02wafer+Preparation

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2. CRYSTAL GROWTH AND2. CRYSTAL GROWTH AND

WAFER PREPARATIONWAFER PREPARATION



Semiconductor MaterialPreparation

High-quality and high-performance semiconductorrequires extremely pure processing materials.

The raw material, Silicon ore, S iO 2 (next to oxygen,it is the most abundant element in nature, 27.8%,and is found in a natural state in rocks and sand)

must be mined and completely purified.



Silicon Refinement: The required siliconpurity can be obtained in the following steps:

1 . Metallurgical Grade Silicon (MGS ± 98% pure)is obtained by heating sand with coal

S iO2 + 2C S i + 2CO(s) (s) (l) (g)

2. Formation of Trichlorosilane: Reaction at hightemperature with hydrogen chloride (HCl) to form a

complex chemical mixture containing trichlorosilane.S i + 3HCl S iHCl3 + H 2(s) (g) (g) (g)



3. Separation and purification of trichlorosilane:This step gives 99.999999% pure trichlorosilane.

4. Hidrogen reduction of ultrapure trichlorosilane:This gives Electronic Grade polycrystalline silicon(EGS) by reaction with hydrogen at 1 , 100 ±1 ,2 00º C.

2S iHCl3 + 2H 2 2S i + 6HCl(gas) (gas) (crystal) (gas)

Silicon Refinement:Silicon Refinement:



Crystal v/s Amorphous

I n some materials, the atoms occupy very definitepositions relative to each other. These positionsare repeated throughout the material. Suchmaterials are called Crystals .

Eg:- Silicon, Germanium.

Materials without a definite arrangement of theiratoms are called Amorphous .

Eg:- Plastic



Unit Cells

There are 2 levels of atomic organization possible for crystalline materials.

First level ± O rganization of individual atoms.

The basic repeating unit of the arrangement of atomsor molecules is a u nit cell .

S ilicon unit cell has 18 atoms arranged into adiamond structure.



Silicon Crystal Structure

Arrangement of Silicon atoms in a Unit Cell, with the numbersindicating the height of the atom above the base of the cube asa fraction of the cell dimension.



Polycrystals v/s Single Crystals

Second level - Organization of unit cells.

Polycrystals ± Unit cells are not in a regulararrangement to each other.Eg.:- I ntrinsic semiconductor.

Single crystals ± Unit cells are all neatly andregularly arranged relative to each other.



Crystal Orientation

D ifferent crystal planes are identified by a 3digit number: Miller I ndices.

Most popular crystal planes/orientations:< 100> : MOSFET , GaAs< 111> : BJT

Every plane differ from others in its chemical,electrical and physical properties.



Crystal Growing

The process of converting the polycrystal

chunks to a large crystal of single crystalstructure, of the correct orientation andcontaining the proper amount of dopant iscalled crystal growing .



Crystal Growing Methods

There are 3 methods to grow crystals:

1. Cz ochralski ( CZ) method.

2. Liquid Crystal EncapsulatedCz ochralski (LE C) method.

3. Float- Zone method.



1. Cz ochralski ( CZ) method



1 . Czochralski (CZ) method

RF heating coils

Crucible



Crystal Defects

1 . Point defectsWhen contaminants in the crystal becomes

jammed in the crystal structure causing strain .Atom missing from a location in the structure± vacancy .

Vacancy



2. D islocations ± misplacement of the unitcells in a single crystal.



3. Growth defects ± structural defect.Crystal slip

Crystal twinning

Crystal slip



W afer Preparation

1 . End cropping2. D iameter grinding3. Crystal orientation, conductivity, and

resistivity check4. Flat grinding5. Wafer slicing6. Rough polish7. Chemical-mechanical polishing (CMP)8. Backside processing9. Edge grinding



W afer Preparation ± Contd.

10 . Wafer evaluation11 . Oxidation

1 2. Packaging

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02wafer+Preparation