Transcript Undergraduate Admissions & College of Engineering

Ion Implantation
ECE/ChE 4752: Microelectronics
Processing Laboratory
Gary S. May
March 4, 2004
Outline
 Introduction
Ion Distribution
 Ion Stopping
 Ion Channeling
 Implant Damage
 Annealing

Basics
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Definition: penetration of ionized dopant atoms into a
substrate by accelerating the ions to very high energies
Allows precisely controlled doses of dopant atoms to be
injected (1011 cm-2 to 1018 cm-2)
Used instead of pre-deposition in industry, but we don't
use because of:
 cost ($2M for one machine)
 equipment maintenance
 limited throughput
 safety
Equipment
Equipment Description
(1) Gas source of material, such as BF3 or AsH3 at high
accelerating potential; valve controls flow of gas to ion
source
(2) Power supply to energize the ion source
(3) Ion source containing plasma with the species of interest
(such as +As, +B, or +BF2), at pressures of ~ 10-3 torr
(4) Analyzer magnet: allows only ions with desired
charge/mass ratio through
(5) Acceleration tube through which the beam passes
(6) Deflection plates to which voltages are applied to scan the
beam in x and y directions and give uniform implantation
(7) Target chamber consisting of area-defining aperture,
Faraday cage, and wafer feed mechanism
Outline
Introduction
 Ion Distribution
 Ion Stopping
 Ion Channeling
 Implant Damage
 Annealing

Implanted Ions
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Implantation energies typically 1 keV - 1 MeV
Ion distributions have depths of 10 nm - 10 µm
Doses vary from 1012 ions/cm2 for threshold
voltage adjustment in MOSFETs to 1018
ions/cm2 for formation of buried insulating
layers.
Advantages of ion implantation:
(1) Precise control and reproducibility
(2) Lower processing temperature
Range
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Energetic ions lose energy through collisions with
electrons and nuclei in substrate
Total distance an ion travels = range (R)
Projection of this distance along axis of incidence =
projected range (Rp).
# collisions per unit distance and energy lost per
collision are random variables; standard deviation in
the projected range = projected straggle (sp)
Statistical fluctuation along an axis perpendicular to
the axis of incidence = lateral straggle (s┴)
Projected
Range
Ion Profile

Implanted profile can be approximated by a
Gaussian distribution function:
n( x ) 

 (x  Rp )2 
exp 

2
2s p 
2 s p

S
where S is the ion dose per unit area
Maximum concentration is at Rp
Example
For a 100 keV boron implant with a dose of 5 × 1014 cm-2,
calculate peak concentration.
Solution:
From Fig. 6a, Rp = 0.31 mm and sp = 0.07 µm
n( x ) 
 (x  Rp )2 
exp 

2
2s p 
2 s p

S
  ( x  Rp )2 
2( x  R p )
dn
S

exp 
0
2
2
dx
2 s p 2s p
 2s p

→ n(Rp) = 2.85 × 1019 cm-3
Outline
Introduction
 Ion Distribution
 Ion Stopping
 Ion Channeling
 Implant Damage
 Annealing

Mechanisms

Nuclear (Sn)– transfer of energy from
incoming ion to target nuclei (collisions)

Ionic (Se) – interaction of incident ion with
electrons surrounding target atoms
(Coulombic interaction)
Stopping and Range

Average energy (E) loss per unit distance:

dE
 S n (E)  Se (E)
dx
Total distance traveled:
R
E0
dE
R   dx  
S (E)  Se (E)
0
0 n
where: E0 = initial energy
Nuclear Stopping
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When spheres collide,
momentum is
transferred
Deflection angle (q)
and velocities, v1 and
v2, can be obtained
from conservation of
momentum and energy
Maximum energy loss
in a head-on collision:
 4M 1 M 2 
1
2
M 2 v2  
E
2 0
2
 (M 1  M 2 ) 
Electronic Stopping

Proportional to velocity of incident ion:
S e (E)  ke E
where ke is a weak function of atomic mass
and number
 ke ~ 107 (eV)1/2/cm for silicon
Total Stopping Power
Outline
Introduction
 Ion Distribution
 Ion Stopping
 Ion Channeling
 Implant Damage
 Annealing

Definition

Channeling occurs
when incident ions
align with
crystallographic
direction and are
guided between rows
of atoms.
Results

Only loss mechanism is electronic stopping,
and range can be significantly larger than it
would be in an amorphous target.

Ion channeling is particularly critical for
low-energy implants and heavy ions.
Minimizing Channeling

Can be minimized by :
(a) A blocking amorphous surface layer
(b) Misorientation of the wafer
(c) Creating a damage layer in wafer surface
Outline
Introduction
 Ion Distribution
 Ion Stopping
 Ion Channeling
 Implant Damage
 Annealing

Cause of Damage
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Nuclear collisions transfer energy to the lattice so
that host atoms are displaced resulting in implant
damage (also called lattice disorder).
Displaced atoms may in turn cause cascades of
secondary displacements of nearby atoms to form
a “tree of disorder” along ion path.
When displaced atoms per unit volume approach
the atomic density, material becomes amorphous.
Tree of Disorder
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Most energy loss for
light ions (e.g., 11B+) is
due to electronic
collisions→ little
damage, most occurs
near final ion position.
For heavy ions, most
energy loss is due to
nuclear collisions →
heavy damage.
Outline
Introduction
 Ion Distribution
 Ion Stopping
 Ion Channeling
 Implant Damage
 Annealing

Basics
Process of repairing implant damage (i.e.,
“healing” the surface) is called annealing
 Also puts dopant atoms in substitutional
sites where they will be electrically active
 2 objectives of annealing:
1) healing, recrystallization (500 - 600 oC)
2) renew electrical activity (600 - 900 oC)

Boron Annealing
Annealing depends on dopant type and dose.
 For a given dose, annealing temperature is
temperature at which 90% of the implanted
ions are activated by a 30 minute annealing
in a conventional furnace.
 For boron, higher annealing temperatures
are needed for higher doses.
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Phosphorus Annealing
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At lower doses, P annealing is similar to B.
When the dose is greater than 1015 cm–2, the annealing
temperature drops to about 600 °C.
At doses greater than 6 × 1014 cm–2, silicon surface
becomes amorphous, and semiconductor underneath
amorphous layer is a seeding area for recrystallization.
A 100 – 500 nm amorphous layer can be recrystallized in a
few minutes.
Full activation can be obtained at relatively low
temperatures.
Rapid Thermal Annealing
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Wafer heated to 600 –
1100°C quickly under
atmospheric conditions
Advantages:
 Short processing time
 Less dopant diffusion
and contamination
Disadvantages:
 Temperature
measurement/control
 Wafer stress and
throughput