Wafer Manufacturing

Farshid Karbassian

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OutlineOutline

• Semiconductor Materials• Purification• Crystal pulling

• Czochralski• Float-Zone

• Grinding• Slicing

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OutlineOutline

• Edge Rounding• Lapping• Etching• Chemical Mechanical Polishing

(CMP)• Epitaxial Deposition

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Semiconductor MaterialsSemiconductor Materials

• ElementalSi, Ge

• Binary CompoundsIV-IV SiC III-V AlP, AlAs, AlSb, GaN, GaP, GaAs,

GaSb, InP, InAs, InSbII-VI ZnO, ZnS, ZnSe, ZnTe, CdS,

CdSe, CdTe, HgSIV-VI PbS,PbSe,PbTe

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Semiconductor MaterialsSemiconductor Materials

• Ternary Compound AlxGa1-x As, AlxIn1-x As, GaAs1-xPx ,

GaxIn1-x As, GaxIn1-xP

• Quaternary Compound AlxGa1-x As1-ySby , GaxIn1-x As1-yPy

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Crystal GrowthCrystal Growth

ShapingShaping

Wafer SlicingWafer Slicing

Wafer Lapping and Edge GrindWafer Lapping and Edge Grind

EtchingEtching

PolishingPolishing

CleaningCleaning

InspectionInspection

PackagingPackaging

PurificationPurification

Basic Process StepsBasic Process Steps

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

Polishing head

PolysiliconSeed crystal

Heater

Crucible

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Purification of SiliconPurification of Silicon

Common quartz sand is mainly silicon dioxide, which can react with carbon at high temperatures.

Carbon used doesn't need very high purity; it can be in the form of coal, coke or even pieces of wood.

At a high temperature carbon starts to react with SiO2 to form carbon mono or dioxide. 2CO )( Si )(SiO )( C2 2 liquidsolidsolid

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Purification of SiliconPurification of Silicon

• This process generates polysilicon with about 98% to 99% purity called crude silicon or MGS.

• MGS has high impurities that makes it inconvenient for electronic applications.

• To purify MGS, the crude silicon is ground into fine powder. Then the powder is introduced into a reactor to react with HCl vapor, forming any of a number of SiHCl.MGS: Metallurgical-Grade Silicon

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Purification of SiliconPurification of Silicon

• The chemical reaction can be expressed as:

23300 H SiHCl 3HCl Si Co

• TCS (SiHCl3) vapor then goes through a series of filters, condensers and purifiers to get ultrahigh-purity liquid TCS. (9s!)

• TCS now has less than one impurity per billion atoms.

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Purification of SiliconPurification of Silicon

• Purified polysilicon is obtained from TCS which is purified earlier by fractional distillation, in a large CVD reactor.

)( 3HCl )( Si )(H )(SiHCl 23 gassolidgasgas

• The high purity polysilicon is called electronic-grade silicon, or EGS.

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Purification of Silicon (cont.)Purification of Silicon (cont.)

H2

LiquidTCS

EGS

Carrier gas bubbles

H2 and TCS

Process chamber

3HCl Si HTCS 2

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Crystal PullingCrystal Pulling

• The EGS which is obtained from CVD has polycrystalline structure whereas Si which is used in fabrication of electronic devices is single crystal.

• The resulting polysilicon may be broken up into pieces to load into crucibles for Czochralski crystal growth or the poly rod itself could be used as the starting material for float-zone crystal growth.

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Crystal Pulling (cont.)Crystal Pulling (cont.)

• Crystallization methods:– Czochralski (CZ)– Float-zone (FZ)

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Czochralski Crystal GrowthCzochralski Crystal Growth

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• During CZ crystal growth, the seed and the crucible are normally rotated in opposite directions to promote mixing the liquid and more uniform growth.

Czochralski Crystal GrowthCzochralski Crystal Growth

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Czochralski Crystal GrowthCzochralski Crystal Growth

Czochralski Crystal GrowthCzochralski Crystal Growth

Why CZ is much more common?• The CZ process is cheaper. • It is capable of producing large diameter crystals, from which large diameter wafers can be cut.

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88 die200-mm wafer

232 die300-mm wafer

Assume 1.5x1.5 cm2 microprocessor

Czochralski Crystal GrowthCzochralski Crystal Growth

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Czochralski Crystal GrowthCzochralski Crystal Growth

• The only significant drawback to the CZ method is that the silicon is contained in liquid form in a crucible during growth and as a result, impurities from the crucible are incorporated in the growing crystal.

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Czochralski Crystal GrowthCzochralski Crystal Growth

• Oxygen and carbon are the most significant contaminants.

• To avoid additional impurities from the ambient, the growth is normally performed in an argon ambient.

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Float-Zone (cont.)Float-Zone (cont.)

RF

Gas inlet (inert)

Molten zone

Traveling RF coil

Polycrystalline rod (silicon)

Seed crystal

Inert gas out

Chuck

Chuck

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Float-ZoneFloat-Zone

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Float-ZoneFloat-Zone

• Not to use any crucible in the FZ method impurity levels particularly oxygen is much lowered in the

resulting crystal. And it makes easier to grow high-resistivity material. Thus the FZ process is used when only high resistivity, low oxygen content or both is required.

