Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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Cathodic Arc Plasma Deposition
1
.O
INTRODUCTION
The cathodic arc plasma deposition (CAPD) method'.' of thin film deposition belongs to
a family
of
ion plating processes that includes evaporative ion plating',4 and sputter ion
plating.5." However, the cathodic arc plasma deposition process involves deposition species
that are highly ionized and possess 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 adhe-
sion, 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
ero-
sion resistance, corrosion resistance, decorative coatings, and architectural and solar
coatings.
2.0
CATHODIC ARC PLASMA DEPOSITION PROCESS
In the cathodic arc plasma deposition process, material is evaporated by the action of
one
or more vacuum arcs, the source material being the cathode in the arc circuit (Fig.
1).
The basic coating system consists of a vacuum 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
15-50
V,
depending on the source material; typical arc currents
in the range
of
30-400
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 in Figure
2
for a titanium source. In this case, an average arc currentkc
spot is about
75
A. The arc spots move randomly
on
the surface of the cathode, typically
at speeds
of
the order of tens
of
meters per second. The arc spot motion and speed can
283
284
RANDHAWA
Arc
SUPPlY
Figure
1
Schematic
of
a
cathode arc deposition system.
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.
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
(30-100%)
of evaporation material is ionized.
3.
The ions exist in multiple charge states (e.g., in case of Ti, Ti', Ti2+. Ti", etc.).
4.
The ions possess very high kinetic energies
(10-100
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:
CATHODIC ARC PLASMA
DEPOSITION 285
Figure
2
Number
of
arc spots on the titanium cathode
arc
source as
a
function
of
arc current.
1.
Good quality films over a wide range of deposition conditions (e.g., stoichiomet-
ric reacted 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.
3.0
CATHODIC ARC SOURCES
A
schematic cross section of a cathodic arc source is shown as an inset in Figure
1;
in a
photograph
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 (Fig.
3)
with predetermined
electronic characteristics may be built much larger and over
a
wide range of sizes. Such
cathode sources provide good uniformity over a wide range of substrate sizes in various
industrial applications.
A
typical thickness uniformity observed using an
8
in.
X
24
in.
286
RANDHAWA
Figure 3
Typical large-area cathodic arc source.
titanium cathode at a source to substrate distance of
10
in. was approximately
10%
over a flat area measuring
5
in.
X
20
in. Furthermore, the target utilization of such arc
sources exceeds 70%-much higher in comparison to the magnetron sputtering source
(N
40%).
4.0
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
Alp?.
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 illustrated in Figure
4.
The
electrons are accelerated toward the cloud of positive ions. The emissions from 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 in Table
1.
CATHODIC ARC PLASMA DEPOSITION
287
Cathode
I
-+
Ion
cloud
Anode
Electrons
Figure
4
Emsslon characteristics
of
a cathodic arc source.
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 craters. The microdroplet emission
is greater for metals with low boiling points. Figure
5
shows such results for copper,
chromium, and tantalum.
Table
1
Physical Conditions: Cathode Arc Spot
Temperature,
K
4
X
103-104
Pressure,
Mpa
0.1-10
Power density, W/cmd2
107-109
Electric field, V/cm-
'
104-105
Current density, Ncm"
106-10*
288
RANDHAWA
Figure
5
Microdroplet emission from metals having different melting pomts.
5.0
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. Microdroplet 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. Zircon-
ium nitride films, deposited under the same conditions as titanium nitride, exhibit a much
lower density of much smaller microdroplets
(N
0.1-0.2
pm). 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, 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
&sec. The
application of suitable external magnetic fields was found to enhance the arc speed to
17
&sec.
A
corresponding reduction in macroparticles was observed. The source design as
CATHODIC ARC PLASMA DEPOSITION
289
Ti
N ZrN
Figure
6
Scanning electron micrographs showing surface topography
of
various
films using
modified arc technology.
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 illumi-
nated. This is illustrated for films of titanium and zirconium nitrides and titanium dioxide,
as shown in Figure
6.
6.0
RECENT DEVELOPMENTS
A major interest in the cathodic arc process till recently has been in the deposition of hard
coatings for tribology, wear, and decorative applications. Deposition, characterization, and
performance evaluations of nitrites, carbides, and carbonitrites of several materials [Ti,
Zr,
Hf, (Ti-Al), (Ti-Zr)(Ti-6Al4V), 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 Ti02 and
ZrOz
were deposited
in a reactive mode using an oxygen-gas mixture.
Zr02
and
Ti02
films with very low
absorption
(-3%)
in the visible light range and excellent adhesion to plastics and glass
290
RANDHAWA
substrates have been deposited. In fact TiOz films with
a
sharp cutoff at approximately
400
nm were found to be very suitable for
UV
filters.
TiN and ZrN films deposited by cathodic arc have also been investigated for architec-
tural 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 Ni-Cr-AI-Y alloys have been
also
successfully deposited at rates
as
high
as
1
mdmin. The film composition
as
analyzed
by spectroscopic techniques (e.g., ESCA and
AES)
was found to be within
10-15%
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, architec-
tural 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,”
Surjkce
Conrings
Techno/..
31, 303
(1987).
2.
J.
L.
Vossen and
W.
Kern, Eds.,
Thin
Film
Processes.
New York: Academic Press, 1978.
3.
J.
A. Thornton, in
Deposition
Technologies
,for
Film
and
Cocrtings.
R.
F.
Bunshah, Ed.
Park
4.
W.
M. Mullarie,
U.S.
Patent No. 4,430,184 (1984).
5.
H.
Randhawa and P. C. Johnson,
Surfuce
Courings
Tecl~rzol..
33,
53
(1987).
6.
H.
Randhawa, presented at ASM International Strategic Machining and Materials Conference.
Ridge, NJ: Noyes Publications, 1982.
Orlando,
FL,
1987.
30
Industrial Diamond and Diamondlike
Films
Arnold
H.
Deutchman and Robert
J.
Partyka
BeurnAlloy
CorporcUior1,
Duhlirl.
Ohio
1
.O
INTRODUCTION
The mechanical, electrical, thermal, and optical properties of diamond make it attractive
for
use in a variety of different applications ranging from wear-resistant coatings for tools
and engineered components to advanced semiconductor structures for integrated circuit
devices.’ Until recently, diamond “coating” was done by bonding single-crystal diamond
grits to the surfaces of the components to be coated. Applications for diamond coatings
were limited therefore to tooling used for cutting and grinding operations.
Recent advancements in plasma-assisted chemical vapor deposition (PACVD) and
ion beam enhanced deposition technologies make it possible to form continuous diamond
and diamondlike carbon films on component surfaces. There new films have many of the
mechanical, thermal, optical and electrical properties of single-crystal diamond, and they
make possible the diamond facing of precision tools and wear parts, optical lenses and
components, and computer disks, as well as the production of advanced semiconductor
devices. The most flexibility, in terms
of
properties
of
the deposited diamond films and
types of material coatable, is found when the films are formed using ion beam deposition
techniques.
2.0
DIAMOND AND DIAMONDLIKE FILMS
The ability to diamond-coat tools and engineered components required that diamond pre-
cursor material be condensed from a vapor phase as a continuous film onto the surface
of
the component to be coated. Furthermore, the deposition must proceed
so
that the vapor-
deposited material condenses with the structure and morphology of diamond. Diamond is
a metastable form of carbon; as such, when condensed from a vapor or from a flux
of energetic particles, it will tend
to
assume its most thermodynamically stable state or
291