ION IMPLANTATION
We saw how dopants were introduced into a wafer by using diffusion
(‘predeposition’ and ‘drive-in’).
This process is limited:
-cannot exceed solid solubility of dopant
-difficult to achieve light doping
Ion implantation is preferred because:
-controlled,low or high dose can be introduced (10
11
- 10
18
cm
-2
)
-depth of implant can be controlled.
Used since 1980,despite substrate damage;
low throughput,and cost,
Plummer Ch,8,Campbell Ch,5
3.155J/6.152J,2003 1
Reminder,Analytical Solutions to Diffusion Equations
Solution for a limitless source of dopant (constant surface concentration),
Bound cond:
C
= D
2
C C (0,t ) = C
surf
C (∞,t ) = 0
t
Bound cond,
Const C
surf
t?z
2
è
z
(Cz,t) = C
surf
erfc
í
2 Dt
˙
,t > 0
z
Initial cond,
C (z,0) = 0
where erfc(x) = 1-erf(x) and t
p
= predep time
Dose Q = (2/√p) C
surf
√Dt
p
Dose in sample increases as t
1/2
3.155J/6.152J,2003 2
Diffusion of a thin surface layer into a solid
When a thin surface layer diffuses into a solid,what is C(z,t)?
Q = initial amount of dopant (‘dose’),
ú
C z,t)dz = Q = const,(# /area)
assumed to be a delta-function
(
-?
t
1
t
2
t = 0
C
Bound cond,
dC(0,t) Bound cond,
= 0
C(?,t) = 0Solution is a Gaussian,dz
2
Q
è
z
(C z,t) = exp -
pDt
í
4 Dt
˙
z
0
(
Initial cond,
Cz,0) = 0 ( z ≠ 0)
diffusion length
a = 2 Dt
Dose in sample constant in time
3.155J/6.152J,2003 3
Example,
wafer originally has a uniform
dopant level,e.g,donor.
Predep plus drive-in
introduces a second dopant,
an acceptor,
At a certain depth,
a p-n junction is formed,
A third pre-dep of donor
can then be done to make
an npn transistor,
Problem,
can only make profiles
consisting of superposed
Gaussians centered at the
substrate surface,
3.155J/6.152J,2003 4
Since the maximum amount of a dopant that can dissolve in the Si is
given by the solid solubility,you may be limited in the amount of
dopant that can be incorporated,
3.155J/6.152J,2003 5
Ion Implantation
Beam of energetic dopant ions is fired into surface of wafer,
Energies are 5 - 200 keV,
This leads to implantation (burial) of the ions into the substrate,
What happens at the substrate?
Ions can,bounce off
absorb
sputter atoms (10 eV - 10 keV)
implant into surface (5 keV - 200 keV)…
and do tremendous damage
3.155J/6.152J,2003
Ion Implantation Equipment
Ions generated in a source (from feed gas,e.g,BF
3
,AsH
3
,PH
3
,.,or
heated solid source,then ionized in arc chamber by electrons from hot filament)
select desired species by q/m,using a magnet,
accelerated by an E-field and focused using electrostatic lenses
and impact substrate
(a bend removes neutrals) in raster pattern,
3.155J/6.152J,2003 7
What happens to ions inside the material?
Ions lose energy,dE/dx,interacting
elastically with nuclei and inelastically with electrons
dE
=-N S (E ) + S (E)
][
n
dx
e
S (E) is Stopping power (eVcm
2
)
i
R
0
dE
Ion range in target,
R =
ú
dx =
1
E
ú
N
0
S
(
E
)
+ S
(
E
)
0 ne
What can we say about
nuclear and electronic stopping…
3.155J/6.152J,2003 8
Nuclear stopping power,Coulomb scattering (assumed elastic)
Incident ion interacts with
b
q
f
b
E
1
,M
1
M
2
Energy lost by incoming ion (microscopic)
DE = E
1
1 -
sin
2
f
cosq sinf + cosf sinq
ì
ó
0 2 4 6 8 10
M
2
/M
1
1.0
0.8
dE/E
0.4
0.2
0.0
nucleus of stationary ion
= impact parameter
The angles depend on masses and on b,
Max,energy loss is when b = 0,f = 0,
4 M
1
M
2
DE = E
1
(
M
1
+ M
2
)
2
3.155J/6.152J,2003 9
q
f
b
E
1
,M
1
M
2
F μ
Q
1
Q
2
r
2
Nuclear stopping power,Coulomb scattering (assumed elastic)
q
f
b
,
1
2
So nuclear scattering
At 100 keV an ion of 15 amu
has velocity,v
ion
≈10
6
m/s!
