Characterization of a large scale RF CCP reactor using Langmuir and
derivative probes
Saša Lazović, Nevena Puač, Kosta Spasić, Gordana Malović and Zoran Lj. Petrović
Institute of Physics, Pregrevica 118,11080 Belgrade, Serbia
University of Belgrade, Studentski trg 1, 11080 Belgrade, Serbia
Abstract: A large scale cylindrical CCP reactor was developed as a prototype of
an industrial device, aiming to show that continuous plasma treatment of textile
rolls at low pressures is possible. Discharge is large in volume (~3 m3),
homogeneous and free from transitions to streamers. Treatment effects are
strongly dependant on the distance between the central electrode and the textile
sample. Powered electrode is made of aluminum (1.5 m long) while chamber
walls are the grounded electrode. In this type of asymmetric discharge,
concentrations of ions coming to the surface are decreasing when samples are
placed further away from the powered electrode. Langmuir probe (Hiden
ESPION) which was placed side-on was used to perform measurements of the
spatial profiles of ions and electrons. Current and voltage derivative probes are
used to obtain U-I characteristics as well as power delivered to the plasma,
providing useful information on plasma operation mode. Measurements are
performed in Argon at 400, 600, 800 and 1000 mTorr, for several distances from
the powered electrode and for different powers delivered to the plasma.
Keywords: large scale asymmetric cylindrical RF CCP reactor, Langmuir probe,
current and voltage derivative probes
1. Introduction
Radiofrequency discharges are necessary for
treatment of isolators and semiconductors [1].
Different kinds of conductive and non-conductive
materials like microelectronics devices [2-4],
biological samples [5] and textiles [6] can be treated
using capacitively coupled RF plasmas. In our
laboratory a large scale CCP RF reactor was
developed in order to cheaply and uniformly treat
textile rolls without damaging the surface of the
fibers. Homogeneous and stable plasma, without
transition to streamers, capable of long term stable
operation (i.e. treatments) was achieved. Detailed
electrical characterization of the plasma reactor is
however required because it can provide information
on the relations between external discharge
properties (current and voltage waveforms,
impedance) and plasma parameters (densities,
energies, fluxes of charged particles).
Textile samples can be placed in the chamber on
several distances from the powered electrode
providing various intensities of treatment. Langmuir
probe measurements are performed in argon, for
different distances, powers and pressures of the
working gas. Ion and electron concentrations are
obtained at the places were textile sample would be
placed. These measurements show complex spatial
dependences of the concentrations and are important
for proper characterization of treating procedures.
Current-voltage properties and power delivered to
plasma can be measured using derivative probes.
2. Experimental setup
The discharge chamber is 2.5 m long and 1.17 m in
diameter and made of stainless steel. Powered
electrode is placed axially in the centre of the
chamber and is 1.5 m long, 3 cm in diameter and
made of aluminium. The chamber has a platform at
the bottom where samples are placed. The distance
between the platform and the powered electrode is
adjustable by moving the platform. Distances for the
Langmuir probe measurements were chosen within
this range. Outer chamber wall is the grounded
electrode. The rest of the electrical circuit consists of
RF power generator Dressler Cesar 1010 in
combination with Variomatch matching network.
Derivative probes are placed into a stainless steel
box opposite to each other. The box is placed as
close as possible to the powered electrode. Low
pressures are maintained using mechanical vacuum
pump with a constant flow of gas air (see Fig.1).
Figure 1. Experimental set-up: (1) Chamber, (2) Powered
electrode, (3) ESPION system, (4) Current probe, (5) Voltage
probe, (6) Variomatch, (7) Power supply, (8) Oscilloscope, (9)
Computer
Instantaneous voltages and currents are monitored
using derivative probes which were connected to the
oscilloscope with cables of identical length. All
waveforms are collected by the computer for further
analysis.
Hiden Analytical ESPION advanced Langmuir
probe system is placed side-on. The system has a
linear motion drive which enables probe positioning
with the minimal spatial resolution of 0.1 mm.
Measurements were made in the range of pressures
from 400 mTorr to 1 Torr. We have used platinum
probe tip, 5 mm long and 0.15 mm in diameter.
Linear motion drive was used to position the probe
at the distances 50.5 cm to 20.5 cm from the
powered electrode. Measurements of U-I curves
were made for all those positions of Langmuir
probe.
At every position 50 measurements were made
each being an average of 10 scans with pre-cleaning
for each measurement. It was observed that even
with pre-cleaning it is better to neglect the first few
measurements because of the probe contamination
until results become stable. After that, the U-I curves
were smoothed and data was processed using Hiden
ESPSoft.
3. Results and discussion
3.1. Derivative probes measurements
Waveforms acquired by current and voltage
derivative probes are further processed using Fast
Fourier Transform procedure. Signals are then
calibrated in the frequency domain and converted
back to time domain using Inverse Fast Fourier
Transform showing the real signal shapes. Figure 2.
shows current and voltage signals after numerical
procedures (for Argon at different pressures 400,
600, 800 and 1000 mTorr). Power at RF generator
was 200 W (forward minus reflected power).
3.2. Langmuir probe measurements
Figure 2. Derivative probe measurements of a) current and b)
voltage waveforms for Argon discharge at 400, 600, 800 and
1000 mTorr. Power at the RF generator was 200 W.
