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32-1

32Cathodic Arc PlasmaDeposition32.1 Introduction ......................................................................32-1

32.2 Cathodic Arc Plasma Deposition Process .......................32-132.3 Cathodic Arc Sources........................................................32-3

32.4 Cathodic Arc Emission Characteristics ...........................32-3

32.5 Microdroplets ....................................................................32-4

32.6 Recent Developments........................................................32-5

References.....................................................................................32-7

32.1 Introduction

The cathodic arc plasma deposition (CAPD) method1,2 of thin film deposition belongs to a family of ion

plating processes that includes evaporative ion plating3,4 and sputter ion plating.5,6 However, the CAPD

process involves deposition species that are highly ionized and posses higher ion energies than other ion

plating processes. All the ion plating processes have been developed to take advantage of the special

process development features and to meet particular requirements for coatings, such as good adhesion,

wear resistance, corrosion resistance, and decorative properties.

The cathodic arc technique, having proved to be extremely successful in cutting tool applications, is

now finding much wider ranging applications in the deposition of erosion resistance, corrosion resistance,

decorative coatings, and architectural and solar coatings.

32.2 Cathodic Arc Plasma Deposition Process

In the CAPD process, material is evaporated by the action of one or more vacuum arcs, the source

chamber, a cathode and an arc power supply, an arc ignitor, an anode, and substrate bias power supply.

Arcs are sustained by voltages in the range of 15 to 50 V, depending on the source material; typical arc

currents in the range of 30 to 400 A are employed. When high currents are used, an arc spot splits into

multiple spots on the cathode surface, the number depending on the cathode material. This is illustrated

spots move randomly on the surface of the cathode, typically at speeds of the order of tens of meters persecond. The arc spot motion and speed can be further influenced by external means such as magnetic

fields, gas pressures during coatings, and electrostatic fields.

Materials removal from the source occurs as a series of rapid flash evaporation events as the arc spot

migrates over the cathode surface. Arc spots, which are sustained as a result of the material plasma

generated by the arc itself, can be controlled with appropriate boundary shields and/or magnetic fields.

H. RandhawaVac-Tec Systems, Inc.

2006 by Taylor & Francis Group, LLC

material being the cathode in the arc circuit (Figure 32.1). The basic coating system consists of a vacuum

in Figure 32.2 for a titanium source. In this case, an average arc current/arc spot is about 75 A. The arc

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32-2 Coatings Technology Handbook, Third Edition

FIGURE 32.1 Schematic of a cathode arc deposition system.

FIGURE 32.2 Number of arc spots on the titanium cathode arc source as a function of arc current.

GasInlet

ArcSupply

ArcSupply

()

()

()

(+)

(+)

(+)

To Anode

To Anode

ArcSource

To Pump

Substrate

??

??

??

??

Bias

Supply


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Cathodic Arc Plasma Deposition 32-3

CAPD is markedly different from the physical vapor deposition process. Some of its characteristic

features are as follows:

1. The material plasma is generated by one or more arc spots.

2. A high percentage (30 to 100%) of evaporation material is ionized.

3. The ions exist in multiple charge states (e.g., in case of Ti, Ti+, Ti2+, Ti3+, etc.).4. The ions possess very high kinetic energies (10 to 100 eV).

These characteristics of CAPD result in deposits that are of superior quality in comparison to other

plasma processes. Some of these advantages are as follows:

1. Good quality films over a wide range of deposition conditions (e.g., stoichiometric reached films

with enhanced adhesion and film density can be obtained over a wide range of reactive gas pressures

and evaporation rates)

2. High deposition rates for metals, alloys, and compounds with excellent coating uniformity

3. Low substrate temperature

4. Retention of alloy composition from source to deposit

5. Ease of producing reacted compound films

32.3 Cathodic Arc Sources

of a typical large area source. The arc source comprises a cathode (source material), an anode, an arc

ignitor, and a means of arc confinement.

The method of arc confinement is a key factor in arc source design and configuration. Cathodes using

magnetic fields or boundary shields are limited to small sizes of the order of a few inches in diameter.This limits the uniformity attained from such sources. It is generally necessary to use a multitude of such

sources to obtain a reasonable coating quality. Arc sources employing confinement passive boarders

range of sizes. Such cathode sources provide good uniformity over a wide range of substrate sizes in

various industrial applications. Typical thickness uniformity observed using an 8 in. 24 in. titanium

cathode at a source to substrate distance of 10 in. was approximately 10% over a flat area measuring 5

in. 20 in. Furthermore, the target utilization of such arc sources exceeds 70% much higher in

comparison to the magnetron sputtering source (N 40%).

32.4 Cathodic Arc Emission Characteristics

The cathodic arc results in a plasma discharge within the material vapor released from the cathode surface.

