Chapter 1 Crystal GrowthCrystal Growth Wafer PreparationWafer Preparation 1.

Chapter 1

• Crystal Growth• Wafer Preparation

1

Process Flow Overview

Starting Material

Polycrystalline semiconductor

Single crystal

Wafer

2OS

S

i

i

Ga,As

GaAs

Distillation andReduction

Crystal growth

Grind, saw , polish

Synthesis

Crystal growth

Grind, saw , polish

2

Overview

• 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 disc that will have all of the processing performed on it.

3

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

Polysilicon Seed crystal

Heater

Crucible

CRYSTAL GROWTH

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Starting Material Preparation

• The starting material for silicon is relatively pure form of sand (SiO2) called quartzite.

• This is placed in a furnace with various forms of Carbon.

• The Silicon treated with HCL at 300° to form tricholosilane.

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Starting Material Preparation

• Fractional Distillation of the liquid removes the unwanted impurities. The purified SiHCL3 is then used in a hydrogen reduction reaction to prepare the electronic-grade silicon ( EGS) .

• EGS ,a polycrystalline material of high purity is the raw material of single-crystal silicon.

6

Silicon Crystal Growth from the Melt

• The Basic Technique , Which is material in liquid form, is Czochralski Technique.

• The Float –Zone Process also can be used to grow silicon that has lower contamination than normally obtained from CZ Technique.

• Most popular technique is CZ.

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Czochralski Technique

• Czochralski Process is a Technique in Making Single-Crystal Silicon

• A Solid Seed Crystal is Rotated and Slowly Extracted from a Pool of Molten Si

• Requires Careful Control to Give Crystals Desired Purity and Dimensions 8

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 of Smaller Circular Pieces Called Wafers

• Each Wafer Yields Hundreds or Thousands of Integrated Circuits

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Czochralski Technique

Crystal Puller

Control system

Furnace

Graphite susceptor

Rotation mechanism

Heating element

Power supply

Crystal-pulling mechanism

Ambient control

Seed Holder

Sio2 crucible

Gas Source

Flow control

Rotation mechanism

Exhaust system

Control process parameters: Crystal Diameter, pull rate and rotation speed.

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Distribution of Dopant• In crystal-growth a known amount of Dopant is added to the melt to

obtain the desired doping concentration in the growth crystal.• At interface, The doping concentration incorporated into the crystal

( solid) is usually different from the doping concentration of the melt ( liquid).

• The ratio of these two concentration is defined as the equilibrium segregation coefficient:

l

s

c

ck 0

Equilibrium concentration of the dopant in the Solid

Equilibrium concentration of the dopant in the Liquid

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Distribution of Dopant

• Note the most values are below 1.• As the crystal grows , the melt becomes enriched.

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• Find the doping concentration in the crystal in CZ technique?

dMC

S

M

C

M

C

s

s

0

0

The doping concentration in the crystal

The initial doping concentration in the crystal

The doping concentration in Melt

Given point of growth

The amount of the dopant remaining in the melt

The reduction of the dopant from the melt

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14

00 CK 10K

10K

10 K

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Example

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Effective Segregation Coefficient

• There is a concentration gradient developed at the interface, when . 10 K

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Effective Segregation Coefficient

1C

CK

se 01

0C

CK

s

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Silicon Float-Zone Process

RF

Gas inlet (inert)

Molten zone

Traveling RF coil

Polycrystalline rod (silicon)

Seed crystal

Inert gas out

Chuck

Chuck

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20


21


22


23

Neutron Irradiation

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GaAs Advantages

Fabrication Cost Noise Breakdown

voltageOperating

PowerCutoff

FrequencyElectron mobility

Saturated electron velocity

Substrate Type

High Low High High High High High GaAs

Low High Low Low Low Low Low Si

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These properties make GaAs circuitry ideal for mobile phones, satellite communications, microwave point-to-point links, and radar systems. However, high fabrication costs and high power consumption have made GaAs circuits unable to compete with silicon CMOS circuits in most applications.

Ga & As

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Native Gallium

Native Arsenic

Elemental GalliumWith 99.9999% purity

Elemental ArsenicWith 99.9999% purity

Bayer Process

Zone passes refining

GaAs Polycrystalline

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Synthesis Process

GaAs Crystal-growth

• The starting materials for the synthesis of polycrystalline gallium arsenide are the elemental , chemically pure gallium and arsenic.

• The behavior of a combination can be described by phase diagram.

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GaAs Crystal-growth

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• Find the fraction of melt that will be solidified?

Example

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aT

bT

mC

mslssll CMMCMCM *)( ss CM

ll CM

sClC

sM

lM

Weight percent scale

Weight of the liquid.

Weight of the solid (GaAs).

Concentration of Dopant in the Solid.

Concentration of Dopant in the Liquid.

Weight of the Arsenic in the Liquid.

Weight of the Arsenic in the solid (GaAs).

Tototal Weight of the Arsenic in Ta and Tb.

Example ( cont.)

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mslssll CMMCMCM *)(

GaAs Crystal-growth

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Bridgman Technique

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Equilibrium Segregation Coefficient

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Wafer Coding

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Packed Wafers

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Chapter Overview• Raw materials (SiO2) are refined to produce electronic grade

silicon with a purity unmatched by any other available material on earth.

• CZ crystal growth produces structurally perfect Si single crystals which are cut into wafers and polished.

• Dopants can be incorporated during crystal growth • Point, line, and volume (1D, 2D, and 3D) defects can be present

in crystals, particularly after high temperature processing.• Point defects are "fundamental" and their concentration

depends on temperature (exponentially), on doping level and on other processes like ion implantation which can create non-equilibrium transient concentrations of these defects.

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Chapter 1 Crystal GrowthCrystal Growth Wafer PreparationWafer Preparation 1.

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Transcript of Chapter 1 Crystal GrowthCrystal Growth Wafer PreparationWafer Preparation 1.