Starting material and wafer production

8/12/2019 Starting material and wafer production

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WAFER PRODUCTIONWAFER PRODUCTIONWAFER PRODUCTIONWAFER PRODUCTION


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Raw materials: Sand,

coke, coal, wood chips

Metallurgical grade silicon

(used in making steel)

Electronic grade

silicon(polysilicon)

Single crystal boule

Single crystal

wafers

Finished wafers

Semiconductor materials used: Silicon

GaAs (only for high frequency applications)

Aim: Obtaining extremely pure silicon to be able to control doping to

control electrical properties accepted level of impurities < 1 over

109 atoms of silicon

Obtaining very regular silicon crystals (defects in the

crystallographic building worsen conduction characteristics of the

semiconductor)

Obtaining a monocrystalline structure rather than a polycrystalline

one (a set of different crystallographic buildings with different

orientations polysilicon): in a polycrystalline structure the

carriers mobility is reduced because of discontinuities betweendifferent crystals !!!

Monocrystals Growth for IntegratedMonocrystals Growth for IntegratedMonocrystals Growth for IntegratedMonocrystals Growth for Integrated

Circuits TechnologyCircuits TechnologyCircuits TechnologyCircuits Technology


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Starting material for silicon is relatively pure quartz based sand (SiO2 orsilica)

Silicon Refining Sand is reduced in an arc furnace with coal, coke, and

wood chips

SiO2 + 2 C Si (solid) + 2CO (gas) @ 2000C for ~ 8 days

This forms metallurgical grade silicon (MGS

): purity ~ 98%

Major impurities (metals):

Fe 0.8%, Al, 0.3%, Cr 0.04%, Ti 0.03%, Mn & V 0.02%

Mg & Ni 0.01%

MGS crushed and reacted with HCl in a fluidized bed to obtain

trichlorosilane (TCS)

3HCl (gas) + Si (solid) --> HSiCl3 (gas) + H2 (gas) @ 325C

Reaction is exothermic, must cool to keep at 325 C to minimize byproduct formation

B and P impurities present in HSiCl3 HSiCl3 is a liquid at room temperature (boiling point at 32C) impurities are

eliminated through fractioned distillation of the liquid

Monocrystals GrowthMonocrystals GrowthMonocrystals GrowthMonocrystals Growth Starting MaterialsStarting MaterialsStarting MaterialsStarting Materials

for Siliconfor Siliconfor Siliconfor Silicon


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Purified TCS is reduced with H2 to obtain electronic-grade silicon (EGS)

HSiCl3 (gas) + H2 (gas) 3HCl (gas) + Si (solid) Reaction (Chemical Vapor Deposition) occurs in a cold chamber of a reactor

containing an heated Si rod which acts as crystalline seed

The result is an extremely pure (~ 1 impurities over 109 atoms) polycrystalline silicon,

which will be the raw material for the production of the monocrystalline one with the

Czochralski (CZ) or with the Float-Zone (FZ) process

The fabrication of slices from pure silicon is based on a large set of steps that are

presented in the following. The approach consists first to fabricate a crystal, precisely

an ingot, which will be further cut in slices that will be transformed afterwards in

wafers.

Monocrystals GrowthMonocrystals GrowthMonocrystals GrowthMonocrystals Growth EGS SiliconEGS SiliconEGS SiliconEGS Silicon


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CzochralskiCzochralskiCzochralskiCzochralski ProcessProcessProcessProcess Crucibles & PullersCrucibles & PullersCrucibles & PullersCrucibles & Pullers

EvolutionEvolutionEvolutionEvolution


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Parts of Ingot DefinitionParts of Ingot DefinitionParts of Ingot DefinitionParts of Ingot Definition


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Doping From The meltDoping From The meltDoping From The meltDoping From The melt

Add dopants to melt to pull doped crystals

dopants added in the form of doped polysilicon

for concentration control.

Concentration of dopant in crystal is not the

same as that in the melt.

segregation coefficient

k=C s

CLCS = Concentration of dopant in solid. [wt dopant/wt solid]

CL = Concentration of dopant in melt. [wt dopant/wt melt]


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Doping from the meltDoping from the meltDoping from the meltDoping from the melt

Since k in general is less than 1, thedopant becomes increasingly more

concentrated in the melt. Dopant concentration changes along the

length of the crystal.

Another problem is oxygen doping due

to the oxygen presents in the crucible(generally made of SiO2)

Dopant P As Bk 0.35 0.3 0.8atomic

weight

30.97 74.92 10.81


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Float Zone (FZ)Float Zone (FZ)Float Zone (FZ)Float Zone (FZ) ProcessProcessProcessProcess

Crystallization is performed by controlling the growth of the seed fixed

at an extremity thanks to a moving of a "fused-zone" (noted "FZ").

