SILICON EPITAXYSILICON EPITAXY

MAIMOONA YASMINMAIMOONA YASMIN

PHYSICS DEPARTMENTPHYSICS DEPARTMENT

UNIVERSITY OF LUCKNOWUNIVERSITY OF LUCKNOW

LUCKNOWLUCKNOW--226 007226 007

maimoonayasmin@yahoo.com

SILICON EPITAXYSILICON EPITAXY1. Definitions.

2. Why Silicon dominates?

3. Why silicon epitaxy?

4. Lattice matching in epitaxial growth.

5. Gases used in Silicon epitaxy.

6. Types of Epitaxy.

7. Silicon on insulator.

8. Applications

- Si on Sapphire device

- Buried Layer Device

- UV Silicon Detector

9. Summary

DefinitionsDefinitions

Epitaxy: “arranged upon”Homoepitaxy: Same substrate and film

e.g. Silicon on Silicon

Heteroepitaxy: Different substrate and film.

e.g. Silicon on Sapphire.

Why Silicon dominates?Why Silicon dominates? Abundant, cheap

Silicon dioxide is very stable, strong dielectric and it is easy to grow on thermal process.

Wider band gap, wide operation temperature

Unit cell of single crystal silicon

Si

2900oCBoiling PointFrom Latin Word “silices”

Origin of name

1414oCMelting point1824Discovery Year

28%ReflectivitySwedenDiscovered at

1000,000 µΩcmElectrical resistivityJons Jacob BerzeliusDiscoverer

2200 m/sVelocity of sound28.0855Atomic Weight

12.06 cm3Molar Volume14Atomic Number

2.33 gm/cm3Density of solidSiSymbol

2.352 AoBond length in single Crystal Si

SiliconName

Si

Si

Si

Si

Why Why Si EpitaxySi Epitaxy??

To enhance the performance of discrete bipolar

transistor.

To improve the performance of dynamic random access

memory devices (RAMs).

Advantages of epitaxial wafers over bulk wafers

Offers means of controlling the doping profile

Epitaxial layers are generally oxygen and carbon free

Lattice matching in Epitaxial Growth

Lattice structure and lattice constant must match for two

materials eg. GaAs and AlAs both have zincblende structure

1.43eV

0.36eV

In .53Ga.47 As

5.65 6.06

Gases used in Silicon Gases used in Silicon EpitaxyEpitaxy

a) Silane (SiH4) Pyrolysis : SiH4 (H2) Si + 2H2

b) Dichlorosilane (DCS) SiH2Cl2

c) Tricholorosilane (TCS) SiHCl3

d) Silicon tetrachloride SiCl4

e) Disilane Si2H6

f) Dopant gases –Diborane (B2H6)

– Phosphine (PH3)

– Arsine (AsH3)

Types of Types of EpitaxyEpitaxy(a) Liquid phase epitaxy

- III-V epitaxial layer GaAs

- Refreeze of laser melted silicon

(b) Molecular beam epitaxy

- Crystalline layer grows in vacuum

- 500o C

(c) Vapor phase epitaxy

- It is performed by chemical vapor deposition (CVD)

- Provides excellent control of thickness, doping and

crystallinity

- High temperature (800o C – 1100oC)

Liquid phase Liquid phase epitaxyepitaxy Growing crystals from a liquid solution below their

melting point .

Melting point of GaAs is 1238oC whereas a mixture

of GaAs with Ga metal has considerably lower

melting point

Single crystal GaAs layer can be grown from

Ga+GaAs melt.

The solution becomes richer in Ga and thus lower

melting point.

Low temperature eliminates many problems of

impurity introduction.

LIQUID PHASE EPITAXYLIQUID PHASE EPITAXYGrowth of Growth of AlGaAs AlGaAs and and GaAs GaAs layer on layer on GaAs GaAs substratesubstrate

Wafer held on carbon slider

Moves into a pocket containing melt

Slider moves the substrate to the next chamber.

Molecular beam epitaxy (MBE)

Substrate is held in high vacuum in the range 10-10 torr

Components along with dopants, are heated in separate

cylindrical cells.

Collimated beams of these escape into the vacuum and are

directed into the surface of a substrate

Sample held at relatively low temperature (600oC for GaAs)

Conventional temperature range is 400o C to 800oC

Growth rates are in the range of 0.01 to 0.3 µm/min

EquipmentEquipment

EquipmentEquipment

An ultra high vacuum chamber holding heated substrate.

Furnaces holding electronic grade silicon and dopants.

Beams of these dopants & EGS directed into the heated wafer.

For attaining vacuum level in the 10–10 torr range, material should have a low vapor pressure and low sticking coefficient.

Silicon volatized by electron beam heating rather than by heating in furnace.

Buffers & shutters shape and control flux.

Resistance heating generates temperature over the range of 400oC to 1100oC.

