Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Deposition Technologies 21
photocatalytic coatings for biomedical, energy, hydrogen production, and self-cleaning
windows
thin film organic electronics, including solar cells; these devices are needed for
low-cost renewable energy and display applications
transparent photovoltaics applied to glazings, roofs, and buildings, using p- and n-type
transparent conducting oxide thin films
thin film lithium batteries for high-density energy storage
superhard tribological coatings and transparent coatings with hardness > 50 GPa for
advanced wear applications, optical applications, aircraft windows, and windscreens
HPPMS for high-density coatings with excellent adhesion
GLAD coatings
thin film thermoelectric structures for power generation and cooling
atomic layer deposition in semiconductor processing
thin film electrolytes for thin film solid oxide fuel cells and batteries
free-standing nanolaminate satellite optics
thin film coatings for microelectromechanical systems (MEMS) and nanotechnology
applications
nanocomposites and low-dimensional structures
carbon nanotubes.
1.11.1 Selection Criteria
The selection of a particular deposition process depends on several factors:
material to be deposited
limitations imposed by the substrate, e.g. material, size, temperature stability
specific application
deposition rate
adhesion of film to substrate
throwing power
purity of source material
22 Chapter 1
Table 1.2: Criteria for deposition processes (taken partially from the Second Edition of this Handbook)
Evaporation Ion plating Sputtering Cathodic arc
deposition
Chemical
vapor
deposition
Polymer
deposition
Electrodeposition Thermal
spraying
Mechanism of
production
of depositing
species
Thermal energy Thermal energy Momentum
transfer
Thermal energy Chemical
reaction
Thermal energy Solution Flames or
plasmas
Deposition
rate
Can be very
high (up to
750,000
A/min)
Can be very
high (up to
250,000
A/min)
Low except for
pure metals
and dual
magnetron
Can be very
high
Moderate
(200–2500
A/min)
Very high
(up to 100,000
A/s)
Low to high Very high
Deposition
species
Atoms & ions Atoms & ions Atoms & ions Ions Atoms Monomers/
polymers
Ions Droplets
Throwing
power
for: Complex
shaped
objects
Poor, line of
sight coverage
except by gas
scattering
Good, but
nonuniforrm
thickness
distribution
Good,
nonuniform,
and uniform
thickness
distributions
Good, but
nonuniform
thickness
distribution
Good Good Good No
Into small
blind holes
Poor Poor Poor Poor Limited Poor Limited Very limited
Metal
deposition
Yes Yes Yes Yes Yes No Yes, limited Yes
Alloy
deposition
Yes Yes Yes Yes Yes No, but
molecular
doping
possible
Quite limited Yes
Refractory
compound
deposition
Yes Yes Yes Yes Yes No Limited Yes
Energy of
deposited
species
Low
(∼ 0.1–0.5 eV)
Can be high
(1–100 eV)
Can be high
(1–100 eV)
Can be high Can be high
with plasma
assisted CVD
Low Can be high Can be high
Bombardment
of sub-
strate/deposit
by inert gas
ions
Generally no Yes Yes or no,
depending on
geometr y
Yes Possible with
PECVD
No No Yes
Growth
inter face
perturbation
Not normally Yes Yes Yes Yes (by
rubbing)
No No No
Substrate
heating (by
external
means)
Yes, normally Yes or no Yes or no Yes or no Yes No, cooling
generally
required
No Not normally
Deposition Technologies 23
apparatus required and availability of same
cost
safety considerations, e.g. toxicity
process stability
manufacturing considerations, e.g. batch size, throughput, process controls
abundance of source materials.
To aid the reader in selecting a suitable deposition process, Table 1.2 lists criteria for each
deposition process (taken partially from the second edition of this handbook). Usually one
process cannot meet all of the above requirements and more than one technique can be used to
deposit a given material. As a result, hybrid processes are now being used to deposit many thin
film materials. Evaluation of each process for the specific application will help lead to a
rational choice of deposition process or hybrid process.
1.12 Summary
In the previous discussion, we noted the following:
There is a very large number and wide variation of deposition techniques.
There is no unique method to classify these techniques. Depending on the viewpoint,
the same process may fall into one or more classes.
Each technique has its advantages and disadvantages.
The choice of technique will depend on the various selection criteria, specific
application, and available resources.
More than one technique can be used to deposit a given thin film material, as shown in
Figure 1.2 for microelectronics [20].
Hybrid techniques are becoming more widely used and available.
Appendix 1.1 Deposition Process Definitions
Definitions of various deposition processes are given below. They are grouped in the same
manner as in the second edition of this handbook.
Conduction and Diffusion Processes
Electrostatic Deposition is the deposition of material in liquid form and the subsequent
evaporation of the solvent to form a solid coating. The liquid is atomized and charged,
and directed onto the substrate using an electrostatic field.
