Chemical-Vapor-Deposited Materials for High Thermal Conductivity Applications
an attractive alternative. In addition, the
high stiffness-to-weight ratio of CVD SiC
allows the susceptor to have low weight
and good surface flatness. This low mass,
coupled with low heat capacity and high
thermal conductivity, keeps the heat
ramps rapid and contributes to tempera-
ture uniformity over the wafer. The CVD
SiC susceptors also do not readily degrade
during hot HCl cleaning cycles, they per-
mit tight susceptor tolerances due to a close
thermal-expansion match with Si, they
generate fewer particulates, and they can
be thermally cycled many more times
than other competing materials. The ex-
cellent machinability and process repro-
ducibility of CVD SiC ensures that the
parts are fabricated to the same shape
with consistent high quality.
In addition to susceptors, slip rings con-
stitute another critical element in RTP re-
actors. Such a ring surrounds the wafer
and is usually slightly offset from its plane.
The slip rings serve to make the radial
temperature profile more uniform in the
wafer. Without a slip ring, the edge of the
wafer is hotter than the rest of it during
ramping, while in steady state, there is a
thermal loss at the edge. CVD SiC provides
definite advantages over other materials
because of its high thermal conductivity,
predictable absorption and emission char-
acteristics, higher purity, and lower par-
ticle generation.
Semiconductor furnaces often employ
high temperatures for oxidation or diffu-
sion and use support components such as
cantilevers, wafer carriers, support tubes,
and paddles. Currently used cantilevers
are heavy, conduct heat poorly, and have a
coefficient of thermal expansion that is
very different from silicon. Quartz is rela-
tively fragile, produces more particulates,
cannot stand HF in wet processing, and
may contain impurities, such as sodium.
For all these reasons, CVD SiC offers an
attractive alternative. Currently, efforts are
being made to fabricate these support
components from CVD SiC.
The resistivity of CVD SiC can be tailored
in the range of 0.01–1000 ꢅ cm without
significantly affecting its other properties.
This large range of values makes CVD SiC
very attractive for fabricating a variety of
support components for use in plasma-
etch chambers. Gas-diffusion plates and
focusing rings are made from high-
resistivity SiC, while the liners and plasma
screens are made from low-resistivity SiC.
Components made from CVD SiC do not
degrade readily and last for a long time in
the hostile plasma environment. This re-
duces equipment downtime and makes
this material very competitive in terms of
cost of ownership.
Low-resistivity SiC may also be used to
make susceptors for the coupling of rf en-
ergy in semiconductor furnaces. The high
thermal-shock resistance of CVD SiC per-
mits heating these susceptors very rapidly
to high temperatures (ꢂ1200ꢃC). These sus-
ceptors are better than graphite susceptors
due to their high purity, extremely low
particulate shedding, and low wear rate.
particularly high thermal conductivity.
This method has successfully produced
polycrystalline diamond and SiC with
thermal conductivities ꢂ75% of the corre-
sponding single-crystal values. The ther-
mal conductivity values of CVD diamond
and CVD SiC depend upon the particular
growth method and specific process con-
ditions used for growth. In general, high
thermal conductivity values are obtained
along the columnar grains in material that
is grown under optimum conditions. For
diamond, these optimum conditions in-
volve low growth rates that produce opti-
cally clear diamond. For SiC, the optimum
conditions are those that produce a low
density of stacking faults. The primary
application of CVD diamond is in semi-
conductor and electronic devices for
thermal-management applications, whereas
CVD SiC is also used for high-heat-load
applications in rapid thermal processing
and plasma-etch systems, rf-heated sus-
ceptors, synchrotron and high-energy
laser mirrors, and optics molds.
Optics,Wear, andThermal-Management
Applications. In optics, CVD SiC has been
used to fabricate lightweight mirrors, x-ray
grazing incidence mirrors, optics standards,
and optics baffles.52–58 The CVD SiC mir-
rors are used in surveillance, high-energy
lasers, laser radar systems, synchrotron
x-ray equipment, vacuum UV telescopes,
large astronomical telescopes, and weather
satellites. Active cooling through heat-
exchanger channels or patterns is employed
in optics to manage high heat loads. These
patterns can be fabricated on the back
side of the CVD SiC mirror face plates di-
rectly in the CVD chamber by a near-net-
shape replication process. Deposition
occurs layer by layer on a molecular scale
and replicates patterns down to very fine
details. This replication of fine features
was demonstrated in CVD SiC during a
thermally controlled tertiary mirror (TCTM)
program.59
High-resistivity CVD SiC can be used
in the electronics industry for thermal-
management applications. Since the co-
efficient of thermal expansion of CVD SiC
is compatible with Si, high-power and
high-performance Si devices can be pro-
duced directly on the SiC substrates.
Furthermore, CVD SiC can be readily met-
allized with a variety of materials such as
silver, gold, TiN, and Mo. This permits
fabrication of more complex patterns and
structures on SiC substrates.
CVD SiC has been successfully used as
a substrate material for making optics
molds because of its high value of thermal
conductivity, elastic modulus, and flexural
strength, and its resistance to abrasion,
scratching, oxidation, and corrosion. The
use of CVD SiC provides a more uniform
temperature over the whole surface of
glass or plastic optics, thus minimizing
residual stress during lens cooldown. The
significant advantages of CVD SiC are
particularly realized when large-area op-
tics molds are used. Furthermore, CVD
SiC molds are robust and have been suc-
cessfully used for fabricating hundreds of
optics parts from a single mold.
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Chemical vapor deposition is an attrac-
tive method for producing a variety of
materials possessing superior properties,
462
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