22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
RF-LF dual frequency uniform dielectric barrier discharge for thin film
processing
F. Massines1, R. Bazinette2 and J. Paillol2
1
CNRS PROMES Processes, Materials Solar Energy, Perpignan, France
2
SIAME, Université de Pau et des Pays de l’Adour, Pau, France
Abstract: The increase of the frequency excitation from the kilohertz to the megahertz
range is a solution to drastically increase the power of a diffuse dielectric barrier discharge
and thus processing time. The next step is to polarise the substrate to try to independently
control the ionisation level and the ions energy. In this work, a usual plane-plane
symmetrical dielectric barrier configuration is used with a radiofrequency voltage applied
on one electrode and a low frequency voltage applied on the other one. Consequences on
the discharge behaviour and the properties of thin film deposited from SiH 4 are considered.
The analysis is mainly based on the discharge optical emission spectroscopy and kinetic
study and on the chemical and morphological observation of the thin film.
Keywords: Diffused DBD, homogeneous DBD, glow, atmospheric pressure discharge,
polarisation, radio frequency, DBD, Argon, ammonia, SiH 4
1. Introduction
Atmospheric pressure diffuse or homogeneous
discharges are streamer or microdischarge free
discharges. Low frequency diffuse dielectric barrier
discharges (DBDs), mainly Townsend or glow DBD,
have been abundantly studied [1]. The objective is to
develop linear DBD having a millimeter gap of great
interest for in line PECVD. Recently efforts have been
made to increase their power in order to increase the
efficiency of processes such as PECVD. As example,
these efforts lead to Glow like DBD [2], square current
Townsend DBD[3], radiofrequency DBD (RFDBD) [4] or
nanopulsed DBD. When modulated, these high power
discharges produce dense films of interest for various
applications like photovoltaics [5], barrier to oxygen or
water, etc…
This work is centered on radiofrequency DBD. When
diffuse, these discharge behavior is similar to that of a
low pressure radio frequency discharge in the alpha mode.
The main difference is the rather high ionization rate in
the gas bulk needed to maintain a constant ionization in
the atmospheric pressure collisional gas. Such capacitive
RF discharge at atmospheric pressure has been studied
through numerical modelling and experiments in helium
and argon [6].These studies shown that a dielectric on the
electrodes and a higher frequency [7] are helpful to
maintain the alpha mode.
At a first glance, the discharge power is a key parameter
according to thin film growth rate. However, the plasma
species making the thin films are created in a boundary
layer defined by the effective excursion distance of
plasma species during their lifetime [8] or during their
residence time in the plasma when it is shorter than their
life time [9]. The thickness of this boundary limit depends
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on the species life time or residence time in the plasma
compared to their diffusion or mobility. The typical value
is a few hundred of micrometer.
Because of the large difference between the electron
energy in the sheath border and in the gas bulk, in a RF
discharge, the power is not a so clear indication of the
density of reactive species created in the boundary layer.
This is enhanced because the boundary layer can be
shorter than the sheath while a non-negligible part of the
power is injected in the bulk. Chemistry occurring in the
sheath and ion extraction are key point according to thin
film processing. To try to modify this sheath and to
extract ions from the bulk, a low frequency sinusoidal
voltage is applied to the electrode supporting the substrate
while the other electrode is powered with a
radiofrequency power supply.
The aim of this work is to determine how the physics of
the homogeneous RFDBD is affected by the voltage
amplitude and frequency of the low frequency voltage
applied on the electrode supporting the thin film substrate.
Consequences on the plasma electrical and optical
characteristics as well as on the thin film properties are
presented. This study is made in Ar/NH 3 Penning mixture
and in Ar/NH 3 /SiH 4 .
2. Experimental set-up
Figure 1 is a schematic view of the experimental reactor
with two plasma areas defined by the two high-voltage
electrodes metalized on an internal face of alumina square
tubes. The ground electrode is on the other side of the
1mm gap. It is also covered with an alumina plate. The
metallization area is twice 14×50mm2 for high voltage
(HV) electrodes and 50×50mm2 for the low-voltage one.
