Chemistry Reference
In-Depth Information
Anodic oxides are thus less dense than thermal oxide films due to the incorpora-
tion of foreign species. The density may vary greatly with the conditions of anodiza-
tion and densities as low as have been reported.
457
The oxide formed in
ammonium borate solution is relatively dense and those formed in oxalic acid or
chromate solutions are less dense.
117
The less dense nature of anodic oxides is also
reflected in the etch rate, which is generally many times higher than for thermal oxides
as shown in Table 3.2. The etching reaction proceeds first by adsorption of the reac-
tants onto the active sites on the oxide surface, which are associated with the defects
and discontinuities of the polyhedra structure. An oxide film with porous struc-
ture thus has more active sites available for the etching reaction, leading to a higher
etch rate.
Anodic films can be densified by annealing in inert gases at high temperatures.
A reduction of 10 to 30% in oxide thickness has been found to result from anneal-
139,457,793
The hydride SiH groups in as-formed anodic oxide films can be reduced
by two orders of magnitude by heat treatment at 300 °C to a level similar to that of
thermally oxidized oxides.
424
The oxide grown in 1.35 M contains about 1 atom
% N and can be reduced to <0.5 atom % by annealing the film at 500 °C for 5min in
forming gas.
449
The properties of anodic oxide films depend on the mode of polarization partic-
ularly at the initial stage of anodization. For example, the number of hydroxyls in as-
grown oxide formed in
ing.
tetrahydrofurfuryl at a constant current
density of
This is reduced to by
switching the oxidation mode to a constant potential from the initial constant current
mode.
424
The potential and how fast the potential is established determine the structure
of the first few layers of oxide, and greatly affect the further growth of oxide. It is found
that in 0.1 M HCl electrolyte, if the initial oxide is grown at a potential below 5.5
then the final oxide will be thin and have a lower hydroxyl content.
98
Structural change
is about
T
424
is found to occur for oxide that is oxidized at a time beyond the maximum forming
ethylene glycol containing 0.04 M
potential in
after anodization at 300 V for
35 h, the oxide becomes opaque.
117
The properties of an anodic oxide are not uniform with distance but change from
the interface to the interface. It has been found that the refrac-
tive index is about 1.37 at the interface but gradually increases to about 1.42
in the bulk oxide.
603
The refractive index of the oxide films formed in ethylene
solution generally decreases with increasing thickness.
919
The anodized
427
silicon surface is more heavily hydrated. Figure 3.16 shows that the concentration of
hydroxyls increases toward the silicon/silicon oxide interface.
424
There are no data on
the potential of zero charge (pzc) for anodic oxides. Data obtained on thermal oxides
indicate that the pzc of silicon oxide lies between pH 2 and 3
903,943
and a linear rela-
tionship exists between the Fermi energy and pzc.
946
Interface.
There is little information on the interface of silicon and an
anodic oxide film. For thermally grown oxides, a transition region exists at the
interface where there is an excess of unoxidized Si bonds with a density on the order
of the surface atom density. The interface structurally consists of two distinct regions.
A few atomic layers near the interface contain Si atoms in intermediate oxidation states,
i.e.,
and
The
atoms are located farther out than
