Chemistry Reference
In-Depth Information
pattern II appears at lower rf power as compared to discharges in hydrogen and argon
[135,136]. In the PIC-MCC simulation, the secondary electron emission process
was not included by purpose, but pattern II was still observed. As one can see in
Figure 9.24, an “electron carpet” starts to appear in the sheath by the time the sheath
potential drop reaches its maximum. In the simulation, these electrons are produced in
detachment collisions of negative ions with background gas (O
−
+
e).
Although the negative ions are constantly expelled from the sheath by the sheath
electric field, it takes several rf cycles for O
−
ions produced in the sheath to get to
the bulk, so there is always a low density background of the negative ions in the
sheath. The negative ions in the sheath act as a reservoir of electrons, which get
released during the collisions with neutrals. These electrons trigger an avalanche,
producing the high-energy tails in the velocity distribution function (Figure 9.25).
The temperature of these high-energy tails at the sheath boundary in the simulation
was estimated as
T
e
hot
O
2
→
O
+
O
2
+
30eV. These energetic electrons are responsible for the
excitation pattern II observed in the simulations. As these electrons are distinctly
hotter than those responsible for excitation pattern I, pattern II is further protruded
into the bulk as compared with pattern I.
At lower rf power, the excitation pattern III appears in the second part of the
rf cycle close to the electrode and reaches its maximum about 2-4 mm in front of
the powered electrode (Figure 9.21). Dynamics of particle densities, charge density,
plasma potential, and the electron velocity distribution during the sheath collapse
obtained from the PIC simulations gives a good insight into this process.
Due to increase of the electron-neutral elastic collision cross section at energies
of about 1eV, the electron flow toward the electrode during the sheath collapse get
inhibited. This results in a buildup of negative space charge close to the sheath
edge (
∼
6 mm) causing a field reversal during the end of the sheath collapse
phase (Figure 9.26). This reversed field accelerates the electrons toward the elec-
trode, self-consistently maintaining the ion-electron current balance, which results
in the excitation pattern III. The electrons heated in this reversed field are visible in
the electron velocity distribution function profile in Figure 9.26 as a broadening of
the distribution in front of the electrode after a clearly observable waist. The heating
mechanism for these electrons is the same as for the electrons during the sheath
growth—the Ohmic heating. The excitation patterns due to heating of the electrons
during the sheath reversal were observed earlier in hydrogen discharges [143].
Several spatiotemporally resolved profiles of electron-impact dissociative excita-
tion rate of the atomic oxygen 3p
3
P level from the simulation were presented earlier.
In none of them is evidence for an excitation process close to the electrode (pattern
IV) as seen in the experiment. This is not really surprising, because, as one can see in
Figures 9.23 through 9.27, energetic electrons can only hardly reach the electrodes.
The electrons are able to reach the electrode only within a short time of the sheath
collapse to balance the ion current that is permanently flowing to the electrode.
For the main part of the rf cycle, there are no electrons near the electrode surface.
While the energetic electrons are responsible for the rf-modulated optical emission
patterns I and II located about 5 mm away from the electrode (Figure 9.28a), the
emission layer closer to the electrode needs another explanation. The excitation due
to heavy particle collisions must be taken into account [124]. By introducing the most
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