24 Chapter 1
Figure 1.2: Applicability of preparation methods to microelectronics. Light shading indicates that
the component can be prepared by the method; dark shading indicates that the method is widely
used [21].
Electrophoric Coating produces a coating on a conducting substrate from a dispersion
of colloidal particles. The piece to be coated is immersed in an aqueous dispersion
which dissociates into negatively charged colloidal particles and cations. An electric
field is applied with the source as anode; colloidal particles are transported to the
anode, where they are discharged and form a film. Curing is required for a paint
coating, which demonstrates that electrophoresis is not a very effective transport
process. Consequently, electrodeposition may be a better term for this coating process.
Electrolytic Deposition is primarily concerned with the deposition of ions rather than
colloidal particles. Two electrodes are immersed in an electrolyte containing an ionic
salt, which dissociates in aqueous solution into its constituent ions. Positive ions are
deposited onto the cathode.
Deposition Technologies 25
Anodization is a process that occurs at the anode (hence its name) for a few specific
metals. The anode reacts with negative ions from an electrolyte and becomes oxidized
and forms a surface coating.
Gaseous Anodization is a process in which the liquid electrolyte of the conventional
wet process is replaced by a glow discharge in a low partial pressure reactive gas,
producing oxides, carbides, and nitrides.
Ion Nitriding is a gaseous anodization that produces a nitride diffusion coating on a
metal surface, usually steel.
Ion Carburizing is a gaseous anodization that produces a carbide diffusion coating on
a metal surface, usually steel.
Plasma Oxidation is gaseous anodization to produce an oxide film on the surface of
metal (e.g. SiO
2
films on Si).
Diffusion Coating is produced by diffusion of material from the surface into the bulk
of the substrate.
Metalliding is a method using electrodeposition in molten fluorides.
Spark-Hardening is a technique in which an arc is periodically struck between a
vibrating anode and the conducting substrate (cathode); material is transferred from
the anode and diffuses into the substrate.
Chemical Processes
Conversion and Conversion/Diffusion Coating is a process in which the substrate is
reacted with other substances (which may be in the form of solids, liquids, or gases) so
that its surface is chemically converted into different compounds having different
properties (anodization could probably be described as an electrochemical conversion
process). Elevated temperatures are often required for the reaction to occur and
diffusion is often an essential feature.
Chemical Vapor Deposition (CVD) is a process in which the substrate is exposed to
one or more volatile precursors, which react and/or decompose on the substrate
surface to produce the desired thin film deposit. The deposit can be a metal,
semiconductor, alloy, nanocomposite, or refractory compound.
Plasma-Enhanced Chemical Vapor Deposition (PECVD) is a form of CVD that
involves creation of a plasma of the reacting gases and subsequent deposition onto a
substrate. The plasma is generally created by an RF, DC or microwave discharge
between two electrodes located in the space into which the reactive gases are
introduced.
26 Chapter 1
Atomic Layer Deposition (ALD) is a self-limiting, sequential surface chemistry that
deposits conformal thin films of materials onto substrates of varying compositions.
ALD is similar in chemistry to CVD, except that the ALD reaction breaks the CVD
reaction into two half-reactions, keeping the precursor materials separate during the
reaction. ALD film growth is self-limited and based on surface reactions, which makes
achieving atomic scale deposition control possible. By keeping the precursors separate
throughout the coating process, atomic layer control of film grown can be obtained as
fine as ∼ 0.1
˚
A per monolayer. ALD has unique advantages over other thin film
deposition techniques, as ALD grown films are conformal, pinhole free, and
chemically bonded to the substrate. With ALD it is possible to deposit coatings
perfectly uniform in thickness inside deep trenches, porous media and around
particles. The film thickness range is usually 1–500 nm.
Plasma-Assisted Chemical Vapor Deposition (PACVD) is a process similar to PECVD
where the reaction between the precursors is stimulated or activated by creating a
plasma in the vapor phase using techniques such as RF, microwave or cyclotron
resonance excitation.
Pyrolysis is a type of CVD which involves the thermal decomposition of volatile
materials on the substrate.
Electroless Deposition is often described as a form of electrolytic decomposition
which does not require a power source or electrodes. It is actually a chemical process
catalyzed by the growing film, so the electroless term is somewhat of a misnomer.
Disproportionation is the decomposition of a film or crystal in a closed system by
reacting the metal with a carrier gas in the hotter part of the system to form a
compound, followed by dissociation of the compound in the colder section of the
system to deposit the metal. Examples are epitaxial deposits of Si or Ge on single
substrates and the Van-Arkel–deBoer process for metal purification and crystal growth.
