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Voltage, current and electron density measurements in an air radiofrequency plasma

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Fixed pressure O.10 Torr. Fig. 14 Electron Density in air discharge. ... Fig. 4 and 5 show Current/Power and Voltage/Power graphs for 0.10 Torr Air discharge. ... – PowerPoint PPT presentation

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Title: Voltage, current and electron density measurements in an air radiofrequency plasma


1
Voltage, current and electron density
measurements in an air radio-frequency plasma
M. Sorokine, D. Hayashi, W.W. Stoffels, G.M.W.
Kroesen
Department of Physics, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands
IV RESULTS Two measurement series have been
carried out the 1st - Power variation, with
keeping the pressure constant (0.10 0.005
Torr), and the 2nd - Pressure variation with
constant amplitude of fundamental voltage
harmonic (100 Volts). On Fig. 13 and 14
calculated electron density values are presented.
Several spectrum peaks were examined during
preliminary experiments. Fig. 7 shows results for
4 of total 7 observed modes. Those that are not
presented showed no change in frequency within
the error of the measurements.
I.       INTRODUCTION Various RF discharges are
widely used in different kinds of production
technologies. Still, questions of quality
improvement often arise. That leads companies to
look for a way of improving and optimising
production processes. This cannot be done without
appropriate diagnostic techniques. Power input
control is really alluring because of its
simplicity in implementation. In most experiments
the main characteristic for the RF discharge is
its consumed power. Nevertheless, in most
theoretical works, the rf voltage is taken to be
constant, which makes comparisons between
experimental and theoretical difficult. Using the
technique mentioned above it is possible to get
the desired results. Microwave methods to measure
electron density are advantageous due to its time
resolution possibilities. Moreover, it is also a
non-intrusive in situ measurement technique. II
EXPERIMENT  The experiment has been carried
out with a 13.56 MHz capacitively coupled air
plasma, confined in an aluminium cylinder cavity
with two symmetrical slits. A schematic drawing
of the plasma chamber is shown on Fig. 1. The
dimensions are diameter 120mm, height 37mm,
slit width 10mm, distance between antennas
90mm, the lower power electrode diameter 107mm.
Fig.10. Gives a schematic drawing of the
experiment.
Fig. 4 and 5 show Current/Power and Voltage/Power
graphs for 0.10 Torr Air discharge.
Frequencies of the cavity modes were calculated
beforehand. As it was expected, the two series
(3036900 kHz and 3924600 kHz the two upper
lines), that were believed to correspond to
Transverse Magnetic Waves (TM) - TM110 and TM210
modes, showed the best agreement between each
other. They were also the best distinguishable in
the cavity spectrum observed and had the most
appropriate frequency for the calculated modes.
So, these two modes were chosen for the
experiments, presented in this work. Fig.8 and 9
show the spatial electric field distribution for
TM110 and TM210 3 within the cylindrical cavity
with dimensions mentioned above.
For the estimation of electric field value FE
has been calculated. As there is E2 under the
integral in the equation (2), obtained behavior
of the value FE needs to be compared to the one
of the E2 obtained from the theory. Taking into
account finite size of a plastic probe (10mm) a
following value has been calculated
It was noticed that by changing the discharge
pressure from 0.6 to 0.8 Torr the discharged
seemed to come to another state.   Also, it was
clearly seen from the graphs for the phase
difference between harmonics of current and
voltage (Fig.15), that in mentioned above
pressure range the value for the first and,
especially, for the third harmonics experiences
dramatic changes. The idea of monitoring certain
processes through the study of the phase
behaviour is not a new one 1 and can be very
promising in commercial production environment.
Fig. 18 presents impedance changes. ImpiVoltage
iRMS/CurrentiRMS Since the current in the odd
harmonics (3f, 5f etc) is low (Fig. 16), no
accurate Impedance can be calculated for those
frequencies. It can be noticed that the Impedance
for the third harmonics (imp3) experiences
dramatic change with pressure goes higher then 4
Torr.
Fig.11 and 12 show both FE and FEt see
equations (3) and (4) for the two different
planes in cavity.
These graphs strongly resemble those that one
would expect to see in case of simple electrical
circuit with direct current and constant
resistance. Though, it is seen that the curves do
not trend to zero, as it would be for a simple
resistor. Microwave technique The resonance
frequency of the cavity, which depends on the
number of free electrons within, is determined by
tuning the microwave generator to maximum
transmission. From the shift ?f f f0 of the
resonant frequency f with respect to its value in
vacuum (f0) the microwave field averaged electron
density (neo) is deduced 2
III.   DIAGNOSTICS A schematic drawing of the
measurements is shown on Fig. 2.
The lines with dots are the experimental curves,
and the thin solid lines are obtained from the
theory.
This method does not provide any spatial
resolution as ne0 is merely a space averaged
density weight with the square of the field
strength (E2),
It has been noticed that the behaviour of the
first harmonics of current, voltage and power
(Fig.16, 17, 18) is pretty much the same as the
behaviour of the electron density.
V ACKNOWLEDGEMENT This work is supported by
the European Commission under contract No.
NNE5-1999-0004 H-alpha solar. The research of
W.W. Stoffels has been made possible by a
fellowship from the Royal Netherlands Academy of
Arts and Sciences (KNAW). ------------------------
------------------------- 1 Kieran Dobbyn,
M.Sc. thesis, Design and Application of a Plasma
Monitor for RF Plasma Diagnostics. Dublin City
University, 2000. 2 E. Stoffels, W.W. Stoffels,
D. Vender, M. Kando, G.M.W. Kroesen, and F.J. de
Hoog. Phys. Rev. 51, 2425-2435 (1995) 3 J.D.
Jackson, Classical Electrodynamics Second
Edition, Wiley, p.335
Power input measurements Using a Plasma Impedance
Monitor (PIM) device by Scientific Systems we
obtain information on the amplitudes of voltage
and current for the driving frequency and first
four harmonics, as well as on the value of phase
shift between them. Along with that, power and
impedance are calculated. Fig. 3 shows the
principle of the PIM IV Sensor .
However, by combining results from several modes
some spatial information can bee obtained. Fig. 6
Represents spectrum of the cavity.
En experimental study of the electric field in
the cavity has also been carried out. If a piece
of plastic is placed within the cavity, then,
according to formula (1), the resonant frequency
will change its magnitude. And, the bigger the
field at the position of the probe, the bigger
the frequency shift. Hence, the space profile of
the microwave electric field within the chamber
cavity can be examined. Measurements have been
made in two vertical planes, perpendicular to
each other the plane with slits (slits plane),
and the antennas plane. A plastic probe was
placed at different places in the cavity.
Dimensions of the probe are 15mm x 15mm x 10mm.
For each probe position a resonance frequencies
for the modes have been found and then used in
further calculations.
A good agreement is seen from the graphs, though
for the middle of the cavity the experimental
electric field is higher than that obtained from
the theory. Probably this can be explained by
other harmonics contributing to the field in the
middle. Also, analysis of the mode structure up
to 5 GHz shows that the TM110 and TM210 modes are
the only ones that can fit the experimental curve.
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