Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Ion Plating 301
Figure 6.3: Fracture cross-section (bottom) and surface morphology (top) of a thick RF sputter
deposited chromium deposit: (A) without a bias (no bombardment) and (B) with concurrent
bombardment (−500 V bias on the substrate) [13].
bombarding energies, the adatoms and surface atoms are knocked about, thus densifying the
deposited material. However, if the atoms are stuffed into the atomic structure by too much
bombardment, they will develop a high compressive stress and in some cases an unusual
atomic arrangement such as diamond-like carbon (DLC). If the bombarding particle is an ion,
it will also release its energy of ionization as it bombards the surface.
302 Chapter 6
Reactive deposition processes present additional considerations. In reactive deposition, the
reacting species may release energy (exothermic) to form the compound. This requires a
three-body collision in the gas phase or a two-body collision on a surface. This is why
chemical reaction generally does not occur in the gas phase. This local energy release on the
surface has to be added to the energies already discussed. In reactive deposition, the depositing
surface atoms must react with the reactive species before they are buried and become
unavailable for reaction. In many cases the reactive species have been ‘activated’ (ionized or
excited) in the plasma, which makes them more reactive. When performing reactive sputter
deposition, it is common to have both a heavy gaseous species (argon) mixed with the lighter
reactive species (nitrogen, oxygen). A key component of reactive sputter deposition is to have
good control of the partial pressure of the reactive gas(es) in the chamber.
In addition to reactive deposition by continuous bombardment, reactive deposition can be
performed by using a rotating drum type substrate fixture and periodically depositing a few
monolayers of one material, then bombarding it with a reactive material, usually from a broad
beam (gridless) ion source [17, 18] as shown in Figure 6.1(d). By repeating the process a thick
deposit can be built up [19]. Figure 6.4 shows a gridless end Hall ion source.
Obtaining the desired properties by bombardment with massive energetic particles often
requires control of the flux and energy of the bombarding particles and the flux of depositing
Figure 6.4: Gridless end Hall ion source. (From SVC Education Guides to Vacuum Coating
Processing (2009) – Broad beam reactive plasma sources – 1: DC sources, with permission.)
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atoms. Unless film ions are primarily being used (as is the case in arc vapor deposition or High
Power Impulse Magnetron Sputtering – HPIMS), the incorporation of gaseous atoms in the
deposit is undesirable, and so the bombarding energy should be less than 300 eV. The flux of
the bombarding species can be difficult to determine. Measuring the energy of massive neutral
particles is complicated and requires a technique such as a time-of-flight mass spectrometry.
6.4 Sources of Depositing Material
The properties of films and coatings deposited by the ion plating process depend primarily on
the deposition conditions on the growing surface and minimally on the source of the
depositing atoms/molecules.
Vacuum evaporation is generally performed either with a resistively heated evaporation source
or using high-voltage electron beam heating. When using resistive heating a plasma can be
established using an electrically isolated substrate holder such as a cathode or radio frequency
(RF) electrode or by having an auxillary plasma source such as an RF induction coil or a
hot filament–cathode plasma source between the evaporation source and the substrate. In order
to establish a plasma the pressure in the chamber must be raised to about 0.5–15 mtorr,
depending on the power source. This pressure does not affect the evaporation of the source
material.
Higher pressures increase the probability of gas scattering and three-body collisions that give
rise to vapor phase nucleation (‘sooting’) of the evaporated material. In a plasma these
particles will become negatively charged and will be suspended in the plasma. If the substrate
holder is electrically isolated and biased negatively the particles will not deposit on the
substrates. Scattering of evaporated particles in the plasma will tend to increase the surface
coverage to out-of-line-of-sight to the evaporation source.
High-current, low-voltage electron beams from hot hollow cathode electron emitting sources
can be used to thermally evaporate material [20, 21]. The electrons will also form a plasma if a
gas (e.g. Ar) is present. Depending on the electron flux, high ion density plasmas can be
formed between the substrate and the evaporating material as shown in Figure 6.5.
Establishing a plasma in a high-voltage, focused electron beam evaporation system is not
generally a good idea since the positive ions rapidly sputter away the electron emitting
filament, which is at a high negative potential. High-voltage electron beam evaporators can be
used with a plasma by having the electron emitter in a separate differentially pumped chamber
where the pressure is low. The electron beam is extracted into the plasma chamber through a
small, low-conductance hole [22].
Hollow cathode arc activated electron beam evaporation is performed by using one or more
electron emitting hollow cathodes to inject electrons into the gas and vapor near the substrate
304 Chapter 6
Figure 6.5: Hollow cathode electron source for electron beam melting/evaporation [38].
