3. crystal growth and wafer fabrication

Silicon Crystal Growth, Wafer Fabrication and

Basic Properties of Silicon Wafers

Crystal Structure

Crystals are characterized by a unit cell which repeats in the x, y, z directions.

Crystal Directions and Planes

Miller Indices Notation Summary

Silicon Crystal Structure

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

Movement of a dislocation by glide in response to shear stress

Defects in Crystals

Silicon Crystal Growth

Czochralski Crystal Growth Method

• 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

Float-Zone Crystal Growth Method

This is an alternative technique which can be used for refining or single crystal growth.

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

Wafer Preparation

Sliced Wafers

Chemical Mechanical Polishing (CMP) Process

Polished Wafer of 300 mm Diameter

Measurements

Hot Point Probe:To determine the N- or P-type of a semiconductor

Resistance:

Measurements

Resistance and Sheet Resistance:

Measurements

Four Point Probe:To determine the Resistivity / Sheet Resistance of semiconductors

Measurements

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,

Measurements

To determine material type, carrier concentration and carrier mobility

Hall Effect Measurements:

Resistivity is given by

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

Modeling Crystal Growth

Czochralski Crystal Growth


Therefore, maximum crystal pull rate is inversely proportional to the square root of the crystal radius.

Modeling Dopant Behavior During Crystal Growth


Equilibrium segregation coefficients during CZ crystal growth for Si


This gives the number of impurities in the melt as a function of how much of the melt has been frozen.


.

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.

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.

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



Behavior of impurities during float-zone growth or zone refining


As the dopants and other impurities tend to stay in the liquid, therefore refining can be accomplished, especially with multiple passes.

Modeling Point Defects in Silicon

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.

Oxygen and Carbon in CZ Silicon

Summary of Key Ideas

3. crystal growth and wafer fabrication

Engineering

Transcript of 3. crystal growth and wafer fabrication