1 Deposition Technologies: An Overview...28 Deposition Technologies for Films and Coatings Research and development expenditures in surface engineering are very extensive. It is reported

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Deposition Technologies: AnOverview

Rointan F. Bunshah

1.0 THE MARKET

Historically, from the late 1950s onward, decorative coatings or aluminumprovided the initial thrust for surface-engineered products for toys, textiles,etc. Since then, the uses of deposition techniques in practically all areas ofengineering and many areas of science have produced a dramatic growth insales of equipment and products produced, particularly in the last decade.According to a recent survey (VDI-Technologiezeutrum-FRG), equipmentwith an estimated value of $6 billion was produced worldwide in 1989 for theirfilm surface technology. Components and devices manufactured with suchequipment amounted to $60 billion and the value of the end-products whichcontained components made possible by surface engineering is estimated at$600 billion. Just one industry, semiconductors, has changed entire productionlines every 5 to 6 years. It is further estimated that only 10% of all items whichcan benefit from surface modifications are being processed today.

Surface engineering will remain a growth industry in the next decade,because surface-engineered products increase performance, reduce costs,and control surface properties independently of the substrate, thus offeringenormous potential due to the following:

! Creation of entirely new products

! Solution of previously unsolved engineering problems

! Improved functionality of existing products—engineering or decorative

! Conservation of scarce materials

! Ecological considerations—reduction of effluent output and powerconsumption

28 Deposition Technologies for Films and Coatings

Research and development expenditures in surface engineering are veryextensive. It is reported that Japan is spending $100 to $150 million for R/Din diamond and diamond-like carbon coatings. The payoff is estimated at $16billion by the end of this decade. In advance thermal barrier coatings by PVDmethods for high temperature operation of turbine blades, it is estimated thatmore than $10 million have been spent in the United States alone. Wear-resistant coatings for disc and heads has attracted much more than $10million in R/D expenditures worldwide. The list continues to expand.

2.0 INTRODUCTION

Most materials used in high technology applications are composites,i.e., they have a near-surface region with properties differing from those of thebulk materials. This is caused by the requirement that the material exhibita combination of various, and sometimes conflicting, properties. For example,a particular engineering component may be required to have high hardness andtoughness (i.e., resistance to brittle crack propagation). This combination ofproperties can be obtained by having a composite material with high surfacehardness and a tough core. Alternately, the need may be for a hightemperature, corrosion-resistant material with high elevated-temperaturestrength as is the case with the hot stage blades and vanes in a gas turbine.The solution again is to provide the strength requirement from the bulk and thecorrosion requirement from the surface.

In general, coatings are desirable, or even necessary, for a variety ofreasons including economics, materials conservation, unique properties, orthe engineering and design flexibility which can be obtained by separating thesurface properties from the bulk properties.

This near-surface region is produced by depositing a coating onto it (i.e.,overlay coating) by processes such as physical or chemical vapor deposition,electrodeposition, and thermal spraying, or by altering the surface material bythe in-diffusion of materials (i.e., diffusion coating or chemical conversioncoating), or by ion implantation of new material so that the surface layer nowconsists of both the parent and added materials.

“Coatings” may also be formed by other processes such as melt/solidification (e.g., laser glazing technique), by mechanical bonding of asurface layer (e.g., roll bonding), by mechanical deformation (e.g., shotpeening), or other processes which change the properties without changingthe composition.

Deposition Technologies: An Overview 29

As stated above, the coating/substrate combination is a compositematerials system. The behavior of this composite system depends not onlyon the properties of the two components (i.e., the coating material and thesubstrate material), but also on the interaction between the two (i.e., thestructure and properties of the coating/substrate interface) which is integral tothe very important factor of adhesion of coatings. In some cases, such as foroverlay coatings, this is a distinct region. For others, such as ion implantationor diffusion coatings, it is not a discrete region.

Historically, most solid metallic and some ceramic materials wereproduced by melting/solidification technology. Since the advent of depositiontechnologies (i.e., production of solid materials from the vapor), the diversityof materials that can be produced has more than doubled because theproperties of solid materials produced from the vapor phase can be varied overa much wider range than the same material produced from the liquid phase.This is because melt techniques produce solid materials with properties closeto equilibrium properties whereas the deposition conditions may be so chosenas to produce materials from the vapor phase with properties close toequilibrium (similar to their melt-produced counterparts), or properties farremoved from equilibrium properties (non-equilibrium properties). Moreover,a much greater variation in microstructure is possible with vapor sourcematerials. For example, a copper-nickel alloy produced by solidification fromthe melt will always consist of a single phase solid solution, whereas the samealloy produced by alternate deposition from two sources may consist ofalternate layers of nickel and copper, i.e., a laminate composite or a solidsolution depending on the deposition temperature.

A large number of materials are used for coatings today. These mayrange from the naturally occurring oxide layer which protects the surfaces ofmany metals such as aluminum, titanium, and stainless steel, to those withvery deliberate and controlled alloying additions to the surface to producespecific properties, as exemplified by techniques such as molecular beamepitaxy or ion implantation. Other examples with increasing degree ofcriticality range from paint coatings applied to wood and metals, electrostaticallypainted golf balls, the print in the daily newspaper, optical coatings on lensesand other elements, vapor deposited microcircuit elements such as resistors,diffusion or overlay coatings on superalloys used in gas turbines for hightemperature corrosion protection, hard overlay coatings of engineeringcomponents and machine tools, etc.


