Samer Elatrache V-17810600 estudiante de CRF
This chapter concludes with a discussion of several PVD processes that are
more complex than the conventional ones considered up to this point. They
demonstrate the diversity of process hybridization and modification possible in
producing films with unusual properties. Ion plating, reactive evaporation, and
ion-beam-assisted deposition will be the processes considered first. In the first
two, the material deposited usually originates from a heated evaporation
source. In the third, well-characterized ion beams bombard films deposited by
evaporation or sputtering. The chapter closes with a discussion of ionized
cluster-beam deposition. This process is different from others considered in
this chapter in that film formation occurs through impingement of collective
groups of atoms from the gas phase rather than individual atoms.
Ion Plating
Ion plating, developed by Mattox (Ref. 29), refers to evaporated film deposition
processes in which the substrate is exposed to a flux of high-energy ions
capable of causing appreciable sputtering before and during film formation. A
schematic representation of a diode-type batch, ion-plating system is shown in
Fig. 3-25a. Since it is a hybrid system, provision must be made to sustain the
plasma, cause sputtering, and heat the vapor source. Prior to deposition, the
substrate, negatively biased from 2 to 5 kV, is subjected to inert-gas ion
bombardment at a pressure in the millitorr range for a time sufficient to
sputter-clean the surface and remove contaminants. Source evaporation is then
begun without interrupting the sputtering, whose rate must obviously be less
than that of the deposition rate. Once the interface between film and substrate
has formed, ion bombardment may or may not be continued. To circumvent
the relatively high system pressures associated with glow discharges, highvacuum
ion-plating systems have also been constructed. They rely on directed
ion beams targeted at the substrate. Such systems, which have been limited
thus far to research applications, are discussed in Section 3.8.3.
Perhaps the chief advantage of ion plating is the ability to promote extremely
good adhesion between the film and substrate by the ion and particle bombardment
mechanisms discussed in Section 3.7.5. A second important advantage is
the high "throwing power" when compared with vacuum evaporation. This
results from gas scattering, entrainment, and sputtering of the film, and
enables deposition in recesses and on areas remote from the source-substrate
line of sight. Relatively uniform coating of substrates with complex shapes is
thus achieved. Lastly, the quality of deposited films is frequently enhanced.
The continual bombardment of the growing film by high-energy ions or neutral
atoms and molecules serves to peen and compact it to near bulk densities.
Sputtering of loosely adhering film material, increased surface diffusion, and
reduced shadowing effects serve to suppress undesirable columnar growth.
A major use of ion plating has been to coat steel and other metals with very
hard films for use in tools and wear-resistant applications. For this purpose,
metals like Ti, Zr, Cr, and Si are electron-beam-evaporated through an Ar
plasma in the presence of reactive gases such as N, , 0, , and CH, , which are
simultaneously introduced into the system. This variant of the process is
known as reactive ion plating (RIP), and coatings of nitrides, oxides, and
carbides have been deposited in this manner.
Reactive Evaporation Processes
In reactive evaporation the evaporant metal vapor flux passes through and
reacts with a gas (at 1-30 X torr) introduced into the system to produce
compound deposits. The process has a history of evolution in which evaporation
was first carried out without ionization of the reactive gas. In the more
recent activated reactive evaporation (ARE) processes developed by Bunshah
and co-workers (Ref. 30), a plasma discharge is maintained directly within the
reaction zone between the metal source and substrate. Both the metal vapor
and reactive gases, such as 0,, N,, CH,, C,H,, etc., are, therefore, ionized
increasing their reactivity on the surface of the growing film or coating,
promoting stoichiometric compound formation. One of the process configurations
is illustrated in Fig. 3-25b, where the metal is melted by an electron
beam. A thin plasma sheath develops on top of the molten pool. Low-energy
secondary electrons from this source are drawn upward into the reaction zone
by a circular wire electrode placed above the melt biased to a positive dc
potential (20-100 V), creating a plasma-filled region extending from the
electron-beam gun to near the substrate. The ARE process is endowed with
considerable flexibility, since the substrates can be grounded, allowed to float
electrically, or biased positively or negatively. In the latter variant ARE is
quite similar to RIP. Other modifications of ARE include resistance-heated
evaporant sources coupled with a low-voltage cathode (electron) emitter-anode
assembly. Activation by dc and RF excitation has also been employed to
sustain the plasma, and transverse magnetic fields have been applied to
effectively extend plasma electron lifetimes.
Before considering the variety of compounds produced by ARE, we recall
that thermodynamic and kinetic factors are involved in their formation. The
high negative enthalpies of compound formation of oxides, nitrides, carbides,
and borides indicate no thermodynamic obstacles to chemical reaction. The
rate-controlling step in simple reactive evaporation is frequently the speed of
the chemical reaction at the reaction interface. The actual physical location of
the latter may be the substrate surface, the gas phase, the surface of the metal
evaporant pool, or a combination of these. Plasma activation generally lowers
the energy barrier for reaction by creating many excited chemical species. By
eliminating the major impediment to reaction, ARE processes are thus capable
of deposition rates of a few thousand angstroms per minute.
Ion-Beam-Assisted Deposition Processes (Ref. 31)
We noted in Section 3.7.5 that ion bombardment of biased substrates during
sputtering is a particularly effective way to modify film properties. Process
control in plasmas is somewhat haphazard, however, because the direction,
energy, and flux of the ions incident on the growing film cannot be regulated.
Ion-beam-assisted processes were invented to provide independent control of
the deposition parameters and, particularly, the characteristics of the ions
bombarding the substrate. Two main ion source configurations are employed.
