Chemistry Reference
In-Depth Information
An analysis of the charged plasma components by using the general equations
(5.19) through (5.25) listed in the preceding text will in principle yield the part of
surface heating caused by positive ions and electrons. Because several heat sources
act together, for example, radiation, chemical reactions, neutrals, and charge carriers,
it is possible to separate the contribution of the charge carriers by variation of the
bias potential.
Example 5.3: Energy balance during thin film deposition
In a process plasma containing reactive species (e.g., N
2
,O
2
and) in addition to
energy transfer by charge carriers, the thermal balance of a substrate may also
be influenced by atomic recombination (association) and exothermic reactions.
Evidence for substrate heating by exothermic reactions on the processed surface
has been reported, for example, with respect to plasma etching of silicon with
fluorine-containing compounds [50] and during plasma cleaning of contaminated
metal surfaces [51]. The percentage of the recombination energy that is used for
surface heating varies with the chemical composition of the surface and of the
plasma.
For a reactive oxygen plasma, the energy influx
J
ass
by atom recombination,
that is, the formation of oxygen molecules, is described by
2
n
O
8
k
B
T
g
1
J
ass
=
j
O
O
E
diss
=
π
m
O
·
E
diss
,
(5.26)
O
where
O
is the association probability of
O
atoms on the substrate surface
T
g
is the gas temperature (room temperature)
n
O
is the density of
O
atoms
m
O
is the mass of
O
atoms
E
diss
is the dissociation energy of O
2
molecules
The contribution
J
chem
due to exothermic chemical reaction (i.e., surface oxidation)
can be calculated by the product of the average energy
E
react
released per reaction
with the flux density
j
react
=
j
O
of the reactive oxygen radicals:
J
chem
=
j
react
E
react
.
(5.27)
The radical flux density
j
O
that has to be known in (5.26) and (5.27) is mainly
determined by electron impact dissociation taking place in the negative glow of
the discharge. If the growth rate of the oxide that is formed during the plasma
process is known, one can easily estimate
J
chem
from the growth rate
R
dep
, the mass
density
ρ
of the layer, and the average specific oxidation enthalpy
h
ox
:
J
chem
=
R
dep
ρ
h
ox
.
(5.28)
Figure 5.6 shows an example for energy influxes
Q
in
measured for a weak
radiofrequency (rf) plasma in argon and oxygen, respectively. The discharge power
was kept constant at 15 W and the gas pressure at 1 Pa, while the substrate voltage
