Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Evaporation 161
Table 4.2: (Continued)
Element and
predominant
vapor species
Temp. (
◦
C) Suppor t materials Remarks
mp p*=10
−1
Torr Wire, foil Crucible
Manganese
(Mn)
1244 940 W, Mo, Ta Al
2
O
2
Wets refractory metals
Molybdenum
(Mo)
2620 2530 – – Small rates by sublimation from Mo
foils. Electron-gun evaporation preferred
Nickel (Ni) 1450 1530 W, W foil lined
with Al
2
O
3
Refractory
oxides
Alloys with refractory metals; hence
charge must be limited. Small rates by
sublimation from Ni foil or wire.
Electron-gun evaporation preferred
Palladium (Pd) 1550 1460 W, W foil lined
with Al
2
O
3
Al
2
O
3
Alloys with refractory metals. Small
sublimation rates possible
Platinum (Pt) 1770 2100 W ThO
2
, ZrO
5
Alloys with refractory metals.
Multistrand W wire offers short
evaporation times. Electron-gun
evaporation preferred
Rhodium (Rh) 1966 2040 W ThO
2
, ZrO
5
Small rates by sublimation from Rh foils.
Electron-gun evaporation preferred
Soleneum (Se
2
,
Se
n
: n = 1–8)
217 240 Mo, Ta,
stainless steel
304
Mo, Ta, C,
Al
2
O
3
Wets all support materials. Wall
deposits spoil vacuum system. Toxic
˛
v
=1
Silicon (Si) 1410 1350 – Beo, ZrO
5
,
ThO
2
,C
Refractory oxide crucibles are attacked
by melten Si and films are contaminated
by Sio. Small rates by sublimation from
Si filaments. Electron-gun evaporation
gives purest films
Silver (Ag) 961 1030 Mo, Ta Mo, C Does not wet W. Mo crucibles are very
durable sources
Strontium (Sr) 770 540 W, Mo, Ta Mo, Ta, C Wets all refractory metals without
alloying
162 Chapter 4
Table 4.2: (Continued)
Element and
predominant
vapor species
Temp. (
◦
C) Suppor t materials
Remarks
mp p*=10
−1
torr Wire, foil Crucible
Tantalum (Ta) 3000 3060 – – Evaporation by resistance heating of
touching Ta wires, or by drawing an arc
between Ta rods. Electron-gun
evaporation preferred
Tellurium (Te
2
) 450 375 W, Mo, Ta Mo, Ta, C,
Al
2
O
3
Wets all refractory metals without
alloying. Contaminates vacuum system.
Toxic. ˛
v
= 0.4
Tin (Sn) 232 1250 W, Ta C, Al
2
O
3
Wets and attacks Mo
Titanium (Ti) 1700 1750 W, Ta C, ThO
2
Reacts with refractory metals. Small
sublimation rates from
resistance-heated rods or wires.
Electron-gun evaporation preferred
Tungsten (W) 3380 3230 – – Evaporation by resistance heating of
touching W wires, or by drawing an arc
between W rods. Electron-gun
evaporation preferred
Vanadium (V) 1920 1850 Mo, W Mo Wets Mo without alloying. Alloys slightly
with W. Small sublimation rates possible
Zinc (Zn) 420 345 W, Ta, Ni Fe, Al
2
O
3
,C,
Mo
High sublimation rates. Wets refractory
metals without alloying. Wall deposits
spoil vacuum system
Zirconium (Zr) 1850 2400 W – Wets and slightly alloys with W.
Electron-gun evaporation preferred
From Handbook of Thin Film Technology [6]. Copyright © 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Company.
Evaporation 163
Table 4.3: Properties of refractory metals
Proper ty Tungsten Molybdenum Tantalum
Melting point (
◦
C) 3380 2610 3000
T (
◦
C) for p*=10
−6
torr 2410 1820 2240
Electrical resistivity, 10
−4
ohm-cm
At 20
◦
C5.55.713.5
At 1000
◦
C333254
At 2000
◦
C666287
Thermal expansion (%)
From 0 to 1000
◦
C0.50.50.7
From 0 to 2000
◦
C1.11.21.5
From Handbook of Thin Film Technology [7]. Copyright © 1970, McGraw-Hill. Used with permission of McGraw-Hill Book
Company.
Platinum, iron, or nickel is sometimes used for materials which evaporate below 1000
◦
C. The
capacity (total amount of evaporant) of such sources is small. The hairpin and wire helix
sources are used by attaching the evaporant to the source in the form of small wire segments.
Upon melting, the evaporant must wet the filament and be held there by surface tension. This
is desirable to increase the evaporation surface area and thermal contact. Multistrand filament
wire is preferred because it increases the surface area. The maximum amount that can be held
is about 1 g. Dimpled sources and basket boats may hold up to a few grams.
Since the electrical resistance of the source is small, low-voltage power supplies, 1–3 kW, are
recommended. The current in the source may range from 20 to 500 A. In some cases, the
evaporant is electroplated onto the wire source.
