Transcript Intro_Implantation

Chapter 8
Ion Implantation
ION IMPLANTATION SYSTEM
Ion implanter is a high-voltage accelerator
of high-energy impurity ions
 Major components are:

– Ion source (gases such as AsH3 , PH3 , B2H6)
– Mass Spectrometer (selects the ion of
interest)
– HV Accelerator (voltage > 1 MeV)
– Scanning System (x-y deflection plates for
electronic control)
– Target Chamber (vacuum)
ION IMPLANTATION SYSTEM

Cross-section of an ion implanter
0 to 175
kV
m/q=(B2R2)/(2V)
Resolving aperture
R
C
R
C
R
Neutral beam trap
and beam gate
C
Beam trap
Neutral beam
Or Faraday cup
2
Integrator
90o
analyzing
magnet
1
Ion
source
25 kV
Q
3
Acceleration
tube
Acceleration energy =
voltage x charge on ion
Focus
5
y-axis
scanner
4
x-axis
scanner
Wafer in process chamber
http://www.bpc.edu/mathscience/chemistry/images/periodic_table_of_elements.jpg
ION IMPLANTATION

High energy ion enters crystal lattice and
collides with atoms and interacts with
electrons
– Types of collisions: Nuclear and electron

Each collision or interaction reduces
energy of ion until it comes to rest
– Amount of energy loss is dependent on ion,
the energy it has at the time of the scattering
event, and the type of scattering.
From Handbook of Semiconductor Manufacturing Technology
by Yoshio Nishi and Robert Doering
From Handbook of Semiconductor Manufacturing Technology
by Yoshio Nishi and Robert Doering
Channeling

Deep penetration by the ion because it traveled
along a path where no semiconductor atoms are
situated
– Process is used for materials characterization:
Rutherford backscattering

To prevent channeling
Implantation is performed at an angle of about 8° off
the normal to the wafer surface.
– The wafer surface is amorphorized by a high dose,
low energy implantation of a nonelectrically active
ion.
–
 Hydrogen, helium, and silicon are common ions used
Determining the Dose

The implanted dose can be accurately
measured by monitoring the ion beam
current using a Faraday cup
– The integrated current during the implant
divided by the charge on the ion is the dose.
Post Implantation Anneals

An annealing step is required to repair
crystal damage (recrystallization) and to
electrically activated the dopants.
– Dislocations will form during the anneal so
times and temperatures must be chosen to
force dislocations disappear.
– If the anneal time is long and the temperature
is high, a drive of the implanted ions may
occur.
ION IMPLANTATION


Projected range (RP): the average distance an ion travels before it
stops.
Projected straggle (RP): deviation from the projected range due to
multiple collisions.
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MODEL FOR ION
IMPLANTATION

Distribution is Gaussian
Cp = peak concentration
Rp = range
Rp = straggle
C ( x)  C P e

( xRp )
2 R p
2
2
MODEL FOR ION
IMPLANTATION

For an implant contained within silicon, the dose
is
Q  2 R pC p
ION IMPLANTATION MODEL

Model developed by Lindhard, Scharff and
Schiott (LSS)
– Range and straggle roughly proportional to
energy over wide range
– Ranges in Si and SiO2 roughly the same

Computer models now available
Range of impurities in Si
1.0
B
Projected range (mm)
P
As
Sb
0.1
Rp
0.01
10
100
Acceleration energy (keV)
1000
Straggle of impurities in Si
Normal and transverse straggle
(mm)
0.10
Sb
B
0.01
P
As
Rp
R
0.002
10
100
Acceleration energy (keV)
1000
Si
Si3N4
SiO2
AZ-7500 resist
http://www.iue.tuwien.ac.at/phd/hoessinger/node22.html
http://www.ensc.sfu.ca/~glennc/e495/e495l7j.pdf
http://www.ensc.sfu.ca/~glennc/e495/e495l7j.pdf
SiO2 AS A BARRIER

The minimum oxide thickness for selective
implantation:
Xox = RP + RP (2 ln(10CP/CBulk))0.5

An oxide thickness equal to the projected range
plus six times the straggle should mask most ion
implants.
Other Materials
A silicon nitride barrier layer needs only be 85%
of the thickness of an oxide barrier layer.
 A photoresist barrier must be 1.8 times the
thickness of an oxide layer under the same
implantation conditions.
 Metals are of such a high density that even a
very thin layer will mask most implantations.

– Nickel is one of the most commonly used metal
masks
ADVANTAGES

Low temperature process
– The wafer is cooled from the backside during high
energy, high current diffusions are performed
– Less change of stress-induced dislocations due to
thermal expansion issues

Wider range of barrier materials
– Photoresist

Wider range of impurities
– No concern about solid solubility limitations
– Implantation of ions such as oxygen, hydrogen,
helium, and other ions with low solid solubility is
possible.
Advantages over Diffusion

Better control and wider range of dose
compared to predep diffusions
– Impurity concentration profile controlled by
accelerating voltage
– Very shallow layers
– Lateral scattering effects are smaller than
lateral diffusion.
Complex-doping profiles can be produced
by superimposing multiple implants
having various ion energies and doses.
NITROGEN CONCENTRATION (ATOMIC PERCENT)

FINAL PROFILE
15
200 KILOELECTRON
VOLTS
100
10
50
10 20
5
0
0
50
100
150
200
DEPTH (NANOMETERS)
250
300
350
RADIATION DAMAGE
Impact of incident ions knocks atoms off
lattice sites
 With sufficient dose, can make amorphous
Si layer

RADIATION DAMAGE
Critical dose to make layer amorphous
varies with temperature and impurity
1018
Critical dose (atom/cm2)

1017
1016
B
1015
P
1014
Sb
1013
0
1
2
3
4
5
6
Temperature, 1000/T
7
8
(K-1)
9
10
Recrystallization

Radiation damage can be removed by
annealing at 800-1000oC for 30 min. After
annealing, a significant percentage of the
impurities become electronically active.
– Point defects coalesce into line dislocations
– Line dislocations merge into loop dislocations
– Loop dislocations slowly disintegrate as
interstitial Si atoms move on to lattice sites
Ion Implantation

Implanting through a sacrificial oxide layer:
– Large ions (arsenic) can be slowed down a little
before penetrating into the silicon.
– The crystal lattice damage is suppressed (at the
expense of the depth achieved).
– Collisions with the thin masking layer tends to cause
the dopant ions to change direction randomly,
thereby suppressing channeling effect.
– The concentration peak can be brought closer to the
silicon surface.
Ion Implantation
For deep diffusion (>1µm), implantation is
used to introduce a certain dose, and
thermal diffusion is used to drive in the
dopants.
 The resulting profile after diffusion can be
determined by:

1
C ( x, t ) 
2
Q
R p  2 Dt
2

e
( x R p )2
2 ( R p 2  2 Dt )