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252
AUFDERHEIDE
Figure
8
Schematic reprcsentation
of
the
relationship between substrate temperature
and
argon
pressure
on
the structure
of
mctal coatings dcposited
by
sputtering
using
21
cylindrical magnetron
source. Tis the substrate temperatures
and
T,,,
is
the melting point
of
the coating
material
in
absolute
dcgrccs.
(From
J.
A.
Thornton,
in
Drpo.sitio~~
Trchr/N/oSirs~i,r
Fibs
urd
Cotr/irlCqs,
R.
F.
Bunshah.
Ed.
Park
Ridge, NJ: Noyes Publications,
1982.
p.
214.)
energy transferred
to
the surface. When depositing alloys or compounds. higher
T
values
my decrease the sticking coefficient
of
one component, changing the overall film compo-
sition.
An
important point
to
remember is that the kinetic energy of sputtered atoms
typically is
10
times that
of
evaporated species. This factor plays a significant role in
determining the difference
in
properties between sputtered and evaporated films.
Most sputtered thin film are in
a
state
of
compressive or tensile stress. The stress
is due to the mismatch
in
thermal coefficients of expansion between the substrate and the
thin film and
to
internal stresses built up within the
film
as
a
result of imperfections in
the crystal lattice. In higher
TI,,
materials, deposited at low
TIT,,,,
internal stresses usually
dominate and may reach the yield strength ofthe film. resulting in fracture. Both magnitude
and type
of
internal stress have been found to be influenced by working gas pressures.
Thermal stress usually predominates
in
higher
TIT,,,
coatings.
6.0
THIN
FILM MATERIALS
Most metals have been deposited by sputtering; some of the more commonly used metals
are aluminum. chromium. copper, gold. molybdenum. nickel. platinum, palladium, silver.
tantalum, titanium, tungsten. vanadium. and zirconium. This group of metals represents
an extremely broad range of electrical. magnetic, mechanical, optical, and other physical
SPUTTERED
THIN
FILM
COATINGS
253
properties. If a single metal does not provide the necessary properties, alloys such as
stainless steel, nichrome, and cobalt chrome may also be sputtered. The different elements
comprising an alloy may have widely ranging sputter rates, but because of concentration
at the target surface of the slower sputtering species, alloys can generally be sputter depos-
ited with the same composition
as
the target. Semiconductors such as silicon and carbon
can also be sputtered, but if conductivity
of
the target is too low, rfpower may be necessary.
Oxides, nitrites, carbides, sulfides, and other compounds
of
the metals, alloys, and semicon-
ductors listed above may be made via reactive sputtering with added oxygen, nitrogen or
ammonia, methane or other gaseous hydrocarbons, and hydrogen sulfide, respectively.
7.0
APPLICATIONS FOR SPUTTERED THIN FILMS
Sputtered coatings are used for
a
wide variety of applications, some of which are listed
below, according to the function performed by the thin film.
7.1
Electrical
Metals and alloys are used as conductors, contacts, and resistors, and in other components
such as capacitors. Transparent conductors such
as
indium tin oxide and thin metals serve
as electrodes for
LCDs,
touch panels, other display devices, and solar cells. Thin films
are also extensively used in microelectronic devices.
7.2 Magnetic
Some high performance magnetic data storage media are deposited via sputtering. Cobalt
alloys such
as
cobalt-chromium and,
to
a
lesser extent, nickel, iron, and samarium alloys
are typically used.
7.3
Optical
Thin metal and dielectric coatings are used to construct mirrors, antireflection coatings.
light valves, laser optics, and lens coatings, and to provide architectural energy control
and optical data storage.
7.4
Mechanical
Hard coatings such
as
titanium carbide, nitride. and carbon produce wear-resistant coatings
for cutting tools. Molybdenum sulfide serves
as
a solid lubricant.
7.5
Chemical
Thin film coatings can be used to provide high temperature environmental corrosion resis-
tance for aerospace and engine parts, catalyst surfaces. gas barrier layers, and lightweight
battery components.
7.6
Decorative
Titanium nitride is deposited on watch bands and jewelry as a hard gold-colored coating.
Metals are deposited for weight reduction in automotive and decorative graphics applica-
tions.
