Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Atmospheric Pressure Plasma Sources and Processing 871
A further option is using special electrode shapes with sharp edges or cones forming
high-intensity local electric fields [11, 12]. A short pulse power preventing the development of
arcs is very efficient, too. Much effort has been put into designing special shapes of pulses,
usually in combination with dielectric barriers, where the correct ratio between positive and
negative parts of the pulse is important to neutralize the charge accumulated on the barrier
surface and decrease the probability of forming streamers.
17.2.4 Space-Charge Sheaths at Low and High Gas Pressures
In all plasma devices the interface between the plasma and solid surfaces (electrode, substrate,
reactor wall) – the space-charge sheath – plays an important role. At low gas pressures the
sheath can be considered collisionless but it is obviously not so at high pressures. In a
low-pressure approach the sheath thickness d
s
depends on the gas pressure p roughly as
d
s
≈ p
−1/2
.Ifd
s
at 1 mbar is about 1 mm it should be 10
−3/2
(≈ 0.032) times shorter at 1 bar, i.e.
about 30 m. Calculations of the sheath thickness at atmospheric pressure based on the
conventional Child–Langmuir approach lead to values of about 10 m at largest. However,
experiments show about one order thicker sheaths, particularly in the atmospheric RF
discharges [7, 14, 15]. This is important, for example, for RF hollow cathode devices, where
the distance between the opposite walls must be sized to at least 2d
s
. A simple realistic model
of the atmospheric pressure RF sheath taking into account secondary and fast electrons [16]
confirms experimental d
s
values of about 100 m. Thus, the intense discharges in RF
electrodes with rectangular openings or holes of 400 m in size are certainly based on the
hollow cathode effect, and explanations using conventional approaches may be rather
confusing.
Recent model studies of electron kinetics in atmospheric RF microdischarges in helium
confirm that the electron energy distribution function in atmospheric pressure microdischarges
is far from the equilibrium, and simulation results from conventional fluid models should
therefore be interpreted with particular caution [17]. For an RF-powered plasma at
atmospheric pressure, the electron collision frequency is much higher than the RF driving
frequency (usually 13.56 MHz). Consequently, the electron energy relaxation time (≈ 5psto
10 ns) is much shorter than the RF period (≈ 74 ns) and the electron energy distribution
function is time modulated. It is therefore possible to consider the RF-coupled capacitive
discharges at atmospheric pressure as a kind of succession of DC discharges sustained at
different voltages [17]. However, similarly to RF plasmas at low pressures, the heavy ions
cannot follow the instantaneous RF field.
17.3 Examples of Cold Atmospheric Plasma Sources
Several examples of plasma sources for generation of cold atmospheric plasma are shown in
Figure 17.3. Probably the simplest and the most widespread cold atmospheric plasma sources
872 Chapter 17
shown in Figure 17.3(a, b) are based on the DBD (see reviews [18, 19]). A dielectric barrier at
one or both electrodes efficiently prevents streamers. There are a great number of different
constructions and shapes of electrodes and dielectric barriers in the DBD plasma sources for
different applications. Typical treatments of moving planar substrates are indicated in Figure
17.3(a, b). Accumulated surface charges on dielectric barriers can be neutralized by a bipolar
pulsed DC power. The DBD of this type can be used arbitrarily in all kinds of gases, in both
static and flowing regimes.
Figure 17.3(c) shows a typical jet plasma source powered by an RF generator [20]. These
sources can also be arranged with a dielectric barrier covering one or both electrode surfaces.
The operating frequency of such coaxial sources can be easily extended up to the microwave
range. Increasing the frequency of the power generator up to the microwave range usually
reduces the problem with streamers substantially and enables generation of the glow plasma in
virtually all molecular gases.
A simple microwave source of cold atmospheric plasma is shown in Figure 17.3(d) [21]. The
source can be assembled from standard waveguide components with a tapered part to increase
the electric field in the axis of dielectric reactor passing through this part. Different microwave
systems and principles can be used for constructions of plasma applicators, including
microwave resonators and radiating structures.
