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In IC technology, thermal oxidation is a way to produce a thin layer
of oxide (usually silicon dioxide) on the surface of a silicon wafer. The
technique forces an oxidizing agent (molecular oxygen or water vapor) to
diffuse into the wafer at high temperature and react with it to consume Si
while forming SiO2.
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Oxide is amorphous and is not as compact as crystalline silicon. Si density is
almost double that of oxide.
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Most thermal oxidation is performed in furnaces, at temperatures between 800
and 1200°C. A single furnace accepts many wafers at the same time, in a
specially designed quartz rack (called a "boat").
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Oxide thickness can be either calculated by simplified growth equations, using
charts or by simulation.
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Notice the difference between dry and wet charts. Wet oxidation is preferred to
dry oxidation for growing thick oxides, because of the higher growth rate.
However, fast oxidation leaves more dangling bonds at the silicon interface,
which produce interface states for electrons and allow lower mobility for
electrons flowing parallel to the interface (MOSFET channel). (This is called a
"dirty" interface.) Wet oxidation also yields a lower-density oxide, with
lower dielectric strength.
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The orientation of the silicon crystal affects oxidation. A <100> wafer oxidizes
more slowly than a <111> wafer, but produces an electrically cleaner oxide
interface. This explains why a CMOS technology starts with <100> wafers.
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This is a method to isolate devices. It is a field thermal oxide (thick oxide)
usually obtained by wet thermal oxidation.
In LOCOS, thermal oxidation is performed on selected areas of a wafer, and
blocked on others. Areas which are not to be oxidized are covered with a film
of silicon nitride, which blocks diffusion of oxygen and water vapor. The
nitride is removed after oxidation is complete. This process cannot produce
sharp features, because lateral (parallel to the surface) diffusion of oxidant
molecules under the nitride mask causes the oxide to protrude into the masked
area.
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When no silicon substrate is available no thermal oxidation will be possible.
Deposited rather than thermally grown oxide is needed. Silicon dioxide
(usually called simply "oxide" in the semiconductor industry) may be
deposited by several different processes. Common source gases
include silane and oxygen, dichlorosilane (SiCl2H2) and nitrous oxide (N2O),
or tetraethylorthosilicate (TEOS; Si(OC2H5)4). The reactions are as follows:
SiH4 + O2 → SiO2 + 2 H2SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2
HClSi(OC2H5)4 → SiO2 + byproducts. The choice of source gas depends on
the thermal stability of the substrate; for instance, aluminium is sensitive to
high temperature. Silane deposits between 300 and 500 °C, dichlorosilane at
around 900 °C, and TEOS between 650 and 750 °C, resulting in a layer of lowtemperature oxide (LTO). However, silane produces a lower-quality oxide
than the other methods (lower dielectric strength, for instance), and it deposits
nonconformally. Any of these reactions may be used in LPCVD, but the silane
reaction is also done in APCVD (Atmospheric Pressure CVD). CVD oxide
invariably has lower quality than thermal oxide, but thermal oxidation can
only be used in the earliest stages of IC manufacturing (gate formation).
Thermal oxidation of any variety produces a higher-quality oxide, with a much
cleaner interface, than chemical vapor deposition of oxide resulting
in LTO layer (reaction of TEOS at about 600 °C). However, the high
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temperatures required to produce High Temperature Oxide (HTO) like thermal oxide
restrict its usability. For instance, in MOSFET processes, thermal oxidation is never
performed after the doping for the source and drain terminals is performed, because it
would disturb the profile of the dopants.
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Differentiate between front end (devices), back end (metal layers), core
module (digital devices which are mainly MOSFETs) and additional analog
modules (caps and high resistances) and RF modules (linear caps MIM and
extremely high thickness M4 layer for spirals).
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