Chemistry Reference
In-Depth Information
R
-
+
A
CNG
CF
F
PC
FIGURE 3.41
Low pressure DC glow discharge. C, cathode; A, anode; CF, cathode fall;
NG, negative glow; F, Faraday dark space; PC, positive column; R, resistance.
anode. The discharge in a long cylindrical vessel with dielectric walls shows different
characteristic regions: cathode fall, negative glow, Faraday dark space, positive col-
umn, and anode fall (Figure 3.41). The homogenous plasma of the positive column
with charge carrier generation by electron impact ionization of the molecules/atoms
is applied as model plasma for fundamental investigations of plasma chemical
processes [62-64].
The transition of the glow into an arc discharge is prevented by a current limiting
resistor in series with the discharge. Another method is the pulsed operation, where
the cathode heating is limited and the time for the development of an arc discharge is
too short.
The generation of a positive column with higher current is possible by changing
thecathodetoahollowcathode(usingahollowcylinder)orexternallyheatedcathode.
Thehollowcathodeitselfisanimportantreactoralsoforplasmachemicalapplications
with its high concentration of charge carriers, radicals and intensive photon emission.
The generation of charge carriers in hollow cathodes is supported by pendulum
electrons (electrons traveling forward/backward within the system) as well as by
additional electron emission by the impact of photons and metastables on the inner
cathode surface, see [65,66] and references cited therein. Such a discharge operates
with lower sustaining voltage and higher currents.
The magnetron discharge mainly applied in sputter devices is characterized by
the application of an external magnetic field approximately parallel to the cathode
surface (Figure 3.42). For sufficient low pressures (1 Pa) the electrons are trapped by
their gyration, because they are magnetized. This prolonged mean free path length
enables a satisfactory ionization rate at very low gas pressures. The ions are not
magnetized due to their larger mass and therefore not trapped. They collide with
sufficient energy for sputtering at the cathode surface. Magnetrons can operate with
medium-frequency (
10 kHz) and RF (13.56 and 27.12 MHz) [67].
Table 3.12 shows some examples of DC discharges, parameters, and applications.
∼
3.8.5 R
ADIO
F
REQUENCY
D
ISCHARGES
Radio frequency discharges usually operate in the frequency range
f
=
1-100 MHz.
The wavelengths (λ
3-300 m) are large in comparison to the reactor dimensions. At
lower frequencies the ions collide with the electrode surface and generate secondary
=
