O/O3 source for Oxide growth at low pressure
Miaomiao Zhang, Xiaoxi Duan and Jiting Ouyang*
School of Science, Beijing Institute of Technology, Beijing 100081, China
Abstract: The active O atoms and O3 molecules are very helpful to improve the
growth rate of oxide film in deposition system which operates at typical pressures
of 0.01 to 1 Torr. We designed a plasma reactor based on dielectric barrier
discharge. The configuration consists of a cylindrical tube and a pair of
electrodes. The driving power is a sinusoidal voltage at audio-frequency and the
system can operate in coplanar discharge mode. Experiments were performed in
pure oxygen at low pressures. The dielectric barrier discharge shows a typical
glow discharge with a single pulse on the current waveform. The chemistry
species reach their equilibrium concentration at given discharge conditions.
Keywords: dielectric barrier discharge, glow discharge, O/O3 source, low
pressure
* Corresponding author; Email: [email protected]
1. Introduction
ZnO has become one of the most important oxides
attract researchers’ eyes in recent years. A popular
measure to prepare the different kinds of oxide
structures is by using a plasma-assisted deposition
system with oxygen as one of the feeding sources, to
deposit or sputter the ZnO films or layers [1-4] on
various substrates. The growth rate of the film is a
key parameter for any deposition or sputtering
system. The oxygen radicals such as O atoms (O3P,
O1D) and O3 molecules will be helpful to improve
the growth rate due to the high activity comparing
with the original O2 molecules. For this purpose, one
can supply O/O3-riched gas environment for the
growth of oxides in the plasma-assisted system. An
available way is to dissociate the O2 molecules by
discharge before it being injected into the deposition
system. To control the species concentration, it is
better to run the discharge system after the oxygen
injected inside the deposition system. In most cases,
the deposition system operates at low pressures. This
requires the discharge system can work at low
pressure in the gas-inlet. The dielectric barrier
discharge (DBD) is very suitable for O/O3
generation at any pressures, which has been used in
many industrial applications [5]. In this paper, we
designed a DBD reactor to produce O/O3 at low
pressure.
2. Experimental set-up
The experimental set-up is shown in Fig.1.
Spectroscope
Glass tube
1.5 cm
electrode
2 cm
10 cm
Power
R
oscilloscope
Figure 1. Experimental arrangement.
The DBD reactor consists of a 20 cm-long glass tube
with 2 cm in inner diameter. The thickness of the
tube is 1 mm. The filling gas is pure O2 at 0.1, 0.5
and 1 Torr. The ring-shaped electrodes are outside
the tube with length of 1.5 cm and a long distance of
10 cm. A sinusoidal voltage is applied between the
two electrodes. The applied voltage is measured by a
high-voltage probe (Tektronik P6015A, 1000× 3pF,
100 MΩ) and monitored with a Tektronik
oscilloscope (TDS 3054B). The discharge current is
sampled by the voltage VR dropped on a non-
inductive resistance R, or I = VR/R. An optical
emission spectroscopy (OES, Zolix Omni λ-5008) is
used to record the light emission and the spectral
lines from the discharge plasma. A digital camera
(Canon A950) is used to record the plasma image of
the discharge.
3. Results and Discussions
The DBD under this condition is glow discharge,
with one current pulse in each half period. The
typical current-voltage waveforms are shown in
figure 2 at pressure of p = 0.5 Torr and peak-to-peak
voltage of VS = 1600 V and frequency of f = 35 kHz.
The discharge current has a long rising time of 2 μs
and duration of 10 μs.
U
I
VS (V)
1000
6
4
500
2
0
0
-2
-500
I (mA)
-4
-1000
-6
0
10
20
30
40
50
t (μs)
Figure 2. Typical current and voltage waveform at 1600 V/ 35
kHz, the gas pressure is 0.5 Torr.
the voltage increasing, as shown in figure 3. The
stratified structure of the discharge channel is very
clear at lower voltage, but will tend to become
homogenous without obvious striations at very high
voltages.
The discharge current and the plasma image at other
pressures (i.e., at p = 0.1 and 1 Torr) are similar to
the above one. In the present coplanar configuration,
the gas pressure can be as low as less than 0.01 Torr
to sustain a stable DBD in our experiments. This can
satisfy the pressure condition for the some plasmaassisted oxide deposition system which operates at
very low pressures.
We also tested the facing electrode configuration in
experiments. In this case, the two electrodes are in
arc-shape. The gap is near the diameter of the glass
tube. The discharge in this condition is just similar
to the traditional planar DBD. But the gas pressure
should not too low to operate. When we reduced the
pressure below 0.05 Torr, it is difficult to turn on the
discharge.