A silicon wafer (CZ (100)) is used as a substrate to
1
Optical emission spectroscopy (OES) with long
exposure time (100 ms) is made with a Maya2000Pro
spectrometer which has a 14 µm square pixel size, an
entrance slit of 25µm and is coupled to an optic fiber of
600 µm with a collimator.
Two Hamamatsu photomultipliers (H10722-210 and 20) working from 300 to 500nm and from 500 to 800nm
respectively have been used to measure the variation of
the light intensity as a function of time.
Thin films are characterized by SEM, profilometer and
infrared absorption (FTIR).
3. Results
3.1. Discharge
800
600
Voltage amplitude (V)
produce SiO 2 coating from SiH 4 . There is no adding of
oxygen to the plasma made in an Ar/NH 3 Penning
mixture.
The discharge cell is in an airtight reactor. The ambient
air is removed with a primary vacuum, and the reactor is
filled with argon up to a pressure of 760 Torr. The Ar
flow rate is set to 6 l /min and the rate of NH 3 and SiH 4
are 280ppm and 67ppm respectively.
The waveforms used are sinusoidal voltages generated
by waveform generator connected to an amplifier. Since
the voltage at the output of the amplifier is not sufficient
to turn on the discharge, the amplifier output is fed into a
high-voltage transformer.
The low frequency (LF) polarization of the lower
electrode is obtained with an audio amplifier (Crest Audio
CC4000) coupled to a high-voltage transformer with a
large bandwidth. This set-up allows the amplitude (V LF )
and the frequency (f LF ) of the voltage to be independently
adjusted in a range from 0 to 700V pp and from 1 kHz to
100 kHz.
For the radio frequency (RF), 2 systems are used (i) a
13,56MHz Advanced Energy CESAR 600W power
supply with matching box, (ii) a power supply which has
the frequency resonance of a RLC circuit defined by the
discharge cell capacitor and the inductance of a
homemade transformer. The frequency is adjusted to
maximize the power injected and to minimize the power
reflected. A homemade transformer allows to work at 5
MHz with a voltage amplitude up to 1 kV. The amplifier
is a PRANA GN500. Standard values of voltage
amplitude (V RF ) are 840 and 340 V pp for 4,88 and
13,56MHz respectively.
400
200
0
-200
-400
-600
-800
-10
-8
-6
-4
-2
0
2
4
6
8
10
Time (µs)
Fig. 1. Voltage applied to the electrodes; V RF =800V pp
and f RF = 5.5 MHz, V LF = 600V pp and f LF = 50kz
8
Power density (W/cm3)
7
6
5
4
3
1kHz
10kHz
25kHz
50kHz
75kHz
100kHz
2
1
0
0
The discharge current is measured close to the LF
electrode with a current probe (Lilco LTD 13W5000
400Hz-100MHz), and the voltage applied to the
electrodes is measured on each electrode with a high
voltage probe Tektronic P6015A 75 MHz on the RF side
and Tektronic P6139A 500MHz the LF side. They are
connected to a digital oscilloscope Tektronix DPO4104 (1
GHz). The average discharge power (P) is determined
from the measurements of the two voltages and the
current over one cycle of the LF (T):
2
1
𝑇
𝑡+𝑇
∫𝑡
(𝑉𝐿𝐿 + 𝑉𝑅𝑅 ). 𝐼 𝑑𝑑
200
300
400
500
600
700
LF Voltage amplitude, VLF (Vpp)
Fig. 1. Discharge cell
𝑃=
100
Equation 1
Fig. 2. Power as a function of the LF voltage amplitude
for different value of the frequency
Measurements presented have been done as a function
of the LF frequency between 1 and 100kHz and amplitude
between 100 and 700V pp , for a 4.88 MHz voltage having
an amplitude ranging between 650 and 1150V pp . Fig. 1. is
a typical oscillogram of the voltage applied to the
electrodes i.e. V LF +V RF . Calculated power presented in
Fig. 2 illustrates that the power does not significantly
change with the LF amplitude or frequency.
