Chapter 3

Crystal Growth, Wafer Fabrication and Basic Properties of Silicon Wafers

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Dr. Wanda WosikECE 6466, F2012

Silicon in microelecronics: Single Crystal Polycrystalline Amorphous periodic small crystals no long range

arrangements order between of atoms atoms

Crystal lattice is described by a unit cell with a base vector (distance between atoms)

Types of unit cells

Face Centered CubeBody Centered Cube

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Basic Crystal Lattice

Directions and Planes in Crystals

lattice constant

Directions (vector components: a single direction is expressed as [a set of 3 integers], equivalent directions (family) are expressed as < a set of 3 integers >Planes: a single plane is expressed as (a set of 3 integers h k l = Miler indices) and equivalent planes are expressed as {a set of 3 integers}

Miler indices: take a,b,c (multiple of basic vectors ex. x=4a, y=3a, z=2a)

reciprocals (1/4, 1/3, 1/2)-> common denominator (3/12, 4/12, 6/12) -> the smallest numerators (3 4 6)

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Wikipedia

Silicon Crystal Structure

Diamond lattice (Si, Ge, GaAs)

Two interpenetrating FCCstructures shifted by a/4 in all three directions

All atoms in both FCCs

Atoms inside one FCC come from the second lattice

Diamond covalent bonding

(100) Si for devices(111) Si not used - oxide charges

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http://www.iue.tuwien.ac.at/phd/hoessinger/node26.html

http://www.tomshardware.co.uk/behind-the-closed-doors-of-amd,review-15637-7.html

http://conceptualphysics.in

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http://www.molecularsciences.org/book/export/html/125

• Silicon has the basic diamond crystal structure - two merged FCC cells offset by a/4 in x, y and z.See 3D models

• Various types of defects can exist in crystal (or can be created by processing steps. In general these are detrimental to device performance.

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http://oes.mans.edu.eg/courses/SemiCond/applets/education/solid/unitCell/home.html

http://cst-www.nrl.navy.mil/lattice/struk.jmol/a4.html

Defects in Crystals

Point defects, line defects, volume defects

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Dislocation Formation by Point Defects Agglomeration

After Shimura

Intrinsic point defects in a crystalNv and NI increase with T

collapseAgglomeration of Interstitials

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After Campbell

After Shimura

Stacking Faults and Grain Boundary

OISF

Perfect stacking

Induced by oxidation

Missing (111) plane

SFs bound by dislocations

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Types of Dislocations Screw Dislocations

Edge Disclocation

inserted plane

Dislocation line | | Burger vector - screwBurger vector - edge

After Shimura

b

b

Dislocation line

In Si 60° dislocations are formed in processing

Start the contour here

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Silicon Crystal w/o Dislocations

Grown Si crystal is dislocation-free

Process induced dislocations deteriorate devices

After Shimura

Dangling bonds of a half plane affect physical and electrical properties (here 60°)

Pure edge dislocation

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Propagation of Dislocations by Climb Motion

After Shimura

Shift

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Motion of Dislocations by Glide

Easy motion of dislocations

Stress induced by • T gradient T• Mismatch of thermal expansion coeff., layers, precipitates

T

After Wolf&Tauber15

Raw Material and Purification

After Wolf&Tauber16

Purification and Preparation of Electronic Grade Semiconductor

98% pure

ppb purity of EGS for CZ or FZ300 °C MGS+HCl →SiHCl3

2SiHCl3+H2→2Si+6HCL

Grind to powder

Refined quartzite (SiO2) Liquid ar RT

gas solid

MGS EGS Si Crystal

MSG

After Wolf&Tauber17

Crystal Growth

• Si used for crystal growth is purified from SiO2 (sand) through refining, fractional distillation and CVD.

• The raw material contains < 1 ppb impurities. Pulled crystals contain O (≈ 1018 cm-3) and C (≈ 1016 cm-3), plus any added dopants placed in the melt.

• 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.

→ Introduces SiO2 in CZ; Oi≈1017-1018cm-3

Ar ambient

→ C ≈ 1015-1016 cm-3

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(Photo courtesy of Ruth Carranza.))(More information on crystal growth at http://www.memc.com/co-as-description-crystal-growth.aspAlso, see animations of http://www.memc.com/co-as-process-animation.asp)

100 kg

Neck confines dislocations

Crystal solidifies

increases - pull rate decreases

•Load EGS+Impurities P, B, As•pump-out, •seed down, •pull fast, •pull slowEGS

seed

Czochralski Growth

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After Shimura

Details of Czochralski Growth

Rotation of crucible and crystal in opposite Directions improves growth and doping uniformity

Oxygen incorporation - important for intrinsic gettering 1017-1018 cm-3

Carbon contributes to native defects 1015-1016 cm-3

Ar

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After Campbell

Oxygen Concentrations in CZ Silicon

Role of temperature

Si melts - C-Si growth

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Requirement for Larger Crystals

