IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998 1685
The Atmospheric-Pressure Plasma Jet: A Review
and Comparison to Other Plasma Sources
Andreas Sch
¨
utze, James Y. Jeong, Steven E. Babayan, Jaeyoung Park, Gary S. Selwyn, and Robert F. Hicks
(Invited Paper)
Abstract—Atmospheric-pressure plasmas are used in a variety
of materials processes. Traditional sources include transferred
arcs, plasma torches, corona discharges, and dielectric barrier
discharges. In arcs and torches, the electron and neutral tem-
peratures exceed 3000
C and the densities of charge species
range from 10
16
–10
19
cm
0
3
. Due to the high gas temperature,
these plasmas are used primarily in metallurgy. Corona and di-
electric barrier discharges produce nonequilibrium plasmas with
gas temperatures between 50–400
C and densities of charged
species typical of weakly ionized gases. However, since these
discharges are nonuniform, their use in materials processing is
limited. Recently, an atmospheric-pressure plasma jet has been
developed, which exhibits many characteristics of a conventional,
low-pressure glow discharge. In the jet, the gas temperature
ranges from 25–200
C, charged-particle densities are 10
11
–10
12
cm
0
3
, and reactive species are present in high concentrations,
i.e., 10–100 ppm. Since this source may be scaled to treat large
areas, it could be used in applications which have been restricted
to vacuum. In this paper, the physics and chemistry of the plasma
jet and other atmospheric-pressure sources are reviewed.
Index Terms—Atmospheric pressure, corona discharge, dielec-
tric barrier discharge, plasma jet, plasma torch, thermal and
nonthermal plasmas, transferred arc.
I. INTRODUCTION
L
OW-PRESSURE plasmas have found wide applications
in materials processing and play a key role in manu-
facturing semiconductor devices [1], [2]. The advantages of
plasmas are well known. They generate high concentrations
of reactive species that can etch and deposit thin films at rates
up to 10
m/min. The temperature of the gas is usually below
150
C, so that thermally sensitive substrates are not damaged.
The ions produced in the plasma can be accelerated toward a
substrate to cause directional etching of submicron features.
In addition, a uniform glow discharge can be generated, so
Manuscript received March 30, 1998. This work was supported in part
by the U.S. Department of Energy, the University of California, the Basic
Energy Sciences, Environmental Management Sciences Program, and the
Office of Science and Risk Policy under Award DE-F5607-96ER45621. The
work of A. Sch
¨
utze was supported by a fellowship from the Deutsche
Forschungsgemeinschaft (DFG). The work of J. Y. Jeong and S. E. Babayan
and was supported by fellowships from the University of California Center
for Environmental Risk Reduction and the Toxic Substances Research and
Training Program.
A. Sch¨utze, J. Y. Jeong, S. E. Babayan, and R. F. Hicks are with the
Department of Chemical Engineering, University of California, Los Angeles,
CA 90095-1592 USA (e-mail: schuetze@seas.ucla.edu).
J. Park and G. S. Selwyn are with Los Alamos National Laboratory, Los
Alamos, NM 87545 USA.
Publisher Item Identifier S 0093-3813(98)09643-X.
that materials processing proceeds at the same rate over large
substrate areas. On the other hand, operating the plasma at
reduced pressure has several drawbacks. Vacuum systems are
expensive and require maintenance. Load locks and robotic
assemblies must be used to shuttle materials in and out of
vacuum. Also, the size of the object that can be treated is
limited by the size of the vacuum chamber.
Atmospheric-pressure plasmas overcome the disadvantages
of vacuum operation. However, the difficulty of sustaining a
glow discharge under these conditions leads to a new set of
challenges. Higher voltages are required for gas breakdown
at 760 torr, and often arcing occurs between the electrodes.
In some applications, such as plasma torches, arcing is in-
tentionally sought [3], [4]. However, to prevent arcing and
lower the gas temperature, several schemes have been devised,
such as the use of pointed electrodes in corona discharges
[5] and insulating inserts in dielectric barrier discharges [6].
