3. crystal growth and wafer fabrication
DESCRIPTION
VLSITRANSCRIPT
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Silicon Crystal Growth, Wafer Fabrication and
Basic Properties of Silicon Wafers
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Crystal Structure
Crystals are characterized by a unit cell which repeats in the x, y, z directions.
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Crystal Directions and Planes
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Miller Indices Notation Summary
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Silicon Crystal Structure
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Silicon Crystal Structure
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Defects in Crystals• Point Defects e.g. Vacancies (V), Interstitials (I)• Line Defects e.g. Dislocations•Area Defects e.g. Stacking Faults (“extrinsic” or “intrinsic” form along {111} planes)•Volume Defects e.g. Precipitates, Collection of Vacancies
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Movement of a dislocation by glide in response to shear stress
Defects in Crystals
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Silicon Crystal Growth
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Czochralski Crystal Growth Method
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• Essentially all Si wafers used for ICs today come from Czochralski grown crystals.
• Polysilicon material is melted, held at close to 1417 °C, and a single crystal seed is used to start the growth.
• Pull rate, melt temperature and rotation rate are all important control parameters.
Czochralski Crystal Growth Method
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Float-Zone Crystal Growth Method
This is an alternative technique which can be used for refining or single crystal growth.
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Wafer Preparation
• Shaping the boule to a uniform diameter
• Formation of “Flats” along boule’s length
• Sawing of the boule into individual wafers
• Mechanical Lapping of individual wafers
•Chemical Etching in a mixture of nitric and hydrofluoric acids
•Chemical Mechanical Polishing (CMP) using a slurry consisting silicon dioxide particles (~ 10 nm) in aqueous solution of NaOH
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Wafer Preparation
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Sliced Wafers
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Chemical Mechanical Polishing (CMP) Process
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Polished Wafer of 300 mm Diameter
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Measurements
Hot Point Probe:To determine the N- or P-type of a semiconductor
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Resistance:
Measurements
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Resistance and Sheet Resistance:
Measurements
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Four Point Probe:To determine the Resistivity / Sheet Resistance of semiconductors
Measurements
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Measurements
Resistivity of a semiconductor is given by
In the case of four point probe,
When t << s, as in the case of diffusion or implant layer,
Sheet Resistance,
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Measurements
To determine material type, carrier concentration and carrier mobility
Hall Effect Measurements:
Resistivity is given by
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The voltage VY is given by
Measurements
The Hall coefficient RH is defined as
Therefore, carrier concentration is determined as
where the + sign applies for holes and the - sign for electrons
The Hall mobility µH is determined as
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Modeling Crystal Growth
Czochralski Crystal Growth
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Czochralski Crystal Growth
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Czochralski Crystal Growth
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Czochralski Crystal Growth
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Czochralski Crystal Growth
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Czochralski Crystal Growth
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Czochralski Crystal Growth
Therefore, maximum crystal pull rate is inversely proportional to the square root of the crystal radius.
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Modeling Dopant Behavior During Crystal Growth
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Modeling Dopant Behavior During Crystal Growth
Equilibrium segregation coefficients during CZ crystal growth for Si
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Modeling Dopant Behavior During Crystal Growth
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Modeling Dopant Behavior During Crystal Growth
This gives the number of impurities in the melt as a function of how much of the melt has been frozen.
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Modeling Dopant Behavior During Crystal Growth
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Modeling Dopant Behavior During Crystal Growth
.
0.01
0.1
1
10
0 0.2 0.4 0.6 0.8 1Fraction of Melt Solidified
Boron
Phosphorus, Arsenic
Antimony
CS/
CO
Plot of Eq. (16)
• Note the relatively flat profile produced by boron with a kS close to 1.
• Dopants with kS << 1 produce much more variation in doping concentration along the crystal.
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Float Zone Growth and Zone Refining
• To prepare high resistivity and high-purity starting silicon wafers, float-zone growth process is used.• This process can also be used to refine silicon and
therefore, it is also sometimes referred to as zone refining.
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If the molten zone moves upward by a distance dx, the change in number of impurities in liquid zone is given by
Integrating this, we findwhere
where IO is the number of impurities in the zone when it is first formed at the bottom. Carrying out the integration and noting that IO = COL and CS = kOI/L, we have finally that
Float Zone Growth and Zone Refining
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Float Zone Growth and Zone Refining
Behavior of impurities during float-zone growth or zone refining
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Float Zone Growth and Zone Refining
As the dopants and other impurities tend to stay in the liquid, therefore refining can be accomplished, especially with multiple passes.
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Modeling Point Defects in Silicon
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Modeling Point Defects in Silicon
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Modeling Point Defects in Silicon
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Modeling Point Defects in Silicon
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Modeling Point Defects in Silicon
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Modeling Point Defects in Silicon
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Oxygen and Carbon in CZ Silicon
• The CZ growth process inherently introduces O and C.
• Typically, CO ≈ 1018 cm-3 and CC ≈ 1016 cm-3.
• The O in CZ silicon often forms small SiO2 precipitates in the Si crystal under normal processing conditions.
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Oxygen and Carbon in CZ Silicon
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Summary of Key Ideas
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Summary of Key Ideas