Chemistry Reference
In-Depth Information
This structure of vitreous silica can be modified by the presence of impurities,
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which can be substitutional and interstitial, as well as by water. Substitutional impu-
and replace the silicon in a silica polyhedron;
they are termed network formers. Since such substitutional cations have a valence of
either 3 or 5, their presence in the lattice results in charge defects. The interstitial impu-
rities are usually large metal ions of low positive charges which enter into the network
interstitially between the polyhedra. As a result, the polyhedra give up their oxygen to
the interstitial, producing non-bridging oxygen ions. Impurities of this type are called
network modifiers. Water is also present in the oxide structure incorporated during a
wet oxidation process or as a contaminant in a dry process. On entering the oxide the
water combines with bridging oxygen ions to form pairs of stable nonbridging hydroxyl
groups. The various types of defects are schematically illustrated in the basic structure
of vitreous silica shown in Fig. 3.1.
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Silicon oxide films can be produced by different methods, such as thermal
oxidation or chemical vapor deposition, and have a diverse range of structures and
rities, the most common being
properties. Characteristically, different types of oxide have distinct densities, for
for quartz,
444
for thermal oxide,
118,310
example,
139
and
in DI water.
457
for the anodic oxide formed in
3.2.1. Thermal Oxide
Growth of thermal silicon oxide is typically carried out in a quartz tube at tem-
peratures between 900 and 1200 °C in dry oxygen or wet oxygen that contains water
vapor, or steam formed by passing dry oxygen and nitrogen through water near the
boiling temperature.
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The oxidation in wet oxygen or steam is much faster than in
dry oxygen. The overall reactions in thermal oxidation are:
