Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Evaporation 141
Figure 4.3: Basic sputtering process.
4.3.2.3 Sputtering
In the sputtering process, illustrated schematically in Figure 4.3, positive gas ions (usually
argon ions) produced in a glow discharge (gas pressure 20–150 mtorr) bombard the target
material (also called the cathode), dislodging groups of atoms which then pass into the vapor
phase and deposit onto the substrate. Alternate geometries of importance in various processing
applications are shown in Figure 4.4. For example, hollow cathode sputtering would be the
ideal geometry for coating the outer surface of a wire. Sputtering is an inefficient way to
induce a solid to vapor transition. Typical yields (atoms sputtered per incident ion) for a 50 eV
argon ion incident on a metal surface are unity. Thus the phase change energy cost is from
three to ten times larger than evaporation [39]. Thornton [39] has provided an excellent review
on sputtering as applied to deposition technology. The reader is also referred to the
proceedings of a special conference on ‘Sputtering and Ion Plating’ [40].
Figure 4.4: Cylindrically symmetric sputter-coating systems.
142 Chapter 4
Table 4.1: Deposition rates for various PVD processes
Evaporation (
˚
A/min) 100–250,000
Ion plating (
˚
A/min) 100–250,000
Sputtering (
˚
A/min) 25–10,000
In special cases to 500,000
˚
A/s.
The deposition rates for the various processes are indicated in Table 4.1. The deposition rates
of the evaporation and ion-plating processes are much higher than those of the sputtering
process. Recently, Schiller and Jasch [41] reported on large-scale industrial applications of
deposition of Al on strip steel continuously at a deposition rate of 20 m/min. It should be
noted that sputter deposition rates at the high side (approximately 10,000
˚
A/min) with diode
sputtering can only be obtained for target materials of high thermal conductivity like copper,
since heat extraction from the target is the limiting parameter. For most materials, it is much
lower, i.e. 50–1000
˚
A/min. With magnetron sputtering, much higher deposition rates are
obtained (see Chapter 5).
4.3.2.4 Inert Gas Condensation
Inert gas condensation (IGC) is a process in which vaporized material is rapidly cooled into
solid phase by interactions with a condenser gas. In 1930 Pfund synthesized bismuth
nanoparticles by evaporating bismuth over tungsten wire [42]. However, a contemporary
version of the IGC process was developed by Gleiter’s group to synthesize Fe nanoparticles
[43, 44]. In this process, the vaporized material is obtained by resistive heating of a refractory
metal boat or crucible which acts as the evaporation source. The material to be evaporated is
supplied to the source either by directly placing it on the source before the evaporation or by a
variety of continuous loading mechanisms during evaporation, e.g. wire insertion. Interactions
between the vaporized material with inert gas molecules and chamber walls provide the rapid
cooling. Upon cooling, vaporized material supersaturates and nucleation starts. Nuclei are
taken away from the growth region by either a convective or a forced flow. Finally, the
particles are collected on a cold surface that is in the form of a solid cold finger or a filter with
porous surface that can trap the particles.
Important parameters controlling particle size
Gas pressure, evaporation temperature, inert gas type, and cooling rate are the main parameters
that control the particle size and shape in the IGC process. Studies show that pressure is the
most important parameter that affects the particle size as compared to all other parameters.
The total gas pressure in the chamber regulates the diffusion rate of the vaporized material
from the growth region and, therefore, the particle size [45–47]. Increasing vapor pressure by
increasing flux of the evaporated species in the growth region forces more coalescence which
results in the formation of larger particles. Evaporation temperature also affects particle size,
Evaporation 143
albeit in a smaller way. Since the kinetic energy of the particles gets larger with the increase of
the evaporation temperature, the nuclei possess excess energy and coalesce after collisions.
Evaporation temperature is also connected with the vapor pressure. Lower evaporation
temperature corresponds to lower vapor pressure, yielding smaller particles [48]. Inert gas type
also plays an important role on the particle size. It is the size of the inert gas molecules that
changes the dynamics. Since massive gas particles can block the escape of the evaporated
particles from the growth region more efficiently than the less massive ones, the resulting
particle size is smaller for inert gas with lower molar (atomic) mass, for instance helium.
Thermal conductivity of the inert gas also has a secondary effect. Helium is usually chosen
owing to its highest thermal conductivity amongst all the noble gases and provides a higher
cooling rate.
