Chemistry Reference
In-Depth Information
Unfortunately, particle formation in low-pressure discharges is not very efficient
due to the low density of active species and due to the limited particle-trapping
capacity. In addition, the injection and manipulation of particles into the vacuum
system as well as collection and extraction of processed particles raise several issues.
Therefore,alsoatmosphericpressureglowdischargesystemsarerecentlyinuse[425].
Particles can be generated either by nucleation and subsequent growth due to gas-
phase reactions in the plasma [426] or they may originate from molecular or cluster-
like species that are produced due to sputtering of electrodes [427] or peeling off
of films deposited on the reactor walls [428]. Besides particle growth, incorporation
of small particles into growing films [429] and coating of particles [430] are also
important issues in plasma chemistry.
8.4.1 P
ARTICLE
S
YNTHESIS IN
P
LASMA
For particle synthesis, the growth of fine particles in reactive silane-, hydrocarbon-,
and fluorocarbon-based plasmas are of interest. Despite a remarkable difference in
the process kinetics and the plasma chemistry involved, the growth scenario can be
very similar. Particle growth in such chemically reactive plasmas starts with the for-
mation of sub-nanometer-sized protoparticles (clusters), which nucleate as a result
of homogeneous or heterogeneous reactions. Then, agglomeration/coagulation pro-
cesses result in the generation of particulates with sizes of a few tens of a nanometer
which quickly acquire a negative electric charge due to collection of free elec-
trons from the plasma [431,432]. These rapidly developing mechanisms result in a
significant reorganization of the entire plasma system due to changes in the parti-
cle and charge carrier balance. As a result of the compensation of electron losses
onto the dust grains, the electron temperature and, therefore, the ionization and
dissociation of the precursor molecules increase. Finally, the dust particles usually
proceed to sub-μm and μm size via slow mechanisms of accretion of monomer
radicals. Detailed description of the particle formation processes and their model-
ing, in particular for silane and fluorocarbon plasmas, can be found, for example,
in [433-435].
Formation of particles in a plasma has been observed often in the course of thin
film deposition of hydrogenated amorphous carbon or DLC films from hydrocarbon-
based precursors as methane, acetylene, etc., [431,436]. Based on numerous data
from the reactive plasma, aerosol, and combustion literature, it is assumed that the
carbon hydride clustering process is induced by the electron-impact abstraction of
hydrogen from the acetylene monomers
e
−
→
e
−
,
C
2
H
2
+
C
2
H
+
H
+
(8.78)
followed by an efficient generation of C
x
H
y
radicals via chain polymerization reac-
tions [437-439]. The measured ion mass spectrum of Ar/C
2
H
2
plasma is displayed in
Figure 8.64a; it is distinctly different from that of the Ar/CH
4
plasma where particle
formation is less likely [437]. Apparently, some difficulty exists to break the carbon-
carbon triple bond of acetylene. Hydrocarbon molecules in acetylene plasmas, hence,
prefer to grow by addition of C
2
-containing molecules as the presence of hydrocarbon
