type
Si Substrate
P-type
Si Substrate
Ion Implantation
Ion Implantation Applications:
Buried Oxide (BOX)
O+ Implant
SIMOX:
~1300 ̊C
O in Silicon
Si Substrate
Si Substrate
Annealing
Si
BOX
Si Substrate
SOI wafer
Smart-Cut:
H+ Implant
Hydrogen
peak
After Bonding of
both wafer
Si donor
Si donor
After thermal or
mechanical cleavage
Si
SiO2
Handle Si wafer
Handle Si wafer
Handle Si wafer
SOI wafer
Ion Implantation
‒ A typical Ion implantation system
 Shockley proposed the concept of Ion Implantation
Process steps:
semiconductor doping in 1956.
Typical ions used for ion implantation are As+
(for Arsenic), P+ (for phosphorous) B+ or BF2+
(for Boron) etc.
Q







for
Generation of Ions
Extraction of Ions
Selection of particular ions
Acceleration of selected ions
Neutral beam trapping
Beam scan/disk scan
Dose monitoring
1
Idt
qA 
Silicon VLSI Technology: Fundamentals, Practice and Modeling
Fig.: Typical mass spectrum for a BF3 source gas
Ion Implantation
‒ Function of different section of Ion implanter

Ion source: operates at a high voltage(25kV) and convert the electrically
neutral dopant atoms in the gas phase into plasma ions and undesired
species. Solid can be sputtered in special ion sources.

Analyzer: a magnetic field is applied to normal to path of ions, and select the
desired impurity ion and filtered out undesired species. Selected ion passes
through an aperture.

Accelerator: The selected ion-beam is accelerated to final energy of
implantation.

Neutralization: Some of the ions on the way from the analyzer may get
neutralized and they will not be electrostatically scanned or counted as
current. The neutral species can not be subjected to deflections using
electric or magnetic fields for raster scan. So a neutral trap is used to
eliminate from the beam.

Scanning system: x and y axis deflection plates are used to scan the beam
across the wafer to produce uniform implantation of desired dose.
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
– A magnetic field B is applied normal to the path of ions
with charge q and velocity v.
– The Force (F) exerted on the charged ions in a
magnetic field is F = qvB.
– Due to exerted force, ions move in a circular path of
radius r and the centripetal force is F = mv2/r
‒ Mass analysis relies on balancing the both force i.e. qvB = mv2/r
‒ Ion velocity is related to the extraction voltage (Vext) by qVext = mv2/2
v
2qVext
,
m
r
1 2mVext
B
q
‒ Magnetic field intensity (B) depends on current (I) in magnetic coil as:
B=αI
q
α depends on magnetic coil design.
r I Selection of mass ions
m
2qVext
depends on current.
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
‒ Typical dose (Q)
and
Ion
Implantation
energy
for
doping
applications.
Q
1
Idt

qA
© Handbook of Semiconductor Manufacturing Technology, 2nd edition
Ion Implantation
© Handbook of Semiconductor Manufacturing Technology, 2nd edition
Ion Implantation
© Handbook of Semiconductor Manufacturing Technology, 2nd edition
Ion Implantation
Few terms used in Ion Implantation :
– Ion implantation is random process.



Range (R): The total distance that an ion travels before coming to rest.
Projected Range (Rp): It is the projection of “R” along the axis of
incidence.
Standard deviation ∆RP: It is the statistical fluctuations in the
projected range RP.
Ion Implantation
Distribution of implanted ions:
– Implantation of Boron (B) in Silicon (Si)

Simulated point response for a 10keV high-dose implantation.
Surface
Peak concentration
region
https://www.iue.tuwien.ac.at/phd/wittmann/node9.html
B concentration
Ion Implantation
Distribution of implanted ions:
– Heavy ions (e.g. Sb) do not travel as far from surface in the crystal
as light ions (e.g. B) for the same implantation energy.
– Projected range depends on the implantation energy, with higher
energy deeper range.
200keV
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Distribution of implanted ions:
– The spread of ions depends on the range travelled.
– Deeper ranges allowing more random stopping events.
–
 This gives rise to distribution on ions.
 Most of the ions are within a standard deviation ± ΔRP of RP.
Heavy ions with smaller range have a narrow distribution than
lighter ions.
C(x)
– The dopants distribution C(x) is C
P
random and often modeled by
Gaussian function.
 x  R P 2 

Cx   C P exp  
2

2

R
P


CP → Peak concentration
RP → Projected Range
ΔRP → Standard deviation or straggle.
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
0.606CP
RP
ΔRP ΔRP
x
Ion Implantation
Distribution of implanted ions:
‒ Total number of ions implanted is defined as dose (Q):

Q   Cx dx  2R P CP

Range (depth) and standard deviation of common dopants in Si
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Distribution of implanted ions:
‒ Total number of ions implanted is defined as dose (Q):

Q   Cx dx  2R P CP

Range (depth) and standard deviation of common dopants in Si
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Distribution of implanted ions:
‒ Comparison of diffusion and ion implantation profile:
Implantation profile
Diffusion Profile
 x  R P 2 

