Semiconductor Grade Silicon

8/17/2019 Semiconductor Grade Silicon

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Semiconductor-Grade Silicon

Steps to Obtaining Semiconductor Grade Silicon (SGS)Step Description of Process Reaction

1

Produce metallurgical gradesilicon (MGS) by heating

silica with carbonSiC (s) + SiO2 (s) Si (l) + SiO(g) + CO (g)

2

Purify MG silicon through a

chemical reaction to producea silicon-bearing gas of

trichlorosilane (SiHCl3)

Si (s) + 3HCl (g) SiHCl3 (g) + H2 (g) + heat

3

SiHCl3 and hydrogen react in

a process called Siemens toobtain pure semiconductor-

grade silicon (SGS)

2SiHCl3 (g) + 2H2 (g) 2Si (s) + 6HCl (g)

• Si is purified from SiO2 (sand) by refining, distillation and CVD.

• It contains < 1 ppb impurities. Pulled crystals contain O (~1018 cm-3) and C

(~1016 cm-3), plus dopants placed in the melt.


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CZ Crystal Puller

Crystal seed

Molten polysilicon

Heat shield

Water jacket

Single crystalsilicon

Quartzcrucible

Carbon heating

element

Crystal pullerand rotation

mechanism


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• All Si wafers come from

“Czochralski” grown crystals.

• Polysilicon is melted, then held

just below 1417 °C, and a

single crystal seed starts the

growth.

• Pull rate, melt temperature androtation rate control the growth


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Silicon Ingot Grown by CZ Method

Photograph courtesy of Kayex Corp., 300 mm Si ingot


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CYLINDER OF MONOCRYSTALLINE

• The Silicon Cylinder is Known as

an Ingot

• Typical Ingot is About 1 or 2

Meters in Length

• Can be Sliced into Hundreds ofSmaller Circular Pieces Called

Wafers

• Each Wafer Yields Hundreds or

Thousands of Integrated Circuits


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An alternative process is the “Float Zone”process which can be used for refining or single

crystal growth.


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• In the float zone process, dopants and other impurities are rejected bythe regrowing silicon crystal. Impurities tend to stay in the liquid and

refining can be accomplished, especially with multiple passes.


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Float Zone Crystal Growth

RF

Gas inlet (inert)

Molten zone

Traveling

RF coil

Polycrystallinerod (silicon)

Seed crystal

Inert gas out

Chuck

Chuck


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Segregation Fraction for FZ Refining


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Dopant Concentration Nomenclature

Concentration (Atoms/cm3)

DopantMaterial

Type< 1014

(Very Lightly Doped)

1014

to 1016

(Lightly Doped)

1016

to 1019

(Doped)>10

19

(Heavily Doped)

Pentavalent n n-- n- n n+

Trivalent p p-- p

- p p

+


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Basic Process Steps for Wafer Preparation

Crystal GrowthCrystal Growth

ShapingShaping

Wafer SlicingWafer Slicing

Wafer Lappingand Edge Grind Wafer Lappingand Edge Grind

EtchingEtching

PolishingPolishing

CleaningCleaning

InspectionInspection

PackagingPackaging

Preparation of Silicon Wafers


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Preparation of Silicon Wafers

1. Crystal Growth

2. Single Crystal Ingot

3. Crystal Trimming and

Diameter Grind

4. Flat Grinding

5. Wafer Slicing

6. Edge Rounding

7. Lapping

8. Wafer Etching

9. Polishing

10. Wafer Inspection

Slurry

Polishing table

Polishinghead

Polysilicon Seed crystal

Heater

Crucible


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Stages of IC Fabrication

Wafer Preparationincludes crystal

growing, rounding,slicing and polishing.

Wafer Fabricationincludes cleaning,

layering, patterning,etching and doping.

Assembly and Packaging:

The wafer is cutalong scribe lines

to separate each die.

Metal connections

are made and the

chip is encapsulated.

Test/Sort includes

probing, testing and sorting of each die on

the wafer.

Final Test ensures IC

passes electrical and environmental

testing.

Defective die

1.

2.

3.

Scribe line

A single die

Assembly Packaging

4.

5.

