Chemistry Reference
In-Depth Information
TABLE 8.15
Plasma Polymerization of Hydrocarbons, Characteristics of the Volume,
and Deposition Reactions
Reaction
Characteristic Features
Type
Chemical Structure
Characteristics
of Deposits
I
Aromatic, heteroaromatic
triple bond
Polymerize readily with little
hydrogen production
High concentration of
dangling bonds and
unsaturation
II
Double bond, cyclic
structure
Polymerize moderate with
moderate hydrogen
generation
Moderate level of dangling
bonds and unsaturation
III
Saturated hydrocarbons
Polymerize with lower rate
and high yield of hydrogen
Low concentration of
dangling bonds and
unsaturation
IV
Oxygen (in aliphatic
structure)
Low deposition rate, high rate
of by-product gases
Very sensitive to energy input
level; often oxygen
containing groups are absent
in polymer
V
Oxygen (in cyclic
structures including
epoxides)
Oxygen atom is preferentially
removed; yielding diradical
for polymerization
Oxygen is absent in polymer
Source:
Yasuda, H.,
Luminous Chemical Vapour Deposition and Interface Engineering
, Marcel Dekker,
New York, 2005.
compounds. The discharge in such vapors is connected with hydrogen generation,
whereas the hydrogen yield is highest for saturated compounds, lower for double
bond hydrocarbons, and lowest for triple bound compounds (see Table 8.15).
Important for the activation of hydrocarbon molecules are electron collision-
induced dissociation processes like
RH
+
e
−
−→
R
•
+
H
+
e
−
,
(8.71)
R
•
+
CH
3
+
RCH
3
+
e
−
−→
e
−
.
(8.72)
Dissociative ionization processes have also to be taken into account. The con-
centration of free radicals may be five or six orders of magnitude higher than that
of ions [322]. The free radicals are transported by diffusion from the volume to
the substrate surface and form the cross-linked polymer film. In the gas phase,
propagation by recombination of two radicals and subsequent activation is possi-
ble. Bifunctionally activated species have to be taken into account as well. A high
hydrogen concentration hinders the formation of higher hydrocarbons and assists
the deposition of carbon-rich films. Selected plasma conditions facilitate diamond
like coatings (DLC, a-C:H) or deposition of diamond films (see Section 8.2.4).
Formation of clusters in the gas phase of hydro carbon plasmas is discussed in
Section 8.4.