3.0 AIM AND SCOPE

The aim of this volume is to give the reader a perspective on severalcoating techniques with emphasis on the techniques which are used in criticalor demanding (i.e., high technology) applications. Consequently, some of thetechniques such as painting, dip coating, or printing will not be emphasizedexcept as they pertain to some special application like thick film electricalcomponents. Nevertheless, a wide variety of techniques and their applicationswill be covered. The material is intended to present a broad spectrum ofdeposition technologies to those who may be familiar with only one or twotechniques. Hopefully, this will help them to select and weigh variousalternatives when the next technological problem involving coatings facesthem.

The specific deposition technologies to be covered are:

1. Physical Vapor Deposition including evaporation, ion plating andsputtering.

2. Chemical Vapor Deposition and Plasma-Assisted ChemicalVapor Deposition

3. Electrodeposition and Electroless Deposition.

4. Plasma Spraying as well as a very special variant calledDetonation Gun Technology.

There are some generic areas common to several of the depositiontechnologies, the most prominent example being the use of plasmas in manyof the deposition technologies. Therefore, a chapter on plasmas in depositionprocesses is included. Another common topic is cleaning of the substrate andadhesion of the coating. A chapter is included on that topic.

A further common topic is the characterization of the chemical compositionand the microstructure of the coating at various levels of resolution. A chapteris included to satisfy this need.

New chapters are added dealing with Metallurgical Applications (Corrosion,Function and Wear), Overview of Plasma-Assisted Deposition Processes,Plasma-Assisted Chemical Vapor Deposition, and Nucleation/Growth of ThinFilms.

It is realized that all specific applications cannot be satisfied within thisframework. For example, specific applications such as coatings for optical ormagnetic applications are not addressed per se. At the other end of thespectrum, coatings for the first wall of thermo-nuclear reactors cannot bediscussed since the development of the subject is in an embryonic stage.


In each of the chapters on deposition technologies, the theory,methodology, advantages, limitations and applications are discussed.

4.0 DEFINITIONS AND CONCEPTS

In order to avoid potential problems, it is necessary to clarify certaindistinctions which are common and pertinent to deposition technologies.These are as follows:

1. Diffusion vs.Overlay Coatings—Diffusion coatings are producedby the complete interdiffusion of material applied to the surfaceinto the bulk of the substrate material. Examples of this are thediffusion of oxygen into metals to form various sub-oxide andoxide layers, the diffusion of aluminum into nickel base alloys toform various aluminides, etc. A characteristic feature of diffusioncoatings is a concentration gradient from the surface to theinterior, as well as the presence of various layers as dictated bythermodynamic and kinetic considerations. Ion implantationmay be considered to be a special case where the coatingmaterial is implanted at a relatively shallow depth (a few hundredangstrom units) from the surface.

An overlay coating is an add-on to the surface of the part, e.g.,gold-plating on an iron-nickel alloy, or titanium carbide onto acutting tool, etc. Depending upon the process parameters, aninterdiffusion layer between the substrate and the overlay coatingmay or may not be present.

2. Thin Films vs. Thick Films—Historically, the physical dimensionof thickness was used to make the distinction between thick filmsand thin films. Unfortunately, the critical thickness value dependedon the application and discipline. In recent years, a "Confucian"solution has been advanced. It states that if a coating is used forsurface properties (such as electron emission, catalytic activity),it is a thin film; whereas, if it is used for bulk properties, corrosionresistance, etc., it is a thick film. Thus, the same coating materialof identical thickness can be a thin film or a thick film dependingupon the usage. This represents a reasonable way out of thesemantic problem.


3. Steps in the Formation of a Deposit—There are three steps in theformation of a deposit:

a. Synthesis or creation of the depositing species

b. Transport from source to substrate

c. Deposition onto the substrate and film growth

These steps can be completely separated from each other or be super-imposed on each other depending upon the process under consideration. Theimportant point to note is that if, in a given process, these steps can beindividually varied and controlled, there is much greater flexibility for such aprocess as compared to one where they are not separately variable. This isanalogous to the degrees of freedom in Gibbs phase rule. For example,consider the deposition of tungsten by CVD process. It takes place by thereaction:

HeatedWF6(vapor) + 3H2(gas) ———" W(deposit) + 6HF(gas)

Substrate

The rate of deposition is controlled by the substrate temperature. At ahigh substrate temperature, the deposition rate is high and the structureconsists of large columnar grains. This may not be a desirable structure. Onthe other hand, if the same deposit is produced by evaporation of tungsten, thedeposition rate is essentially independent of the substrate temperature so thatone can have a high deposition rate and a more desirable microstructure. Onthe other hand, a CVD process may be chosen over evaporation because ofconsiderations of throwing power, i.e., the ability to coat irregularly shapedobjects, since high vacuum evaporation is basically a line-of-sight technique.

5.0 PHYSICAL VAPOR DEPOSITION (PVD) PROCESS TERMINOLOGY

The basic PVD processes are those currently known as evaporation,sputtering and ion plating. In recent years, a significant number of specializedPVD processes based on the above have been developed and extensivelyused, e.g., reactive ion plating, activated reactive evaporation, reactivesputtering, etc. There is now considerable confusion since a particularprocess can be legitimately covered by more than one name. As


an example, if the activated reactive evaporation (ARE) process is used witha negative bias on the substrate, it is very often called reactive ion plating.Simple evaporation using an RF heated crucible has been called gasless ionplating. An even worse example of the confusion that can arise is found in thechapter on ion plating in this volume (Ch. 6) where the material is convertedfrom the condensed phase to the vapor phase using thermal energy (i.e.,evaporation) or momentum transfer (i.e., sputtering) or supplied as a vapor(very similar to CVD processes). Carrying this to the logical conclusion, onemight say that all PVD processes are ion plating! On the other hand, the mostimportant aspect of the ion plating process is the modification of themicrostructure and composition of the deposit caused by the ion bombardmentof the deposit resulting from the bias on the substrate, i.e., what is happeningon the substrate.