254
AUFDERHEIDE
8.0
ADDITIONAL
RESOURCES
The following professional societies include sections dealing with sputtered coatings:
American Vacuum Society (offers short courses
in
sputtering and coatings), Society of
Vacuum Coaters. Electrochemical Society. and Materials Research Society. Journals that
cover developments in sputtered coatings include Jor4r"rl
qf'Vmuur~~
Science n11d Tecknol-
ogy.
Tizit1 Solid
Filrlts,
Joun~nl
qf
Applied Phy~ics, Vncmln, Progress
in
Surjnce
Science,
and the Jo11r11nl
of
the
Electrocher~ziccrl Socids.
BIBLIOGRAPHY
1.
Bunshnh,
R.
F..
ct
al.,
Depo.sitior~
Tf,c/rrlo/ogiL..s,for.
Fibs
urd
Cocrtirlgs.
Park
Ridge, NJ: Noycs
2.
Chapman, B.,
Ch.
Di.schor.ge
Processe.s,
Sputterir~g
trr~d
Pltrsrurr
Erchir~g.
New
York:
Wiley,
3.
Coutts,
T.
J.,
Active
ctrld
Ptrssiw
Tl~ir~
Fih
Dwicrs.
New
York:
Academic
Press,
1978.
4.
Maissel,
L.
I.,
and
R.
Clang,
Hadbook
(If
T/I~II
Film
Twhrrologg.
New
York:
McCraw-Hill,
S.
Vossen.
J.
L..
and
W.
Kcrn,
77rir1
Filru
Proc.e.sse.s.
New
York:
Academic
Press,
1978.
Publications, 1982.
1980.
1970.
28
Reactive Plasma: Deposition and
Etching
1
.O
INTRODUCTION
Plasmas are widely used as the source for ion-bombardment-based techniques for etching.
film deposition, and surface modification. It is the basis of the processes used to etch
semiconductors, to deposit thin layers
of
composite materials, and to synthesize new mate-
rials and alter existing ones.
The two basic classes of processes in this field are etching and deposition. Etching
can be in the form
of
physical sputtering or reactive ion etching. The deposition is based
either on sputtering or on plasma-driven condensation from the gas phase. Both technolo-
gies include similar mechanisms: plasma phase chemical reactions. particle transport. and
surface reactions. For etching applications, the film surface atoms are constantly reacted
with chemicals supplied from the plasma phase reactions. Reaction products are instanta-
neously removed. For deposition applications, chemicals supplied from the plasma phase
react on the film surface. Reaction products remain and accumulate on the surface. In an
actual plasma process. the etching and deposition mechanisms may
not
be exclusive.
With the variation of process parameters such as reactive gases, powerh'requency, ion
bombardment energy, and surface temperature, one
of
the mechanisms may become the
dominant factor. The outstanding advantages
of
a low-pressure plasma technology lie in
the ability to use ions that can be directed to the surface of a growing film
to
give actuation
of adhesion, reaction, cleaning, and structural effects.'.? Ion-assisted processes are widely
used with physical vapor phase (PVD) techniques, although low pressure chemical vapor
deposition/etching (CVD) can also use these effects. In PVD techniques material is trans-
ported to a growing film surface, through a vacuum, after being vaporized
or
by being
ejected from a cold target through a collision with a high-energy inert gas ion, the process
of
sputtering. The use
of
plasmas generated by low-voltage electron beams and those
255
256
PRANEVICIUS
leaked from magnetron sputtering sources' realized the ability
to
create the intense low-
energy bombardment; the films are generally more adhesive with a more dense structure.
Such a deposition process can incorporate reactions with a reactive gas, present in the
residual atmosphere, to provide compound film growth. Low-pressure chemical vapor
phase processes are distinguished by the source material being provided from a volatile
compound which can be dissociated
in
plasma
to
encourage the reaction at low tempera-
tures-plasma enhanced deposition/etching. Ions
in
the plasma provide energy that acti-
vates reactions as well as growth mechanisms. Most
of
these reactions occur on surfaces
where energy conservation is more easily provided.