Particle growth processes
Particle growth in the IGC process can be classified into three stages: nucleation, coalescent
coagulation, and agglomeration. These three stages are depicted in Figure 4.5. Initially, the
temperature in the vicinity of the evaporation source is high to obtain a reasonable vapor
pressure of the evaporant. As the evaporant moves away from the source, the temperature
decreases, causing supersaturation of the vaporized material leading to the homogeneous
nucleation in the gas phase. At high supersaturation, a large amount of small particles is
formed upon rapid nucleation of the vapors. Nucleation usually starts very fast and continues
at a very high rate. The nucleation process reduces the supersaturation and slows down further
nucleation. Particles subsequently grow by Brownian coagulation as they are removed from
the evaporation region to the particle collection region. Particles coalesce quickly after
coagulation. Further decrease in temperature slows down the coalescence rate and agglomerate
growth takes over.
Particles produced by the IGC technique can form agglomerates by either hard or soft
agglomeration depending on the material properties, the atmosphere, and the
temperature–time history. Hard agglomeration is a result of strong bonding of primary
particles via neck formation, whereas soft agglomeration results from the weak bonding of
Figure 4.5: Schematic diagram of the processes that contribute to particle growth in IGC.
144 Chapter 4
primary particles due to van der Waals’ forces. Although soft agglomerates can be separated
into their constituent primary particles, separation of hard agglomerates is extremely difficult.
Details on particle growth mechanism in IGC are reported by Flagan and Lunden [47].
Experimental set-up of IGC system
A schematic diagram of the IGC system is shown in Figure 4.6. It is composed of two main
sections. The left side, where there are two ports for two different evaporation sources, is for
the production of vaporized material, and the right side is for the collection of the particles.
Evacuation of the system can be maintained by mechanical and turbo pumps connected in
series.
Pressure readings are taken by thermocouple and cold cathode gauges. Inert gas circulation is
realized by a roots blower. A suitable power supply is used for the resistive evaporation. The
maximum temperature that can be reached depends on the evaporation source and the power
supply. In order to maintain continuous particle production, a wire feeding mechanism is used
to provide material to the hot boat. Monometallic wires, like Fe [49] and Ag [50], or bimetallic
wire in the desired ratio for alloys like CuAg [51] or NiFe [52], can be evaporated. Ward et al.
reported the preparation of Mn nanoparticles from evaporation of Mn powder by IGC [53].In
the IGC technique the collection of the particles is done by condensing the agglomerates either
on a liquid nitrogen cooled surface or on a cylindrical stainless steel filter that has
micrometer-size pores on its surface, placed in the collection chamber. Circulating inert gas
passes through the filter while leaving the agglomerates on the filter surface. Since the
particles reaching the filter are agglomerates of micrometer size, they are easily captured by
Figure 4.6: Schematic diagram of the IGC system. 1: Evaporation boat; 2: stainless steel filter;
3: hopper for the collection of particles; 4: wire feeding unit; 5: power supply; 6: inert gas
cylinder; 7: turbo pump; 8: roots blower; 9: mechanical pump; 10: gas circulation line.
Evaporation 145
the pores. Particles accumulated on the filter surface are lifted off from the filter surface by
occasionally reversing the inert gas flow direction. Reverse flow knocks the particles off the
filter surface. The displaced particles fall down into the hopper underneath the collection
chamber. There are minor variations in the IGC processes depending on what type of material
is being evaporated. For example, powder evaporation requires a specific evaporator, whereas
a wire evaporation can be done on a continuous basis, as shown in Figure 4.6. Selection of the
evaporation source also depends on the materials being evaporated.
4.3.2.5 Pulsed Laser Deposition
Pulsed laser deposition (PLD) is a useful technique for thin film deposition [54] and was first
used by Smith and Turner [55]. In PLD high-power laser pulses ablate a small amount of
material from a solid target when a focused laser beam is absorbed by a small area of the target
surface. The absorbed energy is utilized to ablate the target. During the ablation, chemical
bonds of the molecules within the solid target are broken and the material is evaporated in a
certain direction. The resultant evaporated material is comprised of ions, molecules, neutral
atoms, and free radicals of the target material in their ground and excited states. This further
absorbs a large amount of energy from the laser beam producing an expansion of hot plasma
(plume). The plume is then expanded away from the target with a velocity distribution of
different particles in the forward direction [56]. Finally, the ablated species condense on a
substrate, placed at an angle opposite to the target, forming a thin film.
A typical set-up for laser ablation and thin-film deposition is shown in Figure 4.7, which is a
Lambda Physik LPX 305 excimer laser with a wavelength of 248 nm and a maximum energy
Figure 4.7: Schematic of a pulsed laser deposition process.