This is 1000 times faster
than speed of sound in solids…
So ion is far past nucleus
before nucleus can displace
in response to Coulomb force
is not strong at high ion velocity;
There are also
only significant
inelastic collisions
when ion slows down,
that transfer energy…
3.155J/6.152J,2003 10
Electronic stopping power,also Coulomb interactions,but inelastic)
Non-local,ion experiences drag due to,free” or polarizable electrons:
incident ion
attracts electron polarization,
Local,
=> energy and moment transfer
E
e
-
e
-
e
-
Ion velocity=> charge separation,drag
passing ion causes internal electronic transitions
Because electrons can follow fields up to optical frequencies,
(velocities of 10
5
m/s - 100 times faster than phonons)
S ( E) = cv
ione
electronic losses dominate at higher ion velocities,
3.155J/6.152J,2003 11
= kE
1 2
Stopping power in Ion Implantation
At each impact,the ion loses some energy,It travels through a
vertical projected range R
p
before stopping,It transfers energy to
target via both electronic
and nuclear interactions
Transitions,
Nuclear
Viscosity,
local
Coulomb
non-local
electrons
collisions
electrons
substrate
velocity
More effective at larger v
ion
More effective
at smaller v
ion
R
p
R
p
= dx
0
R
ú
=
1
N
dE
S
n
E
( )
+ S
e
E
( )
0
E
0
ú
3.155J/6.152J,2003 12
t
Stopping power in Ion Implantation
Most damage is done by nuclear interactions
About 15 eV needed to displace Si from lattice site,
create vacancy/interstitial pair (Frankel defect)
Transitions,
Nuclear
Viscosity,
non-local
local
Coulomb
electrons
electrons
collisions
R
substrate
velocity
More effective
at smaller v
ion
More effective at larger v
ion
R
p
,
ff iMilMMtlargerat
iion
More effective g
ff iMiM eMore effectieffective
ll
Most damage occurs near limit of
effecti e atti
ti
p
3.155J/6.152J,2003 13
3.155J/6.152J,2003 15
From S
e
and S
n
,R
p
and DR
p
R
p
DR
p
in Si
in Si
in Si
in GaAs
DR
p
R
p
R
p
(?)
DR
p
DR
p
R
p
(?)
can be calculated,
(?)
(?)
Composition profile for ion implantation
If the depth is x,the impurity concentration C(x) is
approximated by a gaussian
ê
Cx
)
= C
p
exp
á
-
(
x - R
p
)
2
(
á
2DR
2
p
ˉ
where C
p
is the peak concentration,R
p
the projected range and DR
p
the standard deviation of the projected range (vertical straggle),
The implanted dose is given by Q (Number/area)
(Q =
ú
C x)dx
-?
Q = 2pDR
P
C
P
So a given dose will determine the peak concentration,
3.155J/6.152J,2003 16
200 keV implants in Si
Why do light atoms have greater vertical projected range R
p
and
standard deviation DR?