A drawback of these sources is that the plasmas are not
uniform throughout the volume. Recently, a plasma jet has
been developed which uses flowing helium and a special
electrode design to prevent arcing [7]. This source can etch
and deposit materials at low temperatures and may be suitable
for a wide range of applications.
A number of review articles have been published on
atmospheric-pressure plasmas [3], [4], [6], [8], [9]. However,
these articles are devoted to either thermal, or nonthermal
sources, and do not include recent advances in the field. In
this paper, we give an overview of these plasma sources and
highlight the recent development of the atmospheric-pressure
plasma jet. First, we consider the effect of pressure on the
physical properties of plasmas. Then in the following sections,
each atmospheric-pressure source is examined. Topics of
interest include the current–voltage characteristics, electron
and neutral temperatures, densities of charged particles, and
gas compositions for oxygen-based systems. Finally, in the last
section, a quantitative comparison is made between traditional
sources and the atmospheric-pressure plasma jet.
II. P
ROPERTIES OF PLASMAS
To ignite a plasma, the breakdown-voltage for the gas
must be exceeded. This voltage depends on the electrode
spacing
and the pressure as follows [2], [8], [10]:
(1)
0093–3813/98$10.00 1998 IEEE
1686 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998
Fig. 1. Breakdown potential in various gases as a function of the pressure
and gap distance
p
2
d
for plane-parallel electrodes [8].
Fig. 2. Current–voltage characteristics of a low-pressure DC glow discharge
at 1 torr [13].
Here, and are constants found experimentally, and
is the secondary electron emission coefficient of the cathode.
Paschen curves, showing the dependence of the breakdown
voltage on electrode spacing and pressure, are presented in
Fig. 1, [8], [11], and [12]. Above
, increases
rapidly with pressure at a constant electrode spacing. For
example, the breakdown voltage for argon is estimated to be
2500 V at 760 torr and with a 5 mm gap distance. A narrow
gap is necessary to achieve a reasonable breakdown voltage
at atmospheric pressure.
Insight into the operation of a plasma can be obtained
from the dependence of the applied voltage on current. This
relationship is illustrated in Fig. 2 for a low-pressure DC
glow discharge [13]. The discharge can be divided into four
regions: 1) the “dark” or “Townsend discharge” prior to spark
ignition; 2) “normal glow” where the voltage is constant or
slightly decreasing with current; 3) “abnormal glow” where
the voltage increases with current; and 4) “arc discharge”
where the plasma becomes highly conductive. Low-pressure
plasmas used in materials processing are operated in region
Fig. 3. Schematic of the electron and gas temperature as a function of
pressure in a plasma discharge at constant current [14].
2). Regions 2) and 3) tend to shrink with increasing pressure,
so that for many gases, spark ignition proceeds directly to
arcing at 760 torr.
In a weakly ionized gas at low pressure, the electron density
ranges between 10
–10 cm [2]. Under these conditions,
the collision rate between electrons and neutral molecules is
insufficient to bring about thermal equilibrium. Consequently,
the electron temperature
can be one to two orders of
magnitude higher than the neutral and ion temperatures
and
. However, as the pressure increases, the collision rate will
rise to a point where effective energy exchange occurs between
the electrons and neutral molecules, so that
.In
this case, the electron density usually ranges from 10
–10
cm [2].
The trend in electron and neutral temperatures with pressure
is illustrated in Fig. 3 for a plasma discharge with a mercury
and rare gas mixture [14], [15]. At 1 mtorr, the gas temperature
is 300K, while the electron temperature is 10000K (1 eV =
11 600K). These two temperatures merge together above 5 torr
to an average value of about 5000K. Since the probability for
energy exchange upon collision of an electron and molecule
depends on the nature of the molecule, the pressure at which
approaches should be a sensitive function of the gas
composition. For example, there may be compositions where
and merge together above 760 torr.
Shibata et al. [16] have simulated the distribution of reac-
tive species within a parallel-plate oxygen discharge using a
relaxation-continuum model. Shown in Fig. 4 are the results
obtained during half of the RF cycle with the cathode at
.
