Chemistry Reference
In-Depth Information
dissolution occurs, which is accompanied by significant hydrogen gassing, leading to
the formation of a porous silicon layer (see Chapter 8 for details). At potentials above
the current peak, uniform dissolution occurs without hydrohen evolution, leading to
electropolishing. The effective dissolution valence in the electropolishing region is
close to 4 corresponding to an almost 100% current efficiency. Thus, the etch rate can
be calculated from Faraday's law:
where
i
is the current density
m
the molar mass (28 g/mol for silicon),
the
density
n
the effective dissolution valence which is 4 in the
electropolishing region, and
F
the Faraday constant (96,500 C/mol). According to Eq.
(7.3) a current density of
corresponds to
The etch rate in the electropolishing region, where the silicon surface dissolves
uniformly, is essentially the same for different doping conditions and orientations as
shown in Fig. 8.3. The etch rate in the electropolishing region varies only with HF con-
centration and temperature. At a given temperature, the etch rate of silicon in the elec-
tropolishing region as a function of HF solution is shown in Fig. 8.5. The presence of
has no effect on the etching rate in the electroplishing region.
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Large etch rates
can be obtained in HF containing anhydrous organic solutions at high anodic potentials
due to the lack of oxide formation.
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The effect of various factors on the rate of dis-
solution under anodic potentials is described in Chapter 5.
7.3.2. Effect of
The HF-CrO
3
etching system is widely used for defect sensitive etching and
delineation of junctions between silicon layers of different doping concentrations.
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The etch rate of silicon in pure HF solution is very low due to the lack of holes at the
OCP.
Addition of CrO increases the etch rate due to the increase of surface hole con-
centration resulting from the reduction of dissolves in water to form
and At a concentration of 0.5 M or higher, is the pre-
dominant species.
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The presence of chromic acid suppresses the ionization of hydro-
fluoric acid so that the concentrations of
3
and
are small, less than 1 % of the HF
present in the solution.
Heimann
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suggested that etching in
solutions follows a two-
step reaction mechanism. In the first step, silicon is oxidized to form a silicon subox-
ide (0.67 <
x <
1). The second step consists of the dissolution of the silicon oxide
the dissolution rate of silicon in
solutions is second order with respect to the concentration of HF as shown in Fig. 7.6.
It is first order with respect to ions. Figure 7.6 shows that the
n
-Si and highly
resistive
p-
Si exhibit about the same etching rate whereas the other types of silicon
materials have higher rates. Because the etching process is controlled by the carrier
transfer across the space charge layer, different materials have different hole con-
centrations and thus dissolve at different rates. For
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by HF. According to Chu and Gavaler,
-Si, the hole concentration is high
at the surface, and for heavily doped
n
-Si, electrons can tunnel through the space
charge layer.
p