Many atmospheric plasma sources are based on microdischarges where small size atmospheric
plasma units are multiplied to form arrays or integrated in-line or area-distributed systems.
Very specific systems for generation of the large-area plasmas are based on multiple hollow
cathodes integrated in an FHC. Figure 17.3(e) shows schematically an FHC reactor where
multiple hollow cathodes, powered for example by an RF generator, are positioned close to
each other so that individual plasmas interact with each other [22]. Such an arrangement
enables very low RF powers for keeping the plasma over the whole electrode surface
(≥ 0.2 W/cm
2
in neon) and low breakdown RF voltages. It is therefore possible to ignite the
plasma without any additional tools. Figure 17.4 shows an example of a circular cartridge FHC
system (diameter of 35 mm) with about 900 hollow cathodes producing argon plasma in open
air at an RF power of 70 W [23].
The hybrid plasma source ‘hybrid hollow electrode activated discharge’ (H-HEAD)
(Figure 17.3f) combines a microwave antenna with a hollow cathode powered by an RF or
pulsed DC generator. The microwave plasma is produced by surface waves and the energy of
particles can be controlled by the hollow cathode. Such a source is capable of producing
very long plasma columns (up to 18 cm) at very low gas flows (≤ 250 sccm), and it works
virtually with all types of gas [24, 25]. Recent results [26] proved that it is possible to operate
the source in water vapor or even in water. A photograph of the plasma generated with the
antenna immersed in water held in a simple container is shown in Figure 17.5.
Atmospheric Pressure Plasma Sources and Processing 873
Figure 17.4: Fused hollow cathode (FHC) cold atmospheric discharge in Ar + air. Parameters: RF
power = 70 W, 2 slm Ar, plasma diameter = 35 mm.
Figure 17.5: Water plasma generated by the H-HEAD source at 200 W with the microwave
antenna immersed in the water container. Condensed water is seen at the bottom part of the
source.
17.4 Characteristic Features and Typical Applications of Cold
Atmospheric Plasma
Because of considerable differences between the low-pressure plasmas and atmospheric
pressure plasmas it is not realistic to expect that the low-pressure plasma can be simply
replaced by an atmospheric plasma and used for the same applications.
874 Chapter 17
17.4.1 Typical Features and Application Abilities of Atmospheric Plasma
Important features and typical application abilities for cold atmospheric plasma are briefly
listed and commented on below.
1. Start-up (breakdown) regimes and tools are sometimes needed as the breakdown
voltages in atmospheric plasma systems are often considerably higher than the
maintenance voltages. Similarly, the impedances with and without plasma can be very
different. Figure 17.6 shows an example of ignition of a hybrid microwave plasma
[24] by a high-voltage spark produced by a Tesla coil.
2. Occasional arcing and non-uniform (filamentary) plasmas, mainly in molecular gases,
can be prevented by special designs of both the plasma source and the power generator
(see e.g. [11, 12, 27]). Some of these options have been discussed in Section 17.2.3.
3. Almost no use of magnetic fields can be expected. The mean free path is usually very short
in comparison with the Larmor radii of charged particles in ‘normal’ magnetic fields
(induction B ≤ 0.1 T). However, at a high current density of 1.2 kA/cm
2
in streamers it
may be possible to affect their motions in non-equilibrium filamentary plasmas. Detailed
Figure 17.6: Spark ignition of the plasma plume in the hybrid plasma source (H-HEAD) by the
Tesla coil. A spark ignited between the Tesla coil and the microwave antenna of the H-HEAD
source (upper picture) causes instant formation of the plasma plume (lower picture).
Atmospheric Pressure Plasma Sources and Processing 875
experiments are needed to verify this. Pulsed magnetic fields have already been proven to
affect atmospheric spark discharges [28].
4. The plasma has restricted dimensions due to the short mean free paths of particles. For
example, the calculated mean free path at atmospheric pressure and a gas temperature of
100
◦
C is only 12 nm for air (molecular diameter of 0.97 nm) and 146 nm for helium
(atomic diameter of 0.28 nm) [29]. Processing of 3D substrates often requires large gas
throughputs (already of the order of 10 slm at a laboratory scale) and the use of helium as
a carrier gas. An exception is the H-HEAD source [24, 25], where long plasma plumes can
be generated at very low gas throughputs (about 0.1 slm). For large-area treatments
multiple plasma structures similar to FHC systems should be applied (see e.g. [30]).
