Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
Подождите немного. Документ загружается.
Deposition Technologies 11
Table 1.1: Vacuum deposition techniques [10]
Atomistic deposition Particulate
deposition
Bulk coatings Surface modification
Electrolytic
environment
Thermal spraying Wet processes Chemical conversion
Electroplating Plasma spraying Painting Electrolytic
Electroless plating D-gun Dip coating Anodization (oxide)
Fused salt electrolysis Flame spraying Electrostatic
spraying
Fused salts
Chemical displacement Fusion coatings Printing Chemical–liquid
Vacuum environment Thick film ink Spin coating Chemical vapor
Vacuum evaporation Screen printing Cladding Thermal
Ion beam deposition Jet printing Explosive Plasma
Laser ablation Enameling Roll bonding Leaching
Molecular beam
epitaxy
Electrophoretic Overlaying Mechanical
Cathodic arc Impact plating Weld coating Shot peaning
Vacuum polymer
deposition
Thermal
Plasma environment Surface enrichment
Sputter deposition Diffusion from bulk
Activated reactive
evaporation
Sputtering
Cathodic arc Ion implantation
Plasma polymerization Self-assembly
Ion plating
Chemical vapor
environment
Plasma enhanced
Atomic layer
deposition
Reduction
Decomposition
Spray pyrolysis
Liquid phase epitaxy
configurations and the resulting structure contains high concentrations of structural
imperfections (dangling bonds, voids, lattice mismatch, etc.). The depositing atoms can also
react with the substrate material to form a complex interfacial region.
The energy of the adatoms is a critical factor in determining the microstructure and
morphology, and depends on the deposition process and source of atoms. Sources of atoms can
12 Chapter 1
be thermal evaporation, sputter deposition, vaporized chemical species (CVD), plasma species
(ion plating, cathodic arc), or ionic species in an electrolyte (electrodeposition). Low-energy
atomistic deposition is characterized by depositing species incident on a surface where they
condense, are relatively immobile, and grow into a continuous coating. Mobility of adatoms to
their lowest energy state increases with increased energy. Nucleation and growth mechanisms
of the condensing species also determine the crystallography and microstructure of the thin
film coating. These particles can react with or implant into the substrate in high-energy
processes.
Particulate deposition processes involve molten or solid particles and the resulting
microstructure of the deposit depends on the solidification or sintering of the particles. Bulk
coating processes such as painting involve the application of large amounts of coating material
onto the surface at one time. Surface modification involves ion, thermal, mechanical, or
chemical treatments which alter the surface composition or properties.
1.7 New Deposition Technologies
Several new or advanced deposition technologies have emerged and become important since
publication of the second edition of this handbook. Processes that are included in this edition
are:
vacuum polymer deposition (VPD)
atomic layer deposition (ALD)
high-power pulsed magnetron sputtering (HPPMS or HIPIMS)
filtered cathodic arc deposition
glancing angle deposition (GLAD).
Each will be discussed in detail in future chapters.
VPD and ALD have recently emerged as processes that can achieve molecular doping,
polymer thin films and nanocomposites. Gas jet deposition can also form nanocomposites, as
was presented in the second edition. Processes such as GLAD can achieve unique
microstructures not possible with conventional substrate–source configurations and deposition
processes. Films deposited by HPPMS have high density and excellent adhesion to the
substrate, which makes them desirable for tribological, corrosion-resistant coatings, barrier
coatings, and electronic applications. This should be kept in mind when reading the next
section, Microstructure and Properties.
Deposition Technologies 13
1.8 Microstructure and Properties
The microstructure of thin film materials deposited by vacuum processes depends on a number
of factors, but primarily on the energy of the species (adatoms) incident upon the substrate.
Energies range from a few tenths of eV for thermal evaporation to tens to hundreds of eV for
sputtering, to higher energies used in ion implantation and ion milling. While much of the
understanding of the relationship between adatom energy and microstructure is
phenomenological, a number of models that simulate film growth have been developed.
