Review of plasma-assisted deposition processes

Thin Solid Films. 196 (1991) 329-349 CONDENSED

REVIEW

MATTER

FILM

329

BEHAVIOUR

OF PLASMA-ASSISTED

DEPOSITION

PROCESSES*

H. RANDHAWAt Vuc-Tee Systems Inc., 6101 Lookout

Road, Boulder, CO 80301 (U.S.A.)

Thin film deposition processes are now making increasing use of “plasma” during the film growth. The plasma provides an in situ source of increased ionization and energetic deposition species. This can be suitably used to enhance various physical and chemical processes that may influence the growth and properties of deposited films. In early reviews of such processes the term “glow discharge deposition” was used. In subsequent reviews the process was referred to as “plasma deposition”. However, in order to include a variety of plasma deposition processes such as evaporation, sputtering and chemical vapor deposition, a more generalized term describing such processes has been adopted in the last decade or so. The thin film deposition processes that make use of plasma in the space between the source and substrate are described as “plasma-assisted or plasma-enhanced deposition processes”. In this review such processes using a variety of sources, e.g. resistance heated, electron-beam, cathodic arc, sputtering and gaseous, are described.

1.

INTRODUCTION

The key feature of plasma-assisted processes involves plasmas in which the energy to sustain the ionized state is supplied by external sources. Most applications involve the use of low pressure (glow discharge) plasmas. The energetic species in these plasmas are the free electrons. These electrons can gain energy from the electric field faster than the ions do and are thermally isolated from the atoms and molecules, as far as elastic collisions are concerned. The depositing species, therefore, have an increased average energy, enabling lower deposition temperatures to be used. Early researchers used the term ion plating’ to describe some of these processes, but this term is now less widely used. As process developments occurred in both physical vapor deposition (PVD) and chemical vapqr deposition

* Presented at the 17th International Conference on Metallurgical Conference on Thin Films, San Diego, CA, U.S.A., April 2-6. 1990. t Dr. H. S. Randhawa 0040-6090/91/$3.50

died tragically

on February

Coatings

14. 1990, in the aeroplane

and 8th International

crash at Bangalore,

6 Elsevier Sequoia/Printed

India.

in The Netherlands

330

H. RANDHAWA

(CVD) related processes, researchers have invented new plasma-enhanced techniques and terminologies. Bunshah and Raguran2,3 termed their process activated reactive evaporation (ARE), and incorporated a positive probe electrode to draw electrons emitted by a vapor source, thereby increasing ionization of the condensing species. Matthews and Teer4 called their process a thermionically assisted triode ion plating process; it incorporates a negatively biased substrate and a thermionic electron source. Both of these techniques used a conventional self-accelerated electron beam or a resistively heated source to vaporize the source material. These techniques are a modification of the simple ion plating method introduced by Mattox and McDonald’ in 1963. Thornton’ described a magnetron sputter ion plating technique in which the substrate is negatively biased and positioned within the source plasma in order to produce ion bombardment during film growth and termed it a plasma-assisted deposition process. Recent reviews by Bonifield’ and Sherman* on plasma CVD processes also used the terms plasma-assisted or plasma-enhanced CVD. Recently, Randhawa and Johnson’,” described a vacuum arc process which employs a cathodic arc source which intrinsically produces a highly ionized and energetic plasma of the source material. In this process the substrates are negatively biased and positioned within this active plasma. This process is called the cathodic arc plasma deposition (CAPD) process. 2.

DEPOSITION

PROCESSES

All deposition processes involve three major steps for the formation of a thin film on a substrate: (1) creation of a flux of condensable species (neutral atoms and ions); (2) transport of the species created thus from source to substrate; (3) film growth on the substrate. In the case of an evaporation process, the condensable species are created by physical heating of the source material. This is achieved by resistance heating, electron beam heating, arc evaporation, induction heating or flash evaporation methods. The evaporation rate varies directly with the vapor pressure of the source material which in turn depends on the surface temperature of the source material. In the sputtering process, the species for condensation are created by the positive ions of an inert gas bombarding the source, which is called the target, and thus generating atoms-ions of the target material by momentum transfer. The sputtering rate is thus dependent on the power input to the target, i.e. the cathode voltage and current for sputtering. In reactive sputtering, however, one must consider target Poisoning effects. Further, the sputtering rate is also dependent on the plasma in the vicinity of the target surface. In the case of the CAPD process, the condensable species (mainly ions) are produced by the action of one or more vacuum arcs. The source material to be evaporated is formed as the cathode in the arc discharge circuit. The target (source) material is converted into condensable species as a result of rapid flash evaporation events as the arc spots move on the surface of the target. Arc spots are sustained by the material plasma generated by the arc itself. The evaporation rate is dependent on

