Chemistry Reference
In-Depth Information
and as N and P are detected in the oxide formed in water-free methanol
electrolyte.
424
Once water is present in methanol, the major part of the oxygen required
for the oxidation comes from the water instead of the salts.
Formation of the first layers of oxide (i.e., native oxide) on the surface of silicon,
according to Ozanam and Chazalviel,
692,934
appears to also require the presence of water
even in nonaqueous solutions. On immersion into the solution the silicon surface is
gradually evolving from a H-terminated surface (after HF cleaning) to a silicon oxide-
covered surface due to the residual water present in the nonaqueous electrolyte (10
ppm).
692
Initially the water is molecularly adsorbed at the silicon surface, then slowly
oxidizes the surface silicon atoms to form oxide islands. The oxide islands are about
0.6 nm thick and cover about 60% of the surface area after 1 week of immersion in
various nonaqueous electrolytes.
Knowledge regarding the detailed reaction processes involved in anodic oxida-
tion is still lacking. According to Lewerenz,
602
the oxidation process can be initiated by
formation of hydroxide from water if a hole is supplied. Subsequently, two Si-OH
groups form a Si-O bond splitting off a water molecule. The resulting polarization of
the Si-Si back bonds then possibly leads to the insertion of O by place exchange. The
consecutive steps are described as follows:
The reaction steps involve as an intermediate. It is possible that reactions with higher
oxidation states such as and which are considered to be present at the
silicon/oxide interface, may be involved in addition to reactions (3.6) to (3.8).
702
3.4.2. Ionic Transport within Oxide
During anodic oxidation, either the cation or the anion or both must migrate
across the thickening oxide film. The mobile ions can be identified and if both move,
the relative mobilities can be deduced by tagging a thin surface layer with a completely
immobile marker atom and determining its position after anodizing.
346,427
They may also
be identified by profiling the oxide on the concentration of labeled mobile species such
as oxygen isotopes.
449
The issue of mobile species in the oxide during anodic oxida-
tion has not been fully resolved as cation outward movement is found to dominate in
some studies and anion inward movement in others.
186,346,427,449
The results from many investigations show that inward diffusion of oxygen-
containing anions is responsible for the ionic conduction in the oxide film during
anodization. Mackintosh and Plattner
346
used two different noble gases Kr and Xe
as markers and found that the anodic oxide growth in
in NMA,
in tetrahydrofurfuryl alcohol, and
15%
in tetrahydrofurfuryl alcohol is governed by anion inward movement.
implanted into
186
According to Mende,
who determined the depth distributions of