Cx   C P exp  
2
2R P 

 x2 

Cx, t   C(0) exp 
4
Dt


R p  2Dt
– Implanted Gaussian profile evolution after annealing:
2


x  Rp  

Cx, t  
exp 
2
 2 R P  2Dt 
2 R 2P  2Dt



Q


Gaussian remains a Gaussian upon annealing
of ion implantation profile.
© Silicon VLSI Technology: Fundamentals, Practice and Modeling

Ion Implantation
Distribution of implanted ions:
– Perfect Gaussian distributions are very simple.
– Experimentally they only match central (peak) region of the implant
profile.
‒ Real implanted profiles are more complex
 Light ions backscatter and fill in front side of distribution
 Heavy ions scatter deeper possibly due possibly due to
channeling..
For more accurate description
of different implanted dopants
in Si, four moment equation
can be used.
Ion
implanted
boron
antimony profile
and
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Distribution of implanted ions:
– When a profile deviates from the ideal Gaussian, we can describe it
series of moments taken about Rp.
 measure of the profiles tendency to
lean toward or away from the surface.
 measure of flatness: A perfect
Gaussian has a kurtosis of 3. Larger
kurtosis means the profile is flatter
near it’s peak.
– These characteristics can be used to generate profiles using the
Pearson family of distributions.
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Distribution of implanted ions:
– Comparison of simple Gaussian and 4-moments with measured
data. .
Ion Implantation
Distribution of implanted ions:
– Monte Carlo simulations of the
random trajectories of a group of
ions implanted at a spot on the wafer.
–
Implant Condition: 1000 phosphorus
ions at 35 keV.
– Side view shows Rp and ∆Rp
– Beam direction view shows
lateral straggle.
Beam Direction
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
the
Side View
Ion Implantation
Thickness of a Mask layer
– Masking layer is used to block the transmission of ions through it.
– In an efficient mask layer, thickness of mask should be large enough
to so that tail of the implant profile reach to a specified background
doping in substrate.
Mask layer
xm
Si substrate
Background doping: CB
Ion Implantation
Thickness of a Mask layer
– Considering Gaussian implant profile, doping at x=xm can be written
as:
CB  Background doping
* to indicate doping in masking layer
– By placing C*(xm)=CB, we can get mask thickness xm as follows:
m  it indicate mask thickness should be range + m times of standard deviation
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Thickness of a Mask layer
– If QP is the dose penetrating the mask (dashed region) then:
– Integral or sum of Gaussian function is error function
– QP can be expressed as:
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Ion Beam Heating:
– Temperature rise in Si due to ion implantation is dependent on:



Dose
Acceleration voltage
Thickness of the Si wafer.
– The energy deposited is:
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Energy loss (stopping) mechanism:
– Ions penetrate into substrate.
– Penetrated ions collides with lattice atoms.
– After collisions, gradually lose their energy and stop.
– Two stopping mechanism.
 Nuclear stopping
 Electronic stopping
Ion Implantation
Energy loss (stopping) mechanism:
– Nuclear Stopping
 Collision with nuclei of the lattice atoms
 Scattered significantly
 Causes crystal structure damage.
 Heavier ions at lower energy.
– Electronic Stopping
 Collision with electrons of the lattice atoms
 Incident ion path is almost unchanged
 Energy transfer is very small
 Crystal structure damage is negligible
 Light ions at higher energy.
Ion Implantation
Energy loss (stopping) mechanism:
– Total stopping power (S)
Stotal = Sn + Se
Sn → nuclear stopping
Se → electronic stopping
Silicon lattice viewed along <110> axis
Ion Implantation
Energy loss (stopping) mechanism:
– Total stopping power (S)
Stotal = Sn + Se
Sn → nuclear stopping
Se → electronic stopping
– The rate of energy loss is dependent on stopping power of target:
N  target atoms (51022cm-3 for Si)
– From Sn(E) and Se(E), the range (R) of ion can be calculated:
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Energy loss (stopping) mechanism:
– The nuclear stopping depends on the ion energy
– The nuclear Collison produces most of the damage at end of the
range.
– The nuclear stopping can be simply approximated as:
m1  ion atom mass
m2  Substrate atomic mass
Z1 ion atomic number
Z2 Substrate atomic number
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Energy loss (stopping) mechanism:
– In electronic stopping is depends on the ion
velocity