PLASMA-ASSISTED

DEPOSITION

PROCESSES

331

the arc current, the vapor pressure of the target and the arc speed on the surface of the target material. Similar to reactive sputtering, one must consider target poisoning effects when using reactive gases such as oxygen, methane, etc. Plasma and CVD processes are significantly different from other deposition processes in that the depositing species are created from gases and vapors which, without energy input, are not themselves condensable. In CVD processes, the substrates are heated to a high temperature to cause the gases-vapors to decompose. In plasma-assisted CVD, a non-equilibrium plasma is created through the action of an applied electric field. The substrate is commonly close to an electrode or functions itself as an electrode. This permits coatings to be deposited at much lower temperatures than would be the case if the reactions were simply driven thermally. Several reactions can occur as a result of plasma-condensable species interactions during transport of material from the source to the substrate. The reactions of importance in the plasma-assisted PVD processes are ionization, dissociation, electron impact excitations and the ion energies involved. Thornton’ has described in detail an analytical model illustrating the principal mechanisms of radical formation in glow discharge. The three steps of a deposition process can be completely independent of each other or may occur simultaneously, depending on the deposition process. In general, there is a much higher flexibility for a process if the individual steps are controlled independently. This is generally the case in the PVD process, whereas in the CVD process the individual steps are not necessarily independent of each other. A few of the apparatus configurations that are used for some of the principal plasma-assisted deposition processes are illustrated in Fig. 1. The plasma-assisted ion plating (Fig. 1(a)) process involves ion bombardment of the substrate prior to deposition for substrate cleaning and during deposition to modify film properties 11.12 This is accomplished by making the substrates function as the cathode of a low pressure plasma discharge in a flux of depositing species and inert gas. The depositing species are generally created by evaporation sources such as electron beam, vacuum arc and thermal sources. The working gas may also contain active gas species (reactive ion plating). Low pressure (l-50 mTorr) plasmas are typically used. Plasma-assisted ion plating is commonly applied for metal films, oxides, nitrides, carbides, carbonitrides, alloy films and multicomponent materials. Plasma-assisted sputtering (Fig. l(b)), in which the depositing species are ejected from the target surface as a result of the momentum transfer from the bombarding ions of the plasma and condense on suitably placed substrates, constitutes one of the oldest applications of plasmas in material processing13*r4. Inert gas plasmas (usually argon) at low pressures typically in the range I-70 mTorr are generally used. However, reactive sputtering, in which one of the depositing species used to form a multicomponent material is introduced in the gas phase, is also widely employed. The applied power is generally d.c. for conducting and r.f. (13.56 MHz) for non-conducting targets. A bias of typically 50 V to several hundreds of volts may be applied to induce ion bombardment during film growth. Another new development using sputtering in this area involves “unbalanced magnetrons”15).

332

H. RANDHAWA

TARGET

PLASMA VAPORATION

PLASMA

UBSTRATE

PLASMA

SOURCE

\

,

’

1

,

_,!.___--

(e)

GAS

OR

Vn’POR

PLASMA

SUBSTRATE

SPECIES

Fig. I. Schematic diagrams of the apparatus processes: (a) evaporative ion plating; (b) sputter PVD.

configurations used in plasma-assisted deposition ion plating; (c) CAPD; (d) ARE; (e) plasma-assisted

In ARE (Fig. l(d)), a plasma discharge is sustained in a flux of evaporated material and a reactive gas. The dissociation and ionization which are induced in the low pressure (l-35mTorr) reactive gas promotes reactivity on the surface of the growing film3. ARE is used for depositing metal oxides, carbides, nitrides and carbonitrides. In plasma-assisted CVD, reactant gases are passed through a low pressure (5 mTorr to a few torr) plasma discharge where dissociation, ionization and gas phase reactions are induced, which permits coatings to be deposited at relatively low temperatures. An apparatus with the configuration shown in Fig. l(e) with a parallel plate arrangement is typically used. The discharges are usually driven in the range from 300 Hz to the microwave range (I 3.56 MHz is most common). Plasmaassisted CVD is widely used in the microelectronics industry for the deposition of silicon nitride, boron nitride, aluminum oxide and amorphous silicon films.

PLASMA-ASSISTED

DEPOSITION

PROCESSES

333

3. ROLE OF PLASMA The plasma-assisted processes discussed above involve creation of depositing species of metal or alloys or dissociated gas atoms or vapors in the presence of discharge plasmas. For example, metal nitride and carbide films are obtained by these processes in the presence of evaporated metal and nitrogen-argon and acetylene-argon plasmas respectively. The major role of plasma in this case is twofold: (1) to activate the reactions between evaporating metal species and reactive gas to enhance the formation of compound films; (2) to modify the growth kinetics and hence structure-morphology and thus physical properties of the deposit. In order to understand the role of plasma in enhancing the chemical reactions essential for the formation of particular compounds, one has to consider the reaction kinetics involved. Ionization and ion energies of the depositing species play an important role in the reaction kinetics that takes place on the substrate in the plasma-assisted processes. Ionization of neutral atoms or molecules may occur as a result of the inherent nature of the process such as in the CAPD process and/or as a result of collisions with the mobile and energetic electrons. As shown in Fig. 2, the ionization probability increases with increasing electron energy up to a threshold after which it passes through a maximum value and then decreases markedly I2 . The maximum occurs at 50-100 eV, depending on the nature of the species being ionized. This is typical of the ion enegies that are involved with the evaporating species in the case of CAPD process and can also be attained in most of the plasma-assisted PVD processes.

