Chemistry Reference
In-Depth Information
properties are concerned, there is also a difficulty of finding the physical basis. It is
known that the oxide films formed under anodic polarization are amorphous in nature
and vary in stoichiometry and density. It is unlikely that an amorphous structure would
have sudden phase transformations like a crystalline structure. Amorphous structure,
being random in its atomic arrangement, should change gradually rather than suddenly.
Lacking the physical basis for the models, none of them can rationally explain the con-
ditions for current oscillation to occur.
A New Model.
The results of the studies on anodic oxide films (see section 5.9
and chapter 3 on passive film and anodic oxides) show that anodic oxide properties
(oxidation state, degree of hydration, O/Si ratio, degree of crystallinity, electronic and
ionic conductivities, and etch rate) are a function of the formation field (the applied
potential). Also, they vary from the surface to the oxide/silicon interface, which means
that they change with time as the layer of oxide near the oxide/silicon interface moves
to the surface during the formation and dissolution process. The oxide near the
silicon/oxide interface is more disordered in composition and structure than that in the
bulk of the oxide film. Also, the degree of disorder depends on the formation field which
is a function of thickness and potential. The range of disorder in the oxide structure is
thus responsible for the variation in the etch rate of the oxide formed at different times
during a period of the oscillation. The etch rate of silicon oxides is very sensitive to
the structure and composition (see Chapter 4).
As illustrated in Fig. 5.59, the oxides at points A and B in the same film have
different properties at a given time. The properties of the oxide at A, when it moves
to the surface, gradually change to become identical to those of the oxide at B within