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GrindingGrinding

• The boule is placed in a lathelike machine to grind with a diamond wheel into a perfect cylinder.

• After the boule is ground to an appropriate diameter one or more “flats” are normally ground along its length.

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Flat grind

Diameter grind

Preparing crystal ingot for grinding

GrindingGrinding

Internal diameter wafer saw

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GrindingGrinding

Wafer Orientation

PrimaryFlat

{111} p-type

PrimaryFlat

{100} p-type

Secondary Flat

90o

PrimaryFlat

{111} n-type

45o

Secondary Flat

PrimaryFlat

{100} n-type

SecondaryFlat

180o

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SlicingSlicing

• The boule is sliced into individual wafers by a rapid-rotating, inward-diameter diamond-coated saw which cuts on its inside edge.

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SlicingSlicing

Crystal Ingot

Coolant

Ingot Movement

OrientationNotch

Saw Blade

Diamond Coating

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Edge RoundingEdge Rounding

• After sawing, preventing the wafer chipping during the mechanical handling, the wafer edge is ground in a mechanical process to round the sharp edges created in the slicing process.

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Edge RoundingEdge Rounding

Wafer movement

Wafer

Wafer before edge rounding

Wafer after edge rounding

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LappingLapping

• The lapping operation is done under pressure using a mixture of alumina (Al2O3), water and glycerine to improve the flatness of the wafer to about ±2 µm, removing most of the taper and bow that results from the sawing operation.

• This process removes about 50 µm from both sides of the wafer

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LappingLapping

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EtchingEtching

• To remove any particles and damages that many still remain from sawing and lapping steps chemical etching is done as a batch process, with the wafers are loaded into cassettes and immersed in a mixture of nitric, hydrofluoric and acetic acids. O8H NO4SiF3H HF184HNO Si3 2623

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Etching (cont.)Etching (cont.)

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Chemical Mechanical PolishingChemical Mechanical Polishing• As the wafers are need to have one

mirror finish at least, CMP is the next step.

Upper polishing pad

Lower polishing pad

Wafer

Slurry

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Chemical Mechanical PolishingChemical Mechanical Polishing• The slurry consists of a suspension of

fine silica particles in an aqueous solution of NaOH.

• The rotation and pressure generate heat that drives a chemical reaction in which OH¯ from the NaOH oxidize the silicon. The SiO2 particles abrade the oxide away.

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CMPCMP

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Growth of Epitaxial SiliconGrowth of Epitaxial Silicon

• In some purposes to increase the purity of where devices are supposed to be fabricated, an epitaxial layer of silicon is grown on the wafer.

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Growth of Epitaxial SiliconGrowth of Epitaxial Silicon

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Wafer InspectionWafer Inspection

• Physical dimension• Flatness• Microroughness• Crystal defects• Resistivity• Contaminations

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1234567890

Notch Scribed identification number

Tracking NumberTracking Number

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Wafers Ready for Fabrication ProcessWafers Ready for Fabrication Process

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Concentration (Atoms/cm3)

Dopant Material

Type < 1014

(Very Lightly Doped) 1014 to 1016

(Lightly Doped) 1016 to 1019

(Doped) >1019

(Heavily Doped)

Pentavalent n n-- n- n n+

Trivalent p p-- p- p p+

Dopant Concentration NomenclatureDopant Concentration Nomenclature

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2000

1992

1987

1981

1975

1965

50 mm 100 mm 125 mm 150 mm 200 mm 300 mm

Evolution of Wafer SizeEvolution of Wafer Size

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Diameter (mm)

Thickness (m)

Area (cm2)

Weight (grams)

50.8 (2”) 279 20.26 1.32

76.2 (3”) 381 45.61 4.05

100 525 78.65 9.67

125 625 112.72 17.87

150 675 176.71 28

200 725 314.16 53.08

300 775 706.86 127.64

400 825 1256.64 241.56 / 0.53

Evolution of Wafer Size (cont.)Evolution of Wafer Size (cont.)

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Year (Critical Dimension)

1995 (0.35 mm)

1998 (0.25 mm)

2000 (0.18 mm)

2004 (0.13 mm)

Wafer diameter (mm)

200 200 300 300

Site flatness (mm) Site size (mm mm)

0.23 (22 22)

0.17 (26 32)

0.12 26 32

0.08 26 36

Microroughness of front surface (RMS) (nm)

0.2 0.15 0.1 0.1

Oxygen content (ppm)

£ 24 ± 2 £ 23 ± 2 £ 23 ± 1.5 £ 22 ± 1.5

Bulk microdefects (defects/cm2)

£ 5000 £ 1000 £ 500 £ 100

Particles per unit area (#/cm2)

0.17 0.13 0.075 0.055

Epilayer thickness (± % uniformity) (mm) 3.0 (± 5%) 2.0 (± 3%) 1.4 (± 2%) 1.0 (± 2%)

Improving Si Wafer RequirementsImproving Si Wafer Requirements

Any questions?