Chemistry Reference
In-Depth Information
At high field current, instability occurs and can lead to dielectric breakdown of
films. The breakdown of thermally grown silicon dioxide films depends on the
processing conditions such as oxide thickness, substrate doping concentration and ori-
entation, cleaning procedures, oxidation temperature, and testing conditions.
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The
breakdown fields under a given set of conditions follow a Gaussian distribution skewed
at low fields. The breakdown fields near the maximum (8-16MV/cm) represent the
primary type of breakdown, which is related to the intrinsic dielectric properties of the
oxide; those at fields more than 20% lower than the maximum are low field break-
downs, which are related to the defects in the oxide. Breakdown may also be differen-
tiated in terms of electric breakdown and dielectric breakdown.
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The breakdown at high fields results from the destabilization of the conduction
in the oxide which is associated with carriers generated by the collision ionization
process.
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The tunneling-through electrons drift under the influence of the field and
can gain enough energy above a critical field to create additional carriers through col-
lision, thereby forming a space charge layer. For example, with a mean free path of 34
Å and a field strength of 8 MV/cm the injected carriers are able to excite traps 2.7 eV
below the conduction band. The electron-hole pairs are driven under the field to oppo-
site electrodes. A buildup of the positive space charge in the vicinity of the cathode
further lowers the barrier for tunneling. Such a positive feedback sequence eventually
leads to a large current and insulator breakdown.
Anodic Oxides.
The electrical properties reported in the literature on anodic
oxides, such as breakdown potential, dielectric constant, and leakage current, tend to
vary over a wide range due to the large differences among oxides, which are formed
in the various electrolytes. Some of these parameters are listed in Table 3.2. In general,
the electrical properties of anodically formed oxides are of poor quality relative to those
of thermal oxides due to the high concentration of charges and states associated with
the loose structure and high levels of impurities in anodic oxides.
The dielectric constant of anodic oxides is considerably higher than that of thermal
oxides. This higher dielectric constant can be attributed to the presence of a significant
and silanols and a nonstoichiometric composition.
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The breakdown
potentials of anodic oxide films are generally lower than those of thermal oxides. It also
in general decreases with increasing thickness.
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The breakdown field strongly depends
amount of
on the conditions of anodization. For example, Fig. 3.28 shows that for the oxides
formed in 0.04N
in ethylene glycol, the breakdown field decreases with increas-
Similar results were found in other studies.
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The breakdown potential of the oxide formed in solutions containing trace amounts of
fluorine is inversely proportional to the fluorine content in the oxide.
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The breakdown
228
ing water content in the electrolyte.
voltage of some anodic oxides can be comparable to thermal oxides. For example, values
solution.
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Figure 3.29 shows that the effective interface charge density and the flatband
potential is a function of water content for the oxides formed in ethylene glycol con-
taining 0.04 N at
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The charge density also depends on anodization
mode, postanodization cleaning, and annealing treatment.
Anodic oxide films have high leakage currents.
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The leakage current is a func-
tion of anodization condition. Figure 3.30 shows the
i-V
curves measured on the anodic
oxide films formed in NMA + 0.04 N under a constant current density of
to 300 V and held at 300 V for different times.
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As can be seen, the current
between 5 and 10 MV/cm are found for oxides formed in
