Chemistry Reference
In-Depth Information
due to the difference in hydration number of the solutions.
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Some etch rate
data for these etchants are listed in Table 7.1.
NH
4
OH Solutions.
Among the inorganic alkaline solutions
is special in
two respects. First, when mixed with
it is widely used as a solution (known as
SC
solution) for cleaning organic and metallic contaminants on silicon surface.
Second, it contains ligands that is characteristic of both inorganic alkaline etchants such
as KOH and organic echants such as EDP. It has a low etch rate for thermal
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l
giving
an is the much lower
heavy metal concentrations available in reagent grades of this chemical compared with
other hydroxides.
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However, tends to decompose at elevated temperatures,
especially with more concentrated solutions. Also, in
etch rate ratio of 1:8000.
Another advantage of
mixtures,
decomposes with a half-life of 16min at 70 °C.
208
Figure 7.16 shows the etch rate in solutions as a function of concentra-
tion.
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The etch rate as a function of pH is shown in Fig. 7.17, and is considerably
higher than that in NaOH solutions at the same pH values, indicating that
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plays a
significant role in the etching processes.
The etch rate is dependent on potential with
the peak etch rate of both
p
-Si and
n
-Si occurring near the OCP.
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The etch rate of
p-
Si decreases but maintains a definite value at cathodic potentials whereas that of
n
-Si
decreases to zero at large cathodic potentials.
Figure 7.18 shows the etch rate in as a function of
A sharp drop of etch rate to nearly zero is observed for concentrations above
which is due to the passivation by formation of silicon oxide on the surface.
In solutions containing more than
concentration.
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as an oxidant polarizes
the silicon surface to a potential value above the passivation potential. The
etching of silicon in the solutions with a concentration below the critical value
has a chemical component and an electrochemical component due to the reduction of
the
The relative contribution of the two components changes with the concentration
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of