In
recent times, PVD and CVD techniques for providing relatively low-voltage ion
bombardnlent are made. They involve placing the insulating or isolated substrate in a
dense DC plasma. The self-bias that appears
on
the surface due to the need for electron
and ion currents
to
equalize causes the ions to be accelerated through voltages of up
to
100
volts. This dense plasma is created by passing a large current of electrons through a
low-pressure gas. This current is in the region of
200
A,
at about
80
V. which is close to
the optimum required for ionization of the argon that is commonly used. The pressure
required is below that which would cause significant collisions of evaporatingkputtering
material with the constituents of the residual atmosphere. This bombardment is
of
particular
consequence in reactive processing and is used with plasmas produced by low-voltage
electron beam guns in evaporation systems and the highly ionized product
of
arc evapora-
tion. where
in
this case the high arc current passes through the plasma of the evaporating
material creating ions
of
those atoms rather than those of any residual atmosphere. Redi-
recting the created ions
in
a planar magnetron onto the substrate, by unbalancing the
magnetic field that confines the plasma. another effective way of providing ion bombard-
ment of the growing film is realized.' This plasma is easily manipulated by small magnetic
fields because at the pressures that are used the electron mean free path is sufficiently
long for electrons
to
follow field lines without scattering, while the ions in the plasma
follow the electrons by electrostatic attraction. This technique is used to the direct the
ionized product
of
arc evaporation,
so
as to avoid large particulate contamination
in
a
method called filtered arc.
2.0
REACTIVE
SPUTTERING
The reactive deposition is widely used to synthesize thin dielectric films that have the
required optical and mechanical properties and can be produced on low-temperature sub-
strate materials such as glass and polymer. Considerable progress has been made in devel-
oping enhanced and intensified plasma-assisted PVD to decrease the processing pressure.5
To support a glow discharge at low pressures
(0.5-10
Pa).
it is necessary to increase the
electron path or
to
provide additional electrons. The latter case can be obtained by using
a triode arrangement generally ofa positive electrode and a thermionic source in addition to
the conventional diode system. The electrons produced are then attracted by the positively
charged bias electrode and increase ionization in plasma. The simplest schematic presenta-
tion of deposition in low pressure reactive ambient is presented
in
Figure
l
(triode deposi-
tion
system). Usually the metal target
1
is immersed in plasma supported by gas discharge
between the cathode
2
and the anode
3.
The working gases consist
of
argon and reactive
species. The atoms sputtered from the target
1
are directed to the substrate
4.
The object
of the process is to create films of closely controlled stoichiometry. The rate and composi-
REACTIVE PLASMA: DEPOSITION AND ETCHING
257
tion
of
the sputtered atoms together with the reaction kinetics
of
the surface reaction on
the substrate are important
in
the deposition process.
The advantage
of
the thermionically assisted triode system is due
to
the possibility
of adjusting the current density applied
to
the substrates simply by acting upon parameters
of the electron-emitting source. The substrate voltage and the processing pressure can be
regulated separately. Contrary
to
a
conventional diode discharge, the nature and energy
of
species in interaction with the substrates may be controlled easily.
The general mechanism
of
reactive triode deposition may be considered
as
including
three major steps:
(1)
target sputtering in the presence of reactive species.
(2)
sputtered
particle transport from target to substrate, and
(3)
film growth on the substrate.
2.1
Sputtering and Reactive Adsorption
A
metal target immersed
in
a
mixture of argon and oxygen plasma is sputtered by arriving
energetic ions and is partly oxidized. The surface consists
of
target
cl
and oxygen
c.
atoms, where
cl
and
L'?
are relative surface concentrations
(cI
+
L'?
=
1
).
The sputtering
rate of i-type
atoms
is defined
as
the frequency probability
of
the removal of i-type atoms
bt-l
=
Ylio.
where
Y,
is
the sputtering yield
of
i-type atoms and
ill
is the normalized ion
irradiation intensity
(io
=
lo/C,
where
Io
is the flux of arriving ions and
C
is the surface
concentration
of
atoms). The adsorption rate of i-type atoms to the j-type atoms
on
the
surface
is
defined as
K;,.
It depends
on
the partial pressure of i-type atoms
in
surrounding
IJ;
and is equal
to
where
al,
is the sticking coefficient of i-type to j-type atoms.
Let us denote the target material atoms by index
1
and oxygen atoms by index
2.
Resputtering processes are
not
included
~(~7)
=
K~<)
=
0.
The sticking probability of
an oxygen atom
on
the top of oxygen atoms equals zero
(K$;'
=
0).
It
results that the
target oxidation reactions are defined by one flux of oxygen atoms
(K\;'
=
K,~.)
which
is equal to
Eq.