In the dual-ion-beam system, one source provides the inert or reactive ion
beam to sputter a target in order to yield a flux of atoms for deposition onto
the substrate. Simultaneously, the second ion source, aimed at the substrate,
supplies the inert or reactive ion beam that bombards the depositing film.
Separate film-thickness-rate and ion-current monitors, fixed to the substrate
holder, enable the two incident beam fluxes to be independently controlled.
In the second configuration (Fig. 3-25c), an ion source is used in conjunction
with an evaporation source. The process, known as ion-assisted deposition
(IAD), combines the benefits of high film deposition rate and ion
bombardment. The energy flux and direction of the ion beam can be regulated
independently of the evaporation flux. In both configurations the ion-beam
angle of incidence is not normal to the substrate and can lead to anisotropic
film properties. Substrate rotation is, therefore, recommended if isotropy is
desired.
Broad-beam (Kaufman) ion sources, the heart of ion-beam-assisted deposition
systems, were first used as ion thrusters for space propulsion (Ref. 32).
Their efficiency has been optimized to yield high-ion-beam fluxes for given
power inputs and gas flows. They contain a discharge chamber that is raised to
a potential corresponding to the desired ion energy. Gases fed into the chamber
become ionized in the plasma, and a beam of ions is extracted and accelerated
through matching apertures in a pair of grids. Current densities of several
mA/cm2 are achieved. (Note that 1 mA/cm2 is equivalent to 6.25 x 1015
ions/cm2-sec or several monolayers per second.) The resulting beams have a
low-energy spread (typically 10 eV) and are well collimated, with divergence
angles of only a few degrees. Furthermore, the background pressure is quite
low (-
Examples of thin-film property modification as a result of IAD are given in
Table 3-8. The reader should appreciate the applicability to all classes of solids
and to a broad spectrum of properties. For the most part, ion energies are
lower than those typically involved in sputtering. Bombarding ion fluxes are
generally smaller than depositing atom fluxes. Perhaps the most promising
application of ion bombardment is the enhancement of the density and index of
refraction of optical coatings.
Ionized Cluster Beam (ICB) Deposition (Ref. 33)
The idea of employing energetic ionized clusters of atoms to deposit thin films
is due to T. Takagi. In this novel technique, vapor-phase aggregates or
clusters, thought to contain a few hundred to a few thousand atoms, are
created, ionized, and accelerated toward the substrate as depicted schematically
in Fig. 3-26. As a result of impact with the substrate, the cluster breaks apart,
releasing atoms to spread across the surface. Cluster production is, of course,
the critical step and begins with evaporation from a crucible containing a small
aperture or nozzle. The evaporant vapor pressure is much higher (10-*-10
torr) than in conventional vacuum evaporation. For cluster formation the
nozzle diameter must exceed the mean-free path of vapor atoms in the crucible.
Viscous flow of atoms escaping the nozzle then results in an adiabatic
supersonic expansion and the formation of stable cluster nuclei. Optimum
expansion further requires that the ratio of the vapor pressure in the crucible to
that in the vacuum chamber exceed lo4 to 10'.
The arrival of ionized clusters with the kinetic energy of the acceleration
voltage (0-10 kV), and neutral clusters with the kinetic energy of the nozzle
ejection velocity, affects film nucleation and growth processes in the following
ways:
1. The local temperature at the point of impact increases.
2. Surface diffusion of atoms is enhanced.
3. Activated centers for nucleation are created.
4. Coalescence of nuclei is fostered.
5. At high enough energies, the surface is sputter-cleaned, and shallow
implantation of ions may occur.
6. Chemical reactions between condensing atoms and the substrate or gas-phase
atoms are favored.
Moreover, the magnitude of these effects can be modified by altering the
extent of electron impact ionization and the accelerating voltage.
Virtually all classes of film materials have been deposited by ICB (and
variant reactive process versions), including pure metals, alloys, intermetallic
compounds, semiconductors, oxides, nitrides, carbides, halides, and organic
compounds. Special attributes of ICB-prepared films worth noting are strong
adhesion to the substrate, smooth surfaces, elimination of columnar growth
morphology, low-temperature growth, controllable crystal structures, and,
importantly, very high quality single-crystal growth (epitaxial films). Large Au
film mirrors for CO, lasers, ohmic metal contacts to Si and Gap, electromigration-
(Section 8.4) resistant A1 films, and epitaxial Si, GaAs, Gap, and InSb
films deposited at low temperatures are some examples indicative of the
excellent properties of ICB films. Among the advantages of ICB deposition are
vacuum cleanliness (- lo-' torr in the chamber) of evaporation and energetic
ion bombardment of the substrate, two normally mutually exclusive features.
In addition, the interaction of slowly moving clusters with the substrate is
confined, limiting the amount of damage to both the growing film and
substrate. Despite the attractive features of ICB, the formation of clusters and
their role in film formation are not well understood. Recent research (Ref. 34),
however, clearly indicates that the total number of atoms agglomerated in large
metal clusters is actually very small (only 1 in lo4) and that only a fraction of
large clusters is ionized. The total energy brought to the film surface by
ionized clusters is, therefore, quite small. Rather, it appears that individual
atomic ions, which are present in much greater profusion than are ionized
clusters, are the dominant vehicle for transporting energy and momentum to
the growing film. In this respect, ICB deposition belongs to the class of
processes deriving benefits from the ion-beam-assisted film growth mechanisms
previously discussed.
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