Chemistry Reference
In-Depth Information
The products in the gas phase were generated by reactions between free radicals
and ions.
The state of the early earth atmosphere is discussed by Abelson [37]. An N
2
-
CO-H
2
atmosphere is supposed and HCN and H
2
O were the principal products of a
gas discharge, beside small amounts of CO
2
and CH
4
. HCN in aqueous solution can
lead to other organic compounds.
Also the plasma chemistry in CH
2
-H
2
S [38] and CH
4
-PH
3
[39] atmospheres is
studied to detect prebiochemical substances.
These experiments show the formation of organic compounds in plasma chemical
experiments, but the further reaction steps in the direction of formation of living
structures are largely speculative.
2.3.4 P
LASMA
P
OLYMERIZATION
Plasma polymerization is a process of thin film deposition on electrodes, walls, or
substratesundertheactionofplasmasinanorganicmoleculescontainingatmosphere.
The term “polymerization” is misleading because this product is not a polymer
consisting of equal components. Plasma polymer films consist of highly cross-linked,
brittle material with good dielectric properties. They are pinhole-free, have low sol-
ubility, pronounced chemical inertness, and good adhesion to the surface. A broad
spectrum of organic compounds such as hydrocarbons, fluorocarbon, and silicon
organics was applied for plasma polymerization. In contrary to chemical polymeriza-
tion these starting compounds in the feed gas can be free of double or multiple bonds
or cyclic structures. The starting gas is activated in the plasma by electron collisions
or by collisions with other energetic plasma components as H atoms. Ionic or neutral
radicals are created. The target surface is activated by ion bombardment. The radicals
diffuse (neutral) or drift (ionic) to the surface where they are bonded to the surface.
The starting material can also move directly to the surface where a plasma-induced
polymerization is possible.
Section 8.2.4.1.2 gives more detailed information on plasma polymerization.
2.3.5 T
HIN
F
ILM
D
EPOSITION OF
M
ETAL
C
OMPOUNDS
A method of thin film deposition of simple metal compounds (e.g., oxides, nitrides)
is the plasma-enhanced chemical vapor deposition with metal organic starting gases.
The advantage of this method is the low substrate temperature and is therefore useful
for deposition on temperature sensitive materials [40]. A study of deposition of TiN
using Tetrakis(diethylamine)titanium (TDEAT) shows the importance of H-radicals
in the H
2
plasma for the stripping of TDEAT. The formation of TiN requires N
2
addition to the process [41].
The deposition of thin films of pure metals or simple metal compounds is possible
by sputtering in low-pressure discharges inert gas. An example of reactive sputtering
is the deposition of TiN films. The nitride formation is a surface process of the freshly
deposited Ti with plasma-activated nitrogen.
Plasma-enhanced atomic layer deposition (PEALD) of metals or metallic com-
pounds is based on a sequential use of self-terminating plasma-enhanced gas solid