To resolve this dilemma, it is proposed that we consider all of these basicprocesses and their variants as PVD processes and describe them in termsof the three steps in the formation of a deposit as described above. This willhopefully remove the confusion in terminology.

Step 1: Creation of Vapor Phase Specie. There are three ways to put amaterial into the vapor phase-evaporation, sputtering or chemical vapors andgases.

Step 2: Transport from Source to Substrate. The transport of the vaporspecies from the source to the substrate can occur under line-of-sight ormolecular flow-conditions (i.e., without collisions between atoms andmolecules); alternately, if the partial pressure of the metal vapor and/or gasspecies in the vapor state is high enough or some of these species are ionized(by creating a plasma), there are many collisions in the vapor phase duringtransport to the substrate.

Step 3: Film Growth on the Substrate. This involves the deposition of thefilm by nucleation and growth processes. The microstructure and compositionof the film can be modified by bombardment of the growing film by ions fromthe vapor phase resulting in sputtering and recondensation of the film atomsand enhanced surface mobility of the atoms in the near-surface and surfaceof the film.

Every PVD process can be usefully described and understood in termsof these three steps. The reader is referred to Chapter 9 for a morecomprehensive treatment.


6.0 CLASSIFICATION OF COATING PROCESSES

Numerous schemes can be devised to classify or categorize coatingprocesses, none of which are very satisfactory since several processes willoverlap different categories. For example, the Appendix contains a list anddefinitions of various deposition processes based upon those provided byChapman and Anderson with some additions. These authors classify theprocesses under the general heading of Conduction and Diffusion Processes,Chemical Processes, Wetting Processes and Spraying Processes. Here, theChemical Vapor Deposition process falls under the Chemical Processes, andthe Physical Vapor Deposition Process (Evaporation, lon Plating and Sputtering)falls under the spraying processes. The situation can easily get confused as,for example, when Reactive and Activated Reactive Evaporation, and Reactivelon Plating are all classified as Chemical Vapor Deposition processes byYee[3] who considers them thusly because a chemical reaction is involved andit does not matter to him whether evaporated metal atoms or stable liquid orgaseous compounds are the reactants. Another classification of the methodsof deposition of thin films is given by Campbell.[4] He considers the overlapbetween physical and chemical methods, e.g., evaporation and ion plating,sputtering and plasma reactions, reactive sputtering and gaseousanodization.[5] He classifies the Chemical Methods of Thin Film Preparationas follows:

Chemical Methods of Thin Film Preparation

Basic Class Method

Formation from the Medium Electroplatinglon PlatingChemical ReductionVapor PhasePlasma Reaction

Formation from the Substrate Gaseous AnodizationThermalPlasma Reduction


In addition, he considers the following as chemical methods of thick filmpreparation: Glazing, Electrophoretic, Flame Spraying and Painting.

In contrast to the chemists’ approach given above, the physicists’approach to deposition processes is shown in the following classification ofvacuum deposition techniques by Schiller, Heisig and Goedicke[6] and byWeissmantel.[7]

Figure 1.1. Survey of vacuum deposition techniques (Schiller[6])

A different classification comes from a materials background where theconcern is with structure and properties of the deposits as influenced byprocess parameters. Thus, Bunshah and Mattox[8] give a classification basedon deposition methods as influenced by the dimensions of the depositingspecie, e.g., whether it is atoms/molecules, liquid droplets or bulk quantities,as shown in Table 1.1.

In atomistic deposition processes, the atoms form a film by condensingon the substrate and migrating to sites, where nucleation and growth occurs.Further, adatoms do not achieve their lowest energy configurations and theresulting structure contains high concentrations of structural imperfections.Often the depositing atoms react with the substrate material to form a complexinterfacial region.

Another aspect of coatings formed by atomistic deposition processes isas follows. The sources of atoms for these deposition processes can be bythermal vaporization (vacuum deposition) or sputtering (sputter deposition) ina vacuum, vaporized chemical species in a carrier gas (chemical vapordeposition), or ionic species in an electrolyte (electrodeposition). In lowenergy atomistic deposition processes, the depositing species impinge on thesurface, migrate over the surface to a nucleation site where they condense andgrow into a coating. The nucleation and growth modes of the condensingspecies determine the crystallography and microstructure of


the coating. For high energy deposition processes, the depositing particlesreact with or penetrate into the substrate surface.

Particulate deposition processes involve molten or solid particles and theresulting microstructure of the deposit depends on the solidification orsintering of the particles. Bulk coatings involve the application of largeamounts of coating material to the surface at one time such as in painting.Surface modification involves ion, thermal, mechanical, or chemical treatments,which alter the surface composition or properties. All of these techniques arewidely used to form coatings for special applications.

Table 1.1. Methods of Fabricating Coatings

7.0 GAS JET DEPOSITION WITH NANO-PARTICLES

One of the chapters in this volume (Ch. 11) deals with Plasma Sprayingand Detonation Gun Techniques where a high velocity stream of macro-particles (µm dimensions) impinge on a substrate to form a coating. With the


Figure 1.2. Schematic diagram of gas deposition apparatus.

advent of evaporation[9] and sputtering processes[10] to produce nano-particles (nm dimensions), the same concept can be used to producecoatings by carrying nano-particles in a gas stream and impinging them ona substrate.[11][12] Figure 1.2 shows a schematic of this process wheremetallic nano-particles produced by evaporation are carried in a gas stream,accelerated through a nozzle and impinged on a substrate to produce acoating. Single nozzles or multiple nozzle configurations can be used, thelatter producing an array of dots, for example. The attributes of this processare:

1. Direct write maskless processing to produce dots, lines, andother shapes.

2. High deposition rate, 10 - 20 µm per second over a small area.3. Low temperature (room temperature) deposition.4. Metals, alloys, ceramics, and organic materials can be

deposiited.5. Multiphase films with uniform mixing can be produced.6. The collection officiency is very high, ~90%, i.e. very little waste

or scatter.Examples of applications of this technique are:

1. Electrical connecting lines in circuits including the repair aspect.2. Fabrication of microelectrodes3. Oxide superconductor contacts.4. Capacitors5. Implantation of virus into plants for the bio industry.6. Cell-gene processing technology.


