Precursor Chemistry, Thin Film Deposition, Mechanistic Studies
Thin film Deposition - Las Positas Collegelpc1.clpccd.cc.ca.us/lpc/tswain/lect9.pdf · Thin Film...
Transcript of Thin film Deposition - Las Positas Collegelpc1.clpccd.cc.ca.us/lpc/tswain/lect9.pdf · Thin Film...
Thin Film Deposition
1
7 April 2003
Thin film Deposition!Evaporation
!Sputtering
Field Trip 6:00 PM, Monday April 21Do Not Come to Class!
Semicore
5027 Preston Ave.
Livermore, CA 94550
925-373-8201
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History of Thin Film Deposition
! thermally induced evaporation (by electrical resistance heating, induction heating, and electron beam heating),
! sputtering (diode, triode, magnetron)
! ion beam
! Chemical Vapor Deposition
! Molecular Beam Epitaxy
! laser ablation
PVD: Physical Vapor Depositon
source
evaporant
substrate
vacuum vessel
The three basic steps in any physical vapor deposition process: Evaporationfrom the source, Transport of evaporant, and Condensation of the evaporant.
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Benefits of Vacuum Deposition
! High chemical purity.! Good adhesion between the thin
film and substrate.! Control over mechanical stress
in the film.! Deposition of very thin layers,
and multiple layers of different materials.
! Low gas entrapment.
Parameters You Can Influence
! Incident Kinetic energy.
! Substrate temperature.
! Deposition rate of the thin film.
! Gas scattering during transport of the evaporant
! Augmented energy applied to the film during growth.
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Strike Layers may be necessary
! Gold does not form a chemical bond with many substrate materials.
! depositing a thin (500Å thick) "Binder" layer of chromium or niobium is a remedy to dramatically improve adhesion.
! Chromium and niobium are reactive and will bond with both substrate Si02 and Gold.
Evaporation Temperatures
Material
Evaporation temperature,
°C
Comments
Zinc 325 High vapor pressure at RT Aluminum 1390 Copper 1516 Chromium 1612 Lead 1680 Toxic Iron 1829 Nickel 1848
All materials evaporate, even at room temperatures. Heat simply accelerates the process.
Equilibrium Vapor Pressure. Cadmium…Rhenium
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Requirements for Filaments
"Electrically Conductive"High melting point."Good thermal shock resistance"Filament should be wettable by the charge materials."Low solubility for the charge materials.
helical filament
conical basket
flat boat with dimple
trough style boat
Induction Heated Thermal Evaporation! an electric current is
induced to flow through an electrically conductive charge material
! by the application of radio-frequency (RF) alternating current
! power may range from 1 to 50 kilowatts, depending on the size of the charge.
! The AC current is flowed through the copper coil which surrounds a refractory ceramic crucible.
induction coil
crucible
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Advantages and Disadvantages over Thermal Evaporation
Advantages! Low contamination of the deposited thin
films .! Improved control of deposition rate.! Larger charges can be loaded per
deposition run.
Disadvantages! RF power supplies are large and costly! Chemical interaction between the charge
and crucible can occur.
Work Accelerated Evaporator Concept
filament
focussing apreture
Cruciblecharge
electrons
filament heating power supply
accelerating voltage
vacuum vessel
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Cylindrical Focusing Electrode
filament
charge
evaporant
focussing electrode
electrons
water cooling circuit
Advantages: The evaporant may be directed at a substrate placed above the source without interference by a focusing aperture or filament. Additionally, the focusing aperture and filament do not become heavily overcoated.
Evaporator with steering coils
filament heating power supply
focussing electro- magnets
charge
substrates
evaporant
Advantages: source can be controlled by rastering the electron beam, improving the thickness uniformity and coverage of the substrate.
Note: in this design the electrons emitted from the filament impact the backside of the cathode, heating it so that it will in turn emit electrons. The cathode area which emits electrons is hemispherical,
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Self Accelerated e-beam evaporation
filament
focussing apreture
Cruciblecharge
filament heating power supply
accelerating voltage
vacuum vessel
anode
focussing coilsElectrons are
not accelerated into the charge.
Transverse Design is Commercialized
water inlet
water outlet
Top view
Side view
filamentcrucible liner charge
filament
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The Big Picture
TC 1
IG 2 TC 2
electric motorvacuum rotary feedthrough
substrate holding fixture
deposition shielding
Features (MDC)
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Horizontal Source Assembly on 8” Conflat
Source Assembly (Courtesy MDC)
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Programmable Sweep Controller
~$5000 with some options
Plasma Enhanced DepositionCold Cathode Design
! the cathode is biased negatively from -5 to -20 kV,
! A partial pressure of Argon or Helium from 1 to 100 mTorr is dynamically maintained.
! Electrons emitted by the cathode ionize process gas atoms. These ions are accelerated to the cathode.
vacuum flange
grid lead
grid
cathode
electron beam exit aperture
shield
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Plasma Enhanced DepositionHot Cathode Design
! Much higher deposition rates are permitted with this design.
