EFFECT OF FREQUENCY EXCITATION ON A
HOMOGENEOUS DIELECTRIC BARRIER DISCHARGE IN Ar/NH3
AT ATMOSPHERIC PRESSURE
R. Bazinette1, J. Vallade1, S. Pouliquen1, R. Subileau2, J. Paillol2, F. Massines1
1
Laboratoire PROcédés Matériaux et Energie Solaire, UPR 8521, Tecnosud, 66100 PERPIGNAN, France
Telephone: +33(0)468682228; Fax: +33(0)468682213
Email: [email protected]
2
LGE, Université de Pau et des Pays de l’Adour
Abstract: The aim of this work is to study the transition between low frequency and
radio frequency regime of a homogeneous dielectric barrier discharge in Ar/NH3
mixture at atmospheric pressure. Frequencies of 50 kHz, 250 kHz, 550 kHz, 800 kHz
and 1.5 MHz are compared for a gas gap varying from 1 to 4 mm. Light distribution
is measured by short exposure time photographs synchronized with the excitation
frequency to determine the spatiotemporal distribution of the energy injected in the
gas gap. A transition occurs between 550 and 800 kHz: additional plasma zone
appears out of the interelectrode gap. These plasma zones need larger gas gap and
lower electric field to develop. This is confirmed by their disappearance when the gas
gap increases. Another consequence of the frequency increase is a modification of the
light profile from the anode to the cathode. For 50 kHz, the light is maximum close to
the cathode like in a sub-luminescent discharge, for 250 and 550 kHz, the light is
maximum at the anode like in a Townsend discharge while for 1,5 MHz, the light is
uniform in all the gap.
Keywords: homogeneous DBD, low and radio frequency excitation
1. Introduction
Amorphous hydrogenated silicon nitride (a-SiNx:H)
coatings are usually used as antireflective and
passivation films to improve silicon solar cells
efficiency. The most common technique used to
deposit a-SiNx:H is low pressure Plasma Enhanced
Chemical Vapor Deposition (PECVD). As working
at atmospheric pressure is an advantage for online
deposition, an Atmospheric Pressure Plasma
Enhanced Chemical Vapor Deposition (AP-PECVD)
is under development and used to deposit silicon
nitride on silicon solar cells.
Previously, the sinusoidal excitation frequency was
fixed to 50 kHz and a complete discharge study has
been done in Ar/NH3 to benefit of Penning mixture
and so obtain homogenous discharge [1]. Results
show that the discharge is a subnormal glow
discharge. SiN:H layer having the required refractive
index and homogeneity have been obtained.
However the deposition rate is limited by the
discharge power. The power cannot be increased by
voltage increase because of micro-discharges
formation that induces inhomogeneous plasma and
thus coating. The best solution to increase the power
must be to increase excitation frequency.
Moreover, this solution should also improve the
quality of the silicon nitride layer interface with the
silicon by reducing the ion bombardment of the
cathode.
In this framework, discharge properties at 50 kHz,
250 kHz, 550 kHz, 800 kHz and 1.5 MHz are
discussed. Measurements have been done at these
different frequencies and for different gas gaps.
After a description of the experimental set-up, long
exposure time photographs of the plasma are
presented to show the discharge homogeneity and
the energy distribution in the gas gap. Results
concerning discharge development are presented for
different gas gap. Finally optical emission spectra
are discussed to give a better understanding of the
discharge behaviour.
2. Experimental set-up
Hydrogenated silicon nitride is deposited in an
atmospheric pressure PECVD reactor based on a
homogeneous Dielectric Barrier Discharge (DBD)
technology allowing homogeneous plasma [1].
Figure 1 is a schematic view of the experimental
reactor with two plasma areas defined by the two
high voltage electrodes.
Figure 1: Schematic of experimental setup
The discharge is obtained in a cell made of two
dielectric square tubes metalized on an internal face.
These metallisation are high
igh voltage electrodes.
electrodes On
the other side of the gap, there is an alumina plate
metalized on its backside which is the ground
electrode. The size of metallization is 10x50
10x mm² for
top electrodes and 50x50 mm2 for the bottom one.
The gas flow is introduced
troduced between the two square
tubes, and controlled by mass flow controllers. The
Ar flow rate is fixed to 3 l/min. The NH3
concentration is fixed to 133 ppm. The electrical
circuit (Figure 2) has been modified to obtain the
excitation frequencies
ies of the order of hundred kHz.
Figure 2: Electrical
lectrical circuit
Figure 2: Electrical
al circuit
An amplifier provides power to the primary of a
transformer having an inductance, Lp. The secondary
is directly across the cell discharge.
discharge Its inductance,
Ls, is of few mH. The discharge cell can be modeled
by an electrical capacitor, CDBD ≈ 10 pF. That is an
oscillating circuit defined by a frequency:
fs =
1
2π Ls C DBD
The transformer use inductive coupling and consist
of two coils coupled with air. The main goal of this
electrical circuit is to use forced oscillation to
increase tension on gas mixture (Vs) and create the
plasma.