The main emission of the RF discharge in Ar/NH 3 are
those of (i) c1Π–a1 D NH molecular band system at 324
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10
LF votlage
100kHz 600V
100kHz 100V
9
Plasma emission (a.u.)
8
fRF - fLF
fRF + fLF
80
fRF
60
40
20
4,8
4,9
5
5,1
Frequency (MHz)
Ar
5
Fig. 4 : Detail of the Fast Fourier Transform of the
photomultiplier measurements: fundamental of the RF
and the RF±LF
4
3
Continuum
2
1
0
600
700
800
900
1000
1100
1200
RF Votlage amplitude, VRF (Vpp)
1,0
Emission amplitude (a.u)
100
4,7
6
LF 1kHz
0,8
0,6
b
0,4
LF 50kHz
0,2
LF 100kHz
Continuum
Ar 696nm
0,0
0
100
200
300
400
500
600
700
LF Voltage amplitude (Vpp)
Fig. 3 : Ar (696nm) and Bremsstrahlung continuum
emission intensity normalized to the lower voltage value
as a function of a) the RF voltage amplitude, V RF , for two
100 kHz voltage amplitude b) the 1, 50, 100 kHz low
frequency voltages amplitude, V LF , for an amplitude of
the 5.5MHz voltage, V RF , equal to =840V pp – Gap=1mm
Fig. 3a presents the relative evolution of Ar and
continuum emission as a function of V RF for two values
of the V LF. Ar emission increases exponentially while the
continuum emission increases linearly. That observation
is in agreement with a large increase of the ionization at
the sheath border inducing a higher density of electrons in
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120
0
a
7
the gas bulk. The increase of the low frequency voltage
amplitude, V LF , shifts similarly the two emissions to
higher V RF showing that the V LF decreases the high and
low energy electron density. This can be explained by
electrons and ions drift to the surface reducing the ion
density in the cathode sheath and thus the electric field
gradient.
FFT amplitude (a.u.)
nm , (ii) A3Π–X3Σ NH molecular band system at 336 and
364 nm, (iii) c1Π–b1Σ NH molecular band system at
450nm NH A3Π–X3Σ− NH molecular band system at 330
and 336 nm, (iv) NH (v) Ar corresponding to transitions
between different levels of 3p54p (≈ 13 eV) and the four
levels of 3p54s (from 11.55 to 11.83 eV) (vi) a large
continuum related to Bremsstrahlung. The continuum is
observed in the gas bulk. Ar emission is due to high
energy electrons (≈ 13 eV), the continuum is due to low
energy electrons deceleration induced by interaction with
Ar nucleus. Thus, at a first glance it can be assumed that
Ar emission related to high energy electrons at sheath
boarder reflects the sheath behavior and the continuum
intensity depends on the electron-ions losses due to bulk
recombination or to the drift to the electrodes.
Fig. 3b presents the normalized amplitude or Ar and
continuum emissions as a function of the amplitude of
V LF for 3 frequencies: 1, 50 and 100 kHz. For 1 kHz, the
two emission are rather constant the slight difference in
the mean value is related to the incertitude on the
normalization value. For 50 kHz, the emissions decrease
slowly up to V LF =300V for the continuum and 350V for
Ar. For higher values, the decrease is more rapid. A
similar behavior is observed for 100 kHz, except the
voltage limit between the slow and the rapid variation
which are 250V and 450V for the continuum and Ar
emissions respectively.
To try to better understand these behavior, the kinetic of
the total light has been measured with a PM and its Fast
Fourier Transform (FFT) is analyzed. For 1 kHz,
whatever the amplitude of the low frequency voltage,
there is no FFT components related to the LF which is
surprising but in agreement with the emission variations.