p
3.155J/6.152J,2003 17
B in Si
gaussian
The composition profiles are not always perfect Gaussians,there can be a skew
or distortion (kurtosis) making the profile asymmetric,
3.155J/6.152J,2003 18
Channeling
If the ions are incident parallel to a major crystal direction,they
can pass through the structure with less scattering,so the range is
much larger than expected,
3.155J/6.152J,2003 19
Nuclear Stopping
Need to sum the effects of
all the scattering events,
e.g,using Monte Carlo
modeling,Nuclear
stopping,S
n
,can be
modeled by Coulomb
scattering (so it depends
on impact parameter,
relative masses,and E),
3.155J/6.152J,2003 20
Modeling
Distributions of ions after implant can be modeled using a Monte
Carlo calculation to give projected range,Can include both nuclear
and electronic stopping
Ion implantation can be modeled using SUPREM,which calculates
dopant profiles vs,implant conditions and annealing,
3.155J/6.152J,2003 21
P
*
ê
C
P
** *
2ln
á
á
= R
P
x
m
= R
P
+DR
P
C
B
ˉ
x
m
= range +
some multiple,m,of std dev’n
Dose penetrating mask,
2
˙
Q
P
=
Q
ú
exp
í
í
è
-
á
á
ê
x - R
P
*
*
˙
dx
2pDR
P
*
2DR
P
ˉ
x
m
Ion implantation through a mask
Range,R
p
* and standard deviation,DR * are for ions in mask
p
*
ê
(x - R
P
)
2
* *
m
For an efficient mask,
C
(
x
m
)
= C
P
exp
á
-
￡ C
B
*2
2DR
P
ˉ
Mask thickness,
+ mDR
Background
concentration
in substrate
*
Q
*
Q
P
= erfc
ê
á
á
x
m
- R
P
2DR
P
*
ˉ
2
3.155J/6.152J,2003 22
In GaAs
In SiO2
In Photoresist
Damage in Ion Implantation
Vacancy
V-I pair =
Most damage is done by nuclear interactions
About 15 eV needed to displace Si from lattice site,
Frankel defect
Self
create vacancy/interstitial pair
interstitial
Transitions,
Nuclear
Viscosity,
non-local
local
Coulomb
electrons
electrons
collisions
R
substrate
velocity
More effective
at smaller v
ion
More effective at larger v
ion
R
p
,
ff iMff lttMore effecti e a largereffective at
iion
g
ff iMfftMore effecti eeffective
lltll
Most damage occurs near limit of
p
3.155J/6.152J,2003 24
Implantation damage
The ions damage the crystal structure,and might cause amorphization,
Dose needed to
amorphize a silicon
substrate
Need solid-phase
epitaxy to recrystallize
the amorphous regions
How many Si atoms
does an implant displace?
3.155J/6.152J,2003 25
Implantation damage
A post-implant anneal (e.g,>850?C) must be done to restore
atoms to lattice sites and ‘activate’ the dopant,This causes
diffusion of the dopant profile,and formation of defect clusters,
Transient effect on diffusion are very important!
Effective transient
diffusion distance for
B in Si after
implantation with Si
ions,
As the damage
anneals out,diffusion
const,D,decreases
3.155J/6.152J,2003 26
Example,
A 30 kV implant of B is done into bare Si,The dose is 10
12
cm
-2
,
-what is the as-implanted profile?
log N(x)
c
o
Rp
DRp
From chart,in Si
Rp = 110 nm,DRp = 38 nm
also from Q = c
o
√2pDR
p
you can get c
o
depth
-what thickness of silica mask would you need to keep the B
content below the background level of 10
14
cm
-3
of P?
log N(x)
c
o
From chart,in silica
DRp
DRp = 36 nmRp = 100 nm,
also from Q you can get c
o
Now find the depth at which N(x)
3.155J/6.152J,2003
10
14
cm
-3
Rp
depth
27
reaches the background depth,
How can we make shallow implants (e.g,50 nm)?
e.g,Boron has a large projected range,and channeling is a
problem,How can we reduce this?
-lower energy?
-implant other species?
-preamorphize?
-transient high diffusion due to kickout?
-use predep/drivein using solid source?
-plasma implant?
3.155J/6.152J,2003 28
Some other applications
1) Implant only certain parts of wafer,use a mask such that R
p
lies
within the mask material,Use to form self-aligned source and
drain regions,for example,in a MOSFET,
how thick should the mask be
mask
substrate
what really happens
at the edges of the
implanted region
3.155J/6.152J,2003 29
2) Buried dielectrics,e.g,SOI (Silicon on insulator)
Form Si
3
N
4
using N+ implant,or SiO
2
using O+ implant
Useful to isolate devices,why is this important?
MOSFET built after SOI
G
substrate is implanted and
annealed
Si
insulator
Si
S D
3.155J/6.152J,2003 31
Discussion
When do you prefer to use predep/drive in vs,ion implantation?
Can you think of cases where you could save some process steps
using ion implantation?
What could the ion implant do to the background dopant?
Are there any particular problems in ion implantation for substrates
such as GaAs?