A voltage generator allows defining sinusoidal
excitation frequency (fa).. This frequency is adjusted
a
to fs to benefit of the resonance phenomena to
maximize power injected in the discharge. Voltage
applied on high voltage electrodes
electrode can be close to 1
kV and the discrete frequencies available are 250,
550, 800 and 1500 kHz. They
T
are defined by the
transformer coils.
The plasma is characterised by electrical and optical
measurements. The electrical measurements consists
of discharge current, measured with a current probe,
and voltage applied to the electrodes measured with
a high voltage probe. The plasma photographies are
done with an ICCD camera PI-MAX
PI
II Princeton
instruments. Optical emission spectroscopy (OES)
with long exposure time (100
(10 ms) is made with a
Maya2000Pro Spectrometers with a 14 µm square
pixel size and optic fibber of 600 µm.
3. Results
3.a Influence of the frequency on the
morphology
orphology and properties of the DBD
with a small gap (1 mm)
Photographs
hotographs of the plasma have been carried out
with long and short exposure time. The integration
over more than one cycle allows determining spatial
distribution of the mean energy injected in the
plasma, while 100 ns exposure
e
time gives
information about discharge development and
regime.. The DBD being a transient discharge, a
special attention is given to light distribution when
the current and the light are maximum because it
gives an indication on the maximum ionisation rate
reached by the discharge which determined the
discharge regime. With increasing frequency,
discharge morphology changes
change as shown in figure 3
which presents longs exposure time photographs of
the gas gap for the different frequencies.
frequencies
a)
b)
c)
d)
e)
Figure 3: Long exposure time (4 ms) photographs of the
discharge at: a) 50, b) 250, c) 550,
55 d) 800 and e) 1500 kHz
Whatever the frequency, the discharge is
homogeneous: no micro--discharge is observed.
However, the energy distribution in the gas gap
changes. For 50, 250 and 550 kHz the plasma zone
is limited to the front of each high voltage electrode.
The discharge length reduction between 50 kHz and
250 kHz can be attributed to a voltage decrease from
2 to 1,3kV. For 800 kHz and 1,5 MHz, other plasmas
appear beyond the space where electric field is
applied: one is between the two dielectric barrels
and one is on each side just out of the area where the
gas is confined. For 1,5MHz, the energy density
seems to be larger in these plasma zones even if the
electrical field is lower. It is also interesting to
compare the light distribution across the gas gap.
Results integrated over one half cycle are presented
in figure 4. The origin of the axis corresponds to the
alumina plate which is the cathode.
small compared to the voltage half period to
significantly drift from the anode to the cathode
during half cycle. For 250 kHz the period is of the
same order of magnitude as the time needed for an
ion to move from the anode to the cathode (4 ms
versus 5 ms). Since the beginning to the end of the
discharge the light is maximum in the anode side of
the gas gap. The electrode polarity still influences
the discharge development. This is no more the case
for 1,5MHz excitation. The light of plasmas in front
of each electrode is always maximum in the middle
of the gas gap while additional plasmas are stick on
the outer edge of dielectric square barrels. We
assume that is due to ions trapping. The duration of
half cycle is ten time shorter than the transit time of
ions from one electrode to the other one.
2800
3000
2400
2500
2000
2000
1600
anode
cathode
1200
1500
Intensity (u.a)
Intensity (u.a)
3200
Another point is that at 1500 kHz each additional
plasma is separated from the low emission zone. As
these additional plasmas develop in area where the
electrical field is lower but the gas volume is larger
we assume that the discharge development is limited
by low gas gap. So the effect of the gap on discharge
development is investigated.
1000
800
500
400
0
3.b Evolution of the 1,5MHz
morphology with gas gap
DBD
0
0,0
0,2
0,4
0,6
0,8
1,0
Position in gas gap (mm)
Figures 4: light distribution at the positive alternation in the gas
gap for 50 kHz (blue), 250 kHz (green), 550 kHz (red) and 1500
kHz (black)
This graph confirms that lower is the frequency,
higher is the light maximum between the two
electrodes. This is true whatever the current
alternation.
Three different light distribution profiles are
observed: for 50 kHz, the light is maximum at the
cathode, for 250 and 550 kHz it is at the anode while
a flat profile is measured at 1,5 MHz. According to
[1] the light profile can be related to the discharge
regime: light maximum at the cathode is associated
to a luminescent regime, characterized by a cathode
fall due to electric field deformation by ion
accumulation. Light maximum at the anode has been
associated to a Townsend regime in which the
electric field is quasi uniform across the gas gap and
the light is maximum where the electron density is
maximum i.e. at the anode.