Growth of thin films is addressed in Chapters 12 and 13. Models include Monte Carlo
simulations in two and three dimensions [12], percolation theory, and structure zone model
[13]. Film growth mechanisms can be complicated by the fact that not only is the species from
the source deposited, but additional processes such as resputtering, reflected neutrals,
shadowing, and ion implantation can occur. Mobility of the deposited species on the substrate
is also critical and depends on the energy of incident species, substrate temperature, ion
bombardment, substrate morphology and cleanliness, interfacial reactions at the surface,
deposition angle, and substrate rotation. Adhesion of the film to the substrate also depends on
a number of these factors. If the coating and substrate materials are not chemically reactive
and are insoluble, the interfacial region will be confined to an abrupt discontinuity in
composition and possibly crystalline phase. Adhesion in this case can be poor. This type of
interface can be modified by bombardment with high-energy particles to give high defect
concentrations and implantation of ions, resulting in a ‘pseudodiffusion’ type interface. The
type of interface formed will influence the properties of the deposited film. Because these
interfaces can be very thin, compositional, phase, microstructural, and property analysis can be
challenging.
The microstructure of the coating being deposited in atomic deposition processes depends on
how adatoms are incorporated into the existing structure and can change with each atomic
layer deposited. Surface roughness and geometric shadowing lead to preferential growth of
elevated regions, resulting in a columnar type microstructure. Microstructure can be modified
by substrate temperature, surface diffusion of the atoms, ion bombardment, impurity atom
incorporation, and angle of incidence of the adatom flux. Several structure zone models have
been developed [13–15] to explain the evolution of coating microstructure. An extreme
example of this is GLAD coatings [8]. Adatom flux is incident on the substrate at very acute
angles, resulting in a highly figured and porous chiral, whisker, or convoluted columnar
microstructure. Defects in the substrate and growing film as well as particulates also affect
microstructure.
In CVD, the chemical species being deposited is generally reduced or decomposed on the
substrate surface, usually at high temperatures. Care must be taken to control the interfacial
reactions between coating and substrate and between substrate and gaseous reaction products.
14 Chapter 1
Coating microstructures are very similar to those achieved by vacuum deposition processes
(small-grained columnar structures to large-grained equiaxed or oriented structures).
Because they are non-equilibrium processes, each atomistic deposition process has the
potential to deposit materials that vary significantly from the source material in composition,
microstructure, mechanical properties, tribological properties, and physical properties
(conductivity, optical properties, etc.). The resulting films may have high intrinsic stress, high
concentration of point defects, extremely fine grain size, highly oriented microstructures,
metastable phases, incorporated impurities, and micro- to macroporosity. These properties
have a significant influence on the physical, corrosion resistance, and mechanical and
tribological properties of the deposited film. A unique microstructure of the deposited material
may also lead to anomalously low annealing and recrystallization temperatures where internal
stresses and high defect concentration aid in atomic rearrangement.
Incorporation of impurities or implantation of atoms during deposition can result in high
intrinsic stresses or impurity stabilized phases not observed in bulk. Compounds such as
nitrides, oxides, carbides, and borides can be deposited using reactive species in the vapor.
Graded compositions can be formed by varying or changing the reactive species.
By being able to vary reactive species and source materials in vapor deposition, unique and/or
non-equilibrium microstructures can be formed, including superlattices, nanolaminates, and
nanocomposites. Films with properties superior to bulk or single layer films are readily
deposited. Nanocomposites and nanolaminates have achieved hardness values not possible in
bulk or single homogeneous layers.
1.9 Unique Features of Deposited Materials and Gaps
in Understanding
Several features of materials can only be produced by vacuum deposition technologies,
including:
extreme versatility of range and variety of deposited materials
applied coatings with properties independent of thermodynamic compositional
constraints
ability to vary defect concentration over wide ranges, thus resulting in a range of
properties comparable to or far removed from conventional bulk materials
high quench rate available to deposit amorphous materials
controllable production of microstructures different from conventionally processed
materials, e.g. a wide range of microstructures: ultrafine (superlattice, nanostructures)
to single crystal films
Deposition Technologies 15
fabrication of thin free-standing shapes and foils, even from brittle materials
ecological benefits with certain techniques.
