Chemistry Reference
In-Depth Information
measurement of the decay of CH
3
and SiH
3
, radicals respectively, in the plasma
afterglow by ionization threshold mass spectrometry. In a pulsed plasma, the decay
time of the density of reactive species after switching off the discharge depends on
the efficiency of the chamber walls to act as a sink for these species:
1. If the surface loss probability is large, the species do not survive many wall
collisions, leading to a decay time in the order of the travel time for the
reactive species from the position of their formation in the plasma to the
vessel walls.
2. If the surface loss probability is small, the species survive many wall colli-
sions and the decay time is governed by the pumping speed of the plasma
vessel.
A simple modeling of this decay yields β
=
10
−
3
for CH
3
and β
=
0.25 for
SiH
3
[20].
The surface loss probability can also be measured by depositing films inside
a cavity, which is exposed to a discharge [14-19]. Growth precursors, emanating
from the discharge, enter this cavity via a slit or a hole and build up a layer inside.
From the analysis of the layer thickness profile, the surface loss probability can be
determined. If the surface loss probability is high, films are only deposited in close
proximity to the entrance of the cavity, since the species cannot survive many wall
collisions. If the surface loss probability is small, the species will survive many wall
collisions and the deposition profile inside the cavity becomes uniform. The variation
of the film thickness inside the cavity can be modeled by Monte Carlo simulations,
which follow the reflections of incoming growth precursors among the walls inside
the cavity. Typical results for a cavity with a slit as entrance geometry are shown in
Figure 5.3 for β
=
=
0.1.
It can be seen that for high β (see Figure 5.3a), deposition is observed predom-
inately on the opposite side of the entrance slit, corresponding to the position of the
first wall collision for incoming species. For small β (see Figure 5.3b), a uniform
deposition is observed on all walls inside the cavity.
If the absolute flux of growth precursors to the surface is known, the sticking
coefficient can be calculated from the absolute film thickness. Or, on the other hand,
if the probability of forming a nonreactive volatile product is known, the sticking
coefficient can directly be deduced from the surface loss probability.
As an example for the application of this technique, the deposition inside a cavity
exposed to a hydrocarbon discharge is discussed in Refs. [14,15]. The comparison
with the theoretical model yields for the deposition profile in a methane discharge
β
0.9 and β
0.05.Inamethanedischarge,
the dominant contribution in the radical flux toward the surface consists of CH
3
radicals [21]. However, as mentioned earlier, the surface loss probability for CH
3
radicals is of the order of 10
−
3
. Thereby, the neutral growth precursor responsible for
film formation inside the cavity in the methane discharge cannot be the CH
3
radical.
In an acetylene discharge, the dominant radical should be the C
2
H radical. Therefore,
one assumes that the surface loss probability of β
=
0.65
±
0.15andinaacetylenedischarge β
=
0.92
±
0.92 corresponds to the surface
reaction of C
2
H. In a methane discharge, however, larger hydrocarbon molecules
=
