Chemistry Reference
In-Depth Information
Gas
Activation
Nonreactive
products
Excitation
dissociation
ionization
Plasma
volume
reactions
Reactive
particles
Radicals
Ions
Nonreactive
products
Neutrals
+
Ions
Sheath
Surface reactions
Film
substrate
FIGURE 8.47
Reaction scheme for plasma polymerization of thin films. (From Yasuda, H.,
Plasma Polymerization
, Academi Press, Orlando, FL, 1985. Poll, H.-U. et al.,
Eur. Polym. J.
,
12, 505, 1976.)
the film formation. For small admixtures of monomers in inert gases, the activated
species responsible for thin film deposition may be ions, as investigated for a benzene
argondcdischarge[316].Massspectrometricinvestigationofahexamethyldisiloxane
(HMDSO) discharge leads to the conclusion of a film formation by ions [317,318].
The question of the generally dominant plasma component (neutral radicals or ions)
in the polymerization mechanism is still open [98]. Film formation is also possible by
interaction of surface adsorbed monomer molecules with energetic particles or UV
radiation from the plasma. The activation reaction in the volume, as well as reactions
ofenergeticparticleswithadsorbedmonomermoleculesoralreadydepositedpolymer
film can be associated with the formation of nonreactive products, like hydrogen.
A reaction schema is given in Figure 8.47.
The deposition rate depends on the operating conditions such as reactor geometry
and dimensions, power input, gas pressure, nature and rate of carrier gases, flow rate,
and, last but not least, of the nature of monomer gas. This complex dependence
of the deposition rate and the properties of the plasma polymer on the operating
conditions lead to applications of reactor parameters. The chemical reactivity of
the plasma chemical system may be characterized by the dimensionless reactor
parameter
τ
0
W
pV
A
,
R
=
(8.70)
where
τ
0
is the residence time of the monomer in the active plasma zone
W
is power input into the active plasma zone
p
is the gas pressure
V
A
is the volume of the active plasma zone