(I
).
258
PRANEVICIUS
The kinetics of the surface concentration of target material atoms in the presence
of sputtering and oxygen adsorption is obtained as the solution of the equation
As follows, the surface concentration of nonoxidized atoms exponentially approaches the
steady-state surface composition
The characteristic approach time is equal to
l/(
\v2
+
K.,).
The steady state sputtering rate
v>,,
=
Cw,
c,
is equal to
Equation
(4)
taking into account Eq. (1) can be expressed
as
function of the partial pressure
of
oxygen:
Figure
2
illustrates the calculated dependences of the sputtering rate on the partial pressure
of the reactive species in relative units
(
wZ/a)
for
wl/w2
=
0.5,
1.0,
2.0.
3.0,
4.0,
5.0,
and 6.0 (curves
1-6,
respectively). It follows that with the increase
of
the partial pressure
of
oxygen the sputtering rate changes from the "metallic" mode to the "oxide" mode.
Under external irradiation part of the target material atoms and adsorbed atoms on
the surface are activated. Let us assume that relftive concentrations of activated metal
and oxygen atoms are
c3
and
c4,
respectively
(2.
c,
=
1).
The physical interpretation
of activated atoms is complex.".' and it is related%th the variety of processes initiated
in solids affected by ion irradiation. Activated atoms on the surface have less binding
energy with the bulk and increased chemical activity. It means that the surface
of
a one-
element target in the presence of oxygen plasma consists of four components. The genera-
5.0
2
4.0
5
3.0
0)
M
C
'C
2.0
0.
1.0
9
m
M
_"""""
2
1
0.0
U
10'
1
10
Pressure
of
oxygen,
r.
U.
Figure
2
The
dependences
of
thc sputtering
rate
on
the partial pressure
of
reactive
species,
REACTIVE PLASMA: DEPOSITION AND ETCHING
259
tion rate of i-type activated atoms is characterized quantitatively by the parameter
G,
=
giiO defining the frequency probability of the activation, where
g;
is the coefficient of
activation. The frequency probability of the relaxation
of
i-type activated atoms let us
denote as
R,
=
Ih,,
where
7,
is the mean lifetime of i-type atoms in the activated state.
The atoms are removed from the top monolayer by physical sputtering and thermal
desorption. The nonactivated atoms are removed only by physical sputtering
(W,
=
YjiO,
i
=
1
and
2)
and activated atoms by physical sputtering and thermal desorption with
frequency removal probabilities equal to
-
Ed
kBT
)vi
=
YJ,,
+
v;
exp
-
i
=
3.4
(6)
where
U;
is the characteristic oscillation frequency of atoms in solids
(10”-10’3
S-’),
ELI
is the energy of desorption,
kR
is Boltzmann’s constant, and
T
is the temperature.
Calculations will be done in monolayer approximation assuming that processes of
adsorption and ejection of sputtered atoms take place from the first monolayer. It is shownX
that the interdiffusion between monolayers does not affect the surface composition. At the
beginning the surface reactions will not be included in the general scheme
of
consideration
assuming that adsorbed oxygen reacts with the target material atoms immediately.
The time variations
of
the relative concentrations of constituent components on the
surface during simultaneous sputtering and adsorption, taking into account the presence
of the activated atoms, can be written in the form of the following balance equations:
dc,
dt
dC2
clt
dc3
dt
=
bt’?C?
+
W’~c3
+
WJCJ
-
K~ICI
-
GlCl
f
Rlcj
”
- -
)t’lC?
f
K~ICI
+
K~~CJ
f
K~JCJ
-
G2C2
f
R~CJ
(7)
-
-\t’+3
-
K~~CJ
+
Glcl
-
RlCj
R2c.I
The steady-state solutions of Eqs.
(7)
gives stationary surface concentrations of constituent
components
where
a.
=
W.
bl
=
Ky(bt’3
i-
K23
+
RI)
+
GIK?~
(9)
(1-1
+
K?.,+?
f
R,
+
G;
i
=
1,2
b2
=
bt~?(wJ
+
K?~
+
R2)
+
G2wJ
The steady state target sputtering rate expressed in monolayers per second is equal to
alb,atTI
+
~,bl~f,
+
GIb2(\\,3
-
~’1)
+
G?bl(\t.J
-
)V?)
alb?