The principal use of wire baskets is for the evaporation of pellets or chips of dielectric
materials which either sublime or do not wet the wire on melting. In such cases, if wetting
occurs, the turns of the baskets are shorted and the temperature of the source drops.
The rate of evaporation from such sources may vary considerably owing to localized
conditions of temperature variation, wetting, hot spots, etc. Therefore, for a given thickness of
film, the procedure is to load the source with a fixed weight of evaporant and evaporate to
completion or use a rate monitor and/or thickness monitor to obtain the desired evaporation
rate and thickness.
4.6.3 Sublimation Sources
For materials evaporating above 1000
◦
C, the problem of non-reactive supports may be
circumvented for materials such as Cr, Mo, Pd, V, Fe, and Si which reach a vapor pressure of
10
−2
torr before melting. Hence, they can sublime and produce a sufficiently high vapor
density. The contact area between the evaporant and the source crucible is held to a minimum.
Figure 4.16 shows such a source designed by Roberts and Via.
164 Chapter 4
Figure 4.16: Chromium sublimation source (after Roberts and Via). The electric current flows
through the tantalum cylinder (heavy lines). (From Handbook of Thin Film Technology [7].
© 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
A different type of sublimation source is used for the vaporization of thermally stable
compounds such as SiO which are commonly obtained as powders or loose chunks. Such
source material would release large quantities of gases upon heating, thus causing ejection of
particles of the evaporant which may get incorporated into the film. Figure 4.17 shows two
sources which solve this problem by reflection of the vaporized material.
Figure 4.17: Optically dense SiO sources. (A) The Drumheller source; (B) compartmentalized
source (after Vergara, Greenhouse and Nicholas). (From Handbook of Thin Film Technology [7].
© 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
Evaporation 165
Figure 4.18: Oxide crucible with wire-coil heater. (From Handbook of Thin Film Technology [7].
© 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
4.6.4 Evaporation Source Materials
We have already discussed the potential problems concerned with the reaction between metal
sources and evaporants. Oxides and other compounds are more stable than metals. Table 4.4
gives the thermal stability of refractory oxides in contact with metals. There are many metals
not listed in Table 4.4 which can be evaporated from refractory oxide sources. Note that there
is no such thing as an absolutely stable oxide, nitride, or other compound. Reaction is
controlled by kinetics, i.e. temperature and time.
Oxide crucibles have to be heated by radiation from metal filaments or their contents can be
heated by induction heating. This is illustrated in Figures 4.18 and 4.19 for resistance-heated
sources.
Other source materials are nitrides such as boron nitride. A well-established crucible material
is 50% BN–50% TiB
2
. This material (HAD composite, Union Carbide) is a fairly good
electrical conductor and hence can be directly heated to evaporate materials. It can be readily
machined to shape.
Pyrolytic BN and carbon are also used.
Figure 4.19: DaSilva crucible source. (From Handbook of Thin Film Technology [7]. © 1970,
McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
166 Chapter 4
Table 4.4: Thermal stability of refractory oxides in contact with metals
Metal Temp. f or
10
−2
torr (
◦
C)
Refractory oxides
ThO
2
BeO ZrO
2
Al
2
O
3
MgO
W 3230 Slight reduction at
2200
◦
C
Stable at
1700
◦
C; reac-
tion > 1800
◦
C
Stable 1800
◦
C;
little reaction up
to 2000
◦
C
Limited by the
onset of Al
2
O
3
sublimation at
1800
◦
C
Limited by the
onset of MgO
sublimation at
1600–1900
◦
C
Mo 2530 Little reaction up to
2300
◦
C
Stable at
1900
◦
C but not
active
Stable at
2000
◦
C: ZrO
5
decomposes at
2300
◦
C
Ta 3060 Stable up to 1900
◦
C Stable up to
1600
◦
C
Stable up to
1600
◦
C
Zr 2400 Interaction begins at
1800
◦
C
Interaction
begins at
1600
◦
C
Slight interaction
at 1800
◦
C
Oxide attacked
at 1600
◦
C
Be 1230 Only slight interaction at
1600
◦
C
Oxide discolored
at 1400
◦
C
Stable at
1400
◦
C but not
at 1600
◦
C
Si 1350 Little or no attack at
1400
◦
C; noticeable
reaction at 1600
◦
C
Slight reaction at
1400
◦
C
Slight reaction at
1400
◦
C, strong
at 1600
◦
C
Ti 1750 Slight interaction at
1800
◦
C
Little reaction at
1600
◦
C but
considerable at
1800
◦
C
Little reaction at
1400
◦
C but
considerable at
1800
◦
C
Ni 1530 Ni (l) is stable in contact
with all oxides at 1800
◦
C
Boldface type indicates metal–oxide pairs which can be used for thin film deposition.
After Johnson, Economos and Kingery, and Kohl.
From Handbook of Thin Film Technology [7]. Copyright © 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Company.