[23, 24]. The electrons ionize gases (Ar, O, N, C
2
H
2
) and a small portion of the depositing
vapor. The electrons from the hollow cathode can be distributed in defined ways using a
magnetic field between the cathode and the anode. This technique is particularly useful in
depositing insulating films since charge build-up on the depositing surface (self-bias) will be
negative and controlled by the electron temperature in the plasma.
Plasma-based sputter deposition is ideal for ion plating since the plasma is present to allow
bombardment of the depositing material. When using a non-magnetron DC configuration for
sputtering the substrate will be bombarded with electrons accelerated away from the cathodic
target. In a magnetron sputtering configuration the magnetic field should be configured so as to
let some electrons escape from the magnetic confinement and form a plasma between the
source and the substrate (unbalanced magnetron sputtering, linked field magnetron sputtering)
[25].
The term ionized physical vapor deposition (IPVD) refers to using a plasma generated
between the vapor source (evaporation, sputtering) to enhance the ionization of the vapor
species [26, 27]. The plasma may be formed by an RF coil, a hollow cathode electron emitter,
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or a hot filament electron emitter. Some authors place high-power pulsed magnetron sputtering
(HIPPMS) in this category because of the high ionization fraction of the vaporized material.
In 1999 a technique for HIPPMS, also called high-power impulse magnetron sputtering
(HPIMS or HIPIMS), which uses short pulses of very high power to sputter a target was
published [28–31]. Peak powers may be as high as 3 MW at greater than 1 A/cm
2
when using a
low duty cycle (0.1 5%) and pulsing frequencies up to a few kHz. This technique generates
pulses of highly ionized pulses of the depositing material [32]. Low-power sputtering with a
superimposed high-power pulse may be used to increase the overall deposition rate [33].
Ion plating using ion beam (‘ion gun’) sputtering in a good vacuum may be done by using a
second ion gun in the system to bombard the growing deposit. If the substrate or depositing
material is not electrically conductive the ion beam may be neutralized by adding electrons to
the ion beam, making it a plasma beam to prevent surface charge build-up [34, 35].
Cathodic arc vaporization results when a cathodic arc ‘runs’ over the surface of the source of
material as shown in Figure 6.1(c). A large fraction of the vaporized material is ionized and
may be accelerated to the substrate as ‘film ions’ [36, 37].
Arc vaporization, where a large fraction of the vaporized material is ionized, can be formed in
an arc with a consumable anode (anodic arc). A hollow cathode can be used to evaporate
material through a dense electron cloud so as to increase ionization of the evaporated material,
as shown in Figure 6.5 [38–40].
6.5 Sources of Bombarding Par ticles
The most common energetic particles for bombardment are energetic gaseous ions. When a
plasma is present, ions may be accelerated to the surface by having a negative bias on the
surface. The same types of waveforms used for sputtering [24] can be used for biasing. These
include continuous DC, mid-frequency (MF) pulsed DC, bipolar pulsed power, and RF.
Energetic ions can be formed in ion sources and used to bombard the deposit. Ion sources can
be from a grid extraction ion source (gun) where the ions are relatively monoenergetic or from
a broad-beam ion source where there is a spectrum of ion energies.
High-energy neutral atom bombardment can have the same effects as high-energy ion
bombardment. At low pressure, high-energy neutrals are formed by reflection of bombarding
ions from the sputtering cathode [41, 42]. The sputtered atoms also leave the target surface
with a high energy. These high-energy neutrals reach the substrates with high energies because
of the long mean free path for collision at low pressures [43]. High-energy neutrals can also
be formed by charge exchange between a high-energy ion and a low-energy neutral [44].
Figure 6.6 shows the film stress generated by sputter deposition as a function of gas pressure in
306 Chapter 6
Figure 6.6: Stress as a function of processing pressure due to high-energy neutral bombardment
during deposition [44].
a post cathode magnetron sputtering system [45, 46]. This source of energetic bombardment is
often an uncontrolled variable of sputter deposition. From Figure 6.6 it can be seen that small
changes in pressure can give big changes in the stress in the deposit.
In arc vaporization, IPVD, HIPPMS [47], pulsed laser deposition (PLD) [48, 49], and
hollow-cathode gas-flow sputtering [50, 51], ions of both the gaseous species and the film
material (film ions) may be accelerated to the depositing material. It has been shown that the
thermal load on the substrate is less with HIPIMS than with a DC discharge [52].