8.0 MICROSTRUCTURE AND PROPERTIES

In electrodeposition, typically the growth process involves condensationof atoms at a kink site on the substrate surface, followed by layered growthof the deposit. Adatom mobility is increased by the hydrated nature of the ionsand the adatom mobility may vary with crystal orientation. Field ionmicroscopy stripping studies of copper electrodeposited on tungsten hasshown that there is surface rearrangement of the tungsten atoms during theelectrodeposition process. Electrodeposited material does not grow in auniform manner; rather it becomes faceted, develops dendrites and othersurface discontinuities. Thus the microstructure of electrodeposited coatingsmay vary from relatively defect-free single crystals usually grown on singlecrystal substrates, to highly columnar and faceted structures. In theelectroplating process, organic additives may be used to modify the nucleationprocess and to eliminate undesirable growth modes. This results in amicrostructure more nearly that of bulk material formed by conventionalmetallurgical processes. Electrodeposition from a molten salt electrolyteallows the deposition of many materials not available from aqueous electrolytes.

In vacuum processes, the depositing species may have energies rangingfrom thermal (a few tenths of an electron volt) for evaporation to moderateenergies (ten to hundreds of electron volts) for sputtered atoms to highenergies for accelerated species such as those used in ion implantation.These energies have an important but poorly understood effect on interfacialinteraction, nucleation and growth. Where there is chemical reaction betweenthe substrate atoms and the depositing atoms, and diffusion is possible, adiffusion or compound interfacial region is formed composed of compoundsand/or alloys which modify the effective surface upon which the deposit grows.Low energy electron diffraction studies have shown that this interfacial reactionis very sensitive to surface condition and process parameters. If the coatingand substrate materials are not chemically reactive and are insoluble, theinterfacial region will be confined to an abrupt discontinuity in composition.This type of interface may be modified by bombardment with high energyparticles to give high defect concentrations and implantation of ions resultingin a “pseudodiffusion” type of interface. The type of interface formed willinfluence the properties of the deposited coating. In many circumstances,these interfacial regions are of very limited thickness and pose a challenge tothose interested in compositional, phase, microstructural and propertyanalysis.


The microstructure of the depositing coating in the atomic depositionprocesses depends on how the adatoms are incorporated into the existingstructure. Surface roughness and geometrical shadowing will lead topreferential growth of the elevated regions giving a columnar type microstructureto the deposits.[13] This microstructure will be modified by substratetemperature, surface diffusion of the atoms, ion bombardment during deposition,impurity atom incorporation and angle of incidence of the depositing adatomflux. The structure zone model of Movchan and Demchishin[14] for vacuumdeposited films is discussed in later chapters.

In chemical vapor deposition, the chemical species containing the filmatoms is generally reduced or decomposed on the substrate surface, often athigh temperatures. Care must be taken to control the interface reactionbetween coating and substrate and between the substrate and the gaseousreaction products. The coating microstructure which develops is very similarto that developed by the vacuum deposition processes, i.e., small-grainedcolumnar structures to large-grained equiaxed or oriented structures.

Each of the atomistic deposition processes has the potential of depositingmaterials which vary significantly from the conventional metallurgicallyprocessed material. The deposited materials may have high intrinsicstresses, high point defect concentration, extremely fine grain size, orientedmicrostructures, metastable phases, incorporated impurities, and macro andmicro porosity. These properties may be reflected in the physical propertiesof the materials and by their response to applied stresses such as mechanicalloads, chemical environments, thermal shock or fatigue loading. Metallurgicalproperties which may be affected include elastic constants, tensile strength,fracture toughness, fatigue strength, hardness, diffusion rates, friction/wearproperties, and corrosion resistance. In addition, the unique microstructureof the deposited material may lead to such effects as anomalously lowannealing and recrystallization temperatures where the internal stresses andhigh defect concentration aid in atomic rearrangement.

The high value of grain boundary area to volume ratio found in fine graineddeposited material means that diffusion processes may be dominated by grainboundary rather than bulk diffusion. The fine grained nature of the materialsalso affects the deformation mechanisms such as slip and twinning. For thinfilms, the free-surface to volume ratio is high, and the pinning of dislocation bythe free surface leads to the high tensile strengths often measured in thin filmsof materials.


In vapor deposition processes, impurity incorporation during depositioncan give high intrinsic stresses or impurity stabilized phases which are notseen in the bulk forms of the materials. Reactive species allow the depositionof compounds such as nitrides, carbides, borides and oxides. Gradeddeposits can be formed.

Vapor deposition processes have the capability of producing unique and/or nonequilibrium microstructures. One example is the fine dispersion ofoxides in metals, where the oxide particle size and spacing is very small (100- 500 Å). Alternately, metals and alloys deposited at high substratetemperatures have properties similar to those of conventionally fabricated(cast, worked and heat treated) metals and alloys. A more recent exampleis the nano-scale laminate composites consisting of alternate layers ofrefractory compounds with unusually high hardness values.