Effect of the electrons on the ion is very much
like a particle moving through a fluid.
Ion must do work to move through the media.
The force can also be expressed as an energy
gradient
The energy loss per unit length due to
electronic stopping is:
C & k  constant of proportionality
For an amorphous substrate K= 0.210-15 eV1/2cm2
© Silicon VLSI Technology: Fundamentals, Practice and Modeling
Ion Implantation
Energy loss (stopping) mechanism:
– From Sn(E) and Se(E), the projected range (Rp) of ion can be
calculated:
– The projected range (R or Rp) can be used to estimate Rp:
© Fabrication Engineering at the Micro- and Nanoscale
Ion Implantation
Energy loss (stopping) mechanism:
Q. Find Sn(E) and Se(E) for a 100-keV As implant and calculate the
projected range for E =100 keV. Use the following graph.
Ion Implantation
Energy loss (stopping) mechanism:
Sol.
Ion Implantation
Channeling in crystalline material:
– If the incident angle is right, ion can travel long distance without
collision with lattice atoms.
– It causes uncontrollable dopant profile.
‒ Ways to avoid channeling effect.
 Tilt wafer, 7° is most commonly used
 Screen oxide
 Pre-amorphous implantation, Germanium.
Ion Implantation
Channeling in crystalline material:
– If the incident angle is right, ion can travel long distance without
collision with lattice atoms.
– It causes uncontrollable dopant profile.
– Channeling is characterized by a critical angle Ψ
E0  Incident Energy in KeV
d  Atomic spacing along the
ion direction in Å
Z1 ion atomic number
Z2 Substrate atomic number
© Fabrication Engineering at the Micro- and Nanoscale
Ion Implantation
Channeling in crystalline material:
– Channeling is characterized by a critical angle Ψ
© Fabrication Engineering at the Micro- and Nanoscale
Ion Implantation
Channeling in crystalline material:
Q. Estimate the critical angle for implanting boron into (100) silicon at
100 keV.
Ion Implantation
Channeling in crystalline material:
Sol.
Ion Implantation
Ion Implantation shadowing Problem
Ion Implantation
Implantation damage:
– Implanted ions transfer energy to lattice atoms.

Atoms to break free.
 Freed atoms collide with other lattice atoms.
Free more lattice atoms.
Damage continues until all freed atoms stop.
– One energetic ion can cause thousands of displacements of lattice
atoms.
Before Implantation
After Implantation
Ion Implantation
Annealing Damage and Activating Dopants :
– Annealing step is required to remove damage and to activate
dopants.
– Dopant atoms must be substitutional: for activation.
– Ion implant tends to create Interstitial dopant.
– Interstitial implant ions do not contribute carriers.
– True Interstitial dopant atoms: not activated.
– Need to heat surface to remove damage & activate dopant.
 Heat moves dopant atoms into substitution positions – activates.
Before annealing
After annealing
Lattice atoms
Dopant atom
Ion Implantation
Rapid thermal annealing (RTA):
– Furnace activation leads to unwanted diffusion: changes profile.
– To minimize unwanted diffusion, one can use Rapid Thermal
Processing (RTP) or Rapid Thermal Anneal (RTA).
– Use high intensity light to heat only dopant surface.
– Light penetrates only few microns thus heats only surface.
– Reach high local temperature (~ > 900̊C): rapid healing/activation.
– Rapidly cools when light off – wafer itself is cool.
– Annealing time, few minutes to few seconds, little chance for dopant
diffusion.
Different temperature profile for annealing:
Decreasing thermal budget
RTA
S. Sharma et al. “Thermal Processing for Continued Scaling of Semiconductor Devices” 20th IIT, 2014
Ion Implantation
Rapid thermal annealing (RTA):
‒ RTP/RTA sources/systems.
Rapid heating source:
• high power laser
• electron beam
• high intensity halogen
lamp
Fig.: Applied Materials 300mm RTP System.
© Introduction to Microelectronics Fabrication
Ion Implantation
Rapid thermal annealing (RTA):
‒ Impact of RTA.
– RTA is important for
shallow junction formation.
e.g.: RTA & furnace annealing impact on
source/drain formation in a MOSFET
Ion Implantation
Transient Enhanced Diffusion (TED)
–
TED is the result of interstitial damage from the
implant enhancing the dopant diffusion for a brief
transient period.
–
It is the dominant effect today that determines
junction depths in shallow profiles.
–
It is anomalous diffusion, because profiles can
diffuse more at low temperatures than at high
temperatures for the same Dt.
–
The basic model for TED assumes that all the
implant damage recombines rapidly, leaving only 1
interstitial generated per dopant atom when the
dopant atom occupies a substitutional site (the +1
model) [Giles].
–
–
TED effects may be very non‐local.
After 900˚C, 1 sec anneal, the amorphous As
surface profile recrystalizes by SPE without much
TED.
The buried boron layer is drastically affected by the
+1 interstitials in the As tail region.
–
References
1. J. Plummer, M. D. Deal, and P. B. Griffin “Silicon VLSI Technology:
Fundamentals, Practice and Modeling”, Prentice Hall Upper Saddle River
NJ, 2000.
2. R. C. Jaeger “Introduction to Microelectronics Fabrication” Prentice Hall,
2nd edition
3. G. S. May, S. M. Sze “Fundamentals of Semiconductor Fabrication” John
Wiley & Sons, 2004
4. S. K. Gandhi “VLSI fabrication principles: Silicon and Gallium Arsenide”
2nd Edition, John Wiley & Sons, New York, 1994
5. Stephen A Campbell “Fabrication Engineering at the Micro- and
Nanoscale” Oxford University Press
6. http://www.sfu.ca/~gchapman/e495/e495l7o-2.pdf