Wetting Processes
Wetting processes are coating processes in which material is applied in liquid form and then
becomes solid by solvent evaporation, spinning, curing, baking, or cooling.
Conventional Brush Painting and Dip Coating are wetting processes in which the part
to be coated is literally dipped into a liquid (e.g. paint) under controlled conditions,
including withdrawal rate and temperature.
Spin coating is a process used to apply uniform coatings to flat substrates, usually
wafers. The process is initiated by applying an excess amount of a solution onto the
substrate, which is then rotated at high speed in order to spread the fluid by centrifugal
Deposition Technologies 27
force. Rotation is continued while the fluid spins off the edges of the substrate, until
the desired thickness of the film is achieved. The applied solvent is usually volatile,
and simultaneously evaporates. Thus, the higher the angular speed of spinning, the
thinner the film. The thickness of the film also depends on the concentration of the
solution and the solvent. Spin coating is widely used to create thin films with
thicknesses below 10 nm, and is used intensively in photolithography, to deposit layers
of photoresist about 1 m thick.
Hydrophilic Method is a surface chemical process known as the Langmeir Blodgett
technique which is used to produce monolayers or multimonolayers of low-chain fatty
acids. A film 2.5 nm thick can be deposited on a substrate immersed in water and
pulled through a compressed layer of fatty acid residing on the surface of the water.
Welding Processes are a range of coating techniques all of which rely on wetting.
Spraying Processes
Printing Processes also rely on wetting and are processes in which ink, conventionally
pigment in a solvent, is transferred to and deposited on a paper or other substrate,
usually to form a pattern; the solvent evaporates to leave the required print.
Ink Jet Printing is a process that operates by propelling variably sized (nanometer to
micrometer size) droplets of liquid or molten material (ink) onto almost any medium.
Spraying Processes can be divided into two categories:
Macroscopic, in which the sprayed particle consists of many molecules and is
usually greater than 10 m in diameter
Microscopic, in which the sprayed particles are predominantly single molecules or
atoms (ink jet printing is a form of spraying).
Air and Airless Spraying are the first of the macroscopic processes. When a liquid
exceeds a certain critical velocity it breaks up into droplets, or atomizes. The atomized
droplets, by virtue of their velocity (acquired from high-pressure air or airless source)
can be sprayed onto a substrate.
Flame Spraying is a process in which a fine powder (usually a metal) is carried in a gas
stream and is passed through an intense combustion flame, where it becomes molten.
The gas stream, expanding rapidly because of the heating, then sprays the molten
powder onto the substrate where it solidifies.
Detonation Coating is a process in which a measured amount of powder is injected
into what is essentially a gun, along with a controlled mixture of oxygen and
acetylene. The mixture is ignited and the powder particles are heated and accelerated
28 Chapter 1
to high velocities with which they impinge on the substrate. The process is repeated
several times a second.
Arc Plasma Spraying is a process in which the powder is passed through an electrical
plasma produced by a low-voltage, high-current electrical discharge. Even refractory
metals can be deposited using this technique.
Electric-Arc Spraying is a process in which an electric arc is struck between two
converging wires close to their intersection point. The high-temperature arc melts the
wire electrodes which are formed into high-velocity molten particles by an atomizing
gas flow; the wires are continuously fed to balance the loss. The molten particles are
then deposited onto a substrate similar to other spray processes.
Harmonic Electrical Spraying is a process in which the material to be sprayed must be
in liquid form, which will usually require heating. It is placed in a capillary tube and a
large voltage is applied to the capillary tip. It is found that by adding an AC
perturbation to the DC field, a collimated beam of uniformly sized and charged
particles is emitted from the tip. As a result of being charged, these particles can be
focused by an electric or a magnetic field to produce patterned deposits.
Vacuum Polymer Deposition is a process in which a monomer is atomized in a vacuum
and flash evaporated into a series of baffles. The resulting monomer vapor is
homogeneous and is condensed onto a substrate or moving web substrate. The liquid
monomer is then cross-linked to form a polymer by either ultraviolet radiation or
electron beam heating. Polymer flash evaporation is generally combined with PVD
processes to fabricate a wide range of functional coatings.
Physical Vapor Deposition Processes
Evaporation is a process in which the boiling is carried out in a vacuum where there is
almost no surrounding gas; the escaping vapor atom will travel in a straight line for a
specified distance before it collides with structures in the vacuum chamber or residual
gas atoms. Boiling is caused by thermal heating or electron beam heating of a source
material.
Glow Discharge Evaporation and Sputtering are processes performed in a soft vacuum
(10
−2
–10
−1
torr) operating in the range 10
−1
< pd <10
−2
torr cm, where p is the
pressure and d is the source–substrate distance.
