Chemistry Reference
In-Depth Information
The anodic dissolution of silicon in these solutions occurs by the following reactions
Reaction (6.6) represents creation of an intermediate by a hole, which in the case of
n-
Si is provided by the reduction of Reactions (6.7) and (6.8) represent two possi-
ble reaction routes for the intermediate one by electron injection into the conduction
band and the other by a chemical reaction with hydrogen ions. The injection of elec-
trons into the conduction band accounts for the anodic current on
-Si in the dark. The
reaction with hydrogen ions accounts for the hydrogen evolution during the anodic dis-
solution of
n
-Si.
The chromium complex may also react with the silicon dissolution intermediate
n
This reaction competes with reactions (6.7) and (6.8) and is responsible for the decrease
of photo quantum efficiency to about 1 on
-Si at very low ratios.
For etching at the OCP where the amount of anodic dissolution equals that of
cathodic reduction, reactions (6.3), (6.6), (6.8), and (6.9) operate at high
n
ratios
whereas reactions (6.4) to (6.9) are involved at low ratios.
is a commonly used oxidant in HF solutions for isotropic
etching of silicon. Figure 6.18, reported by Kooij
et
.,
969,981
shows the etch rate and
al
current as a function of potential for
n-
and
p
-type silicon in the dark. The cathodic
current plateau measured on
p-
is not due to a diffusion-controlled process but to
the reduction of involving hole injection in the valence band. This limit-
ing current also appears as a shoulder on the
i-V
curve of
Si
n
-Si indicating that the
reduction of
-Si at
more negative potentials is due to hydrogen evolution. The cathodic plateau current
on
p-
on
n
-Si is similar to
p-
Si.
The large current on the
n
decreases with electrode rotation rate indicating that surface-bond reac-
tion intermediates, rather than the from the bulk, are responsible for the hole
injection into the valence band. Also, during silicon etching, gas evolves and the
composition of the gas is found to be about 80% with the rest being nitrogen
oxides in the form of The etch rate decreases with the rotation speed of the
electrode indicating the removal of active reaction species from the surface
or
Si
The current at anodic potentials on the
p-
Si
is
similar to that in HF solution
without
into the conduction band from the dissolution intermediates. The etch rate decreases
with cathodic polarization for both
The anodic current plateau on
n-
Si
indicates that electrons are injected
due to the more effective removal of
the injected holes as shown in Fig. 6.18. A comparison of the anodic current plateau
n
-Si and
p-
Si