1

2

EleCTA& Fig. 2. Schematic

3 EYZERd:

4

(eV)

plot of ionization

probability

as a function

ofelectron

energy

The plasma consisting of energetic and ionized species provides the necessary activation energy by exciting the deposition species to high energy levels, thereby enhancing the rate and ease of deposition of films with the desired properties. Energies of the depositing species in the case of conventional evaporative sources are typically in the range 0.1-l eV. This energy in sputtering improves to about

334

H. RANDHAWA

2210eV. The deposition species involved in both of these cases are mainly neutrals. Deposition energies can be suitably modified if the substrate and the growing film are continuously bombarded with ions. This is the basis of the plama-assisted PVD processes. Figure 3 summarizes the typical ion enegies involved in various processes.

1o-2

10

1

Fig. 3. Ion energies of various

IO2

10' KINETIC

ENERGY

plasma-assisted

lo3

lo4

(eV)

deposition

processes.

The effect of ion bombardments on growing films was first investigated by of the substrate surface prior Mattox and McDonald’ and Takagi I6 Bombardment to film deposition results in precleaning of the substrate by sputter etching of the surface contaminations such as oxides that are very detrimental to film adhesion. However, care must be exercised so that the sputtering-etching rate is greater than the deposition rate. Substrate bombardment with ions during film growth results in various effects in the deposited films. The dependences of film morphology and microstructure on temperature are significantly reduced. Figure 4 illustrates on a molecular scale the effect of energetic ions on film growth.

(a)

0))

Fig. 4. Effect ofion bombardment during film growth: (a) lowenergyevaporative cathodic arc source. 0. Coating atom; 0, impurity atom.

source; (b) high energy

In the case of low energy deposition such as electron beam evaporation, neutral atoms of the depositing material arrive at the substrate surface with little energy for adatom mobility”, particularly at low substrate temperatures. Thus, the atoms stick almost at the point at which they land on the substrate. The result of this lack of adatom mobility is that the resulting film deposit has low film density and poor adhesion. These film properties can be somewhat improved on by raising the substrate temperature during film deposition. However, the nature of the substrate or other device considerations may limit the maximum substrate temperature that can be used. By contrast, in plasma-assisted processes, positive ions arrive at the substrate surface with increased kinetic adhesion. These and other beneficial effects of plasma-assisted processes are also predicted by recent molecular dynamics

PLASMA-ASSISTED

DEPOSITION

PROCESSES

335

calculations’8~19. Figure 5 shows the results of one such calculation, comparing the type ofcoating structure formed by low energy charged ions with that of high energy charged ions more characteristic of the ion-assisted process.

Ti

i

(no

Ti

ions)

\

l

T?

/ SUBSTRA

(4

TE

(b)

Fig. 5. Molecular

dynamics

modeling

4. INDUSTRIAL

PROCESSES

of plasma-assisted

processes:

(a) no ions; (b) high energy ions.

The plasma-assisted deposition processes used in industrial practice involve a family of ion plating processes. The term ion plating, first used by Mattox and McDonald’, is applied to deposition processes in which the substrate surface is subjected to a flux of ions before and during film growth. Ion plating in its genuine form is a matter of substrate positioning and applied bias only. It can be combined with any vapor source. The vapor source can be a thermally heated source, an electron beam, a vacuum arc or a sputtering source. Historically, Berghaus” was the first to propose the exposure of a negatively biased substrate to a plasma in order to obtain a “perfect structure” and adhesion. A typical ion plating system is described in Fig. 6. The evaporating source may be an electron beam, a sputtering source, a vacuum arc source or a hollow cathode source. An important improvement in this type of process was increasing the ionization efficiency using the triode configuration with a heated filament cathode and thermionic emission supported glow discharge in the neighborhood of the substrate4.‘9P22. As in every triode arrangement, the negatively biased substrates exhibit the typical current-voltage characteristics of a Langmuir probe. Another family of ion plating processes uses an r.f. electrode between the substrate and an evaporation source. The r.f. coil electrode is placed near the vapor source. An induction-heated source23 and even more recently a microwave technique have also been used24. These systems also lead to a more effective ionization in the dense vapor region above the vapor source. The first ion plating systems using a hollow cathode discharge (HCD) gun and plasma activation were developed by Morley 25 in 1968. He applied a guiding magnetic field to produce a 90” bent electron beam. The application of an HCD gun to deposit metal films and reactive films involving nitrides and carbides of chromium and titanium was reported by Komiya and Tsuruoka of the Ulvac Corporation in Japan 26*27. In this process, shown in Fig. 7, a small exchangeable

336

H. RANDHAWA

WORKING

ELECTRON

BEAM

EVAPORATOR

Fig. 6. Plasma-assisted

ion plating

DEFLECTION EQUIPMENT MAGNETIC OR ELECTROSTATIC

processes.

