Chemistry Reference
In-Depth Information
slow attack of ions on the relatively stable Si-H bonds. The Si-H bond is practi-
cally unpolarized and thus has a high degree of chemical stability. In contrast, an Si-O
bond is 46% ionic and a Si-F bond is 67% ionic which results in a strong polarization
and weakening of the back bonds of the silicon atoms. These surface silicon atoms are
not stable and are prone to chemical attack by solution species such as and HF.
Thus, in the etching process in HF solutions, the replacement of the adsorbed hydro-
gen by fluoride or hydroxyl ions is the rate limiting step followed by a fast process of
breaking of the back bonds and removal of the silicon atoms. Such a reaction scheme
is more likely to occur at surface defects resulting in their removal. According to
Willeke and Kellermann,
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a prolonged immersion in HF solution results in a more
atomically smooth surface, and a large decrease of etch rate with time is due to the
decrease of defect density on the surface.
The etch rate of silicon in HF does not seem to have a clear dependence on dopant
concentration as shown in Fig. 7.5. Oxygen in silicon is found to have little effect on
the etch rate in HF, indicating that oxygen is not efficiently reduced at the OCP.
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However, the reactivity of silicon in HF solution may be significantly increased when
active elements such as sodium are present in the silicon. A sodium-containing silicon
powder is found to dissolve in an acidic NaF solution with spontaneous hydrogen evo-
lution.
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The elemental sodium in the silicon reacts with water to remove a protective
silica layer and leaves a reactive silicon surface. The etch rate of silicon has been found
to increase with ion implantation which produces an amorphous structure with a high
density of dangling bonds.
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According to the reaction mechanisms discussed in Chapter 5, silicon dissolves
chemically in KOH solutions by reacting with ions but electrochemically in HF
solutions which requires the participation of electronic carriers. In HF solutions at the
OCP, the chemical dissolution rate is low because the concentration of is low. The
electrochemical reaction is also low because there are few carriers at the OCP. Thus,
the increased etch rate in HF with dilution can be attributed to the increased con-
centration. The lack of direct proportionality between etch rate and concentration
suggests that the rate depends on the surface concentration of , the adsorption of
which is very fast.
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The relative contribution of chemical and electrochemical reac-
tions depends on pH. Figure 7.4 shows that in HF solutions, the dissolution reaction of
silicon is mainly electrochemical but the chemical reaction path becomes more impor-
tant at higher pH values.
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In general, the dissolution of silicon involves two essential steps, namely, oxida-
tion followed by dissolution:
A high etch rate is only obtained when both rates of oxidation and dissolution are high.
The etch rate can be increased either by addition of effective oxidizing agents into the
solution or by anodic polarization which increases the surface concentration of carri-
ers. As shown by the
i-V
curve (Fig. 5.1) the anodic dissolution of silicon in HF solu-
tions consists of three regions. At potentials below the current peak, nonuniform anodic
