Chemistry Reference
In-Depth Information
7.3.3. Effect of
The etching system is perhaps the mostly widely used isotropic etchant
for silicon.
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Figure 7.8 shows the etch rates in different mixtures of concentrated
HF and acids without dilution.
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The etch rate shows a maximum at 33% HF
and 23% (about 1:4.5 molar ratio) with a value of about (equivalent to
a current density of assuming dissolution valence of 4 and 100% current effi-
ciency). The mixtures of these two concentrated acids can be diluted with water or other
dilutent to give a wide range of etch rates as represented by the triaxial plot shown in
Fig. 7.9.
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The etch rate with respect to composition of the etchant shows two extreme
modes of behavior. In the region of high nitric acid composition, etch rates are only a
function of HF concentration, whereas in the region of high HF concentrations, the
nitric acid concentration determines the etch rate. The etch rates decrease with addi-
tion of a diluent but are little affected in the two extreme regions. As a diluent, acetic
acid acts qualitatively the same as water.
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However, acetic acid is preferred because
it results in less dissociation of the nitric acid, which preserves the oxidizing power of
for a wider range of dilution than water.
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The etch rate of both types of
materials are sensitive to the change of potential (see Chapter 6).
The etching system has been systematically investigated by Schwartz
and Robbins.
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The results on the effect of temperature indicate that in the region
of low HF concentration region, the etching process has a single low value of activa-
tion energy, 4kcal/mol, in the temperature range of 0-50 °C. This is characteristic of a
diffusion-controlled process, which is in agreement with the fact that the etch rates of
different silicon materials in this concentration region are essentially the same. In the
region of high HF, two different values of activation energy are observed, one in the
range of 10-14kcal/mol at higher temperatures and the other in the range of 20kcal/
mol at lower temperatures; both are characteristic of a surface-controlled process.
the
