Chemistry Reference
In-Depth Information
potential
V
V
s
. Remark: The term for α given in [83] is not correct. For a deeper
analysis taking into account the change of the entrance velocity of the positive
ions into the probe sheath at
V
−
<
V
s
on replacing the electrons by negative ions
look [72,85-87].
The evaluation described in Section 6.1.2, especially the determination of tem-
perature and density of electrons according to (6.2) and (6.3), may be performed also
at presence of negative ions if the thermal current
i
−
0
of negative ions is much lower
than the thermal electron current
i
e
0
, that means at (
V
−
: temperature of negative ions
in voltage units)
V
−
m
e
V
e
m
−
1
/
2
n
N
e
1.
(6.8)
The portions of negative ions and electrons may be separated sometimes in the second
derivative of the probe characteristic and so temperature and density of negative ions
may be determined directly [72,83,86,88]. But it has to made sure that no rf-distortion
and probe contamination occur [61]. Assuming Maxwellian energy distribution func-
tions for electrons and negative ions the ratio of the portions at
V
=
V
s
is given by
i
−
/
i
e
=
i
−
0
/
i
e
0
V
e
/
V
2
−
leading to
i
−
i
e
V
V
e
3
/
2
m
m
e
1
/
2
n
N
e
=
.
(6.9)
V
=
V
s
The values of
i
−
and
i
e
at
V
V
s
are obtained by extrapolation of the semi-logarithmic
plots of the separated portions
i
−
=
and
i
e
to
V
V
s
.
An interesting method to get the negative ion density for
n
−
<
=
N
e
was described
in [89]. It is based on comparison of two probe characteristics, the first one taken in
pure electropositive (Ar) plasma, the second one measured in electronegative (Ar/O
2
)
plasma. In [90] a more general approach of this method was given.
Langmuir probe independent methods to determine the negative ion density are
mass spectrometry [83], photodetachment [83,84], and a method based on material
etching by negative ions in reactive plasmas [91].
6.2 MICROWAVE INTERFEROMETRY
6.2.1 I
NTRODUCTION
Active probing with electromagnetic waves in the microwave range of frequencies
allows for investigations with negligible perturbation, providing information about
the plasma electron component. Wave plasma interaction can be manifold causing
changes of amplitude, phase, or polarization state of the probing wave; the wave is
being scattered and can completely be reflected. All interactions are being used for
diagnostic purposes [92-96]. Interferometry, the oldest active microwave diagnostic,
probes the dielectric properties of the plasma to determine the sightline integrated
electron density [93,94]. The section describes in an elementary way the physics
basis of the method and gives an overview of the interferometry techniques applied.
