Chemistry Reference
In-Depth Information
a time which is determined by the thickness and the dissolution rate of the oxide,
Such a gradual change of the properties of the anodic oxide from the time
of its formation to the time of its dissolution can explain the phenomenon of current
oscillation. The difference of this model from the previous ones is that it does not
assume micro domain effects nor sudden change of oxide structure. Oscillation is
viewed as a result of inconsistency between fromation and dissolution rates of oxide
in response to current or potential perturbations on the aging process of the oxide as it
moves from A to B.
When the conditions are such that the oxide formed at the Si/oxide interface, A,
as it is moved to the surface, changes to the oxide at point B and has the etch rate of
oxide B, the system is stable and no oscillation occurs. On the other hand, when the
property of the oxide formed at A is not the same as that of the oxide by the time it
reaches point B, the etch rate of the oxide film will vary and current oscillation may
occur. When current oscillation occurs, as illustrated in Fig. 5.59b, the thickness and
variation of properties in the thickness direction change with time. The structure and
etch rate of the oxide formed at A with a thin film are different from that with a thick
film because of the difference in aging time.
In a given solution, the rate of oxide growth is a function of the field, that is,
The dissolution rate of oxide is a function of solution composition, formation
field, and the time lapse between formation and dissolution of the oxide, that is,
At a steady state, the rate of oxide growth equals the rate of
its dissolution, and the thickness of the oxide film is constant. When
that is, the oxide formed at A has different properties and a different etch rate from
that at point B when oxide A reaches the surface,
oscillation can occur when
the following condition exists.
That is, oscillation occurs when the rate of change of the dissolution rate is larger than
that of the formation rate. When the change of the oxide formation rate cannot keep up
with that of the dissolution rate to make up the corresponding change in the oxide thick-
ness, the thickness will change more, which destabilizes the system. Figure 5.59b illus-
trates the relative rates of oxide growth and dissolution and the change of oxide
thickness and density during an oscillation period.
On the other hand, when the oxide formation rate is capable
of keeping up with the change of the oxide film thickness due to the change of the dis-
solution rate. Current oscillation will not occur by a perturbation in the system. The
variation in the occurrence of oscillation, oscillation amplitude, and frequency with
respect to conditions can be further detailed by analyzing the two functions
and
5.11. PARTICIPATION OF BANDS AND RATE-LIMITING PROCESS
The conduction of the charge carriers in the space charge layer during anodiza-
tion of silicon can occur either by electron injection into the conduction band or by
hole capturing from the valence band. For
p
-type silicon, the anodic reactions in HF
