Chemistry Reference
In-Depth Information
3
Anodic Oxide
3.1.
INTRODUCTION
There are many types of silicon oxides such as thermal oxide, CVD oxide, native
oxide, and anodized oxide. Only native oxide and anodic oxide are directly relevant
in the context of this topic. Anodic oxide film, which is involved in most of the
electrochemical processes on silicon electrodes, has not been systematically under-
stood, partly due to its lack of application in mainstream electronic device fabrication,
and partly due to the great diversity of conditions under which anodic oxide can
be formed. On the other hand, thermal oxide, due to its importance in silicon technol-
ogy, has been investigated in extremely fine detail. This chapter will cover some aspects
of thermal oxide such as growth kinetics and physical, electrical, and chemical prop-
erties. The data on anodic oxide will then be described relative to those of thermal
oxide.
The materials discussed in this chapter are limited to the relatively thick oxides
formed at potentials greater than several volts. It thus concerns mainly the growth and
the bulk properties of anodic oxides. The data on thin oxide films are presented in other
chapters. In particular, the oxide films involved in passivation at potentials within a few
volts above OCP are dealt with in Chapter 5. Native oxides, which are almost always
present on the surface of silicon electrodes, are discussed in Chapter 2.
3.2. TYPES OF OXIDES
Silicon oxide can exist in various crystalline forms, such as quartz, as well as
noncrystalline forms, such as vitreous silica. The structure of silicon oxide in a vitre-
constructed from
polyhedra (tetrahedra or triangles) of oxygen ions.
444,720
The centers of these polyhedra
are occupied by silicon ions. The distance between the silicon and oxygen ions is
1.62Å while that between oxygen ions is 2.27Å. polyhedra are joined to one
another by bridging oxygen ions, each of which is common to two such polyhedra. In
crystalline all oxygen ions play this role and all vertices of the polyhedra are tied
to their nearest neighbors by these ions. In vitreous form, however, some of the ver-
tices have nonbridging oxygen ions which belong to only one polyhedron.
ous state consists of a random three-dimensional network of
91
