Chemistry Reference
In-Depth Information
Plotting the current at the maximum slope and the peak current for different
types of silicon as a function of HF concentration, the condition for occurrence of PS
formation and electropolishing is obtained as shown in Fig. 8.5.
2
It is seen that the
three regions in relation to current density and HF concentration are essentially
independent of the silicon substrate doping type and concentration. This means that
the differences in semiconducting properties of the silicon samples have little effect
on the occurrence of these regions. The various parameters involved in PS forma-
tion such as potential, doping, and illumination affect the occurrence of different
regions through their relation to the current density. Low current and high HF concen-
tration favors PS formation whereas high current and low HF concentration favors
polishing.
The occurrence of different regions during anodization of silicon in HF solutions
is attributed to two competing reaction paths: direct dissolution of silicon and indirect
dissolution through formation and dissolution of silicon oxide.
2
Formation of PS is only
possible when the surface is not fully covered with an oxide film and direct electro-
chemical dissolution of silicon is possible. At low potentials, direct dissolution of
silicon dominates and PS is readily formed. On the other hand, at potentials higher than
the current peak, the entire surface is covered with an oxide film and the semi-
conducting properties are masked such that the dissolution is homogeneous across the
surface and PS cannot be formed. Thus, the formation of an oxide film and its surface
coverage determines the occurrence of these three regions. The existence of an oxide
during PS formation and its increasing coverage with potential have been experimen-
tally confirmed.
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The electropolishing region does not occur in anhydrous organic solutions due to
the lack of water required for the formation of oxide film. Figure 8.6 shows that in
anhydrous HF-MeCN solutions the current can increase with potential to a value of
about without showing a peak current. The current increases linearly with
potential due to the resistance in the solution and silicon substrate except for very small
currents near the onset of anodic current where the current increases exponentially with