II, ,

II

TT]

TO VACUUM , PUMP

DEPOSI \

II

u GAS

,

SUPPLY NEL ITRAL VAP OR & IC)NS

SUBSTRATE POWER SUPPLY o-5oov O-500A

1:. -_

Fig. 7. Plasma-assisted Moriyama, Ulvac Corp.,

ion plating U.S.A.)

system

using

a hollow

cathode

gun.

(Courtesy

of Mr.

Koji

PLASMA-ASSISTED

DEPOSITION

PROCESSES

337

hollow cathode is situated in front of the anodic crucible without too much shadowing. The material to be evaporated is heated in this water-cooled crucible by a low voltage (several tens of volts), high current (several hundreds of amperes) electron gun. The evaporated flux is ionized (ionization rate, about 20%) with high density electrons and it also contains high energy neutrals. The argon gas is fed in by the hollow cathode tube made of tantalum and a manifold is used for the reactive gas for compound formation. A dc. bias is applied to the substrate during deposition. Another version of the plasma-assisted ion plating process involves the use of an electron source in the form of a filament28.29 or a conventional electron-beam source. The former process was developed at Balzers, Liechtenstein, in 1977, and right from the beginning has been applied for the production of TIN films’. The details of this process are illustrated in Fig. 8. The material to be evaporated is held

ELECTRON

(TARGET

I Fig. 8. Plasma-assisted ion plating system using an electron E. Bergmann, Balzars A.G., Liechtenstein.)

MATERI

AL)

*source for plasma

activation.

(Courtesy

of

H. RANDHAWA

338

in a crucible that is made as the anode of the circuit and its cathode is a resistanceheated filament situated in a separated chamber. The discharge plasma is confined by a magnetic field, produced by two large coils located outside the vacuum chamber. To prevent the electrons from losing their energy on the way to the anodic crucible by collisions with gas atoms, a lower pressure, about a few millitorr, is maintained in the evaporation chamber. The direction of the magnetic field is such as to guide these electrons to the anodic crucible. This is also a low voltage (typically 50 V), high current (50-I 50 A) source. To achieve high power density, a narrow tube, about several millimeters in diameter, is used to guide the plasma to the crucible to cause the material it contains to melt and thus to evaporate. Normally a negative d.c. bias is applied to the substrate. For reactive processes, a reactive gas, e.g. nitrogen, is introduced into the chamber together with argon. The substrate heating is provided by electron bombardment prior to film deposition. The system using a conventional electron beam gun with plasma activation using a thermionic filament together with an additional electrode for ionization has been developed by Shinko Seiki Co. of Japan. This system, with its important details, is shown in Fig. 9 I

SUBSTRATE

I

I

rl

I

0 --15kV

SHUTTER

II IONIZATION

THERMOELECTRON

Fig. 9. Plasma-assisted Japan.)

II

u+,

HEARTH

ELECTRON

ELECTRODE

GUN

EMITTING

ELECTRODE

ion plating

(FILAMENT)

\ CHAMBER

system using a triode configuration.

(Courtesy

of Shinko-Seiki

Co.,

PLASMA-ASSISTED

DEPOSITION

339

PROCESSES

The material to be evaporated is heated by means of a bent electron beam gun. The evaporated material which consists of mainly neutrals is further ionized when the neutrals collide with electrons emitted by a thermoelectric filament. The positive probe above the filament helps to create further ionization by drawing electrons from the primary electron beam of the evaporating source in a manner similar to the ARE process. The substrates are generally biased to create a glow discharge for ion cleaning prior to deposition as well as during deposition. A reactive gas and argon are fed into the system for the deposition of reactive compound films. The substrate heating is provided by radiation heating. Figure 10 shows a configuration used in a sputter ion plating system in which the substrate is negatively biased and positioned within the plasma in order to obtain the benefits of ion bombardment during film deposition. Such a system was developed in 1980 by Leybold Heraeus G.m.b.H., F.R.G. In this case the substrates are situated between planar magnetron targets. The “cold” or “hot” deposition modes are defined by the spatial extension of the confined plasma in front of the PLASMA FOR HOT COATING

2

l/

PLASMA FOR COLD COATING

//*

\

\ PRINCIPAL

Fig. 10. Configuration

OF

CONFIGURATION: ZONES AND COLD COATING

of a sputter

ion plating

system.

FOR

HOT

I, Cathodes;

\ ‘1

2, Targets.