From 250 kHz up to at least 550 kHz, the light is on
the side of the anode from the breakdown to the
current maximum. Like for 50 kHz, the breakdown
is a Townsend one but ions mobility becomes too
a)
b)
c)
Figure 5: Long exposure time (4 ms) photographs of the
1.5MHz discharge at: a) 2 mm, b) 3 mm and c) 4 mm
Figure 5 shows that larger is the gas gap, more
intense is the plasma in front of the high voltage
electrode. When the gas gap increases, first, low
emitted zone separating the different plasmas (figure
4) disappear, then the additional plasma zones
disappear. These observations confirm that for a
MHz excitation, the discharge development is
limited by the gas gap up to 4 mm. It is to be noted
that even for this gas gap value the energy transfer is
maximum in the gap middle. To have a better
understanding it is interesting to separate the
contribution of the different optical emission.
3.c Studies of excited species emission
A typical emission spectrum is shown figure 7.
Whatever the frequency, NH* at 324 nm and 336
nm, and Ar from 696 nm to 965 are observed. These
emissions correspond to:
- 324 nm: c1Π (5.42 eV) a1∆ (1.55 eV)
- 336 nm: a3Π (3.69 eV) X3∑ (0 eV)
- From 696 nm to 794 nm:: transition between
different level of 3p54p (≈ 13 eV) and the four
fo levels
of 3p54s (from 11.55 to 11.83 eV)
25000
Ar
Intensity (u.a)
20000
15000
NH
10000
but their localization are opposite.
Ar emission is maximum in between the electrodes,
where the electrical field is maximum which is in
agreement with the high electron energy necessary
to create the Ar* excited state NH* emission is
maximum in the additional plasma in agreement
with the fact that the energy needed to create the
NH* excited state is at least 2,5 lower than that of
Ar*. The continuum maximum also corresponds to
the positions of additional plasma showing that the
electron energy involved is low. Its amplitude
increases with the gap while the amplitude of all
peaks of argon decreases. In conclusion, even if the
light emission is more intense in the additional
plasma,, the electron energy is larger between the
two electrodes. For a 1,5MHz excitation and a 1mm
gas gap, the electron density is probably larger in the
additional plasma but their energy is lower.
5000
4. Conclusion
200
400
600
800
1000
Wavelenght (nm)
Figure 7: Optical
ptical emission at 250 kHz and 1 mm gas gap.
A broad UV-Vis-NIR continuum is also observed for
high frequency excitations. Itt extends from 300 to
950 nm, with maximum amplitude around 500 nm.
It is always observed for 1.5 MHz excitation and its
contribution increases with the gap while it is never
observed for frequency of 250 kHz or lower.
lower For 550
kHz it appears for gap larger than 2 mm. Figure 8
compare its amplitude to those of NH* and Ar*
emission as a function of the position along the gas
flow.
NH 324 nm
Ar 696 nm
Continuous spectrum
5100
500
4800
400
300
4200
200
3900
Intensity (u.a)
Intensity (u.a)
4500
100
3600
3300
0
15
20
25
30
35
40
45
50
Optical fiber position (mm)
Figure 8: Intensity of the continuous spectrum (blue) and
emission of Ar (red) and NH (green) for 1500 kHz and 1 mm.
The brown squares represent the dielectric bars.
The first remark is that the amplitude of Ar* and
NH* emission are of the same order of magnitude
The influence of the gas gap and the frequency on
the behavior of a homogeneous DBD in Ar/NH
A
3 was
studied. Photographs with a long exposure time have
shown that whatever the frequency, the discharge is
homogeneous and free of micro discharges. For a 1
mm gap, when the frequency increases from 50 to
250 kHz,, the discharge turns form a sub-luminescent
sub
discharge to a Townsend regime.
regime When it increases
from 500 kHz to 800 kHz it turns from low
frequency to radiofrequency regime. These
evolutions are related to ion trapping. One of the
radiofrequency regime characteristics is that the
ionization level is largely limited by the gap. If the
gas gap is too small additional
dditional plasmas areas
develop beyond the area where the electrical field is
applied,, in area where the distance between the two
dielectrics is larger. The occurrence of these
additional plasmas is related to the apparition of a
large continuum
um in the emission spectra.
5. References
[1] F. Massines, et al, Eur. Phys. J. Appl. Phys.
47(2), 22805, (2009)
[2] J. J Shi and M. G. Kong in Applied physics
letters 90, 111502 (2007), Radio-frequency
Radio
dielectric
barrier glow discharges in atmospheric argon
[3] Steven M. Adler-Golden,
Golden, in J. Phys. Chem.
1989,93, 691-697 The NO+O and NO+O3 reactions.
Analysis of NO2 continuum chemiluminescence
chemiluminesce
The authors thank AIR LIQUIDE and the French
Agency for Environment and Energy Management
(ADEME) for their financial support.