Movements of the plasma source or substrate, or both, are often required.
5. Low ion energies exclude sputtering. Pumping the energy selectively into electrons in
atmospheric plasmas and a high collision frequency naturally limit the energy of ions.
This, in practice, excludes the ion-assisted processes well known in the physical vapor
deposition (PVD) of films in low-pressure plasma systems. Note that theoretical threshold
ion energies for sputtering by noble gas ions exceed 1 eV and experimental values usually
exceed 10 eV [31].
6. Production of dangerous by-products can take place in open systems. Different
nitrogen oxides, NO
x
(N
2
O, NO, NO
2
), can be generated in high-voltage air-assisted
plasmas. Quite low levels of nitrogen oxides (≥ 0.1 ppm) in the air can irritate human
eyes, nose, throat, and lungs, and cause coughing, shortness of breath, tiredness, and
nausea [32]. Another possible by-product is ozone, which can damage the human lung at
levels of ≥ 130 ppb. Cyanogens (CN, HCN) and CO are particularly dangerous. For
certain processes a hermetic chamber and proper ventilation are absolutely necessary.
7. Atmospheric plasma can also be used in processes opposite to those discussed in (6)
above, for removal of dangerous by-products and flue gas abatement (NO
x
, CO, SO
x
,
volatile organic compounds, etc.). Applications for abatement of hazardous components in
the flue gas from power plants and for exhaust aftertreatment from diesel engines have
been tested for more than 20 years (see e.g. [33, 34] and references therein).
8. Atmospheric plasma can be used in particle and film treatment, recrystallization and
sintering. This point can be considered an emerging application despite the fact that
sintering experiments in plasma began more than 20 years ago [35]. As the treatment may
require elevated plasma temperatures, utilization of decaying parts of thermal plasmas can
be more efficient than cold plasma.
9. Short mean free paths in atmospheric plasma are highly compatible with microscale or
nanoscale processes leading to production of microparticles or nanoparticles and objects.
One example is the use of microhollow cathodes for dry etching and diamond deposition
876 Chapter 17
in reactors at slightly reduced pressures [36]. Another example is the production of
nanocrystalline diamond on stainless steel by hybrid plasma at atmospheric pressure in air
mixed with ethanol [37].
10. Atmospheric plasma can serve as a source of ultraviolet (UV) and vacuum ultraviolet
(VUV) radiation. An example is VUV radiation with a wavelength of ≈ 130 nm from
atmospheric microhollow cathodes in argon [38].
11. Atmospheric plasma can be used for disinfection, cleaning, deactivation, and
decontamination of surfaces. The plasma has ability to kill bacteria [39] at least due to an
intense UV light [40]. There are many promising applications in chemical and possibly
also nuclear decontamination.
12. There are many potential medicinal applications of cold atmospheric plasma. Besides
disinfection and treatment of inorganic surfaces, new treatments and cures of human skin
and different biological objects are being tested in numerous laboratories all over the
world (see e.g. [41, 42]). Such treatments obviously cannot be carried out under vacuum
by low-pressure plasma. This new field of research requires thorough, careful, and
long-term multiple experiments.
13. Reactive interactions of cold atmospheric plasma with (and inside) liquids are tested to
study processes inside liquid (submersible arrangements) (see e.g. [43]). Direct contact of
the plasma with or inside liquid can be arranged without interference of the air. An
example of this kind of arrangement has been discussed above (Section 17.3 and Figure
17.5). Experiments confirm, for instance, the applicability of underwater plasma
arrangements for the treatment of cotton yarns [44]. New developments and processes
using submerged plasmas can be expected in the near future.
14. Diesel fuel reforming and related applications in plasma chemistry. Plasma-based fuel
converters enrich the fuel with hydrogen by boosting partial oxidation, which reforms
hydrocarbon fuels into a hydrogen-rich gas for more powerful and efficient combustion
with considerably fewer effluents and particles. Experiments began in the 1990s [45].