3.155J/6.152J,2003 32
We saw how dopants were introduced into a wafer by using diffusion
(‘predeposition’ and ‘drive-in’).
This process is limited:
-cannot exceed solid solubility of dopant
-difficult to achieve light doping
Ion implantation is preferred because:
-controlled,low or high dose can be introduced (10
11
- 10
18
cm
-2
)
-depth of implant can be controlled.
Used since 1980,despite substrate damage;
low throughput,and cost,
Plummer Ch,8,Campbell Ch,5
3.155J/6.152J,2003 1
Reminder,Analytical Solutions to Diffusion Equations
Solution for a limitless source of dopant (constant surface concentration),
Bound cond:
C
= D
2
C C (0,t ) = C
surf
C (∞,t ) = 0
t
Bound cond,
Const C
surf
t?z
2
è
z
(Cz,t) = C
surf
erfc
í
2 Dt
˙
,t > 0
z
Initial cond,
C (z,0) = 0
where erfc(x) = 1-erf(x) and t
p
= predep time
Dose Q = (2/√p) C
surf
√Dt
p
Dose in sample increases as t
1/2
3.155J/6.152J,2003 2
Diffusion of a thin surface layer into a solid
When a thin surface layer diffuses into a solid,what is C(z,t)?
Q = initial amount of dopant (‘dose’),
ú
C z,t)dz = Q = const,(# /area)
assumed to be a delta-function
(
-?
t
1
t
2
t = 0
C
Bound cond,
dC(0,t) Bound cond,
= 0
C(?,t) = 0Solution is a Gaussian,dz
2
Q
è
z
(C z,t) = exp -
pDt
í
4 Dt
˙
z
0
(
Initial cond,
Cz,0) = 0 ( z ≠ 0)
diffusion length
a = 2 Dt
Dose in sample constant in time
3.155J/6.152J,2003 3
Example,
wafer originally has a uniform
dopant level,e.g,donor.
Predep plus drive-in
introduces a second dopant,
an acceptor,
At a certain depth,
a p-n junction is formed,
A third pre-dep of donor
can then be done to make
an npn transistor,
Problem,
can only make profiles
consisting of superposed
Gaussians centered at the
substrate surface,
3.155J/6.152J,2003 4
Since the maximum amount of a dopant that can dissolve in the Si is
given by the solid solubility,you may be limited in the amount of
dopant that can be incorporated,
3.155J/6.152J,2003 5
Ion Implantation
Beam of energetic dopant ions is fired into surface of wafer,
Energies are 5 - 200 keV,
This leads to implantation (burial) of the ions into the substrate,
What happens at the substrate?
Ions can,bounce off
absorb
sputter atoms (10 eV - 10 keV)
implant into surface (5 keV - 200 keV)…
and do tremendous damage
3.155J/6.152J,2003
Ion Implantation Equipment
Ions generated in a source (from feed gas,e.g,BF
3
,AsH
3
,PH
3
,.,or
heated solid source,then ionized in arc chamber by electrons from hot filament)
select desired species by q/m,using a magnet,
accelerated by an E-field and focused using electrostatic lenses
and impact substrate
(a bend removes neutrals) in raster pattern,
3.155J/6.152J,2003 7
What happens to ions inside the material?
Ions lose energy,dE/dx,interacting
elastically with nuclei and inelastically with electrons
dE
=-N S (E ) + S (E)
][
n
dx
e
S (E) is Stopping power (eVcm
2
)
i
R
0
dE
Ion range in target,
R =
ú
dx =
1
E
ú
N
0
S
(
E
)
+ S
(
E
)
0 ne
What can we say about
nuclear and electronic stopping…
3.155J/6.152J,2003 8
Nuclear stopping power,Coulomb scattering (assumed elastic)
Incident ion interacts with
b
q
f
b
E
1
,M
1
M
2
Energy lost by incoming ion (microscopic)
DE = E
1
1 -
sin
2
f
cosq sinf + cosf sinq
ì
ó
0 2 4 6 8 10
M
2
/M
1
1.0
0.8
dE/E
0.4
0.2
0.0
nucleus of stationary ion
= impact parameter
The angles depend on masses and on b,
Max,energy loss is when b = 0,f = 0,
4 M
1
M
2
DE = E
1
(
M
1
+ M
2
)
2
3.155J/6.152J,2003 9
q
f
b
E
1
,M
1
M
2
F μ
Q
1
Q
2
r
2
Nuclear stopping power,Coulomb scattering (assumed elastic)
q
f
b
,
1
2
So nuclear scattering
At 100 keV an ion of 15 amu
has velocity,v
ion
≈10
6
m/s!
