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Previous Lecture
Vacuum & Plasma systems for
Dry etching
Objectives
From this “evaporation” lecture you will learn:
Lecture 9: Evaporation & sputtering
• Evaporator system layout & parts
• Vapor pressure
• Crucible heating techniques
• Deposition rates
• Step coverage
• Multicomponent films
Evaporator system
WaferTilt / rotation stage
Wafer shutter
Charge in
crucible
Film thickness gauge
Film thickness monitor (FTM)
• FTM gauge is a piezo-electric
quartz crystal.
• The acoustic self oscillation
frequency f0 (~6 MHz) can easily
be measured electrically.
• f0 decreases with increasing
deposits.
• The FTM gauge should be
positioned to view the crucible
even when the wafer shutter is
closed.
Quartz crystal
Shutter
Dual head
Vapor pressure
The generation of vapor
pressure is called:
• Evaporation from a liquid
• Sublimation from a solid
E.g.
0.01 Torr - 1 Torr
Normal deposition rate
• Vapor pressure ~ 0.1 Torr
near the charge
C Cr Mg
Sb SiO SnO2
Ag
Vapor pressure
~ 0.1 Torr
The vapor pressure is local
and much higher than the
overall chamber pressure.
Too high vapor pressure
(deposition rates) may cause
the vapor to condense
prematurely, forming droplets
that could hit the wafer and/or
FTM.
Vapor pressure
High vapor pressure can
generate a virtual source
above the crucible.
• Resistive heating
Boats
Wire baskets
Wire heated crucibles
Plated rods
Box sources
• Inductive heating
• Electron beam heating
Focused electron beam
Low flux electron beam
Crucible heating techniques
Choice of crucible and heating
method depend on the material to
be deposited.
A poorly chosen crucible material
may alloy with the charge.
E.g. Al & Ni alloys with W
Resistive heating
I
Resistive heating
Wire basket
Chrome plated Tungsten rod
Resistive heating
Resistive heated crucible
Resistive heating
Baffled box sources
Some charge materials may jump
around during sublimation. Use a box
with chimney. (Baffled box source)
E.g. for SiO
Focused electron beam source
EmitterCrucible
Magnet poles
Direct heating of the
charge.
Temperature gradient
from center out.
Single crucible or
several crucibles in a
linear or revolving indexer.E-beam
Vapor
X-ray emission may
cause wafer damage.
x-rays
Focused electron beam source
~Beam current
control
Ibeam
~300mA
Uacc
~7kV
Ifil ~20A
Cathode
Anode
Beam
shaper
B
e-
Focused electron beam source
~
~
~
Long-sweep
Lat-sweep
Beam current
control
Lat-pos
Long-pos Ibeam
~300mA
Uacc
~7kV
Ifil ~20A
BB
B
e-
Deposition rates
Pe
Atomic (molecular) fluxatoms
time ∙ area
J =Pe
2 π k T M
RME = J ∙ M = Pe
M
2 π k T
Mass evaporation rate (Mass flux)mass
time ∙ area
Mass loss ratemass
time
RML = J ∙ M ∙ A = Pe ∙ AM
2 π k T
A
Equilibrium partial
vapor pressure
Deposition rates
Deposition rate thickness
time
Pe
A
R
Rd = J ∙ M ∙ A π R2
1ρ
1
Deposition rates
Deposition rate thickness
time
Pe
A
Φ
Θ
R
Rd = J ∙ M ∙ A =π R2
cosΘ cosΦ1ρ
=M
2 π k T
Pe ∙ A
ρ π R2
cosΘ cosΦ
Most vapor condenses on the chamber walls
Big material waste
Deposition rates
Deposition rate thickness
time
Pe
A
Φ
Θ
R
Rd =M
2 π k T
Pe ∙ A
ρ 4 π r 2
1
Spherical geometry
r
r
cosΘ = cosΦ =R2 r
Many wafers with equal deposition rate
Less material waste
Independent ofRΦΘ
Deposition rate
What is the deposition rate, in the center of the wafer, for gold at a charge
temperature of 1500°C and 3cm2 crucible area?
The distance from the
charge to the wafer
is 40cm.
0.04
40cm
Deposition rate
Step coverage
Except for near the source, the
vapor travels in straight paths.
Strong shadow effect.
Poor step coverage.
Tim
e
Wafer rotation and wafer heating
increase step coverage.
Wafer heating may also increase
film density and film adhesion.
Multicomponent films
Alloy evaporation Co-evaporation Multilayer evaporation
Conservation of
composition
requires identical
vapor pressure.
Correct
composition
requires well
controlled
deposition rates.
Layers can be heated and
alloyed.
Alloy film Alloy film
Example of co- and multilayer
evaporation is molecular beam
epitaxy. (MBE)
Objectives
From this “sputtering” lecture you will learn:
Lecture 9: Evaporation & sputtering
• DC & RF sputter systems
• Physics of sputtering
• Sputter yield
• DC & RF sputter sources
• Film quality
morphology
step coverage, stress, etc.