The discharge power deposited into the coplanar
DBD reactor increases almost linearly as the voltage
increasing, as shown in figure 4. The power is
calculated by integrating the production of the
current and the voltage in each half period.
1.6
The time-integrated light emission from sideview is
shown in figure 3.
1.4
P (W)
1.2
0.1Torr
0.5Torr
1Torr
1.0
0.8
0.6
0.4
0.2
1400 1600 1800 2000 2200 2400 2600 2800
Figure 3. DBD plasma images at voltage of 1600 V (upper) and
2600 V (bottom), the exposure time is 1 s.
It is seen that the images of the DBD light emission
show typical glow discharge, with bright negative
glow regions near the electrodes and the weak
positive column in the center. It is noticed that the
stationary striations appear in the long positive
column. The striation spacing becomes shorter when
VS (V)
Figure 4. Discharge power changing with applied voltage at
different pressures.
From figure 4, the power consumption is generally
low in this DBD system, with value being less than
1.5 W. The discharge power depends on the gas
pressure in this long-gap coplanar DBD. At the same
voltage supply, a larger energy deposition will be
obtained power value at higher pressures. This is
different from the conversional DBD whose energy
deposition is generally determined by the electrode
area and the applied voltage.
We measured the emission intensity of O*(777.4 nm)
and O*(844.6 nm) at various applied voltage in the
negative glow and the positive column, as shown in
figure 6.
The emission spectrums of the O2 plasma from
negative glow and positive column measured by
OES are shown in figure 5.
Intensity (a.u.)
700
1200
(a)
Intensity (a.u.)
1000
800
0.1Torr
0.5Torr
500
400
300
1Torr
200
0
O
0.1Torr
0.5Torr
1Torr
1400 1600 1800 2000 2200 2400 2600 2800
400
0
VS (V)
O
300
400
500
600
700
Wavelength (nm)
250
7
800
-Solid 777.4nm
-Opened 844.6nm
(b)
0.5Torr
6
300
Intensity (a.u.)
(a)
600
600
200
-Solid 777.4nm
-Opened 844.6nm
100
Intensity (a.u.)
800
(b)
200
5
1Torr
4
0.1Torr
3
2
0.1Torr
0.5Torr
1Torr
150
1
100
0
1400 1600 1800 2000 2200 2400 2600 2800
VS (V)
O
50
O
0
300
400
500
600
700
Figure 6. O atoms emission intensity in (a) the negative glow
and (b) the positive column at different voltages.
800
Wavelength (nm)
Figure 5. Emission spectrum from (a) the negative glow and (b)
(b) the positive column under conditions of figure 2.
The characteristic spectral lines include the typical
oxygen radiation of O*(777.4 nm, 3p5P Æ 3s5S
transition) and O*(844.6 mn, 3p3P Æ 3s3S
transition). This confirms that the O atoms are
produced in the O2 plasma. The emission spectral
lines from the negative glow are not the same as the
positive column. For the O atoms radiations
(O*(777.4 nm) and O*(844.6 nm)), the emission
intensity is generally much stronger in the negative
glow than in the positive column.
Both the O*(777.4 nm) and O*(844.6 nm) increase
their intensity with the applied voltage in the
negative glow (see figure 6(a)). This is reasonable
that at higher voltage, the discharge becomes
stronger, and more O atoms will be dissociated in
the channel. However, the intensity keeps almost
constant as the applied voltage increasing at a given
pressure (see figure 6(b)).
Since the oxygen plasma will be injected slowly into
the deposition region which is the downstream of the
discharge area and has only a weak field there, the
chemistry process of the species is expected to be
similar to that in the positive column. Then, the
density of the active species in the positive column
might be approximately equal to that of the
equilibrium density in the downstream area.
Although the radical concentration is hard to be
determined in experiments, this result suggests that a
lower voltage might help to achieve higher energy
efficiency.
4. Conclusions
We have designed a long-gap coplanar DBD
reactor to produce oxygen radicals (O and O3 etc)
for the plasma-assisted oxide deposition system. The
system can operate in the large range of low
pressures in pure oxygen to satisfy the pressure
condition of the deposition system. The coplanar
DBD is a typical glow discharge. The deposited
power into the DBD reactor is linear to the applied
voltage, and also relates to the gas pressures. It is
suggested that this coplanar DBD might achieve a
higher energy efficiency of O/O3 generation at a
lower voltage.
Acknowledgement
This work was supported in part by the National
Science Foundation of China under Grant No.
10875010.
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