B 3

r t ' Vapor Phase Depos by Philip C. Johnson

j Vac-Tec Systems, Inc., Boulder, CO

d a y , finishers have an array of techniques and materials available

which were only a dream 10 years ago. Advances in surface preparation, elec- troplating, paint systems and vapor phase deposition have created a previ- ously unmatched choice of color, tex- ture, durability and corrosion resistance when using one, or a combination, of these processes. This article features a review of the applications and develop- ments in vapor phase deposition in the OS, summarizes the current status of these processes and looks ahead to pos- sibilities in the '90s Emphasis is placed on newer methods which have been proven in production. Less attention is given to well established and ongoing methods or to techniques which are stili at an early ierearch stage.

VAPOR PHASE PROCESSES AN 5 A P P 1. I CAT1 8 MS

or phase processes, physical deposition (PVD) and chemical deposition (CVD), are used to thin films, typically in the range

5 microns (4 to 200 micro- . Only in a few specific and de- g circumstances arc signifi hicker films deposited by vapor

hods. In both PVC and CVD a vapor of the material to be is generated in a vacuum, thc n condenses on the substrate e the desired film. In the case

, the material is evaporated 01

puttered from a molten or solid source. he case of CVD the depositing spe- s are derived from the dissociation rases introduced into a high femper- re deposition zone. Evaporation 'inti

puttering (PVD) procesjes arts the odr used for film Jeposition 111 ap- lions as divelse as aluminuin Ilizing, microelectronics, ar -

scfural dnd automobile glass, food ging, niemorj disks, audio CD at panel dispiays, to name but CVD prucessea are exrenslvely in microciectronic< p i o d u c tion

photovoltaics, flat panel displays, dep- osition of hard coatings and for fiber and powder coating.

Finishing has represented a relatively small part of an extremely large market for vapor phase processes in the '80s. The most widely used of the finishing methods has been, and continues to be, evaporative aluminum metallizing onto products ranging from automobile components, lighting and drapery fix- tures to packaging materials and toys. Vapor phase processes have also pene- trated the decorative market for watches, pens and eyeglass frames, products which place significantly more stringent d'emands on the coatbig than do many traditional nietalljzing applications.

In a l l of these conventional pruc- esses, condensation of the vapor onlo the substrate is an essentially passive event cliaracterizcti by low arrival ener- gies. These conditions result in films which are only moderately adherent. to the'substrate and which exhibit colum- nar structure and therefore relatively low density and high porosity. While such films are fully adequate for a wide variety of applications they offer little in terms of functional benefits to the substrate such as imparting corrosion or wear resistance. Until the dwelop- ments of the last decade, deposition at significantly elevated substrate ternper- atures was the only means of improving these important film parameters.

PROGRESS IN THE '80s The single most significniit develop-

ment in thin film deposition throughout the '80s was the widespread introduc- tion and iml~lcmcntation of the use of plasma in the deposition environnit:nt to enhance the qualities of tiic growing film.

The presence of plasma in the coat- ing environment leads to ion bombaid- inent of the part being coatcd, ap2" r 1ic:i- tion of a negative voltage: bias to the part incr

bombardment. The concept of ion bom- bardment, before and during film dep- osition, was introduced in the early '60s;' however, it was not until the mid to late '70s that a viable commtxcial application was developed. Thc Bal- zers company in Liechtenstein and the Ulvac company in Japan were the first to develop evaporative ion plating proc- esses for coating high speed steel cut- ting tools with a hard, wear resistant coating of titanium nitride. This was to be the first widely commercialized plasma enhance.d deposition technique.

Ion bombardinent dramatically changes the quality of the deposited film. Propcrties such as adhesion, film density and reactivity, which could only be eiihanced previously hy in- crcasi:rg the temperature, could now be sigr!ificantly enhanced at low substrate fernperature. Adhereni, dense filiiis can now be applied to suhstrates which can- not tolerate the elevated temperatures required to achieve a similar effect in the absence of plasma. For example, films deposited by simple evaporation would lend nothing to the perfoi-mance or service life of a cutting tool. Deposit the same film, however, in the presence of ion bombardment and the film be- come:; well enough adhercd, and suffi- ciently dense, to produce much ex- tended life and performance in the same tool.