! Radio frequency AC electric current is supplied to the cathode from a low voltage, high current power supply.
! Ionization of the process gas occurs as a result of the applied electrical power.
! The cathode needs to be water cooled.
Power supply 1000 A, 50 V
process gas
electrons
electro- magnets
process gas
evaporant
water cooling jacket
detail of cathode
Sputtering
"The verb to SPUTTER originates from Latin SPUTARE (To emit saliva with noise).
"Phenomenon first described 150 years ago… Grove (1852) and plücker (1858) first reported vaporization and film formation of metal films by sputtering.
"Key for understanding discovery of electrons and positive ions in low pressure gas discharges and atom structure (J.J. Thomson, Rutherford), 1897
"Other names for SPUTTERING were SPLUTTERING and CATHODE DESINTEGRATION.
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Penning 1935 (Post Magnetrons)
B
-E
Plasma
Cold Cathode Invention
Clarke 1971
Inversed cylindrical magnetron.Sputter inside a cylinder.Target also cylindrical.
Peter Clarke Founded Sputter Films in Santa Barbara on this Patent
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Materials Res. Co. 1991
DVD(Balzers)
Process Specific Magnetrons
Rotatable cylindrical magnetron (BOC, 1994).
Web coatings and glass coating.
Target materials sometimes difficult to find in cylindrical shape.
Alternative Magnetron Designs
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Diode Sputtering Basics
working distance
cathode
anode
G
e-
G+
C
vacuum vessel wall
mounting flange
electrical power feedthrough
insulator
power supply
- V
+V
Fundamentals of Sputare
! Following evacuation of the vessel to a low base pressure, a process gas (Typ. Argon) is admitted and maintained at a user-selectable pressure between 1-100 mTorr
! An electric bias of from 500 to 5000 V DC is applied to the target. Electrons emitted by the target strike process gas molecules in the vicinity of the target, and may cause the gas to become ionized.
! The positive ions thus created are accelerated towards the cathode by the applied negative bias. When the positive ions collide with the cathode, the kinetic energy transferred is sufficient to eject atoms of the cathode material.
! Secondary electrons, ions, and light (IR, visible, UV and X-rays) are also emitted during this collision.
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Downstream Pressure ControlThrottling Gate Valves permit switching between
100mT and 1E-07 T pressures with a turbopump
Why Argon?
In general, the sputter yield is greatest for the following set of conditions:
! High atomic weight process gas.! Low atomic weight cathode material.! Low concentration of reactive gas species
in the vessel.Argon is the most commonly employed process
gas for sputter deposition processes, as it has a high sputter yield for most metals, is chemically inert and non-toxic, and is relatively inexpensive (compared with the other noble gases (Krypton and Xenon).
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Characteristics of Sputtering
! Emissions from cathode include neutral atoms, ions (both positive and negative), electrons, neutral clusters of atoms and charged clusters of atoms. Of these, the vast majority are neutral atoms. These atoms have kinetic energies approximately 50 to 100 times that of neutral atoms generated from thermal evaporation sources.
! This additional energy is thought to be the reason for the greater adhesion often observed for sputter deposited films over thermally evaporated films.
! Due to the relatively high pressure in an operating sputter deposition chamber, the mean-free path of sputtered species is short. The numerous gas-phase collisions which the sputtered material suffers between the target and substrate tend to reduce the amount of kinetic energy the depositing species have upon arrival. the density and crystal structure of the thin film are affected.
Thermalization
! When sputtered atoms lose energy by gas collisions, they are said to be "thermalized“
! their kinetic energy is reduced to equal that expected for similar atoms at the ambient temperature. 100101.1
.1
1
10
100
Argon Pressure [mTorr]
Dis
tanc
e to
Rea
ch T
herm
al E
nerg
y [c
m]
Ta
Al
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‘Gas’ of charged particles Electrons (negative)Ions (positive)
Plasma ParametersDensity (ne, ni)Electron temperature (Te)Usually measured in electron volts (1eV = 12000°c)Plasma potential (Vp)
“Basic” Concepts
Collision type Mean free path
Sputtered neutral argon 5cm
Sputtered neutral – electron (ionisation) 400m
For 5 mtorr of argon (~300K) with 5ev electrons
The mean free path of the sputtered neutrals depends
on the target material and the background gas
The probability of a sputtered neutral being ionised by electron
collision on the way to the substrate (say 10 cm) is 0.025%
Mean Free Path Issues
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Collision type Mean Free Path
Electron – electron 40 m
Electron – argon (Momentum loss) 50 cm
Electron – argon (Ionisation) 5 m
Electron – argon (Double ionisation) 100 m
Argon – argon 2 cm
For 5 mtorr of argon (~300K) with 5eV electrons
The mean free path is the average distance a particlewill travel before undergoing a collision
Mean Free Path influenced by Particle Size
Triode Sputtering
cathode
anode
vacuum vessel wall
mounting flange
electrical power feedthrough
insulator
cathode power supply
- V
Filament power supply
e-e-e-
e-
anode power supply
substrate
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Magnetron Sputtering
N S N
DC Power supply
target (cathode)
plasma ringmagnetic field lines
Oblique view of planar magnetron cathode.
