Chemistry Reference
In-Depth Information
anodic potentials between OCP and , the data in Table 5.1 indicate that silicon etching
with evolution of hydrogen is still the dominant reaction process. For
-Si, the ratio of
changes little but the etch rate significantly increases with potential
between OCP and
p
.
5.5. HYDROGEN EVOLUTION
Visible hydrogen evolution occurs in HF solutions at anodic potentials in the
exponential region anodic of OCP.
2,34
The rate of hydrogen evolution substantially
decreases as potential approaches the current peak. Hydrogen evolution (visually)
ceases above the above current peak. Since hydrogen evolution is a reduction reaction
and consumes electrons, it is responsible for an apparent dissolution valence smaller
than 4 below the current peak. Thus, the effective dissolution valence can be used as a
measure for the hydrogen efficiency. A silicon dissolution valence of 2 means 100%
efficiency for hydrogen evolution, i.e., for every dissolved silicon atom one hydrogen
molecule is generated.
Results from rotating a ring-disk electrode indicate that the efficiency of hydro-
gen evolution is near 100% in the exponential region but drops to about 50% at the
first peak potential as shown in Fig. 5.22.
599,767
In addition, a considerable number of
hydrogen molecules, corresponding to 10% to 13% efficiency, are generated through-
out the electropolishing region. An equivalent of about 10% of the silicon atoms
10 V.
38,700
dissolve into the solution effectively divalent at potentials up to In the
electropolishing region, silicon dissolution proceeds with oxide formation and dissolu-
tion of the oxide. An apparent dissolution valence less than 4 means that some silicon
atoms are only partially oxidized in the oxide forming stage but the oxidation is com-
pleted during the dissolution stage.