The term plasma enhanced or as- sisted deposition has been coined to en- compiss all of these processes which make use of ion bornbardment to mod- ify the propertics of 3 depositing film. Distinguishing features of the plasiiia

;es art: the high :.it-rival ositing species, coii-

tinuoirs sputter i k m i n g of thc sub- ,tivity of the ionized

species arid excellent throwing power.

peting methods such as sputter ion plat- ing and cathodic arc plasma depositioii were intrcxluced during the ensuing years.

7'he ion plating process employs es- sentially coiivcntional electron beam cvaporation sources to produce the vapor. A plasma discharge is initiated in the space betwcen the source and substrate by the introduction of an inert gas at low pressure. Gas ions from the plasma bombard the substrate and growing film during deposition. Bom- bardment is enhanced by biasing the substrate negative. Enhanced versions of the basic ion plating approach em- ploy more sophisticated electron beam sources and additional electrodes to iii- crease the electron population in the plasma zone and to increase the degree of ionization.

Cathodic arc plasma deposition (CAPD) employs a low voltage, high current arc discharge to generate vapor emission from a target which is the cathode i n the discharge circuit. The arc I T I O V ~ S around the surface of the target, either randoinly or magnetically guidcci. arid produces a series of intense flash evaporation events. Again the substrate is biased negative to enhance the ion bombardment effect. The CAPD process is characterized by:

1. The very high degree of ionization o f the emitted vapor, 30% to 100%.

2. The ernission of ions that are mul- tiply charged.

3. The high kinetic energy of the emitted ions, 10 to 100 eV.

Ion bombardment is achieved in a sputtering situation by overlapping the plasma of two opposed magnetron sources and placing the substrate to be coated in the overlap region. Negative bias is again used to enhance the ion bombardment. When using conven- tional magnetron sources this technique suffers from geometrical constraints since the region of plasma overlap is reIatively confined. Coating of large substrates therefore presents problenis.

A recent development, which holds great promise for the future, is unba- lanced magnetron sputtering. In an un- balanced magnetron the magnetic field is designed to confine the secondary electrons, and thus the plasma, in a zone in front of the source. As a result a significantly more intense plasma is produced which leads to a greatly in- creased ion current density at the sub- strate when compared to convcntional

sputter ion plating. This increased in- tensity of ion bombardment permits an increase in source to substrate distance without compromise in film properties and thus overcomes the geoiner rical constraints of the conventional method. Films produced with the unbalanced magnetron also have superior adhesion and density.

If these plasnia enhanced processes are operated with no gas, or only an inert gas, present then an unmodified film of the source material will be de- posited. The addition of a reactive gas such as oxygen or nitrogen leads to the deposition of the oxide or nitride of the source material due to a reaction be- tween the gas and the source vapor at the substrate surface. Such reactive deposition processes have been, and re- main, the predominant application of these plasma enhanced PVD processes.

In the CVD arena, the introduction of a plasnia discharge during deposition (PECVD) has permitted significant re- duction in processing temperature ( 1 000°C to 200-400°C) and facilitated the deposition of rnaterials which were not previously possible. Conventional CVD relies on thermal energy to dis- sociate the carrier gases and thus create the depositing species. Similar dissoci- ation is induced in the plasma environ- ment thus alleviating the need for high thenna! energy. In addition dissocia- tion can be induced in a plasma which might be impossible to induce ther- mally; hence, the range of niaterial pos- sibilities is expanded.