The arc spot is typically a few micrometers in size and carries current densities as high as 10 A/m2. This

high current density causes flash evaporation of the source material, and the resulting evaporant consists

of electrons, ions, neutral vapor atoms, and microdroplets. Emissions from the cathode spots are illus-

the cathode spots split into a number of spots. The average current carried per spot depends on the

nature of the cathode material. The extreme physical conditions present within cathode spots are listed

It is likely that almost 100% of the material may be ionized within the cathode spot region. These ions

are ejected in a direction almost perpendicular to the cathode surface. The microdroplets, however, have

been postulated to leave the cathode surface at angles up to about 30 above the cathode plane. The

microdroplet emission is a result of extreme temperatures and forces that are present within emission

results for copper, chromium, and tantalum.


inTable 32.1.

A schematic cross section of a cathodic arc source is shown as an inset in Figure 32.1, in a photograph

(Figure 32.3) with predetermined electronic characteristics may be built much larger and over a wide

trated in Figure 32.4. The electrons are accelerated toward the cloud of positive ions. The emissions from

craters. The microdroplet emission is greater for metals with low boiling points. Figure 32.5 shows such

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32.5 Microdroplets

Microdroplets are emitted as one of the products of the flash evaporation events. In an uncontrolled

situation, very high microdroplet densities may be produced and deposited onto the substrates. The

microdroplets are found to be metal-rich in composition in the case of reacted compound films. Micro-droplet size and density can be controlled in the arc deposition process. Parameters and source design

are the key factors that influence the density and size of the microdroplets.

As previously reported, microdroplet density and size vary with the material. Zirconium nitride films,

deposited under the same conditions as titanium nitride, exhibit a much lower density of much smaller

microdroplets (N 0.1 to 0.2 m). It is believed that the smaller microdroplets result from the higher

melting point and low vapor pressure of zirconium coupled with the higher arc spot velocity observed

on a zirconium cathode surface. The higher arc spot velocity results in a low mean residence time of the

arc spot on a given localized area; thus, it minimizes localized overheating and, hence, the size and density

of the microdroplets.The arc motion of a conventional arc source was studied using a very high speed photographic

technique. The arc speed was measured to be approximately 8 m/sec. The application of suitable external

magnetic fields was found to enhance the arc speed to 17 m/sec. A corresponding reduction in macro-

particles was observed. The source design as well as the operating gas pressure during deposition had an

effect on the microdroplet emission. A new arc source using these modified microdroplets could be totally

illuminated. This is illustrated for films of titanium and zirconium nitrides and titanium dioxide, as

FIGURE 32.3 Typical large-area cathodic arc source.


shown in Figure 32.6.

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32.6 Recent Developments

A major interest in the cathodic arc process until recently has been in the deposition of hard coatings

for tribology, wear, and decorative applications. Deposition, characterization, and performance evalua-

tions of nitrites, carbides, and carbonitrites of several materials [Ti, Zr, Hf, (TiAl), (TiZr)(Ti6Al4V),

etc.] using the cathodic arc deposition process have been investigated in detail and will not be discussed

here. Some of the most recent developments involve deposition of oxides and multicomponent materials

for architectural glass, solar reduction applications, barrier films, and so on. Thin films of tin, zirconium

nitride, titanium dioxide, zirconium dioxide, oxide of copper, and other metallic materials have been

investigated for these applications. Films of TiO2 and ZrO2 were deposited in a reactive mode using an

oxygengas mixture. ZrO2 and TiO2 films with very low absorption (

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FIGURE 32.5 Microdroplet emission from metals having different melting points.

FIGURE 32.6 Scanning electron micrographs showing surface topography of various films using modified arc

technology.

Cu

Ta

Cr

1.50 kv 30 kv 002

30 kv 0141.00 kv

TiN ZrN

TiO2

1.50 kv 30 kv 003


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TiN and ZrN films deposited by cathodic arc have also been investigated for architectural glass coatings.

The deposition rates and stoichiometry control were found to be superior to magnetron sputtering. A

deposition rate as high as 10 times that of magnetron sputtering for production scale was demonstrated.

Multicomponent films consisting of Inconel and NiCrAlY alloys have also been successfully depos-

ited at rates as high as 1 mm/min. The film composition as analyzed by spectroscopic techniques (e.g.,

ESCA and AES) was found to be within 10 to 15% of the source material. This makes cathodic arc an

excellent choice for multicomponent materials.

The cathodic arc deposition process has proved to be capable of fulfilling the most exacting demands

in applications as diverse as tool coatings, decorative coatings, architectural glass coatings, and turbine

engine coatings. Developments are continuing to broaden the range of various potential applications of

the cathodic arc.

References

1. H. Randhawa and P. C. Johnson, A review, Surf. Coat. Technol., 31, 303 (1987).2. J. L. Vossen and W. Kem, Eds., Thin Film Processes. New York: Academic Press, 1978.

3. J. A. Thornton, in Deposition Technologies for Films and Coatings. R. F. Bunshah, Ed. Park Ridge,

NJ: Noyes Publications, 1982.

4. W. M. Mullarie, U.S. Patent No. 4,430,184 (1984).

5. H. Randhawa and P. C. Johnson, Surf. Coat. Technol., 33, 53 (1987).

6. H. Randhawa, presented at ASM International Strategic Machining and Materials Conference,

Orlando, FL, 1987.

2006 b T l & F i G LLC

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