The conditions are 0.15 torr O
pressure, 200 power,
and a 20 mm gap spacing. The simulation shows that the
density of charged species varies over a broad range, from
10
–10 cm , depending on position. On the other hand,
the concentrations of oxygen atoms and metastable oxygen
molecules
are much higher, equal to about 0.2 and 2.0
10 cm , respectively. The concentration of metastable
oxygen is constant across the gap, whereas the concentration of
O atoms falls to zero at the walls due to surface recombination
[16]. This simulation is in good agreement with another
SCH
¨
UTZE er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687
Fig. 4. Spatial particle distributions in a parallel plate O
2
RF discharge at
!t
=
=
2
for
V
rms
=200
V [16].
Fig. 5. Time-averaged number densities of each particle in the center of the
discharge as a function of pressure in the range between 0.15 and 1 torr [16].
modeling study by Klopovskii et al. [17]. By comparison,
Selwyn [18] measured the O atom concentration in an oxygen
and argon RF discharge using two-photon laser excitation. He
detected O atom concentrations of about 5
10 cm at 0.1
torr O
, having a gap distance between two plane electrodes
of 55 mm and a self bias of 300 V.
Shown in Fig. 5 is the effect of pressure on the plasma
composition as predicted by Shibata’s model. As the pressure
increases above 0.3 torr, the density of ions diminishes due
to the lower electron energies at higher pressure (cf. Fig. 3),
and thus, a lower production rate of ion-electron pairs. On
Fig. 6. Schematic of a transferred arc apparatus.
the other hand, the concentrations of O metastables and
O atoms increases with pressure, because these species are
created by the excitement and dissociation of O
, which
requires electron energies of only 1–5 eV. The trends shown
in Fig. 5 suggest that at atmospheric pressure, ions will be
relatively insignificant, so that the chemistry will be dominated
by reactive neutral species. In the case of oxygen plasmas,
these species are O atoms, metastable O
, and O .
III. T
RANSFERRED ARCS AND PLASMA TORCHES
Transferred arcs are used to cut [3], [4], [19], and [20], melt
[4] and [21], and weld condensed materials. A schematic of
such a device is shown in Fig. 6. It consists of a cylindrical
shaped cathode, an outer grounded and water-cooled shield,
and a workpiece as the anode. By feeding argon and hydrogen,
oxygen, or air between the cathode and shield, and by applying
DC power of up to 200 kW, an arc between the electrodes
may be ignited and sustained. Typical operation conditions
and properties are 1–15 l/s gas flow, 50–600 A, 10
MW/m ,
gas temperatures between 3000 and 20 000K, and a nozzle-to-
sample distance of 5–10 mm [19]–[22]. These parameters may
vary, depending on the nozzle diameter and materials to be cut.
Transferred arcs can slice steel plates up to 150 mm thick. For
example, a plate, 40 mm thick, was cut at 0.7 m/min with a
600 A arc [23].
Several design variations on the transferred arc have been
developed. A plasma arc heater uses the shield as the anode
instead of the workpiece. In addition, RF induction coils and
advanced gas injection schemes are sometimes employed to
enhance the plasma density and to restrict the cutting area [9].
Plasma torches are the same as plasma arc heaters, except
that they contain a port for injecting precursor compounds
that are used in thin film deposition [3], [24]–[26]. In the
literature, this technique is also referred to as “injection plasma
processing” [25]. The precursors, in the form of solid pellets,
powder, or volatile molecules, are introduced just downstream
of the arc, where they are vaporized and/or dissociated into
reactive species. The resultant mixture is sprayed onto a
substrate, thereby coating it with a film at rates up to 10
m/min. Films which have been deposited with plasma torches
include SiC [26], SiN [27], TiO
[28], Y–Ba–Cu–O [24], [29],
Al
0 [30], and diamond [31]–[36]. The main purpose of these
coatings is to provide materials with added resistance to wear,
1688 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998
Fig. 7. Current–voltage characteristics of an atmospheric-pressure arc.
corrosion, and oxidation and may also have applications in
electrical and electronic fields [37].