+
a&
*!l’
=
(10)
The sputtering rate in dependence on the partial pressure
of
reactive species is obtained
taking into account Eq.
(1).
260
PRANEVICIUS
L
d
5.0
e
.S
4.0
a
$-
3.0
U
M
e,
c
.-a
2
:o
1
oo
1
o2
Pressure, r.
U.
Figure
3
The dependences
of
the sputtering rate on the partial pressure of reactive species.
If
the partial pressure
of
reactive species approaches zero, the sputtering rate is equal
to
If
pi
is high, the sputtering rate approaches the sputtering rate of the oxidized target
(7)
I')
-
M,?).
In the general case the sputtering rate according Eq.
(10)
is expressed as a
third-order algebraic equation and it gives nonmonotonic dependences
of
sputtering rate
on the partial pressure
of
reactive species.
Figure
3
illustrates the calculated dependences of
7)"'
(p)
assuming that
aiI
=
1,
=
2,
=
3,
=
4,
RI
=
R2
=
0.1,
wl
=
4,
=
2.
w3
=
8,
and
wn4
=
4
for different values
of
the parameter
G2
=
1,
10, 100
(curves
1-3,
respectively). It seems
that with the increase of the partial pressure
of
oxygen the sputtering rate monotonically
decreases from the value determined by Eq.
(1
1)
to the value equal to
H*?.
There are many
experimental results that are in qualitative agreement with calculated ones."-" Figure
4
includes experimental results of sputtering rates of different metals in dependence
on
the partial pressure
of
oxygen: Fig.
421,
In"; Fig. 4b. AI, Si, Mn, and
V'"
(curves
1-4,
respectively).
S
k3
d
E?
c
0246
(a)
Pressure
of
oxygen. mTorr
g
c
c
Y
d
E
45
30
15
0
Io-'
lo4
10.'
IO-?
(b)
Pressure of oxygen, Pa
Figure
4
Experimental dependences of sputtering rate
of
different metals on the partial pressure
of oxygen.
REACTIVE PLASMA: DEPOSITION AND ETCHING
261
5.0
3
2.0
.d
a
10’
IO’
10’
IO’
Prcssurc.
r.
U.
Figure
5
The dependences
of
the
sputtering
rate
on
the
partial
pressure
of
reactive species.
However, there are conditions when, for example. if
11’~
=
=
2,
=
4.
and
H‘.,
=
3,
the monotonic dependences of
7I7”(p)
are observed
only
for small values of the
parameter
G?.
Figure
5
illustrates the sputtering rate
in
dependence on the partial pressure
of oxygen calculated for different values of the oxygen activation rate
G?
=
1.
10.
and
100
(curves
1-3,
respectively).
The presented calculated curves give the transfer “metallic” and “oxide” sputtering
modes in dependence
on
the partial pressure of oxygen. The sputtering
of
an oxide-covered
target is generally
not
desirable. because the sputtering rate
of
the oxide is much lower
than that of the metal, the appearance of an insulating surface
on
the cathode causes arc
if
dc is used. and rf power is required. Generally required is the intermediate point between
“metallic” and “oxide” sputtering.
In
many cases it takes place in the narrow interval
of partial pressures
of
oxygen. and different control of the partial pressure of the reactive
gas systems with rapid feedback is employed.
It
follows that this problem can be partially solved if during sputtering
a
high activa-
tion rate of oxygen takes place. The physical meaning
of
the activation can be interpreted
as the result of disruption of metal-oxygen links on the surface with the following thermal
evaporation of molecular oxygen according Eq.
(6).
The stationary flux of sputtered metal atoms leaving the target is equal
to
and the flux of oxygen
atoms
is directed to the substrate
2.2
Sputtering and Heterogeneous Synthesis
Surface reactions have
;I
direct intluence on the film growth and etching rates. They include
several steps: adsorption. reaction. and desorption. The substrate surface is exposed to all
species generated
in
plasma. and each species has its own concentration, energy. and
adsorption efficiency. Although a large portion of species generated
in
the plasma phase
is
not
needed for the process. they are involved
in
the adsorption. reaction. and product
formation steps.
Temperature is one of the
most
critical factors in processes of heterogeneous synthe-
sis because
it
influences each single step
of
the surface reaction. It affects both the film