9.0 UNIQUE FEATURES OF DEPOSITED MATERIALS AND GAPS INUNDERSTANDING

It is useful to state at this point some of the unique features of materialsproduced by deposition technologies. They are:

1. Extreme versatility of range and variety of deposited materials.

2. Overlay coatings with properties independent of thethermodynamic compositional constraints.

3. Ability to vary defect concentration over wide limits, thus resultingin a range of properties comparable to, or far removed fromconventionally fabricated materials.

4. High quench rates available to deposit amorphous materials.

5. Generation of microstructures different from conventionallyprocessed materials, e.g., a wide range of microstructures—ultrafine (submicron grain or laminae size) to single crystal films.

6. Fabrication of thin self-standing shapes even from brittle materials.

7. Ecological benefits with certain techniques.

The first edition lists some of the areas where our understandingof basic processes and phenomena is lacking and which obviously arethe areas where research activities are essential. These are:


1. Microstructure and properties in the range of 500 to 10,000 Å—particularly important for submicron microelectronics, reflectivesurfaces and corrosion.

2. (a) Effect of the energy of the depositing species oninterfacial interaction, nucleation and growth of deposit.

(b) Effect of “substrate surface condition,” i.e.,contamination (oxide) layers, adsorbed gases,surface topography.

3. Residual stresses—influence of process parameters.

Considerable progress and understanding has developed in the lastdecade.

10.0 CURRENT APPLICATIONS

The applications of coatings in current technology may be classed intothe following generic areas:

Optically Functional—Laser optics (reflective and transmitting),architectural glazing, home mirrors, automotive rear view mirrors,reflective and anti-reflective coatings, optically absorbing coatings,selective solar absorbers.

Electrically Functional—Electrical conductors, electrical contacts,active solid state devices, electrical insulators, solar cells.

Mechanically Functional—Lubrication films, wear and erosionresistant coatings, diffusion barriers, hard coatings for cutting tools.

Chemically Functional—Corrosion resistant coatings, catalyticcoatings, engine blades and vanes, battery strips, marine useequipment.

Decorative—Watch bezels, bands, eyeglass frames, costumejewelry.

A few examples are chosen to illustrate them in greater detail.

10.1 Decorative/Functional Coating

Weight reduction is a high priority item to increase gas mileage inautomobiles. Therefore, heavy metallic items such as grills are being


replaced with lightweight plastic, overcoated with chromium by sputtering forthe appearance to which the consumer is accustomed.

Another extensive application is aluminum-coated polymer films for heatinsulation, decorative and packaging applications.

A rapidly growing application is the use of a gold-colored wear-resistantcoating of titanium nitride on watch bezels, watch bands and similar items.

A new application is black wear-resistant hard carbon films.

10.2 High Temperature Corrosion

Blades and vanes used in the turbine-end of a gas turbine engine aresubject to high stresses in a highly corrosive environment of oxygen-, sulfur-and chlorine-containing gases. A single or monolithic material such as a hightemperature alloy is incapable of providing both functions. The solution is todesign the bulk alloy for its mechanical properties and provide the corrosionresistance by means of an overlay coating of an M-Cr-AI-Y alloy where Mstands for Ni, Co, Fe or Ni + Co. The coating is deposited in production byelectron beam evaporation and in the laboratory by sputtering or plasmaspraying. With the potential future use of synthetic fuels, considerableresearch will have to be undertaken to modify such coating compositions forthe different corrosive environments as well as against erosion from theparticulate matter in those fuels.

10.3 Environmental Corrosion

Thick ion plated aluminum coatings are used in various irregularly-shaped parts of aircraft and space-craft as well as on fasteners: (a) to replaceelectroplated cadmium coatings which sensitize the high-strength parts tohydrogen embrittlement or (b) to prevent galvanic corrosion which would occurwhen titanium or steel parts contact aluminum or (c) to provide goodbrazeability. New alloy coatings in the micron thickness range have beendeveloped.

10.4 Friction and Wear

Dry-film lubricant coatings of materials such as gold, MoS2, WSe2 andother lamellar materials are deposited on bearings and other sliding parts bysputtering or ion plating to reduce wear. Such dry-film lubricants are


especially important for critical parts used in long-lifetime applications sinceconventional organic fluid lubricants are highly susceptible to irreversibledegradation and creep over a long time.

10.5 Materials Conservation

Aluminum is continuously coated on a steel strip, 2 feet wide and 0.006inches thick to a 250 micro-inch thickness in an air-to-air electron-beamevaporator at the rate of 200 feet/minute. The aluminum replaces tin, whichis becoming increasingly scarce and costly. The strip then goes to the lacquerline and is used for steel can production. With the change in Eastern Europe,this line has switched to deposition of Cr and Cu on steel.

10.6 Cutting Tools

Cutting tools are made of high-speed steel or cemented carbides. Theyare subject to degradation by abrasive wear as well as by adhesive wear. Inthe latter mode, the high temperatures and forces at the tool tip promotemicrowelding between the steel chip from the workpiece and the steel in thehigh-speed steel tool or the cobalt binder phase in the cemented carbide. Thesubsequent chip breaks the microweld and causes tool surface cratering andwear. A thin layer of a refractory compound such as TiC, TiN, Al2O3 preventsthe microwelding by introducing a diffusion barrier. Improvements in tool lifeby factors of 300 to 800% are possible as well as reductions in cutting forces.The coatings are deposited by chemical vapor deposition or physical vapordeposition. Some idea of the importance of such coatings can be assessedfrom the fact that the yearly value of cutting tools purchased in the U.S. is $1billion and the cost of machining is approximately $60 billion.

