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Wafer Manufacturing

Reading Assignments:

Plummer, Chap 3.1~3.4

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Periodic Table of the Elements

Roman letters give valence

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Why Silicon?

• Abundant, cheap– 26% of earth’s crust vs. 1.8ppm of Germanium

• Stable dielectric for gate dielectric and doping mask– Silicon dioxide is very stable, strong dielectric, and it is easy to

grow in thermal process. Perfect interface with Silicon. Germanium oxide is not stable at >800oC and water soluble.

• Large band gap– Wide operation temperature and doping range. Higher breakdown

voltage.

First IC, T.I. Jack Kilby 1958First transistor, Shockley, Bardeen, Brattain1947

Made by Germanium

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• Amorphous (no short/long-range order)• Poly-crystalline (short-range order, typically, 1nm - 50μm

range)• Crystalline (long-range order, irregularity treated as defects,

faults, stacked faults or line faults)

Structure of Solids

Amorphous Polycrystalline Crystal

Grain/domainGrain boundary

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Crystal Structures (Crystallography)Example: Simple Cubic (SC)

Space lattice (SC) Basis (one atom) Crystal Structure

Example: Face-Centered Cubic (FCC)

There are total 14 distinct Bravais lattices.

NaCl

Na+

Cl-

a1

a2

a3

a1 a2 a3 : primitive translation vectors

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Simple Cubic Crystal

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Diamond Crystal Structure

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•Diamond (covalent, Si, Ge, C, etc.) and Zinc-blend (ionic, GaAs, InP, etc.) consist of 2 interleaved FCC with 1/4 diagonal offset

Zincblende lattice cell(GaAs, AlAs, GaP, ZnS, etc.)

Diamond lattice cell(C, Si, Ge, etc.)

Diamond Crystal Structure

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Crystal Planes/Direction

1 2

1

2

3

1

2

Plane (233)

x

y

zIndexing Procedure for Planes

1. Record where the plane intercepts the axes in the unit of the unit cell length. → 3, 2, 2

2. Invert the intercept values → 1/3, ½, ½

3. Convert to the smallest possible set of whole numbers → 2, 3, 3

4. Enclose the whole-number set in parentheses→ (233)Direction [100]

Direction [001]

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Crystal Planes

(110) (111)

corner atom in FCCface atom in FCC

(100)

(200) (100)1

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Use of Crystal Orientation• Electrical anisotropy: (100) has the

highest mobility, also scattering rates are slightly different

• Mechanical anisotropy: different surface has different modulus components (later)

• Chemical anisotropy: e.x., KOH will not etch (111), Etch rate (110)>(100)>(111). 54.74° V-groove etch on (100) plane.

• Angles: (100) vs. (110): 45°, 90 ° ; (100) vs. (111): 54.74°; (110) vs. (111): 35.26°, 90 ° or 144.74°

• Important for strain engineering

Newer wafers (8” and 12”) now use a groove to mark the orientation, instead of a cut to save some area for test structures

<110>

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Newer wafers (8” and 12”) now use a groove to mark the orientation, instead of a cut to save some area for test structures.

Manufacturers Data of incoming wafers:orientation, thickness, doping type, resistivity.

Wafer Flat

Wafer flats in 4” and 6” silicon wafers

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Defects in Crystals

0D

0D

1D

2D

3D

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Commercial Silicon Wafer

100mm

150mm

200mm

300mm

150mm 200mm 300mm 450mm

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Starting Material Cost

300mm bare silicon ~ $100/pcs

200mm bare silicon ~ $40/pcs

200mm ASP ~ $1000/pcs

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Si + HCl

→ TCS Silicon Powder

Hydrochloride

FiltersCondenser

Purifier Pure TCS with 99.9999999%

Reactor, 300 °C

Heat (2000 ̊ C)

SiO2 + C → Si + CO2

Sand Carbon MGS Carbon Dioxide

Heat (2000 ̊ C)

SiO2 + C → Si + CO2

Sand Carbon MGS Carbon Dioxide

Silicon Purification

MGS: Metallurgical Grade Silicon, 98% pure, Major impurity Al and Fe

Heat (1100 ̊ C)

SiHCl3 + H2 → Si + 3HCl

TCS Hydrogen EGS Hydrochloride

Heat (1100 ̊ C)

SiHCl3 + H2 → Si + 3HCl

TCS Hydrogen EGS Hydrochloride

TCS: Trichlorosilane SiHCl3

EGS: Electronic Grade Silicon, 99.9999999% pure, impurity 1013-1014/cm3

MGS:

EGS:

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Czochralski (CZ) Crystal Growth

Graphite Crucible

Single Crystal silicon Ingot

Single Crystal Silicon Seed

Quartz Crucible

Heating Coils1415 ˚C

Molten Silicon

Most common. Use in the large-diameter wafer manufacturing.The ingot diameter is determined by the pulling speed.The dislocation due to the thermal stress is terminated at the neck region.Impurities comes from crucibles (O; 1017-1018/cm3) and susceptors (C; 1015-1016/cm3)

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Czochralski (CZ) Crystal Growth

Source: http://www.fullman.com/semiconductors/_crystalgrowing.html

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Floating (FZ) Crystal Growth

Heating Coils

Poly Si Rod

Single Crystal Silicon

Seed Crystal

Heating Coils Movement

Molten Silicon

Local melting. No crucible needed.Low impurity. High resistive wafer possible.Hard to scale up.

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Wafer Finishing

Flat, 150 mm and smaller Notch, 200 mm and larger

Flat, Notch

Orientation Notch

Crystal Ingot

Saw Blade

Diamond Coating

Coolant

Ingot Movement

Wafer Sawing

Wafer Wafer movement

Wafer Before Edge Rounding

Wafer After Edge Rounding

Wafer Rounding

Slurry

Polishing Pad

Pressure

Wafer HolderWafer

Surface Flatten

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Surface Flatten

76 μm

914 μmAfter Wafer Sawing

After Edge Rounding76 μm914 μm

12.5 μm814 μm<2.5 μm750 μm

725 μmVirtually Defect Free

After Lapping

After Etch

After CMP

Etch-off 20-25μm (DS)

Polish away 25μm (SS)

Polish away 50μm (DS)

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Resistivity and Mobility

Sheet resistance : Rs(Ω/□)= ρ(Ω．cm) / t (cm)

t

pn qpqn μμρ

+=

1

Four Point Probe

Jdrift = (qp μp +qn μn )F =σF