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1. Silicon crystal structure and defects.
2. Czochralski single crystal growth.
3. Growth rate and dopant incorporation for CZ method.
4. Float zone single crystal growth and doping.
5. Wafer fabrication.
6. Measurement methods.

Chapter 3 Crystal growth, wafer fabrication and basic properties of silicon wafers

NE 343: Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

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Single crystal silicon

• Three types of solids - amorphous, polycrystalline, mono-crystalline (single-crystal).
• Semiconductor devices and VLSI (very large scale integrated) circuits require high-purity single-crystal semiconductors. Because:
• Difficult to control properties of amorphous or poly-crystals.
• By doping, electronic properties (carrier density, mobility, conductivity, carrier lifetime) of a single crystal can be controlled more precisely.
• Amorphous silicon is used in photovoltaic cells, electronic displays (large-area).
• Polycrystalline silicon is used as a gate contact in MOSFETs (VLSI circuits).

Single crystal Si wafers

Diameters: currently up to 300mm (500mm?)

Wafer thickness: ~650μm

Wafer purity: 150 parts/trillion

Impurities: 99.99999999% Si

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Microstructure of electronic materials

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

Crystallography - introduction

Planes and directions are defined using an x, y, z coordinate system.

[111] direction is defined by a vector having components of 1 unit in x, y and z.

Planes are defined by Miller indices - reciprocals of the intercepts of the plane with the x, y and z axes.

For the (110) plane, it has intercepts with x, y, z-axis of 1, 1, ∞ (i.e. no intercept with z-axis). So its Miller indices are (1/1, 1/1, 1/∞)=(1,1,0).

For any plane (l, m, n), it is always perpendicular to the direction [l, m, n].

E.g. [111] direction is perpendicular to (111) plane.

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Diamond Structure of Si

Silicon has the basic diamond crystal structure:

Two merged FCC cells offset by a/4 in x, y and z.

http://jas.eng.buffalo.edu/education/solid/unitCell/home.html

This website has a 3D structure at various viewing angles.

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Point defects: vacancy, interstitial, substitutional, Frankel defects

Linear defects: edge and screw dislocations

Planar defects: stacking fault, grain boundaries, surfaces

Bulk defects: cracks and pores, precipitate

Defect in crystals

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Point defects

Point defects.

1. Substitutional.
2. Interstitial.
3. Vacancy (Schottky defect).
4. Frenkel-type defect (interstitial - vacancy pair).

Point defects dictate most dopant diffusion mechanisms, and thus determine the impurity profile.

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The number of neutral vacancies is thermodynamically determined by,

​

​

where N_o is the density of atoms/cm³ and E_a is the activation energy for the formation of the vacancy (in silicon, N_o=5.02×10²²/cm³ and E_a=2.6 eV)

Point defects: vacancies and substitutional

Substitutional can be foreign unwanted impurities, or intentionally introduced impurities.

You may want a dopant impurity to be on a substitutional site, but you may not want a heavy metal atom or other unwanted impurity to be on a substitutional site (harder to remove).

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Interstitials can be foreign unwanted impurities, intentionally introduced impurities, or “misplaced” host atoms (an intrinsic defect, the self-interstitial).

Dopant atoms diffuse through the semiconductor faster as interstitials, but we need to place them in substitutional sites to make use of them (i.e. they act as dopants only when in substitutional sites).

Point defects: interstitial

Energies involved for the atomic diffusion of interstitial impurities

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A missing line or additional line of atoms is called a dislocation.

Dislocations are either pure edge, pure screw or a combination of both types.

Line defects: dislocation

Screw dislocations give a helical structure to the planes, often show up in crystal growth

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Dislocation affects deformation properties - to slide upper block over lower now only requires a line of bonds to break at a time, not a whole plane process of slip.

It causes low yield strength of solids.

Dislocation sources: by stress due to temperature gradient, due to agglomeration.

Dislocation move

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A stacking fault is a disruption in the stacking of layers in the crystal.

It is terminated by dislocation.

Planar defects: stacking faults

Volume defects: precipitates

Precipitates are three dimensional defects that have a different chemical makeup from the host lattice.

