wafer prep ration 1
TRANSCRIPT
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Crystal Growth and WaferPreparation
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Material Preparation
Material preparation is the beginning of
the process in making an IC chip .
The goal for this part of the process is to
grow the ingot that will be sliced up into
wafers.
The wafer is a round solid silicon discthat will have all of the processing
performed on it.
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Step 1: Obtaining the Sand
The sand used to grow the wafers has
to be a very clean and good form of
silicon.
For this reason not just any sand
scraped off the beach will do.
Most of the sand used for theseprocesses is shipped from the beaches
of Australia.
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Step 2: Preparing the MoltenSilicon Bath
The sand is taken and put into a pot
where it is heated to about 1600degrees C where it melts.
The molten sand will become the
source that will ultimately produce theraw poly-crystalline silicon.
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Raw Polysilicon
Raw polycrystalline silicon
produced by mixing refined
trichlorosilane with hydrogengas in a reaction furnace.
The poly-crystalline silicon is
allowed to grow on the
surface of electrically heatedtantalum hollow metal wicks
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Polysilicon Ingots
The polycrystalline silicon
tubes refined by dissolving in
hydrofluoric acid producing
polysilicon ingots.
Polycrystalline silicon has
randomly oriented crystallites,
electrical characteristics not
ready for device fabrication.
Must be transformed into
single crystal silicon using
crystal pulling
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Examples of completed ingots
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IngotSizes
Most ingots
produced today are
150mm (6") and200mm (8")
diameter,
For the most current
technology 300mm(12") and 400mm
(16") diameter ingots
are being
developed.
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Ingot Characterization
Single Crystal Silicon ingots are
characterized by the orientation of their
silicon crystals. Before the ingot is cut
into wafers, one or two "flats" are ground
into the diameter of the ingot to mark
this orientation.
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Different flats for orientation
Each of the wafers
is given either a
notch or a flat edge This will be used in
orienting the wafer
into the exact
position for laterprocedures.
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Lattice Orientation
This lattice
orientation is
used for MOS(metaloxide
semiconductor),
Bi-CMOS, &
GaAs types ofchips.
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Lattice Orientation
The lattice orientation refers to the
organized pattern of the silicon crystals in
the wafer and their orientation to thesurface.
The orientation is obtained based on the
orientation of the crystal that is placed intothe molten silicon bath.
The different orientations have different
benefits and are used in different types of
chips.
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Lattice Orientation
This
orientationis used for
Bipolar
types of
chips
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Step 4: Preparing the Wafers
The ingot is ground
into the correct
diameter for thewafers.
Then it is sliced into
very thin wafers.
This is usually done
with a diamond saw.
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Some wafers in storage trays
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Wafer Lapping
The sliced wafers
are mechanically
lapped using a
counter-rotatinglapping machine
and aluminum oxide
slurry. This flattens
the wafer surfaces,makes them parallel
and reduces
mechanical defects
like saw markings
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Wafer LappingMachine
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WaferEtching
After lapping, wafers are etched in a
solution ofnitric acid/ acetic acid or
sodium hydroxide to removemicroscopic cracks or surface damage
created by the lapping process.
The acid or caustic solution is removedby a series ofhigh-purity RO/DI water
baths
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Wafer polishing
Next, the wafers are
polished in a series of
combination chemical
and mechanical polishprocesses called CMP
The wafers are held in
a hard ceramic chuck
using either wax bondor vacuum and buffed
with a slurry of silica
powder, RO/DI water
and sodium hydroxide
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Wafer Cleaning
Most wafer manufacturers use a 3-step
process which starts with an SC1 solution
(ammonia, hydrogen peroxide and RO/DI
water) to remove organic impurities andparticles from the wafer surface.
Next, natural oxides and metal impurities are
removed with hydrofluoric acid. Finally, the SC2 solution, (hydrochloric acid
and hydrogen peroxide), causes super clean
new natural oxides to grow on the surface.