The last decade has seen major advances in this area and some of theseare:

! Ti alloy nitrides, e.g., (Ti, Al) N

! Ti carbonitrides, e.g., Ti (C,N)

! Multilayer coatings of different nitrides

! Diamond coated tools by CVD and PACVD processes formachining of non-ferrous metals and polymer-matrixcomposites. A bond layer such as silicon nitride has to be usedto attach the diamond coating to the carbide cutting tool.


! Hard diamond-like carbon for heads and discs

! Cubic boron nitride coatings by plasma-assisted PVD andCVD methods for cutting of hard ferrous materials

10.7 Nuclear Fuels

Pyrolytic carbon is deposited on nuclear fuel particles used in gas-cooledreactors by chemical vapor deposition in fluidized beds. The coating retainsthe fission products and protects the fuel from corrosion.

10.8 Biomedical Uses

Parts for implants such as heart valves are made of pyrolytic carbon byCVD techniques. Metal parts are coated with carbon by ion plating in orderto obtain biological compatibility.

10.9 Electrical Uses

High temperature cuprate superconductors with transition temperaturesof 85° to 115°K. This permits the operation of liquid nitrogen cooled devices.Various PVD techniques such as co-evaporation in an oxygen plasma,sputtering from simple or multiple targets and laser ablation have been usedto fabricate films, ranging from 1 to 50 cm2. Microwave devices such as delaylines, quasioptical filters have been fabricated and are being marketed.

11.0 “FRONTIER AREAS” FOR THE APPLICATION OF THE PRODUCTSOF DEPOSITION TECHNOLOGY

The following were listed in the first edition published in 1982.

1. Reflective surfaces, e.g., for laser mirrors.

2. Thermal barrier coatings for blades and vanes operating at hightemperatures.

3. Corrosion/erosion resistant coatings at high temperatures,e.g., valves and other critical compounds in coal gasificationplants.

4. Advanced cutting tools.


5. Wear-resistant surfaces without organic lubricants, particularlyat high temperatures where lamellar solid state lubricants suchas MoS2 are ineffective.

6. First wall of thermonuclear reactor vessels.

7. High-strength/high-toughness ceramics for structural use.

8. Ultrafine powders.

9. Super conducting materials:

High transition temperatures >23.2°K.

Fabricability of these brittle materials into wire or ribbons.

10. CataIytic materials.

11. Thin film photovoltaic devices.

12. Transparent conductive coatings in opto-electronics devices,photo detectors, liquid crystal and electrochromic displays,solar photo thermal absorption devices, heat mirrors.

13. Biomedical devices, e.g., neurological electrodes, heart valves,artificial organs.

14. Materials conservation.

15. Sub-micron microelectronic devices. In this context, a goodquestion is, How far can dimensions be reduced without runninginto some limit imposed by physical phenomena?

In 1992, new additions to the above list are:

16. Diamond and diamond-like carbon for various applications:

! Tribology, particularly cutting tool

! Heat management–heat sinks of diamond sheet currentlyseveral square inches in area are on the market

! Hard protective coatings for infrared applications such asthe protection of germanium and sodium chloride optics

17. Cubic boron nitride for various applications:

! High temperature use (up to 1200°) semiconductordevices. Very perfect device quality single crystal filmshave been grown epitaxially on lattice matched TiCsubstrates

! Tribological uses for machining of hard steels

! Optical and opto-electronic devices


18. Film deposition using a high velocity gas jet. Hayashi andcoworkers[9] have developed a process where ultra-fine powders(~10 nanometer diameter) are carried on a high velocity gas jetand impinged on a substrate to “write” lines of depositedmaterials, e.g., YBCO superconductors. The usage of materialis very high, almost 97% is collected as a deposit. Variousapplications are envisioned.

19. Unbalanced magnetron deposition—very useful new developmentwhere some of the electrons are allowed to escape from themagnetic trap at the sputtering target and from a plasma nearthe substrate from which ions can be extracted to bombard thegrowing film.

12.0 SELECTION CRITERIA

The selection of a particular deposition process depends on severalfactors. They are:

1. The material to be deposited

2. Rate of deposition

3. Limitations imposed by the substrate, e.g., maximum depositiontemperature

4. Adhesion of deposit to substrate

5. Throwing power

6. Purity of target material since this will influence the impuritycontent in the film

7. Apparatus required and availability of same

8. Cost

9. Ecological considerations

10. Abundance of deposition material in the world

In order to aid the reader in the task of selection, Table 1.2 lists severalcriteria for each of the processes. It is obvious that there are very fewtechniques which can deposit all types of materials. It is also impossible todetail the advantages and limitations of each of the techniques. However, inthe evaluation of each application, the above factors will lead to a rationalchoice of the deposition technique to be used.

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Table 1.2. Some Characteristics of Deposition Processes


13.0 SUMMARY

In the above discussion, we have noted the following:

1. There are a very large number of deposition techniques.

2. There is no unique way to classify these techniques. Dependingon the viewpoint, the same technique may fall into fall into one ormore classes.

3. Each technique has its advantages and limitations.

4. The choice of the technique to be used depends on variousselection criteria which have been given above.

5. More than one technique can be used to deposit a given film asshown in Figure 1.3 below from Campbell’s article on preparationmethods in microelectronic fabrication.

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Electro- Chemical Vapor Anodization Thermal Evaporation Sputteringplating Reduction Phase

Conductors,

resistiors

Insulators,

capacitors

Active

devices

Magnetic

materials

Super-

conductors

Figure 1.3. Applicability of preparation methods to microelectronics. Light shadingindicates that the component can be prepared by the method; Dark shading

indicates that the method is widely used.