Areas where more understanding and research are required are:
relationships between plasma density, pulse length, coating density, and
microstructure for high-power pulsed plasma deposition
role of plasmas in large area deposition
microstructure and properties in the thickness range 10–10,000 nm, particularly for
low-dimensional structures, submicroelectronics, nanocomposites, tribological
coatings, corrosion-resistant materials, reflective surfaces, and thermoelectrics
ion energy and ion flux on properties of deposited material
effect of energy of the depositing species on interfacial interaction, nucleation, and
growth of the deposit
effect of substrate surface condition and morphology on adhesion and mechanical
properties of the deposit
influence of process parameters on residual stress
sculpted thin films and highly porous films.
1.10 Current Applications
Although there is significant overlap, current coating applications may be classified into the
following generic areas:
optically functional – laser optics (reflective, semi-transmitting and transmitting),
phase separation, telecommunication filters (WDWM), architectural glazing,
residential mirrors, automotive rear-view mirrors and headlamps, reflective and
antireflection coatings, optically absorbing materials, low-e coatings, solar selective
coatings, free-standing reflectors, transparent conductive films
energy related – thin film battery, thin film fuel cell, thin film solar cell, thermoelectric
thin films, superlattice, electrochromic coatings, low-e coatings, solar absorbers,
barrier coatings (oxygen and water permeation barriers), transparent solar cells,
organic solar cells, photocatalytic coatings
electrically functional – electrical conductors, electrical contacts, semiconductor films,
active solid state devices, electrical insulators, photovoltaics, transparent electrical
contacts
16 Chapter 1
mechanically functional – tribological coatings, lubrication films, nanocomposites,
diffusion barriers, hard coatings for dies and cutting tools, wear- and erosion-resistant
coatings, biomedical coatings
chemically functional – corrosion-resistant coatings, catalytic coatings, biomedical
coatings, photocatalytic coatings, thin film electrolytes, organic materials.
1.10.1 Decorative/Functional Coating
Decorative coatings continue to be a large segment of the vacuum coating market. In addition
to chromium coatings on plastic automotive parts such as grills, new color-shift automotive
paints have flakes with Fabrey-Perot filters which change color with angle of incidence of the
observer. Color shift paints are also used on high-end athletic shoes, eyeglass frames and
jewelry. A wide variety of mirrors is produced using aluminum and silver films on glass and
plastics. Electrochromic coatings are now used on automobile sun roofs, rear-view mirrors
(interior and exterior) and glazings. It should be noted that because of environmental concerns,
major efforts are underway to replace chromium with suitable thin film materials.
Refractory metal nitrides (TiN, ZrN, TaN, HfN) have wide ranges of decorative, as well as
tribological, applications. Titanium nitride (TiN) has the reflective properties of gold and has
decorative, tribological and optical applications. TiN coatings are used to give jewelry a gold
color as well as protect it from corrosion and wear. Tantalum nitride (TiN) and zirconium
nitride (ZrN) have reflective properties of silver and brass. Dichroic multilayer filters are
applied to windows and mirrors to provide a range of colors in reflection and transmission.
Flexible polymers such as polyethylene terephthalate (PET) are metallized by vacuum web
processes for heat insulation, food packaging, and decorative applications.
Black coatings, such as chromium carbon nitride (CrCN), titanium carbon nitride (TiCN), and
titanium aluminum nitride (TiAlN), are becoming very fashionable, especially on small
electronic devices such as mobile phones, personal digital assistants (PDAs), and cameras.
Black coatings are deposited by reactive arc deposition or reactive sputter deposition.
1.10.2 Transparent Conductive Thin Films
Transparent conductive coatings and transparent conductive oxides (TCOs) in particular have
a wide range of optical, device, photovoltaic, energy, and display applications. Cadmium oxide
(CdO) was the first transparent conductive coating and was used in solar cells in the early
1900s. Tin oxide (SnO
2
) was deposited on glass by pyrolysis and CVD in the 1940s for
electroluminescent panels. Indium tin oxide (ITO) has the best combination of transparency
and conductivity to date, but materials such as aluminum-, gallium-, and indium-doped zinc
oxide (ZnO:Al, ZnO:Ga, and In
2
O
3
:ZnO) are also widely used. A number of ternary
compounds have been developed in the past ten years, including Zn
2
SnO
4
, ZnSnO
3
, MgIn
2
O
4
,
Deposition Technologies 17
(GaIn)
2
O
3
,Zn
2
In
2
O
5
, and In
4
Sn
3
O
12
. They are critical for energy-efficiency applications such
as low-e windows, solar cells, and electrochromic windows. Transparent conductive thin films
are used as the transparent electrical contacts in flat panel displays, sensors, and optical
limiters. While most TCOs are n-type semiconductors, p-type TCOs such as CuFeO
2
,
CuAlO
2
, AgCoO
2
, NiCo
2
O
4
, ZnIr
2
O
4,
Ga-doped SnO
2
,Cu
2
O-CoO:CuAlO
2
, CuGaO
2
,
CuInO
2
, SrCu
2
O
2
, and LaCuOCh (Ch = chalcogen) have also been developed, creating new
applications such as transparent p-n diodes, transistors, and photovoltaics.