340

H. RANDHAWA

targets 3o, The plasma extension itself depends strongly on the power dissipation at the cathode, the magnetic field strength and the gas pressure. The substrates are heated prior to deposition using radiation heaters. The reactive deposition is carried out using a mixture of reactive gas and argon. A recent development involving magnetron sputtering consists of unbalancing the magnetic field in a standard magnetron cathode. An “unbalanced magnetron” has been recently shown” to extend the plasma to the substrates at larger distances and thus to provide high density-low energy ion and electron bombardment that could be beneficial for film properties. This technique has significant merits over conventional magnetron sputtering for industrial applications. Figure 11 shows a CAPD process developed at Vat-Tee Systems Inc., Boulder, CO, U.S.A. The system includes a vacuum chamber, a cathode (material to be evaporated), an anode, an arc ignitor and a substrate bias power supply. The arc ignitor consists of a high voltage pulse or a mechanical striker rod made of molybdenum or tungsten and is connected to the anode potential through a low value (a few ohms) high wattage resistance. The ignitor initiates the arc spot(s) on the surface of the cathode material at very low pressures. The arc spot(s) thus generated move randomly on the surface of the cathode at speeds typically of the order of 8m s-l. The arc speed motion and speed can be further influenced by external magnetic fields. The randomly moving or magnetically guided arc spot(s)

GAS

INLET

TO PUMP

SUBSTRATE I

Fig.

I I Schematic diagram of a CAPD system

PLASMA-ASSISTED

DEPOSITION

PROCESSES

341

evaporate sauce material in a series of flash evaporation events as the arc spot(s) migrate on the cathode surface. The material thus eroded helps to sustain the cathodic arc spot(s). Typical arc voltages and currents range between 15 and 50 V and from 30 to 400A respectively, depending on the cathode material. The mechanisms of cathode spot arc evaporation have been studied extensively over recent decades31-34. Several papers in which the details of spot initiation, spot evolution, spot extinction and movement of the arc spot are described have been on the feasibility of vacuum published 32.35. All the earlier work had concentrated 36*37. Research on the cathode arc as an evaporation switching and interruptions source was for a long time limited to the U.S.S.R 33,38. Only recently has the work been actively pursued elsewhere 39-41. Several reviews dealing with CAPD as a viable deposition process for production applications have also been recently published42-44. Some of the characteristic features of the CAPD process are as follows: (1) a very high percentage of the evaporated material is ionized, 30°/0-1000/0 depending on the source material; (2) the ions in the emitted plasma are multiply charged; (3) the emitted ions possess very high kinetic energies, 1O-l 00 eV. The ion plating techniques that have been successfully employed on a commercial basis use activated plasma sources and intense ion bombardment of the substrate. In fact, the critical parameter for reactivity is not the amount of argon ions but the density of ionized metal evaporants hitting the substrate and the ion fraction of the reactive and argon gases. It may be pointed out here that the plasmaassisted processes using electron beam sources that are commercially available are being described as arc discharge processes. This is to be distinguished from the CAPD systems which employ vacuum arcs for the creation of highly ionized and energetic metal plasmas for film deposition. In the conventional arc process, the material is evaporated by the action of one or more vacuum arcs. The vacuum arc(s) themselves are characterized by a highly localized intense high current discharge. This arc discharge is further self-sustained by the material plasma emitted from such sources. As the definition implies, the vacuum arc(s) do not require a gas medium and can be sustained even in high vacuum. These arc spot(s) are suitably confined on the target surface. For more details, the reader is referred to various reviews 9.10 of this process. On the contrary, the Balzers process described in Fig. 8 a filament situated in a separate chamber under a relatively high argon gas pressure is heated to emit electrons. To prevent the electrons from losing their energy on the way to the crucible through collisions with gas atoms, a lower gas pressure is maintained in the evaporation chamber. The direction of the magnetic field is chosen such as to guide the electrons in a confined plasma beam in a straight line to the crucible. The material to be evaporated is kept in this anodic crucible. Using an aperture diameter less than IOmm, at a given suitable argon pressure and increasing the power density of the electron beam, sufficient current (about 100 A) can be passed through the material held in the crucible, and the material will melt and thus evaporate. This process thus generates an ionized vapor cloud. Even though the evaporating species are ionized, the mode of their production is bombardment using an intense beam of electrons. The process is a plasma-assisted family of processes and not an arc process. Similar arguments

H. RANDHAWA

342 apply to the Shinko process. 5.

