Chemistry Reference
In-Depth Information
is considered to be less susceptible to rf-fluctuations, because it floats with the
fluctuations [8] and therefore often no rf-compensation is used at double probe
measurements in rf-plasmas [76]. Nevertheless sometimes an rf-compensation is
employed [74,75]. The error in the electron temperature determination derived from
a double probe characteristic measured in an atmospheric pressure discharge and
using the evaluation valid for low pressure is less than 20 % [77].
A very short measurement of electron temperature and ion density is possi-
ble with a floating
triple probe
consisting of three identical probe tips [78,79]
(plasma: Ar/hydrocarbons, layer: polymer, probe tips: wire loops heated directly
prior measurement).
6.1.3.6.4 Other Probe Principles
With the
self-excited electron resonance spectroscopy
(SEERS) the volume averaged
electron density and the electron to neutral collision rate in a cylindrical and strong
asymmetrical rf-discharge at frequencies
f
50 Pa may be
determined. The measured signal is a small part of the rf-discharge current detected
with a sensor head mounted flat in the chamber wall made of the same material as
the wall and therefore not disturb the technological process. Because an rf-current is
measured there is no influence of polymer or other thin insulating layers on the mea-
surement. The information about the plasma parameters is extracted from harmonics
of the discharge current and a plasma physical model including the Maxwellian equa-
tions and the first three moments of the Boltzmann equation [80-82]. Some further
probe principles applicable in chemically active plasmas are listed in [4,8,11].
>
5 MHz and pressures
p
<
6.1.3.7 Probe Measurements in Discharges Containing Negative Ions
Electronegative gases are used as isolators in high-voltage technology, for laser
generation, and in most surface processing discharges. Negative ions in sometimes
large concentrations
n
−
may occur in discharges containing NH
3
,CH
4
,SiH
4
,O
2
,
H
2
, halogens, or halocarbons [83]. Temperature and mass of positive and negative
charge carriers become similar for
n
−
/
in electronegative plasmas. Thus
also the absolute values of positive and negative saturation currents of a Langmuir
probe characteristic become similar and floating and plasma potential coincide in this
case [68,72]. At the other hand the result
n
+
>
N
e
→∞
N
e
where both
n
+
and
N
e
are derived
from the same characteristic do not always indicate the existence of negative ions
because the positive ion density
n
+
derived from the ion saturation current often is
overestimated even in electropositive plasmas [11,14].
The relation
i
s
∝
n
s
(
e
0
|
V
−
V
s
|
/
2π
m
s
)
1
/
2
for the current of particles of species s
(
s
=+
,
−
,
e
)
is valid in thermal plasmas for a thin probe (
r
p
/
λ
D
>
1 with λ
D
as the
Debye length) at
e
0
|
kT
e
[72]. But it may be used also in nonthermal plasmas
as a rough approximation [83,84]. Then the ratio α
V
−
V
s
|
=
n
−
/
N
e
may be estimated in a
quasi-neutral plasma (
n
+
=
N
e
+
n
−
)as
n
N
e
=
(
m
+
/
m
e
)
1
/
2
−
(
m
+
/
m
−
)
1
/
2
−
1,
(6.7)
R
−
(
m
+
/
m
−
)
1
/
2
where
R
i
−
to the positive
i
+
par-
ticle current at the same absolute values of probe potential with respect to plasma
=
(
i
e
+
i
−
)/
i
+
is the ratio of the negative
i
e
+