APPENDIX 1: DEPOSITION PROCESS DEFINITIONS

The definitions of various deposition processes are given below. Theyare grouped as proposed by Chapman and Anderson[1] and many of them arethose proposed by these authors.

Conduction and Diffusion Processes

Electrostatic Deposition is the deposition of material in liquid form, thesolvent used then being evaporated to form a solid coating. At the source, theliquid is atomized and charged, and then it can be directed onto the substrateusing an electrostatic field.

Electrophoretic Coating produces a coating on a conducting substratefrom a dispersion of colloidal particles. The article to be coated is immersedin an aqueous dispersion which dissociates into negatively charged colloidalparticles and positive cations. An electric field is applied with the article asanode (positive electrode); the colloidal particles are transported to the anode,where they are discharged and form a film. In the case of a paint coating, thisrequires curing, which further shows that electrophoresis itself is not a veryeffective transport process, so that electrodeposition may be a better term forthe coating process.

Electrolytic Deposition is primarily concerned with the deposition of ionsrather than of colloidal particles. Two electrodes are immersed in anelectrolyte of an ionic salt which dissociates in aqueous solution into itsconstituent ions; positive ions are deposited onto the cathode (negativeelectrode).

Anodization is a process which occurs at the anode (hence its name) for a fewspecific metals. The anode reacts with negative ions from the electrolyte and becomesoxidized, i.e., it forms a surface coating.

Gaseous Anodization is a process in which the liquid electrolyte of theconventional wet process is replaced by a glow discharge in a low partial pressure ofa reactive gas. Oxides, carbides and nitrides can be produced this way.

Ion Nitriding is a gaseous anodization to produce nitride diffusion coating on ametal surface, usually steel.

Ion Carburizing is a gaseous anodization to produce a carbide diffusion coatingon a metal surface, usually steel.

Plasma Oxidation is gaseous anodization to produce an oxide film on the surfaceof metal, e.g., SiO2 films on Si.

Diffusion Coating is produced by diffusion of material from the surface into thebulk 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); materialis transferred from the anode and diffuses into the substrate.

Chemical processes

Conversion and Conversion/Diffusion Coating is a process in which thesubstrate is reacted with other substances (which may be in the form of solids,liquids or gases) so that its surface is chemically converted into differentcompounds having different properties. (Anodization could probably bedescribed as an electrochemical conversion process). Conversion coatingusually takes place at elevated temperatures and diffusion is often an essentialfeature.

Chemical Vapor Deposition (CVD) is a chemical process which takesplace in the vapor phase very near the substrate or on the substrate so thata reaction product is deposited onto the substrate. The deposition can be ametal, semiconductor, alloy or refractory compound.

Pyrolysis is a particular type of CVD which involves the thermaldecomposition of volatile materials on the substrate.

Plasma-Assisted CVD is a process where the reaction between thereactants is stimulated or activated by creating a plasma in the vapor phaseusing means such as R F excitation from a coil surrounding the reactionvessel.

Electroless Deposition is often described as a variety of electrolyticdeposition which does not require a power source or electrodes, hence itsname. It is really a chemical process catalyzed by the growing film, so thatthe electroless term is somewhat a misnomer.

Disproportionation is the deposition of a film or crystal in a closed systemby reacting the metal with a carrier gas in the hotter part of the system to formthe compound, followed by dissociation of the compound in the colder part ofthe system to deposit the metal. Examples are epitaxial deposits of Si or Geon a single crystal substrate and the Van-Arkel-deBoer process for metalpurification and crystal growth.

Wetting Process

Wetting Processes are the coating processes in which material isapplied in liquid form and then becomes solid by solvent evaporation or cooling.


Conventional Brush Painting and Dip Coating are wetting processes inwhich the part to be coated is literally dipped into a liquid (e.g., paint) undercontrolled conditions of, for example, withdrawal rate and temperature.

Hydrophilic Method is a surface chemical process known as theLangmeir Blodgett technique which is used to produce multimonolayers oflong chain fatty acids. A film 25 Å thick can be deposited on a substrateimmersed in water and pulled through a compressed layer of the fatty acid onthe surface of the water. The process can be repeated to build up many layers.

Welding Processes are the range of coating techniques all of which relyon wetting.

Spraying Processes

Printing Process also relies on wetting and is a process in which the ink,conventionally pigment in a solvent, is transferred to and is deposited on apaper or other substrate, usually to form a pattern; the solvent evaporates toleave the required print.

Spraying Processes can be considered in two categories; (i) macroscopicin which the sprayed particle consists of many molecules and is usually graterthan 10 µm in diameter; (ii) macroscopic in which the sprayed particles arepredominantly single molecules or atoms.

Air and Airless Spraying are the first of the macroscopic processes.When a liquid exceeds a certain critical velocity, it breaks up into smalldroplets, i.e., it atomizes. The atomized droplets, by virtue of their velocity(acquired from a high pressure air or airless source) can be sprayed onto asubstrate.

Flame Spraying is a process in which a fine powder (usually of 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 theheating, then sprays the molten powder onto the substrate where it solidifies.

Detonation Coating is a process in which a measured amount of powderis injected into what is essentially a gun, along with a controlled mixture ofoxygen and acetylene. The mixture is ignited, and the powder particles areheated and accelerated to high velocities with which they impinge on thesubstrate. The process is repeated several times a second.

Arc Plasma Spraying is a process in which the powder is passed throughan electrical plasma produced by a low voltage, high current electricaldischarge. By this means, even refractory materials can be deposited.


Electric-Arc Spraying is a process in which an electric arc is struckbetween two converging wires close to their intersection point. The hightemperature arc melts the wire electrodes which are formed into high velocitymolten particles by an atomizing gas flow; the wires are continuously fed tobalance the loss. The molten particles are then deposited onto a substrateas with the other spray processes.