This is 1000 times faster
than speed of sound in solids…
So ion is far past nucleus
before nucleus can displace
in response to Coulomb force
is not strong at high ion velocity;
There are also
only significant
inelastic collisions
when ion slows down,
that transfer energy…
3.155J/6.152J,2003 10
Electronic stopping power,also Coulomb interactions,but inelastic)
Non-local,ion experiences drag due to,free” or polarizable electrons:
incident ion
attracts electron polarization,
Local,
=> energy and moment transfer
E
e
-
e
-
e
-
Ion velocity=> charge separation,drag
passing ion causes internal electronic transitions
Because electrons can follow fields up to optical frequencies,
(velocities of 10
5
m/s - 100 times faster than phonons)
S ( E) = cv
ione
electronic losses dominate at higher ion velocities,
3.155J/6.152J,2003 11
= kE
1 2
Stopping power in Ion Implantation
At each impact,the ion loses some energy,It travels through a
vertical projected range R
p
before stopping,It transfers energy to
target via both electronic
and nuclear interactions
Transitions,
Nuclear
Viscosity,
local
Coulomb
non-local
electrons
collisions
electrons
substrate
velocity
More effective at larger v
ion
More effective
at smaller v
ion
R
p
R
p
= dx
0
R
ú
=
1
N
dE
S
n
E
( )
+ S
e
E
( )
0
E
0
ú
3.155J/6.152J,2003 12
t
Stopping power in Ion Implantation
Most damage is done by nuclear interactions
About 15 eV needed to displace Si from lattice site,
create vacancy/interstitial pair (Frankel defect)
Transitions,
Nuclear
Viscosity,
non-local
local
Coulomb
electrons
electrons
collisions
R
substrate
velocity
More effective
at smaller v
ion
More effective at larger v
ion
R
p
,
ff iMilMMtlargerat
iion
More effective g
ff iMiM eMore effectieffective
ll
Most damage occurs near limit of
effecti e atti
ti
p
3.155J/6.152J,2003 13
3.155J/6.152J,2003 15
From S
e
and S
n
,R
p
and DR
p
R
p
DR
p
in Si
in Si
in Si
in GaAs
DR
p
R
p
R
p
(?)
DR
p
DR
p
R
p
(?)
can be calculated,
(?)
(?)
Composition profile for ion implantation
If the depth is x,the impurity concentration C(x) is
approximated by a gaussian
ê
Cx
)
= C
p
exp
á
-
(
x - R
p
)
2
(
á
2DR
2
p
ˉ
where C
p
is the peak concentration,R
p
the projected range and DR
p
the standard deviation of the projected range (vertical straggle),
The implanted dose is given by Q (Number/area)
(Q =
ú
C x)dx
-?
Q = 2pDR
P
C
P
So a given dose will determine the peak concentration,
3.155J/6.152J,2003 16
200 keV implants in Si
Why do light atoms have greater vertical projected range R
p
and
standard deviation DR?