For higher LF values, LF fundamental and harmonics pics
and RF ± LF pics (Fig. 4) are observed whatever the LF
voltage values. The observation of the plasma emission
FFT pics at RF ± LF indicates a coupling between the two
frequencies. According to Fig. 5, the amplitude of the
fundamental LF pic increases linearly with the amplitude
of V LF but it is not at all influenced by the RF amplitude
while the RF amplitude increase the FFT RF pic. The RF
± LF pics have the same behavior as the LF one with a
lower amplitude.
The amplitude of the plasma emission oscillating at the
LF is not affected by the RF voltage which drastically
increases the emission of Ar. On the other hand, the
amplitude of the RF emission is not affected by the LF
3
voltage. This seems in contradiction with the results of the
Fig. 3. However, Fig. 3 results have been obtained for a
1mm gap and Fig. 5 for a 3mm gap which can largely
affect the part of the gap from which the LF voltage
extract the ions. More measurements for 1 and 3 mm gap
including the FFT of Ar and continuum emissions are
necessary to really understand the consequence of the
dual frequency on the plasma.
0,25
VRF = 1.1kVpp
VRF = 0.9kVpp
FFT pic amplitude
0,20
f0 LF
0,15
f0 RF-f0 LF
0,10
0,05
f0 RF
0,00
0
100
200
300
400
500
600
700
800
900
LF votlage amplitude, VLF (Vpp)
Fig. 5. Variation of the PM measurements FFT pics
amplitude of the fundamental LF, RF and RF-LF as a
function of the low frequency voltage amplitude and for
two RF voltage amplitude (open and filled symbol).
LF=75kHz, RF=4.8MHz, gap=3mm.
amplitude increases. It shows that even when there is no
signature on the optical emission of the plasma like for 1
kHz, the LF voltage induces modification of the thin film
i.e. of the species interacting with the surface.
Infrared measurements (Fig. 6) show that the thin film is
an hydrogenated silicon oxide. However OH fully
disappeared in the intermediate range of the low
frequency voltage as it is illustrated in Fig. 7.
4. Conclusion
For the first time a kHz range sinusoidal voltage has
been applied on a DBD at the same time as a
radiofrequency one. The discharge is a radiofrequency
discharge having a large continuum emission
characteristic of the plasma bulk and Ar emission more
related to the high energy electrons at the sheath boarder.
The FFT of the light intensity appears like a powerful tool
to characterise the plasma.
The amplitude of the low frequency voltage important
but also its frequency. 1kHz has not a significant effect on
the plasma characteristics but induces a large amount of
thin film buckling showing that the coating stress is
important. For 4.8MHz and 1mm gap, the adding of a
voltage having a frequency larger than 20 kHz induces a
smother plasma. Another important conclusion is that,
according to FTIR, all OH can be removed which is not
so obvious with an atmospheric pressure PECVD.
5. Acknowledgements
The authors would like to acknowledge FUI and OSEO
for their financial support (Project :F11211007U)
3.2 Silicon oxide thin film
1,0
LF 50kHz 200Vcc
Without LF
0,8
a.u
0,6
0,4
0,2
0,0
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Fig. 6. Absorption Infrared spectra of thin film made with
and without LF voltage added to the RF one.
The morphology and the chemical composition of the
thin film are analyzed as a function of the low frequency
amplitude and frequency.
SEM pictures shows buckling structure on the thin film
which, for 4.8MHz, is not at all observed without the low
frequency voltage. Buckling occurs several minutes after
the end of the coating. It is attributed to a compressive
stress relaxation and increases with the amplitude of V RF.
It is more present for 1 and 100 kHz decreases for the
intermediate low frequency especially when V LF
4
6. References
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and N. Gherardi, Plasma Processes and Polymers 9 (1112), 1041 (2012).
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