Chemistry Reference
In-Depth Information
Such reaction processes are responsible for the transition with increasing poten-
tial from PS formation to electropolishing as typically revealed in an
i-V
curve.
2
Once
the whole surface is covered with an oxide film, further reaction can only proceed
through the formation of oxide followed by its dissolution by HF and electropolishing
rather than PS formation occurs. Increasing further the potential will only increase the
oxide film thickness. On the other hand, increasing HF concentration will increase the
dissolution rate of oxide (see Chapter 4). The presence of oxide on the silicon surface
in the PS formation region and its increase with potential have been experimentally
observed.
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When the surface is completely covered with an oxide film, dissolution becomes
independent of the geometric factors that are responsible for the formation and direc-
tional growth of pores, such as surface curvature and orientation. Fundamentally, unlike
silicon, which does not have an atomic structure identical in different directions, anodic
silicon oxides are amorphous in nature and thus show intrinsically identical structure
in all orientations. Also, on the oxide-covered surface the rate-determining step is no
longer electrochemical but rather the chemical dissolution of the oxide.
Distribution of Reactions and Their Rates on Pore Bottoms.
For a stably
growing PS the reactions and rates are different on the pore walls and on the pore
bottoms. Furthermore, they are different at different positions of a pore bottom due to
the difference in the radius of curvature. The current is the largest at the pore tip because
there the radius of curvature is the smallest. It decreases from the pore tip to the pore
wall as the radius of curvature increases. On the other hand, because the reactions
involved on a silicon surface in HF solution depend on the current density, for a given
condition, direct dissolution of silicon dominates at a relatively low current range
whereas oxide formation and dissolution dominate at a higher current range. Thus,
oxide formation and dissolution tend to occur at the pore tips at a lower applied poten-
tial than at the side of the pore bottom. The distribution of the kinds of reactions along
the pore bottom at different current densities is shown in Fig. 8.69.
For a pore to propagate under a steady state the current density on the side of the
pore bottom,
and that at the pore tip,
as illustrated in Fig. 8.68 have the follow-
ing relation:
