MSE 4121 Thin Film Technology
and Nanocrystalline Coating
5. Application of Cold Plasma to
Thin Film Deposition
Lecture: Dr. Zhiyuan Zeng
Office: BOC-R7190
E-mail: zhiyzeng@cityu.edu.hk
1
Outline
Classification of Deposition Process
PVD
Physical Vapor Deposition
Sputter deposition (also reactive)
Ion plating
Reactive ion plating
Activated reactive
evaporation
CVD
Chemical Vapor Deposition
Plasma enhanced CVD
Plasma decomposition
Chemical transport in plasma
Plasma stream transport
Surface Modification
Ion nitriding
Ion carburizing
Plasma oxidation
Plasma anodization
Plasma surface treatment
for polymers
2
Physical Vapor Deposition under Plasma
Conditions
•
•
•
•
•
PVD (e.g. sputter deposition or ion plating) can be used in thin film
deposition of
Elemental metals
Metal alloys
Compounds such as carbides, nitrides or oxides
PVD - used in industry
Sputter Deposition - Features
• Deposition - physical sputtering by ion bombardment
• Various materials - metals, alloys and insulators can be formed
• High melting point materials
3
IB: Thin Film Technology/Sputtering
5.1 Sputtering
A sputtered particle induced at
energetic impact of an ion
Momentum transferred to atoms of a solid
Relaxation: Repulsive forces among compressed atoms
Ejection of surface atoms
4
Definition of Sputtering Yield
•
•
Average number of atoms sputtered per incident ion
Sputtering Yield is affected
• Surface structure
• Ion mass
• Incident energy
• Rather insensitive to temperature (in certain cases, decreasing
sputtering yield with increasing the target temperature)
5
Sputtering Yield Depends on the Ion Energy
•
•
•
•
•
•
Rises rapidly from a threshold energy
Threshold 10 - 30 eV for metals
Above 100 eV, increases ~ linearly
Thereafter, a broad maximum and then
decreases slowly
Decrease with increasing ion energy –
penetration depth too large to eject atoms
from the deeper regions
Light ions (H2, He) maximum at a few
thousand eV – large penetration depth.
Heavy ions (Xe, Kr ) - maximum at
around 50 keV.
Fig.5.1.1 Sputtering yield of
some metals for Ar+ ions as a
function of their energy.
6
Sputtering Induced by Different Ions
•
•
•
•
•
•
Sputtering Yield for Different Ions
Maximum for inert ions.
Minimum for C, Mg, Al, Ca and Sc ions - sticking coefficient.
Sputtering by neutrals – lesser significance.
Energetic neutrals formed by neutralization of an ion beams for
sputtering insulators.
No sputtering is initiated by electrons.
7
Table 5.4.1 Sputtering Yield for argon ion bombardment at 600 eV
Target
Be
Al
Ti
V
Cr
Fe
Co
Ni
Cu
Ge
Zr
Nb
Yield
0.56
0.83
0.54
0.55
1.05
0.97
0.99
1.34
2.00
0.82
0.42
0.42
Target
Mo
Ru
Rh
Pd
Ag
Hf
Ta
W
Os
Ir
Pt
Au
Yield
0.54
0.67
0.77
1.32
1.98
0.39
0.30
0.32
0.41
0.46
0.7
1.18
Target
Al2O3
SiO2
TiO2
V 2 O3
Cr2O3
Fe2O3
ZrO2
Nb2O3
In2O3
SnO2
Sb2O3
Ta2O5
Yield
0.18
1.34
0.96
0.45
0.18
0.71
0.32
0.24
0.57
0.96
1.37
0.15
Target
CdS
GaAs
GaP
GaSb
InSb
SiC
Yield
1.2
0.9
0.95
0.9
0.55
1.8
8
Deposition and Etching
Relation between the deposition rate d’ of a film on the substrate and the
etching rate r’ of a target is
d’ = c r’
(5.4.1)
c- constant determined from the geometrical configuration of the target and
substrate, and the effect of scattering target atoms by gas particles in
plasma.
Etching rate r’ is proportional to the ion current density at the target
surface ji, thus
ji M a
r'  Y
 Nae
r’ - etching rate
ji - current density
Y- sputtering yield
Ma - atomic weight of the target
 - density in g/cm3.
(5.4.2)
9
•
•
Sputtering yield Y - maximum at
10 – 50 keV.
For the high deposition rate pressure must be optimized.
•
•
Several keV at 10-3 – 10-2 Torr.
•
Current density ji = 0.1 – 1.0
mA/cm2 is necessary to obtain
etching on the order of hundreds
Å/min.
MFP of the sputtered atoms and
ions in comparison with the
distance between target and
substrate.
Fig.5.4.1 Energy distribution of Cu
atoms ejected from a single crystal Cu at
Kr+ ion bombardment with energies from
80 – 1200 eV.
10
KE of sputtered atoms
KE of sputtered atoms varies with
• Mass of ions: the heavier the bombarding ion
- the larger energy of the sputtered atoms.
KE1>KE2>KE3
• Ion energy
• Target material
•
•
•
Average energy of sputtered atoms and their atomic
numbers
Average velocity of sputtered atoms: v = (4 - 8)  105 cm/s.
Remains a nearly constant even if the atomic number changes.
Therefore, atoms with larger atomic number have larger average
energies.
v1 = v2 = v3
M1 > M 2 > M 3
Sputtered
11
Energy Distribution
•
•
•
•
•
Sputtered atoms slow down in collisions.
Effect is called the thermalization.
As sputtered atoms travel from the target
to substrate their distribution is changed.
Energy distribution of sputtered atoms at
the substrate is sharper than for evaporated
atoms.
Distribution at which the sputtered atoms
arrive at a substrate is strongly affected by
the gas, the pressure and the distance
between the substrate and the target.
(a)
d= 0 cm
(b)
d= 3 cm
(c)
d= 6 cm
Fig.5.4.2 Change in Nb energy
distribution with distance from
target with Ar pressure 10 mTorr;
a) 0 cm, b) 3 cm, c) 6 cm.
12
IB: Thin Film Technology/Sputtering
5.5 Reactor Configuration
Sputter deposition system
Configurations
• Diode
• Triode – 3rd electrodes with
substrate.
• Magnetron - sputtering device
with magnetic field B crossing
perpendicularly the E-field lines.
Fig.5.5.1 I-V characteristics of three
different methods used for sputtering
13
IB: Thin Film Technology/Sputtering
DC Diode System
-ve few kV
target
Water cooling
Electrons
Ar Ions
+
+
+
+
+
+
+
+
Substrates
Fig.5.5.1b
•
•
•
•
•
Power density 0.5 -15 W/cm2
Widely used because of simplicity
DC glow discharge well understood
Sputtering limited to metals and semiconductors
DC sputter deposition requires ion current > 1mA/cm2 to obtain
reasonable deposition rates
14
RF Diode System
RF
Water cooling
Electrons
Ar Ions
+
+
+
+
+
+
+
+
Substrates
15
Effective
bias
Plasma Potential –Electrode Potential: Determine
the Energy of Impact Ions
Ion energy is proportional to e(Vp –VE)
16
Target is Surrounded by a Shield
(normally grounded)
• To avoid discharges at undesirable
regions
• To localize the sputtering to desired
surface
• Distance between the electrode and
shield must be smaller than the twice
of the width of the dark space in the
pressure range employed
• Shield acts as the floating capacitor of
the electrode system - causes losses of
RF power
Fig.5.5.2 Cross sectional view of the
electrode used for DC or RF diode
sputtering
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IB: Thin Film Technology/Sputtering
Schematic Diagram of Sputtering Deposition System
Fig.5.5.3 Schematic diagram of sputter deposition system
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IB: Thin Film Technology/Sputtering
Magnetron Sputtering
•
Magnetron – device with a magnetic field perpendicular to an electric field
– crossed electric and magnetic fields.