Evaporation 167
Figure 4.20: RF-heated aluminum source with boron-nitride/titanium-diboride crucible (after
Ames, Kaplan and Roland). (From Handbook of Thin Film Technology [7]. © 1970, McGraw-Hill.
Used with permission of McGraw-Hill Book Co.)
4.6.5 Induction-Heated Sources
Figure 4.20 shows the induction-heated sources using a BN–TiB
2
crucible. Figure 4.21 shows
an induction-heated evaporation sublimation source using a water-cooled copper crucible [19].
This is suited to the evaporation of reactive metals such as Ti, Be, etc., which will react with all
the refractory oxides, nitrides, etc.
4.6.6 Electron Beam-Heated Sources
Electron beam-heated sources have two major benefits. One is a very high power density and
hence a wide range of control over evaporation rates from very low to very high. The other is
that the evaporant is contained in a water-cooled copper hearth, thus eliminating the problem
of crucible contamination.
The evaporation rate for pure metals like Al, Au, and Ag, which are good thermal conductors,
from water-cooled copper crucibles decreases owing to heat loss to the crucible walls. In such
cases, crucible liners of carbon and other refractory materials are used.
Any gun system must consist of at least two elements: a cathode and an anode. In addition, it is
necessary to contain these in a vacuum chamber in order to produce and control the flow of
electrons, since they are easily scattered by gas molecules. A potential difference is maintained
between the cathode and the anode. This varies from as little as a few kilovolts to hundreds of
kilovolts. In melting systems, a normal operational range is of the order of 10–40 kV. In the
simple diode system, the cathode emits electrons, which are then accelerated to the anode
across the potential drop. Where the anode is the workpiece to be heated, this is termed a
work-accelerated gun. It is shown schematically in Figure 4.22(a). In a self-accelerated gun
structure, an anode is located fairly close to the cathode, and electrons leave the cathode
168 Chapter 4
Figure 4.21: Schematic representation of the distillation set-up.
surface, are accelerated by the potential difference between the cathode and anode, pass
through the hole in the anode, and continue onward to strike the workpiece. Self-accelerated
guns have become the more common type in use and offer more flexibility than
work-accelerated guns.
Electron beam guns may be further subdivided into two types depending on the source of
electrons: thermionic guns and plasma guns.
4.6.6.1 Thermionic Guns
In thermionic guns, the source of electrons is a heated wire or disk of a high-temperature metal
or alloy, usually tungsten or tantalum. Such guns have the limitation of a minimum operating
gas pressure of about 1 × 10
−3
torr. Higher pressures cause scattering of the e-beam as well as
a pronounced shortening of the cathode life (if it is a wire or filament) owing to erosion by ion
bombardment. Figure 4.23 shows examples of thermionic e-beam-heated work-accelerated
sources. The close cathode gun shown in Figure 4.23(A) is not a desirable configuration since
molten droplet ejection from the pool impinging on the cathode will terminate the life of the
Evaporation 169
Figure 4.22: Simple electron beam guns. (a) Work-accelerated gun; (b) self-accelerated gun.
cathode owing to low melting alloy formation. Thus cathodes are hidden from direct line of
sight of the molten pool and the e-beam is bent by electrostatic fields (Figure 4.23B, C) or a
magnetic field (Figures 4.24 and 4.25) generated by electromagnets. The latter is a preferred
arrangement since variation of the X and Y components of the magnetic field can be used to
scan the position of the beam on the molten pool surface.
Figures 4.23–4.25 show linear cathodes (i.e. wires or rods) and are referred to as transverse
linear cathode guns. Figure 4.26 shows a disk cathode which is characteristic of a high-power
Pierce-type electron beam gun. Low-power Pierce type guns may have a hairpin filament or a
wire loop as the cathode. In either case the beam geometry of the Pierce gun is different from
that of the transverse linear cathode guns. In some instances, the electron emitter assembly is
located at a distance from the crucible in a separately pumped chamber to keep the pressure
below 1 × 10
−3
torr, with a small orifice between the emitter chamber and the crucible
chamber for the passage of electrons.
170 Chapter 4
Figure 4.23: Work-accelerated electron-bombardment sources: (A) pendant-drop method; (B)
shielded filament (Unvala); (C) shielded filament (Chopra and Randlett). (From Handbook of
Thin Film Technology [7]. © 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
4.6.6.2 Plasma Electron Beam Guns
A plasma is defined as a region of high-temperature gas containing large numbers of free
electrons and ions. By a proper application of electrical potential, electrons can be extracted
from the plasma to provide a useful energy beam similar to that obtained from thermionic
guns. There are two types of plasma e-beam gun:
Cold cathode plasma electron beam – the plasma e-beam gun has a cylindrical cathode
cavity made from a metal mesh or sheet (Figure 4.27) containing the ionized plasma
Figure 4.24: Bent-beam electron gun with water-cooled evaporant suppor t (with permission of
Temescal Metallurgical Co., Berkeley, CA). (From Handbook of Thin Film Technology [7].
© 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Co.)