process described

in Fig. 7. This is indeed a triode ion plating

FILM PROPERTIES

Ion bombardment during film growth provides sufficient activation energy to modify the film properties and the plasma processes can be controlled to provide the required film properties. The specific conditions for deposition and film properties depend on the applications. The function of thin films in electronic applications is quite different from that of metallurgical applications. For example, TIN films for barrier applications require pinhole-free coatings that are very fine grained and have good adhesion. On the contrary, TIN for wear applications require high intrinsic strength, high hardness and adhesion. Further, the films required for barrier properties are very thin (about 1000 A), whereas films for wear applications are typically 2-5 urn thick. In this section a brief discussion of film property enhancement due to plasma is presented. 5.1. Film growth charucteristics The effect of ion bombardment on film growth has been studied in detail in the last few years. In general, this results in films with improved adhesion and density. This also facilitates low substrate temperatures needed during deposition by activating the reaction species especially in reacted films. Bunshah and Raguran45 found that when titanium was evaporated in the presence of acetylene gas, TIC films were formed only in the presence of the plasma. In the absence of the plasma, there was no activation of the reacting species present and the deposits consisted of mainly unreacted titanium and carbon. Similar results were also reported by Abe et ~1.~~ Other workers studied the effect of d.c. and r.f. plasmas on the deposition of TIC. It was reported that it was easier to obtain stoichiometric TIC when a dc. plasma was used. In the case of an r.f. plasma, the films were found to be carbon rich. The preferred orientation of the films was also found to be different. This was attributed to differences in ionization and activated reactions that occurred in d.c. and r.f. plasmas. The bombardment of the growing films with energetic ions and neutrals can also result in modification of film morphology, crystal orientation, grain size, etc. The films grown using plasma-assisted processes are generally fine grained and have a dense structure. As one would expect, the films subjected to a high flux of energetic atoms and ions tend to be in compressive stress. This results into a very high hardness of such films. However, the brittleness of the film is found to be a function of the material and also the deposition conditions, which can be suitably modified. Bombardment by energetic atoms and ions may therefore be advantageous in obtaining a particular film property (e.g. morphology or crystal orientation). However, the associated effects (e.g. stress) must also be considered. 5.2. Adhesion The plasma-assisted

processes

yield films with outstanding

adhesion

resulting

PLASMA-ASSISTED

DEPOSITION

343

PROCESSES

from the energetic deposition species involved. High energies result in strong atomic mixing at the interface and also result in films with much fewer defects. Substrate cleaning, however, also plays an important role in adhesion. The increase in the substrate temperature resulting from ion bombardment further promotes good adhesion. The high energy ions sputter etch the material from the surface biased with a high negative voltage. This helps to remove any residual thin oxide layers and any foreign contaminations. A further result of the high energy bombardment stage, particularly important in the deposition of reacted hard films, is the deposition of a thin adhesion layer that has a better match to substrate than the final film itself. This is particularly true in the case of the CAPD process in which high energy metal ions may be used for initial bombardment of substrates that are held at a high negative bias. A thin layer of titanium-rich phase (Ti,N) as the adhesion interface between substrate and TiN films enhances adhesion and hence performance of the coated part47. 6. APPLICATIONS Plasma-assisted deposition processes are used in a wide variety of applications. The widespread applications of plasma-assisted PVD techniques have been in the area of perishable cutting tools, decorative coating of jewelry, watch cases and bands, eye glass frames etc., corrosion applications and optical applications. On the contrary, plasma-assisted CVD has found most of its widespread use in microelectronics and optoelectronics areas. Table I provides a range of applications for plasma-assisted processes. In the following sections, these applications are briefly discussed.

TABLE

I

VARIOUS

THIN

APPLICATIONS

Moterid

FILMS DEPOSITED OF THE THIN

USING

PLASMA-ASSISTED

FILMS DEVELOPED

deposited

PHYSICAL

VAPOR

UEPOSITION

ARE ALSO LISTED

Applicurion

Metals: Cr. Cu, Al, Ni. a-C, a-Si, MO etc.

Electronic: printed circuits, through-hole boards. magnetic discs, flex circuits, metallization

Nitrides: TiN, ZrN, HfN, CrN. CrN. Ti6Al4VN. TiAIN, TiZrN etc.

Cutting tools, turbine blades. forming tools, decorative, pump components

Carbides:

Cutting tools, corrosion temperature

Tic, WC, TaC

Carbonitrides:

TiCN. ZrCN

Oxides: CuO, TiO,, ZrO,, ITO, TO Alloys: MCrAIY, Inconel. high Ni alloys

Decorative,

cutting

resistance.

high

tools

Hard transparent electrode, automobile windows Turbine blades. high temperature corrosion

METHODS:

THE

344

TABLE

H. RANDHAWA

II TiN

INCKEASEUPRODUCT~VITYUSING

COATEDTOOLS

Work materid

Diunwlrr

1045 steel, I5 HRC

0.5312 x 2.25

(in) x dpprh (in)

cu

0.2656 x 1.O

Al bronze

0.6562 x I .50

H I3 tool steel, 200 HB

12L14 steel, 200-250

41 L40 steel, 206250

Hastelloy,

0.5625 x 3.50

HB

0.4062 x 3.25

HB

0.4062 x 3.0

240-3 10 HB

0.28 12 x 0.25

Properr>

Feed (in rev-‘) rev min- ’ (surface inmin~’ Number of holes Feed (in rev- ‘) rev mini (surface in min-’ Number of holes Feed (in rev-‘) rev min _ ’ (surface in min-’ Number of holes Feed (in rev- i) rev min-’ (surface in min-’ Number of holes Feed (in rev-‘) rev min- ’ (surface in min-’ Number of holes Feed (in rev- ‘) rev min-’ (surface in min-’ Number of holes Feed (in rev-') rev min-'

