Wafer Prep Ration 1

8/6/2019 Wafer Prep Ration 1

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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.

Wafer Prep Ration 1

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