1.10.3 Thin Film Solar Cells and Batteries
Thin film solar cells and batteries have emerged as critical applications for many processes
described in this handbook. While efficiencies of thin film solar cells are not as high as those
of single crystal cells, they are significantly less expensive to fabricate and can be made in
large areas on glass and polymer substrates. Amorphous silicon solar cells are now being
deposited in large areas using primarily PECVD processes and have efficiencies near 11%.
Copper indium diselenide (CuInSe
2
, CIS) and copper indium gallium diselenide (CuInGaSe
2
,
CIGS) have efficiencies near 14%. Cadmium telluride (CdTe)-based cells also show promise
and are amenable to large-scale production. Thin film lithium-polymer batteries promise to
revolutionize energy storage technology with very high-energy current densities and the added
advantage that they are rechargeable.
1.10.4 Friction and Wear: Nanolaminates and Superlattices
In addition to MoS
2
, WSe
2
, and lamellar low-friction thin film coatings used on bearings and
other sliding parts, DLC and metal-containing carbon (C:Me) coatings such as W–C:H are
extensively used in automotive engines. DLC coatings can have a COF as low as 0.01. DLC
and other low-friction coatings, and nanocomposites are also being developed for medical
implants, and artificial knees and hips. The majority of low-friction and low-wear coatings are
deposited by magnetron sputtering, unbalanced magnetron sputtering, closed-field magnetron
sputtering, and dual/mid-frequency magnetron sputtering. TiN, TiC, TiCN, TiYN, TiAlN,
CrN, CrMoC
x
N
1−x
, and Ti
1−x
Cr
x
N coatings have demonstrated good wear-resistance
performance and low COF. Nanolaminates and superlattices have significantly increased wear
resistance, often by a factor of 2.
1.10.5 Cutting Tools
Cutting tools are made of high-speed steel or cemented carbides, and are subject to
degradation by abrasive wear as well as by adhesive wear. In adhesive wear, high temperatures
and forces at the tool tip promote microwelding between the steel chip from the work piece
and steel in the high-speed steel tool or cobalt binder phase in the cemented carbide. The
resulting chip breaks the microweld and causes cratering and wear in the tool. Real
18 Chapter 1
improvement in tribological properties and corrosion resistance comes with deposition of
multilayers, superlattices, nanolaminates, and nanocomposites [16–19]. A thin refractory
metal coating such as TiC, TiN, CrC, or ZrN forms a diffusion barrier and prevents
microwelding and can improve tool life by a factor of 300–800%. Cutting forces are also
reduced. Tribological coatings are deposited by PVD and CVD processes.
Improvements in this area and superhard coatings have been achieved by new composition,
composites and nanolaminate structures, such as:
Ti alloy nitrides: (Ti,Al)N
Ti alloy carbonitrides: Ti(C,N)
nanocomposites: SiC/Si
3
N
4
nanolaminates: CrN/W
x
, TiAlN/VN, AlN/Si
3
N
4
, NiAlN/TiBCN/Ti, C/Cr/CrN,
NbN/CrN, nc-TiN/aSi
3
N
4
/nc-&TiSi
z
, and nc-TiN/aSi
3
N
4
.
Several of these structures are deposited by a hybrid unbalanced magnetron/cathodic arc
process.
1.10.6 Gas and Water Permeation Barriers on Plastic
Encapsulation of atmospherically sensitive organic electronic devices, thin film lithium
batteries, and thin film solar cells is critical in achieving significant operating lifetimes. These
structures are also now being deposited onto thin flexible plastic webs using vacuum web
processes. Multilayer polymer/oxide coatings have demonstrated water and oxygen
permeation values < 10
−6
gm/m
2
/d, and are now being deposited onto these structures, on
glass and plastic, by the VPD process.