Several types of magnetrons
• (i) Cylindrical; (ii) Planar: circular, rectangular
• Operated using either DC or RF.
• Plasma density increases in magnetically confined region - decrease in
discharge impedance.
• Higher ion current density, 10 - 100-times
than that in the RF diode type.
• Increase in deposition rate.
• Deposition rate per unit power density at
the target also increases.
Fig.5.5.4
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IB: Thin Film Technology/Sputtering
Paschen Curve discharge threshold voltage Vg = f(pd) in Ar
Non-magnetron discharge
• (Vg)min at (pd)min = 1 – 3 Torr.cm
Magnetron discharge
• (pd)min shifts towards the lower
side while maintaining the same
value of (Vg)min.
•
•
•
•
Fig.5.5.7 Comparison between the
Pashen curves for magnetron (o) and
non-magnetron (x) discharges.
Lower value spans a wider range.
Indicates more stable discharge.
Maximum plasma density is ~ 3.5  1012 cm-3 at p = 0.3 Torr.
Ionization is ~ 310-4, (~10-6 in the non-magnetron discharge).
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IB: Thin Film Technology/Sputtering
Circular Planar Magnetron
A deposition area with 100 mm in diameter needs a target with ~120 mm in
diameter and in distance ~6 cm to provide the thickness uniformity of ~5%.
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IB: Thin Film Technology/Sputtering
Rectangular Planar Magnetron
Target
Grounded shield
Vacuum flange
Water out
Water in
22
IB: Thin Film Technology/Sputtering
Assembly of Rectangular Planar Magnetron
Grounded shield
Flange
Target fixture
Target
Cathode plate
Magnet assembly
Cathode casing
Water out
Water in
23
Deposition Systems with Two Magnetrons
• Two unbalanced magnetrons
• Closed mg field
• Planetary motion
Unbalanced Magnetrons?
• Two magnets: outer magnet is strong while inner
magnet is week magnetic field more open.
• Trap fast electrons emitted from the hot electrode.
• Yield higher ionization and higher ion current 
Higher sputtering yield, economical use of the
target.
24
The Magnetron Sputtering of Magnetic Materials
• Reducing saturation magnetization sputtering by
heating to Curie temperature
– It requires a special assembly with spacing and heating
• Using thin target
25
Confocal sputtering
Uniform coating provided by confocal sputter
deposition and rotation of substrates
Rotary substrate
26
IB: Thin Film Technology/Sputtering
Cylindrical Magnetron
3D Magnetic Field Modeling for
optimized “turn-around” erosion
27
IB: Thin Film Technology/Sputtering
Cylindrical Magnetron
28
IB: Thin Film Technology/Sputtering
5.6 Reactive Sputter Deposition
•
•
Target compounds - multiple elements with different volatilities
(e.g. metal oxides or nitrides).
Deposited film often differs from the composition of the target.
•
•
Compounds decompose during the sputtering process.
Decomposition varies according to the bond strength.
•
•
Concentration of the volatile component such as oxygen or
nitrogen, is reduced in the deposited film.
Thus, both the target and film composition change.
•
Loss of dissociated volatile component is compensated by adding
this component into plasma.
29
Reactive Sputter Deposition
•
•
•
•
Sputter deposition technique employing plasma with a reactive gas and
elemental metal target.
Various compound films have been prepared by reactive sputter deposition
and the reactive gases.
Composition of the compound can be controlled by changing the partial
pressure of the reactive gas added into an inert gas (Ar).
Partial pressure of reactive gases (N2 or O2) affects the deposition rate and
film properties.
