Chemistry Reference
In-Depth Information
6.1 PROBE MEASUREMENTS
6.1.1 O
VERVIEW
An electrical single or Langmuir probe is a small auxiliary electrode immersed into
the plasma, the voltage current characteristic of which is measured with respect to a
big reference electrode, the chamber wall, or one of the discharge electrodes. Posi-
tive or negative charge carriers (positive ions, electrons, negative ions) are attracted
or repelled by the probe depending if the probe bias
V
is lower or higher than the
potential
V
s
of the probe surrounding plasma.
V
s
is called space or plasma poten-
tial. The currents of the different charge carriers depend on the plasma parameters.
Therefore the evaluation of the characteristic gives internal plasma parameters as
floating and plasma potential, electron temperature, electron and ion density, mean
electron energy and electron energy distribution function (EEDF) in spatial and
temporal resolution. Monitoring of these parameters during a technological plasma
process is of great importance to enhance reproducibility and process understanding,
optimization, and control. Langmuir probes are well established in plasma diagnos-
tics since tens of years [1]. There are some excellent reviews on probe diagnostics,
such as [2-12].
Usually electric probes are of spherical, cylindrical, or plane geometry, see
Figure 6.1. Because of the simple assembly cylindrical probes of few mm length are
most commonly used. Usual probe materials are tungsten (high melting point) and
platinum (high chemical resistivity). A possible probe circuit is shown in Figure 6.2.
Here PMD is a probe measuring device performing the generation of the probe bias
as a staircase voltage and the probe current measurement. A computer controls the
measuring procedure, stores the measured current and voltage values, and processes
the acquired data.
The use of Langmuir probes in chemically active plasmas is restricted because
of etching of the probe material or deposition of the probe surface with insulating or
metallic layers. The first problem especially occurs in fluorine-containing media and
it may be solved using platinum as probe material; in chlorine containing plasmas
tungsten may be used [13]. The second problem may be overcome or at least reduced
1
2
r
p
p
2
r
p
FIGURE 6.1
Probe shapes.