p
3.155J/6.152J,2003 17
B in Si
gaussian
The composition profiles are not always perfect Gaussians,there can be a skew
or distortion (kurtosis) making the profile asymmetric,
3.155J/6.152J,2003 18
Channeling
If the ions are incident parallel to a major crystal direction,they
can pass through the structure with less scattering,so the range is
much larger than expected,
3.155J/6.152J,2003 19
Nuclear Stopping
Need to sum the effects of
all the scattering events,
e.g,using Monte Carlo
modeling,Nuclear
stopping,S
n
,can be
modeled by Coulomb
scattering (so it depends
on impact parameter,
relative masses,and E),
3.155J/6.152J,2003 20
Modeling
Distributions of ions after implant can be modeled using a Monte
Carlo calculation to give projected range,Can include both nuclear
and electronic stopping
Ion implantation can be modeled using SUPREM,which calculates
dopant profiles vs,implant conditions and annealing,
3.155J/6.152J,2003 21
P
*
ê
C
P
** *
2ln
á
á
= R
P
x
m
= R
P
+DR
P
C
B
ˉ
x
m
= range +
some multiple,m,of std dev’n
Dose penetrating mask,
2
˙
Q
P
=
Q
ú
exp
í
í
è
-
á
á
ê
x - R
P
*
*
˙
dx
2pDR
P
*
2DR
P
ˉ
x
m
Ion implantation through a mask
Range,R
p
* and standard deviation,DR * are for ions in mask
p
*
ê
(x - R
P
)
2
* *
m
For an efficient mask,
C
(
x
m
)
= C
P
exp
á
-
￡ C
B
*2
2DR
P
ˉ
Mask thickness,
+ mDR
Background
concentration
in substrate
*
Q
*
Q
P
= erfc
ê
á
á
x
m
- R
P
2DR
P
*
ˉ
2
3.155J/6.152J,2003 22
In GaAs
In SiO2
In Photoresist
Damage in Ion Implantation
Vacancy
V-I pair =
Most damage is done by nuclear interactions
About 15 eV needed to displace Si from lattice site,
Frankel defect
Self
create vacancy/interstitial pair
interstitial
Transitions,
Nuclear
Viscosity,
non-local
local
Coulomb
electrons
electrons
collisions
R
substrate
velocity
More effective
at smaller v
ion
More effective at larger v
ion
R
p
,
ff iMff lttMore effecti e a largereffective at
iion
g
ff iMfftMore effecti eeffective
lltll
Most damage occurs near limit of
p
3.155J/6.152J,2003 24
Implantation damage
The ions damage the crystal structure,and might cause amorphization,
Dose needed to
amorphize a silicon
substrate
Need solid-phase
epitaxy to recrystallize
the amorphous regions
How many Si atoms
does an implant displace?
3.155J/6.152J,2003 25
Implantation damage
A post-implant anneal (e.g,>850?C) must be done to restore
atoms to lattice sites and ‘activate’ the dopant,This causes
diffusion of the dopant profile,and formation of defect clusters,
Transient effect on diffusion are very important!
Effective transient
diffusion distance for
B in Si after
implantation with Si
ions,
As the damage
anneals out,diffusion
const,D,decreases
3.155J/6.152J,2003 26
Example,
A 30 kV implant of B is done into bare Si,The dose is 10
12
cm
-2
,
-what is the as-implanted profile?
log N(x)
c
o
Rp
DRp
From chart,in Si
Rp = 110 nm,DRp = 38 nm
also from Q = c
o
√2pDR
p
you can get c
o
depth
-what thickness of silica mask would you need to keep the B
content below the background level of 10
14
cm
-3
of P?
log N(x)
c
o
From chart,in silica
DRp
DRp = 36 nmRp = 100 nm,
also from Q you can get c
o
Now find the depth at which N(x)
3.155J/6.152J,2003
10
14
cm
-3
Rp
depth
27
reaches the background depth,
How can we make shallow implants (e.g,50 nm)?
e.g,Boron has a large projected range,and channeling is a
problem,How can we reduce this?
-lower energy?
-implant other species?
-preamorphize?
-transient high diffusion due to kickout?
-use predep/drivein using solid source?
-plasma implant?
3.155J/6.152J,2003 28
Some other applications
1) Implant only certain parts of wafer,use a mask such that R
p
lies
within the mask material,Use to form self-aligned source and
drain regions,for example,in a MOSFET,
how thick should the mask be
mask
substrate
what really happens
at the edges of the
implanted region
3.155J/6.152J,2003 29
2) Buried dielectrics,e.g,SOI (Silicon on insulator)
Form Si
3
N
4
using N+ implant,or SiO
2
using O+ implant
Useful to isolate devices,why is this important?
MOSFET built after SOI
G
substrate is implanted and
annealed
Si
insulator
Si
S D
3.155J/6.152J,2003 31
Discussion
When do you prefer to use predep/drive in vs,ion implantation?
Can you think of cases where you could save some process steps
using ion implantation?
What could the ion implant do to the background dopant?
Are there any particular problems in ion implantation for substrates
such as GaAs?
3.155J/6.152J,2003 32