Langmuir probe (Hiden ESPION) was placed
perpendicular
to
the
powered
electrode.
Measurements were performed for distances of
20.5 cm, 30.5 cm, 40.5 cm and 50.5 cm from the
powered electrode in Argon at 400 mTorr and
1000 mTorr and powers at RF generator in range
from 100 W to 300 W. At the lowest pressure
(400 mTorr) both electron and ion concentrations are
slightly lower than at 1000 mTorr (compare Figure
3. a) and b) ). Electron concentrations are almost
constant for all probe positions (see Figure 3.), while
ion concentrations are decreasing by more than an
order of magnitude as probe is placed closer to the
grounded chamber wall (40.5 cm and 50.5 cm).
1E18
1E17
1E17
-3
Ni [m ]
-3
1E16
1E16
1E15
20
25
30
35
40
45
50
1E15
55
1E18
1E18
1E17
1E17
1E16
1E16
-3
Ne [m ]
distance from the powered electrode [cm]
Ni [m ]
-3
Voltage waveforms clearly indicate a presence of
higher harmonics, especially at 400 and 600 mTorr.
At higher pressures, waveforms become more
sinusoidal. Generation of higher harmonics is due to
geometrical asymmetry of the discharge chamber
and due to the nonlinear nature of plasma
impedance. In this configuration, having grounded
electrode with a large area, and metal platform at the
bottom for placing the samples, current paths can be
very different in different parts of the chamber [7].
More detailed derivative probe and Langmuir probe
measurements in different current branches would
prove useful in an attempt to determine equivalent
discharge chamber circuit, putting more light on the
links between external and internal plasma
parameters. Our main objective was to obtain
homogeneous and stable plasma, find optimal
treatment conditions and provide reproducible
treatments based on electrical measurements. Power
delivered to plasma, V-I characteristic and
impedance of the discharge can also be calculated.
1E18
Ne [m ]
Current signals are in the range of 5.5 A to 8.5 A
peak to peak, and voltage is ranging from 130 V to
200 V peak to peak. At lower pressures (400 and
600 mTorr) current waveform has a saw tooth like
shape and at higher pressures (800 and 1000 mTorr)
it becomes more sinusoidal.
1E15
1E15
20
25
30
35
40
45
50
55
distance from the powered electrode [cm]
Figure 3. Electron and ion concentrations in Argon discharge at
a) 400 mTorr and b) 1000 mTorr. Power at the RF generator
was 300 W for both pressures.
Treatment effects of different kinds of materials
strongly depend on ion concentrations and energies.
We can see that by placing the samples at different
positions from the powered electrode we can control
the concentrations of ions coming to the sample
surface and therefore achieve different treating
effects. Fine ion concentration adjustment in a range
of almost two orders of magnitude (from 1e15 to 1e17
m-3) can be achieved by precise positioning of the
samples without changing power or pressure.
4. Conclusion
A large scale asymmetric RF CCP reactor at
13.56 MHz has been diagnosed by using derivative
and Langmuir probes. Working gas was Argon and
measurements by using derivative probes were
committed for pressures of 400, 600, 800 and
1000 mTorr. Presented Langmuir probe results are
obtained at 400 and 1000 mTorr for the power of
300 W given by the RF generator. Electron and ion
concentrations were measured for several distances
from the powered electrode (20.5, 30.5, 40.5 and
50.5 cm). It was found that shape of the current and
voltage waveforms is changing with changing the
gas pressure due to changing in plasma impedance
and generation of higher harmonics. Current signals
are in the range from 5.5 A to 8.5 A and voltage
from 130 V to 200 V peak to peak.
For proper plasma treatment characterization and
reproducibility, electron and ion concentrations were
measured. Electron concentrations are found to be
almost constant with changing the distance between
the Langmuir probe and the powered electrode. Ion
concentrations are changing in the range from 1e15 to
1e17 m-3 and are decreasing rapidly when moving
away from the powered electrode. Changing of the
distance between the sample and the powered
electrode, ma be used to control ion concentrations
and treatment of the surfaces.
References
[1] A. Bogaerts et al. / Spectrochimica Acta Part B
57 (2002) 609–658
[2] M.A. Lieberman, A.J. Lichtenberg, Principles of
Plasma Discharge and Materials Processing, (2005)
(Wiley:Hoboken);
[3] T. Makabe, Z.Lj. Petrović, Plasma Electronics,
(2006) (Taylor and Francis:New York);
[4] U. Cvelbar, K. (Ken) Ostrikov and M. Mozetic,
Nanotechnology 19 (2008) 405605.
[5] N. Puač, Z.Lj. Petrović, S. Živković, Z. Giba, D.
Grubišić and A.R. Ðorđević, Plasma Processes and
Polymers, (2005) 193, (Whiley)
[6] M. Radetić, P. Jovančić, N. Puač and Z.Lj.
Petrović, Workshop on Nonequilibrium Processes in
Plasma Physics and Studies of the Environment,
SPIG 2006, Journal of Physics: Conference Series
71 (2007) 012017
[7] M. A. Sobolewski, IEEE Transactions on Plasma
Science 23 (1995) 1006