12 inches

After Wolf&Tauber22

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Wafer Preparation and Specification

• Grind crystal to a diameter (200mm750µm) … 850µm thick• Grind flats (the primary and secondary)• Saw of the boule into wafers• Lapping, etching (batch process in acids etching Si) 20 µm, polishing (chemical-mechanical) 25µm removes damage and improves flatness ±2µm

CMP

3Si +4HNO3+18HF 3H2SiF6+4NO+8H2O

Suspension Al2O3

SiO2 10nm in NaOH/DI

Mark wafer earlier (laser process) to track their process flow

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Orientation of ICs on Silicon Wafers

Primary and Secondary Flats

After Shimura

Si cleaves along {111}

For (100) the {111} planes are along <110>

plane

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Float Zone Method for Crystal Growth

No crucible - no impurities

High resistivity Si

EGS

Add Dopants (gas) PH3 B2H6

ESG

• An alternative process is the float zone process which can be used for refining or single crystal growth.

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• After crystal pulling, the boule is shaped and cut into wafers which are then polished on one side.

Crystal Growth and Wafers Fabrication

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After Wolf&Tauber28http://www.memc.com/index.php?view=Process-Animations-

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Modeling CZ Crystal Growth

• We wish to find a relationship between pull rate and crystal diameter.• Freezing occurs between isotherms X1 and X2.• Heat balance (A B C): latent heat of crystallization + heat conducted from melt to crystal = heat conducted away.

(1)

B=conduction in solid

A=Heat of crystallization

Freezing interface

Liquid to solid → HEAT

C=Radiation

For crystal uniformity T uniformity is important Vpmax ~1√r

Large crystal requires slow pull rates 31

• The rate of growth of the crystal is (2)

where vP is the pull rate and N is the density.

• Neglecting the middle term in Eqn. (1) we have: (3)

• In order to replace dT/dx2, we need to consider the heat transfer processes.

• Heat radiation from the crystal (C) is given by the Stefan-Boltzmann law

(4)

• Heat conduction up the crystal is given by

(5)

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• Differentiating (5), we have (6)

• Substituting (6) into (4), we have (7)

• kS varies roughly as 1/T, so if kM is the thermal conductivity at the melting point,

(8)

(9)

• Solving this differential equation, evaluating it at x = 0 and substituting the result into (3), we obtain (see text):

(10)

• This gives a max pull rate of ≈ 24 cm hr-1 for a 6” crystal (see text). Actual values are ≈ 2X less than this.

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Dopant Segregation During Crystal Growth

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“k” affects doping uniformity

After Wolf&Tauber34

Modeling Dopant Behavior During Crystal Growth• Dopants are added to the melt to provide a controlled N or P doping level in the wafers.

• However, the dopant incorporation process is complicated by dopant segregation.

• Most k0 values are <1 which means the impurity prefers to stay in the liquid.

• Thus as the crystal is pulled, NS will increase.

Segregation Coefficients of Various Impurities in Silicon

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After Campbell

Solid Solubility

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• If during growth, an additional volume dV freezes, the impurities incorporated into dV are given by

(12)

(13)

(14)

• We are really interested in the impurity level in the crystal (CS), so when incremental volume freezes

(15)

(16)

where f is the fraction of the melt frozen (Vs/V0).

Dopant Incorporation During CZ Growth

Initial in melt During the growth

solidified

Removed →from the melt

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• 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.

Uniformity of Crystal Doping

where f is the fraction of the melt frozen.

ko=0.8seed

tail

k0=0.35

k0=0.023

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After Wolf&Tauber39

Radial Doping Nonuniformity

After Shimura

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Zone Refining and FZ GrowthSegregation of Impurities Between Solidus and Liquidus (in FZ Growth)

RF -> melt=zone

CrystalPoly-Si

I - the number of impuritiesin the liquid dI=(C0-k0CL)dx

Distribution of a dopant along the crystal

C0 original concentration in the rod

• In the float zone process, dopants and other impurities tend to stay in the liquid and therefore refining can be accomplished, especially with multiple passes

• See the text for models of this process.

moving

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Float Zone Growth; Removal of Impurities

For one pass only → k0 small gives better refining

Better crystal purity

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Modeling Point Defects in Silicon

• Point defects (V and I) will turn out to play fundamental roles in many process technologies.• The total free energy of the crystal is minimized when finite concentrations of these defects exist.

(17)

• In general and both are strong functions of temperature.• Kinetics may determine the concentration in a wafer rather than thermodynamics.

• In equilibrium, values for these concentrations are given by:

(18)

(19)

Sf entropy & Hf enthalpy of defect formation

Smaller concentrations than those of dopants/carriers(hard to measure) @1000°C C I0≈1012cm-3 and

CV0≈5x1013 cm-3

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• V and I also exist in charged states with discrete energies in the Si bandgap.