1.10.7 Biomedical
Biomedical applications for thin film materials have mushroomed in the past decade, involving
low-friction and wear-resistant coatings for medical implants and prostheses, Ta coatings on
stents, photocatalytic coatings in an artificial lung, DLC coatings on hypodermic needles,
ophthalmic coatings, and tribological coatings on surgical instruments. Many of these
materials are Ta and Ti based because Ti and Ta have been shown to be biocompatible and do
not cause thrombus. Alpha alumina, titanium carbide, ta-C, and DLC coatings also show
promise. Most of these materials are deposited by some form of magnetron sputtering or CVD.
1.10.8 Thin Film Solid Oxide Fuel Cells
The operating temperature of a solid oxide fuel cell can be lowered and the efficiency
increased by constructing it with thin films. The basic structure is a porous nickel or
Deposition Technologies 19
gallium-doped nickel, lanthanum strontium cobalt iron oxide (LSCF) cathode (for low
operating temperatures, since it offers both electronic and ionic conductivity), and yttria
stabilized zirconia (YSZ) electrolyte. The cell is operated at a temperature range between 370
and 550
◦
C, and maximum output power densities near 7 mW/cm
2
are obtained at 400
◦
C.
1.10.9 Flat Panel Displays and Molecular Electronics
Thin film polymers, light-emitting polymers, and small molecule organic materials are used
extensively in OLEDs for flat panel displays, organic electronics, and organic solar cells.
Organic materials have the advantage of a wide range of molecular doping combinations to
achieve a wide range of electrical, optical, and electro-optical properties. Conductive polymers
are also used extensively in organic circuitry and photovoltaics.
1.11 ‘Frontier Areas’ for Applications of the Products
of Deposition Technology
The following were listed in the first and second editions, published in 1982 and 1994,
respectively:
First edition:
reflective surfaces, e.g. for laser mirrors
thermal barrier coatings for blades and vanes operating at high temperatures
corrosion/erosion-resistant coatings at high temperatures, e.g. valves and other critical
compounds in coal gasification plants
advanced cutting tools
wear-resistant surfaces without organic lubricants, particularly at high temperatures
where lamellar solid state lubricants such as MoS
2
are ineffective
first of all thermonuclear reactor vessels
high-strength/high-toughness ceramics for structural applications
ultrafine powders
superconducting materials:
high transition temperatures > 23.2 K
manufacturability of these brittle materials into wire or ribbons
catalytic materials
thin film photovoltaic devices
20 Chapter 1
transparent conductive coatings in optoelectronic devices, photodetectors, liquid
crystal displays, electrochromic windows and optical devices, solar photothermal
absorption devices, heat mirrors
biomedical devices, e.g. neurological electrodes, heart valves, artificial organs
materials conservation
submicrometer microelectronic devices; in this context, a good question is, ’How far
can dimensions be reduced without running into some limit imposed by physical
phenomena?’
Second Edition additions to the above list are:
diamond and DLC for various applications:
tribology, particularly cutting tools
heat management – heat sinks of diamond sheet currently several square inches in
area are on the market
hard protective coatings for infrared applications such as the protection of
germanium and sodium chloride optics
cubic boron nitride for various applications:
high-temperature use (up to 1200
◦
C) semiconductor devices; very perfect device
quality single crystal films have been grown epitaxially on lattice matched TiC
substrates
tribological uses for matching of hard steels
optical and optoelectronic devices
films deposition using a high-velocity gas jet: Hayashi et al. [19] developed a process
in which ultrafine powders (∼ 10 nm diameter) are carried on a high-velocity gas jet
and impinge on a substrate to ‘write’ lines of deposited materials, e.g. YBCO
superconductors. The usage of material is very high; almost 97% is collected as a
deposit. Various applications are envisioned
unbalanced magnetron sputter deposition – very useful new development where some
of the electrons are allowed to escape from the magnetic trap at the sputtering target
and form a plasma near the substrate from which ions can be extracted to bombard the
growing film.
Third Edition additions are:
ultra-low emissivity broadband high reflectors for advanced telescopes with ultra-high
reflectance at near ultraviolet wavelengths