30
IB: Thin Film Technology/Sputtering
Table 5.6.1 Compound films formed by reactive sputtering
31
IB: Thin Film Technology/Sputtering
Application Examples of Sputtering
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Wear resistant materials for tribological and cutting tool applications: binary and ternary
composites TiC, TiN, iC-graphitic, MoST, ZrB2, AlTiN etc.
Optical materials for lens characteristics: Typical MgF2, MgO, CeO2.
Transparent conducting materials for example ITO (indium tin oxides used as large
electrode in flat panel displays), SnO2, In2O3, ZnO, AlZnO, F:SnO2 (FTO) or SiO2.
Metalization paths and levels in microelectronic devices: W-Ti, Al-Si, Al-Cu, etc.
Thin film resistors: Ni-Cr, Cr-Si, Cr-SiO.
Amorphous bubble memory devices. Typical are Gd-Co, Lu3Fe5O12, Gd3Ga5O12.
Microcircuit mask blanks using such materials as Cr.
Magnetic materials for data storage tapes employing Co-Ni, Tb-Fe, Co-Ni-NiCr.
32
Use of Sputtering
Energy
• Efficient signage
• Gas turbine blade coatings
• Outdoor display systems
• Solar panels
Lightening
• Emission filters
• Energy-efficient lighting
• IR intensifiers
• Traffic signals
Optics
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•
•
•
•
Anti-reflective/Anti-glare coatings
Cable communications
Laser lenses
Optical filters for achromatic lenses
Spectroscopy
Medical
• Angioplasty devices
• Anti-rejection coatings
• Radiation capsules
Security
• Markings and holograms for currency, etc.
• Night vision/infrared equipment
• One-way security windows
Wear coatings
• Anti-corrosion coatings
• Anti-seize coatings
• Dies and molds
• Sewing needles
• Tool and drill bit hardening
Aerospace and defense
• Heads-up cockpit displays
• Jet turbine engines
• Mirrors for optical and x-ray telescopes
33
• Night vision equipment
Use of Sputtering
Architectural Glass
• Energy-producing glass
• Low-emissivity glass
Electronics Microelectronics
• Flip-chip back side metallization
• Gate dielectric
• Interlayer dielectric
• On-chip interconnects and interlevel bias
• Passive thin film components
• Printed circuit boards
• Sensors
• Surface Acoustic Wave (SAW) devices
Automotive
• Auto headlights and taillights
• Auto trim components
• Drive train bearings and components
Data storage
• CDs
• DVDs
• Laser disks
• Magnetic disks (hard and floppy)
• Microelectronic flash memory
• Read/Write heads
Decorative
• Appliance trim
• Building glass
• Building hardware
• Clothing
• Jewelry
• Packaging
• Plumbing fixtures
34
• Toys
Thin Films: Volmer – Weber growth
•
•
•
•
Thin films do not growth in perfect ways. Only epitaxial growth is
exception
The first atomic layer forms islands on nucleation sites (Volmer –
Weber growth)
Islands laterally expands until they touch each other which is called
percolation threshold.
The film properties in this growth stage are very different from the
bulk material properties.
Percolation
threshold
5 - 15 nm
35
Morphology and Characteristics of the Films
•
Films prepared by sputter deposition are often polycrystalline.
Zone 1
• Structure caused by limited migration
of incident atoms.
• Effected by adsorbed atoms.
• Structure is constructed from
tapered crystallites with domed heads
and contains voids in the grain
boundaries.
Thornton’s structural model
Atoms of particle leave the sputter source with energies of several eV. Increasing the
pressure reduces the energy particles to be deposited via scattering process- thermalization.
36
Morphology and Characteristics of the Films
• Films obtained by sputter deposition are usually polycrystalline.
Zone T
• Appears only in sputter films
• Regarded as a transition region
• Film reveals fibrous structure
crystallites grown perpendicular
to the surface
• Crystallites develop close each other
• Density is nearly equal to that of
the bulk material
• Surface is relatively smooth
•
Film has large tensile strength
and hardness values
37
Morphology and Characteristics of the Films
• Films obtained by sputter deposition are usually polycrystalline.