Vapor phase processes which are now well established, and which are believed to hold the greatest promise for growth in the '90s are, p l a s m en- hanced CVD, unbalanced magnetron sputtering, cafhodic arc plasma deposi- tion and enhanced evaporative ion plat- ing processes. Hybrid systems and processes also have been demonstrated and show great promise, deposition sources which can be operated in unbal- ed magnetron and arc modes have al- ready been successfully demonstrated and produce films of extremely high quality. Combinations of sputtering and cathodic arc and sputtering and ion plating piovide flexibility in materials and surface engineering which has not previously been available

ATERIALS The advent of plasma enhanced proc-

esses has brought with it a range and

combination ofrmterials and propei ties which could not previously he achieved. Perhaps the most drarnatic results have been achieved in the c i ~ c c i g

of dimiond anci diarnonrl-like fjfni< ,rnd in thc preparation of thin film h1gh tern- peratuic supcrcoiiductors Alloy 111-

tridcs, oxynitrides and c;irbonitrida< of the refractory metals are providing weal performance and decorativc flcx- ibility previously uninatched. Cubic boron nitride, deposited by PECVD is

of great interest as a semiconductor and as a wear resistant coating. CBN 15 an extremely hard material which i i also chemically stable; unlike diamond. i t

can be used for the machining of steel

ACTIVE RESEARCH AREAS In the areas of process technique dnd

equipment, attention is currently fo- cused on unbalanced magnetron sput- tering, cathodic arc plasma deposition and various plasma enhanced CVD techniques. While the unbalanced mag- netron is a relatively recent innovation, sufficient work har already been re- ported to confirm the potential of the approach. Work on CAPD is directed towards refining the technique through hardware improvements. The CAPD process is known to generate nucro- droplets which deposit on the substrate as macroparticles of a size and number which cannot be tolerated in many ap- plications. Ion optjcs have been used to separate the ion flux from the drop- lets and to produce coatings of optical quality. A practical resolution of this issue will considerably expand the range of application potential for the CAPD process.

Applications and process develop- ments currently receiving the most at- tention include diamond and diamond- like carbon, cubic boron nitride, super- conducting thin films, ambient and high temperature corrosion resistant coatings, thermal barrier coatings and optical coatings.

EQUIPMENT EVOLUTION The coating equipment required for

p l ama enhanced processes IS consider- ably more complex than that required for less sophisticated processes. In the operation of aluminum metallizing sys- terns there are only one or two process variables which need to be controlled and it is entirely practical to run this type of equipment under manual con- trol. By contrast, plasma enhanced

62 METAL FINISHING

Table 1. Plasma Enhanced Vapor Deposition, Materials and Applications

Materials Applications ___ ~ __________ _____ Metals: Cr, Cu, AI, Ni, Electronic and microelectronic

a-C, a-Si, Mo Magnetic and optical media Decorative metallization

Nitrides TIN, ZrN, HfN, CrN, Cutting and forming tools TiAIN, TiZrN, CBN, TiNbN, TiAlON

Decorative and wear parts

Carbides TIC, WC, TaC Carbide cutting tools

Carbonitrides TiCN, ZrCN Cuttings tools and decorative

Oxides CuO, TiO,, ZrO,, Hard transparent electrodes ITO, ZnO Scratch resistant transparent coatings, automobile

windows

Alloys: MCrAIY, Inconel, High temperature corrosion barriers, turbine blades high Ni alloys

Diamond: Wear resistance, acoustic and microelectronic - -

processes require precise monitoring and control of many processing vari- ables. As a consequence, it has become necessary to develop sophisticated con- trol systems and to operate these sys- tems under automatic computer con- trol. Process parameters which are nor- mally under automatic control include source power, substrate bias and tem- perature ~ reactive gas pressure and flow ratc and subs!rate motion.

The semiconductor and memory disk ma.rliets have led the way i n driving the development of some of the most sophisticated equipment intended for deposition of the highest quality films coupled with very high throughput. ‘These markets require equipment which is extremely clean, highly reli- able and able to deposit complex film structures with very tight tolerances and performance specifications. Fortuit- ously, not all markets impose the same technical deniands; however, other markets have benefitted from the equip- ment advances created by semiconduc- tor and memory demands.

APPLICATIONS IN THE ’90s Some of the cstablished applic, ‘I t’ lolls

for conventional PVD and CVD proc- esses wcre mentioned earlier. It is an- ticipated thal many of these applica- tions will continue essentially unrnod-

able future, Most of the new opportunities will derive from the capability and performance of the plasma enhanced processes which hsve been discussed. The advent and rcfinement of plasma enhanced deposi-

tion processes has already positively impacted a range of applications (see Table I). Coating of cutting tools with refractory nitrides and carbides and the decorative finishing of jewelry with similar coatings is now commonplace and is expected to see further expan- sion.