Molecular Beam Epitaxy is an evaporation process performed in an ultra-high vacuum
for the deposition of compounds of extreme regularity of layer thickness and
composition from well-controlled deposition rates.
Deposition Technologies 29
Reactive Evaporation is a process in which small traces of a reactive gas are added to
the vacuum chamber; the evaporating material reacts chemically with the gas so that
the compound is deposited onto the substrate.
Activated Reactive Evaporation (ARE) is the reactive evaporation process carried
out in plasma which converts some of the neutral atoms into ions or energetic
neutrals thus enhancing reaction probabilities and rates to deposit refractory
compounds.
Biased Activated Reactive Evaporation (BARE) is the same process as ARE with the
substrate held at a negative bias voltage.
Sputter Deposition is a vacuum process which uses a different physical phenomenon
to produce the microscopic spray effect. When a fast ion strikes the surface of a
material (target), atoms of that material are ejected by a momentum transfer process.
As with evaporation, the ejected atoms or molecules can be condensed on a substrate
to form a surface coating.
Reactive Sputter Deposition is sputter deposition that involves a partial pressure of a
reactive gas which reacts with the sputtered material to form a compound surface
coating.
High-Power Pulsed Magnetron Sputtering (HPPMS), also known as high-power
impulse magnetron sputtering (HIPIMS), utilizes extremely high-power densities of
the order of kW/cm
2
in short pulses (impulses) of tens of microseconds at a low duty
cycle (on/off time ratio) of < 10%.
Dual Magnetron/Mid-Frequency Magnetron Sputtering process achieves both high
deposition rates and improved materials utilization. Dual magnetron sputtering uses a
mid-frequency (∼ 40–300 kHz) pulsed power source and two magnetron cathodes. In
its simplest form, the power source supplies a positive pulse to one magnetron cathode
during the first half of the cycle while negatively biasing the other cathode and then
supplying a positive pulse to the other magnetron cathode while negatively biasing the
other cathode. In this manner, one cathode acts as an anode while the other is the
sputtering cathode. Sputtering only occurs during negative bias. This process is very
amenable to reactive sputtering.
Ion Plating is an atomistic vacuum coating process in which the depositing film is
continuously or periodically bombarded by energetic atomic-sized inert or reactive
particles that can affect the growth and properties of the film. The source of depositing
atoms can be from vacuum evaporation, sputtering, arc vaporization, or a chemical
vapor precursor.
30 Chapter 1
Reactive Ion Plating is ion plating that involves a partial pressure of a reactive gas
which reacts with the sputtered material to form a compound surface coating.
Chemical Ion Plating is similar to reactive ion plating but uses stable gaseous reactants
instead of a mixture of evaporated atoms and reactive gases. In most cases, the
reactants are activated before they enter the plasma zone.
Ion Beam Deposition is a process in which a beam of ions generated from an ion beam
source, impinge and are deposited on the substrate.
Ion Beam Assisted Deposition (IAD) – two versions are possible. In dual ion beam
assisted deposition an ion beam is used to sputter a target and a second beam is used to
bombard the growing film to change microstructure and properties. The other version
uses an ion beam to bombard the growing film to change the structure and properties.
In this case, conventional evaporation or sputtering techniques are used to generate a
flux of the depositing species.
Cluster Ion Beam Deposition is ion beam deposition in which atomic clusters are
formed in the vapor phase and deposited on the substrate.
Filtered and Unfiltered Cathodic Arc Deposition (FCA) processes can be considered a
form of ion plating. Vacuum arc ion sources produce a plasma of the source material
by microexplosions at the surface of the solid cathode, in contrast to production by
gaseous ionization. The vacuum arc is a discharge between two metallic electrodes in
a vacuum, which is characterized by a low burning voltage (about 20 V) and a high
current (35–500 A). The current transfer is made possible by the production of
plasma produced at micrometer-size cathode spots on the cathode surface. Vacuum
arc is an efficient way of generating plasma. Plasmas of intense energetic ions can be
used to carry out high-current ion plating for material surface modification
applications.
Ion Implantation is very similar to ion plating, except that now all of the depositing
material is ionized, and accelerating energies are significantly higher. The depositing
ions are thus able to penetrate the surface barrier of the substrate and become
implanted in the substrate rather than on it.
Plasma Polymerization is a process in which organic and inorganic polymers are
deposited from a monomer vapor by the use of an electron beam, ultraviolet radiation,
or glow discharge. Excellent insulating films can be prepared in this manner.
References
[1] K. Robbie et al., J. Vac. Sci. Technol. A13(3) (1995) 1032–1034.
[2] J. Hwan et al., J. Vac. Sci. Technol. A26(1) (2008) 146–150.