Fig. 7 shows the current–voltage characteristics of a plasma
torch. After exceeding the breakdown voltage (the maximum
in the curve), the gas becomes highly ionized and conductive.
This produces a rapid drop in voltage with increasing current.
Notice that 300 kV is required to achieve breakdown, whereas
at normal operating currents of 200–600 A, only about 100 V
is needed to sustain the arc [19], [38].
Although plasma torches have been considered to be in
equilibrium, Kruger et al. [39], [40] have shown that this is
not normally the case. With regard to thermal equilibrium,
Kim et al. [41] measured the electron and ion temperatures in
a plasma torch as a function of the radial and axial positions.
They found that
varied from 7.0–9.0 eV, while was
an order of magnitude lower at 0.3–0.9 eV. In addition, they
observed electron densities near 3
10 cm .
In transferred arcs and plasma torches, extremely high tem-
peratures promote the complete dissociation of the feed gas.
For example, Fig. 8 shows the effect of the gas temperature
on the distribution of oxygen species and electrons in an
arc at 0.95 atm of oxygen [42]. The oxygen molecules are
approximately 99% dissociated with predominantly all of these
molecules being converted into O atoms. By contrast, the
concentrations of O
,O, and O are several orders of
magnitude lower. These results are consistent with the energies
required to dissociate and ionize oxygen:
(O ) 5.11 eV,
(O ) 6.48 eV, and (O) 13.62 eV [43].
IV. C
ORONA DISCHARGE
A corona discharge appears as a luminous glow localized
in space around a point tip in a highly nonuniform electric
field. The physics of this source is well understood [5], [8],
[12], [44]–[47]. The corona may be considered a Townsend
discharge or a negative glow discharge depending upon the
field and potential distribution [45]. Fig. 9 shows a schematic
of a point-to-plane corona. The apparatus consists of a metal
tip, with a radius of about 3
m, and a planar electrode
separated from the tip by a distance of 4–16 mm [47]. The
Fig. 8. Composition of an oxygen plasma arc at 0.95 atm as a function of
the temperature [42].
Fig. 9. Schematic of a corona discharge.
plasma usually exists in a region of the gas extending about 0.5
mm out from the metal point. In the drift region outside this
volume, charged species diffuse toward the planar electrode
and are collected.
The restricted area of the corona discharge has limited
its applications in materials processing. In an attempt to
overcome this problem, two-dimensional arrays of electrodes
have been developed. Some applications of coronas include the
activation of polymer surfaces [48], [49], and the enhancement
of SiO
growth during the thermal oxidation of silicon wafers
[50], [51].
Shown in Fig. 10 is the dependence of the voltage on the
current for a positive point-to-plane corona operating in air
at 760 torr [5], [52]. The plasma ignites at a voltage of 2–5
kV and produces an extremely small current of 10
–10 A.
Above 10
A, the voltage rapidly increases with current. This
coincides with the generation of micro-arcs, or “streamers,”
that extend between the electrodes. A maximum voltage is
recorded at about 5
10 A, where the device begins
to arc. Coronas are operated at currents below the onset
of arcing.
SCH
¨
UTZE er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1689
Fig. 10. Current–voltage characteristics for a positive point-to-plane corona
discharge with a gap of 13 mm in 1 atm of air [5].
Fig. 11. Concentrations of species in a corona discharge as a function of the
radial distance between the electrodes for a potential difference of
V
=
4
:
25
kV [53].
In the plasma near the tip, the density of charged species
rapidly decreases with distance from about 10
–10 cm [5],
[8]. The electron temperature within the plasma averages about
5 eV. In the drift region outside the discharge, the electron
density is much lower, near 10
cm .
