Chemistry Reference
In-Depth Information
1000
800
700
600
500
400
T
( K )
300
Cl
2
+ GaAs
GaCl + AsCl
10
-1
ε
T
3 Cl
2
+ GaAs
GaCl
3
+ AsCl
3
1/
T
(10
-3
K
-1
)
10
-2
1.0
1.5
2.0
2.5
3.0
3.5
FIGURE 5.5
Total reaction probability of Cl
2
on GaAs in dependence on the substrate
temperature.
CS. In the first reaction channel, etching occurs by conversion from a precursor
weakly bound to the surface, and the latter etching occurs directly by the reactants
arriving from the gas phase. A characteristic example for the combination of both
reaction channels is the dry etching of silicon by fluorine-containing compounds.
A nonmonotonous
R
-curve due to the different desorption behavior of
the formed products has been observed, for instance, during etching of GaAs by
chlorine [48]. Starting at low temperature, the etch rate increases because of the
large surface coverage of reactive species and increasing chemical reaction rates
for the formation of GaCl
3
and AsCl
3
. At a higher temperature, the fast decreasing
coverage of the origin species (Cl
2
) and of the products (e.g., GaCl
3
) vanishes
and the formation of trichlorides stops. However, at these high temperatures des-
orption of the very fast formed layer of monochlorides (AsCl, GaCl) starts. The
situation for the total reaction probability
ε
T
that is directly related to the etch rate
R
(
T
S
)
j
Cl2
is illustrated in Figure 5.5. The comparison of the measure-
ments (points) with a simple model on the basic assumptions mentioned earlier
yields typical quantities of the etch process like activation energies, residence
times, etc.
One can conclude that the several elementary mechanisms of adsorption,
diffusion, and desorption as well as the chemical reactions (CS, SFR) depend
sensitively on the surface temperature
T
S
of the substrate, while the mechanisms
of activation, physical sputtering, and implantation (at least in the relevant range)
are almost independent of
T
S
.
(
ε
T
)
=
(
1
/
2
)
R
/
5.9 ENERGY BALANCE AT SURFACES
For the energy balance during the plasma-surface interaction, one has to include the
internal heat sources (energetic particle bombardment, radiation, surface reactions,
