Chemistry Reference
In-Depth Information
is low, particularly in organic solutions where it is generally less than a few percent.
(3) Except for an initial period, the growth of oxide increases linearly with time, giving
a yield in the range of 2-40 Å/V. (4) For a given solution, there is a maximum voltage
above which physical breakdown of the oxide film occurs. (5) Water plays a critical
role in the growth and properties of the anodic oxides and thus the properties of the
oxides formed in aqueous solutions and organic solutions differ greatly.
3.5.2. Effect of Solution Composition
Anodization of silicon to produce oxide film was first tried in solutions such as
and and borate aqueous solutions.
117
In these solutions, the formation
voltage was limited to less than 200 V. In a 1 M KOH solution, the forming voltage can
only reach 20 V, above which the reaction is dominated by oxygen evolution.
925
Later,
organic solvent-based solutions such as methylacetamide, glycols, and alcohols were
used, which allowed the formation at higher voltages, and thus thicker oxide. The
V
t
curves during anodization in different electrolytes at constant current density are shown
in Fig. 3.2. Since the anodic oxides obtained in organic solutions generally have better
properties, most later investigations of anodic oxides were conducted in these solutions.
Among the organic solvents, ethylene glycol and
N
-methylac-
etamide are the most commonly used for anodic oxidation of
silicon. A small amount of salt, typically 0.04 N is added to increase the con-
ductivity of the solvent. Usually a small amount of water, either as an addition or as
an impurity, is also present in the solution and is important in determining the growth
and properties of the anodic oxides. In terms of formation characteristics, ethylene
glycol and NMA are very similar. Oxidation in both solutions at a constant current
density can be continued to voltages in excess of 500 V. The growth of oxide is a linear
function of potential over a wide range as seen in Fig. 3.3, which shows a yield of
5.3 Å/V for ethylene glycol and 5.6 Å/V for NMA. Also, the current efficiency for the
two solutions and its dependence on water content, lowest at 1-3% water, are very
similar for the two solutions as shown in Fig. 3.4.
107,919
Anodization in other types of
-
