Chemistry Reference
In-Depth Information
3.4. GROWTH MECHANISMS
3.4.1. Reactions
The oxidation reaction of silicon is a process involving breaking Si-Si bonds and
formation of Si-O bonds with conversion of the silicon atoms from a valence of 0
to a valence of 4. In anodization the four valence electrons on a silicon atom are
transferred in the overall reaction
The water involved in the above reaction is the main source of oxygen in the
formation of anodic oxide in various electrolytes, aqueous and nonaqueous. According
to Croset
et al
.,
301
in NMA and glycol solutions, over 80% of the oxygen in the oxide
comes from the water and the rest from the salts, while the solvents play little role
as a direct source of oxygen. Therefore, when water is present in the electrolyte the
source of oxygen does not depend on the nature of the solvents. However, when
water is not present such as in dry organic solutions, the main reaction at the anode
is the production of water via oxidation of the solvents, which supply the water to
sustain the oxidation reaction, and this is responsible for the low oxidation efficiency
in the growth of anodic oxide. The amount of water available depends on the solvent:
about 0.5 mol/Faraday of water is created at the anode in NMA and 1 mol/Faraday in
glycol.
301
In dry electrolytes (initially water free) the oxygen required for oxidation can
either come from the water due to the anodic decomposition of the solvent or come
from the dissolved salts. According to Madou
et al
.,
404
most oxygen required for oxide
growth in dry glycol stems from the water derived from the decomposition reaction of
glycol. Oxygen-containing electrolytes such as or are not essential for
the anodization process in NMA solutions as anodization is found to be possible in a
solution containing NaF and
107
On the other hand, in dry methanol which does
not decompose into water easily, the oxygen may originate from oxyanions such as