146 Chapter 4
of 1.2 J per pulse [57]. The ablation process takes place in a vacuum chamber, either under
base vacuum or in the presence of a gas. A laser beam is focused (via convex lenses) on the
surface of a target through a transparent fused silica port on the chamber. The stoichiometry of
the material is preserved in the ablation process owing to the simultaneous evaporation of all
the components in the target, irrespective of their binding energies. This is mainly due to the
fast and intense heating of the target surface by the laser beam. The accumulation of target
material on the substrate from a large number of laser pulses leads to the gradual formation of
a film. The micrograph of a typical thin film of Co–TiO
2
, grown by the set-up described above,
is shown in Figure 4.8. The cross-sectional view of the same film is shown in Figure 4.8(b).
The uniformity of the film is obvious from these scanning electron micrograph (SEM)
pictures, which reveal that PLD can be used to obtain good-quality films. Moreover, the PLD
process is a non-equilibrium process which helps the deposition of the materials without
segregation or spinodal decomposition, e.g. it is quite helpful in obtaining Co-doped TiO
2
having Co substituted for Ti.
Deposition by laser ablation is known worldwide to produce quality films of various kinds
such as semiconductors, high T
c
superconductors, ceramics, ferroelectrics, multilayer
polymers, etc. [58–61]. In particular, PLD is useful for ablating materials in the combinatorial
forms that cannot be easily produced by other methods [62, 63]. Recently, PLD has also been
used to synthesize nanotubes [64], nanopowders [65], quantum dots [66], and some organic
thin films that have applications in optoelectronics [67].
Parameters that can be controlled during PLD are fluence (energy per unit area), wavelength,
pulse duration, pulse repetition, and preparation conditions, as well as system configurations
including target to substrate distance, substrate temperature, background gas, and pressure
[57]. All of these parameters and configurations influence the film growth, film quality and
film properties. Usually the raw laser beam is of rectangular shape of size 2 × 3cm
2
and is
Figure 4.8: (a) SEM micrograph of the surface of the PLD grown Co–TiO
2
thin film;
(b) cross-sectional view of the same film.
Evaporation 147
further shaped, collimated, and focused by converging lenses. The maximum fluence of the
laser is determined by the focal length of the lens. For a given focal length f of a convex lens,
the demagnification factor is:
M
d
=
d
0
f
− 1 (4.1)
where d
0
is the distance of the mask from the lens. Thus, using the above expression we can
calculate the appropriate demagnification and, therefore, get the right fluence at the target
surface.
PLD can be used in combination with the IGC set-up (called laser-assisted inert gas
condensation (LAIGC)). With this technique one can prepare different particles, for example
with core/shell structure for environmental applications, etc. This method makes it possible to
prepare nanoparticles that cannot be made with the conventional IGC method described above
and is shown in the next subsection, on Co-evaporation. Furthermore, modification of the
conventional PLD technique has made possible the deposition of complex materials in
complex configurations such as multilayers. For example, the combination of PLD with
molecular beam epitaxy (MBE) and/or sputtering systems and CVD techniques has been
developed for the deposition of particulate-free and conformal thin films of several materials
[68]. There are other modifications to conventional PLD. One such modification uses a
combinatorial approach to explore novel material properties. Such an approach is best
implemented in the form of a thin film combinatorial library, where on a substrate as small as
1cm
2
, different compositions can be integrated, synthesized, and screened, for the desired
physical properties [69].
Although laser evaporation is an attractive approach for the synthesis of high-purity metal
alloys and compound films, it suffers from the following limitations:
complex transmitting and focusing systems need to be employed to direct the beam
from the laser located outside the vacuum system onto the evaporant placed inside the
system. This involves special optical path designs and increases the cost of the set-up.
Also, a window material which efficiently transmits the wavelength band of the laser
must be found and mounted in such a way that it is not rapidly covered up by the
evaporated material
it is not always possible to find a laser with a wavelength compatible with the
absorption characteristics of the material to be evaporated because material properties,
such as reflectivity and absorption coefficient of the solid target, depend on the
wavelength of the laser used. Thus the wavelength of the laser has a significant effect
on the yield of the ablated particles
energy conversion efficiency is very low – usually around 1–2%
148 Chapter 4
the size of the deposited film is small (10–20 mm or 0.4–0.8 inches in diameter),
resulting from the small size of the laser impact spot
the ‘splashing effect’ [70], which involves the production of microparticles between
0.1 and 10 m in size, diminishes film quality.
The main advantages of this technique are:
the production of high-energy species, which enhances film quality
reflectivity of most materials for lasers with short wavelengths (ultraviolet (UV)) is
much lower than long infrared (IR) wavelengths [71]. When the reflectivity decreases,
a larger part of a laser pulse is absorbed, increasing the number of ablated particles.