• Reactive sputtering
• Ion beam sputter deposition
• Evaporation / sputtering comparison
DC sputter system
Ar gas
inletWafer
Shutter
Source
DC source works for conducting targets only.
Difficult to fit a film
thickness monitor.
RF sputter system
Ar gas
inletWafer
Shutter
Source
RF source works also for non conducting targets.
~
z-match
Difficult to fit a film
thickness monitor.
Physics of DC sputtering
Sputtered target atoms Deposit on the wafer
Electrons & ions Create new ions through collisions
Target (Electrode)
Wafer
E
Sputtered target
atom
Secondary
electron emission
Ion impactAr
IonizationAr
Ar
Target dependence of sputter yield
Threshold energy
Sputter yield increases with
ion energy but starts to
saturate near 1keV.
Argon
Sputter yield for some target
materials depend strongly on the
angle of the incident ion.
Projectile dependence of sputter yield
Highest sputter yield for:
• Heavy ions
• Ions with full valence shell
45keV !!!!
Much higher than magnetron
sputtering.
Ar
Ne
Kr
Xe
Magnetically confined plasma
Magnetron, commonly used for sputter deposition sources.
E
Review from Lecture 6.
Magnetron sputter source
• DC and RF sources have targets with negative potential.
• Free electrons try to get away from the target proximity.
• The escape is less successful where the magnetic field is perpendicular
to the escape route.
e-
B
Target erosion
• The target of a typical magnetron source erodes
in a ring pattern.
Eroded target area
• Uneven erosion reduces the lifetime of the target.
• Uneven erosion may change film thickness
uniformity over time.
Target erosion
A new gold sputter target
will be expen$ive!
Eroded target area
W. Disney
Alternative magnetron orientation
Horizontal sputtering Sputter-up
T
T1
2
3
Morphology
Zone 1: Amorphous, porous film.
Zone T: Small grains, smooth and relatively dense film.
Zone 2: Tall narrow columnar grains. Moderately rough surface.
Zone 3: Large 3-D grains. Moderately rough surface.
Au evaporated onto
wafer @85°C
3-zone model
Movchan &
Demchishin
Example:
T/TM = 358K / 1335K =
Evaporation
= 0.27
Morphology
Zone 1: Amorphous, porous film.
Zone T: Small grains, smooth and relatively dense film.
Zone 2: Tall narrow columnar grains. Moderately rough surface.
Zone 3: Large 3-D grains. Moderately rough surface.
3-zone model
1Pa = 7.5mTorr
T
Ti sputtered on wafer
@ 250°C, 7mTorr
Example:Thornton
T/TM = 523K / 1948K =
= 0.27
Directionality
Magnetron sputter deposition has low directionality.
Table
Wafer
Target
The size of the target is about the same as the distance between the
target and the wafer. Approx. 10cm – 15cm
Most material deposits on the wafer.
Step coverage
Deposition progress of sputtered film
All surfaces covered
Low directionality Good step coverage
Directionality
The deposition directionality can be increased by the
use of a collimator.
Table
Wafer
Target
Collimator
A substantial fraction of target material is wasted.
Film stress
Compressive film stress
The film expands if detached
from the wafer.
Tensile film stress
The film contracts if detached
from the wafer.
Film stress causes
Thermal mismatch Tensile or Compressive
The film and the substrate have different thermal expansion
coefficients. The deposition is not done at room temperature.
Atomic attraction Tensile
Atomic attraction seeks to bridge microvoids during deposition.
Mechanical deformation Compressive
Ion impacts during deposition may cause permanent deformation of
the film.
Examples
Measuring film stress
= Film stress
= Bow change
t = Film thickness
E = Young’s modulus of wafer
= Poisson’s ratio of wafer
T = Wafer thickness
R = Wafer radius
2
2
31 R
TE
t
Wafer with
stressed film
R
Reactive sputter system
Ar gas
inlet
~z-match
Process
gas inlet
Why reactive sputtering? Examples of reactive sputtering:
SiO2O2
Si Si3N4N2
Ti TiNN2
In/Sn In2O3 / SnO2O2
Si
Target Gas Film
(ITO)
• Variety of films from few targets.
• Often higher deposition rates.
• DC sputter deposition of oxides possible.
Ion beam sputter deposition system
Ion gun
Ar gas
inlet
Ar gas
inlet
Ion gun
Targets
Wafer
Widely used for dense, high-quality optical coatings.
Evaporation-Sputtering comparison
E-beam
evaporationMagnetron
sputtering
High film density
Good adhesion
Good step coverage
High lift-off compatibility
High film thickness uniformity
Low material consumption (waste)
Low material investments
Low x-ray emission
In-situ thickness & rate control
Good compound deposition control
High system layout flexibility
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Next Lecture
Thin film deposition
using
Pulsed Laser Deposition
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