Advantages of MBEAdvantages of MBE Low temperature processing (400oC-800oC)

Precise control of doping

No chemical reactions along with high thermal velocities results in

properties rapidly changing with source

A wider choice of dopants

Mostly used dopants are Sb, Ga, Al

Vapor phase Vapor phase epitaxyepitaxy

Crystallization from vapor phase

Better purity and crystal perfection

Offers great flexibility in the actual fabrication of devices

Epitaxial layers are generally grown on Si substrates by the

controlled deposition of Si atoms onto the surface from a

chemical vapor containing Si

e.g. SiCl4 + 2H2 Si + 4HCl

(for deposition as well as for etching)

Vapor Phase Vapor Phase EpitaxyEpitaxy

Four silicon sources have been used for growing epitaxial

silicon

Silicon tetrachloride (SiCl4)

Dichlorosilane (SiH2Cl2)

Trichlorosilane (SiHCl3)

Silane (SiH4)

Four species in a reaction

SiCl4 (gas) + 2H2 (gas) Si (solid) + 4HCl (gas)

at 1200o C were detected

Concentration of species at different Concentration of species at different

positions along a horizontal reactorpositions along a horizontal reactor

Overall reaction in VPEOverall reaction in VPE

SiCl4 concentration decreases while the other three

constituents (SiHCl3, SiH2Cl2, HCl) increase.

SiCl4 + H2 SiHCl3 + HCl ……….. (1)

SiHCl3 + H2 SiH2Cl2 + HCl……….. (2)

SiH2Cl2 + H2 SiCl2 + H2 …………(3)

SiHCl3 SiCl2 + HCl …………. (4)

SiCl2 + H2 Si + 2HCl ……………(5)

EquipmentEquipment

Weight 2000 Kg

Occupy 2m2 or more of floor space.

Quartz reaction chamber with

susceptors

Graphite susceptors for physical

support

A coating of silicon carbide (50 to

500 µm) applied by CVD process

on susceptors.

Rf heating coil or tungsten halogen

lamps.

Radiant heating

Water cooling

A radiant barrel reactor

Three basic reactor configurationsThree basic reactor configurations

VPE processVPE process

Hydrogen gas purges of air from the reactor .

Reactor is heated to a temperature.

After thermal equilibrium, an HCl etch takes place at

1150oC and 1200oC for 3 minutes nominally.

Temperature is reduced to growth temperature.

Silicon source and dopant flows are turned on.

After growth, temperature is reduced by shutting off

power.

Hydrogen flow replaced by nitrogen flow.

Depending on wafer diameter and reactor type, 10 to 50

wafer per batch can be formed.

Process cycle times are about one hour .

Doping

Intentional addition of impurities or dopants to the crystal

to change its electronic properties (varying conductivity of

semiconductors)

Doping of 1014 to 1017 atom/cm3

Typically hydrides of atoms are used as the source of

dopants eg. PH3, AsH3 or B2H6 for controlled doping

2AsH2AsH33 (gas) (gas) 2As (solid) + 3H2As (solid) + 3H22 (gas)(gas)

2As (solid) 2As2As (solid) 2As++ (solid) + 2e(solid) + 2e--

Doping: Schematic representation of Doping: Schematic representation of

arsine doping and growth processesarsine doping and growth processes

2AsH2AsH33 (gas) (gas) 2As (solid) + 3H2As (solid) + 3H22 (gas)(gas)

2As (solid) 2As2As (solid) 2As++ (solid) + 2e(solid) + 2e––

Doping: Impurity concentrationDoping: Impurity concentration

Interaction between doping process & growth process

Growth rate influences the amount of dopant incorporated in Si

Equilibrium established at low growth rates.

AutodopingAutodoping Outdiffusion from heavily doped substrate

Impurity incorporation from dopant in gas phase

Autodoping limits the minimum layer thickness

Generalized doping profile of an epitaxial layer detailing various

regions of autodoping

Minimizing Minimizing AutodopingAutodoping

Fast growth to minimize outdiffusion.

Low temperature deposition reduces boron

autodoping (not As however).

Seal backside of substrate with highly doped

polyoxide.

Avoid the use of HCl etching.

Reduced pressure epitaxy.

Silicon on insulatorsSilicon on insulators

Fabrication of devices in small islands of silicon on an insulating substrate eg. Silicon on Sapphire (Al2O3)

Substrates have the appropriate thermal expansion match to silicon.

Epitaxial films grown by CVD (eg. Pyrolysis of silane)

Junction capacitance is reduced thus improve the high frequency operation of circuits

Silicon on sapphireSilicon on sapphire

SiHSiH44 Si Si + 2H+ 2H22 (low temperature)(low temperature)

Temperature 1000o C – 1050oC

Growth rate 0.5 µm/min

Film thickness 1 µm or less

Doping range 1014 to 1016 atoms/cm3

High defect density permits only majority

carrier devices

Carrier mobility is reduced.

Buried layerBuried layer• The higher collector series resistance of an integrated

transistor can be easily reduced by a process known as

“buried layer”

Silicon on sapphire devices

Ultraviolet Silicon DetectorUltraviolet Silicon Detector

SUMMARY Silicon is the most preferable material for epitaxial growth.

Three basic methods for growing epitaxial layer on silicon.

(a.) LPE (b). MBE (C). VPE

In all these, lattice structure and lattice constant should match for the two

materials.

The advantages of SOI techniques are compelling for high density and high

speed circuits

MBE is advantageous in ion implanted VLSI circuits

While growth of crystal the growth rate and doping rate should be well

controlled.

Autodoping can be controlled by low temperature epitaxial growth.

Fast growth to minimize outdiffusion.

The trend to thinner layers for bipolar and unipolar ICs will result in

incremental process improvements and continued steady of flatness changes,

defect generation, and autodoping effect.

Contamination free epitaxy will be a worthwhile process improvement.