Zone 2
• Migration of atoms on the substrate
surface becomes considerable.
• Structure is constructed of the
columnar grains.
• Grain size increases with
increasing T/Tm
Zone 3
• Structure controlled by interdiffusion of atoms.
• Thus, the film surface becomes smooth.
• Recrystallization progress in the film during film formation.
• Film becomes, therefore, isotropic and randomly oriented polycrystals
38
Stress in Deposited Films
•
•
•
Energy of the sputtered atoms is large: Inter-mixing and mutual
diffusion between the substrate and incoming atoms.
Therefore, the adhesion is stronger than that by evaporation or
plasma enhanced CVD.
Adhesion strong enough to bend substrate
Stress
1. THERMAL STRESS
• Film prepared at temperatures higher than RT temperatures. Upon
cooling to RT the difference between expansion coefficient causes
thermal stress.
2. INTRINSIC (INTERNAL) STRESS: up to 0.1 - 10 GPa
• Subtraction of thermal stress from apparent stress: arises from
microstructure; may dominate at T less than 20% of melting point,
because of incomplete structural ordering
39
Deposition Temperature Affects the Film Stress
Intrinsic Stress
associated with microstructure
 Depends on deposition
temperature since the
microstructure is affected by T
 Often decreases with increasing T.
Thermal Stress
 Different thermal expansion coefficients of
substrate and film invoke thermal stress upon
cooling.
 Significant whenever the intrinsic stress is small
 Example: Ni/glass deposition
 25 oC: Thermal stress is 5% of intrinsic
 75 oC: Thermal stress is prevalent
 Total stress has a minimum.
 Ratio between the intrinsic and
thermal stress depends on the
preparation method.
40
Parameters Affecting Intrinsic Stress of Films
•
•
•
•
•
•
•
•
•
•
•
•
Substrate temperature
Difference between the temperatures of the deposited film and substrate
temperature (influences thermal stress).
Variation in temperature during deposition
Deposition rate
Incident angle
Film thickness
Gas environment
Phase transitions- crystallinity, stoichiometry
Electrostatic effect – free charges consequences – modification of stress
Lattice mismatch
Dislocations, lattice defects
Variation in stress with depth – curling when it peels off
41
Two Types of Internal Stress
Substrate
Tensile
•
concave
Tensile stress
convex
(look at interface)
Substrate bends concave
High tensile stress:
• Microvoids in the thin film, because of the
attractive interaction of atoms across the voids.
• Defects in deposited films
Compressive
•
Compressive Stress
(look at interface)
•
Substrate bends convex
Energetic particle bombardment during
deposition results in packing the atoms more
tightly.
42
Gas Environment and Internal Stress
Compressive
Substrate bends convex
at low Ar pressure
-higher particle energies.
Tensile
•
•
•
•
Substrate bends concave
Changes to a tensile stress
as pressure increases
-possible void formation
Transition pressure increases
nearly linearly with increasing
atomic number.
Transition pressure is higher
for heavier inert gases.
Figure 5.7.2 Ar pressure dependence
on internal stress
43
Transition Pressure: Compressive to Tensile Stress
Fig.5.7.3 The transition pressure
from compressive stress to tensile
stress in films as a function of
atomic weight of sputtering metals
44
Internal stress in sputter deposited metal films can be caused by
•
•
•
•
Sputtered atoms with high energy and neutral gas atoms reflected back from
cathode.
Knocking the metal atoms into interstitial lattice sites.
Sputtered atoms and gas atoms themselves incorporated into the substrate
material as interstitial atoms.
Both phenomena are called the atomic shot peening.
Internal stress is induced by atomic shot peening
•
•
When +ve bias voltages are applied - tensile stress.
When -ve bias voltages are applied - compressive stress.