Wafers sliced from ingot

Single crystal silicon

I t Di t G i d


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Ingot Diameter Grind

Flat grind

Diametergrind

Preparing crystal ingot for grinding


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Internal Diameter Saw

Internal diameterwafer saw

Wafer Polishing


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Wafer Polishing

D bl Sid d W f P li h


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Double-Sided Wafer Polish

Upper polishing pad

Lower polishing pad

Wafer

Slurry


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Polished Wafer Edge

Chemical Etch of Wafer Surface


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Chemical Etch of Wafer Surfaceto Remove Sawing Damage

W f Di i & Att ib t


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Wafer Dimensions & Attributes

Diameter

(mm)

Thickness

(µm)

Area

(cm2)

Weight

(grams/lbs)

Weight/25

Wafers (lbs)

150 675 ± 20 176.71 28 / 0.06 1.5

200 725 ± 20 314.16 53.08 / 0.12 3300 775 ± 20 706.86 127.64 / 0.28 7

400 825 ± 20 1256.64 241.56 / 0.53 13

Increase in Number of Chips


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Increase in Number of Chipson Larger Wafer Diameters

(Assume large 1.5 x 1.5 cm microprocessors)

88 die200-mm wafer

232 die300-mm wafer

Quality Measures


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Quality Measures

• Physical dimensions

• Flatness

• Microroughness

• Oxygen content

• Crystal defects• Particles

• Bulk resistivity


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“Backside Gettering” to Purify Silicon

Polished Surface

Backside Implant: Ar (50 keV, 1015/cm2)

The argon amorphizes the back side of the silicon. The wafer is

heated to 550oC, which regrows the silicon. However, the

argon can not be absorbed by the silicon crystal so it

precipitates into micro-bubbles and prevents some damagefrom annealing. The wafer is held at 550oC for several hours,

and all mobile metal contaminants are attracted to and then

captured by the argon stabilized damage. Once captured, theynever leave these sites.

Measurement of Wafer Characteristics


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Darkfield and Brightfield Detection

Brightfield imaging

Two-way mirror

Light source

Lens

Viewingoptics

Viewingoptics

Darkfield imaging

L i g h t s

o u r c

eLens

Light reflected bysurface irregularities

Principle of Confocal Microscopy


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Principle of Confocal Microscopy

Detector

Pinhole

Wafer is driven up and

down along Z-axis

Laser

Pinhole

Beam splitter

Objective lens

Center of focus+Z

-Z0

Particle Detection by Light Scattering


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Particle Detection by Light Scattering

Incident light

Beam scanning

Photo detector

Particle

Wafer motion Scattered light

Reflected lightDetection of

scattered light

Four Point Probe


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Four Point Probe

Wafer

R

Voltmeter

Constant current source

V

Iρs =

V

Ix 2πs (ohms-cm)

“Van der Pauw” Sheet Resistivity


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y(similar to 4-point probe, but uses shapes on wafer)

I

(a)

(c) (d)

ContactConductive material

V

(b)


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Conceptual representation of Hall effect measurement.The right sketch is a top view of a more practicalimplementation.

Epitaxy


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Epitaxy

SPE (Solid Phase Epitaxy)

LPE (Liquid Phase Epitaxy)

LPEE (Liquid Phase Electroepitaxy)

VPE (Vapour (Vapor) Phase Epitaxy)

CVD (Chemical Vapour Deposition)

PVD (Physical Vapour Deposition)

Epitaxy


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Epitaxy

Molecular epitaxy -

MBE (Molecular beam epitaxy)

SSMBE = SolidSource MBE,

CBE = ChemicalBeamEpitaxy,GSMBE = GasSource MBE (HydrideSource MOMBE, MetalOrganic

MBE),

UHV ALE = UltraHighVacuum AtomicLayerEpitaxy

Organometalic epitaxy -

MOVPE (MetalOrganic Vapour Phase Epitaxy)

MOCVD (MetalOrganic Chemical Vapour Deposition)

Photo-MOVPE

Plasma-MOVPE


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CVD


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http://www.fzu.cz/oddeleni/povrchy/mbe/mbe.php


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http://www.fzu.cz/oddeleni/povrchy/mbe/mbe.php


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http://www.fzu.cz/texty/brana/movpe/movpe.php


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Semiconductor Grade Silicon

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