304 stainless steel, 225-275

3 16 stainless steel, 300 HB

4130steel

18-22 HRC

4140 steel, 20 HRC

HB

0.375 x 5.50

0.375 x 1.75

0.3125 x 0.88

0.1875 x 1.0 (uncoated is cobalt high speed steel)

Values qfproperlies

(surface

0.010

ft min

ft mini i)

ft min

ft min

‘)

ft mini’)

ft min-‘)

ft min

rev min-'

ftmin~‘)

(surface

’)

ft min-‘)

in min-’ Number of holes Feed (in Rev-‘) rev min- ’ (surface inmin~’ Number of holes Feed (in rev-') in min ’ Number of holes Feed (in rev-‘) rev min ’ (surface inmin~’ Number of holes Feed (in rev- i) rev min ’ (surface in min 1 Number of holes

‘)

ft min

i)

‘)

ft min _ ’ )

300 (41) 3.0 185 0.008 1200 (83) 9.6 200 Manual llOO(188) Manual 2 0.005 350 (52) 1.75 9 0.008 800 (85) 6.4 800 0.006 900 (95) 5.4 38Om400 0.004 430 (30) 1.7 36 0.008 500 (48) 4.0 6 0.010 425 (41) 4.2 7 0.008 500 (41) 4 100 0.0045 1400 (69) 6.3 230

0.010 300 (41) 3.0 185 0.008 1200 (83) 9.6 200 Manual llOO(188) Manual 2 0.005 350 (52) 1.75 9 0.008 800 (85) 6.4 800 0.006 900 (95) 5.4 380400 0.004 430 (30) 1.7 36 0.008 500 (48) 4.0 6 0.010 425 (41) 4.2 7 0.008 500 (41) 4 100 0.0045 1400 (69) 6.3 230

PLASMA-ASSISTED

DEPOSITION

345

PROCESSES

6.1.

Wear resistance applications The performance of cutting and forming tools can be significantly improved by applying a thin film of refractory metal nitride or carbide. Titanium nitride is the most commonly used for general machining applications. The use of hard coatings results in increased productivity, increased tool life and shorter machining times. The increased productivity results from coated tools as these tools can be run at much higher feeds and speeds that are not possible using uncoated tools. This is illustrated in Table II using a TIN film. While TIN continues to be the most widely used of the hard coatings, alternatives are now being considered. It has been demonstrated4’ that, in applications involving high hardness and workpiece materials with a greater abrasive content, TiAlN is superior to TIN. This is because TiAlN has higher stability against oxidation at high temperatures. The performance of several nitride, carbides and carbonitride films is summarized in Table III. TABLE

III

COMPARISON

OF TOOL LIFE OBTAINED

USING DIFFERENT

HARD COATINGS

Drilling: 4 I50 alloy steel, 275-300 HB

70 standard ft min ‘. 0.010 in rev- I; depth, 4 x diameter through holes; collant, Drascool (20: I ratio)

Uncoated TiN ZrN TiC TiC,,N,, TiC,,N,,

2s 153 167 5x 160 102

30 18 IO 36 15 20

Drilling: TiAI,V,, 30&350 HB

70 standard ft min _ ’ , 0.010 in rev- ‘; depth. 4 x diameter through holes; collant, Drascool (20: I ratio)

Uncoated TiN ZrN TiC

90 95 231

25 150 5

850 I565 I680

20 I5 IO

Tapping ( 114 20 tap): 4340 alloy steel, 300-320 HB

Thickness, 0.22W.225 tap setting. B-3 through holes: coolant. oil based

TiC, 5N85 TiC,,N,, in;

Uncoated TIN ZrN

The tool life of coated tools is a strong function of film thickness4’. There is a minimum thickness of 1.5 urn above which the tool life increase is less significant for a further increase in thickness. Although the hard coatings are used on cutting tools world wide, only minimal attention is given to wear protection on engineered components. 6.2. Corrosion resistance applications There are many environments