They can result from an impurity exceeding the maximum solubility of the crystal (much like super-saturation of sugar in water).

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1. Silicon crystal structure and defects.
2. Czochralski single crystal growth.
3. Growth rate and dopant incorporation for CZ method.
4. Float zone single crystal growth and doping.
5. Wafer fabrication.
6. Measurement methods.

Chapter 3 Crystal growth, wafer fabrication and basic properties of silicon wafers

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Raw material preparation

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Economical value

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Quartzite (sand, SiO₂) is placed in a hot (~1800^oC) furnace with carbon releasing materials, and reacts as shown, forming metallurgic grade silicon (MGS):

2SiO₂(solid) + 2C(solid) → Si(liquid) + 2CO(gas)

Metallurgical grade silicon

Metallurgical grade silicon (~98% pure) production and typical impurity levels.

Over 50% MGS is used to make Al alloys.

The fraction used for semiconductors is very small.

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Electrical grade silicon (polycrystalline)

Basically, the solid Si is first converted into a liquid form (SiHCl₃) for purification, then converted back into solid Si.

Both reactions occur at high temperatures.

Metallurgical grade silicon is treated with hydrogen chloride to form trichlorosilane:

Si + 3HCl → SiHCl₃(g) + H₂(g) (use catalyst)

SiHCl₃ is liquid at room temperature, boiling point 32^oC. Multiple distillation of the liquid removes the unwanted impurities (99.9999% pure).

The purified SiHCl₃ is then used in a hydrogen reduction reaction to prepare the electronic grade Si (EGS):

SiHCl₃(g) + H₂(g) → Si(s) + 3HCl(g)

(this is the reverse reaction of the above reaction)

EGS is the raw material for Si single crystal production.

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Jan Czochralski (cho-HRAL-skee) (1885 - 1953) was a Polish chemist who invented the Czochralski process, which is used to grow single crystals and is used in the production of semiconductor wafers.

He discovered the Czochralski method in 1916 when he accidentally dipped his pen into a crucible of molten tin rather than his inkwell. He immediately pulled his pen out to discover that a thin thread of solidified metal was hanging from the nib. The nib was replaced by a capillary, and Czochralski verified that the crystallized metal was a single crystal.

Si single crystal growth

Two methods used: Czochralski (CZ) and float zone (FZ).

http://en.wikipedia.org/wiki/Jan_Czochralski

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Czochralski method (CZ)

• It is widely employed for Si, GaAs, and InP.
• The EGS is broken into small pieces and placed in an SiO₂ crucible.
• In an argon ambient, the crucible is heated to just above 1417^oC.
• Dopant is added to the melt to intentionally dope the resulting crystal.
• A single crystal seed is then lowered into the melt (crystal orientation and wafer diameter determined by seed orientation and pull rate), and withdrawn slowly.
• Melt flows up the seed and cools as crystal begins to grow.
• Seed rotated about its axis to produce a circular cross-section crystal. The rotation inhibits the natural tendency of the crystal to grow along certain orientations to produce a faceted crystal.
• Long ingots (boules) ~100 kg, with very good circular cross-section are produced.
• The oxygen and carbon (from graphite furnace components), contribute about 10¹⁷-10¹⁸/cm³ contaminants.

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Czochralski method (CZ)

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A commercial CZ puller Early in the growth process Later in the growth process

Czochralski growth of silicon

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Oxygen and carbon in CZ silicon

• The CZ growth process inherently introduces O (from SiO₂ crucible) and C (from graphite susceptor/supporter).
• Typically, C_O ≈ 10¹⁸ cm^-3 and C_C ≈ 10¹⁶ cm^-3.
• The O in CZ silicon often forms small SiO₂ precipitates in the Si crystal under normal processing conditions.
• O and these precipitates can actually be very useful: provide mechanical strength, internal gettering.

Figure 3-25 Point defect and diffusion mechanisms that contribute to the growth or shrinkage of SiO₂ precipitates in silicon.

Critical size is about 1nm: stable precipitates above 1nm, may shrink and disappear below 1nm.