Harmonic Electrical Spraying is a process in which the material to besprayed must be in liquid form, which will usually require heating. It is placedin a capillary tube and a large electrical field is applied to the capillary tip. Itis found that by adding an AC perturbation to the DC field, a collimated beamof uniformly sized and uniformly charged particles is emitted from the tip.Sense these particles are charged, they could be focused by an electrical fieldto produce pattern deposits.

Evaporation is a process in which the boiling is carried out in vacuumwhere there is almost no surrounding gas; the escaping vapor atom will travelin a straight line for some considerable distance before it collides withsomething, for example, the vacuum chamber walls or substrate.

Glow Discharge Evaporation and Sputtering are processes in softvacuum (10-2 to 10-1 torr) operating in the range 10-1 < pd < 10-2 torr cm wherep is the pressure and d is the cathode fall dimension.

Molecular Beam Epitaxy is an evaporation process for the deposition ofcompounds of extreme regularity of layer thickness and composition from wellcontrolled deposition rates.

Reactive Evaporation is a process in which small traces of an active gasare added to the vacuum chamber; the evaporating material reacts chemicallywith the gas so that the compound is deposited on the substrate.

Activated Reactive Evaporation (ARE) is the Reactive EvaporationProcess carried out in the presence of plasma which converts some of theneutral atoms into ions or energetic neutrals thus enhancing reactionprobabilities and rates to deposit refractory compounds.

Biased Activated Reactive Evaporation (BARE) is the same process asActivated Reactive Evaporation with substrate held at a negative bias voltage.

Sputter Deposition is a vacuum process which uses a different physicalphenomenon to produce the microscopic spray effect. When a fast ion strikesthe surface of a material, atoms of that material are ejected by a momentumtransfer process. As with evaporation, the ejected atoms or molecules can becondensed on a substrate to form a surface coating.

Ion Beam Deposition is a process in which a beam of ions generated froman ion beam gun, impinge and deposit on the substrate.


Ion Beam Assisted Deposition—two versions are possible. One, an ionbeam is used to sputter a target and a second beam is used to bombard thegrowing film to change structure and properties. This is dual Ion BeamAssisted Deposition. The other version uses an ion beam to bombard thegrowing film to change structure and properties. In this case, conventionalevaporation or sputtering techniques are used to generate a flux of thedepositing species.

Cluster Ion Beam Deposition is an ion beam deposition in which atomicclusters are formed in the vapor phase and deposited on the substrate.

Ion Plating is a process in which a proportion of the depositing materialfrom an evaporation, sputtering or chemical vapor source is deliberatelyionized. Once changed this way, the ions can be accelerated with an electricfield so that the impingement energy on the substrate is greatly increased,producing modifications of the microstructure and residual stresses of thedeposit.

Reactive Ion Plating is ion plating with a reactive gas to deposit acompound.

Chemical Ion Plating is similar to Reactive Ion Plating but uses stablegaseous reactants instead of a mixture of evaporated atoms and reactivegases. In most cases, the reactants are activated before they enter theplasma zone.

Ion Implantation is very similar to ion plating, except that now all of thedepositing material is ionized, and in addition, the accelerating energies aremuch higher. The result is that the depositing ions are able to penetrate thesurface barrier of the substrate and be implanted in the substrate rather thanon it.

Plasma Polymerization is a process in which organic and inorganicpolymers are deposited from monomer vapor by the use of electron beam,ultraviolet radiation or glow discharge. Excellent insulating films can beprepared in this way.


REFERENCES

1. Science and Technology of Surface Coating, (B. N. Chapman and J. C.Anderson, eds.), Academic Press (1974)

2. Adhesion Measurement of Thin Films, Thick Films and Bulk Coatings,(K. D. Mittal, ed.), Am. Soc. for Testing Materials (1978)

3. Yee, K. K., International Metal Reviews, No. 226, The Metals Society andAmerican Society for Metals (1978)

4. Campbell, D. S., Handbook of Thin Film Technology, (L. Maissel and R.Glang, eds.), Ch. 5, McGraw-Hill (1970)

5. Handbook of Thin Film Technology, (L. Maissel and R. Glang eds.),McGraw-Hill (1970)

6. Schiller, S., Heisig, O., and Geodick, K., Proc. 7th Int’l. VacuumCongress, (R. Dobrozemsky, ed.), p. 1545, Vienna (1977)

7. Weissmantel, C., ibid, p. 1533,

8. Bunshah, R. F. and Mattox, D. M., Physics Today (May 1970)

9. Hayashi, C., Paper presented at the International Vacuum Congress,Hague, Netherlands (Oct. 1992); also: Hayashi, C., J. Vac. Sci. Tech.,A5(4):1375 (1987)

10. Suc, T. G., Umarjee, D. M., Prakash, S., and Bunshah, R. F., Surfaceand Coatings Technology, 13:199 (1991)

11. Hayashi, C., Kashu, S., Oda, M., and Naruse, F., presented at the Int'lVac. Cong., The Hague, Netherlands (Nov. 1992) - to be published in Mat.Sci. Eng., (1993)

12. Oda, M., Katsu, I., Tsuneizumi, M., Fuchita, E., Kashu, S., and Hayashi,C., presented at Fall Mtg. Mat. Res. Soc., Boston, 1992

13. Thornton, J. A., Proc. 19th National SAMPLE Symposium, Buena Park,Ca. (April 23-25, 1974)

14. Movchan, B. A., and Demchishin, A. V., Phys. Met Metallogr., 28:83(1969)

Documents

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