Emissive Probe Measurements in the DC Low Temperature

A. Marek1, R.P. Apetrei2, S.B. Olenici2,3, R. Gstrein3, I. Picková1, P. Kudrna1,
M. Tichý1, R. Schrittwieser3
1Charles University in Prague, Faculty of Mathematics and Physics – V Holešovičkách 2, 18000 Prague, Czech Republic
2Al. I. Cuza University, Faculty of Physics, 11 Carol I Blvd., RO-700506, Iasi, Romania
3Institute for Ion Physics and Applied Physics, Leopold-Franzens University of Innsbruck,
Technikerstr. 25, A-6020 Innsbruck, Austria
Abstract. We report on emissive probe measurements in a cylindrical magnetron. Our contribution consists of two parts:
In the first part we focus on the emissive probe technique, particularly on the experimentally observed changes of the
electron saturation current of the emissive probe current-voltage characteristic caused by the variation of the probe heating
current. We propose two possible causes for the observed changes of electron saturation current: (a) the space charge
around the probe shaft, (b) the change of the work function of the probe material due to heating. We have tried to find
sufficient experimental evidence in the cylindrical magnetron plasma to make a decision between the two abovementioned
causes. The second part of our contribution is devoted to systematic measurements of radial plasma potential
profiles of the magnetron discharge carried out by emissive probes. They were aim at complementing and supporting the
results of Langmuir probe measurements. The emissive probe measurements confirm the plasma potential profiles derived
from Langmuir probe data that showed relatively narrow anode and cathode fall regions and a broad region of positive
column in between.
Keywords: Emissive Probe, Cylindrical Magnetron, Particle In Cell.
PACS: 52.25.Xz, 52.65.-y, 52.65.Rr, 52.70.Ds
INTRODUCTION
Emissive probes are used for direct measurements of the plasma potential and its fluctuations in various types of
plasma – e.g. [1,2,3]. These quantities cannot be directly obtained by Langmuir probes since they allow direct measurement
of the floating potential only. However, the floating potential of an emissive probe approaches the plasma
potential due to the influence of thermionic electrons flowing from the probe towards the plasma at sufficient probe
heating. Emissive probe current-voltage characteristics differ substantially from cold probe characteristic, especially
in the region of the ion saturation current, since the electron emission current superimposes on the ion saturation current.
According to simple theory [4], only the current in the electron-retarding region of the characteristic should increase
due to the electron emission. The electron saturation current should remain unaffected because the emitted
electrons are reflected back to the probe when the probe voltage becomes positive with respect to the plasma potential.
However, changes in the electron saturation current of the emissive probe with the variation of the probe heating
current were frequently observed in experiments – e.g. [5]. That is the reason why we focus the first part of our work
on this phenomenon which, to our knowledge, has not yet been systematically studied and explained.
In the second part of our contribution we report on systematic measurements of radial plasma potential profiles in
the cylindrical magnetron discharge performed by emissive probe that were aimed at complementing and supporting
the results of Langmuir probe measurements. The motivation for this work were discrepancies between the results of
our 2D PIC-MCC simulations of the magnetron discharge [6] and Langmuir probe data, particularly concerning the
plasma potential profile.
EXPERIMENTAL SETUP
Cylindrical magnetrons are used for plasma generation and thin film deposition. Our experimental system is described
in detail e.g. in [7] and its schematics are shown in Fig. 1. The outer cylindrical discharge electrode has
58 mm diameter and serves as anode. The coaxially positioned cathode has a diameter of 18 mm. The discharge region
is limited to a length of 300 mm by a pair of limiters on the cathode potential. Our magnetron operates in rare
gases at pressures of typically 1 – 10 Pa. The magnetic field is created by coils and can be varied up to 40 mT. It is the order of 1016 m-3. The presented experiments were performed in argon.
The construction of the emissive probes is described in detail e.g. in [8,9,10].
RESULTS OF THE EXPERIMENTS
Electron Saturation Current of the Emitting Probe
An example of the current-voltage characteristic of an emissive probe for different probe heating current is
shown in Fig 2(a). Discharge conditions and probe parameters are given in the figure caption. The stability of the
discharge conditions was monitored during the experiment by the floating Langmuir probe that was placed in the
middle port of the magnetron. In order to characterize the changes of the electron saturation current we sorted out the
electron saturation current at a fixed voltage with respect to the plasma potential and plotted it vs. the probe heating
current in Fig 2(b). In order to suppress the noise, we used a linear fit in the vicinity of the chosen value of the Radial Profiles of the Plasma Potential
in the Magnetron
Radial profiles of the plasma potential in
the magnetron device were measured by
emissive probes in order to support the
Langmuir probe data. Fig. 5 shows the results
of the emissive probe measurements
for different discharge conditions. It is
worth noting that for all discharge conditions
the plasma potential profile has approximately
the same shape and differs
only in the extension of the positive column. We obtained profiles with extended regions of the positive column but
narrow regions of the anode and cathode falls. This is consistent with our former Langmuir probe measurements [7].
In contrast to that the plasma potential profiles derived from our PIC-MCC model of the magnetron [6] show
relatively broad regions of the anode fall which even protrude up to one third of the discharge area. Data were obtained
in simulation with grid dimensions comparable to the Debye length, but with only approx. 2 superparticles per
cell.
CONCLUSION
We performed emissive probe measurements in a low temperature magnetized plasma in a cylindrical magnetron.
We tried to find sufficient experimental evidence to explain the changes in the electron saturation current caused by
the variation of the probe heating current. From our experiments we were unable to draw clear conclusions on the
causes of this phenomenon and further measurements are needed for its clarification. Measurements of radial profiles
of the plasma potential in the magnetron proved our former Langmuir probe data.
ACKNOWLEDGMENTS
The work was supported by Czech Science Foundation, grants 202/03/H162, 202/06/0776 and 202/04/0360, by
the Ministry of Education, Youth and Sports, Research plan MSM 0021620834, by the Austrian-Czech Scientific-
Technical Collaboration project A-14/2004 and by EURATOM.
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