Pontiga et al. [53] have modeled the distribution of species
within a negative corona discharge, operating with pure oxy-
gen at 760 torr and an applied voltage of 4.25 kV. His model
is based on a discharge between two coaxial electrodes, a wire
cathode, and an outer cylinder. Fig. 11 shows the concentration
of these species as a function of the radial distance. Ozone
is the dominant product at a concentration of 5
10
cm . Metastable oxygen molecules and O atoms are five to
six orders of magnitude lower in concentration. The ionized
Fig. 12. Schematic of a silent discharge; (1) metallic electrodes and (2)
dielectric barrier coating.
species are still lower at an average density of about 10
cm . This distribution of reaction products differs greatly
from that seen in a low-pressure glow discharge (cf. Figs. 4
and 5).
V. D
IELECTRIC BARRIER DISCHARGE
Dielectric barrier discharges are also called “silent” and
“atmospheric-pressure-glow” discharges [6], [54]–[56]. A
schematic of this source is shown in Fig. 12. It consists
of two metal electrodes, in which at least one is coated with
a dielectric layer. The gap is on the order of several mm, and
the applied voltage is about 20 kV. The plasma is generated
through a succession of micro arcs, lasting for 10–100 ns, and
randomly distributed in space and time. These streamers are
believed to be
100 m in diameter and are separated from
each other by as much as 2 cm [6], [56]. Dielectric barrier
discharges are sometimes confused with coronas, because the
latter sources may also exhibit microarcing.
Dielectric barrier discharges have been examined for several
material processes, including the cleaning of metal surfaces
[57] and the plasma-assisted chemical vapor deposition of
polymers [56] and glass films [58]. However, since the plasma
is not uniform, its use in etching and deposition is limited to
cases where the surface need not be smooth. For example, in
the study of SiO
deposition, it was found that the surface
roughness exceeded 10% of the film thickness [58].
Eliasson and coworkers [6], [54] have modeled silent dis-
charges and have concluded that the electron temperature
ranges from 1–10 eV. They have also simulated air and oxygen
discharges, and their results for pure oxygen at 760 torr are
shown in Fig. 13. The
axis is time following the ignition of
a single micro arc, and the
axis is the concentration relative
to the initial amount of oxygen
2 10 cm . Eliasson’s
model predicts that the electrons and ions exist for a very short
time, from 2–100 ns. These charged species produce O atoms,
which in turn react with oxygen molecules to produce ozone.
Beyond about 20 ms, the discharge achieves a steady-state
production of ozone of 0.1% of the initial oxygen density.
Silent discharges are efficient ozone generators, and this has
proven to be their principal industrial application [6].
VI. P
LASMA JET
Shown in Fig. 14 is a schematic of an atmospheric-pressure
plasma jet [7], [59]. This new source consists of two concentric
electrodes through which a mixture of helium, oxygen, and
other gases flow. By applying 13.56 MHz RF power to the
inner electrode at a voltage between 100–250 V, the gas
discharge is ignited.
![Fig. 16. Numerical simulation of the concentrations of species in the effluent of the plasma jet as a function of the distance from the nozzle in a 1.0% O2/He plasma at 760 torr [61].](/figures/fig-16-numerical-simulation-of-the-concentrations-of-species-14cbw1cs.webp)
![Fig. 14. Schematic of the atmospheric-pressure plasma jet for the deposition of silica films [59].](/figures/fig-14-schematic-of-the-atmospheric-pressure-plasma-jet-for-snly1w9p.webp)
![Fig. 13. Numerical simulation of the concentrations of species in a silent discharge in pure oxygen as a function of time at 760 torr and 300K [54].](/figures/fig-13-numerical-simulation-of-the-concentrations-of-species-1bb3ejfg.webp)
![Fig. 1. Breakdown potential in various gases as a function of the pressure and gap distancep d for plane-parallel electrodes [8].](/figures/fig-1-breakdown-potential-in-various-gases-as-a-function-of-2p9d0sdl.webp)
![Fig. 3. Schematic of the electron and gas temperature as a function of pressure in a plasma discharge at constant current [14].](/figures/fig-3-schematic-of-the-electron-and-gas-temperature-as-a-5jce978t.webp)
![Fig. 2. Current–voltage characteristics of a low-pressure DC glow discharge at 1 torr [13].](/figures/fig-2-current-voltage-characteristics-of-a-low-pressure-dc-2fzcbycm.webp)