Chemistry Reference
In-Depth Information
can be formed by passing an anodic current across the surface in an electrolyte; such
oxides are called anodic oxides.
3.2.5. Use of Oxides in Device Fabrication
In the fabrication of semiconductor devices, silicon dioxide is widely used as gate
insulator of metal oxide semiconductor (MOS) devices, planarization of interlayer
dielectrics, isolation, passivation layers, and masking material.
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In particular, the
ability of thermally grown dioxide films to reduce the number of electrically active
surface defects is most important.
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Since the current is essentially carried along the
interface, passivation is required for the operation of MOS devices. films
are also used to insulate conducting layers from each other or to isolate individual
devices on a common silicon substrate. In addition, thin oxides are used in making
capacitors in the circuit. The amount of charge stored on these capacitors is inversely
proportional to the oxide thickness. Silicon dioxide films doped with phosphorus or
boron are also used as doping sources for the underlying silicon. Phosphorus-doped
films find many uses in protecting the underlying circuits against corrosion, ionic
impurities, and mechanical damage.
Passivation coatings may be classified as primary if they are directly in contact
with the single-crystal silicon from which the device is fabricated, and as secondary if
they are separated from the silicon by an underlying dielectric layer.
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The function of
the primary passivation layer is to provide good dielectric properties, low surface
recombination velocity, controlled immobile charge density, and device stability at the
elevated temperatures under bias or operating conditions. The secondary passivation
layer provides additional stability in various ambients, in both production and use, and
serves as getter, impurity barrier, or mechanical shield. Anodic oxides, due to their
relatively poor electrical properties, have not found much use in device technology.
3.3. FORMATION OF ANODIC OXIDES
3.3.1. General
Anodization of silicon to form a silicon oxide film can be carried out in differ-
ent organic and inorganic solutions under a constant potential or a constant current
density. During anodization under a constant current, the voltage typically increases
with time along with an increase in oxide thickness. At a certain voltage depending on
the anodization conditions, the voltage stops increasing or starts oscillating and decreas-
ing. At this voltage, the oxide film is physically broken down often accompanying the
formation of pits and nodules.
Anodization systems of various electrolyte compositions and polarization condi-
tions have been explored as can be seen in Table 3.1. They are characterized by the
yield (thickness per volt), current efficiency (CE; ionic current responsible for oxide
growth), and maximum voltage, (highest attainable anodic voltage). Several
general remarks may be made based on the data in Table 3.1. (1) A high field is required
for the growth of anodic film, on the order of
(2) Ionic current efficiency
