Chemistry Reference
In-Depth Information
100
Δ
Δ
plasma
pl
sheath
0
ground
grounded
Δ
sheath
(t)
-100
rf
powered
-200
selfbias
-300
-400
-500
0
2
4 6 8 0 2 4
16
8
ω
t
FIGURE 3.29
Electric potentials idealized for an asymmetric capacitively coupled RF
plasma (CCP). The differences between the plasma potentials ϕ
pl
and ϕ
RF
,aswellasthe
ground potential
(
ϕ
=
)
represents the sheath voltage at the powered and grounded electrode,
respectively. RF amplitude: 125 V, self-bias potential:
0
−
200 V, floating potential: 10 V.
voltage, whereas no significant influence can be observed at the grounded electrode.
Due to the weak plasma potential modulation, the sheath properties at the grounded
electrode are comparable with a DC plasma sheath.
The transport of positive ions to the RF powered electrode is strongly influenced
by the oscillating plasma sheath voltage. Therefore, the flux of positive ions at the RF
electrode depends on the ratio between the ion transit time over the RF sheath region
and the RF-cycle. Furthermore, charge transfer collisions and elastic collisions in
dependence on the nature of the positive ions and the total pressure affects the ion
energy distribution in the sheath and at the electrode surface. Direct ion extraction at
the discharge electrode and energy selective mass spectrometry was used to measure
the flux of positive ions depending on their kinetic energy. Figure 3.30 shows the
time-averaged Ar
+
energy distributions at the driven and grounded electrode of an
RF-discharge in argon. As expected from the sheath potential, the maximum ion
energy at the powered electrode is strongly coupled to the RF voltage due to the
increasing self-bias voltage. The maximum ion energy is of the order of several
hundred eV, whereas the maximal energy of positive ions at the grounded electrode
is lower than about 20 eV.
Significant differences in the shape of the ion energy distribution at the two
electrodes are observed. Ions coming from the bulk plasma need several RF cycles
fortransitiontothepoweredelectrode.InFigure3.30athearrowsidentifythesituation
for entering the ions from the bulk plasma into the sheath at the low and high sheath
voltage, respectively. In result a saddle-shaped structure in the ion energy distribution
is found. The observed multiple peak structure in the low energetic part results
from charge transfer collisions in the sheath region. In the time-averaged ion energy