In some cases, where the vapor originates from a sputtering source having two components of
different electronegativities, negative ions may be formed and accelerated away from the
negative source. If the substrate is at ground potential and in-line-of-sight, it will be
bombarded by high-energy negative ions [53–55]. This is a particular problem with depositing
superconducting compounds containing oxygen (e.g. YBa
2
Cu
3
O
7
).
6.6 Substrate Potential
Figure 6.7 shows some of the types of fixtures used in vacuum deposition technologies. In
some cases the substrate holder is fixed and may be easily electrically isolated and a bias
potential applied. In other cases the substrate holder is moving and biasing the substrates
becomes more difficult. If water cooling is used on the substrate holder there should be an
electrical break in the tubing and deionized (DI) water should be used.
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Figure 6.7: Fixtures for holding substrates. (From SVC Education Guides to Vacuum Coating
Processing (2009) – Fixtures and Tooling, with permission.)
Figure 6.8 shows a ‘barrel ion plating’ fixture that allows coating of small parts being
‘tumbled’ inside a barrel/cage at high voltage [56]. This technique is used for aluminum
coating fasteners for the aerospace industry to prevent galvanic corrosion [5].
As with sputtering there are a number of types of voltage waveforms that can be used to bias
substrates. These include continuous DC, low-frequency (50 Hz) AC, pulsed DC, bipolar
pulsed power,
1
mid-frequency (20–250 kHz) AC, RF, high-impulse power, etc.
As with reactive sputter deposition of insulating compounds a pulsed DC, mid-frequency AC,
and bipolar pulsed AC,
1
or RF bias on the depositing insulating material is used to prevent
arcing. The pulse bias may be at the same or different frequency [57] as the sputtering
frequency.
1
Some authors use the term bipolar pulsed DC for this waveform. The author prefers the term bipolar pulsed
power since if you sent the waveform through a full wave rectifier it would not come out bipolar.
308 Chapter 6
Figure 6.8: Barrel ion plating configuration [63].
In pulsed DC sputtering there is a significant large positive overshoot as the pulse is turned off
owing to the inductance in the circuit. This raises the plasma potential and causes energetic
ions to bombard the substrate [58].
The electron temperature in a plasma determines the self-bias on an insulating surface or a
surface with a floating potential (ungrounded) in contact with the plasma. The self-bias may be
from a few volts to several hundred volts. Figure 6.9 shows a technique to bias an insulating
film or floating substrate holder using an electron emitting source and magnetic confinement to
form a plasma and induce a high negative potential on the electrically floating or insulating
surface [59].
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Figure 6.9: Self-bias generated by accelerating electrons away from a source and magnetically
confining them so they strike an electrically floating substrate or substrate holder [59].
310 Chapter 6
6.7 Some Applications of Ion Plating
Ion plating is used to deposit adherent coatings, dense coatings, and conformal coatings on
complex surfaces, and to tailor coating properties such as stress, density, grain size, and
crystallography. Some specific examples are:
adhesion of low shear metal lubricants on metal surfaces, e.g. bearings for X-ray tubes
aluminum coating of aircraft fasteners to prevent galvanic corrosion (IVD)
corrosion protective coatings on materials with coherent oxides without removing too
much material [60, 61]
coatings on strip steel [62]
coating on aero engine parts [51]
densification of high and low index of refraction films on optical components [59, 63]
hard coatings of compound materials (Ti, Al, Zr, Cr – O, N, C, B) on tools, injection
molds, etc. [64]
adhesion layer (‘strike’) for subsequent electroplating of materials such as Zr, U, and
Ti [65].
References
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[3] A. Matthews, Developments in ionization assisted processes, J. Vac. Sci. Technol. A3(6) (1985) 2354.
[4] S.M. Rossnagel, J.J. Cuomo, W.D. Westwood, Handbook of Plasma Processing Technology: Fundamentals,
Etching, Deposition, and Surface Interactions, Noyes Publications (1990).
[5] IVD (Ion Vapor Deposition) – Military Specification MIL-C-83488.
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deposition (IPVD): a review of technology and applications, Thin Solid Films 513 (2006) 1–24.
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(1979) 315.
[8] S. Schiller, U. Heisig, K. Goedicke, Alternating ion plating – a method of high-rate ion vapor deposition,
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[9] H.R. Kaufman, J.M.E. Harper, J.J. Cuomo, Developments in broad-beam, ion-source technology and
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(1989) 1105.
[11] J.A. Thornton, High rate thick film growth, Annu. Rev. Mat. Sci. 7 (1977) 239.
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[13] R.D. Bland, G.J. Kominiak, D.M. Mattox, Effects of ion bombardment during deposition on thick metal and
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