• In N type Si, V= and V- will dominate; in P type, V+ and V++ will dominate.

(20)

(21)

• Shockley and Last (1957) first described these charged defect concentrations (see text).

Note: • The defect concentrations are always << ni. ( doping EF point defect concentrations)

• As doping changes, the neutral point defect concentrations are constant.

• However, the charged defect concentrations change with doping. \ the total point defect concentrations change with doping.

Point Defects

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Point Defects

I and V increase with T & are very mobile and affect many various processing

•Concentrations of I and V are different (surface generation & recombination= sinks, defects)• Equilibrium concentrations change instantaneously with T• Non-equilibrium concentrations possible (after implantation, CZ growth - freeze leads to swirls, oxidation)

1012 cm-3

5x1013 cm-3

@1000°C

Point Defects are very fast:Diffusivities of V and I >> dopants’ diffusivities

Very difficult to measure

These concentrations can be measured (ex. RBS)

ni≈7.14x1018cm-3

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Fermi-Diracdistributionnot sharp at high T

Neutral – do not depend on F-level

Still extrinsic semiconductor so F-level in the upper half.

interstitials

vacancies

Low doped Highly doped

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Continue the Example

• At 1000 ˚C, the P region will be intrinsic, the N region is extrinsic.

Note: • ni relative to doping in the two regions. • V0 is the same in the two regions. • Different charge states dominate in the different regions.

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Oxygen and Carbon in Silicon

10-20 ppm (5x1017-1018cm-3) • Interstitial oxygen improves crystal strength (≈25%)• Oxygen (thermal 450°C) donors TD (1016cm-3) affect resistivity; TD dissolve T>500°C • Precipitations weakens strength, getter impurities, contribute for dislocation and SF formation

Carbon CC ≈ 1016 cm-3 is usually substitutional in Si, affects SiO2 precipitates and high T thermal donors (650-1000°C), interacts with point defects and some dopants

grow

volume expansion (stress)alleviated by V and I

30 Å embryos grow @700 °C

compensate stress

Solubility of oxygen

1.2x1018cm-3 in melt

precipitation at T •heterogenous or •homogeniuos

Growing precipitates consume V release I

Si-Si → Si-O-Si

Processes that generate V cause SiO2 precipitates’ growth I cause smaller growth

Co*

1000°C=1017cm-3

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Measurements of the Grown Crystal

Resistivity

>>s

r

xj<<t

Sheet resistance

Average resistivity

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Measurements of the Grown Crystal

Conductivity Type

Seebeck voltage

Electrons move - Donors stay

Pn is the thermoelectric powerSign of the measured voltage V tells what is the conductivity type

Electron current

25-100°C hotter

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Hall Effect Measurement

Measurements of majority carrier concentrations (and type) and their mobility

w/o B field

Electrons at the bottom

Test structure

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E field

•w

av. carriers concentrations

Fourier Transport Infrared Spectroscopy (FTIR)

For Interstitial Oxygen incorporated during CZ growth and Substitutional Carbon (detection limit O- 2x1015cm-3, C- 5x1015cm-3 )

Fourier transform

Sweep the wavelength of incident energy look for absorption by specific molecules to identify their presence (ex. Wavenumbers of Oxygen as SiO2 is 1106 cm-1, C it is 607cm-1)

Transmittance

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Electron Microscopy: SEM and TEM

SEM - 1-40 keV e-beam (primary)Secondary or backscattered e are collected and displayed on CRT -> up to 300,000X

TEM - 100-300 keV e-beam passes through a sample -> atomic resolution

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Defects Etches

Defect free crystals are grown: no dislocations, no stacking faults! Crystals contain Native Defects (Swirls - role of oxygen and carbon) due to point defects’ agglomeration and metal contaminants (seen as Saucer Pits after annealing)

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Limits and Future Trends in Technologies and Models

Crystal growth *magnetic field to reduce thermal convection - more uniform doping and size*double crucible

Wafers *epi-layers (N/N+ or P/P+)*SOI

*SIMOX (Separation by IMplanted OXygen)*BESOI (Bonded and Etch-Back Technology)*Smart-cut*ELO (Epitaxial Lateral Overgrowth)

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Summary of Key Ideas

• Raw materials (SiO2) are refined to produce electronic grade silicon with a purity unmatched by any other commonly available material on earth.

• CZ crystal growth produces structurally perfect Si single crystals which can then be cut into wafers and polished as the starting material for IC manufacturing.

• Starting wafers contain only dopants, O, and C in measurable quantities.

• Dopant incorporation during crystal growth is straightforward except for segregation effects which cause spatial variations in the dopant concentrations.

• 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.

• For more information see papers @ http://www.memc.com/t-technical-papers.asp

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