The opportunities for new applica- tions essentially fall into two categories: replacement of , existing methods for economic or perfoimance reasons and the development of entirely new applications based upon newly de- veloped capabilities and materials. The former category is anticipated to in- clude replacement of conventional processes in automobile fjnishes and the more extensive replacement of elec- troplating than has already been achieved. Specific issues are already being addressed in both of these areas.

In the case of automobile finishes, alternatives are being sought to the use of aluminum in headlanip reflectors. Aluminum provides a superb reflector, but is quickly oxidized if left unpro- tected. For this reason aluminum films are almost always protected with a transparent organic topcoat. Elimina- tion of the organic topcoat is the objec- tive. The answer, as yet unfound, is to usc a coating which combines high re- flectivity with high resistance to corro- sion . Keplaccirient of electroplating has already occurred in a large proportion of the watch, pen and eyeglass frame industries worldwide. Efforts are now directed towards developing coating combinations which have the durabil-

ity, corrosion resistance arid decorative qualities to completely replace elec- troplating in a wide range of industrial and consumer products. Vapor phase processes have the great environmental benefit of producing no polluting by- products.

New applications of these processes are being actively pursued in Europe and the Far East. Hard coatings which can extend the life of cutting tools can also dramatically enhance the perfor- mance and working life of engineered components. In both Germany and Japan high throughput in-line CAPD deposition systems are in production, coating automobile engine and other wear components. Another new appli- cation of these processes, which has great promise, is the continuous coating of strip steel with combinations of coat- ings which provide both decorative and corrosion resistant finishes. Nippon steel has had a large pilot plant operat- ing for some years which incorporates plasma enhanced sputtering and PECVD to deposit various combina- tions of coatings. Recently introduced processes and equipment employ plasma enhanced processes to deposit optical coatings with increased density and high refractive index.

New moteriais with superior perfor- mance continue to be introduced. .Titmiurn nitride, which has been the standard cutting tool coating for a dec- ade, is now being challenged by higher performance coatings such as zir- conium nitride, titanium aluminum ni- tride, titanium carbonitride and titanium niobium nitride. In specific applications, each of these coatings significantly outperforms titanium nit- ride. These new materials also offer interesting decorative possibilities, ranging from the gold and gray colors of zirconium carbonitride to the black color of titanium aluminum oxynitride.

Plasma enhanced CVD is used in the manufacture of solar cells, on both glass and flexible stainless steel sub- strates, and in the production of flat panel displays and semiconductor de- vices. Films routinely depositeti in- clude silicon nitride, silicon dioxide and amorphous silicon. Developing ap- plications for PECVD include diamond and diamond-like carbon films and cubic boron nitride. Both of these ma- terials offer great promise in applica- tions ranging from hard coatings to semiconductors.

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CONCLUSION Plasma cnhnnced dcposition proc-

esses wil l be central to the development of new t h i n tilrn materials anti i\ppllca- tioiis 111 thc 905 I'hr choice ot mite- rials m i qu ' i l~t~zs of depoiltcii t i l i l ~ s

will continuc to cxpand and iiiipiove 1l)cvelopment contiriucs iicrosj d broad front to pro! itie mswcrj to the rriost dernandlng electronic, optical, !+car, coi-ro>ion and tliennal coating require- ments

In his c i r~~clz In the Apiil 1990 issue of M ~ ~ ~ I ~ F Z I I I J I I I I I ~ , ilowaid Smith pre- dicted inigrdtion ot large 5cctois of the tinishlng indujtry to less developed na- tion< 111 icsponic to the increas~ngly burdeniomc cojt of coinpil*t ‘rice h l t h s a f et y 4 11 d en v i i on inen tal reg ul d t ions It is bellcved th'it some of the capabil- ities discusicd, md dcveloprnent~ in

hand, may provide alternatlves to the "inevit~bil~ty"of 5Lich migiatlon MF

References I M a t t c i \ 11 hf J 4ppI I'tib5 '34 7 193

(1963) -- --

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64 METAL FINISHING