Also, the absorption coefficient is larger in the UV region, such that the beam energy is
absorbed in a thin surface layer and the ablation occurs more efficiently
excellent transfer of stoichiometry between the target and the film, for example, the
deposition of hydroxyl apatite thin films for biomedical applications such as implants.
The macroparticle density can be decreased by lowering the power level at the expense
of deposition rate. The latter may not be important for many thin film applications.
The question of large area deposition has been recently addressed by Greer [72].He
constructed a vacuum deposition system in which the laser beam is scanned on a
rotating YBCO target and the substrate is itself rotated. This rather complex apparatus
is capable of depositing YBCO films onto 2- or 3-inch diameter substrates.
4.3.2.6 Co-evaporation Processes
Preparation of thin films by magnetron sputtering has been a very popular method for decades.
There have been a few successful attempts to prepare a variety of nanoparticles with
sputtering. Qiu and Wang reported that the L1
0
phase of FePt nanoparticles can be controlled
by modifying magnetron sputtering [73]. Also, non-convention (FeCo)
core
Au
shell
and
(FeCo)
core
Ag
shell
nanoparticles can be prepared by combined sputtering and evaporation
techniques [74]. (FeCo)
core
Au
shell
nanoparticles prepared by this technique showed a three-fold
increase in saturation magnetization compared with that of iron oxide nanoparticles and a
better oxidation resistance of the FeCo nanoparticles. Bai and Wang [75] used the same
technique to prepare (FeCo)
3
Si–SiO
x
core shell nanoparticles by natural oxidation after the
preparation of the particles. The set-up, which combines sputtering and evaporation, is shown
in Figure 4.9. This system combines two different PVD processes and has proven to be useful
in enhancing different properties of materials, e.g. in the case of magnetic nanoparticles
oxidation of the nanoparticles could be prevented. As reported by Bai and Wang [75],two
kinds of in situ methods, namely sputtering and evaporation, were used to deposit the shell
layers on the surface of FeCo nanoparticles during their flight in the shell-deposition chamber.
The formation of the core/shell structure needs low velocity of FeCo nanoparticles, high
Evaporation 149
Figure 4.9: Schematic configuration of the integrated nanocluster deposition system for
fabrication of the core-shell-type nanoparticles [75].
Au/Ag deposition rate, and a long deposition zone, which favors the shell formation, and vice
versa.
Similarly, IGC and laser ablation techniques can be combined to synthesize core/shell
structures [76]. The set-up of this system is the same as described above (Inert gas
condensation subsection) with the addition of a laser target incorporated, as shown in Figure
4.10. The core/shell structured nanoparticles can be synthesized by simultaneously using
resistive evaporation and laser ablation techniques in an IGC system. Usually metals can be
evaporated by IGC and a subsequent oxygen passivation leads to a corresponding metal and its
oxide core-shell structure. However, the need for the synthesis of one type of metal and
another type of oxide shell requires a combination of IGC and laser ablation techniques. Laser
ablation provides the deposition of a wide range of target materials at room temperature. The
process often takes place as a sequence of steps, initiated by the laser beam interaction with the
solid target, absorption of energy and localized heating of the surface, and subsequent material
evaporation. The system shown in Figure 4.10 is composed of two parts. In the first part,
resistive evaporation is used to synthesize core material which is carried to the next stage,
where a shell of the particles is formed on the core as it passes through the laser-ablated
material. The synthesis of Ni as core and CoO as shell has been reported [76].
4.3.3 Advantages and Limitations
There are several advantages of PVD processes over competitive processes such as
electrodeposition, CVD, and plasma spraying. They are:
150 Chapter 4
Figure 4.10: Schematic of the IGC system. 1: Evaporation boat; 2: laser ablation t arget;
3: stainless steel filter; 4: hopper for collection of particles; 5: wire feeding unit; 6: laser source;
7: power supply; 8: inert gas cylinder; 9: turbo pump; 10: roots blower; 11: mechanical pump;
12: gas circulation line.
extreme versatility in composition of the deposit – virtually any metal, alloy, refractory
or intermetallic compound, some polymeric type materials and their mixtures can be
easily deposited. In this regard, they are superior to any other deposition process
the ability to produce unusual microstructures and new crystallographic modifications,
e.g. amorphous deposits
the substrate temperature can be varied within very wide limits from subzero to high
temperatures
the ability to produce coatings or self-supported shapes at high deposition rates
deposits can have very high purity
excellent bonding to the substrate
excellent surface finish, which can be equal to that of the substrate
elimination of pollutants and effluents from the process, which is a very important
ecological factor.
The present limitations of PVD processes are:
inability to deposit polymeric materials, with certain exceptions, e.g. PLD
a higher degree of sophistication of the processing equipment and hence a higher
initial cost.