This zone is heated at a temperature just under the fusion point by a

high frequency power supply via a coil surrounding the ingot.


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This FZ technique is also involved for purifying the CZ ingot; the principle is to

take benefit of the difference of diffusion of species at high temperature in a

crystal. In this case, up to three coils are set in the system to save time.

Several passes are usually performed for purification.

This technique allows fabricating very lightly doped ingots, very useful for

high power devices or high voltage integrated circuits (quasi-intrinsic zones

allowing high voltage strength).

Float Zone (FZ)Float Zone (FZ)Float Zone (FZ)Float Zone (FZ) ProcessProcessProcessProcess

Technique more expensive than CZ:

Used really very seldom (about 2% of the wafer market are produced by FZ process)

Only for applications where very pure silicon is required


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IIIIIIIIIIII----V Monocrystals GrowthV Monocrystals GrowthV Monocrystals GrowthV Monocrystals Growth

LIQUID ENCAPSULATED CZ (LEC): CZ technique modification for the growth of GaAs

GaAs has a low boiling point and evaporation could result in toxic fumes and non-

uniform crystal growth

Solution: to pressurize the chamber and melt B2O3 (about 1 cm thick) on top to seal and

suppress evaporation

B2O3 does not react with GaAs during growth

Ga melt at 30C, B2O3 melt at 500C and seal the crucible, while Ga and As startreacting at 800C to produce GaAs


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IIIIIIIIIIII----V Monocrystals GrowthV Monocrystals GrowthV Monocrystals GrowthV Monocrystals Growth

LIQUID ENCAPSULATED CZ (LEC):

High pressure As atmosphere in the reactor to avoid Asevaporation (very volatile element)

Crystalline seed introduction and monocrystal growth

Slow cooling down (30-80C/h)

Pyrolitic BN crucibles do not react with GaAs


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Ingot inspection

Undersize

Control of the resistivities on the top and bottom faces of the ingot: due to the variation

of the doping concentration during the pulling, the final resistivity varies in function of

location. A check of resistivity and an agreement with specifications are needed. A four

probe method equipment makes these measurements.

Check of the crystallographic orientation of the ingot

~ 50% rejected

Shaping of ingot

Make it round and of the correct diameter: during the pulling, due to the very large set of

physical parameters to control, the diameter of the ingot slightly varies. That creates

some waves at the ingot surface. To get slices of calibrated diameter suitable for

automatic equipment, a cylindrical polishing is needed.

From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers Ingot cropping

This operation consists in cutting, or cropping,the extremities of the ingot, which are the high

defect concentration regions with a variable

diameter


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From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers

Flat(s) grinding

One or two flat zones on the edge of the ingot areprocessed to get a crystallographic orientation reference

for the wafer fabrication. This reference will be used during

the wafer process (orientation of conducting zones,

crystallographic axes for the die cutting).

Ingot sawing

This sawing is proceeded with a diamond tooth saw (creation of a thickness of powder

equivalent to the saw thickness)


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Edge grinding

After sawing, some "peaks of matter" remain on theperipheral zones of the slices. One has to remove them. In

addition, to make easier the manipulation of the wafers

during the IC's fabrication process, a circle shaped edge is

proceeded; this avoids the degradation of the wafer transfer

equipment but also creating cracks, or dislocations in thecrystal, which lead to definitive brakes.

Wafer lapping and grinding

The thicknesses after sawing can be significantly

different. To decrease the cost, the quantity of matter

to grind has to be minimized. Usually, the wafers aresorted by thickness range of ten micrometers (10

m).

In order to improve their surface quality, the slices

are polished using a mixture that contains alumina or

diamond grains for which the size is about severalmicrometers (final roughness < 2 m).


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Wafer polishing

This polishing can be mechanical or chemico-mechanical based. This operation is performed with an

equipment similar to the lapper, but the polishing

solution is less agressive with a mixture containing

smaller grains of alumina or diamond (the grain

diameters can be as low as 0.1 m) and acid or basicchemical agents.

Mirror finishing is necessary for lithographic steps.


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Wafer cleaning

This step consists in removing the abrasive species and contaminants by ultra puredesionized water rinsing.

Laser marking for ID

The writting of the lots mentionning ingot number, date, etc.. , is performed

through a laser beam scanning. These indications will allow controllingeach wafer during the fabrication process of circuits and devices.


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Finished WaferFinished WaferFinished WaferFinished Wafer

300 mmdiameter

wafer

200 mm

diameterwafer

450 mm

diameter

wafer

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Starting material and wafer production