Chemistry Reference
In-Depth Information
2.1.2 P
LASMA
A
SHING
The interaction of an oxygen plasma with hydrocarbon compounds leads to CO
2
and
H
2
O. In microelectronic industry, e.g., the photoresist mask is removed (stripped)
by an oxygen plasma. Damage of the semiconductor material by high-energy ions
must be avoided by low ion energies and high fluxes of neutral radicals, i.e., oxygen
atoms to the resist surface. The low-temperature plasma ashing procedure is used
for preparation of samples for electron microscopy [9] and for quantitative analysis
of lignite [10]. Oxygen plasmas are applicable to precision cleaning of metallic
surfaces contaminated by organic substances such as grease or oil [11]. Hazardous
gaseous organic molecules as volatile organic compounds (VOCs) may be destroyed
by reactive species like O
2
(a
1
D
g
), O(
1
D), O(
3
P),H,OH,N
2
(A
3
S
u
), N
2
(B
3
P
g
), and
N best into CO
2
or H
2
O[12].
2.2 PLASMA ETCHING
Plasma etching is the key technology for patterning in every chip production in
the microelectronic industry. It enables nonisotropic etching in sub-μm range with
significant increasing of packed density of electronic elements in integrated circuits
applied in computer production. For this process a nonreactive gas is fed into the
plasma where it is activated. The interaction of this activated gas with a solid substrate
generates in a chemical reaction a volatile compound which contains atoms of the
substrate. Exemplary is the silicon etching by a fluorine compound feed gas as
CF
4
. The plasma activation leads to generation of fluorine atoms by electron impact
dissociation of the CF
4
molecule.
e
−
−→
e
−
.
CF
4
+
CF
3
+
F
+
(2.2)
The fluorine atoms react with silicon and produce volatile SiF
4
SiF
4
.
Si
+
4F
→
(2.3)
The activation energy for desorption of the etch product SiF
4
is transferred from the
plasma to the surface by ion bombardment. Because of the existence of fluorocarbon
radicals (CF
n
) a polymer film is deposited on the silicon surface, also on sidewalls
of trenches. The sidewall protection is important for the anisotropy of trench etching
with high aspect ratio (ratio depth to width).
Fundamental starting processes of activating the etching gases are the electron-
molecule collisions. A critical review of data of electron collision processes for a lot
of fluorine and chlorine containing gases is given by Christophorou and Olthoff [13].
An extensive presentation of plasma etching can be found in Section 8.2.1.
2.3 HYDROCARBON PLASMA CHEMISTRY
Importantreactionchannelsinhydrocarbonsareinducedbycollisionsofhotelectrons
with gaseous molecules in a cold environment. It offers a broad spectrum of appli-
cations beyond standard organic chemistry. The spectrum covers from complicated