in which

parts

and components

come

into

346

I-1.RANDHAWA

contact with chemical fluids which may be corrosive. TiN films are found to be highly resistant to chemical attack from concentrated acids or other such corrosive chemicals. However, the choice of substrate used can aggravate these problems. If the substrate has a lot of free iron or nickel, it is found that galvanic corrosion tends to dominate the situation. In such cases, CrN films were found to be more effective. CrN films are also found to be very effective in controlling the wear and corrosion in automobile bearings. Gold colour coatings are also finding increasing use, not only for corrosion protection, but also for their decorative appeal. TIN and TIC films have been found effective in controlling the localized corrosion in engine oil environments containing chlorine. Thin films of Cr:Pt are routinely used for corrosion protection on shaving blades and are being deposited primarily using a sputtering process. A variety of surgical tools and instruments14 are being coated with wearresistant and corrosion-resistant coatings. The coatings primarily being used are TiN and ZrN. The coated instruments not only provide improved life but also are more compatible with the chemicals used for sterilization and storage. The resistance to erosion of refractory hard films has been investigated4” in relation to the life-increasing properties of these coatings for compressor stage airfoils of a turbine engine. This erosion is a particularly serious problem in helicopter engines where the airfoils have to be refurbished on a regular basis. A IO urn thick TIN coating in such application was found to enhance the life of the airfoils threefold. In the hot stage of the turbine engines. the major problem is high temperature corrosion. This problem has been combated with MCrAlY coatings. These coatings are currently being applied in the industry using plasma-assisted ion plating methods.

Plasma-assisted ion plating processes are used in a very wide range of industries as diverse as automobiles and jewelry. The most common techniques used are sputter ion plating and CAPD. Most nitrides. carbides and carbonitrides of refractory metals exhibit very interesting colors as shown in Table IV: these are in great demand in the decorative market. The range of colors can be easily adjusted by control of the deposition parameters. As is clear from Table IV{ the film composition can be suitably adjusted to match the actual gold colors. Figure 12 shows the reflectance in the visible region of some hard films. The effect of doping the TiN or ZrN films with carbon (from acetylene as a reactive gas) has the desirable effect of increasing the red content of the color while lowering the green content at the same time. Care must be exercised. however. to obtain acceptable brilliance and color.

Thin films for solar control are being used on a large scale in diverse applications such as architectural glass and automobile windows. The films for such applications provide tremendous energy conservation. Automobile windows are routinely

PLASMA-ASSISTED

TABLE

DEPOSITION

IV

UECORATIVE

PROPERTIES

OF VARIOUS

COh

TiN ZrN Hm TiC TiC,N I_.r. .v = 0.05- 50 Ti,Al, \-=O.l

347

PROCESSES

,N, 70

&,N, A. .\-= 0.05 20 Ti,Zr, _,N. .I.= 2OMO

HARD

COATINGS

HUi4W.S.S (kgfmm2)

Compo.rilion

Golden yellow Silver-yellow Yellow-green Steel grey Brown

2500 3300 3200 2900 2700

TIN TiC 0 05 N 0.95 TiC 0.1 oNo.9,

Black

2500 3250-3450

ZG.,SN, ~5 10 karat Au 24 karat Au (pure)

Golden

TG.,5No.85 ZrN ZrC 0 10N 0.90

Range L*

u*

fl*

77-80 7&79 71-75 66 69 86-89 81 ~84 79-8 I

2-S 5.5-8 8.5-l I I I-16 -3to -I -I to -0.4 t&3

81-86 X8-91

- I .6 to -3.7 to

33-37 30-33 23m28 21-22 23-25 26-29 17-19 19-30 27-34

I I

240@3250 Golden

90 80 70 60 M

20 4

-

DOPED liN

WAVELENGTH nm

Fig. 12. Reflectance

as a function

of wavelength

in the visible region for a variety of gold color films.

coated with a ZnO-Ag-ZnO structure using a sputter ion plating technique. This structure provides the selective feature of transmitting visible light through while blocking most of the IR radiation. The other materials that are also used involve TiN, TiO,, ZrO, etc.

H.

348

RANDHAWA

Another application of plasma-assisted processes involves transparent conducting oxides for applications ranging from deicing of airplane and automobile windows. to transparent electrodes and transparent scratch-resistant coatings. 6.5.

Microdectronic

upplicutims

The PACVD technique is very widely used in the microelectronics industry. Plasma-deposited silicon nitride and silicon dioxide are used for the encapsulation of very-large-scale integrated circuits. These films are also widely used as diffusion barriers and moisture barriers. Amorphous silicon deposited by this method is being used for the fabrication of solar cells, transistors and photoreceptors. A wide range of plasma-assisted processes are currently being investigated for the deposition of superhard films such as diamond and boron nitride for a variety of applications including tools, barrier coatings. acoustics and microelectronics. 7. CONCLUSIONS Plasma-assisted deposition processes have become established for deposition in a variety of applications. Equipment and processes have both been exhaustively proven in production environments. At the time of writing, these processes display great promise for the synthesis of polycrystalline diamond and superconducting thin films in the near future. Major gaps continue to exist in detail understanding of the plasma kinetics and in the way the process affects the properties of the film and the devices produced. In this regard, research in the field of these processes will continue to expand. ACKNOWLEDGMENT

The author wishes to acknowledge the assistance her help in the preparation of this manuscript.

of Ms. Barbara

Trapani

for

REFERENCES

I 2 .i 4

5 6 7 8 9 IO II I2 13

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DEPOSITION

PROCESSES

349

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1982. 48 49