45
•
•
•
•
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•
•
Ion Plating
Methods often used:
– Evaporation carried out in a plasma.
– Cathodic Arc deposition with 3rd biased electrode
Evaporated metal atoms are ionized in plasma.
Ions are accelerated toward the substrate.
Substrate negatively biased - Simultaneous deposition and sputtering.
Deposition rate is higher than the sputter-etched rate.
Working gas - inert (Ar)
Plasma without a working gas in only metal vapor.
Through the introduction of the reactive gases such as N2 or O2, compound
films can be deposited.
Many types of ion plating systems have been developed.
Their characterization – in accordance with the combination of discharge
type for generating plasma and evaporation source.
46
Ion Plating using Resistance Heated Source
•
•
Resistance heated filament as an
evaporation source
DC glow discharge generated by
applying negative high potential
to the substrate.
Fig.5.8.1 Diagram of ion plating DC glow discharge for generating
plasma and resistance heated
filament as an evaporation
Fig.8.1 Ion Plating
47
Ion Plating using EBE
•
•
Resistance heated evaporation source –
materials below ~ 1300 oC.
Electron beam evaporation - materials
up to ~ 3000 oC.
Fig. 5.8.2 Ion plating apparatus
using DC glow discharge and
an EBE.
48
Activated Reactive Evaporation (ARE)
•
•
•
•
•
Metal evaporated by an EBE source
reacts with a reactive gas plasma and
deposit as a compound film
Plasma is generated between the
grounded chamber wall and probe
with positive potential
DC voltage Probe currents 40 - 170
mA/ 20 – 80 V.
Reactive gases, as in sputter
deposition - listed in Table
Pressure during deposition
~ 10-3 – 10-4 Torr.
Fig.5.8.3Activated
Activated reactive
reactive evaporation
Fig.5.8.3
evaporation
49
Ion Plating using an Inductively Coupled RF
•
•
•
Ion plating technique that uses an
inductively coupled RF discharge for
generating plasma
Coil 7 cm in dia
Turns 4 -7
Fig.5.8.4 Schematic setup of an ion
plating apparatus using RF
inductively coupled discharge
50
Effects at Ion Plating
•
•
•
•
•
•
Substrate is sputter cleaned by Ar+ ion bombardment.
Films are continuously exposed to the bombarding ions.
Substrate temperature increases due to this ion bombardment.
Mutual diffusion layer forms in the film –substrate interface.
Higher ion energies – intermixing evaporated and substrate
materials.
Stronger adhesion than at conventional evaporation.
Characteristic features of ion plated films
•
•
Strong adhesion, low porosity, high corrosion resistance and
significant mechanical properties (hardness and wear
resistance).
Surface roughness depends on the method of preparations.
51
Variety of metals and compound films produced by ion plating:
• (a) Metals - Cr, Cu, Ag, Au; (b) Oxides – Y2O3, Al2O3, In2O3, Bi2O3; (c)
Nitrides – TiN, Ti2N, HfN, TaN, CrN, Cr2N, Si3N4;(d) Carbides – TiC, ZrC,
HfC, VC, NbC, TaC;
• (e) Others – CdS, Nb3Ge.
Some applications - Examples
1. Metal films such as Ag and Au for anticorrosion, wear resistance, or for
use as high quality optical and decorative coatings.
2. Refractory compounds such as metal carbides or nitrides represented by
TiN, ZrN, TiC, or Al2O3 for cutting tools and watch parts - extreme
hardness and excellent wear resistance under adhesive, abrasive and erosive
wear conditions.
3. Metal oxides such as TiO2, In2O3, SnO2 or SiO2 for optical coating for
laser mirrors and antireflection coatings.
52
Variation of microhardness with nitrogen pressure during
deposition of TiNx film
Fig.5.8.5 Microhardness of TiNx
film as a function of N2 pressure
1) Reactive ion plating
2) Activated reactive evaporation
3) Reactive evaporation
N2
53