Chemistry Reference
In-Depth Information
oxide/electrolyte interfaces, silicon/oxide interfaces, and localized states in the oxide
may result in photoemission.
3.4.6. Growth Kinetics
Thermal Oxides.
Before discussing anodic oxides it is beneficial to provide a
brief description of the classic theory of growth kinetics of thermal oxides. The growth
of thermal oxides follows different kinetics depending on the thickness of the oxide.
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Thick films, from about several hundred angstroms upwards, grow according
to linear-parabolic kinetics where the transport of oxidant through the oxide is the
rate-limiting step. Below this oxidation regime is a thin oxide regime in which the
kinetics is not described by linear-parabolic kinetics but is determined by the inter-
face reactions. This regime can be further divided into two regimes: one from 0 to
about 10 Å, representing the native oxide, and the other from 10 to several hundred
angstroms.
While the kinetics of the thick oxide regime is well established, that of the native
oxide and thin oxide regimes is not, due to the complexity of the reactions involved at
the interface. As discussed in the section on native oxide, the thickness and growth rate
of native oxides strongly depend on surface preparation and the environmental condi-
tions regarding exposure. It is thus very difficult to prepare a native oxide of a certain
quality. The thin oxide regime has been studied to a great extent not only because
of its importance in affecting the nature of the thermal oxide but also because of its
potential use for very thin silicon oxide/silicon systems.
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In the thin oxide regime,
the oxidation kinetics for wet and dry oxidants is, unlike in the thick oxide regime, very
different. The kinetics in this regime is influenced by the nature of oxidants for the
surface reaction such as the presence of both O and or by the strain present due to
the lattice mismatch between Si and or by the transport through micropores, or
by the presence of impurities at the interface regime to alter the oxidation reaction.
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The transport of oxygen consists of two parts, one related to the interstitial transport of
through the network and the other to a step-by-step motion of network oxygen
atoms.
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A significant part of the oxygen diffusion may be of ionic nature since an
electric field is found to affect the growth of oxide.
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The general growth kinetics of thermal silicon oxide was originally proposed
by Deal and Grove.
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It is assumed that the transported species go through three
stages: (1) from bulk of the gas to the outer surface, (2) across the oxide film
toward the silicon, and (3) reaction at the silicon surface to form a new layer of
Figure 3.20 illustrates the physical situation. It is further assumed that the fluxes of
oxidant in each of the three steps in the oxidation process are identical beyond an
initial transient period. The flux of the oxidant from the gas to the vicinity of the outer
surface is
where
h
is a gas-phase transport coefficient,
is the concentration of the oxidant at
the outer surface, and
is the equilibrium concentration of the oxidant in the oxide.
