Chemistry Reference
In-Depth Information
Anodic Oxides.
Except for the first few layers of oxide, which almost always
exist on the surface of silicon in an anodization electrolyte as the native oxide, the
growth of the oxide film requires the diffusion and/or migration of the reactants in the
form of cations or anions or both through the oxide film. In comparison with thermal
oxidation, anodization operates at low temperatures at which the diffusion of either
silicon species or oxidants (as can be estimated based on the information about thermal
oxidation at high temperature) must be too slow to account for the oxide growth
observed during anodization of silicon. Mass transport required during anodization
must rely on field assisted migration. The field strength required to produce the ionic
current for oxide growth on silicon is very high, in the range from
to
depending on growth rate and electrolyte composition.
Young and Zobel
961
found that the growth of anodic oxide on silicon follows the
high field mechanism which describes the relationship between the ionic current and
the field as: where is the magnitude of the
charge on the ion,
a
is half of the distance between successive sites occupied by the
ions, and is a constant. Figure 3.21 shows the data for anodization in 0.0025 N
in anhydrous NMA giving a line
with
a
being 0.6 to
1.5 Å.
Ghowsi and Gale
201
developed an analytical model, which agrees with the phys-
ical model illustrated in Fig. 3.19, for the growth kinetics of anodic taking into
account both ionic current and electronic current. The total current density, including
the ionic component and electronic component,
is
The inward motion of anions is assumed to be the dominant ionic transport across the
oxide. The ionic movement is field-assisted drifting and is the rate-limiting process.
The diffusion of ions at room temperature is considered to be too slow to account for
the oxide growth rates. The current density is written as