17.4.2 Applications of Atmospheric Plasma in Coating and Surface Processing
Cold atmospheric plasmas are often used for the efficient pretreatment and activation of
surfaces, particularly webs. Such treatment is about ten times faster than in low-pressure
plasmas. However, traces of streamers in the high-voltage filamentary discharges can damage
the surfaces of plastic webs. The low-power FHC discharges can activate plastic webs without
damage and without undesirable changes in polymer crystallinity [46]. Cold atmospheric
plasma pretreatment of surfaces is already used as a standard commercial technology [47].
Film parameter control is limited in atmospheric plasma coating because of low ion energies
and high collision frequency at atmospheric pressure. It is not realistic, for example, to expect
Atmospheric Pressure Plasma Sources and Processing 877
efficient control of coating properties (texture, hardness) by bias-controlled ion bombardment.
The films deposited in an atmospheric plasma are mostly amorphous [48], unless an elevated
substrate temperature is used. Production of hard coatings without substantial heating of the
substrate or additional post-treatment (e.g. annealing) is a problem.
Feasible coating regimes in cold atmospheric plasma are based on plasma-enhanced chemical
vapor deposition (PECVD). All coating regimes reported so far in the literature are based on
PECVD using precursors diluted mainly in He carrier. Large-area coating of 2 m wide webs
using liquid precursors can be performed in commercial DBD arrangements with multiple flat
electrodes [49]. The PECVD regimes are based on the formation of reactive radicals.
Evaporation of material targets can be applied only in special arrangements. Note that high
voltages and high collision frequency in the atmospheric plasma systems can produce intense
UV radiation from the plasma, which may assist in surface activation, photo-assisted
chemistry, enhanced gas activation, etc.
Atmospheric plasma coating systems in arrangements open to the ambient air are suitable for
deposition of oxides and oxynitrides. Other film compositions require hermetic chambers
protecting the plasma from the ambient atmosphere. Our experiments show that even when the
plasma is generated in an inert gas the optical emission spectra confirm high emission
intensities of the molecular nitrogen and oxygen band systems, excited oxygen atoms, etc. The
active oxygen produced in the plasma can be used for surface oxidation. We have observed
rapid surface oxidation of molybdenum substrates in H-HEAD air atmospheric plasma in the
presence of ethanol vapor. Polycrystalline MoO
3
formations shown on the scanning electron
microscope image in Figure 17.7 were observed after several tens of minutes of air plasma
Figure 17.7: Typical formations of polycrystalline MoO
3
created on Mo surface by oxidation of
Mo substrate in the hybrid atmospheric air plasma generated by the H-HEAD source. Microwave
power 300 W, pulsed DC voltage 30 ns, 8 kV, 100 Hz.
878 Chapter 17
Figure 17.8: Multiple nucleation of diamond crystals on MoO
3
formations observed in air hybrid
plasma interacting with an alcohol vapor. (See [37].)
treatment by a hybrid plasma source (shown in Figure 17.3f) at much larger areas than
corresponding to the diameter of the interacting plasma on the Mo substrate, and even on the
opposite side of the substrate. As shown in Figure 17.8, the MoO
3
surface can be used for
nucleation of diamond crystals formed from an alcohol vapor in the air plasma generated by
the hybrid source [37].
17.5 General Conclusions
Low-temperature, non-equilibrium atmospheric plasma shows very promising potential for a
number of new, non-conventional applications, including new surface treatments and
processing of materials. However, the principles, characteristic features, and feasible processes
in cold plasma at atmospheric and higher pressures are often considerably different from those
in low-pressure, low-temperature plasmas. It is therefore reasonable to conclude that cold
atmospheric plasma extends the potential of plasma technology without considerable
competing with conventional and well-established low-pressure plasma systems.
Acknowledgments
Financial support from the Foundation for Strategic Environmental Research (MISTRA) and
Johan Gustaf Richerts Scientific Foundation in Sweden is gratefully acknowledged.
Atmospheric Pressure Plasma Sources and Processing 879
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