Side view of planar magnetron cathode showing magnetic structure.
Confinement between a negatively biased target and ‘closed’ magnetic field produces a dense plasma.
High densities of ions are generated within the confined plasma, and these ions are subsequently attracted to negative target, producing sputtering at high rates.
+ +-
Neg
ativ
ely
bias
ed ta
rget
-V
High density plasmagenerated by the combineelectrical and magnetic field
Resulting erosionof the sputter target
Magnetron Sputtering
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Plasma is a fluid of positive ions and electrons in a quasi-neutral electrical state
The vessel that contains this fluid is formed by electric and magnetic fields.
In many plasma coating applications positive ions are generatedby collisions between neutral particles and energetic electrons.
The electrons in a plasma are highly mobile,especially compared to the larger ions (typically argon for sputtering)
Control of these highly mobile plasma electrons is the key to all formsof plasma control
0+ e-1+
e-
Conversion of a neutral atom into an ion by electron collision
in a plasma
What is a Plasma
e-
xB
B
e-
E
B
ExB
S N N S
e-
Electron motion in a combined electric & magnetic field
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Rotating arrays (Garrett ,1983- Fujitsu, 1985 -Varian,Applied Materials 1990)# Semiconductor industry
Planar Rotating Magnetron
Basic Rectangular Magnetron
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Material will be sputtered at a rate that depends on the current of ions to the target, and the sputter yield:
But the energy of the incoming ions is just the voltage on the cathode, and the yield is approximately linear with energy:
So we can say:
( )EYIR ⋅∝
PowerVIR =⋅∝( ) VEEY ∝∝
Planar Magnetrons allow high Deposition rates
Target Uniformity also drives cost
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Magnetron Structure Cooling
N
S
N
SS
N
S S
water cooling circuit
magnetic field lines
sputtered material
ground plane shield
Typical Operating Parameters of a Magnetron
! Current density at the cathode: 10-50 mA/sq-cm
! Process gas pressure (Argon): 3 to 50 mTorr
! Cathode bias: -400 to -2000 VDC
! Working distance: 2 to 20 cm
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In-situ Film Metrics
! Quartz Crystal Microbalance
! Optical Monitoring
! Mechanical Stress Measurment
Ar+
Electrons lost from the discharge can be replaced by secondary emission - electrons liberated from the target due to ion impact.
This is characterised by the parameter γ. For every ion impacting on the target γ electrons are released by secondary emission.
For most metals, γ ~ 0.1, and is insensitive to ion impact energy (for ion energies < 1keV).
Secondary Ion Emission
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e-
ArAr+
e-
e-
Ions and electrons lost from the discharge can be replaced by ionising collisions
e- + Ar ⇒ e- + e- + Ar+
Main Menu
Ionizing Collisions
If an electron collides with an Argon ion, there is a possibility of the Argon losing another electron,
becoming doubly ionised
e- + Ar+ ⇒ e- + e- + Ar++
e-
Ar+
Ar++
e-
e-
Double Ionization
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B B
e- Ar+
Magnetic field traps electrons in the discharge,resulting in an electron having more ionising collisions
before being lost from the discharge.Ions, being much heavier, have a much larger gyro-radius,
and are relatively unaffected by the field.
Increasing Trajectory through Magnetic Fields
Electrons trapped by magnetic field, so lower pressures can be used.
Lower pressure means that sputtered atoms are less likely
to have a collision on the way to the substrate
High Pressure Discharge Low Pressure Discharge (idealised)
Magnetic Files allow a lower sustainable plasma pressure
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Sheath
Distance
Pote
ntia
l
Quasineutrality and the Wall Boundary Condition
He Ne Ar Kr Xe
Si
Ti
Al
Cu
0
0.5
1
1.5
2
2.5
3
Plasma Gas
TargetMaterial
Sputter YieldsFor Selected Plasmas Sputtering Different Target Materials (Ion Energy = 600 eV)
Ions impacting on the target can liberateneutrals from the target
Relative Sputter Yields
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The energy distribution of the sputtered neutrals is given by:
( ) ( )3BEE
EEf+
∝
E = energy of sputtered neutral
EB = surface binding energy of target ~ 5 eV
F(e) = probability of a sputtered neutral
being emitted with energy E
Thompson Sigmund Distribution
0
1
2
3
4
5
6
7
0 2 4 6 8 10 12 14 16 18 20
Energy Of Sputtered Neutral (eV)
f(E) /
Arb
itary
Uni
ts
EB/2
Thompson Sigmund Distribution